The importance of immunoregulatory T cells has become increasingly apparent. Both CD4+CD25+ T cells and CD1d-restricted NKT cells have been reported to down-regulate tumor immunity in mouse tumor models. However, the relative roles of both T cell populations have rarely been clearly distinguished in the same tumor models. In addition, CD1d-restricted NKT cells have been reported to play a critical role not only in the down-regulation of tumor immunity but also in the promotion of the immunity. However, the explanation for these apparently opposite roles in different tumor models remains unclear. We show that in four mouse tumor models in which CD1d-restricted NKT cells play a role in suppression of tumor immunity, depletion of CD4+CD25+ T cells did not induce enhancement of immunosurveillance. Surprisingly, among the two subpopulations of CD1d-restricted NKT cells, Vα14Jα18+ (type I) and Vα14Jα18− (type II) NKT cells, type I NKT cells were not necessary for the immune suppression. These unexpected results may now resolve the paradox in the role of CD1d-restricted NKT cells in the regulation of tumor immunity, in that type II NKT cells may be sufficient for negative regulation, whereas protection has been found to be mediated by α-galactosylceramide–responsive type I NKT cells.

Negative immunoregulation is a major barrier to successful cancer immunotherapy. The NKT cell is known to be one such regulator. In this study we explored the roles of and interaction between the classical type I NKT cell and the poorly understood type II NKT cell in the regulation of tumor immunity. Selective stimulation of type II NKT cells suppressed immunosurveillance, whereas stimulation of type I NKT cells protected against tumor growth even when responses were relatively skewed toward Th2 cytokines. When both were stimulated simultaneously, type II NKT cells appeared to suppress the activation in vitro and protective effect in vivo of type I NKT cells. In the absence of type I, suppression by type II NKT cells increased, suggesting that type I cells reduce the suppressive effect of type II NKT cells. Thus, in tumor immunity type I and type II NKT cells have opposite and counteractive roles and define a new immunoregulatory axis. Alteration of the balance between the protective type I and the suppressive type II NKT cell may be exploited for therapeutic intervention in cancer.

Purpose: Transforming growth factor-β (TGF-β) is an immunosuppressive cytokine, having direct suppressive activity against conventional CD4 + and CD8 + T cells and natural killer cells, thereby inhibiting tumor immunosurveillance. Here, we investigated possible synergy between anti-TGF-β (1D11) and a peptide vaccine on induction of antitumor immunity, and the mechanisms accounting for synergistic efficacy. Experimental Design: The effect of combination treatment with a peptide vaccine and anti-TGF-β was examined in a subcutaneous TC1 tumor model, as well as the mechanisms of protection induced by this treatment.Results: Anti-TGF-β significantly and synergistically improved vaccine efficacy as measured by reduction in primary tumor growth, although anti-TGF-β alone had no impact. The number of tumor antigen-specific CTL with high functional avidity as measured by IFN-γ production and lytic activity was significantly increased in vaccinated mice by TGF-β neutralization. Although TGF-β is known to play a critical role in CD4 + Foxp3 + Treg cells, Treg depletion/suppression by an anti-CD25 monoclonal antibody (PC61) before tumor challenge did not enhance vaccine efficacy, and adding anti-TGF-β did not affect Treg numbers in lymph nodes or tumors or their function. Also, TGF-β neutralization had no effect on interleukin-17-producing T cells, which are induced by TGF-β and interleukin-6. Absence of type II NKT cells, which induce myeloid cells to produce TGF-β, was not sufficient to eliminate all sources of suppressive TGF-β. Finally, the synergistic protection induced by anti-TGF-β vaccine augmentation was mediated by CD8 + T cells since anti-CD8 treatment completely abrogated the effect. Conclusions: These results suggest that TGF-β blockade may be useful for enhancing cancer vaccine efficacy. (Clin Cancer Res 2009;15(21):6560-9)