Development of antigen-specific CD8+ CTL in MHC class I-deficient mice through CD4 to CD8 conversion

CD8(+) CTL are the predominant tumoricidal effector cells. We find, however, that MHC class I-deficient mice depleted of CD8(+) T cells are able to mount an effective antitumor immunity after immunization with fused dendritic/tumor cells. Such immunity appears to be mediated by the generation of phenotypic and functional CD8(+) CTL through CD4(+) to CD8(+) conversion, which we have demonstrated at the single cell level. CD4(+) to CD8(+) conversion depends on effective in vivo activation and is promoted by CD4(+) T cell proliferation. The effectiveness of this process is shown by the generation of antitumor immunity through adoptive transfer of primed CD4 T cells to provide protection against tumor cell challenge and to eliminate established pulmonary metastases.