Addition of TLR9 agonist immunotherapy to radiation improves systemic antitumor activity

•High-dose RT upregulated pDCs within the tumor microenvironment.•The administration of intratumoral TLR9 agonist (CMP-001) after stereotactic RT significantly enhanced the anti-tumor immune response both locally and at secondary tumor site.•CMP-001 Post-RT delayed the abscopal tumor growth and extended the survival rate via increasing the percentages of activated CD4+ and CD8+ T-cells within the tumor microenvironment.•The treatment proved efficacious in both lung adenocarcinoma and colon carcinoma syngeneic models used. Radiotherapy (RT) has been used to control tumors by physically damaging DNA and inducing apoptosis; it also promotes antitumor immune responses via neoantigens release and augmenting immune-oncology agents to elicit systemic response. Tumor regression after RT can recruit inflammatory cells, such as tumor-associated macrophages and CD11b+ myeloid cell populations, a major subset of which may actually be immunosuppressive. However, these inflammatory cells also express Toll-like receptors (TLRs) that can be stimulated to reverse suppressive characteristics and promote systemic antitumor outcomes. Here, we investigated the effects of adding CMP-001, a CpG-A oligodeoxynucleotide TLR9 agonist delivered in a virus-like particle (VLP), to RT in two murine models (344SQ metastatic lung adenocarcinoma and CT26 colon carcinoma). High-dose RT (12Gy x 3 fractions) significantly increased the percentages of plasmacytoid dendritic cells within the tumor islets 3- and 5-days post-RT; adding CMP-001 after RT also enhanced adaptive immunity by increasing the proportion of CD4+ and CD8+ T cells. RT plus CMP-001-mediated activation of the immune system led to significant inhibition of tumor growth at both primary and abscopal tumor sites, thereby suggesting a new combinatorial treatment strategy for systemic disease. [Display omitted]