The overall hypothesis of this program is that persistent inflammation, immunosuppression and catabolism (PICS) are the hallmarks of pathophysiologic processes leading to decreases in long-term survival and functional capacity in patients with chronic critical illness (CCI). While persistent expansion of myeloid-derived suppressor cells (MDSC;Project #2) is a key underlying mechanism of immunosuppression and inflammation in CCI, this project investigates the mechanism by which kidney damage in sepsis initiates an anti-angiogenic state that augments and perpetuates inflammation, immunosuppression, and catabolism in CCI. During sepsis, infection, via toll-like receptors, and hypoxia leads to activation of hypoxia inducible factor (HIF)-1 and subsequent upregulation of angiogenic factors (erythropoietin (EPO) and vascular endothelial growth factor (VEGF)). We have previously shown that the heterodimeric EPO receptor (consisting of the EPO receptor and ?-common receptor (?cR)) interacts with VEGF receptor 2 (VEGFR-2) to mobilize bone marrow derived angiogenic cells, which can contribute to the endothelial repair. EPO and VEGF can both initiate the anti- angiogenic response of upregulation of soluble VEGR-2 (sFlt-1) and angiopoietin-2 (ANG-2). While sFlt-1 binds VEGF reducing its circulating levels and counteracting its effect, unopposed EPO leads to persistent sFlt-1 and ANG-2 elevation and VEGF suppression. Our hypothesis, is that patients in whom kidney damage in sepsis results in an exaggerated EPO response, relative to VEGF, the stimulation of sFlt-1 leads to a persistence of an anti-angiogenic (low levels of VEGF and elevated ANG-2), inflammatory (elevated EPO) state. The investigators propose to examine kidney damage in septic patients as a predictor of anti-angiogenic imbalance and to determine whether anti-angiogenic balance is associated with increased expansion of MDSCs (as determined in Project #2) and increased likelihood of PICS, characterized by morbid long-term outcome (Project #1). The direct effect of increased EPO production on MDSC expansion will be tested in murine models of chronic sepsis using the ?cR knockout mouse.