Regulation by Innate Immune T Lymphocytes in the Host Defense against Pulmonary Infection with Cryptococcus neoformans

Recently, innate immune lymphocytes, such as natural killer (NK) T cells and γδ antigen receptor-bearing T (γδ T) cells, have garnered much attention, and their biological significance in the tumor immunity, allergic diseases and infectious diseases is extensively exploited. We have addressed the role of these cells in the host defense using a mouse model of pulmonary infection with Cryptococcus neoformans, which frequently causes fatal meningoencephalitis in AIDS patients. Host defense to this fungal pathogen is largely mediated by cellular immunity, and type-1 helper T (Th1) cells play a central role in this process. This infection causes a prompt accumulation of both NKT and γδ T cells in the lung tissues in a monocyte chemoattractant protein (MCP)-1-dependent or -independent manner, respectively. Genetic deletion of Vα14+ NKT cells ameliorates the Th1 response and clearance of microorganisms in the lungs, whereas these host protective responses are rather enhanced in mice lacking γδ T cells. Thus, in some aspect, these innate immune lymphocytes may co-regulate the Th I -mediated response for induction of the moderate host defense. γδ T cells may act to keep the balance of Th1-Th2 responses in a proper manner by suppressing the exaggerated Th I response caused by NKT cells. In this review, I describe the recent research development in the innate immune host defense against cryptococcal infection in respiratory organs with emphasis on our data in the regulatory role of NKT cells and γδ T cells.