The A2b receptor (A2bR) belongs to the
adenosine receptor family. Emerging evidence suggest that A2bR is implicated in
tumor progression in some murine
tumor models, but the therapeutic potential of targeting A2bR in
melanoma has not been examined. This study first shows that
melanoma-bearing mice treated with
Bay 60-6583, a selective A2bR agonist, had increased
melanoma growth. This effect was associated with higher levels of immune regulatory mediators
interleukin-10 (IL-10) and
monocyte chemoattractant protein 1 (MCP-1) and accumulation of
tumor-associated CD11b positive Gr1 positive cells (CD11b(+)Gr1(+)) myeloid-derived suppressor cells (MDSCs). Depletion of CD11b(+)Gr1(+) cells completely reversed the protumor activity of
Bay 60-6583. Conversely, pharmacological blockade of A2bR with
PSB1115 reversed immune suppression in the tumor microenvironment, leading to a significant
melanoma growth delay.
PSB1115 treatment reduced both levels of
IL-10 and MCP-1 and CD11b(+)Gr1(+) cell number in
melanoma lesions. These effects were associated with higher frequency of
tumor-infiltrating CD8 positive (CD8(+)) T cells and natural killer T (NKT) cells and increased levels of T helper 1 (Th1)-like
cytokines. Adoptive transfer of CD11b(+)Gr1(+) cells abrogated the antitumor activity of
PSB1115. These data suggest that the antitumor activity of
PSB1115 relies on its ability to lower accumulation of
tumor-infiltrating MDSCs and restore an efficient antitumor T cell response. The antitumor effect of
PSB1115 was not observed in
melanoma-bearing nude mice. Furthermore,
PSB1115 enhanced the antitumor efficacy of
dacarbazine. These data indicate that A2bR antagonists such as
PSB1115 should be investigated as adjuvants in the treatment of
melanoma.