Post-stroke immunodepression and infection: an emerging concept

Infections occur commonly following stroke and adversely influence outcome. Dysphagia, greater stroke severity and increasing age are associated with post-stroke infection, but post-stroke immunodepression is now recognised as an independent factor associated with increased susceptibility. Counter-regulatory responses, triggered by the pro-inflammatory response to stroke, appear to effect systemic immunodepression via suppression of both innate and adaptive immune responses. Experimental and clinical studies have identified a range of anti-inflammatory and immunosuppressive changes, including reduced mononuclear phagocyte and natural killer cell function, induction of anti-inflammatory cytokines, apoptotic lymphocyte loss and altered T lymphocyte activity. A range of mechanisms has been proposed, including hypothalamo-pituitary-adrenal axis (HPAA) and sympathetic nervous system (SNS) activation. The post-stroke balance of pro- and anti- inflammatory mechanisms may be aimed at restricting the extent of inflammation and contributing to the restoration of immune homeostasis. However, severe inflammation in the brain may trigger major systemic, counter-inflammatory responses that ultimately compromise immune mechanisms required to combat pathogens. Although key pathways have been identified, the extent to which the various elements of post-stroke immunodepression are clinically relevant remains to be discovered. The identification of markers of immunodepression in the early post-stroke phase may prove useful for identifying patients that may have increased susceptibility to infection. It also seems likely that post-stroke immunodepression will need to be taken into account where stroke treatments impact upon inflammatory and immune pathways.