Lyme disease is the most common vector-borne illness in the United States. It is caused by the spirochete Borrelia burgdorferi transmitted by the Ixodes tick. A proportion of patients with Lyme disease develop arthritis. The investigator has previously shown that the synovial fluid alpha/beta T-cells have a skewed alpha/beta T-cell repertoire (similar to RA) compared to peripheral blood. He has also observed that gamma/delta T-cells from synovial fluid have high expression of Fas ligand and induce apoptosis of C4+ T-cells. This suggests a possible immunoregulatory network in the synovial fluid. The investigator proposes to extend the studies on gamma/delta T-cells to elucidate their role in Lyme arthritis. He has isolated more than 20 gamma/delta T-cell clones from the synovial fluid of patients with Lyme arthritis and has developed a cell culture assay comprised of dendritic cells, IL-2 and T-cell clones to evaluate activation of the T-cell by B. Burgdorferi. In Aim 1, the investigator will perform fractionation of B. Burgdorferi sonicates into lipid, aqueous and protein components to identify the nature of the antigen responsible for T-cell activation. He will then examine whether antigen recognition is restricted by classical or non-classical MHC molecules and whether antigen processing is required. He will also study whether the B. Burgdorferi directly activates dendritic cells by upregulating costimulatory molecules. In the second aim, he will isolate the cDNA encoding the gamma and delta chains and transfect these into variant Jurkat or murine T-cell hybridomas to identify which chain is responsible for recognition and activation in response to B. Burgdorferi. In the third aim, the investigator will examine whether gamma/delta T-cells induce preferential Th1 cytokine production by CD4 T-cells. Cytokine expression will be analyzed following activation of peripheral blood T-cells and synovial T-cells from patients where available. Gamma/delta T-cells will be depleted to determine whether this influences cytokine production skewing it to Th2 functional phenotype. To understand the role of IL-4 in protection of Fas ligand mediated apoptosis, he will determine whether signalling through the common gamma chain or the alpha chain (shared with IL-13) mediates resistance to apoptosis in murine CD4 splenic T-cells. STAT6-deficient mice will also be used to define the signal transduction pathway.