Relevance of anti-C1q autoantibodies to lupus nephritis

The first component of the classical pathway of the complement system (C1q) is considered to have a crucial role in the clearance of immune complexes (ICs) as well as in the removal of waste material originating from apoptotic cells. A prolonged exposure of C1q epitopes to the immune system could eventually lead to an autoimmune response against itself. Although autoantibodies against C1q are found in several diseases, their clinical interest originates from their strong association to active lupus nephritis (LN). Several studies indicate that anti-C1q autoantibodies could serve as a reliable serologic marker in the assessment of LN activity compared to other immunological tests. Additionally, it was suggested that anti-C1q autoantibodies could play a role in LN pathogenesis. Their potential pathogenic actions likely depend on genetic background, titers, Ig classes and subclasses, and specific epitopes of anti-C1q autoantibodies as well as C1q availability and allocation. It is still unclear which different types of anti-C1q autoantibodies dominate in each case and if their upregulation is pathogenic, an epiphenomenon of aberrant tissue damage, or compensatory to an uncontrolled immune response.