Immunoglobulin-Antiimmunoglobulin Interactions and Immune Complexes in IgA Nephropathy

Immunoglobulin-Antiimmunoglobulin Interactions and Immune Complexes in IgA Nephropathy Susan Jackson, PhD • IgA nephropathy (lgAN) is characterized by mesangial co-deposition of IgA and C3. Elevated levels of circulating immune complexes containing these components in significant numbers of patients have been found in several studies; IgAN is therefore assumed by many investigators to be an immune complex-mediated disease. Our studies have shown that IgG is often co-complexed with IgA within circulating immune complexes, and we have begun to examine the potential mechanisms for these observations. In this regard, elevated levels of IgA rheumatoid factor and of IgG anti-lgA antibodies were found in some patients. Nevertheless, we were unable to correlate levels of circulating immune complexes with any clinical index of disease. Furthermore, many individuals with the acquired immune deficiency syndrome (AIDS) also have elevated levels of circulating immune complexes containing IgG and IgA, IgA rheumatoid factor, and IgG anti-lgA antibodies, although these patients apparently do not have mesangial IgA deposits. Therefore, the role of circulating IgA-containing immune complexes in the pathogenesis of IgAN requires further evaluation. © 1988 by the National Kidney Foundation, Inc. INDEX WORDS: IgA nephropathy; acquired immune deficiency syndrome; circulating immune complexes; antiimmunoglobulin.

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HE DIAGNOSTIC hallmark of IgA nephropathy (lgAN) is the mesangial deposition of IgA, which is usually accompanied by C3. Other immunoglobulin isotypes are often found in the glomeruli. Because of the frequent co-deposition of complement with immunoglobulin, IgAN is considered to be an immune complex-mediated disease.! In this regard, several studies have shown increased levels of circulating immune complexes containing IgA in patients with IgAN.!-4 Some reports additionally indicated that levels of circulating immune complexes containing IgG and/or IgM were also increased,2-5 but usually in fewer patients than had IgA-containing circulating immune complexes.2.4.5 Our results agreed with this pattern; we found elevated levels of IgAcontaining immune complexes in 48 % of patients with IgAN. At the same time, 24% and 9% of the patients had increased levels ofIgG- and IgM-containing immune complexes, respectively.6 Using subclass-specific monoclonal antibodies, we showed that all of the IgA within these complexes was of the IgAI subclass. These data are summarized in Fig 1. The next question addressed was whether patients with elevated levels of circulating immune complexes containing multiple immunoglobulin isotypes had complexes with different antibody classes within the same complex matrix. To answer this question, we designed a solid-phase assay to detect immune complexes within polyeth-

ylene glycol-precipitable material (which is enriched for circulating immune complexes) that contain two immunoglobulin isotypes. Our findings indicated that, indeed, a significant proportion of patients with IgAN had complexes that contained mixtures of IgA 1 and IgG. 6 We found no evidence that IgA2 or IgM could be identified within the "mixed" complexes. IMMUNOGLOBULIN-ANTI IMMUNOGLOBULIN INTERACTIONS AS THE BASIS FOR MIXED CIRCULATING IMMUNE COMPLEXES IN IGAN

Several possible mechanisms might be envisioned as the basis for mixed immune complexes. One could be an antigen "bridge" between IgAI and IgG molecules of the same specificity. Alternatively, mixed complexes with different immunoglobulin isotypes might arise from a specific immunoglobulin-antiimmunoglobulin interaction. Such an interaction could be the result of an idiotype-antiidiotype recognition, the presence of IgA rheumatoid factor (an IgA autoantibody specific for the Fc region of IgG), or IgG anti-IgA From the Department of Microbiology, University of Alabama at Birmingham. Supported in part by Grants AI 23952 and AI 18745 from the National Institutes of Health. Address reprint requests to Susan Jackson, PhD, Department of Microbiology, UAB, Birmingham, AL 35294. © 1988 by the National Kidney Foundation, Inc. 0272-6386/88/1205-0018$3.00/0

American Journal of Kidney Diseases, Vol XII, No 5 (November), 1988: pp 425-429

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antibodies. All of these possible immunoglobulinanti immunoglobulin interactions have been investigated in IgAN.

with IgAN, as defined by the presence or absence of hematuria. 8

Idiotypes and Antiidiotypes in Circulating Immune Complexes of Patients With IgAN

Czerkinsky et al first described IgA rheumatoid factor in IgAN; about one fourth of the patients assayed had elevated levels of this autoantibody. 9 The same investigators subsequently showed that the IgA rheumatoid factor was restricted to the IgAl subclass and was both polymeric and monomeric in configuration. 6 Subsequently, Sinico et apo found IgA rheumatoid factor in the sera of 41 % of their patients. This IgA was mostly polymeric; the subclass of the IgA was not examined. Czerkinsky et al 6 found a marginal correlation between the presence of IgA rheumatoid factor and the levels of IgA-containing circulating immune complexes. They asked the next logical question: can these IgA/IgG mixed complexes be shown to contain IgA rheumatoid factor? This is indeed the case. Sucrose gradient analyses of IgAI IgG complexes isolated from polyethylene glycolprecipitable material contained active IgA rheumatoid factor (Fig 2).

The presence of spontaneously appearing (ie, not artificially induced) autoantiidiotypic antibodies has been well-documented in several systems, including human disease models. 7 The potential regulatory role of such autoantibodies has been the subject of intense speculation and investigation for several years. One study has shown that patients with IgAN often have serum or circulating immune complex-bound antibodies against a common food antigen, bovine serum albumin (BSA), and that these antibodies have shared idiotypes. 8 The levels of idiotype-positive anti-BSA antibodies correlated with the presence of IgA-containing circulating immune complexes, suggesting the possibility of the contribution of idiotype-antiidiotype admixtures in their formation. 8 Of special interest in this study was the apparent autoantiidiotypic activity in the sera containing anti-BSA (idiotypic) antibodies. It should be noted that the antiidiotype reagents used to detect shared idiotypes in these experiments were raised against affinity-purified IgG anti-BSA antibodies; it is unclear whether IgA antibodies of the same specificity would also bear shared idiotypic determinants. Nevertheless, the levels of idiotype-positive antibodies correlated with clinical status of patients

IgA Rheumatoid Factor in IgAN

Auto-Anti-IgA Antibodies in IgAN

In the study of Czerkinsky et al 6 it was shown that IgA/IgG mixed circulating immune complexes usually associated with detectable levels of serum IgA rheumatoid factor. This raised the question of whether other IgG-IgA autorecognition mechanisms could be implicated in IgAN. In other

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ANTIIMMUNOGLOBULINS AND IMMUNE COMPLEXES Fig 2. Sedimentation profile of IgA rheumatoid factor in circulating immune complexes from two patients with IgAN. Complexes containing IgA and IgG were isolated by affinity chromatography on a column of anti-lgG coupled to Sepharose 48. After acid elution and sucrose gradient ultracentrifugation at acid pH, fractions containing polymeric IgA (plgA) or monomeric IgA (mlgA) were neutralized and assayed for IgA rheumatoid factor activity. Details of all methods are in reference 6.

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words, was it possible that a mechanism reverse to that of IgA rheumatoid factor, eg, IgG autoantiIgA antibodies, might be responsible for the formation of the IgA/IgG mixed immune complexes? We had previously demonstrated the ubiquitous presence of IgG and IgM antibodies directed at IgA, specifically at the Fab portion of the IgA molecule, in widely varying amounts. II We considered the possibility of altered regulation of antiIgA antibody production in IgAN, whereby excessive levels of IgG anti-IgA antibodies might contribute to the formation of mixed immune complexes. The results of these experiments showed highly significant differences in the levels of both IgG and IgM anti-Fab a-antibodies in patients with IgAN as compared with normal controls l2 (Fig 3) IgG antibody levels were elevated in the patients with IgAN at the same time that IgM antibody levels were decreased. There was no significant correlation between levels of IgG anti-Fab a antibodies and levels of IgAlIIgG mixed circulating immune complexes. Similarly, there was no correlation of antibody levels with any clinical index of disease, other than that IgM antibody levels were negatively correlated with serum creatinine concentration. 12 IMMUNOGLOBULIN-ANTI IMMUNOGLOBULIN INTERACTIONS AND IGA-CONTAINING CIRCULATING IMMUNE COMPLEXES IN THE ACQUIRED IMMUNE DEFICIENCY SYNDROME

The acquired immune deficiency syndrome (AIDS) is characterized by a panoply of abnormal clinical findings, including notably elevated levels of serum IgA and circulating immune com-

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plexes. 13 These observations are reminiscent of IgAN. We have recently studied the circulating immune complexes of patients with AIDS with respect to immunoglobulin isotypes and have also investigated the potential role of IgA rheumatoid factor and anti-IgA antibodies in the formation of these complexes in this disease. Our initial findings were strikingly similar to those we had obtained in patients with IgAN: 57 % of the patients with AIDS had elevated levels of IgA-containing circulating immune complexes. Comparable to patients with IgAN, the IgA in the complexes was exclusively IgAI, and the complexes usually also contained IgG. 14 Additional experiments also provided results analogous to those obtained from patients with IgAN. When sera from patients with AIDS were assayed for IgA rheumatoid factor, 26% of the patients had elevated levels of this autoantiglobulin (compared with about one third of patients with IgAN6). The rheumatoid factor was restricted to the IgAI subclass,15 as we had shown in IgAN.6 Furthermore, we found IgA rheumatoid factor activity within immune complex-enriched polyethylene glycol precipitates. 15 Finally, the sera of the patients with AIDS appeared to have remarkably elevated levels oflgG antibodies directed at autologous IgA (Fig 4). Therefore, there were notable similarities in AIDS and IgAN with respect to alterations in the IgA system leading to the formation of circulating immune complexes containing IgA I or mixtures of IgA I and IgG. However, the striking difference in the two disorders was the apparent nephritogenicity of these complexes in IgAN as compared with AIDS. This difference

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ing. 6 However, it is clear that several "autoimmune" mechanisms of an immunoglobulin-antiimmunoglobulin nature are at work in IgAN. These include idiotype-antiidiotype interactions as shown by Gonzalez-Cabrero et alB in their BSA-anti-BSA system. Our studies 6,9 and those of Sinico et al lO have demonstrated the presence ofIgA rheumatoid factor in subgroups of patients with IgAN. The experiments of Czerkinsky et al indicated that this IgA rheumatoid factor correlated with the incidence of IgA/IgG mixed immune complexes, but was not an "all or none" observation. 6 Finally, some patients with IgAN have elevated levels of IgG antibodies directed against the Fab region of IgA, and at the same time a significant number of them have decreased levels of IgM antibodies of the same specificity. 12 The meaning of the latter observations is unclear, since no statistical correlations with clinical findings were found. Clearly, further studies are necessary to define the roles of

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leads to the conclusion that high levels of IgA-containing circulating immune complexes are insufficient to explain the pathogenesis of the mesangial deposition leading to renal damage in IgAN. In summary, although some investigators have observed a correlation between immune complex activity and clinical manifestations of disease in IgAN,5 not all studies have corroborated this find-

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ANTIlMMUNOGLOBULINS AND IMMUNE COMPLEXES

429

these various autoantiimmunoglobulins in IgAN, especially because similar abnormalities have been demonstrated in patients with AIDS, in whom these antibodies apparently do not lead to mesangial deposition of IgA.

8. Gonzalez-Cabero J, Egido J , Sancho J: Presence of shared idiotypes in serum and immune complexes in patients with IgA nephropathy. Clin Exp Immunol 68:694-702, 1987 9. Czerkinsky C, Crago S, Koopman W, et al: The occurrence of circulating IgA-IgG immune complexes in IgA associated r enal diseases, in Revillard J-p, Voisin C, Wierzbicki N (eds): Mucosal Immunity, IgA and Polymorphonuclear Neutrophils, Local Immunity : International S ymposium Series, Suresnes, France, Fondation Franco-Allemande , 1985, p 176 10. Sinico R , Fornasieri A, Oreni N, et al : Polymeric IgA rheumatoid factor in idiopathic IgA mesangial nephropathy (Be rger 's Disease). J Immunol 137:536-541, 1986 11. Jackson S, Montgomery R, Mestecky J, et al: Normal human sera contain antibodies directed at Fab of IgA. J Immunol 138:2244-2248, 1987 12 . Jackson S, Montgomery R, Julian B, et al: Aberrant synthesis of antibodies directed at the Fab fragment of IgA in patients with IgA nephropathies. Clin Immunol Immunopathol 45 :208-213 , 1987 13 . Fauci AS, Macher AM, Longo DL, et al : Acquired immunodeficiency syndrome: Epidemiologic, clinical , immunologic, and therapeutic considerations. Ann Intern Med 100:92106, 1984 14. Jackson S, Dawson LM , Kotler DP: IgAI is the major immunoglobulin component of immune complexes in the acquired immune deficiency syndrome. J Clin Immunol 8:64-68, 1988 15. Jackson S, Tarkowski A, Collins JE , et al: Occurrence of polymeric IgAI rheumatoid factor in the acquired immune deficiency syndrome. J Clin Immunol 1988 (in press)

REFERENCES I. Emancipator S, Gallo G, Lamm M: IgA nephropathy: Perspectives on pathogenesis and classification. Clin Nephrol 24: 161-179 , 1985 2. Woodroffe A, Gormly A, McKenzie P, et al : Immunologic studies in IgA nephropathy. Kidney Int 18:366-374, 1980 3. Stachura I, Singh G, Whiteside T:Immune abnormalities in IgA nephropathy (Berger's Disease). Clin Immunol Immunopathol 20:373-388, 1981 4. Hall R, Stachura I, Cason J , e t al : IgA-containing circulating immune complexes in patients with IgA nephropathy. Am J Med 74:56-63 , 1983 5. Coppo R, Basolo B, Martina G, et al : Circulating immune complexes containing IgA , IgG and IgM in patients with primary IgA nephropathy and with Henoch-Schoenlein nephritis. Correlation with clinical and histologic signs of activity. Clin Nephrol 18:230-239, 1982 6 . Czerkinsky C , Koopman W, Jackson S, et al : Circulating immune complexes and immunoglobulin A rheumatoid factor in patients with mesangial immunoglobulin A nephropathies. J Clin Invest 77:131-138, 1986 7 . Male D: Idiotypes and autoimmunity. Clin Exp Immunol 65 :1-9, 1986