Experimental infection of mice with the protozoan parasite, Leishmania major, has proven to be an extremely useful tool for investigating immune response mechanisms. Most notably, this model has demonstrated the functional dichotomy of T helper cell subsets also present in human immune responses to pathogenic infections. T cell-mediated immune responses are initiated by antigen presenting cells (APCs), which display antigens derived from intracellular pathogens on surface MHC molecules. Macrophages have long recognized as important MHC class II APCs, but antigen presentation by macrophages infected with real pathogens is still largely uncharacterized. The Leishmania model provides considerable potential for exploring the role of macrophages in activating T helper cells, because Leishmania parasites replicate exclusively within host macrophages and elicit a direct and well-defined MHC class II-restricted response. The experiments proposed here use a unique antigen presentation system to explore three stages in the initiation of a T cell response to infected macrophages. First, the intracellular compartment where MHC class II molecules bind Leishmania antigens will be defined. Experiments will address whether this compartment is identical to the parasitophorous vacuole where Leishmania parasites reside. Second, the co-stimulatory capacity of Leishmania-infected macrophages will be characterized. The constitutive and induced expression of B7 and CD40 molecules will be determined and their role in T cell activation will be tested. These experiments may reveal an effect of intracellular Leishmania or T cell engagement upon the co-stimulatory capacity of the infected macrophage. Finally, the transcription of IL-12 by infected macrophages will be investigated. IL-12 is necessary for the differentiation of responding T cells into Th1 cells, the appropriate subset for controlling L. major infection. Macrophages are the most likely source of IL-12 but Leishmania may interfere with IL-12 induction. T cell dependent IL-12 induction via CD40 will be tested at two stages of macrophage differentiation, immediately after infection and following activation by IFNgamma and TNFalpha. These studies will demonstrate a distinct pattern of macrophage differentiation in response to infection and antigen-specific T cell engagement. Techniques will be established for further exploration of the molecular crosstalk between intracellular pathogens, macrophage APCs, and responding T cells.
