A critical driving force for optimal development of T helper type 1 (TH1) lymphocytes is signaling through the IL-12 receptor. The IL-12 receptor is composed of two subunits, with expression of the IL-12 receptor beta 2 chain (IL-12RB2) dictating a high affinity IL-12 receptor complex. Signaling through this high affinity IL-12 receptor controls the development of TH1 lymphocytes and the maintenance of this phenotype, while limiting lineage commitment to the TH2 phenotype. Since TH1 lymphocytes mediate cellular immunity, while TH2 lymphocytes enhance humoral responses, early expression of the high affinity IL-12 receptor is critical for a commitment to cell mediated immune responses. Salmonella is an intracellular pathogen of macrophages, epithelial cells and possibly dendritic cells, and requires cell-mediated immunity for clearance. Based on recently published work, we demonstrated that Salmonella-infected macrophages can significantly limit IL-12RB2 expression on T lymphocytes early in the response. This finding has profound implications for the early development and commitment of T lymphocytes to the TH1 lineage during Salmonella infection. The overall goal of this proposal is to define the mechanisms for Salmonella-induced reductions in IL-12RB2 expression in vitro and in vivo. At present, it is not clear whether induced reductions in IL-12RB2 expression are solely mediated by soluble factors or require macrophage-T cell contact. IL-12RB2 expression will be quantified at the level of mRNA using quantitative RT-PCR, and at the protein level using Western blot, FACS and radioreceptor analyses. Furthermore, reductions in T lymphocyte function associated with the loss of IL-12RB2 will be assessed, and a functional assessment of developing TH1 and TH2 lymphocytes will be determined by following STAT-4 activation, and T-bet, GATA-3 and c-maf mRNA expression, respectively. Whether infected dendritic cells can induce such alterations in CD4+ T cells will also be determined. Taken together these studies represent the first to define mechanisms whereby an intracellular bacterial pathogen can adversely affect the early development of TH1 lymphocytes upon infection.