C1q, but not other complement components, supplies stimulatory signal for mast-cell activation by Listeria-immune complex. (A) Purified PMCs (2 × 10⁴ cells/well) isolated from WT (WT) mice were assayed for adhesion to a matrix consisting of Listeria plus anti-Listeria antibody, Listeria, anti-Listeria antibody alone, and 50% murine serum obtained from either WT mice or mice deficient in the complement components, C1q, C3, C4, C5, or factor B (C1q^−/−, C3^−/−, C4^−/−, C5^−/−, FB^−/−) or type I collagen. (B) Purified PMCs (5 × 10⁴) from WT and mice were incubated for 1 hour with a washed suspension of Listeria, anti-Listeria antibody, and 50% murine serum from either WT or mice deficient in the complement components C1q, C3, C4, C5, and factor B. Supernatants were collected and analyzed for IL-6 production by ELISA. All adhesion and activation experiments were carried out in the presence of 2 mM MgCl₂. Results are mean plus or minus SEM from triplicate wells of a single experiment and represent 1 of at least 3 experiments demonstrating similar results. (C,D) WT and C1q^−/− mice were infected for 1 or 6 hours with 5 × 10⁴Listeria intraperitoneally. At the indicated time points, the percentage PMN and IL-6 in the peritoneal fluid were determined. Shown is representative of at least 3 experiments (mean ± SEM), carried out in triplicate. Statistics were performed using unpaired Student t test (***P < .001).