Lupus nephritis: the roles of C1q and C3 in preventing antibody mediated injury

Abstract

Abstract Lupus nephritis (LN) is a common and serious complication of systemic lupus erythematosus (SLE). While defective clearance of apoptotic cells (AC) and immune complexes (IC) is implicated in LN pathogenesis, the precise way in which different complement components impact kidney protection and injury is unknown. To address this, we created C57BL/6 mice with defective clearance of AC (Mfge8 deficiency) and anti-chromatin antibodies (sle1 interval) that were also deficient in either C1q [C1q Triple mutant (C1qTM)] or C3 (C3TM). Both C1q and C3 TM mice spontaneously developed anti-DNA antibodies and glomerulonephritis associated with glomerular IgG deposits after the age of 6 months. To dissociate the effects of complement on B cells versus effects on the kidney, we examined the effects of antibody mediated kidney injury (nephrotoxic nephritis, NTN) in mice deficient in the clearance of AC with complement deficiency [double knockout (DKO) (Mfge8 C1q or Mfge8 C3) mice]. In NTN, 50% of C1q DKO and 25% of C3 DKO, but no Mfge8 KO, died after NTN induction and the surviving C1q DKO mice developed severe proteinuria. Surprisingly, both spontaneous and NTN-induced nephritis in C1q TM or C1q DKO had strong glomerular C3 and C3d staining, while strong staining for the membrane attack complex (MAC) was observed in C3 TM or C3 DKO mice. These findings indicate that: 1) C1q and C3 independently protect against kidney injury separate from their effects on B cell function 2) In the presence of excess AC and low C1q, antibody mediated kidney injury is associated with C3 activation, presumably by the lectin or alternative pathway and 3) In the presence of excess AC and low C3, antibodies can cause MAC activation by pathways that remain to be determined.