Topical treatment of all-trans retinoic acid inhibits murine melanoma partly by promoting CD8 T-cell immunity.

All-trans retinoic acid (atRA), the main biologically active metabolite of vitamin A, has been implicated in immunoregulation and anti-cancer. A recent finding that vitamin A could decrease the risk of melanoma in humans indicates the beneficial role of atRA in melanoma. However, it remains unknown whether topical application of atRA could inhibit melanoma growth by influencing tumour immunity. We demonstrate topical application of tretinoin ointment (atRA as the active ingredient) effectively inhibited B16F10 melanoma growth. This is accompanied by markedly enhanced CD8 T-cell responses, as evidenced by significantly increased proportions of effector CD8 T cells expressing granzyme B, tumour necrosis factor-α, or interferon-γ, and Ki67 proliferating CD8 T cells in atRA-treated tumours compared with vaseline controls. Furthermore, topical atRA treatment promoted the differentiation of effector CD8 T cells in draining lymph nodes (DLN) of tumour-bearing mice. Interestingly, atRA did not affect tumoral CD4 T-cell response, and even inhibited the differentiation of interferon-γ-expressing T helper type 1 cells in DLN. Importantly, we demonstrated that the tumour-inhibitory effect of atRA was partly dependent on CD8 T cells, as CD8 T-cell depletion restored tumour volumes in atRA-treated mice, which, however, was still significantly smaller than those in vaseline-treated mice. Finally, we demonstrated that atRA up-regulated MHCI expression in B16F10 cells, and DLN cells from tumour-bearing mice had a significantly higher killing rate when culturing with atRA-treated B16F10 cells. Hence, our study demonstrates that topical atRA treatment effectively inhibits melanoma growth partly by promoting the differentiation and the cytotoxic function of effector CD8 T cells.