Sepsis describes the life-threatening systemic inflammatory response (SIRS) of an organism to an infection and is the leading cause of mortality on intensive care units (ICU) worldwide. An acute episode of sepsis is characterized by the extensive release of cytokines and other mediators resulting in a dysregulated immune response leading to organ damage and/or death. This initial pro-inflammatory burst often transits into a state of immune suppression characterised by loss of immune cells and T-cell dysfunction at later disease stages in sepsis survivors. However, despite these appreciations, the precise nature of the evoked defect in T-cell immunity in post-acute phases of SIRS remains unknown. Here we present an in-depth functional analysis of T-cell function in post-acute SIRS/sepsis. We document that T-cell function is not compromised on a per cell basis in experimental rodent models of infection-free SIRS (LPS or CpG) or septic peritonitis. Transgenic antigen-specific T-cells feature an unaltered cytokine response if challenged in vivo and ex vivo with cognate antigens. Isolated CD4+/CD8+ T-cells from post-acute septic animals do not exhibit defects in T-cell receptor-mediated activation at the the level of receptor-proximal signalling, activation marker upregulation or expansion. However, SIRS/sepsis induced transient lymphopenia and gave rise to an environment of immune attenuation at post acute disease stages. Thus, systemic inflammation has an acute impact on T-cell numbers and adaptive immunity, but does not cause major cell-autonomous enduring functional defects in T-cells.

Sepsis is characterized by a disproportionate host response to infection that often culminates in multiple organ failure. Current concepts invoke a deregulated immune reaction involving features of hyperinflammation, as well as protracted immune suppression. However, owing to the scarcity of human data, the precise origin of a long-term suppression of adaptive immunity remains doubtful. We report on an explorative clinical study of chronic critical illness (CCI) patients aimed at assessing the long-term consequences of sepsis on T cell function. Blood was drawn from 12 male CCI patients (median age 67 y, range 48-79 y) receiving continuous mechanical ventilation and renal replacement therapy in a long-term care hospital who had been treated in an external acute care hospital for severe sepsis. T cells were purified and subjected to flow cytometric immune-phenotyping and functional assays. We found that T cells from CCI patients featured higher basal levels of activation and stronger expression of the inhibitory surface receptor programmed cell death 1 compared with controls. However, T cells from CCI patients exhibited no suppressed TCR response at the level of proximal TCR signaling (activation/phosphorylation of PLCγ, Erk, Akt, LAT), activation marker upregulation (CD69, CD25, CD154, NUR77), IL-2 production, or clonal expansion. Rather, our data illustrate an augmented response in T cells from CCI patients in response to TCR/coreceptor (CD3/CD28) challenge. Thus, the present findings reveal that CCI sepsis patients feature signs of immune suppression but that their T cells exhibit a primed, rather than a suppressed, phenotype in their TCR response, arguing against a generalized T cell paralysis as a major cause of protracted immune suppression from sepsis.

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