Interleukin-18 and host defense against infectious pathogens

Host defense to infectious pathogens is largely mediated by neutrophil-, cellular, or humoral immunity or eosinophil-dependent mechanism. Each mechanism preferentially acts against extracellular or intracellular microbial pathogens, viruses, or helminthes. These host defense responses are strictly regulated by two different categories of cytokines, T helper (Th)1 and Th2 cytokines. Interleukin-18, originally found as interferon-gamma-inducing factor, has now been identified to be involved in the development of Th1 and Th2 cells, which suggests the considerable involvement of this cytokine in the protective immune responses against infection. This review focuses on the role of interleukin-18 in the development and regulation of host resistance to infectious pathogens, with an emphasis on the infection with Cryptococcus neoformans, an intracellular fungal pathogen, as determined by recent studies from our laboratory.