Introduction: Severe sepsis is characterized by an initial hyper-inflammatory response that may progress to an immune-suppressed state associated with increased susceptibility to nosocomial infection. Analysis of samples obtained from patients who died of sepsis has identified expression of specific inhibitory receptors expressed on lymphocytes that are associated with cell exhaustion. The objective of this study was to prospectively determine the pattern of expression of these receptors and immune cell function in patients with acute sepsis.Methods: Twenty-four patients with severe sepsis were enrolled within 24 hours of the onset of sepsis, as were 12 age-matched healthy controls. Peripheral blood was obtained at enrollment and again seven days later. Immune cell subsets and receptor expression were extensively characterized by quantitative flow cytometry. Lymphocyte function was assayed by stimulated cytokine secretion and proliferation assays. Results were also correlated to clinical outcome.Results: At the onset of severe sepsis, patients had decreased circulating innate and adaptive immune cells and elevated lymphocyte expression of receptors associated with cell activation compared to controls. Samples analyzed seven days later demonstrated increased expression of the inhibitory receptors CTLA4, TIM-3 and LAG-3 on T lymphocytes accompanied by decreased expression of the IL-7 receptor. Functional assays revealed impaired secretion of interferon γ following stimulation in vitro, which was reversible by incubation overnight in fresh media. Impaired secretion of IFNγ correlated with death or development of secondary infection.Conclusions: Lymphocytes from patients with acute sepsis upregulate expression of receptors associated with cell exhaustion, which may contribute to the immune suppressed state that occurs in protracted disease. Therapy that reverses T cell exhaustion may restore immune function in immunocompromised patients and improve survival in sepsis.