CD16 promotes Escherichia coli sepsis through an FcRg inhibitory pathway that prevents phagocytosis and facilitates inflammation Fabiano Pinheiro da Silva 1–3,8 , Meryem Aloulou 1,2,8 , David Skurnik 4 , Marc Benhamou 1,2 , Antoine Andremont 4 , Irineu T Velasco 3 , Murilo Chiamolera 3,5 , J Sjef Verbeek 6 , Pierre Launay 1,2,7 & Renato C Monteiro 1,2 Sepsis, a leading cause of death worldwide, involves proinflammatory responses and inefficient bacterial clearance 1,2 . Phagocytic cells play a crucial part in the prevention of sepsis by clearing bacteria through host innate receptors 3 . Here we show that the FcRc adaptor, an immunoreceptor tyrosine-based activation motif (ITAM)-bearing signal transduction subunit of the Fc receptor family, has a deleterious effect on sepsis. FcRc –/– mice show increased survival during peritonitis, owing to markedly increased E. coli phagocytosis and killing and to lower production of the proinflammatory cytokine tumor necrosis factor (TNF)-a. The FcRc-associated receptor that inhibits E. coli phagocytosis is FccRIII (also called CD16), and its absence protects mice from sepsis. FccRIII binds E. coli, and this interaction induces FcRc phosphorylation, recruitment of the tyrosine phosphatase SHP-1 and phosphatidylinositide-3 kinase (PI3K) dephosphorylation. Decreased PI3K activity inhibits E. coli phagocytosis and increases TNF-a production through Toll-like receptor 4. We identified the phagocytic receptor negatively regulated by FcRc on macrophages as the class A scavenger receptor MARCO. E. coli-FccRIII interaction induces the recruitment of SHP-1 to MARCO, thereby inhibiting E. coli phagocytosis. Thus, by binding FccRIII, E. coli triggers an inhibitory FcRc pathway that both impairs MARCO-mediated bacterial clearance and activates TNF-a secretion. Cellular responses and inflammation are essential in sepsis and are controlled by a balance of activating and inhibitory signals delivered by transmembrane receptors, such as those associated with adaptors containing ITAMs, including FcRg (encoded by Fcer1g) and DAP12 (also known as TYRO protein tyrosine kinase binding protein, or TYROBP) 4,5 . We used the well-characterized model of acute perito- nitis caused by cecal ligation and puncture (CLP) in FcRg –/– mice to examine whether FcRg regulates receptors for bacteria during systemic infections 6 . Unexpectedly, mortality was far lower among FcRg –/– mice than it was among wild-type (WT) animals (Fig. 1a). Sepsis severity has been linked to immunological overreaction, which produces an excess of proinflammatory cytokines 1,2 . Serum and peritoneal levels of TNF-a, but not of IL-10, were markedly lower in FcRg –/– mice than in WT mice 8 h after CLP (Fig. 1b and data not shown). Therefore, an FcRg-associated receptor promotes a robust inflammatory response in sepsis that is at least partly responsible for the high mortality rate from sepsis in WT mice 1 . As TNF-a indirectly influences bacteremia by attracting and acti- vating phagocytes 2 , we wanted to determine which commensal bacteria are involved in the septic shock in this model. Before CLP, comparative cultures of feces from WT and FcRg –/– mice showed no differences in the abundance of E. coli, Enterococcus fecalis or anae- robes (data not shown). By contrast, 24 h after CLP, when E. coli were abundant in the peritoneal lavage fluid from WT animals, they were nearly absent from FcRg –/– mice (Fig. 1c) and were detected in the blood of only one of ten FcRg –/– mice (Fig. 1d). Polymicrobial flora (anaerobes and E. fecalis) predominated in FcRg –/– mice. To explore the mechanisms underlying these differences in bacter- emia, we evaluated phagocytosis by peritoneal neutrophils and macro- phages and by bone-marrow–derived macrophages (BMMs). Relative to WT cells, FcRg –/– neutrophils and macrophages showed a six- to tenfold increase in their capacity to phagocytose E. coli (Fig. 2a–c). This FcRg inhibitory signal was observed with non-opsonized E. coli and was partially abrogated by opsonization (Fig. 2a–c). No signifi- cant differences between the two genotypes were found in the phagocytosis of Staphylococcus aureus (data not shown) or zymosan (Fig. 2a), indicating that phagocytosis inhibition by FcRg might be specific for Gram-negative bacteria. These results were confirmed with peritoneal macrophages (data not shown). Killing of internalized E. coli was at least as efficient in FcRg –/– BMMs as in WT BMMs Received 6 April; accepted 11 September; published online 14 October 2007; doi:10.1038/nm1665 1 Institut National de la Sante ´ et de la Recherche Me ´ dicale U699, 16 rue Henri Huchard, Paris F-75018, France. 2 University of Paris 7, Bichat Medical School, 16 rue Henri Huchard, Paris F-75018, France. 3 Emergency Medicine Department, University of Sa ˜o Paulo, Av Dr Arnaldo 455, Sa ˜ o Paulo 01246-903, Brazil. 4 Bacteriology Department, Bichat-Claude Bernard Hospital and EA 6934 University of Paris 7, 46, rue Henri Huchard, Paris F-75018, France. 5 Division of Rheumatology, University of Sa ˜o Paulo, Av Dr Arnaldo 455, Sa ˜ o Paulo 01246-903, Brazil. 6 Department of Human Genetics, Leiden University Medical Center, Postzone S4-P PO Box 9600, 2300 RC, Leiden, the Netherlands. 7 Equipe Avenir Institut National de la Sante ´ et de la Recherche Me ´ dicale, 16 rue Henri Huchard, Paris F-75018, France. 8 These authors contributed equally to this work. Correspondence should be addressed to R.C.M. (monteiro@bichat.inserm.fr). 1368 VOLUME 13 [ NUMBER 11 [ NOVEMBER 2007 NATURE MEDICINE LETTERS © 2007 Nature Publishing Group http://www.nature.com/naturemedicine