C1q-bearing immune complexes detected by a monoclonal antibody to human C1q in rheumatoid arthritis sera and synovial fluids.

Using a monoclonal antibody directed against the C-chain of human C1q, we detected C1q-bearing immune complexes (IC) in sera and synovial fluids of rheumatoid arthritis (RA) patients. In a sandwich-ELISA, C1q-bearing IC were captured by the solid-phase monoclonal antibody and then detected with peroxidase-labeled F(ab')2-antibodies to either human IgG or IgM. The results of this assay were compared to an ELISA-modification of the C1q-solid-phase binding assay (C1q-SPBA). C1q-bearing IC were detected in 81.1% of RA-sera and the 65.2% of RA-synovial fluids. IgG as well as IgM was present in 72.6% of the sera and 70% of the synovial fluids which were positive in both assays. Most RA sera that were only positive for C1q-bearing IC, contained IgG alone (81.5%). The corresponding synovial fluids showed IgG alone (53%) or both IgG and IgM (41.1%). IgM alone (25%) could be detected in sera, e.g. in juvenile forms of RA. The levels of IC were higher in synovial fluid than in paired serum. In comparison to normal human serum (NHS) and patients with osteoarthritis, complement activity (CH50 titers) and C1q-values in patients with RA were frequently elevated. Since the formation of C1q-bearing IC is an indicator for the classical complement pathway activation, an assay with monoclonal anti-C1q antibody may be a useful tool in the diagnosis of rheumatoid diseases.

[Autoantibodies against the complement component C1q in systemic lupus erythematosus]. / Autoantikörper gegen die Komplementkomponente C1q beim Systemischen Lupus Erythematodes.

Autoantibodies against C1q, a subcomponent of the first complement component C1, could be detected in 49.4% of sera from patients with systemic lupus erythematosus (SLE). They are directed against the collagen-like portion of the C1q molecule and recognize only bound, but not fluid-phase C1q. The appearance of these autoantibodies in the course of SLE correlates with the detection of IgG in the C1q-Solid-Phase-Binding-assay, with high titres of dsDNA-antibodies and with depressed total complement activity (CH50) and C1q-values. Our investigations show that autoantibodies against the collagen-like portion of bound C1q but not immune complexes are the main constituent of C1q-binding IgG in SLE.

Evidence for the presence of autoantibodies to the collagen-like portion of C1q in systemic lupus erythematosus.

We investigated the connection between the C1q solid-phase binding assay (C1q SPBA) and double-stranded DNA antibodies, and analyzed the immune complex material in systemic lupus erythematosus (SLE) sera. Comparison with a new monoclonal assay for C1q-bearing immune complexes (the 242G3 assay) revealed that the immune complexes in SLE bind specifically to solid-phase C1q, and not to fluid-phase C1q. The C1q solid-phase binding activity sedimented as 7S IgG, was insensitive to DNase treatment, and could be selectively absorbed by C1q-coupled beads and by bovine serum albumin-anti-bovine serum albumin C1q beads, but not by DNA. Thus, antibodies to double-stranded DNA do not interfere in the C1q SPBA. Isolated IgG from SLE serum precipitated the collagen-like portions, and not the globular, Fc-recognizing portions, of C1q. F(ab')2 fragments of IgG from SLE patient serum were able to bind C1q. These data show that in SLE sera, especially in those with low levels of CH50 and C1q, autoantibodies that react with the collagen-like part of C1q are detectable. Since in the C1q SPBA, the C1q molecule is randomly fixed to the solid phase, we can detect not only immune complexes, but also antibodies that react with the collagen part of C1q; this may explain the high percentage of positive results for SLE sera in the C1q SPBA, in contrast to results of other immune complex assays.

Detection of C1q-bearing immune complexes by a monoclonal anti-C1q ELISA system.

A monoclonal antibody directed against the collagenous portion of human C1q was used to detect C1q-bearing immune complexes in patients with rheumatic disorders. Sera of patients with rheumatoid arthritis, systemic lupus erythematosus (SLE), osteoarthritis, as well as normal human sera (NHS) used as controls were tested in an ELISA system. C1q-bearing immune complexes were bound to a solid-phase monoclonal anti-C1q antibody, and detected with F(ab')2 antibodies to human IgG. Heat-aggregated human IgG was adjusted to the same concentration as the WHO standard for immune complexes and used for the standard curve in NHS. The mean value in NHS was 19.5 micrograms/ml equivalents of aggregated IgG. Using 2 SD over the mean as the upper limit for normal values, samples greater than 43 micrograms/ml were considered positive. Patients with osteoarthritis were negative; high levels of C1q-bearing immune complexes were detected in patients with rheumatoid arthritis (up to 800 micrograms/ml equivalents of aggregated IgG). With our assay C1q-bearing immune complexes were detected with high frequency (81%) in the sera of patients with rheumatoid arthritis, while a C1q solid-phase binding assay (C1q SPBA) revealed positive results only in 67% of rheumatoid arthritis sera. Compared to NHS, CH50 titers and C1q values of sera from patients with rheumatoid arthritis were frequently high. In contrast, the sera of SLE patients with low CH50 titers and low C1q levels had IgG immune complexes which could be detected only in the C1q-SPBA. C1q-bearing immune complexes were not detectable in the sera of patients with SLE. Since C1q triggers activation of the classical C pathway, this assay with monoclonal anti-C1q antibody appears to be useful for detecting immune complexes in rheumatoid arthritis patients with normal or elevated CH50 and C1q values, especially in the early stage of the disease.

Enzyme-linked immunosorbent assay for C1q in human serum by use of monoclonal antibodies.

A sandwich ELISA system has been developed for the detection of C1q in human serum. It is specific, uses monoclonal antibodies, is sensitive into the nanogram range and is rapidly performed. Therefore, it may be a helpful tool for clinical routine diagnosis, e.g., detecting abnormal C1q levels in patients with rheumatic disorders. Various combinations of poly- and monoclonal antibodies were tested in a sandwich assay. One of these combinations, in particular, resulted in a highly reproducible standard curve: C1q bound to solid-phase polyclonal anti-C1q was detected by the monoclonal antibody 242 G3. In this assay, the C1q concentration in sera of normal individuals was found to be 160 micrograms/ml (mean value of 70 normal human sera). This ELISA detected nanogram levels of C1q and gave results comparable to those obtained by haemolytic C1q titration. One nanogram of C1q corresponded to ca. 2.6 X 10(10) effective C1q molecules. With this technique, selective C1q deficient sera as well as sera from patients with rheumatoid diseases were analysed.