Mast cell activation and IL-6 secretion is α₂β₁ integrin- and C1q-dependent. (A) Purified PMCs (5 × 10⁴) from WT and KO mice were incubated for 1 hour with a washed suspension of Listeria, anti-Listeria antibody, and 50% murine serum. Supernatants were analyzed by ELISA for IL-6. Unfractionated peritoneal cells (5 × 10⁴ cells, consisting mostly of peritoneal macrophages and lymphocytes and containing approximately 2% PMCs) from WT (WT Unf) and KO (KO Unf) mice were also assayed, and are designated by the vertical dashed line. (B) Purified PMCs (5 × 10⁴) from WT and KO mice were incubated for 1 hour with a washed suspension of Listeria and anti-Listeria antibody alone (Control) or Listeria, anti-Listeria antibody, plus 50% serum from either WT or C1q^-/- mice. Supernatants were analyzed by ELISA for IL-6. All results are presented as mean ± SEM from triplicate wells of a single experiment and represent 1 of at least 3 experiments demonstrating similar results.