444 Post-septic immune-suppression following Gram positive sepsis is mediated by TLR dependent induction of myeloid derived suppressor cells

Abstract

Systemic Gram+ bacterial infection (sepsis) is a leading cause of death among critically ill patients. Treatment of acute sepsis has been improved, but secondary infections due to post-septic immune suppression are still associated with high mortality and underlying mechanisms are poorly understood. Therefore, we established a mouse model of Gram+ sepsis. Mice were iv. infected with Staphylococcus aureus SA113. Post-septic immune status was assessed by determining a cutaneous T-cell mediated recall response to FITC (FITC-contact hypersensitivity; CHS). Indeed, immune suppression in these mice was evident as FITC-CHS was significantly reduced. Strikingly, we found a massive expansion of the Gr1+ CD11b+ so called myeloid derived suppressor cells (MDSC). We found induction of Ly6G- CD11b+ granulocytic MDSC (grMDSC) but stronger and longer upregulation of Ly6C+Ly6G-CD11b+ monocytic MDSC (mMDSC) paralleled by reduced numbers of plasmacytoid, CD8- and activated dendritic cells (DC), suggesting inhibition of DC differentiation to be associated with MDSC accumulation. Importantly, mMDSC and grMDSC differed in their functional properties. Compared to grMDSC, mMDSC showed a more immature phenotype during early infection with increased differentiation capacity later (expression of CD115, MHCII and F4/80). Only mMDSC, but not grMDSC completely blocked T-cell activation ex vivo, depending on NO and oxygen radicals. Elimination of MDSC by anti-Gr1 depleting antibody or blocking their differentiation by vitamin A abrogated post-septic immune suppression. MDSC induction after sepsis in vivo was clearly reduced in IL-6-ko mice and when mice deficient in TLR-adaptor protein MyD88 or in TLR2/3/4/7/9 were infected. Furthermore, numbers of nitrotyrosine+MHC-II+ and nitrotyrosine+ CD3+ cells were reduced in TLR-ko mice. In summary, we show for the first time post-septic immune suppression after Gram+ sepsis to be mediated by mMDSC induced via MyD88 and TLR signaling and dependent on IL-6.