Memory phenotype CD8+ T cells are typically thought to have undergone an immune response to foreign antigen and to have differentiated from antigen-specific precursors in the naïve pool. However, using a peptide-MHC I tetramer enrichment technique, we identified foreign antigen-specific memory-phenotype CD8+ T cells in unimmunized mice. These cells (termed "virtual memory" T or VM cells) were observed in mice maintained in both specific-pathogen- and germ-free (SPF and GF respectively) housing.
This thesis focuses on the relationship between VM cells and "conventional" memory cells: memory cells arising from homeostatic proliferation (HP), and innate-like memory CD8+ T cells such as IL-4 bystander memory CD8+ T cells.
Our data indicate physiological HP and IL-4-driven bystander processes are the main mechanisms that drive the generation of VM cells and not the exposure to foreign antigens. VM cells arise in the periphery during the neonatal period and are maintained long term. We also show that VM cells respond in vitro to innate cytokines (similar to conventional memory CD8+ T cells) and they outcompete antigen-specific naive CD8+ T cells in in vivo responses. Overall our observations suggest that VM cells arise out of normal homeostatic and IL-4-driven bystander processes in unimmunized SPF and GF mice, and express at least some memory-like capabilities.