IL6 produced by tumor cells promotes their survival, conferring a poor prognosis in patients with cancer. IL6 also contributes to immunosuppression of CD4(+) T cell-mediated antitumor effects. In this study, we focused on the impact of IL6 trans-signaling mediated by soluble IL6 receptors (sIL6R) expressed in tumor-bearing hosts. Higher levels of sIL6R circulating in blood were observed in tumor-bearing mice, whereas the systemic increase of sIL6R was not prominent in tumor-bearing mice with myeloid cell-specific conditional deletion of IL6R even when tumor cells produced sIL6R. Abundant sIL6R was released by CD11b(+) cells from tumor-bearing mice but not tumor-free mice. Notably, IL6-mediated defects in Th1 differentiation, T-cell helper activity for tumor-specific CD8(+) T cells, and downstream antitumor effects were rescued by myeloid-specific deletion of sIL6R. Expression of the T-cell transcription factor c-Maf was upregulated in CD4(+) T cells primed in tumor-bearingmice in an IL6-dependent manner. Investigations with c-Maf loss-of-function T cells revealed that c-Maf activity was responsible for IL6/sIL6R-induced Th1suppression and defective T-cell-mediated antitumor responses. In patients with cancer, myeloid cell-derived sIL6R was also possibly associated with Th1 suppression and c-Maf expression. Our results argued that increased expression of sIL6R from myeloid cells and subsequent c-Maf induction were adverse events for counteracting tumor-specific Th1 generation. Overall, this work provides a mechanistic rationale for sIL6R targeting to improve the efficacy of T-cell-mediated cancer immunotherapy. (C) 2017 AACR.