Abstract
Though TGF‐β inhibition enhances antitumor immunity mediated by CD8^+^ T cells in several tumor models, it is not always sufficient for rejection of tumors. In this study, to maximize the antitumor effect of TGF‐β blockade, we tested the effect of anti‐TGF‐β combined with an irradiated tumor vaccine in a subcutaneous CT26 colon carcinoma tumor model. The irradiated tumor cell vaccine alone in prophylactic setting significantly delayed tumor growth, whereas anti‐TGF‐β antibodies alone did not show any antitumor effect. However, tumor growth was inhibited significantly more in vaccinated mice treated with anti‐TGF‐β antibodies compared to vaccinated mice without anti‐TGF‐β, suggesting that anti‐TGF‐β synergistically enhanced irradiated tumor vaccine efficacy. CD8^+^ T‐cell depletion completely abrogated the vaccine efficacy, and so protection required CD8^+^ T cells. Depletion of CD25^+^ T regulatory cells led to the almost complete rejection of tumors without the vaccine, whereas anti‐TGF‐β did not change the number of CD25^+^ T regulatory cells in unvaccinated and vaccinated mice. Though the abrogation of CD1d‐restricted NKT cells, which have been reported to induce TGF‐β production by MDSC through an IL‐13‐IL‐4R‐STAT6 pathway, partially enhanced antitumor immunity regardless of vaccination, abrogation of the NKT cell‐IL‐13‐IL‐4R‐STAT‐6 immunoregulatory pathway did not enhance vaccine efficacy. Taken together, these data indicated that anti‐TGF‐β enhances efficacy of a prophylactic vaccine in normal individuals despite their not having the elevated TGF‐β levels found in patients with cancer and that the effect is not dependent on TGF‐β solely from CD4^+^CD25^+^ T regulatory cells or the NKT cell‐IL‐13‐IL‐4R‐STAT‐6 immunoregulatory pathway.