CD8+ T cells specific to a single Yersinia pseudotuberculosis epitope restrict bacterial replication in the liver but fail to provide sterilizing immunity

• Perforin and CD8+ T-cells are required to protect against Yersinia pseudotuberculosis.
• CD8+ T-cells provide preferential protection in the liver vs. the spleen.
• Immunity to a single antigen is insufficient to generate sterilizing immunity.

CD8+ T cells use contact-dependent cytolysis of target cells to protect the host against intracellular pathogens. We have previously shown that CD8+ T cells and perforin are required to protect against the extracellular pathogen Yersinia pseudotuberculosis. Here we establish an experimental system where CD8+ T cells specific to a single model antigen are the only memory response present at time of challenge. Using mice immunized with a vaccine strain of Listeria monocytogenes that expresses secreted ovalbumin (Lm-OVA), we show that OVA-specific CD8+ T cells are generated and provide limited protection against challenge with virulent OVA+Y. pseudotuberculosis. Perforin expression by OVA-specific CD8+ T cells was required, as Lm-OVA-immunized perforin-deficient mice showed higher bacterial burden as compared to Lm-OVA-immunized perforin-sufficient mice. Surprisingly, antigen-specific T cell protection waned over time, as Lm-OVA-immune mice eventually succumbed to Yersinia infection. Kinetic analysis of infection in mice with and without OVA-specific CD8+ T cells revealed that bacterial numbers increased sharply in OVA-naïve mice until death, while OVA-immune mice held bacterial burden to a lower level throughout the duration of illness until death. Clonal analysis of bacterial populations in OVA-naïve and OVA-immune mice at distinct time points revealed equivalent and severe bottle-neck effects for bacteria in both sets of mice immediately after intravenous challenge, demonstrating a dominant role for other aspects of the immune system regardless of CD8+ T cell status. These studies indicate that CD8+ T cells against a single antigen can restrict Y. pseudotuberculosis colonization in a perforin-dependent manner, but ultimately are insufficient in their ability to provide sterilizing immunity and protect against death.