REVIEW ARTICLE
Renal Involvement in Monoclonal Gammopathy Turki Al-Hussain, MD,* Maged H. Hussein, MD,w Hadeel Al Mana, MD,* and Mohammed Akhtar, MD, FCAP, FRCPA, FRCPath*
Abstract: Monoclonal gammopathy is produced by neoplastic or non-neoplastic expansion of a clone of plasma cells or B lymphocytes. Monoclonal gammopathy of unknown significance is characterized by low levels of the monoclonal protein and a relatively small population of clonal lymphocytes or plasma cells in the bone marrow. In these cases, the patient is asymptomatic with no evidence of overt myeloma or lymphoma. The abnormal serum protein may be present as a complete immunoglobulin molecule or may consist of Z1 of its components such as light chains or heavy chains. These proteins may cause a variety of diseases in various tissues and organs, of which the kidney appears to be the most vulnerable. Renal involvement in monoclonal gammopathy may occur as part of a generalized disease such as amyloidosis, immunoglobulin deposition disease, and cryoglobulinemia. In addition, there may be evidence of kidney damage by processes which are renal specific. These include light chain proximal tubulopathy, light chain cast nephropathy, and a variety of glomerulopathies encompassing a wide spectrum of disease patterns. Key Words: monoclonal, gammopathy, unknown significance, immunoglobulin deposition, casts, light chains, amyloid, cryoglobulin, tubulopathy, glomerulonephritis
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onoclonal gammopathies reflect a wide spectrum of related diseases in which increased amounts of immunoglobulins are produced by a clone of plasma cells or B lymphocytes. The monoclonal immunoglobulin is recognized as an abnormal band of restricted migration on serum or urine electrophoresis and is termed as M component or paraprotein. The abnormal protein may be in the form of intact immunoglobulin or immunoglobulin fragments, such as free light and/or heavy chains. This may be accompanied by the presence of monoclonal B cells or plasma cells in the bone marrow.1,2 These proteins may be responsible for tissue damage in a range of body organs. Among these organs, the kidney seems to be especially vulnerable and is indeed the most frequently involved organ in patients with monoclonal gammopathy.3–7 The purpose of this review is to describe the pathophysiology of immunoglobulins and discuss the pathology and pathogenesis of a wide spectrum of renal diseases associated with monoclonal gammopathies. From the *Department of Pathology and Laboratory Medicine; and wDepartment of Medicine, King Faisal Specialist Hospital and Research Center, Riyadh, Kingdom of Saudi Arabia. The authors have no funding or conflicts of interest to disclose. Reprints: Mohammed Akhtar, MD, FCAP, FRCPA, FRCPath, Department of Pathology and Laboratory Medicine (MBC 10), King Faisal Specialist Hospital and Research Centre, P.O. Box 3354, Riyadh 11211, Kingdom of Saudi Arabia (e-mails: [email protected]; [email protected]). All figures can be viewed online in color at http://www.anatomic pathology.com. Copyright r 2015 Wolters Kluwer Health, Inc. All rights reserved.
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NORMAL STRUCTURE OF IMMUNOGLOBULINS Immunoglobulins are antibodies consisting of glycoproteins arranged as Z1 units. Each of these units contains 4 polypeptide chains: 2 identical heavy chains and 2 identical light chains (Fig. 1). The amino terminal ends of these polypeptide chains show considerable variation in amino acid composition and are referred to as the variable regions, which determine the antigen specificity of the antibodies. Each light chain is made up of around 220 amino acids and has a molecular weight of 25 kD. The genes coding for k and l light chains are situated on chromosomes 2 and 22, respectively. There is very little variation within the constant (CL) region of k light chain and l light chain; k CL is coded for by a single gene and l CL by one of several gene segments. In contrast, the variable (VL) region of a light chain comprises 4 framework regions which form a hydrophobic core, and within which are scattered 3 segments of hypervariable amino acid sequences called complementarity determining regions. The remaining parts of the light and heavy chains are composed of Z1 constant regions. The amino acid sequences in the constant regions do not manifest any variation. Each light chain consists of 1 variable domain VL and 1 constant domain CL. The heavy chains consist of a variable domain, VH, and 3 constant domains CH1, CH2, and CH3. Each heavy chain has about twice the number of amino acids and molecular weight (B50,000 kD) compared with light chains (B25,000 kD). Heavy and light chains are bound by covalent disulfide bonds, forming a bilaterally symmetric structure. Each Ig monomer is bivalent as it contains 2 antigen-binding sites. The hinge region is the area of the H chains between the first and second C region domains and is held together by disulfide bonds (Fig. 1). This flexible hinge is found in IgG, IgA, and IgD, but not in IgM or IgE. Each immunoglobulin monomer has an approximate molecular weight of 150,000 kD.11 The 5 primary classes of immunoglobulins are IgG, IgM, IgA, IgD, and IgE. These are distinguished by the type of heavy chain found in the molecule. IgG molecules have heavy chains known as g-chains; IgMs have m-chains; IgAs have a-chains; IgEs have E-chains; and IgDs have dchains. The heavy chains differ from each other in the amino acid sequences.8–12 Antibody classes differ in valency as a result of different numbers of units (monomers) that join to form the complete protein. IgG usually exists as a monomer, IgA as a dimer in secretions but as a monomer in serum, and IgM as a pentamer, thus functioning IgM antibodies have 5 Yshaped units containing a total of 10 light chains, 10 heavy chains, and 10 antigen-binding sites.8–12
PRODUCTION OF IMMUNOGLOBULINS Immunoglobulins are normally produced in response to exposure to foreign substances (antigens), such as www.anatomicpathology.com |
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their maturation pathway they differentiate into plasma cells which secrete large amounts of soluble antibody, with the same unique antigen-binding site as the cell-surface antibody that served earlier as the surface antigen receptor on the naive or memory B cell. Plasma cells seem to have completely and irreversibly committed most of their protein-synthesizing machinery to making antibody and are incapable of further growth and division. Many of the plasma cells die after several days, although some survive longer in the bone marrow and continue to secrete antibodies into the blood for months and even years.8–12 Within the effector B cells and plasma cells, light and heavy chains are separately produced within the endoplasmic reticulum and are then assembled to produce the complete antibody before secretion. Around two thirds of light chain production is k and this is reflected in a serum ratio of k isotype to l isotype of 1.8:1. Normally, there is a 40% overproduction of light chains compared with heavy chains, but the majority of light chains in the serum is bound to heavy chains in the form of immunoglobulin. Free light chains normally exist in the serum at low levels. FIGURE 1. Diagram depicting the structure of an antibody composed of 2 light chains and 2 heavy chains joined by several disulfide bonds (S-S). Each of the chains has 1 variable (yellow) and several constant regions (red). The variable regions of heavy and light chains determine the antigen specificity of the antibody. For further details please see the text.
bacteria, viruses, fungi, and a variety of other protein molecules. Each B cell is capable of producing a single species of antibody, characterized by a unique antigenbinding site. A naive or memory B cell is activated by exposure to an antigen matching its surface receptor, following which the cell proliferates (with the aid of a helper T cell), and differentiates into an antibody-secreting effector cell (Fig. 2). Effector B cells usually start secreting antibody while they are still small lymphocytes. At the end stage of
FIGURE 2. The process of antibody production starts with a naive B lymphocyte with a surface receptor coming in contact with the corresponding antigen triggering the cell to proliferate. This process is also aided by T-helper cells which produce cytokines that stimulate cell proliferation. This gives rise to active germinal centers, where the antigen-specific B cells undergo proliferation and later differentiate into plasma cells, which are dedicated to produce an antigen-specific antibody.
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MONOCLONAL GAMMOPATHY In contrast to the great diversity of normal immunoglobulins, the immunoglobulin in patients with monoclonal gammopathies contains an immunologically homogenous immunoglobulin commonly referred to as a paraprotein or monoclonal M-protein. The circulating M-protein may consist of an intact immunoglobulin, the light chain only, or rarely the heavy chain only. The heavy chain in monoclonal gammopathy may be from any 1 of the 5 immunoglobulin classes G, A, M, D, or E, whereas the light chain is either k or l in type, but not both.1,2 Monoclonal gammopathies result from an overproduction of a single abnormal clone of a plasma cell or B lymphocyte. Clonal expansion of these cells is the underlying abnormality among the monoclonal gammopathies. These cells may be found in the bone marrow, peripheral circulation, or soft tissue. Usually, the production of an M-protein component does not seem to be a response by the immune system to an offending antigen. If a disease is caused by a monoclonal line of plasma cells, derived either from a malignant clone or from a proliferative population of cells, the condition is called plasma cell dyscrasia. In some cases, monoclonal gammopathies may occur as a result of abnormal B cells, which have not yet developed into plasma cells. Demonstration of clonality depends on light chain restriction, excess of k-expressing or l-expressing plasma cells, or lymphocytes resulting in an abnormal k to l ratio.1,2 The monoclonal gammopathies may be encountered in a number of diseases, the most common of which is multiple myeloma representing approximately 60% of cases. Other less common conditions associated with monoclonal gammopathy are Waldenstro¨m macroglobulinemia (WM) and B-cell lymphoma/leukemia, each representing approximately 10% of the cases. Autoimmune diseases such as systemic lupus erythematosis, Sjogren disease, rheumatoid arthritis, and mixed connective tissue disease are usually associated with polyclonal increase in gammaglobulins, but may occasionally be accompanied by monoclonal gammopathy. Similarly, HIV, viral hepatitis, or other chronic infections may also be associated with monoclonal gammopathy. The presence of M-protein is usually detected by serum and/or urine protein electrophoresis (Fig. 3A).
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clinical symptoms and disorders unrelated to myeloma. The incidence of MGUS seems to be 2 to 3 times higher in blacks than that in whites and is more frequent in men as compared with women. Follow-up studies have revealed that MGUS population in a given community acts as a substrate from which cases of multiple myeloma, WM, and B-cell lymphoma/ leukemia may develop at the rate of about 1% to 2% a year. As this rate does not decrease with time, a patient with MGUS requires lifetime follow-up surveilance.18 MGUS may be divided into 3 distinct subtypes based on the composition of the abnormal protein produced by the monoclonal cell population. Thus, MGUS may be IgM, non-IgM, and light chain type. Non-IgM MGUS is the most common subtype and has the potential to progress to multiple myeloma with all of its complication. IgM MGUS has the potential to progress to WM. Light chain MGUS (LC-MGUS) is a unique subtype of MGUS in which the secreted protein lacks heavy chain component. LC-MGUS may progress to light chain multiple myeloma.12–18
MONOCLONAL GAMMOPATHY OF RENAL SIGNIFICANCE FIGURE 3. Immunoelectropheresis patterns in monoclonal gammopathy (A) and MGUS (B). In a patient with monoclonal gammopathy, a prominent monoclonal peak (M) is present in the g region. In cases of MGUS, a similar but much smaller peak is present. MGUS indicates monoclonal gammopathy of unknown significance.
Immunofixation is used to identify the type of protein in the M-component and to distinguish it from a polyclonal gammopathy. Immunofixation may also be useful for detecting low levels of M-component that is not detectable by protein electrophoresis. In recent years, measurement of free light chains in the serum has become a common method to screen and follow patients with plasma cell dyscrasias.1,2
MONOCLONAL GAMMOPATHY OF UNDETERMINED SIGNIFICANCE Monoclonal gammopathy of undetermined significance (MGUS) is a condition in which an M-protein is found in the blood during standard laboratory tests. However, as compared with multiple myeloma and related diseases, the levels of antibody and the number of monoclonal cells in the bone marrow is lower. Furthermore, patients with MGUS have no symptoms or other apparent clinical problems related to multiple myeloma or other lymphoproliferatve diseases. It is defined by the presence of a serum monoclonal protein (M-protein), at a concentration 10% bone marrow involvement by clonal plasma cells in the absence of end-organ damage.13–18 MGUS is the commonest form of monoclonal gammopathy which occurs in over 3% of the general white population over the age of 50, and is typically detected as an incidental finding when patients undergo a protein electrophoresis as part of an evaluation for a wide variety of Copyright
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The term “monoclonal gammopathy of renal significance” (MGRS) has been proposed by the International Kidney and Monoclonal Gammopathy Research Group to describe cases that would otherwise meet the criteria for MGUS, but demonstrate renal insufficiency and monoclonal immunoglobulin deposits in the kidney. The distinction between MGUS and MGRS is important because the monoclonal protein seems to be the direct cause of kidney disease in such cases and treatment that targets the responsible clone is associated with restoration and preservation of kidney function.19 The natural history of MGUS is variable with some patients having a normal lifespan without ever progressing to overt multiple myeloma or other related lymphoproliferative disorder. Some ultimately manifest as a malignant lymphoplasmacytic cell proliferation accompanied by evidence of tissue damage due to deposition of abnormal proteins. The third group may never manifest a malignant process but instead develops tissue damage because of deposition of monoclonal protein (MGRS) (Fig. 4).
PATTERNS OF RENAL INVOLVEMENT BY MONOCLONAL GAMMOPATHY Patients with MG can develop a large variety of related renal lesions. The diversity of renal lesions in these cases may reflect the spectrum of the immunoglobulin abnormalities in these disorders. The renal abnormalities in MG may result from direct deposition of varying abnormal proteins or their components, such as heavy chain, light chain, or complete immunoglobulin in blood vessels, tubules, or glomeruli.3–7,20,21 There may be tubular toxicity or obstruction along with abnormal recruitment of inflammatory cells by monoclonal protein and stimulation of cytokines and/or chemokines resulting in damage of parenchymal cells. It must be pointed out, however, that in some patients with MG, the renal lesion may be the result of an unrelated disease process.7 Patients with renal involvement due to MG may be divided into 2 groups. The first group includes cases where kidney involvement is part of a systemic disease affecting several other organs and tissues. This group includes
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FIGURE 4. Patients with MGUS may have a variable outcome. In some patients the gammopathy remains stable; others progress to overt lymphoproliferative disorder (usually multiple myeloma). In a third group, deposition of abnormal serum proteins may result in tissue damage (usually kidney involvement) without any clinical manifestations of a neoplastic process. MGUS indicates monoclonal gammopathy of unknown significance.
amyloidosis, monoclonal immunoglobulin deposition disease (MIDD), and cryoglobulinema. The second group of patients manifests a renal-specific disease pattern, which includes conditions such as light chain proximal tubulopathy (LCPT), light chain cast nephropathy (LCCN), and various types of glomerulopathies (Table 1). The lesions in the 2 groups, however, are not mutually exclusive and different combinations may exist. For example, a patient may have systemic light chain deposition disease (LCDD) along with a renal-specific lesion such as LCCN.22
AL AMYLOIDOSIS AL amyloidosis is a condition in which an abnormal proteinaceous material derived from immunoglobulin light chains is deposited in tissues. The most commonly affected
TABLE 1. Renal Involvement in Monoclonal Gammopathy Part of systemic disease AL amyloidosis Monoclonal immunoglobulin deposition disease Cryoglobulinemia Renal-specific disease Light chain proximal tubulopathy Light chain cast nephropathy Glomerulonephritis With organized deposits Immunotactoid GN Fibrillary GN With nonorganized deposits Proliferative GN with monoclonal IgG Membranoproliferative GN Membranous GN C3 nephritis Other glomerulopathies GN indicates glomerulonephritis.
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FIGURE 5. Diagram depicting the cellular mechanism for generation of amyloid by mesangial cells. Following exposure to increased amounts of light chains, mesangial cells assume a macrophage phenotype. The light chains are taken up by the mesangial cells through endocytosis and are then partially broken down within the lysosomes to produce amyloid fibrils, which are then deposited in the interstitial spaces.
organs in amyloidosis are the kidney, heart, liver, and gastrointestinal tract. AL amyloidosis is usually associated with plasma cell dyscrasias including multiple myeloma. Approximately 5% of patients with WM may also develop amyloidosis.23–26 AL amyloid is derived from free light chains and is formed when the circulating light chains are taken up and internalized by tissue macrophages. The mesangial cells, in the presence of light chains, seem to acquire a macrophage phenotype and thus participate actively in the genesis of amyloid (Fig. 5). The light chains are partially metabolized within lysosomes and are then secreted into the surrounding matrix.27,28 The resulting misfolded light chain fragments form the characteristic eosinophilic tissue deposits of amyloid. Under the polarizing microscope, amyloid appears as a Congo red–staining material that exhibits apple-green birefringence (Figs. 6A–C). Congo red staining is the most common method used to identify amyloid, although the electron microscope is more sensitive and may be helpful in suspected cases when light microscopy is nondiagnostic. At the ultrastructural level, amyloid is characterized by the presence of randomly disposed nonbranching fibrils, 8 to 12 nm in diameter (Fig. 7). Amyloid fibrils when examined by x-ray diffraction reveal an antiparallel conformation with a bpleated sheet structure. l light chains appear to have a greater affinity to form amyloid than k chains. Rarely heavy chains with or without participation by light chains may also give rise to amyloid deposits.29,30 In renal amyloidosis there is a predominant involvement of the glomeruli along with variable amyloid deposits in blood vessels, tubular basement membranes, and the interstitium. In rare cases glomerular involvement may be minimal or completely absent; these patients may not manifest significant proteinuria. In cases with predominantly tubular deposition there may be signs of tubular dysfunction occasionally leading to acquired Fanconi syndrome. Rarely amyloid deposits may be limited to blood
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FIGURE 6. Amyloidosis. A, The mesangial matrix is expanded with eosinophilic amorphous material (H&E stain). B and C, The glomerulus stains with Congo red and with polarized microscopy it shows birefringence (apple-green). Please see this image in color online.
vessels.31 These patients present with slowly progressive chronic kidney disease with little or no proteinuria. Prognosis in such patients appears to be more favorable. In a study focusing on the predominant site of amyloid deposition in cases of renal amyloidosis, 84.6% showed glomerular, 9.4% vascular, and 6% tubulointerstitial distribution pattern. Within the glomeruli, amyloid is initially deposited in a focal segmental manner that becomes diffuse and global in later stages.32 Serum creatinine correlates well with the extent of interstitial fibrosis and tubular atrophy, whereas the degree of proteinuria reflects the glomerular amyloid load.
MONOCLONAL IMMUNOGLOBULIN DEPOSITION DISEASE The term Monoclonal Immunoglobulin Deposition Disease is used to encompass a multitude of deposition
FIGURE 7. Amyloidosis. Electron micrograph demonstrates nonbranching, randomly arranged amyloid fibrils (8 to 12 nm in diameter) in the glomerular basement membrane.
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diseases including LCDD, heavy chain deposition disease (HCDD), as well as mixed LCDD and HCDD.33–44 In a series of 64 patients with MIDD reported by Nasr et al,45 51 had LCDD, 7 had HCDD, and 6 had LCDD and HCDD. These diseases differ from amyloidosis in that the deposits do not have a fibrillar organization and lack affinity for Congo red. Patients with these disorders typically present with the nephrotic syndrome and renal insufficiency. All the MIDDs have essentially similar clinical and pathologic features with prominent renal dysfunction and usually asymptomatic involvement of several other organs. LCDD is by far the most common type among the immunoglobulin deposition diseases and may serve as a prototype for the description of the clinicopathologic features of the entire group. LCDD was first recognized by Randall et al46 as an infiltration of light chains involving multiple organs, including kidney, heart, liver, and GI tract. Renal involvement is a constant feature which may present as renal insufficiency, proteinuria, and nephrotic syndrome. Approximately 50% to 60% of patients with LCDD have associated lymphoproliferative disorder, most commonly multiple myeloma. The remaining cases develop LCDD in the setting of progression of MGUS or with no evidence of neoplastic plasma cell proliferation.39–45 Extrarenal involvement by LCDD is primarily noted at autopsy and is usually confined to the perivascular regions of the affected organs. The manner of renal involvement by LCDD is somewhat different from that seen in other organs where deposition of light chain is the only pathology. In the kidney, light chains are deposited along the basement membranes of the glomerular capillaries and along the tubular basement membranes and on electron microscopy appear as flocculent to granular electron dense material (Figs. 8A, B). In addition, there are multiple nodular lesions within the mesangial areas. These nodules are composed of mesangial matrix with variable amounts of light chain deposition and may be indistinguishable from those seen in diabetic nephropathy (Fig. 9A). On electron
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FIGURE 8. A and B, Light chain deposition disease. Electron micrographs demonstrate granular electron-dense material in the inner aspect of the glomerular basement membranes (A) and outer aspect of the tubular basement membranes (B).
microscopic examination, these nodules are composed predominantly of mesangial matrix with variable deposits of light chains (Fig. 9B). Immunofluorescent microscopy usually reveals deposition of k light chain within the mesangium, along the capillary walls and along tubular basement membranes (Fig. 9C). The mechanism of the development of nodular mesangial lesions has been attributed to interaction between light chains and receptors on the surface of mesangial cells (Fig. 10). This leads to activation of the
mesangial cells, which transform into a myofibroblastic phenotype with marked increase in profiles of rough endoplasmic reticulum, which produces excess amount of mesangial matrix giving rise to the nodular lesions.26–27 In rare cases the glomerular lesion may lack nodular changes and may be characterized by massive subendothelilal and mesangial deposits resembling that in lupus nephritis.47 The frequency of LCDD is unknown. In a renal biopsy study by Mallick et al48 of 260 patients with idiopathic proteinuria, 5 had LCDD. A renal biopsy study by Pirani
FIGURE 9. Light chain deposition disease. A, Prominent mesangial nodules resembling diabetic glomerulosclerosis (H&E stain). B, Electron micrograph demonstrating a mesangial nodule with scattered deposits of granular electron-dense material (light chains) within the mesangial matrix nodule. C, Immunofluorescence microscopy demonstrates mesangial staining with an antiserum for k light chain. In addition, there is linear staining of the tubular basement membranes. Please see this image in color online.
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FIGURE 10. Diagram depicting the cellular processes that give rise to the nodular lesions in the mesangium in light chain deposition disease. The light chains interact with receptors on the surface of mesangial cells. The receptor activation triggers a cell response, whereby the mesangial cells assume a myofibroblastic phenotype with abundant profiles of rough endoplasmic reticulum. This results in production of increased amounts of matrix proteins, which are deposited in the mesangium with resulting mesangial nodular lesions. Please see this image in color online.
et al49 reported 47 patients with plasma cell dyscrasia, in whom 24 had cast nephropathy and 10 had LCDD. The disease is found in approximately 5% of patients with multiple myeloma at autopsy.7 Light chains in patients with LCDD appear to have affinity for a variety of tissues thus explaining the distribution of these deposits in specific locations in several organs. This affinity for tissues is dependent on the sequence of amino acids in the variable region of the light chains, which seem to determine the physicochemical properties of the light chains. Approximately 85% of cases of LCDD are associated with k light-chain deposition.50–52
Renal Involvement in Monoclonal Gammopathy
FIGURE 12. Electron micrograph of cryoglobulinemic glomerulonephritis demonstrates subendothelial deposits. On high magnification (inset), these deposits exhibit an organized substructure composed of approximately 30 nm paired microtubules.
A monoclonal protein of the same light-chain isotype is usually demonstrated in serum or urine.
CRYOGLOBULINEMIA Cryoglobulins are immunoglobulins that precipitate when cooled below body temperature and redissolve on heating. Cryoglobulins may be composed of only immunoglobulins or may contain a mixture of immunoglobulins and complement components. Cryoglobulinemias are classified into 3 types according to the immunoglobulin composition. Type I cryoglobulin consists typically of monoclonal IgM, does not activate complement in vitro, and leads to symptoms of hyperviscosity; type II cryoglobulins contain a monoclonal IgM (presenting with rheumatoid factor activity) and polyclonal IgG; and type III cryoglobulins contain only polyclonal immunoglobulins and thus is not included among monoclonal gammopathies.53–57
FIGURE 11. Cryoglobulinemic glomerulonephritis. A, The glomerulus shows proliferative changes with MPGN type-1 pattern (H&E stain). B, The glomerulus shows numerous intracapillary protein thrombi (PAS stain). Please see this image in color online.
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Type I cryoglobulins are seen mostly in combination with or as a result of plasma cell dyscrasias, including multiple myeloma and WM, and less commonly with other lymphoproliferative disorders. Cryoglobulins type II and III are seen mostly in combination with autoimmune and infectious diseases, such as hepatitis C. Cryoglobulins type II may occasionally be seen in patients with plasma cell dyscrasia. Clinical manifestations in cryoglobulinemia include arthritis, skin lesions such as vasculitis and thrombosis, neuropathies, and glomerulonephritis.53–57 The most frequent pattern of glomerular lesion is an appearance reminiscent of membranoproliferative glomerulonephritis (Figs. 11A, B). The typical histopathologic feature of renal involvement of cryoglobulinemia is the presence of massive intracapillary and subendothelial immunoglobulin aggregates, which may almost obliterate the glomerular lumina.13 Other patterns include focal
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segmental or diffuse proliferative, mesangial proliferative, and membranous glomerulonephritis. Renal lesions are less frequently associated with type I cryoglobulinemia than with type II or type III. Cryoglobulins are generated by the clonal expansion of B cells, in the context of either lymphoproliferative disorders or persistent immune stimulation triggered by chronic infections or autoimmune diseases. Types I and II cryoglobulinemias result from the monoclonal expansion of a clone that can be overtly malignant (multiple myeloma), smoldering (WM, plasmacytoid lymphoma), or indolent (as in MGUS). By contrast, B-cell expansion is polyclonal in type III cryoglobulinemia. When renal biopsy specimens are examined with electron microscopy, amorphous or granular dense deposits may be found within the capillary lumina (hyaline thrombi) and in subendothelial location. Ultrastructure may also show deposits with organized substructure with paired,
FIGURE 13. Light chain proximal tubulopathy. A, The proximal tubules reveal numerous intracytoplasmic crystals (Masson trichrome stain). B, Immunofluorescence microscopy demonstrates staining of these crystals with an antiserum for k light chain, whereas negative for l light chain in (C).
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FIGURE 14. Electron micrograph of light chain proximal tubulopathy. The tubular epithelial cell contains numerous rhomboid crystals.
curved, microtubular, and/or annular forms measuring 20 to 30 nm in diameter (Fig. 12). In some cases the immune deposits may have a “fingerprint”-like pattern, which has also been noted in other types of renal diseases, particularly lupus nephritis in the absence of cryoglobulinemia.58,59 Two major mechanisms may be at play to varying degrees across the different types of cryoglobulinemia: cryoglobulin precipitation in the microcirculation, and immune complex–mediated inflammation of blood vessels. Vascular occlusion is more frequent in type I cryoglobulinemia, which is usually accompanied by high cryoglobulin concentrations, and can be associated with hyperviscosity syndrome and cold-induced acral necrosis. Immune complex–mediated vasculitis is more frequent in mixed cryoglobulinemias, particularly type II, in which the monoclonal IgM component generates large immune complexes with IgG and complement fractions, particularly C1q. Three types of renal-specific lesions may be seen in patients with MG.3–7 These include LCPT, LCCN, and several types of glomerulonephritis.
FIGURE 15. Intratubular casts in light chain cast nephropathy are formed by interaction of uromodulin with light chain filtered through the glomerulus. Uromodulin, which is normally produced in the ascending loop of Henle, forms a strong bond with the light chains resulting in formation of insoluble casts.
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FIGURE 16. Light chain cast nephropathy featuring intratubular light chain casts with alteration of the tubular epithelium and scattered giant cells (H&E stain). Please see this image in color online.
LIGHT CHAIN PROXIMAL TUBULOPATHY Light chains are relatively small proteins (molecular weight 22 to 25 kD) and are freely filtered through the glomerulus. Light chains in the glomerular filtrate are normally reabsorbed by the proximal tubules, which have an efficient system for reabsorption of proteins by endocytosis mediated by cubulin and megalin endocytic receptors.60 Within the proximal tubules, the light chains are catabolized by lysosomal degradation and their amino acids are reabsorbed and recycled. Normally the amount of light chains in the glomerular filtrate is relatively small (< 0.5 g), which is well within the absorptive capacity of the tubules. As a result there is little, if any, excretion of light chains in the urine. In patients with overt multiple myeloma, the amount of light chains may reach >20 g/d exceeding the absorptive capacity of the tubules resulting in light chain excretion in the urine as Bence Jones proteins. Excess amounts of light chains may on occasion be produced in association with other related malignant processes such as malignant lymphoma and WM. Processing of pathologically large quantities of light chains leads to accumulation of large numbers of phagolysosomes within the cytoplasm of proximal tubular cells, which may interfere with absorption of other substances in the glomerular filtrate.61–68 Furthermore, some of the light chains may be resistant to proteolysis resulting in their accumulation within the tubular cells (Fig. 13A). The presence of the abnormal proteins is recognizable at light microscopic examination and light chain may be demonstrated by immunofluorescent microscopy, which demonstrates predominant presence of one of the light chains (Figs. 13B, C). Precipitation of partially degraded proteins results in accumulation of crystalline structures within the lysosomes of the tubular cells. Crystal formation may be a prominent feature of LCPT. The crystals are electron dense, usually rhomboid, square, or rectangular in configuration, and found in the lysosomes of the tubular cells by EM (Fig. 14). Occasionally the crystals may have fibrillar architecture. Rarely, crystals can be seen in glomerular cells including podocytes as well as in interstitial cells and histiocytes. Resistance to complete intralysosomal breakdown is a feature almost exclusively seen in cases with k light chains, although amino acid sequences at the variable region of the light chains may also influence the physicochemical properties of the light chains. The presence of crystals within the proximal tubular cells, however, is not a requirement for diagnosis of LCPT, as several cases of LCPT without
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LIGHT CHAIN CAST NEPHROPATHY
FIGURE 17. Fibrillary glomerulonephritis with mesangial matrix expansion and thickening of the peripheral capillary walls (PAS stain). Please see this image in color online.
crystals have now been reported.63,64 Most of these cases are due to l light chains which are more amenable to lysosomal degradation. These cases may be diagnosed by demonstration of monoclonality of intratubular protein on immunofluorescent microscopy and by recognition of excessive numbers of large granules of reabsorbed proteins within the lysosomes by light microscopy and electron microscopic examination. Immunoelectron microscopy may also be helpful in determining the clonality of the intracellular light chains. The first description of LCPT in the proximal tubular epithelial cell cytoplasm was reported in 1957. Needle-shaped crystals were noted in the proximal tubular cells on electron microscopic examination. Subsequently,
Renal Involvement in Monoclonal Gammopathy Turki Al-Hussain, MD,* Maged H. Hussein, MD,w Hadeel Al Mana, MD,* and Mohammed Akhtar, MD, FCAP, FRCPA, FRCPath*
Abstract: Monoclonal gammopathy is produced by neoplastic or non-neoplastic expansion of a clone of plasma cells or B lymphocytes. Monoclonal gammopathy of unknown significance is characterized by low levels of the monoclonal protein and a relatively small population of clonal lymphocytes or plasma cells in the bone marrow. In these cases, the patient is asymptomatic with no evidence of overt myeloma or lymphoma. The abnormal serum protein may be present as a complete immunoglobulin molecule or may consist of Z1 of its components such as light chains or heavy chains. These proteins may cause a variety of diseases in various tissues and organs, of which the kidney appears to be the most vulnerable. Renal involvement in monoclonal gammopathy may occur as part of a generalized disease such as amyloidosis, immunoglobulin deposition disease, and cryoglobulinemia. In addition, there may be evidence of kidney damage by processes which are renal specific. These include light chain proximal tubulopathy, light chain cast nephropathy, and a variety of glomerulopathies encompassing a wide spectrum of disease patterns. Key Words: monoclonal, gammopathy, unknown significance, immunoglobulin deposition, casts, light chains, amyloid, cryoglobulin, tubulopathy, glomerulonephritis
(Adv Anat Pathol 2015;22:121–134)
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onoclonal gammopathies reflect a wide spectrum of related diseases in which increased amounts of immunoglobulins are produced by a clone of plasma cells or B lymphocytes. The monoclonal immunoglobulin is recognized as an abnormal band of restricted migration on serum or urine electrophoresis and is termed as M component or paraprotein. The abnormal protein may be in the form of intact immunoglobulin or immunoglobulin fragments, such as free light and/or heavy chains. This may be accompanied by the presence of monoclonal B cells or plasma cells in the bone marrow.1,2 These proteins may be responsible for tissue damage in a range of body organs. Among these organs, the kidney seems to be especially vulnerable and is indeed the most frequently involved organ in patients with monoclonal gammopathy.3–7 The purpose of this review is to describe the pathophysiology of immunoglobulins and discuss the pathology and pathogenesis of a wide spectrum of renal diseases associated with monoclonal gammopathies. From the *Department of Pathology and Laboratory Medicine; and wDepartment of Medicine, King Faisal Specialist Hospital and Research Center, Riyadh, Kingdom of Saudi Arabia. The authors have no funding or conflicts of interest to disclose. Reprints: Mohammed Akhtar, MD, FCAP, FRCPA, FRCPath, Department of Pathology and Laboratory Medicine (MBC 10), King Faisal Specialist Hospital and Research Centre, P.O. Box 3354, Riyadh 11211, Kingdom of Saudi Arabia (e-mails: [email protected]; [email protected]). All figures can be viewed online in color at http://www.anatomic pathology.com. Copyright r 2015 Wolters Kluwer Health, Inc. All rights reserved.
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NORMAL STRUCTURE OF IMMUNOGLOBULINS Immunoglobulins are antibodies consisting of glycoproteins arranged as Z1 units. Each of these units contains 4 polypeptide chains: 2 identical heavy chains and 2 identical light chains (Fig. 1). The amino terminal ends of these polypeptide chains show considerable variation in amino acid composition and are referred to as the variable regions, which determine the antigen specificity of the antibodies. Each light chain is made up of around 220 amino acids and has a molecular weight of 25 kD. The genes coding for k and l light chains are situated on chromosomes 2 and 22, respectively. There is very little variation within the constant (CL) region of k light chain and l light chain; k CL is coded for by a single gene and l CL by one of several gene segments. In contrast, the variable (VL) region of a light chain comprises 4 framework regions which form a hydrophobic core, and within which are scattered 3 segments of hypervariable amino acid sequences called complementarity determining regions. The remaining parts of the light and heavy chains are composed of Z1 constant regions. The amino acid sequences in the constant regions do not manifest any variation. Each light chain consists of 1 variable domain VL and 1 constant domain CL. The heavy chains consist of a variable domain, VH, and 3 constant domains CH1, CH2, and CH3. Each heavy chain has about twice the number of amino acids and molecular weight (B50,000 kD) compared with light chains (B25,000 kD). Heavy and light chains are bound by covalent disulfide bonds, forming a bilaterally symmetric structure. Each Ig monomer is bivalent as it contains 2 antigen-binding sites. The hinge region is the area of the H chains between the first and second C region domains and is held together by disulfide bonds (Fig. 1). This flexible hinge is found in IgG, IgA, and IgD, but not in IgM or IgE. Each immunoglobulin monomer has an approximate molecular weight of 150,000 kD.11 The 5 primary classes of immunoglobulins are IgG, IgM, IgA, IgD, and IgE. These are distinguished by the type of heavy chain found in the molecule. IgG molecules have heavy chains known as g-chains; IgMs have m-chains; IgAs have a-chains; IgEs have E-chains; and IgDs have dchains. The heavy chains differ from each other in the amino acid sequences.8–12 Antibody classes differ in valency as a result of different numbers of units (monomers) that join to form the complete protein. IgG usually exists as a monomer, IgA as a dimer in secretions but as a monomer in serum, and IgM as a pentamer, thus functioning IgM antibodies have 5 Yshaped units containing a total of 10 light chains, 10 heavy chains, and 10 antigen-binding sites.8–12
PRODUCTION OF IMMUNOGLOBULINS Immunoglobulins are normally produced in response to exposure to foreign substances (antigens), such as www.anatomicpathology.com |
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their maturation pathway they differentiate into plasma cells which secrete large amounts of soluble antibody, with the same unique antigen-binding site as the cell-surface antibody that served earlier as the surface antigen receptor on the naive or memory B cell. Plasma cells seem to have completely and irreversibly committed most of their protein-synthesizing machinery to making antibody and are incapable of further growth and division. Many of the plasma cells die after several days, although some survive longer in the bone marrow and continue to secrete antibodies into the blood for months and even years.8–12 Within the effector B cells and plasma cells, light and heavy chains are separately produced within the endoplasmic reticulum and are then assembled to produce the complete antibody before secretion. Around two thirds of light chain production is k and this is reflected in a serum ratio of k isotype to l isotype of 1.8:1. Normally, there is a 40% overproduction of light chains compared with heavy chains, but the majority of light chains in the serum is bound to heavy chains in the form of immunoglobulin. Free light chains normally exist in the serum at low levels. FIGURE 1. Diagram depicting the structure of an antibody composed of 2 light chains and 2 heavy chains joined by several disulfide bonds (S-S). Each of the chains has 1 variable (yellow) and several constant regions (red). The variable regions of heavy and light chains determine the antigen specificity of the antibody. For further details please see the text.
bacteria, viruses, fungi, and a variety of other protein molecules. Each B cell is capable of producing a single species of antibody, characterized by a unique antigenbinding site. A naive or memory B cell is activated by exposure to an antigen matching its surface receptor, following which the cell proliferates (with the aid of a helper T cell), and differentiates into an antibody-secreting effector cell (Fig. 2). Effector B cells usually start secreting antibody while they are still small lymphocytes. At the end stage of
FIGURE 2. The process of antibody production starts with a naive B lymphocyte with a surface receptor coming in contact with the corresponding antigen triggering the cell to proliferate. This process is also aided by T-helper cells which produce cytokines that stimulate cell proliferation. This gives rise to active germinal centers, where the antigen-specific B cells undergo proliferation and later differentiate into plasma cells, which are dedicated to produce an antigen-specific antibody.
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MONOCLONAL GAMMOPATHY In contrast to the great diversity of normal immunoglobulins, the immunoglobulin in patients with monoclonal gammopathies contains an immunologically homogenous immunoglobulin commonly referred to as a paraprotein or monoclonal M-protein. The circulating M-protein may consist of an intact immunoglobulin, the light chain only, or rarely the heavy chain only. The heavy chain in monoclonal gammopathy may be from any 1 of the 5 immunoglobulin classes G, A, M, D, or E, whereas the light chain is either k or l in type, but not both.1,2 Monoclonal gammopathies result from an overproduction of a single abnormal clone of a plasma cell or B lymphocyte. Clonal expansion of these cells is the underlying abnormality among the monoclonal gammopathies. These cells may be found in the bone marrow, peripheral circulation, or soft tissue. Usually, the production of an M-protein component does not seem to be a response by the immune system to an offending antigen. If a disease is caused by a monoclonal line of plasma cells, derived either from a malignant clone or from a proliferative population of cells, the condition is called plasma cell dyscrasia. In some cases, monoclonal gammopathies may occur as a result of abnormal B cells, which have not yet developed into plasma cells. Demonstration of clonality depends on light chain restriction, excess of k-expressing or l-expressing plasma cells, or lymphocytes resulting in an abnormal k to l ratio.1,2 The monoclonal gammopathies may be encountered in a number of diseases, the most common of which is multiple myeloma representing approximately 60% of cases. Other less common conditions associated with monoclonal gammopathy are Waldenstro¨m macroglobulinemia (WM) and B-cell lymphoma/leukemia, each representing approximately 10% of the cases. Autoimmune diseases such as systemic lupus erythematosis, Sjogren disease, rheumatoid arthritis, and mixed connective tissue disease are usually associated with polyclonal increase in gammaglobulins, but may occasionally be accompanied by monoclonal gammopathy. Similarly, HIV, viral hepatitis, or other chronic infections may also be associated with monoclonal gammopathy. The presence of M-protein is usually detected by serum and/or urine protein electrophoresis (Fig. 3A).
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clinical symptoms and disorders unrelated to myeloma. The incidence of MGUS seems to be 2 to 3 times higher in blacks than that in whites and is more frequent in men as compared with women. Follow-up studies have revealed that MGUS population in a given community acts as a substrate from which cases of multiple myeloma, WM, and B-cell lymphoma/ leukemia may develop at the rate of about 1% to 2% a year. As this rate does not decrease with time, a patient with MGUS requires lifetime follow-up surveilance.18 MGUS may be divided into 3 distinct subtypes based on the composition of the abnormal protein produced by the monoclonal cell population. Thus, MGUS may be IgM, non-IgM, and light chain type. Non-IgM MGUS is the most common subtype and has the potential to progress to multiple myeloma with all of its complication. IgM MGUS has the potential to progress to WM. Light chain MGUS (LC-MGUS) is a unique subtype of MGUS in which the secreted protein lacks heavy chain component. LC-MGUS may progress to light chain multiple myeloma.12–18
MONOCLONAL GAMMOPATHY OF RENAL SIGNIFICANCE FIGURE 3. Immunoelectropheresis patterns in monoclonal gammopathy (A) and MGUS (B). In a patient with monoclonal gammopathy, a prominent monoclonal peak (M) is present in the g region. In cases of MGUS, a similar but much smaller peak is present. MGUS indicates monoclonal gammopathy of unknown significance.
Immunofixation is used to identify the type of protein in the M-component and to distinguish it from a polyclonal gammopathy. Immunofixation may also be useful for detecting low levels of M-component that is not detectable by protein electrophoresis. In recent years, measurement of free light chains in the serum has become a common method to screen and follow patients with plasma cell dyscrasias.1,2
MONOCLONAL GAMMOPATHY OF UNDETERMINED SIGNIFICANCE Monoclonal gammopathy of undetermined significance (MGUS) is a condition in which an M-protein is found in the blood during standard laboratory tests. However, as compared with multiple myeloma and related diseases, the levels of antibody and the number of monoclonal cells in the bone marrow is lower. Furthermore, patients with MGUS have no symptoms or other apparent clinical problems related to multiple myeloma or other lymphoproliferatve diseases. It is defined by the presence of a serum monoclonal protein (M-protein), at a concentration 10% bone marrow involvement by clonal plasma cells in the absence of end-organ damage.13–18 MGUS is the commonest form of monoclonal gammopathy which occurs in over 3% of the general white population over the age of 50, and is typically detected as an incidental finding when patients undergo a protein electrophoresis as part of an evaluation for a wide variety of Copyright
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The term “monoclonal gammopathy of renal significance” (MGRS) has been proposed by the International Kidney and Monoclonal Gammopathy Research Group to describe cases that would otherwise meet the criteria for MGUS, but demonstrate renal insufficiency and monoclonal immunoglobulin deposits in the kidney. The distinction between MGUS and MGRS is important because the monoclonal protein seems to be the direct cause of kidney disease in such cases and treatment that targets the responsible clone is associated with restoration and preservation of kidney function.19 The natural history of MGUS is variable with some patients having a normal lifespan without ever progressing to overt multiple myeloma or other related lymphoproliferative disorder. Some ultimately manifest as a malignant lymphoplasmacytic cell proliferation accompanied by evidence of tissue damage due to deposition of abnormal proteins. The third group may never manifest a malignant process but instead develops tissue damage because of deposition of monoclonal protein (MGRS) (Fig. 4).
PATTERNS OF RENAL INVOLVEMENT BY MONOCLONAL GAMMOPATHY Patients with MG can develop a large variety of related renal lesions. The diversity of renal lesions in these cases may reflect the spectrum of the immunoglobulin abnormalities in these disorders. The renal abnormalities in MG may result from direct deposition of varying abnormal proteins or their components, such as heavy chain, light chain, or complete immunoglobulin in blood vessels, tubules, or glomeruli.3–7,20,21 There may be tubular toxicity or obstruction along with abnormal recruitment of inflammatory cells by monoclonal protein and stimulation of cytokines and/or chemokines resulting in damage of parenchymal cells. It must be pointed out, however, that in some patients with MG, the renal lesion may be the result of an unrelated disease process.7 Patients with renal involvement due to MG may be divided into 2 groups. The first group includes cases where kidney involvement is part of a systemic disease affecting several other organs and tissues. This group includes
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FIGURE 4. Patients with MGUS may have a variable outcome. In some patients the gammopathy remains stable; others progress to overt lymphoproliferative disorder (usually multiple myeloma). In a third group, deposition of abnormal serum proteins may result in tissue damage (usually kidney involvement) without any clinical manifestations of a neoplastic process. MGUS indicates monoclonal gammopathy of unknown significance.
amyloidosis, monoclonal immunoglobulin deposition disease (MIDD), and cryoglobulinema. The second group of patients manifests a renal-specific disease pattern, which includes conditions such as light chain proximal tubulopathy (LCPT), light chain cast nephropathy (LCCN), and various types of glomerulopathies (Table 1). The lesions in the 2 groups, however, are not mutually exclusive and different combinations may exist. For example, a patient may have systemic light chain deposition disease (LCDD) along with a renal-specific lesion such as LCCN.22
AL AMYLOIDOSIS AL amyloidosis is a condition in which an abnormal proteinaceous material derived from immunoglobulin light chains is deposited in tissues. The most commonly affected
TABLE 1. Renal Involvement in Monoclonal Gammopathy Part of systemic disease AL amyloidosis Monoclonal immunoglobulin deposition disease Cryoglobulinemia Renal-specific disease Light chain proximal tubulopathy Light chain cast nephropathy Glomerulonephritis With organized deposits Immunotactoid GN Fibrillary GN With nonorganized deposits Proliferative GN with monoclonal IgG Membranoproliferative GN Membranous GN C3 nephritis Other glomerulopathies GN indicates glomerulonephritis.
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FIGURE 5. Diagram depicting the cellular mechanism for generation of amyloid by mesangial cells. Following exposure to increased amounts of light chains, mesangial cells assume a macrophage phenotype. The light chains are taken up by the mesangial cells through endocytosis and are then partially broken down within the lysosomes to produce amyloid fibrils, which are then deposited in the interstitial spaces.
organs in amyloidosis are the kidney, heart, liver, and gastrointestinal tract. AL amyloidosis is usually associated with plasma cell dyscrasias including multiple myeloma. Approximately 5% of patients with WM may also develop amyloidosis.23–26 AL amyloid is derived from free light chains and is formed when the circulating light chains are taken up and internalized by tissue macrophages. The mesangial cells, in the presence of light chains, seem to acquire a macrophage phenotype and thus participate actively in the genesis of amyloid (Fig. 5). The light chains are partially metabolized within lysosomes and are then secreted into the surrounding matrix.27,28 The resulting misfolded light chain fragments form the characteristic eosinophilic tissue deposits of amyloid. Under the polarizing microscope, amyloid appears as a Congo red–staining material that exhibits apple-green birefringence (Figs. 6A–C). Congo red staining is the most common method used to identify amyloid, although the electron microscope is more sensitive and may be helpful in suspected cases when light microscopy is nondiagnostic. At the ultrastructural level, amyloid is characterized by the presence of randomly disposed nonbranching fibrils, 8 to 12 nm in diameter (Fig. 7). Amyloid fibrils when examined by x-ray diffraction reveal an antiparallel conformation with a bpleated sheet structure. l light chains appear to have a greater affinity to form amyloid than k chains. Rarely heavy chains with or without participation by light chains may also give rise to amyloid deposits.29,30 In renal amyloidosis there is a predominant involvement of the glomeruli along with variable amyloid deposits in blood vessels, tubular basement membranes, and the interstitium. In rare cases glomerular involvement may be minimal or completely absent; these patients may not manifest significant proteinuria. In cases with predominantly tubular deposition there may be signs of tubular dysfunction occasionally leading to acquired Fanconi syndrome. Rarely amyloid deposits may be limited to blood
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FIGURE 6. Amyloidosis. A, The mesangial matrix is expanded with eosinophilic amorphous material (H&E stain). B and C, The glomerulus stains with Congo red and with polarized microscopy it shows birefringence (apple-green). Please see this image in color online.
vessels.31 These patients present with slowly progressive chronic kidney disease with little or no proteinuria. Prognosis in such patients appears to be more favorable. In a study focusing on the predominant site of amyloid deposition in cases of renal amyloidosis, 84.6% showed glomerular, 9.4% vascular, and 6% tubulointerstitial distribution pattern. Within the glomeruli, amyloid is initially deposited in a focal segmental manner that becomes diffuse and global in later stages.32 Serum creatinine correlates well with the extent of interstitial fibrosis and tubular atrophy, whereas the degree of proteinuria reflects the glomerular amyloid load.
MONOCLONAL IMMUNOGLOBULIN DEPOSITION DISEASE The term Monoclonal Immunoglobulin Deposition Disease is used to encompass a multitude of deposition
FIGURE 7. Amyloidosis. Electron micrograph demonstrates nonbranching, randomly arranged amyloid fibrils (8 to 12 nm in diameter) in the glomerular basement membrane.
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diseases including LCDD, heavy chain deposition disease (HCDD), as well as mixed LCDD and HCDD.33–44 In a series of 64 patients with MIDD reported by Nasr et al,45 51 had LCDD, 7 had HCDD, and 6 had LCDD and HCDD. These diseases differ from amyloidosis in that the deposits do not have a fibrillar organization and lack affinity for Congo red. Patients with these disorders typically present with the nephrotic syndrome and renal insufficiency. All the MIDDs have essentially similar clinical and pathologic features with prominent renal dysfunction and usually asymptomatic involvement of several other organs. LCDD is by far the most common type among the immunoglobulin deposition diseases and may serve as a prototype for the description of the clinicopathologic features of the entire group. LCDD was first recognized by Randall et al46 as an infiltration of light chains involving multiple organs, including kidney, heart, liver, and GI tract. Renal involvement is a constant feature which may present as renal insufficiency, proteinuria, and nephrotic syndrome. Approximately 50% to 60% of patients with LCDD have associated lymphoproliferative disorder, most commonly multiple myeloma. The remaining cases develop LCDD in the setting of progression of MGUS or with no evidence of neoplastic plasma cell proliferation.39–45 Extrarenal involvement by LCDD is primarily noted at autopsy and is usually confined to the perivascular regions of the affected organs. The manner of renal involvement by LCDD is somewhat different from that seen in other organs where deposition of light chain is the only pathology. In the kidney, light chains are deposited along the basement membranes of the glomerular capillaries and along the tubular basement membranes and on electron microscopy appear as flocculent to granular electron dense material (Figs. 8A, B). In addition, there are multiple nodular lesions within the mesangial areas. These nodules are composed of mesangial matrix with variable amounts of light chain deposition and may be indistinguishable from those seen in diabetic nephropathy (Fig. 9A). On electron
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FIGURE 8. A and B, Light chain deposition disease. Electron micrographs demonstrate granular electron-dense material in the inner aspect of the glomerular basement membranes (A) and outer aspect of the tubular basement membranes (B).
microscopic examination, these nodules are composed predominantly of mesangial matrix with variable deposits of light chains (Fig. 9B). Immunofluorescent microscopy usually reveals deposition of k light chain within the mesangium, along the capillary walls and along tubular basement membranes (Fig. 9C). The mechanism of the development of nodular mesangial lesions has been attributed to interaction between light chains and receptors on the surface of mesangial cells (Fig. 10). This leads to activation of the
mesangial cells, which transform into a myofibroblastic phenotype with marked increase in profiles of rough endoplasmic reticulum, which produces excess amount of mesangial matrix giving rise to the nodular lesions.26–27 In rare cases the glomerular lesion may lack nodular changes and may be characterized by massive subendothelilal and mesangial deposits resembling that in lupus nephritis.47 The frequency of LCDD is unknown. In a renal biopsy study by Mallick et al48 of 260 patients with idiopathic proteinuria, 5 had LCDD. A renal biopsy study by Pirani
FIGURE 9. Light chain deposition disease. A, Prominent mesangial nodules resembling diabetic glomerulosclerosis (H&E stain). B, Electron micrograph demonstrating a mesangial nodule with scattered deposits of granular electron-dense material (light chains) within the mesangial matrix nodule. C, Immunofluorescence microscopy demonstrates mesangial staining with an antiserum for k light chain. In addition, there is linear staining of the tubular basement membranes. Please see this image in color online.
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FIGURE 10. Diagram depicting the cellular processes that give rise to the nodular lesions in the mesangium in light chain deposition disease. The light chains interact with receptors on the surface of mesangial cells. The receptor activation triggers a cell response, whereby the mesangial cells assume a myofibroblastic phenotype with abundant profiles of rough endoplasmic reticulum. This results in production of increased amounts of matrix proteins, which are deposited in the mesangium with resulting mesangial nodular lesions. Please see this image in color online.
et al49 reported 47 patients with plasma cell dyscrasia, in whom 24 had cast nephropathy and 10 had LCDD. The disease is found in approximately 5% of patients with multiple myeloma at autopsy.7 Light chains in patients with LCDD appear to have affinity for a variety of tissues thus explaining the distribution of these deposits in specific locations in several organs. This affinity for tissues is dependent on the sequence of amino acids in the variable region of the light chains, which seem to determine the physicochemical properties of the light chains. Approximately 85% of cases of LCDD are associated with k light-chain deposition.50–52
Renal Involvement in Monoclonal Gammopathy
FIGURE 12. Electron micrograph of cryoglobulinemic glomerulonephritis demonstrates subendothelial deposits. On high magnification (inset), these deposits exhibit an organized substructure composed of approximately 30 nm paired microtubules.
A monoclonal protein of the same light-chain isotype is usually demonstrated in serum or urine.
CRYOGLOBULINEMIA Cryoglobulins are immunoglobulins that precipitate when cooled below body temperature and redissolve on heating. Cryoglobulins may be composed of only immunoglobulins or may contain a mixture of immunoglobulins and complement components. Cryoglobulinemias are classified into 3 types according to the immunoglobulin composition. Type I cryoglobulin consists typically of monoclonal IgM, does not activate complement in vitro, and leads to symptoms of hyperviscosity; type II cryoglobulins contain a monoclonal IgM (presenting with rheumatoid factor activity) and polyclonal IgG; and type III cryoglobulins contain only polyclonal immunoglobulins and thus is not included among monoclonal gammopathies.53–57
FIGURE 11. Cryoglobulinemic glomerulonephritis. A, The glomerulus shows proliferative changes with MPGN type-1 pattern (H&E stain). B, The glomerulus shows numerous intracapillary protein thrombi (PAS stain). Please see this image in color online.
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Type I cryoglobulins are seen mostly in combination with or as a result of plasma cell dyscrasias, including multiple myeloma and WM, and less commonly with other lymphoproliferative disorders. Cryoglobulins type II and III are seen mostly in combination with autoimmune and infectious diseases, such as hepatitis C. Cryoglobulins type II may occasionally be seen in patients with plasma cell dyscrasia. Clinical manifestations in cryoglobulinemia include arthritis, skin lesions such as vasculitis and thrombosis, neuropathies, and glomerulonephritis.53–57 The most frequent pattern of glomerular lesion is an appearance reminiscent of membranoproliferative glomerulonephritis (Figs. 11A, B). The typical histopathologic feature of renal involvement of cryoglobulinemia is the presence of massive intracapillary and subendothelial immunoglobulin aggregates, which may almost obliterate the glomerular lumina.13 Other patterns include focal
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segmental or diffuse proliferative, mesangial proliferative, and membranous glomerulonephritis. Renal lesions are less frequently associated with type I cryoglobulinemia than with type II or type III. Cryoglobulins are generated by the clonal expansion of B cells, in the context of either lymphoproliferative disorders or persistent immune stimulation triggered by chronic infections or autoimmune diseases. Types I and II cryoglobulinemias result from the monoclonal expansion of a clone that can be overtly malignant (multiple myeloma), smoldering (WM, plasmacytoid lymphoma), or indolent (as in MGUS). By contrast, B-cell expansion is polyclonal in type III cryoglobulinemia. When renal biopsy specimens are examined with electron microscopy, amorphous or granular dense deposits may be found within the capillary lumina (hyaline thrombi) and in subendothelial location. Ultrastructure may also show deposits with organized substructure with paired,
FIGURE 13. Light chain proximal tubulopathy. A, The proximal tubules reveal numerous intracytoplasmic crystals (Masson trichrome stain). B, Immunofluorescence microscopy demonstrates staining of these crystals with an antiserum for k light chain, whereas negative for l light chain in (C).
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FIGURE 14. Electron micrograph of light chain proximal tubulopathy. The tubular epithelial cell contains numerous rhomboid crystals.
curved, microtubular, and/or annular forms measuring 20 to 30 nm in diameter (Fig. 12). In some cases the immune deposits may have a “fingerprint”-like pattern, which has also been noted in other types of renal diseases, particularly lupus nephritis in the absence of cryoglobulinemia.58,59 Two major mechanisms may be at play to varying degrees across the different types of cryoglobulinemia: cryoglobulin precipitation in the microcirculation, and immune complex–mediated inflammation of blood vessels. Vascular occlusion is more frequent in type I cryoglobulinemia, which is usually accompanied by high cryoglobulin concentrations, and can be associated with hyperviscosity syndrome and cold-induced acral necrosis. Immune complex–mediated vasculitis is more frequent in mixed cryoglobulinemias, particularly type II, in which the monoclonal IgM component generates large immune complexes with IgG and complement fractions, particularly C1q. Three types of renal-specific lesions may be seen in patients with MG.3–7 These include LCPT, LCCN, and several types of glomerulonephritis.
FIGURE 15. Intratubular casts in light chain cast nephropathy are formed by interaction of uromodulin with light chain filtered through the glomerulus. Uromodulin, which is normally produced in the ascending loop of Henle, forms a strong bond with the light chains resulting in formation of insoluble casts.
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FIGURE 16. Light chain cast nephropathy featuring intratubular light chain casts with alteration of the tubular epithelium and scattered giant cells (H&E stain). Please see this image in color online.
LIGHT CHAIN PROXIMAL TUBULOPATHY Light chains are relatively small proteins (molecular weight 22 to 25 kD) and are freely filtered through the glomerulus. Light chains in the glomerular filtrate are normally reabsorbed by the proximal tubules, which have an efficient system for reabsorption of proteins by endocytosis mediated by cubulin and megalin endocytic receptors.60 Within the proximal tubules, the light chains are catabolized by lysosomal degradation and their amino acids are reabsorbed and recycled. Normally the amount of light chains in the glomerular filtrate is relatively small (< 0.5 g), which is well within the absorptive capacity of the tubules. As a result there is little, if any, excretion of light chains in the urine. In patients with overt multiple myeloma, the amount of light chains may reach >20 g/d exceeding the absorptive capacity of the tubules resulting in light chain excretion in the urine as Bence Jones proteins. Excess amounts of light chains may on occasion be produced in association with other related malignant processes such as malignant lymphoma and WM. Processing of pathologically large quantities of light chains leads to accumulation of large numbers of phagolysosomes within the cytoplasm of proximal tubular cells, which may interfere with absorption of other substances in the glomerular filtrate.61–68 Furthermore, some of the light chains may be resistant to proteolysis resulting in their accumulation within the tubular cells (Fig. 13A). The presence of the abnormal proteins is recognizable at light microscopic examination and light chain may be demonstrated by immunofluorescent microscopy, which demonstrates predominant presence of one of the light chains (Figs. 13B, C). Precipitation of partially degraded proteins results in accumulation of crystalline structures within the lysosomes of the tubular cells. Crystal formation may be a prominent feature of LCPT. The crystals are electron dense, usually rhomboid, square, or rectangular in configuration, and found in the lysosomes of the tubular cells by EM (Fig. 14). Occasionally the crystals may have fibrillar architecture. Rarely, crystals can be seen in glomerular cells including podocytes as well as in interstitial cells and histiocytes. Resistance to complete intralysosomal breakdown is a feature almost exclusively seen in cases with k light chains, although amino acid sequences at the variable region of the light chains may also influence the physicochemical properties of the light chains. The presence of crystals within the proximal tubular cells, however, is not a requirement for diagnosis of LCPT, as several cases of LCPT without
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LIGHT CHAIN CAST NEPHROPATHY
FIGURE 17. Fibrillary glomerulonephritis with mesangial matrix expansion and thickening of the peripheral capillary walls (PAS stain). Please see this image in color online.
crystals have now been reported.63,64 Most of these cases are due to l light chains which are more amenable to lysosomal degradation. These cases may be diagnosed by demonstration of monoclonality of intratubular protein on immunofluorescent microscopy and by recognition of excessive numbers of large granules of reabsorbed proteins within the lysosomes by light microscopy and electron microscopic examination. Immunoelectron microscopy may also be helpful in determining the clonality of the intracellular light chains. The first description of LCPT in the proximal tubular epithelial cell cytoplasm was reported in 1957. Needle-shaped crystals were noted in the proximal tubular cells on electron microscopic examination. Subsequently,
