Cytotoxic T Lymphocyte–associated Antigen 4 (CTLA-4) Can Regulate Dendritic Cell–induced Activation and Cytotoxicity of CD8⁺ T Cells Independently of CD4⁺T Cell Help

The mechanisms that regulate the strength and duration of CD8⁺ cytotoxic T cell activity determine the effectiveness of an antitumor immune response. To better understand the antitumor effects of anti-cytotoxic T lymphocyte–associated antigen 4 (CTLA-4) antibody treatment, we analyzed the effect of CTLA-4 signaling on CD8⁺ T cells in vitro and in vivo. In vitro, cross-linking of CTLA-4 on purified CD8⁺ T cells caused decreased proliferative responses to anti-CD3 stimulation and rapid loss of activation marker expression. In vivo, blockade of CTLA-4 by neutralizing anti–CTLA-4 mAb greatly enhanced the accumulation, activation, and cytotoxic activity of CD8⁺ T cells induced by immunization with Ag on dendritic cells (DC). This enhanced response did not require the expression of MHC class II molecules on DC or the presence of CD4⁺ T cells. These results demonstrate that CTLA-4 blockade is able to directly enhance the proliferation and activation of specific CD8⁺ T cells, indicating its potential for tumor immunotherapy even in situations in which CD4⁺ T cell help is limited or absent.