212: Innate neutrophil function is regulated locally through IL-6 control of prostaglandins

Abstract

Interleukin-6 (IL-6) is vital for competent anti-microbial host defense. Mice deficient in IL-6 showed an inability to appropriately control bacterial peritonitis (5 × 108 cfu Staphylococcus epidermidis) and displayed both increased dissemination and reduced bacterial clearance. This response was reversed through local reconstitution of IL-6 trans-signaling using a chimeric sIL-6R-IL-6 fusion protein (termed HDS). Administration of HDS (10–1000 ng/ml) to infected IL-6−/− mice promoted bacterial clearance and reduced bacteremia in a dose-dependent fashion. This response was associated with improved resolution of the acute phase response and a more rapid normalization of body temperature. To define the mechanisms governing IL-6 involvement in anti-microbial immunity, we first developed an ex vivo flow cytometric assay to simultaneously monitor neutrophil respiratory burst and phagocytosis. Bacterial stimulation of circulating blood neutrophils from IL-6−/− and IL-6R−/− mice showed comparable effector function to that displayed by WT controls. Thus, IL-6 deficiency is not associated with an inherent defect in neutrophil function. However, evaluation of infiltrating neutrophils isolated by lavage from the peritoneal cavity of infected IL-6−/− and IL-6R−/− mice showed significantly reduced respiratory burst and phagocytic activity. Interleukin-6 therefore controls responses local to the site of infection and is consistent with the activation of non-hematopoietic stromal cells by IL-6 and sIL-6R. In human peritoneal mesothelial cells and peritoneal membranes from infected mice, IL-6 trans-signaling promoted cyclooxygenase-2 (COX2) expression. Blockade of IL-6 signaling in infected WT mice with the antagonist sgp130 or anti-IL-6R antibodies inhibited peritoneal COX2 expression. Mass spectrometry for the COX2 products, prostaglandin-E2 (PGE2) prostaglandin-D2 (PGD2) in lavage samples from infected mice showed that the generation of PGD2, but not PGE2, was impaired in IL-6 deficiency. Here, PGD2 levels correlated with the number of infiltrating neutrophils undergoing respiratory burst. These studies suggest IL-6 signaling regulates the effector characteristics of infiltrating neutrophils through local control of immuno-regulatory prostaglandin production.