Variability in the survival to sepsis: inhibitory immunoglobulin G in plasma blocks macrophage mediated killing of enteric bacteria
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The humoral response of an individual can act as a critical factor in mediating survival during sepsis. We hypothesized that the presence of pre-existing, plasma IgG antibodies in mice prior to the onset of sepsis accounts for the differences in their survival and provides a mechanism for the variability in survival in the Cecal Ligation and Puncture (CLP) model of sepsis. Naive pre-CLP plasma was classified as Killing-Plasma or Non-Killing-Plasma based on its ability to kill enteric bacteria in presence of elicited phagocytes using a Plasma Enhanced Killing (PEK) assay. The constant parameters in the assay were bacteria and phagocytes. The variable parameter was pre-CLP plasma obtained from individual mice. The PEK capacity correlated with the CLP survival. Plasma IL-6 levels 24 hours post CLP were used to predict mortality (Die-P) or survival (Live-P). Die-P mice had Non-Killing-Plasma (determined by PEK) and developed higher peritoneal bacterial counts 24 hours post CLP. To understand the defense mechanism conferred by the pre-existing antibodies, IgM immunodepletion did not alter PEK but removing IgG improved PEK capacity suggesting that pre-existing IgG inhibited killing in Non-Killing- Plasma. To evaluate the mechanism of action for lgG, the steps of antibody mediated bacterial killing were evaluated. Plasma by itself was unable to kill bacteria. Addition of macrophages post-opsonization inhibited killing by the Non- Killing-Plasma. Removal of unbound plasma fraction post-opsonization improved killing for the Non-Killing-Plasma. Incubation of macrophages with plasma made bacterial killing worse indicating that lgG in Non-Killing-Plasma interfered with the macrophage mediated killing of opsonized bacteria. De-glycosylation of IgG, which limits its ability to bind to Fcy receptors (FcyRs), improved the PEK capacity of the Non-Killing-Plasma. Blocking FcyRs on macrophages inhibited bacterial killing suggesting that the antibody mediated bacterial killing is through the Fc-FcyR pathway. This study indicates that the possibility of the presence of an inhibitory IgG in naive plasma which may block plasma enhanced killing of bacteria and may be one of the factors conferring increased susceptibility to sepsis induced mortality by CLP.
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