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Renal lesions associated with plasma cell dyscrasias: practical approach to diagnosis, new concepts, and challenges.

Patients with circulating monoclonal proteins may or may not have systemic manifestations with associated organ damage. Those patients without recognizable organ involvement are considered to exhibit a monoclonal gammopathy of unknown significance (MGUS). Monoclonal gammopathy of unknown significance occurs in approximately 0.15% of the population. (1) Conceptually, MGUS is considered to be an asymptomatic premalignant disorder. Patients with MGUS are administered no therapy but are closely followed, as a percentage of them progress to develop organ damage with time and transform into having a full-blown plasma cell dyscrasia (myeloma). For a diagnosis of MGUS, the serum monoclonal protein levels should be less than 3 mg/dL and plasma cells in the bone marrow aspirate/biopsy less than 10%, and there should be an absence of end organ damage, including lytic bone lesions, anemia, hypercalcemia, or renal failure. Monoclonal gammopathy of unknown significance is found in 3% of individuals aged 50 years, 5% among those 70 years or older, and 7.5% among those 85 years or older. (2)

Patients with MGUS have an overall 1% lifelong risk per year of progression to multiple myeloma or related conditions. (3,4) To change a patient from the MGUS category into a diagnosis of plasma cell dyscrasia and initiate appropriate treatment for the malignancy, the burden of proof rests essentially on the demonstration of organ involvement or overt abnormal plasmacytic proliferation in the bone marrow. Bone marrow aspiration and biopsy may not show definitive findings to support a morphologic diagnosis of plasma cell dyscrasia and trigger therapy.

In part, this is due to the misconception that a significant increase of plasma cells, atypical cellular changes, and sheets of plasma cells are absolute requirements for a diagnosis of plasma cell dyscrasia/myeloma. The use of ancillary diagnostic techniques to typify plasma cells and demonstrate a clonal population in the bone marrow, even when the percentage of plasma cells is within acceptable or so-called normal limits, is not a widespread practice.

Because renal damage is the most common, and in most cases the earliest expression of systemic involvement in these patients, proper evaluation of renal biopsies is very important.

In order to link the renal biopsy findings with an underlying plasma cell dyscrasia, pathologists have to be aware of the vast and heterogeneous spectrum of renal pathologic manifestations. (5-11) Subtle and early cases can be truly challenging and require use of a variety of diagnostic techniques. (12,13) Unfortunately, in some instances doubt still remains even after a thorough evaluation of the renal biopsy is completed because of existing diagnostic limitations.


Challenges that are inherent to the interpretation of the renal biopsies from these patients will be discussed in this article. One of the confounding factors that may affect the accurate interpretation of renal samples is that these patients may have other concurrent renal diseases that may mask the proper identification of alterations that may be associated with monoclonal deposition of immunoglobulin components and related renal damage.


To link any renal alterations with an underlying plasma cell dyscrasia, there is a need to demonstrate the association of the particular lesion detected with deposition of monoclonal immunoglobulin components and to establish a cause-effect relationship. Therefore, immunofluorescence evaluation becomes extremely important in these cases. The routine use of antibodies to [kappa] and [lambda] light chains in the evaluation of renal biopsies is essential so that unusual, early, and subtle manifestations can be detected. Once routine testing for light chains became the norm, these conditions were detected with much more frequency, and the spectrum of morphologic manifestations expanded. It is also crucial that the staining for immunoglobulins be carefully evaluated, as the confirmation of the deposition of heavy-chain components is key to diagnosing heavychainassociated disorders. Commercially available antibodies to [kappa] and [lambda] light chains and heavy chains are routinely used to evaluate these cases.

Ultrastructural evaluation is also very important, as the specific morphologic correlates of these conditions must be carefully assessed to either support the suspected diagnosis or to provide additional evidence.

Another technique that can provide extremely useful information in some cases is immunogold labeling at the ultrastructural level. (12) There are cases that lack definitive morphologic findings by light or electron microscopy, and the immunofluorescence data are inconclusive; the presence of monoclonal immunoglobulin components in the kidney can be confirmed objectively using immunogold labeling. This is particularly useful in early cases and in circumstances where immunomorphologic parameters are doubtful, suboptimal, or fulfill less than diagnostic criteria. If the immunofluorescence data are definitive (ie, de fine a pattern characteristic of monoclonal light chain deposition disease), this finding essentially suffices to make an unequivocal diagnosis.


Because morphologic features encountered in cases associated with plasma cell dyscrasias overlap with those seen in a variety of other conditions, and a generic morphologic diagnosis can be made in virtually every case, the association of a particular case with a plasma cell dyscrasia can be easily missed. Unfortunately, this important association is often ignored, delaying proper patient management.


Although in some cases there is documented clinical evidence that the patient has a plasma cell dyscrasia, in a significant number of the cases the clinical information available at the time of the renal biopsy is incomplete. Occasionally, the pertinent laboratory tests to explore whether a patient may have a plasma cell dyscrasia have been requested, but the results are not available at the time the biopsy is interpreted. In other cases, the clinician does not suspect a plasma cell dyscrasia, and therefore has not requested the appropriate tests to be performed.

Renal manifestations associated with these disorders are quite variable depending on the pattern of renal damage. If cast nephropathy or acute proximal tubular damage is present, acute renal failure is a common manifestation. If a glomerulopathy (ie, associated with light chain deposition disease or amyloidosis) is the main lesion, proteinuria with or without nephrotic syndrome is the most common clinical presentation. (5-11) In both situations the clinical presentation may be characterized by slowly progressive renal dysfunction.

A rather common scenario is that a patient who is being followed after a diagnosis of MGUS (presence of monoclonal spike on serum protein electrophoresis) has been established, or the patient has been diagnosed recently with MGUS and develops renal manifestations. In these settings, the most common clinical finding that leads to a renal biopsy is proteinuria with or without associated nephrotic syndrome. In patients with renal dysfunction, a kidney biopsy is performed to assess whether the renal problem is related to the paraproteinemia, thus requiring the treatment of the plasma cell dyscrasia.

It is also important to understand that some of the patients with plasma cell dyscrasia do not exhibit a monoclonal spike in the serum due to the fact that they have circulating light chains. Because of their low molecular weight (generally 25 kd or less), light chains are freely filtered and appear in the urine. In these cases it is very important to detect the presence of abnormal light chains (Bence-Jones proteins) in the urine, where the monoclonal spike can be detected. To do this effectively, there is a need to concentrate urine in instances when the amounts of light chains are small. This is particularly important in cases with glomerulopathies, because the light chains become entrapped in the mesangium, and only a small amount appears in the urine.


Immunofixation was until recently the most reliable methodology available to detect small amounts of light chains. Determination of free light chains in the serum has become a common method to diagnose and follow patients with plasma cell dyscrasias. A similar test to detect abnormal circulating heavy chains in the serum is being developed. A recent comparison of immunofixation and serum-free light chains showed advantages and disadvantages of each technique. (14) Free light-chain assays are sensitive and very specific in detecting serum monoclonal light chains. Serum-free light-chain assays combined with immunofixation virtually eliminate the need for urine screening for monoclonal gammopathies. (15)




Any of the renal compartments can be affected. In some cases, more than one compartment is involved. (5-11) There are some morphologic patterns that are readily recognizable at the light microscopic level, especially when the manifestations are florid, including that of myeloma cast nephropathy, nodular glomerulosclerosis associated with light or heavy chain deposition disease, and light and heavy chain (AL and AH) amyloidosis. Subtle and early manifestations of the above-mentioned conditions and other less well-recognized patterns can be missed easily if the data available emanating from light, electron, and immunofluorescence microscopies are not properly collated and evaluated. This article will focus on these lesscommon manifestations, and will relate them to the more readily recognizable ones in an effort to provide a conceptual framework that can help in the understanding of the heterogeneity of these conditions.

Another complicating factor is that these conditionsmay be found in combination. For example, a given patient may have light chain deposition disease and light chain cast nephropathy, or AL/AH amyloidosis and light/heavy chain deposition disease. Other combinations also have been reported. (16-19) Light chain cast nephropathy has been traditionally considered the most common pattern of renal damage in patients with plasma cell dyscrasias. This information emerged primarily from autopsy studies. (20-22) However, AL amyloidosis has been found to be more common in a recent renal biopsy series. (23)


The typical clinical presentation is acute renal deterioration or frank renal failure. There may be precipitating factors, such as dehydration, hypercalcemia, hyperurice mia, infections, or exposure to contrast media, nonsteroidal anti-inflammatory drugs, nephrotoxins, or loop diuretics, such as furosemide.


Manifestations at the light microscopic level diagnostic of light chain (myeloma) cast nephropathy may be difficult to assess with certainty. Cast nephropathy has not been described as associated with heavy-chain disease, except in very rare instances (ie, associated with Waldenstrom macroglobulinemia, which could be conceptualized as a peculiar heavy-chain [immunoglobulin M]-associated disorder). (24) The pathology of Waldestrom macroglobulinemia is different from that of other gammopathies and will not be discussed in this article. Therefore, for all practical purposes cast nephropathy is only associated with abnormal light chains.



On light microscopic examination, typical myeloma casts show fracture planes (Figure 1, A) and polymorphonuclear cells. In addition, there is generally a reaction of surrounding tubular cells (Figure 1, B), which become enlarged and may acquire prominent nucleoli. Sometimes, the casts are associated with multinucleated giant cells of histiocytic origin, even when the tubular basement membranes appear intact and the casts' contents have not extravasated into the interstitium (Figure 1, C). (25) Not all casts in patients with myeloma exhibit the entire spectrum of morphologic features noted above. The casts also may break through the tubular basement membrane and elicit an interstitial reaction, which may include multinucleated giant cells. They are usually in the distal nephron and are formed as a result of interactions of the pathologic light chain and Tamm-Horsfall protein. (26,27) Large casts may also be found in proximal tubules and even in the urinary space, arriving there by retrograde extension. An interstitial inflammatory infiltrate, which may contain eosinophils, is commonly associated with the tubular casts.

One of the most important unsettled points is that there are no guidelines as to how many tubular casts should be present to make such a diagnosis. The number of casts may be quite variable in a biopsy specimen due to sampling, and even in kidneys sampled extensively at autopsy. The morphologic findings associated with the casts may also pose some diagnostic problems. Occasionally, when tubular casts are present in other conditions, primarily in patients with end-stage renal disease (Figure 1, D) or in those patients taking certain drugs, such as rifampin (28) and tacrolimus/rapamycin, (29) they may mimic those seen in association with myeloma. The cells lining the tubules associated with casts in these 2 situations are generally flattened and do not reveal reactive features. Furthermore, multinucleated giant cells are not present.


Because there is relatively little change in the structure of the light chains that are delivered to the distal nephrons, there are usually no difficulties in identifying these light chains using the commercially available antibodies. However, the immunofluorescence findings associated with the tubular casts may be confusing, difficult to interpret, or nondiagnostic. Casts that have formed relatively rapidly and generally a short time before the renal biopsy was performed may reveal light-chain restriction (Figure 2) or distinct preponderance of one light chain over the other, findings that can be helpful to establish a definitive diagnosis or to support a diagnostic impression. (5) However, in most cases, both [kappa] and [lambda] light chains can be found in the casts, making it impossible to establish monoclonality. This occurs because the nonpertinent light chain becomes trapped in the casts. Casts with crystalline material may be diagnostic in the proper clinicopathologic setting, (5,13) especially when monoclonality can be demonstrated in the crystals using routine fluorescence microscopy, immunohistochemistry, or ultrastructural immunolabeling.

Electron microscopy is usually not very helpful in this condition, as the casts may vary substantially in their appearance (30) and not uncommonly exhibit a rather nonspecific appearance (Figure 3, A). Rarely, the casts may contain crystalline structures. (5,20) In a small subset of these patients the casts stain with Congo red, are apple green birefringent, and contain fibrillary material with characteristics most consistent with amyloid fibrils. (31) Ultrastructural immunogold labeling may be of value in assessing light-chain monoclonality associated with the cast contents (Figure 3, B), (12,13) even when immunofluorescence has failed.


The typical clinical presentation is progressive renal insufficiency or acute renal failure. In cases associated with Fanconi syndrome, the most classical presentation includes tubular dysfunction with aminoaciduria, phosphaturia, and glucosuria. They may also have nonnephrotic-range proteinuria, uricosuria and, at times, renal tubular acidosis.

Acute tubular damage (acute tubulopathy) mimics acute tubular necrosis by light microscopy. (5,13,32) At the light microscopic level the alterations in the proximal tubules can be quite subtle or overt, and the degree of tubular damage does not correlate directly with the functional abnormalities. Two morphologic patterns can be seen in these cases that correlate with the presence or absence of Fanconi syndrome.



On the hematoxylin-eosin stain, needle-shaped empty spaces can be noted in the cytoplasm of proximal tubular cells (Figure 4, A) in those cases associated with Fanconi syndrome. In the non-Fanconi type, the cytoplasm of the proximal tubular cells may be markedly eosinophilic and granular due to an abundance of lysosomes (Figure 4, B). Mitoses can be seen in tubular cells. Localization of monoclonal light-chain components within the damaged proximal tubular cells becomes important in making this diagnosis. Light-chain monoclonality can be demonstrated in the cytoplasm of proximal tubules using immunofluorescence or immunogold labeling at the ultrastructural level. (13,32)

At the ultrastructural level there is prominence of the lysosomal system with variably sized and shaped lysosomes and, in some instances, markedly atypical lysosomes are identified (Figure 5) in the non-Fanconi type of acute tubular damage. The monotypic light chains are localized to the lysosomes using ultrastructural labeling techniques, (13,32) and may be [kappa] or [lambda]. Vacuolization, apical blebbing, fragmentation, and desquamation of proximal tubular cells can be present. This pattern of proximal tubular damage has been reproduced in the laboratory, and the molecular events that take place have been elucidated. (33,34)

One of the unique clinical and pathologic manifestations associated with proximal tubular damage is represented by Fanconi syndrome. (35) In this particular type of proximal tubular injury, needle-shaped, often described as crystallinelike, inclusions are virtually always present in the cytoplasm of proximal tubular cells, which show evidence of damage. These inclusions are difficult to detect at the light microscopic level in many of the cases, and are much easier to identify ultrastructurally (Figure 6). The number of these peculiar inclusions per tubular cell can be quite variable. (36) All but one of the reported cases associated with Fanconi syndrome have been [kappa] light chain restricted. The pathologic light chains in these cases belong to the [kappa]1 subgroup. (37,38) Biochemical analysis of the primary structure of these light chains has demonstrated substitutions in residues 28 and 38 of the complementarity-determining region 1 (CDR 1), which results in a resistance to normal catabolism of these light chains in the proximal tubules. As a result, intracytoplasmic inclusions form. In a minority of the cases, the aforementioned cytoplasmic inclusions are highlighted and easily detectable on the immunofluorescence stain for [kappa] light chains. (10) Ultrastructurally, the inclusions may at times exhibit a somewhat fibrillary appearance, and their shape and electron density can be variable from case to case (Figure 6). The cytoplasmic inclusions are rather characteristic in most cases, and allow a definitive diagnosis. Immunogold labeling at the ultrastructural level is extremely helpful in demonstrating light-chain monoclonality associated with the inclusions (Figure 6, B and C), solidifying the diagnosis. (12,13,36)



Renal Fanconi syndrome associated with plasma cell dyscrasia has also been reproduced in the research laboratory, resulting in insightful observations that have revolutionized our overall understanding of this entity. This translational effort has provided a solid platform for the future development of novel therapeutic interventions for this disorder. (37-39)


Patients with this pattern of renal injury are usually older than 50 years and present with acute renal failure. (40,41) Nonnephrotic-range proteinuria may be found, and serum creatinine is generally increased.

This interstitial manifestation of plasma cell-associated renal disease mimics acute tubular interstitial nephritis by light microscopy. (39) Interstitial inflammatory infiltrates associated with variable degrees of tubulitis represent the typical light microscopic finding associated with this condition (Figure 7, A). Eosinophils may be present in the interstitial inflammatory infiltrates, and although tubulitis is a constant finding, the inflammatory process may be focal. (40,41)

Demonstration of an association with a monoclonal plasma cell process is crucial to make a definitive diagnosis; therefore, demonstrating light-chain deposition and restriction associated with the interstitial inflammatory process is key (Figure 7, B). There are no tubular casts.

Immunofluorescence and electron microscopy individually may or may not be helpful, but both combined should provide sufficient evidence to support the diagnosis in most cases. (40)

Ultrastructural labeling has also been used to assess light chain monoclonality associated with the tubular interstitial manifestations with excellent results, even in cases when other techniques have failed. (40) Most of these cases are [kappa] light chain restricted.

Glomeruli and vasculature are entirely normal in this pattern of light-chain-associated interstitial disease. This pattern of interstitial disease could be conceptualized as an interstitial form of light chain deposition disease without manifestations in other renal compartments. (41)


The average age of patients with these disorders is 55 to 60 years. At the time of diagnosis, acute renal failure is present in 30% of patients, and a similar percentage is already dialysis dependent. More than 90% of the patients exhibit proteinuria, with nephrotic syndrome only occurring in a minority. A high percentage are hypertensive, and most also have hematuria and/or varying degrees of renal insufficiency.

The spectrum of glomerular manifestations in light and heavy chain deposition disease is broad. Light and heavy chain deposition disease can coexist, albeit uncommonly. (9-11,42,43) Our understanding of light chain deposition disease is far more advanced than that of heavy chain deposition disease.

Recently, an in vitro mesangial cell culture model of light-chain-associated disease reproduced in the laboratory the sequence of events that likely occur in vivo, and the sequential molecular mechanisms involved have been dissected. (44) This disease represents a good example of the important contribution of basic research to the understanding of pathologic morphologic expressions of this disorder. Such translational efforts have contributed much to our progress in conceptualizing mechanisms involved45 and understanding of the key events that take place and are amenable to therapeutic intervention.

Proper interpretation of some of the light morphologic expressions in these disorders and linking the pathology findings to an underlying plasma cell dyscrasia require a high degree of sophistication, use of ancillary diagnostic techniques, and careful integration of all of the data obtained from the renal biopsy.


Among the glomerular expressions, "minimal" change, mesangioproliferative (Figure 8), membranoproliferative, and crescentic patterns have been documented in light chain deposition disease. (5,10,12,46) These morphologic patterns are easily conceptualized as occurring as a result of the interplay of 2 growth factors: platelet-derived growth factor [beta], promoting mesangial cell proliferation, and transforming growth factor [beta], responsible for matrix deposition. 47 Because the morphologic expressions are by no means specific for these disorders, other generic descriptive diagnoses may be rendered, and the association with an underlying neoplastic plasmacytic process may not be recognized.

Nodular glomerulosclerosis represents the most recognized manifestation of light and heavy chain deposition disease (Figure 9, A). (5,9,48) The mesangial nodules may be focal and small in the early stages of nodular glomerulosclerosis (Figure 10). The glomerular morphology in light and heavy chain deposition disease is similar, although cellular proliferation is in our experience more pronounced in heavy chain deposition disease (Figure 11). Secretion and activation of transforming growth factor [beta] produced by mesangial cells, leading to an increase in mesangial matrix rich in tenascin, (49,50) is responsible for the mesangial nodularity.

When comparing the typical appearance of nodular glomerulosclerosis in light/heavy chain deposition disease with that of diabetic nephropathy, there are some differences that may be helpful in suggesting the correct diagnosis. The mesangial nodules in light and heavy chain deposition disease tend to be uniform in size in all glomeruli, whereas in diabetic nephropathy they are commonly irregular in distribution and vary in size and shape in a given glomerulus and among different glomeruli. A peculiar thickening of tubular basement membranes resulting from the deposition of monotypic light chains is also a very helpful finding in making a diagnosis of light or heavy chain deposition disease (Figure 9, B). Hyaline caps and capsular drops, as well as hyalinosis in afferent and efferent arterioles are features characteristic of diabetic nephropathy.

An important differential diagnosis is the recently recognized idiopathic nodular glomerulosclerosis, which is not associated with light or heavy chains or diabetes mellitus. (51) Morphologically, this condition is identical to light and heavy chain deposition disease but lacks immunofluorescence and ultrastructural features associated with these conditions. Its diagnosis is essentially made when diabetic nephropathy can be excluded based on clinical and laboratory data. The pathogenesis of the idiopathic type of nodular glomerulosclerosis remains speculative, but the combined effect of hypertension and smoking has been suggested to play an important role. (51)

Although the glomerular manifestations are similar to those seen in diabetic nephropathy by light microscopy, immunofluorescence and electron microscopy should provide crucial additional information that allows establishing an unequivocal diagnosis in the great majority of cases. Typically, linear staining for the involved light (usually [kappa]) or heavy (most commonly [gamma]) chain is present along peripheral capillary walls and in the mesangium in glomeruli and along tubular basement membranes (Figure 12). Staining can also be noted outlining Bowman capsule, in the interstitium proper, and in the vasculature. [kappa]4 is overrepresented in light chain deposition disease.

However, there are challenges in association with proper interpretation of the information that is obtained from these 2 diagnostic techniques. It is essential that pitfalls that may be encountered be understood fully. Because the abnormal light and heavy chains are often truncated in tissue deposits, the commercially available antibodies may not be able to pick them up. Therefore, no staining for [kappa] and [lambda] light chains in the renal biopsy does not rule out these conditions. Patients with combined diabetic nephropathy and light/heavy chain deposition disease are also difficult to diagnose. The glomerular milieu in these patients can be such that commercially available antibodies to light and heavy chains may not detect antigenic sites in the deposited abnormal light and heavy chains (unpublished data, March 2000). The use of antibodies to specific light-chain subtypes increases the ability to detect deposits of abnormal light chains in light and heavy chain deposition disease. Demonstration of absence of CH1 (most common), CH2, or hinge region epitopes of the heavy chain in the renal glomeruli using antibodies to these regions can further confirm a diagnosis of heavy chain deposition disease. (9,42,43)


The ultrastructural features associated with light- and heavy-chain deposits in the renal parenchyma are also variable. The electron density associated with the punctate, powdery light-, and heavy-chain deposits can vary (Figures 13 through 15). In some instances, the deposits are very subtle (Figure 13, A) and can be easily missed, and in others they are so massive that they may be confused with immune complexes. (52) Light-chain deposits are typically in subendothelial zones along peripheral capillary walls and/or in mesangial areas. However, the deposits may also be located on top of the lamina densa of the glomerular basement membranes, mimicking dense deposit disease (53) (Figures 16 and 17, A), or on the subepithelial side (Figure 17, B), albeit rarely. Deposits along Bowman capsule can also be found. The light-chain deposits are also typically seen along tubular basement membranes (Figure 18) and can be present in the interstitium proper and along vessel walls. In early cases of light chain deposition disease, when the glomeruli appear essentially unremarkable by light microscopy and fluorescence for monoclonal light chains is inconclusive, lightchain deposits may not be recognizable ultrastructurally.

Although in some of these cases, interstitial and vascular deposits may be noted and provide evidence to make or support a definitive diagnosis, in a significant number of cases they are absent in these locations. Ultrastructural immunogold labeling has been proven to be very valuable in confirming the diagnosis in a variety of settings (5,6,12) in a small number of selected cases (Figure 19). In contrast, in diabetic nephropathy the lamina densa of the glomerular basement membranes is classically thickened, and subepithelial lamellation is often also present. No deposits are present along the thickened glomerular basement membranes or in the expanded mesangial areas.

There are well-documented cases in which the immunofluorescence findings are diagnostic of light/heavy chain deposition disease and unequivocal deposits cannot be identified by electron microscopy. (9,10) A typical fluorescence pattern (ie, monoclonal linear staining along peripheral capillary walls and/or tubular basement membranes and/or mesangial areas for light or heavy chains) is sufficient to make a definitive diagnosis.

Light and heavy chain deposition disease may be associated with generalized systemic manifestations and deposition of monoclonal light chains in many organs, not only kidneys. (12,54,55)


Patients with amyloidosis usually present with proteinuria with or without associated nephrotic syndrome. It is typically a disease of individuals in the age range of 50 to 70 years. More than 20 precursor proteins have been found to be amyloidogenic. The clinical setting is variable, depending on the type of amyloidosis. (8-11) Regardless of the precursor protein, all types of amyloid appear the same by light and electron microscopies and exhibit identical tinctorial characteristics: Congo red and thioflavins T and S positivity. Amyloidosis, regardless of type, is associated with the amyloid-P component, which can be detected using immunohistochemistry.

The diagnosis of amyloidosis may be difficult in early stages of the disease process because the amount of amyloid deposition may be small and very focal and segmental (Figure 20, A and B). Mesangium and blood vessel walls are the most common early sites of amyloid deposition in the renal parenchyma. Nodular mesangial expansion is seen commonly (Figure 20, C). The normal mesangial matrix is destroyed by activated metalloproteinases, (56) and amyloid fibrils replace it (Figure 20, D). Silver stain is helpful in showing expanded mesangial areas that have lost their silver positivity as a result of the replaced mesangial matrix (Figure 20, B). Amyloid is usually weakly periodic acid-Schiff positive (Figure 20, A) and stains blue with the trichrome stain. Congo red stain may not show positivity, or the expected apple green birefringence may be very difficult to demonstrate due to the small quantities of amyloid present. Sections should be cut at 9 [micro]m to maximize the ability to detect small amounts. Thioflavin T or S stains are more sensitive if small amounts of amyloid are to be detected (Figure 21). A helpful "trick" to identify small amounts of amyloid is to place the Congo red stain under fluorescence light, which will make amyloid deposits (even very small ones) appear bright red (Figure 22). In addition, there may be significant challenges, even after a definitive diagnosis of amyloidosis is made, to establish a connection with monoclonal light- or heavy-chain immunoglobulin components, as discussed later in this article.

Although in most cases light microscopy allows one to make a definitive diagnosis of amyloidosis, detection of early amyloidosis requires a high level of suspicion and a very careful evaluation of the material. (5) These cases may be confused with minimal-change glomerulopathy, be cause the amyloid deposition can be very focal and segmental, and therefore easy to miss (Figures 20, A, and 21, A). Correlation of light, immunofluorescence, and ultrastructural findings is extremely important to make the correct diagnosis. Because amyloid deposits can be small and subtle and confused with hyalinosis in glomerular and vascular locations, detecting light- or heavy-chain monoclonality by immunofluorescence can be very informative and the triggering event to lead to a more careful evaluation of the light microscopic findings. Ultrastructural confirmation is crucial in some of these cases. Ultrastructural immunoelectron microscopy can label the precursor protein associated with the amyloid fibrils (Figure 20, D).



Amyloid deposition can occur in any of the renal compartments (Figure 23). Although in most cases amyloid deposits are present in all compartments, albeit with variation in quantity, there are a minority of cases in which only one compartment is affected. Tubular casts with material exhibiting fibrillary material with staining and ultrastructural characteristics diagnostic of amyloid can be seen in cases with widespread renal amyloidosis. However, tubular casts containing amyloid as the only manifestation of renal amyloidosis have been reported in rare cases (Figure 24). (31)

Amyloid is composed of randomly arranged, nonbranching fibrils that measure 8 to 12 nm. Amyloid is first detected in mesangial areas in the glomeruli, and with time the fibrils are also noted to extend into peripheral capillary walls. They can be seen in subepithelial and subendothelial areas (Figure 25, A and B), and in some cases the fibrils replace the glomerular basement membranes and are seen extending transmurally (Figure 25, C).

Once a diagnosis of amyloidosis is established, staining for the precursor protein allows proper identification of the type. (57) Although AL amyloidosis is the most common form of amyloidosis in the United States and the Western hemisphere, fewer than 10 cases of AH amyloidosis have been reported in the literature (Figures 26 and 27). (58) Immunofluorescence and histochemical stains are routinely used to evaluate amyloid protein precursors. The routine panel of immunofluorescence stains used in renal biopsies includes antibodies to detect light/heavy chains (Figures 26, B, and 27, B) and fibrinogen, all of which can be amyloid precursor proteins. Commercially available antibodies to light and heavy chains of immunoglobulins may not always detect light and heavy chains because truncated forms of these are generally present in the amyloid tissue deposits. Negative stains for these do not rule out AL or AH amyloidosis. (59) Ultrastructural labeling techniques can result in characterizing a subset of amyloid cases that cannot be typed at the light microscopic level with certainty (Figure 28).

The use of antibodies to different subgroups of light chains may help in detecting amyloid deposits, because their chance of detecting abnormal subtype-specific antigenic epitopes is significantly increased. Identifying the wrong type of amyloid may have significant adverse consequences.

Rare cases have been reported where more than one type of amyloid is present. The use of tandem mass spectroscopy on formalin-fixed, paraffin-embedded specimens is superior to other techniques and provides precise characterization by identifying specific protein sequences of the various types of amyloid. Unfortunately, this methodology is still in the hands of research laboratories and not widely available for general diagnostic use. (57)


At the ultrastructural level, amyloidosis must be distinguished from fibrillary glomerulopathy. (59-63) In both conditions, the fibrils are nonbranching and randomly arranged, but in amyloidosis they measure 8 to 12 nm in diameter, whereas in fibrillary glomerulopathy they range from 12 to 25 nm (Figures 28, B, and 29). (60-63) Congo red and thioflavins T and S stain amyloid but not the fibrils in fibrillary glomerulopathy.

A possible source of confusion is that the amyloid-P component is also present in fibrillary glomerulopathy. Other important distinctions are between amyloid fibrils and mesangial matrix accentuation resulting in a fibrillary appearance (Figure 30), and precollagen and collagen fibers. Matrix fibrils are approximately 6 nm in diameter. Collagen fibers exhibit their classical periodicity at 65 nm, and precollagen fibers are usually present in a background where typical collagen is also noted. (60) Immunotactoid glomerulopathy can also be in the differential diagnosis. In this condition, there is deposition of microtubular structures which measure 20 to 60 nm in diameter, predominantly in the mesangium. Congo red, thioflavins T and S, and stain for amyloid-P component are all negative in this condition. Interestingly, immunotactoid glomerulopathy is often associated with an underlying plasma cell dyscrasia. (62)

An unusual morphologic manifestation of amyloidosis that mimics an immune complex-mediated process has been documented in the literature. (64) The glomerular capillary walls exhibit a bright red appearance on the hematoxylineosin stain (Figure 31). The unusual deposits in this type of glomerular amylodosis are Congo red positive with apple green birefringence and thioflavin T positive, and are composed of randomly disposed, nonbranching 8- to 12-nm fibrils, fulfilling all criteria for the diagnosis.

Amyloidosis is a systemic disorder, and amyloid deposits may be found in any organ. Pathologists are sometimes asked to examine fat pad aspirates or gingival or rectal biopsies to diagnose systemic amyloidosis. The same criteria for amyloid diagnosis as in the kidney apply in other sites.

Glomerular amyloid formation generally occurs first in the mesangium. Mechanisms involved have been eluci dated, and key steps in the process have been delineated. (65) One of the most fundamental steps involves the transformation of mesangial cells from their normal smooth muscle into a macrophage phenotype and engaging in avid endocytosis and delivery of the amyloidogenic light chains to the lysosomal compartment, where fibril formation occurs. (65)



Renal lesions associated with plasma cell dyscrasias are quite varied in terms of morphologic manifestations. They can occur in any of the renal compartments, and in some cases in more than one compartment. A significant factor associated with the variability of morphologic manifestations arising from the damage to various portions of the nephron emanates from the physicochemical characteristics of the abnormal light and heavy chains present in these patients. (37-39,66-70) Glycosylated light chains are 4 times more amyloidogenic than nonglycosylated ones. (67) [lambda]6 Light chains display a unique tropism for renal amylodosis. (70) Host factors, (71) including genetic polymorphism, may also be important in explaining the propensity of a particular type of pathologic lesion to develop in a given patient and the diversity of renal lesions that may be encountered in different patients. Other factors may also predispose a given patient to develop a particular type of pathology. For example, patients with plasma cell dyscrasias with certain types (not any kind) of circulating light chains who become dehydrated, hypercalcemic, or are given loop diuretics, such as furosemide, increase significantly their risk of developing light chain cast nephropathy. (72) Although our understanding of the pathogenesis and ability to make diagnoses of light-chain- or heavy-chain-associated renal diseases has increased substantially in the last 2 decades, there are still significant challenges that remain. It should also be noted that not all conditions associated with plasma cell dyscrasias have been covered in this review.


This article has focused on highlighting advances that have taken place in the field and pointing out areas where additional work is needed. Now that a molecular understanding of the various morphologic manifestations of these diseases has been achieved by effective translational efforts that have taken data from the research laboratory into the clinical arena, new therapeutic modalities aimed at stopping, ameliorating, or preventing renal damage in these conditions can be developed. The understanding of the pathogenesis of these disorders that has been acquired will lead to the development of sound and rational therapeutic strategies, primarily aimed at molecular targets.

Accepted for publication June 11, 2008.


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Guillermo A. Herrera, MD

From the Pathology Department, Nephrocor Laboratory, Tempe, Ariz.

The author has no relevant financial interest in the products or companies described in this article.

Reprints: Guillermo A. Herrera, MD, Pathology Department, Nephrocor Laboratory, 1700 N Desert Dr, Tempe, AZ 85281 (e-mail:
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Author:Herrera, Guillermo A.
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Date:Feb 1, 2009
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