Exploiting death: apoptotic immunity in microbial pathogenesis

Innate immunity typically is responsible for initial host responses against infections. Independently, nucleated cells that die normally as part of the physiological process of homeostasis in mammals (including humans) suppress immunity. Specifically, the physiological process of cell death (apoptosis) generates cells that are recognized specifically by viable cells of all types and elicit a profound transient suppression of host immunity (termed 'innate apoptotic immunity' (IAI)). IAI appears to be important normally for the maintenance of self-tolerance and for the resolution of inflammation. In addition, pathogens are able to take advantage of IAI through a variety of distinct mechanisms, to enable their proliferation within the host and enhance pathogenicity. For example, the protist pathogen Leishmania amazonensis, at its infective stage, mimics apoptotic cells by expressing apoptotic-like protein determinants on the cell surface, triggering immunosuppression directly. In contrast, the pathogenic bacterium Listeria monocytogenes triggers cell death in host lymphocytes, relying on those apoptotic cells to suppress host immune control and facilitate bacterial expansion. Finally, although the inhibition of apoptotic cell death is a common attribute of many viruses which facilitates their extended replication, it is clear that adenoviruses also reprogram the non-apoptotic dead cells that arise subsequently to manifest apoptotic-like immunosuppressive properties. These three instances represent diverse strategies used by microbial pathogens to exploit IAI, focusing attention on the potency of this facet of host immune control. Further examination of these cases will be revealing both of varied mechanisms of pathogenesis and the processes involved in IAI control.