Vaccine immunotherapy consisting of tumor antigens combined with an immune-enhancing adjuvant fosters cytotoxic T cell (CTL) proliferation. Clinically, polyI:C has been used as an adjuvant to enhance cancer vaccine protocols. However, according to its long history, polyI:C promotes inflammation that causes cytokine toxicity. Although checkpoint inhibitor immunotherapy has improved the prognoses of patients with progressive cancer, over 75% of patients continue to experience resistance to antibody (Ab) against anti-programmed cell death-protein 1 (PD-1) or its ligand, PD-L1 therapy. In most cases, patients suffer from adverse events resulting from inflammation during anti-PD-1/L1 Ab therapy, which is a serious obstacle to patients' quality of life. We have studied the functional properties of double-stranded (ds)RNA and polyI:C, and developed a nucleic acid adjuvant that barely induces a significant increase in the level of serum inflammatory cytokines in mouse models. This adjuvant, termed ARNAX, consists of DNA-capped dsRNA that specifies the endosomal target for Toll-like receptor 3 (TLR3) in dendritic cells (DCs). We expect that this adjuvant is safe for administration in elderly patients with cancer receiving immunotherapy. Here, we summarize the properties of ARNAX for immunotherapy in mice. We suggest that DC-priming is essential to induce anti-tumor immunity; neither exogenous inflammation nor the administration of tumor antigens is always a prerequisite for DC-mediated CTL proliferation. If our mouse data can be extrapolated to humans, ARNAX and the liberated endogenous tumor antigens may facilitate effect of current therapies on patients with therapy-resistant tumors.