| Summary: | Successful immunotherapy for melanoma depends on the recruitment of effector CD8+ T cells to the tumor microenvironment. Factors contributing to T cell regulation in melanoma have recently been recognized, including the stimulator of interferon genes (STING). Agents that can activate STING or enhance T cell infiltration into established tumors have become an important focus for further clinical development. Talimogene laherparepvec (T-VEC) is an oncolytic herpes simplex virus, type 1 (HSV-1) encoding granulocyte-macrophage colony stimulating factor (GM-CSF) and is approved for the treatment of melanoma and has shown therapeutic activity in murine tumors known to express high levels of STING. The mechanism of action for T-VEC has not been fully elucidated but is thought to include induction of immunogenic cell death (ICD) and activation of host anti-tumor immunity. Thus, we sought to investigate how T-VEC mediates anti-tumor activity in a melanoma model. To determine if T-VEC induced ICD we established the relative sensitivity of a panel of melanoma cell lines to T-VEC oncolysis. Following T-VEC infection in vitro, melanoma cell lines released of HMGB1, ATP, and translocated ecto-calreticulin. To identify potential mediators of this effect, we found that melanoma cell sensitivity to T-VEC was inversely related to STING expression. CRISPR/Cas9-STING knockout was also associated with increased T-VEC cell killing. In the D4M3A melanoma, which has low expression of STING and is resistant to PD-1 blockade therapy, T-VEC was able to induce therapeutic responses in both injected and non-injected tumors and demonstrated recruitment of viral- and tumor-antigen specific CD8+ T cells, and induction of a pro-inflammatory gene signature at both injected and non-injected tumors. These data suggest that T-VEC induces ICD in-vitro and promotes tumor immunity and can induce therapeutic responses in anti-PD-1-refractory, low STING expressing melanoma.
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