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-gamma, IL-12, and TNF-alpha. 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 gamma/delta T cells. These cultures showed a 10- to 50-fold expansion of gamma/delta 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 gammadelta T cell or NK cell expansion, but induced significant alphabeta T cell expansion. Vgamma9/Vdelta2 were expressed on all gamma/delta T cells expanded by exposure of PBMC to Leptorspira: Leptospira stimulation of purified TCRgammadelta(+) T cells, obtained from 8-day cultures of Leptospira-stimulated PBMC, induced high levels of IFN-gamma production, but no cell proliferation, suggesting that such stimulation of gammadelta 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 TCRgammadelta(+) T cells. These results indicate that Leptospira can activate gammadelta T cells and alphabeta T cells and will guide further investigations into the roles of these T cell populations in host defense and/or the pathology of leptospirosis.