New insight into the autoimmunogenicity of the complement protein C1q

C1q along with its physiological role in maintenance of homeostasis and normal function of the immune system is involved in pathological conditions associated with repetitive generation of anti-C1q autoantibodies. The time and events that cause their first appearance are still unknown. We addressed this issue by analyzing the immunogenicity of C1q in two target groups-one of non-diseased humans and the other of lupus nephritis (LN) patients whose autoimmune disorder is associated with high titers of anti-C1q autoantibodies. The non-diseased humans were represented by pregnant women because the sex hormones are thought to be involved in triggering autoimmune pathologies by their ability to tip the balance of female adaptive immune response to production of antibodies. We screened, using ELISA, 31 sera from healthy pregnant women for the presence of IgM and IgG classes of autoantibodies, recognizing epitopes within the native C1q molecule, its collagen-like region (CLR) and globular head fragment (gC1q). The latter was represented by recombinant analogs of the three globular fragments of A, B and C chains, comprising C1q-ghA, ghB and ghC. We did not find IgM antibodies for all test-antigens which suggest that the natural IgM antibodies are not involved in triggering autoimmunity to C1q. Still more, we did not detect anti-CLR antibodies which have been proved pathogenic in already manifested LN. We completed the analysis with comparative epitope mapping of gC1q and we found similar immunogenic behavior in both target groups-ghA and ghC contained the immunodominant epitopes. This implies that the initial immune response to C1q might occur when the molecule has interacted with its ligands via ghB as part of gC1q. The presence of anti-gC1q in both healthy and diseased humans also implies that these antibodies, unlike anti-CLR, may have a contribution to an onset of autoimmunity.