Activated protein C targets CD[8.sup.+] dendritic cells to reduce the mortality of endotoxemia in mice

Introduction The recombinant form of human activated protein C (aPC) is used in clinical practice to treat patients suffering from the most severe forms of sepsis. Proposed aPC candidate mechanisms [...]
Activated protein C (aPC) therapy reduces mortality in adult patients with severe sepsis. In mouse endotoxemia and sepsis models, mortality reduction requires the cell signaling function of aPC, mediated through protease-activated receptor-1 (PAR1) and endothelial protein C receptor (EPCR; also known as Procr). Candidate cellular targets of aPC include vascular endothelial cells and leukocytes. Here, we show that expression of EPCR and PAR1 on hematopoietic cells is required in mice for an aPC variant that mediates full cell signaling activity but only minimal anticoagulant function (5A-aPC) to reduce the mortality of endotoxemia. Expression of EPCR in mature murine immune cells was limited to a subset of CD[8.sup.+] conventional dendritic cells. Adoptive transfer of splenic CD[11c.sup.hi]PDCA-1- dendritic cells from wild-type mice into animals with hematopoietic EPCR deficiency restored the therapeutic efficacy of aPC, whereas transfer of EPCR-deficient CD[11c.sup.hi] dendritic cells or wild-type CD[11c.sup.hi] dendritic cells depleted of [EPCR.sup.+] cells did not. In addition, 5A-aPC inhibited the inflammatory response of conventional dendritic cells independent of EPCR and suppressed IFN-γ production by natural killer-like dendritic cells. These data reveal an essential role for EPCR and PAR1 on hematopoietic cells, identify EPCR-expressing dendritic immune cells as a critical target of aPC therapy, and document EPCRindependent antiinflammatory effects of aPC on innate immune cells.