The Lyme disease spirochete Borrelia burgdorferi is the only known human pathogen that directly activates invariant NKT (iNKT) cells. The number and activation kinetics of iNKT cells vary greatly among different strains of mice. We now report the role of the iNKT cell response in the pathogenesis of Lyme disease using C57BL/6 mice, a strain with optimal iNKT cell activation that is resistant to the development of spirochetal-induced inflammation. During experimental infection of B6 mice with B. burgdorferi, iNKT cells localize to the inflamed heart where they are activated by CD1d-expressing macrophages. Activation of iNKT cells in vivo results in the production of IFN-γ, which we demonstrate ameliorates the severity of murine Lyme carditis by at least two mechanisms. First, IFN-γ enhances the recognition of B. burgdorferi by macrophages, leading to increased phagocytosis of the spirochete. Second, IFN-γ activation of macrophages increases the surface expression of CD1d, thereby facilitating further iNKT activation. Collectively, our data demonstrate that in the resistant background, B6, iNKT cells modulate the severity of murine Lyme carditis through the action of IFN-γ, which appears to self-renew through a positive feedback loop during infection.