Impaired mast cell-dependent natural immunity in complement C3-deficient mice

The complement system is widely regarded as essential for normal inflammation, not least because of its ability to activate mast cells1,2,3,4,5. However, recent studies have called into question the importance of complement in several examples of mast cell-dependent inflammatory responses6,7,8,9. To investigate the role of complement in mast cell-dependent natural immunity, we examined the responses of complement-deficient mice10,11 to caecal ligation and puncture12, a model of acute septic peritonitis12,13 that is dependent on mast cells and tumour necrosis factor-α (TNF-α). We found that C4- or C3-deficient mice10,11 were much more sensitive to caecal ligation and puncture than wild-type (WT) controls (100% versus 20% in 24-h mortality, respectively). C3-deficient mice also exhibited reductions in peritoneal mast cell degranulation, production of TNF-α, neutrophil infiltration and clearance of bacteria. Treating the C3-deficient mice with purified C3 protein enhanced activation of peritoneal mast cells, TNF-α production, neutrophil recruitment, opsonophagocytosis of bacteria and resistance to caecal ligation and puncture, confirming that the defects were complement-dependent. These results provide formal evidence that complement activation is essential for the full expression of innate immunity in this mast cell-dependent model of bacterial infection.