All five animals vaccinated with the obligate intracellular bacterium Ehrlichia (ex Cowdria) ruminantium inactivated by azide and emulsified in Montanide ISA50 adjuvant (group I) were protected against a homologous challenge. In contrast, none of the three animals vaccinated with denatured E. ruminantium survived (group II). Using flow cytometry based intracellular staining of IFN-[gamma], we observed in Group I animals before the booster infection, a comparable contribution of CD4+ and CD8+ T cells to the production of IFN-[gamma] triggered by in vitro stimulation with E. ruminantium antigens. After the booster injection, IFN-[gamma] production was confirmed in stimulated immune CD4 in all animals and in CD8 in two animals. In group II, we detected IFN-[gamma] only in two out of three animals, and only in CD8. Simultaneous staining of intracellular IFN-[gamma] and IL-4 indicated that stimulated immune CD4+ T cells produced IFN-[gamma] in the absence of detectable IL-4, which is suggestive of a strict Th1 response. On the other hand, when the same cells were stimulated with concanavalin A, Th1, and Th2 and to a lesser extent Th0 responses were detected. We confirmed by ELISA that stimulated immune but not naïve cells also secreted IFN-[gamma]. Again, cells from all goats immunised with inactivated E. ruminantium secreted IFN-[gamma], while only one animal vaccinated with denatured antigen responded. Finally, the contribution of CD8 to the secretion of IFN-[gamma] was confirmed in two out of three animals from group 1, by blocking experiments using antibodies against MHC class I molecules. However, IFN-[gamma] secretion was entirely dependent on CD4 as shown by addition of anti-MHC class II antibodies. In conclusion, our data strongly suggest that protective immunity induced in goats by the killed E. ruminantium vaccine is associated with a dominant Th 1 response. (Texte intégral)