STING signaling induces interferon-beta (IFNbeta) production by intratumoral dendritic cells (DCs), driving T-cell priming and recruitment into the tumor microenvironment (TME). We examined to what extent pre-existing antigen tolerance influenced the efficacy of in situ delivery of a potent STING-activating cyclic dinucleotide (CDN), ADU S-100, against established HER-2+ breast tumors. ADU S-100 induced HER-2-specific CD8+ T-cell priming and durable tumor clearance in 100% of nontolerant parental FVB/N mice. In contrast, ADU S-100 did not sufficiently prime HER-2-specific CD8+ T cells in tolerant neu/N mice, resulting in only delayed tumor growth and tumor clearance in 10% of the mice. No differences in IFNbeta production, DC priming, or HER-2-specific CD8+ T-cell trafficking were detected between FVB/N and neu/N mice. However, activation and expansion of HER-2-specific CD8+ T cells was defective in neu/N mice. Immune cell infiltrates of untreated tumor-bearing neu/N mice expressed high numbers of PD1 and OX40 receptors on their CD8+ T cells, and PD-L1 was highly expressed on both myeloid and tumor cells. Modulating PD-L1 and OX40 receptor signaling combined with intratumoral ADU S-100 administration enhanced HER-2-specific CD8+ T-cell activity, clearing tumors in 40% of neu/N mice. Thus, intratumoral STING agonists could potently prime tumor antigen-specific CD8+ T-cell responses, and adding PD-L1 blockade and OX40 receptor activation can overcome antigen-enforced immune tolerance to induce tumor regression.