CD1d-restricted "NKT" cells and myeloid IL-12 production: an immunological crossroads leading to promotion or suppression of effective anti-tumor immune responses?

CDld-restricted T cells are remarkable for their unusual ability to respond to self-antigens and to contribute to both immunostimulatory and immunosuppressive responses. Their effects in different cancer models have appeared contradictory; in some cases, they are linked to the generation of effective tumor clearance, and in others, they seem to contribute to suppression of antitumor responses. Recent results suggest CDld-restricted T cells are involved in critical interactions with myeloid dendritic cells (DCs) that can affect the subsequent course of the immune response, and that factors such as the strength of the antigenic signal and the presence or absence of proinflammatory cytokines may determine the outcome of these interactions. In the presence of a strong antigenic signal, CDld-restricted T cells induced myeloid DCs to secrete interlenkin (IL)-12, and these DCs in turn activated naive T cells to secrete Thl cytokines. When exposed to the weak antigenic stimulus of self-antigens, CDld-restricted T cells induced DCs to secrete IL-10 but not IL-12, and these DCs failed to stimulate Thl cytokine production by naive T cells. In contrast, CDld-restricted T cells that were stimulated by self-antigens in the presence of IL-12 potently secreted interferon-gamma (IFN-gamma) and were among the first lymphocytes to become activated in vivo. Hence, CDld-restricted T cells may promote or prevent effective anti-tumor responses that are mediated by other lymphocytic effector cells by influencing IL-12 production by myeloid DCs and by their own production of early IFN-gamma in response to IL-12.