Selective dependence of H2-M3-restricted CD8 responses on IL-15.

We studied whether CD8 T cell responses that are mediated by unconventional MHC class Ib molecules are IL-15 dependent in mice. CD8(+) T cell responses to Listeria monocytogenes infection that are restricted by the MHC class Ib molecule H2-M3 decreased in the absence of IL-15, whereas other primary MHC class Ib- and MHC class Ia-restricted responses were IL-15 independent. This result was confirmed in MHC class Ia-deficient mice in which IL-15 deficiency also reduced H2-M3-restricted but not all CD8 T cell responses to L. monocytogenes. IL-15 deficiency did not affect proliferation or survival of responding H2-M3-restricted CD8(+) T cells, but IL-15 was necessary to detect H2-M3-restricted CD8(+) T cells in naive mice. This finding suggests that these CD8(+) T cells require IL-15 during development, but become IL-15 independent after activation. IL-15 was necessary for the survival of most class Ib-restricted CD8(+) T cells, starting at the mature thymocyte stage in naive mice, but does not affect a distinct CD44(low)/CD122(low) subpopulation. These data suggest that the nature of the selecting MHC class Ib molecule determines whether CD8(+) T cells acquire IL-15 dependence during thymic development.