Human monocytic ehrlichiosis (HME) is a relatively new tick-borne disease caused by obligate intracellular bacteria of the genus ehrlichia. Although immunodeficient individuals are particularly susceptible to severe disease, we have a very rudimentary understanding of the immune components that regulate disease susceptibility and resistance, and the mechanisms of host defense. The proposed studies will utilize Ehrlichia muris, which causes a non-fatal persistent infection, and an ehrlichia known as Ixodes ovatus ehrlichia (IOE), that causes fatal disease in immunocompetent mice that resembles HME. Our studies have revealed that IFNgamma production by Type 1 helper CD4 T cells, but not CD8 cells, is essential for resistance to low dose IOE infection. Unexpectedly, secondary challenge with a sublethal (low dose) IOE inoculum causes fatal disease that is mediated by CD8 T cells, and requires TNFalpha. Although the lOE-specific memory CD8 T cell responses were deleterious, it was possible to generate highly effective challenge immunity against high dose IOE challenge by prior infection with E. muris. The observations reveal that we lack a fundamental understanding regarding how memory responses are regulated during this disease. Therefore, in Aim 1 memory lymphocyte subsets and functions critical for protection against high dose IOE challenge will be identified and characterized. These studies will help to define correlates of protective immunity against the ehrlichiae that will likely be relevant for understanding human susceptibility. Aim 2 will address the hypothesis that cytokine production by CD8 T cells contributes to immunopathology during failed IOE recall responses. These studies together will reveal how CD8 T cells, even though inessential during primary IOE infection, may nevertheless play significant roles in regulating secondary immune responses. Immunity to the ehrlichiae is relatively unexplored in both humans and animals models, so the proposed studies will provide important new information relevant to ehrlichiosis and immune regulation that will be generally applicable to other established and emerging intracellular bacterial pathogens controlled by cell mediated immunity. By defining optimal CD4 T cell immune correlates, and the role of memory CD8 T cells during recall responses, the proposed studies will contribute to the development of therapies and vaccines for ehrlichioses and related infections.