MHC class I and II deficiencies Suheir Hanna, MD, and Amos Etzioni, MD
Haifa, Israel
Deficiencies of MHC complex class I or II are rare primary immunodeficiencies, both of which are inherited in an autosomal recessive pattern. MHC class II deficiency is a prototype of a disease of gene regulation. Defects in transacting regulatory factors required for expression of MHC class II genes, rather than the genes themselves, are responsible for the disease phenotype. The affected genes are known to encode 4 distinct regulatory factors controlling transcription of MHC class II genes. These transacting factors are the class II transactivator and 3 subunits of regulatory factor X (RFX): RFX containing ankyrin repeats (RFXANK), the fifth member of the RFX family (RFX5), and RFX-associated protein (RFXAP). Mutations in one of each define 4 distinct complementation groups termed A, B, C, and D, respectively. MHC class I deficiency is extremely rare and has been reported in less than 30 patients worldwide. Here we review the clinical, genetic, and molecular features that characterize these primary immunodeficiencies and discuss therapy options. Beyond the description of MHC class I and II deficiencies, their discovery has fascinated scientists and clinicians because of their ability to reveal the molecular basis of MCH regulation. (J Allergy Clin Immunol 2014;134:269-75.) Key words: MHC class I and II, immunodeficiency, regulatory factor X, class II transactivator, transporter associated with antigen processing I and II, tapasin
MHC class II molecules, also called HLAs, are heterodimeric transmembrane glycoproteins consisting of a and b chains. In human subjects there are 3 different isotypes (HLA-DR, HLA-DQ, and HLA-DP), all of which are encoded by distinct a and b genes clustered on the short arm of chromosome 6. MHC molecules are characterized by a very high level of polymorphism.1 This great allelic diversity seems largely caused by gene duplication. Many evolutionary biologists have tried to explain this enormous diversity, but until now, the evolution of the MHC complex has been unclear.2 All 3 MHC class II isotypes serve the same main function, namely the presentation of exogenous peptides to the T-cell receptor (TCR) of CD4 T helper cells. Recognition of the MHC-peptide complex by the TCR is crucial for the normal adaptive immune response.3 Furthermore, MHC class II expression on thymic epithelial cells promotes the direction of From the Department of Pediatrics and the Pediatric Immunology Unit, Rambam Medical Center, and the B. Rappaport Faculty of Medicine, Technion. Disclosure of potential conflict of interest: S. Hanna and A. Etzioni are employed by the Israeli government. A. Etzioni receives royalties from FRIM Books. Received for publication February 27, 2014; revised June 2, 2014; accepted for publication June 2, 2014. Available online July 7, 2014. Corresponding author: Amos Etzioni, MD, Ruth Children Hospital, Rambam Medical Campus, Bat-Galim, Haifa, Israel 31096. E-mail: [email protected]. 0091-6749/$36.00 Ó 2014 American Academy of Allergy, Asthma & Immunology http://dx.doi.org/10.1016/j.jaci.2014.06.001
Abbreviations used B2M: b2-Microglobulin CG: Complementation group CID: Combined immunodeficiency CIITA: Class II transactivator ER: Endoplasmic reticulum HSCT: Hematopoietic stem cell transplantation NK: Natural killer PID: Primary immunodeficiency RFX: Regulatory factor X RFX5: Fifth member of the RFX family RFXANK: RFX containing ankyrin repeats RFXAP: RFX-associated protein SCID: Severe combined immunodeficiency TAP: Transporter associated with antigen processing TCR: T-cell receptor
positive and negative selection events, thereby leading to the establishment of the TCR repertoire of the CD41 T-cell population. The lifespan of CD41 cells in the periphery is also dependent on interactions with MHC class II–positive cells.4 On the other hand, HLA class I molecules present peptides driven from protein synthesized in the cell to cytotoxic CD8 T lymphocytes and thus are involved in immune defense against intracellular pathogens.5 Taking into account these important functions of MHC class I and II in the normal immune response, it is clear why defects in these molecules will lead to immunodeficiency. However, in both patients with class I and those with class II deficiency, although mainly those with class I deficiency, the clinical presentation is not as severe as one would expect, and thus these are classified as combined immunodeficiencies (CIDs) and not as severe combined immunodeficiency (SCID). This might be due to the fact that low levels of MHC class I and II molecules are always present.
MHC CLASS I DEFICIENCY MHC class I deficiency is an extremely rare autosomal recessive primary immunodeficiency (PID) that thus far has been reported in only a few cases worldwide. It represents a heterogeneous group of disorders that collectively share a decreased surface expression of HLA class I molecules.5,6 Interestingly, the first reported case of MHC class I deficiency by Touraine et al7 in 1978 was later found to be a typical MHC class II deficiency. The rest of the documented cases, all sharing defective class I molecule surface expression but intact class II molecule presentation, are further categorized into the following subgroups based on genetic, biochemical, and/or clinical characteristics: transporter associated with antigen processing (TAP) 1 and TAP2 and tapasin deficiencies.5,6 The aforementioned genes are not involved in HLA class I transcription or synthesis but rather are essential for the process of peptide transport and loading, a 269
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prerequisite step for subsequent proper surface presentation of HLA class I molecules. Consequently, intracellular levels of HLA class I molecules are normal, although their surface expression is compromised.5,8 In the fourth group the causative genetic defect has not been elucidated yet is presumed to involve HLA class I–related unidentified transcriptional defects.6
Clinical manifestations The larger and best studied groups of MHC class I deficiency are those resulting from a defect in either the TAP19 or TAP28 subunits. Patients are usually asymptomatic through infancy. The clinical course typically presents during the first decade of life, with recurrent bacterial infections confined to the respiratory tract evolving into chronic inflammatory lung disease and bronchiectasis. Aside from the respiratory manifestations, skin involvement by sterile necrotizing granulomatous lesions resembling granulomatosis with polyangiitis have also have been described in several patients.5,6 These lesions might be related to vasculitis and mainly involve the legs. Lesions occurring in the upper respiratory tract causing midface deformity have also been described. Some asymptomatic TAP1- and TAP2-deficient cases were also identified (Table I).5,10 Tapasin deficiency was reported in only 1 patient who had late-onset chronic primary glomerulonephritis without any of the symptoms associated with TAP deficiency.11 The fourth group of MHC class I deficiency was described in only 2 brothers. One had unexplained steroid-responsive anemia, and the defective HLA class I surface expression was incidentally seen on HLA-matching studies. His brother was later discovered but was completely asymptomatic.6 Few other systemic diseases that present with chronic bacterial infection of the respiratory tract, bronchiectasis, and/or granulomatous skin disease can be considered in the differential diagnosis, such as chronic granulomatous disease, common variable immunodeficiency, granulomatosis with polyangiitis, and sarcoidosis. Molecular basis HLA class I molecules are composed of a polymorphic heavy chain encoded by HLA-A, HLA-B, and HLA-C genes, which are associated with b2-microglobulin (B2M). They play important roles in immune surveillance by interacting with CD81 T and natural killer (NK) cells. The HLA class I genes are located on chromosome 6. The assembly of heavy chains with B2M occurs in the lumen of the endoplasmic reticulum (ER). After assembly, the complexes are loaded with peptides derived from the degradation of intracellular proteins.12 The antigenic peptides are transported from the cytosol into the lumen of the ER by a transporter protein named TAP. TAP is composed of 2 subunits: TAP1 and TAP2. The genes encoding the TAP subunits are located in the HLA class II genetic region on chromosome 6. Tapasin is a TAP-associated glycoprotein that links TAP to the heavy chain.11 In patients with TAP deficiency, the HLA class I heavy chain/B2M complexes are peptide free. These unoccupied HLA class I molecules are retained between the ER and the cis-Golgi compartment.8
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Diagnosis Until recently, HLA class I deficiency was diagnosed by means of serologic HLA typing. However, with the decreased use of serologic testing for ‘‘routine’’ HLA phenotyping, molecular typing has widely replaced such techniques. HLA homozygosity in suspected indexes suggests TAP or tapasin deficiency because both molecules are located within the MHC locus on chromosome 6. Flow cytometric analysis of PBMCs labeled with a pananti–HLA class I mAb (W6/32) is an additional valuable diagnostic tool.5 Further experiments might facilitate differentiating the specific type of defect, irrespective of whether it involves transcription or assembly errors. Intact intracellular HLA type I molecular levels associated with the absence of MHC sialylation along with degradation tendency and complex instability at 378C highly support protein assembly defects as seen in patients with TAP or tapasin deficiency rather than a primary HLA I molecular defect. Accordingly, their extracellular expression is not significantly affected on cytokine stimulation. However, in the fourth group transcriptional defects are involved, the intracellular expression of HLA class I mRNA and molecules are decreased, and their extracellular expression can be induced by stimulation with various cytokines.5,8 Immunologic aspects The evaluation of T-lymphocyte subpopulations reveals a normal or increased CD41/CD81 cell ratio. This is mainly due to decreased CD81 ab T-cell counts. It would be reasonably anticipated that the CD81 ab T-cell repertoire would be compromised because of defective thymic positive selection, but such a repertoire is surprisingly normal in most cases.8,13 Immunoglobulin levels are normal. Although the number of NK cells was found to be within normal ranges, a phenotypic NK cell study showed differences. NK and T cells displayed less cytotoxic reactivity; however, on activation, they presented with overreactivity toward autologous cells. These autoreactive characteristics might explain the inflammatory nature of the disease.14 Pathologic hypotheses Although the genetic background has been revealed in most of the cases of MHC class I deficiency, the way it is translated into clinical outcomes requires further clarification. Because HLA class I molecules mediate intracellular protein presentation to CD81 T lymphocytes, patients are expected to present with susceptibility to viral infections, cancer, or both. However, these patients seem to maintain intact antiviral immunity with no significant propensity for malignancy. One of the proposed explanations of such dissociation is that NK cells and some ab CD81 cells can promote efficient immune defense in a TAP-independent manner.5 Moreover, an expansion of CD81, NK, and gd T cells was observed on stimulation, suggesting that they can be recruited in immune responses. The potential pathophysiology behind the noninfectious phenotype and recurrent bacterial infections that are confined to the respiratory tract are proposed to be related to secondary inflammatory reactions. It is presumed that inefficient clearance of viral infections from lung tissues induces the synthesis of several cytokines and chemoattractants that promote neutrophil influx
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TABLE I. Characteristics of patients with MHC class I and II deficiency
Disease
Genetic defect/presumed pathogenesis Inheritance
Circulating T-cell counts
Serum Circulating immune B-cell globulin counts level
Clinical features
Functional analysis
MHC class I Mutations in TAP1 and deficiency TAP2 genes involved in protein loading of MHC class I molecules
AR
Total normal Normal CD8 low CD4 normal
Normal Bacterial respiratory tract Normal responses on mitogen infections, bronchiectasis, stimulation. Normal specific skin granulomatous lesions immune reactivity to foreign (can be asymptomatic) antigens
MHC class II Mutation in transcription deficiency factors for MHC class II proteins (CIITA, RFX5, RFXAP, and RFXANK genes)
AR
Total normal Normal CD4 low CD8 normal
Low
Recurrent severe infections. Normal responses on mitogen Failure to thrive, diarrhea, stimulation. Absence of liver/biliary tract disease specific cellular and humoral Autoimmune cytopenias immune reactivity to foreign antigens
AR, Autosomal recessive.
and inflammation. This can lead to destruction of ciliary cells and lung fibrosis, resulting in defective bacterial elimination, which further increases neutrophil chemotaxis, thus maintaining a chronic lung inflammatory state.14 Activated NK and gb T cells, which kill autologous cells, might be involved in development of the sterile granulomatous skin lesions observed in some patients and are also presumed to take part in lung injury evolution. Although taking into consideration the limitations of extrapolating animal data to human subjects, several animal models displayed similar mechanisms.5,14
Treatment and prognosis No specific therapy can yet be proposed. Prevention and treatment of bronchial infections are the main therapeutic strategies and are virtually identical to the approach used in patients with other chronic respiratory inflammatory disorders, such as cystic fibrosis. As for chronic sinus involvement, a conservative approach is recommended, including local washing, use of high-dose topical steroids, and consideration of antibiotics for acute exacerbations. Only basic antiseptic care can be recommended for skin ulcers. Skin grafting had no positive effects, and lesions reappeared. IFN-a treatment trials worsened the lesions.6 MHC CLASS II DEFICIENCY MHC class II deficiency is a rare primary autosomal recessive immunodeficiency disorder (PID; MIM 209920) first described in the late 1970s. The disease is primarily characterized by the absence of MHC class II molecules on the surfaces of immune cells. Because these molecules have a pivotal role in the control of various immune responses, their absence results in severely impaired cellular and humoral immune responses, leading to significant susceptibility to severe infections and, frequently, death in early childhood.15 MHC class II determinants, also called HLAs, are polygenic and highly polymorphic cell-surface glycoproteins that form a peptide-binding cleft after their assembly as a and b chain heterodimers. In human subjects there are 3 MHC class II isotypes, designated HLA-DR, HLA-DQ, and HLA- DP, each of which is
composed of a distinct pair of a and b chains. All genes encoding the 3 isotypes are clustered in the D region of the MHC on the short arm of chromosome 6. Two modes of MHC class II expression are observed: constitutive and inducible. Constitutive expression is restricted to the surfaces of 3 distinct types of antigen-presenting cells: dendritic cells, monocyte/macrophage lineage, and B lymphocytes. In human subjects activated T cells express MHC class II as well. Most other cell types do not usually express MHC class II molecules, but such expression can be induced by a variety of stimuli, of which the most potent and known is IFN-g. MCH class II molecules mediate several key functions in the adaptive immune system, directing the development, activation, and homeostasis of CD41 T helper cells. These include the MHC class II– dependent presentation of peptides to the TCRs of CD41 T helper cells and the thymic shaping of the CD41 TCR repertoire.16
Clinical features and worldwide distribution Although MHC class II deficiency is not considered classical SCID according to the International Union of Immunological Societies classification criteria, patients usually present with clinical findings of typical CID. It is a rare CID, with approximately 200 patients reported worldwide. The majority of patients are of North African origin (Algeria, Tunisia, and Morocco), and the remaining patients are of diverse ethnic backgrounds.17 Comparing annual reports regarding the prevalence of the different subtypes of SCIDs revealed an approximately 5% frequency of MHC class II in the Canadian survey, whereas approximately 20% and 30% of SCID cases occurred in Kuwait and in North African countries, respectively.18,19 This discrepancy is related to the higher consanguinity rates in such countries along with the determination of a founder effect of a certain mutation prevalent in the North African population.20 Patients exhibit an extreme vulnerability to infections, including a broad spectrum of bacterial, viral, fungal, and protozoan pathogens. Typical clinical manifestations include an early onset of severe and recurrent infections, mainly of the respiratory and gastrointestinal tracts. Protracted diarrhea with failure to thrive is often present. Recurrent pneumonia and upper respiratory tract infections, including rhinitis, otitis, and sinusitis, were observed in almost
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all patients. The most common infectious agents are intracellular pathogens, such as cytomegalovirus, herpes simplex virus, Pneumocystis jirovecii, Salmonella and Cryptosporidium species, and extracellular pathogens, such as Pseudomonas species, Staphylococcus species, streptococci, and Candida species. Intestinal and hepatic involvement caused by Cryptosporidium species colonization appears to be more frequent in patients with MHC class II deficiency than in patients with other immunodeficiencies. Hepatic involvement, including sclerosing cholangitis, is frequent and associated with poor prognosis. The absence of BCGitis can be accounted for by the presence of residual immunity in the form of CD81 T and NK cells.19-21 Autoimmune manifestations were observed in approximately 20% of patients, such as autoimmune cytopenias, anemia, neutropenia, and/or thrombocytopenia (Table I). The associated autoimmune phenomena were not correlated with poor prognosis. In one series dysmorphic features were seen in 3 unrelated patients, all of whom had normal karyotypes. This should be researched more carefully to evaluate its relevance.21 Although most patients were given a diagnosis in the first year of life, some were given a diagnosis later at up to 15 years of age. This might be related to a milder onset in some patients, delays in the referral of patients to specialized centers, or both. Immunologic investigations revealed no significant differences from other patients. Innate or CD81 T cell–mediated immunity might account for this more favorable outcome. Both genetic factors and environmental factors, such as hygiene, might be responsible for differences in the clinical expression of the disease state.20
Immunologic features The immunologic hallmark of the disease is the absence of constitutive and inducible expression of MHC class II molecules on all cell types. The most constant immunologic feature is the absence of specific cellular and humoral immune reactivity to foreign antigens with intact responses to mitogen stimulation because the latter is not MHC class II–TCR complex restricted. The most prominent finding on lymphocytic phenotypes is the absence or very low HLA-DR expression on lymphocytes, with reduced CD4 cell counts leading to an inverted CD4/CD8 ratio. The low CD4 count presumably reflects abnormal thymic selection and maturation from the lack of MHC class II expression on cortical epithelial cells in the thymus. The reversed CD41/CD81 ratio is due to an absolute CD41 T-cell count reduction and a concomitant increase in CD81 T-cell counts, which might result in normal total numbers of circulating T cells. CD81 T-cell counts, MHC class I levels, or both can be decreased as well.16 Interestingly, patients with MHC class II deficiency have detectable TCR excision circles in both total lymphocytes and sorted CD4 cells.22,23 This reflects the fact that early T-cell development is normal in patients with this condition. Thus it might be missed by the TCR excision circle quantification assay, which has been proposed to be a sensitive neonatal screening test capable of identifying SCID and other T-cell lymphopenias. However, some clonal abnormalities, along with reduced DNA rearrangement events, have been reported. The majority of patients have a decrease in levels of 1 or 2 immunoglobulin isotypes, although panhypogammaglobulinemia is also common.16
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FIG 1. Molecular defects in patients with bare lymphocyte syndrome (BLS) type II. Four different transacting factors on MHC class II promoters have been associated with BLS type II. A defect in any of them defines a distinct CG. The gene encoding CIITA is mutated in CG A. The genes encoding the RFX subunits RFXANK, RFX5, and RFXAP are defective in CGs B, C, and D, respectively.
Genetic and molecular background The genetic basis of MHC class II deficiency exhibits a unique feature because the genes implicated in the phenotypic manifestation of the disease, namely the family of MHC class II genes on chromosome 6, are intact, although the mutated genes reside outside the MHC class II locus (Fig 1). On the basis of somatic cell fusion experiments with transformed B cells from patients, 4 distinct groups of MHC class II deficiency were identified: complementation groups (CG) A, B, C, and D. The genetic background of the disease was ultimately confirmed only several years later after the identification and cloning of 4 different mutated genes corresponding to 4 distinct key MHC class II regulatory genes encoding transacting regulatory factors that coordinately control the expression of MHC class II at the transcriptional level.15 The class II transactivator (CIITA) is mutated in CG A, whereas the 3 subunits of the regulatory factor X (RFX), namely RFX containing ankyrin repeats (RFXANK), the fifth member of the RFX family (RFX5), and RFX-associated protein (RFXAP), are defective in CGs B, C, and D, respectively. MHC class II deficiency is a monogenic disease in which a single defective gene is responsible for the entire clinical picture. Despite the genetic heterogeneity, the clinical presentation is quite homogeneous, with no obvious correlation between the CG and the clinical picture.16 The first MHC class II deficiency gene to be discovered was CIITA, and it is classified in group A. It was isolated based on complementation studies. RJ2.2.5 is one of the in vitro–generated mutant MHC class II–negative cell lines allocated to CG A. Transfection of this cell line with a B-cell cDNA library restored MHC expression, leading to the isolation of cDNA clones encoding the MHC CIITA.24 The human gene MHC2TA is localized on chromosome 16. Expression vectors encoding CIITA can reactivate expression of all 3 MHC class II isotypes when transfected into cell lines from CG A. Nine mutations have been characterized thus far. The expressions of CIITA and MHC class II are tightly correlated: the majority of cells do not express the MHC2TA gene and are MHC class II negative. CIITA expression is cell specific and limited to MHC class II–positive
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cells. Transfection of cells with CIITA expression vectors is sufficient to induce MHC class II expression. Silencing of the CIITA gene in the differentiated mature plasma cells is correlated with loss of MHC class II expression.16,25 The RFX–DNA-binding complex consists of 3 subunits encoded by 3 distinct genes called RFXANK, RFX5, and RFXAP and are mutated in MHC class II deficiency CGs B, C, and D, respectively.16 Unlike CIITA, it is expressed ubiquitously in all cell types examined, even in cells that do not express MCH class II. RFX5 cDNA clones were isolated because they restored MHC class II expression when transfected into SJO cells, a cell line derived from a patient with MHC class II deficiency in CG C. It derived its name because it is the fifth isolated member of the DNA-binding proteins interacting with the MHC class II X box. The RFX 5 gene is located on chromosome 1.16 Six mutations of the RFX5 gene have been identified. Defects in the RFXAP gene (on chromosome 13) account for CG D.26 Thus far, 3 different mutations have been identified. An in vitro–generated mutant (6.1.6) has also been shown to contain mutations in RFXAP. The RFXANK defect, also called RFXB, defines CG B, the most common group, accounting for more than 70% of all known patients. RFXANK was identified by using a biochemical approach after purification of the RFX complex. It is located on chromosome 19. Transfection of the RFXANK cDNA into cell lines from group B restored expression of all MHC class II isotypes. The RFXANK gene’s most frequent mutation is a 26-bp deletion at the boundary between intron 5 and exon 6 called c.338-25_338del26 (also known as 752delG-25). It is also the most frequent mutation responsible for MHC class II deficiency. It has been found in less than 90% of North African families, including many unrelated patients, indicating the existence of a founder effect. Indeed, a founder event has been identified and dated as responsible for the RFXANK deletion 15E6-25_15E611 in the North African population. The mutation was dated to approximately 2250 years ago, a period concurrent to the Berber civilization.20 Eight other mutations in the RFXANK gene have been described in patients of different ethnic backgrounds. RFXANK mutations dominated also in Middle Eastern populations. There is a correlation between certain groups and ethnic origin. Patients from group B are predominantly of North African origin, whereas those from group A are mainly Hispanic. Most mutations occur in splice sites that cause frameshifts, which result in truncated or absent proteins, providing an easy explanation for the lack of transcription and surface expression of MHC class II determinants. The loss of MHC expression suggests that both CIITA and the RFX complex are essential and that no bypass or other pathways can compensate for their absence. Moreover, no other major systems are critically dependent on RFX and CIITA, indicating their high specificity for MHC class II expression. No clinical/immunologic correlation was found with the different CGs. Mouse models for MHC class II deficiency have been constructed, including the MHC class II knockout mouse and CIITA and RFX5 knockout mice. All exhibited a similar phenotype to that of human disease. However, a significant discrepancy regarding the CD41 T-cell population was noticed among the 2 models and found to be nearly absent in mouse models, although it was only mildly reduced in MHC class II–deficient patients. This might be explained by different CD41 T-cell selection mechanisms among species.5
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Diagnosis The mainstay of diagnosis is the absence of MHC class II molecule expression on all cell types. Other tests, including the inability to produce specific immune reactions, such as antibodies, in response to immunization and delayed-type hypersensitivity skin tests, are decisive as well. Lymphopenia, mainly of the CD41 subclass, and hypogammaglobulinemia are frequently associated.17 The identification of the molecular defects (by using sequence analysis in specific genetic laboratories) underlying MHC class II deficiency permits reliable prenatal and postnatal diagnosis and allows accurate carrier detection. This is remarkably valuable in high-risk populations and families with previously identified cases or consanguineous marriage. Umbilical vein puncture and analysis of MHC class II expression on fetal leukocytes is an alternative when genetic background is unknown. Because all the patients reported thus far belong to one of the known complementary groups, the use of whole-exome sequencing is not needed in patients with MHC class II deficiency. Therapy and prognosis As for other CID disorders, allogeneic hematopoietic stem cell transplantation (HSCT), preferably from an HLA-identical sibling, is currently the only available curative treatment and is considered the treatment of choice.20 Aggressive treatment of acute infections and other complications can reduce the frequency and severity of the associated clinical manifestations. This includes intravenous anti-infectious medications, administration of intravenous immunoglobulins, parenteral nutrition, and prophylactic antibiotics. However, in most of the cases, these methods do not prevent progressive organ dysfunction. Unfortunately, without HSCT, patients will ultimately die between the ages of 5 and 18 years. The overall success rate of HSCT has been reported to be relatively poor compared with that seen in patients with other PIDs, with survival rates of less than 60%.27-29 Several protocols were used, and it seems that standard busulfan/cyclophosphamide conditioning resulted in higher donor engraftment. A reduced-intensity conditioning regimen might be particularly suited for patients with comorbid conditions.27 Umbilical cord blood transplantation was used in 4 cases, and all of them had good engraftment.28,30 Patients undergoing HSCT before the age of 2 years had better prognosis. The main causes of treatment failure are poor engraftment caused by residual host immunity, old age at diagnosis and/or treatment, and persistent viral infections.20 Thus it is highly recommended to perform HSCT in young patients independently of whether an HLA-identical sibling is available.1 Recently, a better outcome was reported, especially in HLA-identical transplant recipients, in whom the overall disease-free survival rate was 66% to 69% after a median follow-up of approximately 6 years. Interestingly, despite the lack of MHC class II expression, the risk of graft-versus-host disease is similar to that observed in patients with other forms of immunodeficiency. Among HSCT long-term survivors, CD41 T-cell counts remain low, with a moderate decrease in naive CD41 T-cell counts. This finding is probably related to defective MHC class II expression on the thymic epithelium of the host.20 Identification of the affected genes raised the hope
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that gene therapy might present an alternative treatment strategy. One probable concern is that the therapeutic transgene could induce ectopic levels of MHC class II expression, potentially causing harmful consequences, such as autoimmunity or tissue destruction. This is of particular importance in CG A because CIITA expression is cell specific, and it might be difficult to obtain the endogenous pattern of expression. Another potential problem is the possibility of abnormal selection of CD41 T cells resulting from the absence of MHC class II expression on thymic epithelial cells. However, because classical bone marrow transplantation can cure MHC deficiency without the requirement of functional thymus, this issue might not pose an obstacle. This strategy needs further optimization before its clinical implementation.31 Although most patients who do not undergo HSCT have a poor prognosis, with a life expectancy not exceeding the first decade, this is not always the case, with some patients surviving through teenage or adulthood. In some groups this preferable prognosis could be explained by residual expression of surface MHC class II determinants on patients’ immune cells retaining some antigen presentation function and thus protection. Several distinct mutations have been linked to such a milder course because they contain missense single point mutations rather than termination or nonsense mutations.32 However, in another groups of patients, the less severe manifestations could not be explained by such residual MHC class II expression. In a large series of North African patients, even within the group sharing the same mutation of the 26-bp deletion of the RFXANK gene, some affected cases have survived to the second decade. Similar findings were observed in a large Tunisian study.21 Collectively, these data suggest that their capacity to cope with infections must depend on other unknown genetic and immunologic factors.20 These newly observed patients demonstrate that with the better diagnostic techniques (mainly lymphocyte markers and genetic molecular analysis) now available, physicians should be able to detect less severe cases. It is likely that such patients will not require the same aggressive treatment used for typical cases of MHC class II deficiency.
SUMMARY MHC class I and II deficiencies are extremely rare PIDs, both of which share defective antigen presentation to CD81 and CD41 T cells, respectively. On the other hand, they differ enormously in their clinical manifestations. MHC class II deficiency presents as a CID, whereas MHC class I deficiency is mainly characterized by chronic lung disease along with skin granulomatous disorder. Although in patients with MHC class I deficiency the clinical presentation might vary widely in relation to the genetic defect, in patients with MHC class II deficiency, the phenotypic characteristics are largely uniform, although the genetic background is heterogeneous. Such discrepancy was clarified by revealing the regulatory nature of the involved genes. The elucidation of MHC class II deficiency disease has made remarkable contributions to what we currently know about the molecular mechanisms controlling MHC class II genes and is considered a textbook model for the regulation of gene expression in general. Because of its diverse presentation, MHC class I and II deficiencies should always be included in the differential diagnosis of unclear cases of CIDs.
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REFERENCES 1. Reith W, Picard C, Fischer A. Molecular basis of major histocompatibility complex class II deficiency. In: Ochs H, Smith E, Puck J, editors. Primary immunodeficiency diseases. A molecular and genetic approach. New York: Oxford University Press; 2014. pp. 241-57. 2. Piertney SB, Oliver MK. The evolutionary ecology of the major histocompatibility complex. Heredity 2006;96:7-21. 3. Batista FD, Harwood NE. The who, how and where of antigen presentation to B cell. Nat Rev Immunol 2009;9:15-27. 4. Waldburger JM, Masternak K, Muhlethaler-Mottet A, Villard J, Peretti M, Landmann S, et al. Lessons from the bare lymphocyte syndrome: molecular mechanisms regulation MHC class II expression. Immunol Rev 2000;178:148-65. 5. de la Salle H, Donato L, Hanau D. Peptide transporter defects in human leukocyte antigen class I deficiency. In: Ochs H, Smith E, Puck J, editors. Primary immunodeficiency diseases. A molecular and genetic approach. New York: Oxford University Press; 2014. pp. 258-68. 6. Gadola S, Moins-Teisserenc H, Trowsdale J, Gross WL, Cerundolo V. TAP deficiency syndrome. Clin Exp Immunol 2000;121:173-8. 7. Touraine J-L, Betuel H, Souillet G, Jeune M. Combined immunodeficiency disease associated with absence of cell-surface HLA-A and -B antigens. J Pediatr 1978;93: 47-51. 8. de la Salle H, Hanau D, Fricker D, Urlacher A, Kelly A, Salamero J, et al. Homozygous human TAP peptide transporter mutation in HLA class I deficiency. Science 1994;265:237-41. 9. de la Salle H, Zimmer J, Fricker D, Angenieux C, Cazenave JP, Okubo M, et al. HLA class I deficiencies due to mutations in subunit I of the peptide transporter TAP I. J Clin Invest 1999;103:R9-13. 10. de la Salle H, Saulquin X, Mansour I, Klayme S, Fricker D, Zimmer J, et al. Asymptomatic deficiency in the peptide transporter associated to antigen processing (TAP). Clin Exp Immunol 2002;128:525-31. 11. Yabe T, Kawamura S, Sato M, Kashiwase K, Tanaka H, Ishikawa Y, et al. A subject with a novel type I bare lymphocyte syndrome has tapasin deficiency due to deletion of 4 exon by Alu- mediated recombination. Blood 2002;100: 1496-8. 12. Pamer E, Cresswell P. Mechanisms of MHC class I-restricted antigen processing. Annu Rev Immunol 1998;16:323-53. 13. Matamoros N, Mila J, Llano M, Balas A, Vicario JL, Pons J, et al. Molecular studies and NK cell function of a new case of TAP2 homozygous human deficiency. Clin Exp Immunol 2001;125:274-82. 14. Markel G, Mussaffi H, Ling KL, Salio M, Gadola S, Steuer G, et al. The mechanisms controlling NK cell autoreactivity in TAP 2-deficient patients. Blood 2004;103:1770-8. 15. Nekrep N, Fontes J, Geyer M, Peterlin BM. When the lymphocyte loses its clothes. Immunity 2003;18:453-7. 16. Reith W, Mach B. The bare lymphocyte syndrome and the regulation of MHC expression. Annu Rev Immunol 2001;19:331-73. 17. Elhasid R, Etzioni A. Major histocompatibility complex class II deficiency: a clinical review. Blood Rev 1996;10:242-8. 18. Rozmus J, Junker A, Laffin Thibodeau M, Grenier D, Turvey SE, Yacoub W, et al. Severe combined immunodeficiency (SCID) in Canadian children: a national surveillance study. J Clin Immunol 2013;33:1310-6. 19. Al-Herz W, Alsmadi O, Melhem M, Recher M, Frugoni F, Notarangelo LD. Major histocompatibility complex class II deficiency in Kuwait: clinical manifestations, immunological findings and molecular profile. J Clin Immunol 2013;33:513-9. 20. Quederni M, Vincent Q, Frange P, Touzot F, Scerra S, Bejaoui M, et al. Major histocompatibility complex class II expression deficiency caused by a RFXANK founder mutation: a survey of 35 patients. Blood 2011;118:5108-17. 21. Ben-Mustapha I, Ben-Farhat K, Guirat-Dhouib N, Dhemaied E, Largueche B, Ben-Ali M, et al. Clinical, immunological and genetic findings of a large Tunisian series of major histocompatibility complex class II deficiency patients. J Clin Immunol 2013;33:865-70. 22. Kuo CY, Chase J, Garcia Lioret M, Stiehm MR, Moore T, Aguilera MJ, et al. Newborn screening for severe combined immunodeficiency does not identify bare lymphocyte syndrome. J Allergy Clin Immunol 2013;131:1963-5. 23. Lev A, Simon AJ, Broides A, Levi J, Garty BZ, Rosenthal E, et al. Thymic function in MHC class II-deficient patients. J Allergy Clin Immunol 2013;131:831-9. 24. Steimle V, Otten LA, Zufferey M, Mach B. Complementation cloning of an MHC class II transactivator mutated in hereditary MHC class II deficiency (or bare lymphocyte syndrome). Cell 1993;75:135-46. 25. Devaiah B, Singer D. CIITA and its dual roles in MHC gene transcription. Front Immunol 2013;4:1-6. 26. Villard J, Lisowska-Grospierre B, van den Elsen P, Fischer A, Reith W, Mach B. Mutation of the RFXAP, a regulator of the MHC class II genes, in primary MHC class II deficiency. N Engl J Med 1997;337:748-53.
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27. Renella R, Picard C, Neven B, Ouachee-Chardin M, Casanova JL, Le Deist F, et al. Human leukocyte antigen identical hematopoietic stem cell transplantation in major histocompatibility complex class II immunodeficiency: reduce survival correlated with an increase incidence of acute graft versus host disease and pre-existing viral infections. Br J Hematol 2006;134:510-6. 28. Small TN, Qasim W, Friedrich W, Chiesa R, Bleesing JJ, Scurlock A, et al. Alternative donor SCT for the treatment of MHC Class II deficiency. Bone Marrow Transplant 2013;48:226-32. 29. Al-Mousa H, Al-Shammari Z, Al-Ghonaiium A, Al-Dhekri H, Al-Muhsen S, Al-Saud B, et al. Allogenic stem cell transplantation using myeloablative and
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reduced intensity conditioning in patients with major histocompatibility complex class II deficiency. Biol Blood Marrow Transplant 2010;16:818-23. 30. Siepermann M, Gudowius S, Beltz K, Strier U, Feyen O, Troeger A, et al. MHC class II deficiency cured by unrelated mismatched umbilical cord blood transplantation: case report and review of 68 cases in the literature. Pediatr Transplant 2011;15:E80-6. 31. Matheux F, Villaed J. Cellular and gene therapy for major histocompatibility complex class II deficiency. News Physiol Sci 2004;19:154-8. 32. Nekrep N, Jabrane-Ferrat N, Wolf H, Eibl MM, Geyer M, Peterlin BM. Mutation in a winged-helix DNA-binding motif causes atypical bare lymphocyte syndrome. Nat Immunol 2002;3:1075-81.
Haifa, Israel
Deficiencies of MHC complex class I or II are rare primary immunodeficiencies, both of which are inherited in an autosomal recessive pattern. MHC class II deficiency is a prototype of a disease of gene regulation. Defects in transacting regulatory factors required for expression of MHC class II genes, rather than the genes themselves, are responsible for the disease phenotype. The affected genes are known to encode 4 distinct regulatory factors controlling transcription of MHC class II genes. These transacting factors are the class II transactivator and 3 subunits of regulatory factor X (RFX): RFX containing ankyrin repeats (RFXANK), the fifth member of the RFX family (RFX5), and RFX-associated protein (RFXAP). Mutations in one of each define 4 distinct complementation groups termed A, B, C, and D, respectively. MHC class I deficiency is extremely rare and has been reported in less than 30 patients worldwide. Here we review the clinical, genetic, and molecular features that characterize these primary immunodeficiencies and discuss therapy options. Beyond the description of MHC class I and II deficiencies, their discovery has fascinated scientists and clinicians because of their ability to reveal the molecular basis of MCH regulation. (J Allergy Clin Immunol 2014;134:269-75.) Key words: MHC class I and II, immunodeficiency, regulatory factor X, class II transactivator, transporter associated with antigen processing I and II, tapasin
MHC class II molecules, also called HLAs, are heterodimeric transmembrane glycoproteins consisting of a and b chains. In human subjects there are 3 different isotypes (HLA-DR, HLA-DQ, and HLA-DP), all of which are encoded by distinct a and b genes clustered on the short arm of chromosome 6. MHC molecules are characterized by a very high level of polymorphism.1 This great allelic diversity seems largely caused by gene duplication. Many evolutionary biologists have tried to explain this enormous diversity, but until now, the evolution of the MHC complex has been unclear.2 All 3 MHC class II isotypes serve the same main function, namely the presentation of exogenous peptides to the T-cell receptor (TCR) of CD4 T helper cells. Recognition of the MHC-peptide complex by the TCR is crucial for the normal adaptive immune response.3 Furthermore, MHC class II expression on thymic epithelial cells promotes the direction of From the Department of Pediatrics and the Pediatric Immunology Unit, Rambam Medical Center, and the B. Rappaport Faculty of Medicine, Technion. Disclosure of potential conflict of interest: S. Hanna and A. Etzioni are employed by the Israeli government. A. Etzioni receives royalties from FRIM Books. Received for publication February 27, 2014; revised June 2, 2014; accepted for publication June 2, 2014. Available online July 7, 2014. Corresponding author: Amos Etzioni, MD, Ruth Children Hospital, Rambam Medical Campus, Bat-Galim, Haifa, Israel 31096. E-mail: [email protected]. 0091-6749/$36.00 Ó 2014 American Academy of Allergy, Asthma & Immunology http://dx.doi.org/10.1016/j.jaci.2014.06.001
Abbreviations used B2M: b2-Microglobulin CG: Complementation group CID: Combined immunodeficiency CIITA: Class II transactivator ER: Endoplasmic reticulum HSCT: Hematopoietic stem cell transplantation NK: Natural killer PID: Primary immunodeficiency RFX: Regulatory factor X RFX5: Fifth member of the RFX family RFXANK: RFX containing ankyrin repeats RFXAP: RFX-associated protein SCID: Severe combined immunodeficiency TAP: Transporter associated with antigen processing TCR: T-cell receptor
positive and negative selection events, thereby leading to the establishment of the TCR repertoire of the CD41 T-cell population. The lifespan of CD41 cells in the periphery is also dependent on interactions with MHC class II–positive cells.4 On the other hand, HLA class I molecules present peptides driven from protein synthesized in the cell to cytotoxic CD8 T lymphocytes and thus are involved in immune defense against intracellular pathogens.5 Taking into account these important functions of MHC class I and II in the normal immune response, it is clear why defects in these molecules will lead to immunodeficiency. However, in both patients with class I and those with class II deficiency, although mainly those with class I deficiency, the clinical presentation is not as severe as one would expect, and thus these are classified as combined immunodeficiencies (CIDs) and not as severe combined immunodeficiency (SCID). This might be due to the fact that low levels of MHC class I and II molecules are always present.
MHC CLASS I DEFICIENCY MHC class I deficiency is an extremely rare autosomal recessive primary immunodeficiency (PID) that thus far has been reported in only a few cases worldwide. It represents a heterogeneous group of disorders that collectively share a decreased surface expression of HLA class I molecules.5,6 Interestingly, the first reported case of MHC class I deficiency by Touraine et al7 in 1978 was later found to be a typical MHC class II deficiency. The rest of the documented cases, all sharing defective class I molecule surface expression but intact class II molecule presentation, are further categorized into the following subgroups based on genetic, biochemical, and/or clinical characteristics: transporter associated with antigen processing (TAP) 1 and TAP2 and tapasin deficiencies.5,6 The aforementioned genes are not involved in HLA class I transcription or synthesis but rather are essential for the process of peptide transport and loading, a 269
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prerequisite step for subsequent proper surface presentation of HLA class I molecules. Consequently, intracellular levels of HLA class I molecules are normal, although their surface expression is compromised.5,8 In the fourth group the causative genetic defect has not been elucidated yet is presumed to involve HLA class I–related unidentified transcriptional defects.6
Clinical manifestations The larger and best studied groups of MHC class I deficiency are those resulting from a defect in either the TAP19 or TAP28 subunits. Patients are usually asymptomatic through infancy. The clinical course typically presents during the first decade of life, with recurrent bacterial infections confined to the respiratory tract evolving into chronic inflammatory lung disease and bronchiectasis. Aside from the respiratory manifestations, skin involvement by sterile necrotizing granulomatous lesions resembling granulomatosis with polyangiitis have also have been described in several patients.5,6 These lesions might be related to vasculitis and mainly involve the legs. Lesions occurring in the upper respiratory tract causing midface deformity have also been described. Some asymptomatic TAP1- and TAP2-deficient cases were also identified (Table I).5,10 Tapasin deficiency was reported in only 1 patient who had late-onset chronic primary glomerulonephritis without any of the symptoms associated with TAP deficiency.11 The fourth group of MHC class I deficiency was described in only 2 brothers. One had unexplained steroid-responsive anemia, and the defective HLA class I surface expression was incidentally seen on HLA-matching studies. His brother was later discovered but was completely asymptomatic.6 Few other systemic diseases that present with chronic bacterial infection of the respiratory tract, bronchiectasis, and/or granulomatous skin disease can be considered in the differential diagnosis, such as chronic granulomatous disease, common variable immunodeficiency, granulomatosis with polyangiitis, and sarcoidosis. Molecular basis HLA class I molecules are composed of a polymorphic heavy chain encoded by HLA-A, HLA-B, and HLA-C genes, which are associated with b2-microglobulin (B2M). They play important roles in immune surveillance by interacting with CD81 T and natural killer (NK) cells. The HLA class I genes are located on chromosome 6. The assembly of heavy chains with B2M occurs in the lumen of the endoplasmic reticulum (ER). After assembly, the complexes are loaded with peptides derived from the degradation of intracellular proteins.12 The antigenic peptides are transported from the cytosol into the lumen of the ER by a transporter protein named TAP. TAP is composed of 2 subunits: TAP1 and TAP2. The genes encoding the TAP subunits are located in the HLA class II genetic region on chromosome 6. Tapasin is a TAP-associated glycoprotein that links TAP to the heavy chain.11 In patients with TAP deficiency, the HLA class I heavy chain/B2M complexes are peptide free. These unoccupied HLA class I molecules are retained between the ER and the cis-Golgi compartment.8
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Diagnosis Until recently, HLA class I deficiency was diagnosed by means of serologic HLA typing. However, with the decreased use of serologic testing for ‘‘routine’’ HLA phenotyping, molecular typing has widely replaced such techniques. HLA homozygosity in suspected indexes suggests TAP or tapasin deficiency because both molecules are located within the MHC locus on chromosome 6. Flow cytometric analysis of PBMCs labeled with a pananti–HLA class I mAb (W6/32) is an additional valuable diagnostic tool.5 Further experiments might facilitate differentiating the specific type of defect, irrespective of whether it involves transcription or assembly errors. Intact intracellular HLA type I molecular levels associated with the absence of MHC sialylation along with degradation tendency and complex instability at 378C highly support protein assembly defects as seen in patients with TAP or tapasin deficiency rather than a primary HLA I molecular defect. Accordingly, their extracellular expression is not significantly affected on cytokine stimulation. However, in the fourth group transcriptional defects are involved, the intracellular expression of HLA class I mRNA and molecules are decreased, and their extracellular expression can be induced by stimulation with various cytokines.5,8 Immunologic aspects The evaluation of T-lymphocyte subpopulations reveals a normal or increased CD41/CD81 cell ratio. This is mainly due to decreased CD81 ab T-cell counts. It would be reasonably anticipated that the CD81 ab T-cell repertoire would be compromised because of defective thymic positive selection, but such a repertoire is surprisingly normal in most cases.8,13 Immunoglobulin levels are normal. Although the number of NK cells was found to be within normal ranges, a phenotypic NK cell study showed differences. NK and T cells displayed less cytotoxic reactivity; however, on activation, they presented with overreactivity toward autologous cells. These autoreactive characteristics might explain the inflammatory nature of the disease.14 Pathologic hypotheses Although the genetic background has been revealed in most of the cases of MHC class I deficiency, the way it is translated into clinical outcomes requires further clarification. Because HLA class I molecules mediate intracellular protein presentation to CD81 T lymphocytes, patients are expected to present with susceptibility to viral infections, cancer, or both. However, these patients seem to maintain intact antiviral immunity with no significant propensity for malignancy. One of the proposed explanations of such dissociation is that NK cells and some ab CD81 cells can promote efficient immune defense in a TAP-independent manner.5 Moreover, an expansion of CD81, NK, and gd T cells was observed on stimulation, suggesting that they can be recruited in immune responses. The potential pathophysiology behind the noninfectious phenotype and recurrent bacterial infections that are confined to the respiratory tract are proposed to be related to secondary inflammatory reactions. It is presumed that inefficient clearance of viral infections from lung tissues induces the synthesis of several cytokines and chemoattractants that promote neutrophil influx
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TABLE I. Characteristics of patients with MHC class I and II deficiency
Disease
Genetic defect/presumed pathogenesis Inheritance
Circulating T-cell counts
Serum Circulating immune B-cell globulin counts level
Clinical features
Functional analysis
MHC class I Mutations in TAP1 and deficiency TAP2 genes involved in protein loading of MHC class I molecules
AR
Total normal Normal CD8 low CD4 normal
Normal Bacterial respiratory tract Normal responses on mitogen infections, bronchiectasis, stimulation. Normal specific skin granulomatous lesions immune reactivity to foreign (can be asymptomatic) antigens
MHC class II Mutation in transcription deficiency factors for MHC class II proteins (CIITA, RFX5, RFXAP, and RFXANK genes)
AR
Total normal Normal CD4 low CD8 normal
Low
Recurrent severe infections. Normal responses on mitogen Failure to thrive, diarrhea, stimulation. Absence of liver/biliary tract disease specific cellular and humoral Autoimmune cytopenias immune reactivity to foreign antigens
AR, Autosomal recessive.
and inflammation. This can lead to destruction of ciliary cells and lung fibrosis, resulting in defective bacterial elimination, which further increases neutrophil chemotaxis, thus maintaining a chronic lung inflammatory state.14 Activated NK and gb T cells, which kill autologous cells, might be involved in development of the sterile granulomatous skin lesions observed in some patients and are also presumed to take part in lung injury evolution. Although taking into consideration the limitations of extrapolating animal data to human subjects, several animal models displayed similar mechanisms.5,14
Treatment and prognosis No specific therapy can yet be proposed. Prevention and treatment of bronchial infections are the main therapeutic strategies and are virtually identical to the approach used in patients with other chronic respiratory inflammatory disorders, such as cystic fibrosis. As for chronic sinus involvement, a conservative approach is recommended, including local washing, use of high-dose topical steroids, and consideration of antibiotics for acute exacerbations. Only basic antiseptic care can be recommended for skin ulcers. Skin grafting had no positive effects, and lesions reappeared. IFN-a treatment trials worsened the lesions.6 MHC CLASS II DEFICIENCY MHC class II deficiency is a rare primary autosomal recessive immunodeficiency disorder (PID; MIM 209920) first described in the late 1970s. The disease is primarily characterized by the absence of MHC class II molecules on the surfaces of immune cells. Because these molecules have a pivotal role in the control of various immune responses, their absence results in severely impaired cellular and humoral immune responses, leading to significant susceptibility to severe infections and, frequently, death in early childhood.15 MHC class II determinants, also called HLAs, are polygenic and highly polymorphic cell-surface glycoproteins that form a peptide-binding cleft after their assembly as a and b chain heterodimers. In human subjects there are 3 MHC class II isotypes, designated HLA-DR, HLA-DQ, and HLA- DP, each of which is
composed of a distinct pair of a and b chains. All genes encoding the 3 isotypes are clustered in the D region of the MHC on the short arm of chromosome 6. Two modes of MHC class II expression are observed: constitutive and inducible. Constitutive expression is restricted to the surfaces of 3 distinct types of antigen-presenting cells: dendritic cells, monocyte/macrophage lineage, and B lymphocytes. In human subjects activated T cells express MHC class II as well. Most other cell types do not usually express MHC class II molecules, but such expression can be induced by a variety of stimuli, of which the most potent and known is IFN-g. MCH class II molecules mediate several key functions in the adaptive immune system, directing the development, activation, and homeostasis of CD41 T helper cells. These include the MHC class II– dependent presentation of peptides to the TCRs of CD41 T helper cells and the thymic shaping of the CD41 TCR repertoire.16
Clinical features and worldwide distribution Although MHC class II deficiency is not considered classical SCID according to the International Union of Immunological Societies classification criteria, patients usually present with clinical findings of typical CID. It is a rare CID, with approximately 200 patients reported worldwide. The majority of patients are of North African origin (Algeria, Tunisia, and Morocco), and the remaining patients are of diverse ethnic backgrounds.17 Comparing annual reports regarding the prevalence of the different subtypes of SCIDs revealed an approximately 5% frequency of MHC class II in the Canadian survey, whereas approximately 20% and 30% of SCID cases occurred in Kuwait and in North African countries, respectively.18,19 This discrepancy is related to the higher consanguinity rates in such countries along with the determination of a founder effect of a certain mutation prevalent in the North African population.20 Patients exhibit an extreme vulnerability to infections, including a broad spectrum of bacterial, viral, fungal, and protozoan pathogens. Typical clinical manifestations include an early onset of severe and recurrent infections, mainly of the respiratory and gastrointestinal tracts. Protracted diarrhea with failure to thrive is often present. Recurrent pneumonia and upper respiratory tract infections, including rhinitis, otitis, and sinusitis, were observed in almost
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all patients. The most common infectious agents are intracellular pathogens, such as cytomegalovirus, herpes simplex virus, Pneumocystis jirovecii, Salmonella and Cryptosporidium species, and extracellular pathogens, such as Pseudomonas species, Staphylococcus species, streptococci, and Candida species. Intestinal and hepatic involvement caused by Cryptosporidium species colonization appears to be more frequent in patients with MHC class II deficiency than in patients with other immunodeficiencies. Hepatic involvement, including sclerosing cholangitis, is frequent and associated with poor prognosis. The absence of BCGitis can be accounted for by the presence of residual immunity in the form of CD81 T and NK cells.19-21 Autoimmune manifestations were observed in approximately 20% of patients, such as autoimmune cytopenias, anemia, neutropenia, and/or thrombocytopenia (Table I). The associated autoimmune phenomena were not correlated with poor prognosis. In one series dysmorphic features were seen in 3 unrelated patients, all of whom had normal karyotypes. This should be researched more carefully to evaluate its relevance.21 Although most patients were given a diagnosis in the first year of life, some were given a diagnosis later at up to 15 years of age. This might be related to a milder onset in some patients, delays in the referral of patients to specialized centers, or both. Immunologic investigations revealed no significant differences from other patients. Innate or CD81 T cell–mediated immunity might account for this more favorable outcome. Both genetic factors and environmental factors, such as hygiene, might be responsible for differences in the clinical expression of the disease state.20
Immunologic features The immunologic hallmark of the disease is the absence of constitutive and inducible expression of MHC class II molecules on all cell types. The most constant immunologic feature is the absence of specific cellular and humoral immune reactivity to foreign antigens with intact responses to mitogen stimulation because the latter is not MHC class II–TCR complex restricted. The most prominent finding on lymphocytic phenotypes is the absence or very low HLA-DR expression on lymphocytes, with reduced CD4 cell counts leading to an inverted CD4/CD8 ratio. The low CD4 count presumably reflects abnormal thymic selection and maturation from the lack of MHC class II expression on cortical epithelial cells in the thymus. The reversed CD41/CD81 ratio is due to an absolute CD41 T-cell count reduction and a concomitant increase in CD81 T-cell counts, which might result in normal total numbers of circulating T cells. CD81 T-cell counts, MHC class I levels, or both can be decreased as well.16 Interestingly, patients with MHC class II deficiency have detectable TCR excision circles in both total lymphocytes and sorted CD4 cells.22,23 This reflects the fact that early T-cell development is normal in patients with this condition. Thus it might be missed by the TCR excision circle quantification assay, which has been proposed to be a sensitive neonatal screening test capable of identifying SCID and other T-cell lymphopenias. However, some clonal abnormalities, along with reduced DNA rearrangement events, have been reported. The majority of patients have a decrease in levels of 1 or 2 immunoglobulin isotypes, although panhypogammaglobulinemia is also common.16
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FIG 1. Molecular defects in patients with bare lymphocyte syndrome (BLS) type II. Four different transacting factors on MHC class II promoters have been associated with BLS type II. A defect in any of them defines a distinct CG. The gene encoding CIITA is mutated in CG A. The genes encoding the RFX subunits RFXANK, RFX5, and RFXAP are defective in CGs B, C, and D, respectively.
Genetic and molecular background The genetic basis of MHC class II deficiency exhibits a unique feature because the genes implicated in the phenotypic manifestation of the disease, namely the family of MHC class II genes on chromosome 6, are intact, although the mutated genes reside outside the MHC class II locus (Fig 1). On the basis of somatic cell fusion experiments with transformed B cells from patients, 4 distinct groups of MHC class II deficiency were identified: complementation groups (CG) A, B, C, and D. The genetic background of the disease was ultimately confirmed only several years later after the identification and cloning of 4 different mutated genes corresponding to 4 distinct key MHC class II regulatory genes encoding transacting regulatory factors that coordinately control the expression of MHC class II at the transcriptional level.15 The class II transactivator (CIITA) is mutated in CG A, whereas the 3 subunits of the regulatory factor X (RFX), namely RFX containing ankyrin repeats (RFXANK), the fifth member of the RFX family (RFX5), and RFX-associated protein (RFXAP), are defective in CGs B, C, and D, respectively. MHC class II deficiency is a monogenic disease in which a single defective gene is responsible for the entire clinical picture. Despite the genetic heterogeneity, the clinical presentation is quite homogeneous, with no obvious correlation between the CG and the clinical picture.16 The first MHC class II deficiency gene to be discovered was CIITA, and it is classified in group A. It was isolated based on complementation studies. RJ2.2.5 is one of the in vitro–generated mutant MHC class II–negative cell lines allocated to CG A. Transfection of this cell line with a B-cell cDNA library restored MHC expression, leading to the isolation of cDNA clones encoding the MHC CIITA.24 The human gene MHC2TA is localized on chromosome 16. Expression vectors encoding CIITA can reactivate expression of all 3 MHC class II isotypes when transfected into cell lines from CG A. Nine mutations have been characterized thus far. The expressions of CIITA and MHC class II are tightly correlated: the majority of cells do not express the MHC2TA gene and are MHC class II negative. CIITA expression is cell specific and limited to MHC class II–positive
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cells. Transfection of cells with CIITA expression vectors is sufficient to induce MHC class II expression. Silencing of the CIITA gene in the differentiated mature plasma cells is correlated with loss of MHC class II expression.16,25 The RFX–DNA-binding complex consists of 3 subunits encoded by 3 distinct genes called RFXANK, RFX5, and RFXAP and are mutated in MHC class II deficiency CGs B, C, and D, respectively.16 Unlike CIITA, it is expressed ubiquitously in all cell types examined, even in cells that do not express MCH class II. RFX5 cDNA clones were isolated because they restored MHC class II expression when transfected into SJO cells, a cell line derived from a patient with MHC class II deficiency in CG C. It derived its name because it is the fifth isolated member of the DNA-binding proteins interacting with the MHC class II X box. The RFX 5 gene is located on chromosome 1.16 Six mutations of the RFX5 gene have been identified. Defects in the RFXAP gene (on chromosome 13) account for CG D.26 Thus far, 3 different mutations have been identified. An in vitro–generated mutant (6.1.6) has also been shown to contain mutations in RFXAP. The RFXANK defect, also called RFXB, defines CG B, the most common group, accounting for more than 70% of all known patients. RFXANK was identified by using a biochemical approach after purification of the RFX complex. It is located on chromosome 19. Transfection of the RFXANK cDNA into cell lines from group B restored expression of all MHC class II isotypes. The RFXANK gene’s most frequent mutation is a 26-bp deletion at the boundary between intron 5 and exon 6 called c.338-25_338del26 (also known as 752delG-25). It is also the most frequent mutation responsible for MHC class II deficiency. It has been found in less than 90% of North African families, including many unrelated patients, indicating the existence of a founder effect. Indeed, a founder event has been identified and dated as responsible for the RFXANK deletion 15E6-25_15E611 in the North African population. The mutation was dated to approximately 2250 years ago, a period concurrent to the Berber civilization.20 Eight other mutations in the RFXANK gene have been described in patients of different ethnic backgrounds. RFXANK mutations dominated also in Middle Eastern populations. There is a correlation between certain groups and ethnic origin. Patients from group B are predominantly of North African origin, whereas those from group A are mainly Hispanic. Most mutations occur in splice sites that cause frameshifts, which result in truncated or absent proteins, providing an easy explanation for the lack of transcription and surface expression of MHC class II determinants. The loss of MHC expression suggests that both CIITA and the RFX complex are essential and that no bypass or other pathways can compensate for their absence. Moreover, no other major systems are critically dependent on RFX and CIITA, indicating their high specificity for MHC class II expression. No clinical/immunologic correlation was found with the different CGs. Mouse models for MHC class II deficiency have been constructed, including the MHC class II knockout mouse and CIITA and RFX5 knockout mice. All exhibited a similar phenotype to that of human disease. However, a significant discrepancy regarding the CD41 T-cell population was noticed among the 2 models and found to be nearly absent in mouse models, although it was only mildly reduced in MHC class II–deficient patients. This might be explained by different CD41 T-cell selection mechanisms among species.5
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Diagnosis The mainstay of diagnosis is the absence of MHC class II molecule expression on all cell types. Other tests, including the inability to produce specific immune reactions, such as antibodies, in response to immunization and delayed-type hypersensitivity skin tests, are decisive as well. Lymphopenia, mainly of the CD41 subclass, and hypogammaglobulinemia are frequently associated.17 The identification of the molecular defects (by using sequence analysis in specific genetic laboratories) underlying MHC class II deficiency permits reliable prenatal and postnatal diagnosis and allows accurate carrier detection. This is remarkably valuable in high-risk populations and families with previously identified cases or consanguineous marriage. Umbilical vein puncture and analysis of MHC class II expression on fetal leukocytes is an alternative when genetic background is unknown. Because all the patients reported thus far belong to one of the known complementary groups, the use of whole-exome sequencing is not needed in patients with MHC class II deficiency. Therapy and prognosis As for other CID disorders, allogeneic hematopoietic stem cell transplantation (HSCT), preferably from an HLA-identical sibling, is currently the only available curative treatment and is considered the treatment of choice.20 Aggressive treatment of acute infections and other complications can reduce the frequency and severity of the associated clinical manifestations. This includes intravenous anti-infectious medications, administration of intravenous immunoglobulins, parenteral nutrition, and prophylactic antibiotics. However, in most of the cases, these methods do not prevent progressive organ dysfunction. Unfortunately, without HSCT, patients will ultimately die between the ages of 5 and 18 years. The overall success rate of HSCT has been reported to be relatively poor compared with that seen in patients with other PIDs, with survival rates of less than 60%.27-29 Several protocols were used, and it seems that standard busulfan/cyclophosphamide conditioning resulted in higher donor engraftment. A reduced-intensity conditioning regimen might be particularly suited for patients with comorbid conditions.27 Umbilical cord blood transplantation was used in 4 cases, and all of them had good engraftment.28,30 Patients undergoing HSCT before the age of 2 years had better prognosis. The main causes of treatment failure are poor engraftment caused by residual host immunity, old age at diagnosis and/or treatment, and persistent viral infections.20 Thus it is highly recommended to perform HSCT in young patients independently of whether an HLA-identical sibling is available.1 Recently, a better outcome was reported, especially in HLA-identical transplant recipients, in whom the overall disease-free survival rate was 66% to 69% after a median follow-up of approximately 6 years. Interestingly, despite the lack of MHC class II expression, the risk of graft-versus-host disease is similar to that observed in patients with other forms of immunodeficiency. Among HSCT long-term survivors, CD41 T-cell counts remain low, with a moderate decrease in naive CD41 T-cell counts. This finding is probably related to defective MHC class II expression on the thymic epithelium of the host.20 Identification of the affected genes raised the hope
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that gene therapy might present an alternative treatment strategy. One probable concern is that the therapeutic transgene could induce ectopic levels of MHC class II expression, potentially causing harmful consequences, such as autoimmunity or tissue destruction. This is of particular importance in CG A because CIITA expression is cell specific, and it might be difficult to obtain the endogenous pattern of expression. Another potential problem is the possibility of abnormal selection of CD41 T cells resulting from the absence of MHC class II expression on thymic epithelial cells. However, because classical bone marrow transplantation can cure MHC deficiency without the requirement of functional thymus, this issue might not pose an obstacle. This strategy needs further optimization before its clinical implementation.31 Although most patients who do not undergo HSCT have a poor prognosis, with a life expectancy not exceeding the first decade, this is not always the case, with some patients surviving through teenage or adulthood. In some groups this preferable prognosis could be explained by residual expression of surface MHC class II determinants on patients’ immune cells retaining some antigen presentation function and thus protection. Several distinct mutations have been linked to such a milder course because they contain missense single point mutations rather than termination or nonsense mutations.32 However, in another groups of patients, the less severe manifestations could not be explained by such residual MHC class II expression. In a large series of North African patients, even within the group sharing the same mutation of the 26-bp deletion of the RFXANK gene, some affected cases have survived to the second decade. Similar findings were observed in a large Tunisian study.21 Collectively, these data suggest that their capacity to cope with infections must depend on other unknown genetic and immunologic factors.20 These newly observed patients demonstrate that with the better diagnostic techniques (mainly lymphocyte markers and genetic molecular analysis) now available, physicians should be able to detect less severe cases. It is likely that such patients will not require the same aggressive treatment used for typical cases of MHC class II deficiency.
SUMMARY MHC class I and II deficiencies are extremely rare PIDs, both of which share defective antigen presentation to CD81 and CD41 T cells, respectively. On the other hand, they differ enormously in their clinical manifestations. MHC class II deficiency presents as a CID, whereas MHC class I deficiency is mainly characterized by chronic lung disease along with skin granulomatous disorder. Although in patients with MHC class I deficiency the clinical presentation might vary widely in relation to the genetic defect, in patients with MHC class II deficiency, the phenotypic characteristics are largely uniform, although the genetic background is heterogeneous. Such discrepancy was clarified by revealing the regulatory nature of the involved genes. The elucidation of MHC class II deficiency disease has made remarkable contributions to what we currently know about the molecular mechanisms controlling MHC class II genes and is considered a textbook model for the regulation of gene expression in general. Because of its diverse presentation, MHC class I and II deficiencies should always be included in the differential diagnosis of unclear cases of CIDs.
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