Mast-cell activation by Listeria immune complex does not require FcRγ. (A) A proposed model is a 2-site, 2-receptor model in which concurrent activation of the α2β1 integrin and secondary receptor (complement receptor, FcRγ, Listeria receptor) stimulates mast-cell activation. Adapted from Edelson et al¹  with permission. (B) Purified PMCs (2 × 10⁴) from either WT (WT) mice or from mice lacking FcRγ (FcRγ^−/−) were assayed for adhesion to a matrix consisting of (1) Listeria plus anti-Listeria antibody alone, (2) Listeria, anti-Listeria antibody and 50% murine serum, or (3) type I collagen in the presence or absence of 1 mM EDTA. (C) Purified PMCs (5 × 10⁴) from WT and FcRγ^−/− mice were incubated for 1 hour with a washed suspension of Listeria, anti-Listeria antibody, and 50% murine serum. 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. (D,E) WT and FcRγ^−/− mice were infected for 1 or 6 hours with 5 × 10⁴Listeria intraperitoneally. At the indicated time points, the percentage of PMNs and concentration of IL-6 in the peritoneal fluid were determined. Shown is a representative example of at least 3 experiments (mean ± SEM), all carried out in triplicate.