Memory phenotype CD8⁺ T cells in IL-15 transgenic mice are involved in early protection against a primary infection with Listeria monocytogenes

We recently constructed IL-15 transgenic (Tg) mice using cDNA encoding a secretable isoform of the IL-15 precursor protein under the control of an MHC class I promoter. The IL-15 Tg mice exhibited resistance against a primary infection with Listeria monocytogenes. The numbers of memory CD8⁺ T cells were markedly increased in the IL-15 Tg mice following Listeria infection accompanied by sustained IL-15 production. The increased CD44⁺CD8⁺ T cells in the infected IL-15 Tg mice were not specialized to recognize Listeria-specific antigen but produced a large amount of IFN-γ in response to bystander stimulation exogenous IL-15 in combination with IL-12. Furthermore, Listeria-specific Th1 response by CD4⁺ T cells was significantly augmented in the IL-15 Tg mice compared with control mice following Listeria infection. In vivo depletion of the CD8⁺ T cells by anti-CD8 monoclonal antibody and adoptive transfer of the T cells from naive IL-15 Tg mice indicated that the CD8⁺ T cells functioned not only to eliminate bacteria at the early stage of infection but also to promote Th1 response to L. monocytogenes. Overexpression of IL-15 shed light on a novel role of memory CD8⁺ T cells in early protection and promotion of Th1 response against a primary infection with L. monocytogenes.