Innate and adaptive immune responses induced by leptospirosis have not been well characterized. In this study we show that in vitro exposure of naive human PBMC to Leptospira interrogans results in cell proliferation and the production of IFN-γ, IL-12, and TNF-α. Cell proliferation was highest when using high numbers of Leptospira. Optimal cell proliferation occurred at 6-8 days, and the majority of cells contained in these cultures were γ/δ T cells. These cultures showed a 10- to 50-fold expansion of γ/δ T cells compared with the initial cellular input. Additionally, these cultures contained elevated numbers of NK cells. In contrast, exposure of PBMC to low numbers of Leptospira failed to induce γδ T cell or NK cell expansion, but induced significant αβ T cell expansion. Vγ9/Vδ2 were expressed on all γ/δ T cells expanded by exposure of PBMC to Leptospira. Leptospira stimulation of purified TCRγδ+ T cells, obtained from 8-day cultures of Leptorspira-stimulated PBMC, induced high levels of IFN-γ production, but no cell proliferation, suggesting that such stimulation of γδ T cells did not depend on specialized accessory cells or Ag processing. Finally, in patients with acute leptospirosis, there was a significant (4- to 5-fold) increase in the number of peripheral blood TCRγδ+ T cells. These results indicate that Leptospira can activate γδ T cells and αβ T cells and will guide further investigations into the roles of these T cell populations in host defense and/or the pathology of leptospirosis.