The immune response to infection must be controlled to ensure it is optimal for defense while avoiding the consequences of excessive inflammation, which include fatal septic shock. Mice deficient in FcγRIIb, an inhibitory immunoglobulin G Fc receptor, have enhanced immune responses. Therefore, we examined whether FcγRIIb controls the response to Streptococcus pneumoniae. Macrophages from FcγRIIb-deficient mice showed increased antibody-dependent phagocytosis of pneumococci in vitro, and consistent with this infected FcγRIIb-deficient mice demonstrated increased bacterial clearance and survival. In contrast, previously immunized FcγRIIb-deficient mice challenged with large inocula showed reduced survival. This correlated with increased production of the sepsis-associated cytokines tumor necrosis factor α and interleukin 6. We propose that FcγRIIb controls the balance between efficient pathogen clearance and the cytokine-mediated consequences of sepsis, with potential therapeutic implications
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