Experimental Hematology 2015;43:149–157

REVIEW

Do human leukocyte antigen E polymorphisms influence graft-versus-leukemia after allogeneic hematopoietic stem cell transplantation? Ehteramolsadat Hosseinia,b, Anthony P. Schwarerb, and Mehran Ghasemzadeha a

Blood Transfusion Research Center, High Institute for Research and Education in Transfusion Medicine, Tehran, Iran; bDepartment of Immunology, Alfred Medical Research and Education Precinct, Monash University, Melbourne, Australia (Received 27 May 2014; revised 16 November 2014; accepted 19 November 2014)

Hematopoietic-stem-cell transplantation (HSCT) is complicated by histocompatibilitydependent immune responses such as graft-versus-host disease, relapse, and graft rejection. The severity of these common adverse effects is directly related to the degree of human leukocyte antigen (HLA) incompatibility. In addition to the key role of classic HLA matching in influencing HSCT outcome, several lines of evidence suggest an important role for nonclassic major histocompatibility complex class I molecule, HLA-E. The interaction of HLA-E with NKG2A, its main receptor on natural killer cells, modulates cell-mediated cytotoxicity and cytokine production, an important role in innate immune responses. In addition, the HLAE molecule can present peptides to different subtypes of T cells that may either support graft-versus-leukemia effects or be involved in bridging innate and acquired immunity. To date, the role of HLA-E and its polymorphisms in HSCT outcomes such as graft-versushost disease, transplant-related mortality, and improved survival has been published by a number of groups. In addition, these data suggest an association between HLA-E polymorphisms and relapse. Whether the engagement of the HLA-E molecule in the modulation of donor T cells is involved in the graft-versus-leukemia effect, or whether a different mechanism of HLA-E dependent reduction of relapse is involved, requires further investigation. Copyright Ó 2015 ISEH - International Society for Experimental Hematology. Published by Elsevier Inc.

Major histocompatibility complex (MHC) class I antigens comprise two subtypes: classic and nonclassic, both of which have essential roles in innate and adaptive immune systems. The classic subtype includes human leukocyte antigen (HLA) A, B, and C, which account for immunologic recognition mechanisms, particularly through the presentation of antigenic peptides to the ab T-cell receptor (TCR) on T cells [1–3]. The nonclassic MHC class I mainly in-

Offprint requests to: Dr. Mehran Ghasemzadeh, Blood Transfusion Research Center, High Institute for Research and Education in Transfusion Medicine, Iranian Blood Transfusion Organization Building, Hemmat Express Way, Next to the Milad Tower, Tehran 14665-1157, Iran; E-mail: [email protected]

volves the regulation of innate immune responses [4], and HLA-E plays an important role via natural killer (NK) cells [1]. The autoreactivity of NK cells is controlled by the expression of the inhibitory receptors, the killer-cell immunoglobulin-like receptor group, and NKG2A/CD94, on cells that interact with self-MHC class I molecules on normal cells [5]. This control mechanism is important to avoid exaggerated responses by NK cells, which lead to tissue damage. The interaction of HLA-E with inhibitory receptor NKG2A as its main receptor on NK cells modulates NKcell-mediated cytotoxicity and cytokine production [6–9]. In addition to interacting with the NK-cell-inhibitory receptor NKG2A/CD94, the HLA-E molecule can interact with activatory receptors NKG2C/CD94 and NKG2E/CD94

0301-472X/Copyright Ó 2015 ISEH - International Society for Experimental Hematology. Published by Elsevier Inc. http://dx.doi.org/10.1016/j.exphem.2014.11.007

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Figure 1. HLA-E and CD94/NKG2 interaction. The cell-surface expression of the HLA-E*0103 allele is higher than that of HLA-E*0101. Human leukocyte antigen E interacts with CD94/NKG2A to inhibit NK-cell activity, whereas its interaction with CD94/NKG2C leads to activation. Proinflammatory cytokines released from activated immune cells result in the upregulation of HLA-E on the surfaces of target cells. The increased levels of soluble HLA-E in the microenvironment also occur in response to cytokine stimuli. Signaling molecules: immunoreceptor tyrosine-based inhibition motifs (ITIM), adapter molecule DAP-12 containing immunoreceptor tyrosine-based activation motifs (ITAM), Zeta-chain-associated protein kinase 70 (ZAP-70), and SH2-containing expressed tyrosine-specific protein phosphatase (SHP-1).

[10,11]. However, it is worthwhile to note that members of the NKG2/CD94 family show different levels of affinity to HLA-E: the inhibitory NKG2A/CD94 binds more tightly to HLA-E than NKG2C/CD94 or NKG2E/CD94 [11–13] (Fig. 1). This finding may highlight a pivotal role for

NKG2A in the modulation of NK-cell function. In addition to being a ligand for the NKG2/CD94 receptors, HLA-E has also been shown to react with CD8þ T cells expressing conventional ab TCRs [12] (Fig. 2B). This phenomenon suggests HLA-E involvement in the adaptive immune

Figure 2. T cell mediated graft-versus-leukemia (GvL) effect. (A) Following T-cell-depleted haploidentical HSCT, high cell-surface expression of NKG2A on immature NK cells and its interaction with HLA-E inhibits cytolytic activity, leading to reduced GvL effect. (B) In non-T-cell-depleted HSCT, the presence of donor T cells interacting with HLA-E supports a GvL effect.

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responses mediated by T cells [1,14,15]. Several lines of evidence have shown an association of different genotypes of HLA-E with a variety of clinical conditions [16–22]. These observations provide evidence of the importance of the HLA-E molecule in immune responses. In line with this research, there is also a growing body of evidence that supports a role for HLA-E polymorphisms in the clinical outcomes of hematopoietic-stem-cell transplantation (HSCT). This is an important topic which is of interest for further discussion in this review. Human leukocyte antigen E Human leukocyte antigen E is a member of the class Ib group and plays an important role in cell identification by NK cells [1]. Unlike other nonclassic MHC class I molecules, HLA-E is expressed in most tissues; however, its level of expression is lower than that of classic MHC class I molecules [12]. Structurally, HLA-E is a trimeric complex including a light chain (b2m) and an MHC class Ia heavy chain associated with a subset of peptides derived from the leader sequence of other class I molecules. The formation of this trimeric complex is necessary for efficient transport of HLA-E to the cell surface and its subsequent stabilization [23]. It has been shown that HLA-E expression only occurs when peptides from the leader sequence of other class I molecules bind to the peptide-binding groove of HLA-E a chain [1]. It is thought that these aforementioned peptides are cleaved from related MHC molecules presented on endoplasmic reticulum membrane by the effect of signal peptide peptidase. These peptides are then released into the cytoplasm [24] and processed by the proteasome [25]. Subsequently, these peptides are transported into the endoplasmic reticulum by transporter associated with antigen processing (TAP), where they assemble with the HLA-E a heavy chain and b2m [26]. Thus, the cellsurface expression of HLA-E is firmly dependent on the expression of other MHC class I molecules (as a peptide provider), the TAP transporter function, and other related antigen processing-machinery [12]. Human leukocyte antigen E polymorphisms Unlike the classical MHC class I molecules, which exhibit high levels of polymorphism [27], the nonclassic molecules HLA-E, -F and -G display a restricted degree of polymorphism in their encoding genes [28]. Human leukocyte antigen E exhibits a limited polymorphism with only two functional alleles, HLA-E*0101 and HLA-E*0103, identified to date [29,30]. It appears likely that other reported alleles of HLA-E are the result of sequencing artifacts [31]. The confirmed HLA-E*0101 and HLA-E*0103 alleles allow only three possible genotypes: HLA-E*01010101, HLA-E*01030103, and HLAE*01010103. These two alleles have just one amino acid difference, at position 107 on the a2 domain of the heavy chain; an arginine at this position (HLA-ER) defines

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HLA-E*0101, whereas a glycine at this position (HLAEG) defines HLA-E*0103 [28,32]. Human leukocyte antigen E*0101 and HLA-E*0103 alleles are present with a frequency of nearly 50% for each among the Caucasian, African-American, and Hispanic populations, whereas, in Japanese and Chinese populations, the frequency of HLAE*0103 allele was shown to be higher than HLA-E*0101 allele [12,14,15,31,33,34]. Human leukocyte antigen E alleles and cell-surface expression Although HLA-E transcripts have been identified in almost all tissues and cell types, significant expression of this molecule is mainly limited to endothelial and immune cells, including T and B lymphocytes, monocytes, and macrophages. Nonetheless, endothelial cells express only a low level of HLA-E molecules, which are upregulated following exposure of the cell to proinflammatory cytokines, such as tumor necrosis factor a, interleukin 1b, and interferon g, that are released from sensitized immune cells. It is noteworthy that this upregulation is also associated with HLAE secretion; the biological effects of this phenomenon remain to be determined [35] (Fig. 1). In addition to leukocytes and endothelial cells, platelets and megakaryocytes, but not erythrocytes, exhibit HLA-E protein expression at a basal level. In line with this data, using flow cytometry analysis, HLA-E protein has been constitutively detected on CD3þ, CD4þ, CD8þ, CD19þ, and CD14þ leukocyte subsets of freshly isolated peripheral blood mononuclear cells and lymphoid-originated cell lines including Jurkat, Raji, U937, and NKL [35]. From the myeloid lineage, the HL-60 human promyelocytic leukemia cell line also significantly expresses HLA-E, whereas the K562 human erythroleukemia cell line is deficient in this protein expression [32]. Several lines of evidence have shown HLA-E overexpression in different types of neoblastic cells [36,37]. In an ex vivo model, Nguyen et al. have shown the upregulation of HLA-E in acute myeloid leukemia (AML) blasts after haplo-mismatched stem cell transplantation. Another study has also demonstrated significantly higher expression of MHC-I A/B and HLA-E in acute lymphoblastic leukemia patients than that in AML patients [38]. Further investigation has also revealed that the cellsurface expression of HLA-E*0103 is higher than that of HLA-E*0101 in the majority of human-transfected cells. This observation has been confirmed in untransfected normal cells using a comparative analysis of HLA-E expression. However, Western blot analysis has revealed almost equal levels for each allelic heavy chain of HLAE in the cell line transfected with either HLA-E*0101 or HLA-E*0103, suggesting that the observed difference in the polymorphic cell-surface expression is not due to the different levels of the protein production [28]. To further investigate, the physical basis for differential surface expression levels of HLA-E*0103 and HLA-

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E*0101 in the presence of diverse peptides was examined. The evaluation of their three-dimensional structure, peptide affinity, and thermal stability showed that HLA-E structural changes due to the amino acid differences appear to be limited. However, HLA-E*0103 has a stronger affinity for various peptides than HLA-E*0101. In addition, HLAE*0103 has a higher melting temperature compared with HLA-E*0101, which suggests a higher thermal stability of HLA-E*0103. These differences in both peptide affinity and thermal stability between HLA-E*0103 and HLAE*0101 may explain the differences in their expression levels on the surface of the cell [28]. In another study using mouse cells transfected with the human HLA-E alleles, it was shown that allelic variation of HLA-E greatly affected the intracellular transport and cell-surface expression of HLA-E [23]. Some studies have suggested that the observed higher cell-surface expression of HLA-E*0103 compared with HLA-E*0101 might influence the signaling pathway of the counterreceptor NKG2/CD94, which might create functional differences between these molecules. It is possible that the allelic variation of HLA-E affects ligand interaction, through the differences in the number of HLA-E molecules expressed on the cell surface, and that it results in different affinities to the receptor [28]. Human leukocyte antigen E in adaptive immunity The classic MHC class I molecules, which are highly polymorphic, are best known for their involvement in adaptive immunity [4]. These molecules play a central role to distinguish self from nonself, which is mediated by the presentation of peptides to the T cells [1]. Conversely, the nonclassic MHC class I molecules, which exhibit a restricted polymorphism, are prominently involved in the regulation of innate immune responses. These molecules are ligands for NK-cell receptors [4], however it has recently reported that a subset of the nonclassical MHC class I molecules can also present peptides to the T cells, suggesting their involvement in the acquired immunity [1]. The distinctive roles of these molecules in response to tumors and pathogens, such as listeria and mycobacteria, are some examples of their contribution to the acquired immunity [4]. As a member of the nonclassic MHC, HLA-E can be recognized by both NK cells and ab CD8þ T cells. The recognition of this molecule through T-cell receptor is a phenomenon that highlights the role of HLA-E in the acquired immune response to the pathogens [12]. The observation of HLA-E–restricted T cells directly supports such a role. These T cells are able to recognize HLA-E in complex with the peptides from either bacteria, such as Mycobacterium tuberculosis [39], Salmonella enterica serovar typhimurium [40], and Listeria monocytogenes [39], or viruses, such as human cytomegalovirus (hCMV) [41] and EpsteinBarr virus [42]. Human leukocyte antigen E is also capable

of presenting the prostate tumor antigen to CD8þ T cells [43]. Taken together, these observations suggest a dual role for the nonclassical MHC class I molecule HLA-E in both the innate and adaptive immune systems [12]. Human leukocyte antigen E in innate immunity Human leukocyte antigen E plays a greater role in the regulation of innate immunity than adaptive immunity [4,6,44– 46]. As already described, HLA-E needs to bind specific peptides derived from the leader sequences of other MHC class I molecules to be expressed on the cell surface. Therefore, the downregulation of MHC class I expression can reduce the cell-surface expression of HLA-E complex [47,48]. Since the HLA-E molecule acts as a major ligand for the inhibitory NKG2A/CD94 receptor, the binding of MHC class I peptides to HLA-E allows the NK cells, via NKG2A/CD94, to indirectly monitor the overall cellsurface expression of other MHC class I molecules [6,45]. The peptides derived from the leader sequence of MHC class I molecules are influenced by malignant transformation or viral infection. Such events can downregulate the expression of MHC class I molecules or inhibit the function of TAP. Consequently, this may decrease the surface expression of HLA-E molecules, leading to an increase in target-cell susceptibility to the NK cell lysis [12]. Since the function of NK cells is mostly regulated by HLA-E– dependent inhibitory receptors that are specific for self-MHC class I molecules, lower engagement of inhibitory receptors may lead to the lysis of target cells [49]. This is a mechanism by which NK cells become sensitive to infected or transformed cells [11,50], highlighting the role of NK cells in immune surveillance [12]. Several lines of evidence suggest different mechanisms by which viruses can downregulate the synthesis of class I proteins, either by disruption of the transcriptional mechanisms of classic MHC class I [51] or by reducing the stability of the heavy chain of the molecule [52]. This process also indirectly decreases the peptide source for HLA-E complexes, which consequently downregulates its expression on the cell surface. This phenomenon results in less inhibition of NK cell cytotoxicity. Interestingly, in some viral infections, the converse phenomenon may occur, wherein the NKG2A/CD94-mediated inhibition of NK cells may lead to viral evasion from the innate immune response. For instance, glycoprotein UL40 from hCMV includes a peptide similar to the one that originates from the leader sequence (VMAPRTLIL) of many HLA-C alleles. This peptide can bind to the HLA-E molecule and consequently facilitates its expression and interaction with NKG2A/ CD94, leading to NK cell inhibition [48], protecting hCMV-infected cells from NK cell lysis. In a similar order, other viruses such as hepatitis C virus and human immunodeficiency virus (HIV) even exhibit peptides, different from canonical leader peptides, that are also capable of binding to HLA-E and consequently interact with NKG2A/CD94.

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Evidence has shown that the peptide of core 35-44 derived from HCV can bind to HLA-E, which stabilizes the expression of the HLA-E/b2m complex, in turn better interacting with inhibitory NKG2A/CD94 receptor and finally inhibiting NK-cell-mediated cytotoxicity [53]. According to another report, upregulation of HLA-E expression on lymphocytes in patients infected with HIV and the reduction of susceptibility to NK cell cytotoxicity may contribute to the virus’ potential for creating chronic infection [54]. In addition to the inhibitory NKG2A/CD94 receptor, the HLA-E molecule can also interact with other receptors, such as NK-cell activatory receptors NKG2C/CD94 and NKG2E/CD94 [10] (Fig. 1), which may have a different functional role for HLA-E compared with those already described. For example, it has been shown that the activatory NKG2C/CD94 receptor has a specific but weak interaction with the hCMV MHC-homologue UL18, which is thought to be important in the immune system response to viral evasion strategy [55]. Role of human leukocyte antigen E complex in the stem cell transplantation Hematopoietic stem cell transplantation is commonly complicated by histocompatibility-dependent adverse effects such as graft-versus-host disease (GvHD), relapse, and graft rejection. Alloreactive T cells have been described as mediators of acute GvHD. The activity of these cells can cause tissue damage in recipients, particularly in skin, intestines, and liver [56]. However, studies using T-cell depletion to reduce GvHD have consistently shown an increased incidence of both graft rejection and disease relapse. This suggests a beneficial role of alloreactive T cells in favor of graft acceptance and antitumor responses [57–59]. In line with these observations, further studies have shown that alloreactive T cells also mediate the graft-versus-leukemia (GvL) effect through the reaction with histocompatibility antigens on leukemic cells [49]. In contrast to the unfavorable effect of GvHD, GvL is often crucial to the eradication of residual malignant cells that have survived the conditioning regimen before HSCT. Although GvL has been thought to be exerted via donor T cells, several lines of evidence have suggested that NK cells also play an important role in this phenomenon [49,60–62]. Donor-derived NK-cell cytolytic potential against residual leukemic cells (GvL) is a critical phenomenon that highlights the involvement of these cells in reconstitution of the immune system after HSCT [49,60–63]. It has been shown that activation of NK cells depends on a fine balance between activatory and inhibitory signals transduced by different types of NK-cell receptors [64]. Considering the fact that HLA-E is an important ligand for TCRs in CD8þ T cells [12] and inhibitory receptor NKG2A in NK cells [6,45], its role in GvL seems to be dependent on the type of HSCT. In addition, the presence of immature NK cells with overexpression of NKG2A after

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transplantation and upregulation of HLA-E in tumor cells [38,65,66] are the mechanisms that might favor leukemic cell escape from immune surveillance, especially in Tcell-depleted HSCT. Having declared these, several lines of evidence support a role for HLA-E in HSCT. During the last decade, the observation that HLA-E–specific alloreactive T-cell proliferation induced by allogeneic stimulation and their cytolytic effect on donor cells in transgenic mice (HLA-E*0103 and b2-microglobulin gene) suggested a potential role for HLA-E in the immunologic response following allogeneic HSCT [67,68]. Today, it is well understood that, in HLA-mismatched allogeneic HSCT, donor T cells directly recognize major histocompatibility antigens, whereas, in transplants in which both donor and recipient are matched for HLA-A, -B and -C alleles, the T cells react to minor histocompatibility antigens (mHags) that are associated primarily with the classic HLA molecules this process may activate T cells. In addition, HLA-E also can outcompete classic HLA molecules in binding to mHags; this phenomenon may disrupt the classic pathway of T-cell activation. On the other hand, HLA-E may bind nonstandard mHag peptides, a process that could specifically generate GvL rather than GvHD [14,69]. Bogunia and Kubik have shown that, in patients undergoing allogeneic HSCT who are matched at highresolution level for five classic HLA loci (HLA-A, -B, -Cw, -DRB1 and -DQB1), a significantly higher incidence of acute GvHD was observed in those who received donor cells that were HLA-E mismatched [69]. These findings are in line with other research that has also shown patient– donor compatibility with respect to the HLA-E*0103 allele can significantly reduce the development of posttransplant complications [14,15,70,71]. Other studies have suggested an important role for NKG2A/CD94-HLA-E interactions in the regulation of NK-cell cytotoxicity using the blockade of either receptor or ligand in both AML and ALL patients after allogeneic HSCT. These observations also suggest that the involvement of HLA-E in NK-cell function may affect clinical outcomes after HSCT [28,38]. So far, several studies have reported the association of different genotypes of HLA-E with the clinical conditions in different diseases [16–22], of which those related to clinical outcomes after HSCT are of significant interest. Therefore, further studies have investigated whether HLA-E polymorphisms might also influence the clinical outcomes of HSCT. Human leukocyte antigen E polymorphisms in the stem cell transplantation The observation of different clinical outcomes in various situations for patients with different HLA-E genotypes suggests that the one amino acid difference may affect the function of HLA-E. For instance, the association of the HLA-E*01010101 genotype with an increased rate of

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recurrent spontaneous abortion in Indian women [16] and a higher frequency of this genotype in patients with type I diabetes mellitus have been demonstrated [21]. In addition, it has been reported that there is a protective effect of the HLA-E*01030103 genotype on HIV-1 infection in Zimbabwean women [17], a significant increase in the frequency of the HLA-E*0103 allele in patients with Behcet’s disease (a chronic inflammatory disorder) [18], and an increased frequency of both the HLA-E*0103 allele and the HLAE*01030103 genotype in the patients with nasopharyngeal carcinoma (a tumor of epithelial cell lining nasopharynx) [19,20]. Our current published data showed a comparable frequency of both HLA-E*0101 and HLA-E*0103 alleles in healthy Australian individuals, leukemic patients, and their HSCT donors [14,15,70,71]. These findings are in line with other studies among Caucasian, AfricanAmerican, and Hispanic populations, whereas, in Japanese and Chinese populations, the frequency of the HLAE*0103 allele was shown to be higher than that of the HLA-E*0101 allele [12,14,15,31,33,34]. The presence of the HLA-E*01030103 genotype has been shown to be associated with a low incidence of acute GvHD and transplant-related mortality, as well as improved survival, in HLA-genoidentical bone marrow transplantation [14] and in related and unrelated HSCT [15]. In addition, our current published results found, for the first time, a significant association between the presence of the HLAE*01030103 genotype and low incidence of extensive chronic GvHD. We also suggested a protective role for the HLA-E*01030103 genotype against acute GvHD (grade II or more), as well as an association between this genotype and a better overall survival after HLA-E– matched allogeneic HSCT [14,15,70]. These observations all provide evidence for the role of HLA-E polymorphisms in the outcomes after allogeneic HSCT. twb 0.26wFor GvHD, the possible mechanism of the aforementioned associations could be related to the importance of endothelium in transplant rejection, where the upregulation of HLA-E molecules on endothelial cells of vessels by cytokines released in the inflammatory circumstances after transplant involves this immune-regulatory molecule in vascular homeostasis [15,35]. Given that, and owing to lower cell-surface expression of HLA-E*0101 compared with HLA-E*0103, the endothelial cells of vessels having the HLA-E*0101 allele might be more susceptible to the destruction mediated by NK and T cells, leading to a higher frequency of acute GvHD in the patients with HLAE*01010101 genotype [15]. Conversely, higher cellsurface expression of HLA-E*0103 compared with the HLA-E*0101 allele in the healthy cells of patients with the HLA-E* 01030103 genotype might better protect those cells from tissue damage mediated by NK cells, resulting in a lower risk of acute GvHD. Mechanistically, there is some evidence that highlights the importance of the interaction between CD94/NKG2A and HLA-E in this phenomenon.

Using a murine model of acute GvHD, it has been shown that, like NK cells, the CD94/NKG2A receptor is expressed on donor T cells. In humans, a similar observation has been also reported for donor T cells after allogeneic HSCT. Thus, considering the role of activated T cells in the pathology of acute GvHD, it seems that the interaction of HLA-E on target cells with CD94/NKG2A expressed on donor T cells can restrain alloreactivity of these cells against the host, limiting acute GvHD [72]. Having said this, the higher expression of HLA-E in patients with the HLA-E*01030103 genotype compared with those with the HLA-E*01010101 genotype might favor the lower incidence of GvHD in patients with HLA-E*01030103 genotype. The mechanism for the lower incidence of extensive chronic GvHD for this genotype might be similar to that for acute GvHD. Nevertheless, the lack of any association between limited chronic GvHD and HLA-E polymorphisms in these patients suggests that there are other conditions or mechanisms involved, which will need further investigation [71]. Besides GvHD, relapse is also one of the major causes of mortality following allogeneic HSCT [73]. This complication can be due to the escape of residual malignant cells from the GvL effect [49,61,62,70,71], which is often crucial for the eradication of malignant cells that have survived the conditioning regimen (radiotherapy and/or chemotherapy) before the HSCT. Even so, HLA-E molecule can play an important role in the modulation of NK-cell function, which might be essential for the eradication of residual malignant cells in some situations. The role of this molecule in the GvL effect is of interest to scientists. Nguyen et al. have stated that, following T-cell-depleted haploidentical HSCT, immature NK cells generated after transplant show impaired cytotoxicity, which makes them ineffective at destroying AML blasts. These immature NK cells express a high level of NKG2A, whereas their production of interferon g upregulates cell-surface expression of HLA-E on AML blasts, leading to the inhibition of NK cell-mediated lysis through the interaction of CD94/ NKG2A and HLA-E; this phenomenon is associated with both relapse and death in these patients [38,65] (Fig. 2A). Based on this mechanism, Iwaszko et al. suggested that, since patients with the HLA-E*01030103 genotype express higher levels of HLA-E on their cell surfaces compared with those with the HLA-E*01010101 genotype, their NK cells received more inhibitory signals, which might be associated with the higher rate of relapse in these patients [69]. However, to our knowledge, this hypothesis has not been examined in T-cell-depleted haploidentical HSCT as yet. Following cytokine-induced upregulation of HLA-E, the increased level of soluble HLA-E in the microenvironment and its possible antagonizing effect on NKG2A receptor would still benefit from further investigation and discussion [35]. Other studies in patients undergoing HLA-matched allogeneic HSCT without T-cell depletion did not report

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an association between the incidence of relapse and HLA-E*01030103 genotype [14,15], whereas, in our study, we found a lower incidence of relapse in patients with this genotype [70]. Our results are supported by the fact that, in a non-T-cell-depleted, HLA-matched allogeneic HSCT, the presence of donor T cells that react to mHags associated with HLA-E can induce a significant GvL effect [14] (Fig. 2B) that may prevent relapse. This is especially the case in patients with the HLA-E*01030103 genotype, expressing higher level of HLA-E associated with mHags that can react to T-cell-activatory receptors. This is a phenomenon that supports a GvL effect that is much more potent than that which might be induced by immature NK cells in these patients. Conclusion Despite the ongoing advances in transplant medicine, HSCT is still complicated by histocompatibilitydependent immune-adverse effects such as GvHD, relapse, and graft rejection. So far, using high-resolution HLA matching, various methods of transplantation, immunecell depletion, cellular therapy, and various conditioning regimens, scientists have struggled to minimize the risk of complications for patients undergoing HSCT. In line with these efforts, the evidence for the involvement of HLA-E in transplant outcomes is of interest to both clinical and academic researchers. It has been suggested that HLAE matching of donor and recipient can improve transplant outcomes. Interestingly, this finding is in line with a growing body of evidence that indicates that HLA-E polymorphisms have a role in HSCT outcomes.

Acknowledgments We thank Prof. Anthony P. Schwarer for his assistance with this manuscript.

Conflict of interest disclosure No financial interest/relationships with financial interest relating to the topic of this review have been declared.

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