Antibiotic treatment-resistant Lyme arthritis is a chronic inflammatory joint disease that follows infection with Borrelia burgdorferi (Bb). A marked Ab and T cell response to Bb outer surface protein A (OspA) often develops during prolonged episodes of arthritis. Furthermore, cross-reaction between the bacterial OspA and human LFA-1α L at the T cell level and the inability to detect Bb in the joint implicate an autoimmune mechanism. To analyze the nature of response to OspA and LFA-1α L, we used OspA-specific T cell hybrids from DR4 transgenic mice, as well as cloned human cells specific for OspA 165–184, the immunodominant epitope, from five DRB1* 0401+ patients, using OspA-MHC class II tetramers. Although OspA 165–184 stimulated nearly all OspA-specific human T cell clones tested to proliferate and secrete IFN-γ and IL-13, LFA-1α L326–345 stimulated∼ 10% of these clones to proliferate and a greater percentage to secrete IL-13. Assays with LFA-or OspA-DR4 monomers revealed that higher concentrations of LFA-DR4 were needed to stimulate dual-reactive T cell hybrids. Our analysis at the clonal level demonstrates that human LFA-1α L326–345 behaves as a partial agonist, perhaps playing a role in perpetuating symptoms of arthritis.