Effect of temperature and opsonin density on particle binding. (A) Effect of opsonin density on binding in the presence and absence of the actin-depolymerizing agent latrunculin B. RAW macrophages were maintained at 37°C and treated with either vehicle control (DMSO, black bars) or latrunculin B (2 µM, gray bars) and then exposed to sRBCs for 3 min. The final dilution of the opsonizing IgG used for each condition is indicated below the abscissa. The percentages above the bars indicate the reduction in binding induced by latrunculin; data are presented as a binding index normalized to DMSO-treated cells. (B) Effect of opsonin density on binding at varying temperatures. RAW macrophages maintained at either 37°C or 4°C (to inhibit actin rearrangements) were incubated with IgG-opsonized sRBCs for either 40 s (black and gray bars) or 20 min (white bar) as indicated. The final dilution of opsonin used for each condition is indicated below the abscissa. The percentages above the bars indicate the reduction in particle binding induced by cooling, relative to control. Data are presented as a binding index normalized to cells maintained at 37°C. (C) RAW macrophages pretreated with latrunculin B (gray bars) or with vehicle only (DMSO; black bar) were exposed to IgG-opsonized beads for either 30 s or 30 min, as indicated, before washing. Data are means ± SEM of three independent experiments.