Recycling of IgG is not responsible for enhanced phagocytosis mediated by FcRn. (A) Effect of IgG level on phagocytosis of pneumococci by wild-type and β2M^-/- mouse PMNs. Note that phagocytosis by β2M^-/- PMNs is more impaired at higher concentrations of human IgG1. (B) Recycling of human IgG1 by wild-type and β2M^-/- mouse PMNs. Wild-type mouse PMNs and β2M^-/- PMNs were allowed to take up human IgG1 antiserotype 6B mAb (10 μg/mL) for 5 minutes at 37°C in the presence of serotype 6B pneumococci. After extensive washing, cells were incubated for an additional 30 minutes in medium to allow for exocytosis/recycling of IgG, and supernatants were subsequently analyzed for the presence of human anti-6B pneumococcal antibodies by ELISA. Control samples in which mouse PMNs were incubated with IgG1 only (without bacteria) did not result in significant recycling. Experiments were repeated 4 times, with essentially identical results. Data represent means plus or minus standard deviation.