Legionella pneumophila pathogenesis and immunity.

Legionella pneumophila is a ubiquitous intracellular bacterium found widely in the environment and is the cause of sporadic outbursts of opportunistic infection, mainly in immunocompromised individuals, including young children as well as aged persons. The host response to this organism is similar to responses to other opportunistic intracellular microbes and features both innate and adoptive immune mechanisms. Innate immunity includes the responses of a variety of host cells and cytokines, including those produced by macrophages stimulated by microbial antigens. Adoptive immunity consists of activated lymphocytes and the cytokines they produce, such as interferon and other cytokines that activate macrophages to restrict the growth and spread of intracellular bacteria. The role of cytokines specifically in resistance and immunity to Legionella is exemplified by studies concerning the nature and mechanism whereby interferon produced by activated T lymphocytes influences macrophages to resist infection by this bacterium, not only by restricting growth but also killing this bacterium. This cytokine is considered to have a key role in activating macrophages in adoptive immunity to Legionella and other intracellular bacteria. In particular, interferon is known to have a crucial role in activating macrophages to resist infection by L. pneumophila. This review also describes newer findings that demonstrate that various cytokines that define Th1 vs Th2 helper cell activity also are important in regulating resistance versus susceptibility to this ubiquitous microorganism.