IgG subclasses of antibodies to the collagen-like region of C1q in patients with systemic lupus erythematosus

Role of anti-C1q autoantibodies in the pathogenesis of lupus nephritis

Anti-C1q autoantibodies can be found in the sera of patients with several autoimmune diseases, but also in healthy individuals. Although these anti-C1q autoantibodies were already identified several decades ago, they still puzzle both immunologists and nephrologists. The main reason for this puzzling effect are observations that seemed to indicate quite clearly that anti-C1q should be pathogenic to the kidney and the observation on the other hand that anti-C1q autoantibodies can be found in several disease conditions, as well as in healthy individuals, and are then unrelated to overt renal inflammation. This puzzle is the focus of the current review, which will provide an overview of the historical data, define the clinical interests and, importantly, will try to put several aspects in perspective based on recent observations in patients and in murine models. In addition, the paper will discuss therapeutic intervention possibilities regarding anti-C1q-mediated damage in systemic lupus erythematosus, as well as the therapeutic potential of anti-C1q antibodies in other conditions.

Diagnostic and prognostic significance of anti-C1q antibodies in systemic lupus erythematosus

PURPOSE OF REVIEW

The presence of a wide variety of autoantibodies is a characteristic finding in systemic lupus erythematosus. Autoantibodies against nuclear proteins, such as anti-nuclear and anti-double-stranded DNA antibodies, are used as diagnostic markers in systemic lupus erythematosus. Renal involvement is frequently found in systemic lupus erythematosus and is an important risk factor for death. Therefore, markers for the diagnosis and follow-up of nephritis are very important. Anti-C1q autoantibodies are strongly associated with renal involvement in systemic lupus erythematosus. This study will review recent findings on the pathogenic role and clinical importance of anti-C1q antibodies in lupus nephritis.

RECENT FINDINGS

Recent clinical studies have clearly emphasized the diagnostic relevance of anti-C1q autoantibody levels in patients with lupus nephritis. With a possible negative predictive value of 100%, anti-C1q autoantibodies are the only exclusive antibodies associated with the involvement of a single organ in systemic lupus erythematosus. Next to the clinical findings, the pathogenic significance of anti-C1q antibodies has been shown in an animal model. The deposition of autologous C1q in healthy glomeruli of mice after the infusion of anti-C1q antibodies induces moderate tissue damage.

SUMMARY

The latest insight into the pathogenesis of anti-C1q autoantibodies in the development of lupus nephritis and the recently demonstrated clinical importance of anti-C1q autoantibodies for the diagnosis of lupus nephritis support the value of further investigations. New diagnostic methods for the detection of anti-C1q and an accurate follow-up of antibody levels might be of use in clinical practice.

C1q as antigen in humoral autoimmune responses

The observation of anti-C1q antibodies (C1qAb) in patients with various autoimmune diseases has led to the establishment of a strong correlation between these antibodies and renal involvement in patients with systemic lupus erythematosus (SLE). The measurement of anti C1q antibodies requires detailed insight in the reactivity of C1q with immune complexes and the methods to detect C1q as an antigen by solid phase assays. In this overview we describe the pitfulls of the anti C1qAb assay and its use in the measurement of C1qAb. Further we discuss the relevance of C1qAb in the pathogenesis of SLE and especially in relation to lupus nephritis.

Antibodies to C1q in systemic lupus erythematosus: characteristics and relation to Fc gamma RIIA alleles

Autoantibodies to the collagen-like region of the first complement component (C1qAB) are found in patients with systemic lupus erythematosus (SLE), particularly those with renal disease. In a cohort of 46 SLE patients with diffuse proliferative glomerulonephritis, we found declining C1qAB titers in 77% of treatment responders and in only 38% of treatment non-responders (P < 0.03). To further characterize this autoantibody, we tested 240 SLE patients for the presence of C1qAB. Positive titers were found in 44% of patients with renal disease and 18% of patients without renal disease (chi2 P < 0.0003). Analysis of IgG subclass revealed IgG2 C1qAB alone in 34%, IgG1 C1qAB alone in 20%, and both IgG1 and IgG2 in 46% of patients. Fewer than 10% of patients had measurable titers of IgG3 or IgG4 C1qAB. The pathogenic role of these IgG2-skewed C1qAB may relate to impaired immune complex clearance by the mononuclear phagocyte system: IgG2 antibodies are efficiently recognized by only one IgG receptor, the H131 allele of Fc gamma RIIa (Fc gamma RIIa-H131). In contrast, Fc gamma RIIa-R131, which is characterized by minimal IgG2 binding, has recently been associated with lupus nephritis. In our C1qAB positive patients, the presence of Fc gamma RIIA-R131 was associated with an increased risk for renal disease. Autoantibodies to C1q may have pathogenic significance in SLE patients with genetic defects in the ability to clear IgG2 containing immune complexes.

IgG subclass restriction of autoantibody to solid-phase C1q in membranoproliferative and lupus glomerulonephritis

The IgG subclass distribution for autoantibodies to solid-phase C1q (anti-spC1q) in sera from 14 patients with membranoproliferative glomerulonephritis (MPGN) and 10 patients with systemic lupus erythematosus (SLE) nephritis was determined by an enzyme-linked immunosorbant assay employing C1q as the immunosorbant in the presence of 2 M NaCl to prevent Fc binding and monoclonal anti-human IgG subclass reagents. The autoantibody to spC1q in MPGN, especially in types I (7 patients) and II (3 patients), was almost entirely restricted to IgG3. In contrast, in SLE anti-spC1q was completely restricted to IgG2 in 3 patients while predominantly IgG2 in the other 7 patients. The different subclass restriction of anti-spC1q in these two disorders suggests that antibody formation is either in response to different epitopes on the collagen-like region of C1q or that patients with SLE and MPGN mount different immunologic responses to the same antigenic stimulus.

Antibody formation against human collagen and C1q in response to a bovine collagen implant

The humoral immune response to commercially available bovine collagen implants (Zyderm, Zyplast) is characterized in a 45-year-old female patient. Circulating anti-collagen antibodies were detected after eight injections of Zyderm and after two injections of Zyplast given during a period of 3 years. The specificity of these antibodies for bovine and human collagens as well as for the collagen-like region of C1q (a subcomponent of the first component of complement), was investigated by affinity chromatography. Serum levels of anti-collagen and anti-C1q antibodies were measured using ELISA. High levels of antibodies to bovine collagens, showing a strong cross-reactivity with human collagen type III were detected in the patient's serum. Only weak cross-reactivity with human collagen type I and IV and no reactivity with type II were observed. In addition, these antibodies specifically cross-reacted with the collagen-like region of C1q. The antibody levels decreased continuously and disappeared 1 year after cessation of treatment. These results demonstrate for the first time the formation of autoantibodies upon treatment with a bovine collagen implant. Although antibodies to collagens and C1q have been found in various autoimmune diseases, neither adverse reactions to the bovine collagen implant nor any other clinical symptoms were observed in association with the described antibody response.

Heavy and light chain utilization in autoantibodies of elderly patients with systemic lupus erythematosus

To determine whether age-related changes in immune function affect patterns of autoantibody production, we have examined the isotype and light chain utilization in autoantibodies of elderly patients with systemic lupus erythematosus (SLE). Enzyme-linked immunosorbent assays (ELISA) were used to determine the frequencies of IgG and IgM antibodies to single-stranded DNA (ssDNA), Sm, and the 70K protein component of RNP in the sera of 53 patients with SLE older than age 60. The IgG subclass distributions and kappa/lambda ratios for each of these autoantibodies were also determined and compared to measurements performed on the sera of 53 young adult patients with SLE. The frequencies of autoantibodies of each specificity, except IgM anti-ss DNA antibodies, were higher among the young adult patients, although the magnitudes of the responses were similar in both age groups. IgG anti-Sm antibodies were composed of both IgG1 and IgG2 subclasses, while IgG anti-70K RNP and IgG anti-ssDNA were predominantly of the IgG1 subclass. There were no differences in the IgG subclass distributions of any of the three autoantibodies between the elderly and young adult patient sera. The kappa/lambda ratios for each of the three autoantibodies were similar to that present in total serum immunoglobulins, and kappa/lambda ratios of autoantibodies, standardized to the kappa/lambda ratios of serum, were not different between elderly and young adult groups. Few patient sera of either age group (9 elderly, 7 young adult) demonstrated even midly skewed light chain ratios in their autoantibody responses. Thus, despite developing in an immunological environment that may have altered the clonality and isotype distribution of their responses, the autoantibodies produced by elderly patients with SLE were qualitatively similar to autoantibodies of younger patients.