Characterizing ATF6 activation-driven mechanisms improving ICB efficacy

Melanoma is the most fatal skin cancer and incidence continues to rise, particularly among young people. Among the current clinical standards of care for advanced melanoma, immune checkpoint blockade (ICB) therapy has been the most successful for improving the survival of melanoma patients. However, the benefits of ICB therapy are limited to a subset of melanoma patients, with only 50% of melanoma patients responding ICB due to primary resistance. Using a genetic model of activating transcription factor 6 (ATF6), our laboratory has found that constitutively active ATF6 improved ICB efficacy in multiple preclinical, ICB resistant mouse melanoma models and increased T cell mediated-killing (TCK) of melanoma tumor cells in vitro . As a translational strategy we used a previously reported small molecule ATF6 activator, AA147, and found an AA147-mediated increase in TCK in vitro and an improvement of ICB efficacy in preclinical ICB resistant subcutaneous and metastatic mouse melanoma models. To identify ATF6-dependent mechanisms improving anti-tumor immune response, we performed FactorPath (Active Motif) chromatin immunoprecipitation coupled with sequencing (ChIP-seq) to map the binding of FLAG-ATF6 at a genome-wide scale. We found that the constitutively active form of ATF6 binds to the promoter region of anti-tumor immune regulatory genes. In parallel with ChIP-seq findings, we found constitutively active ATF6 significantly upregulates the protein expressions of anti-tumor immune regulatory genes as determined through quantitative mass spectrometry. These findings indicate that ATF6 is a transcriptional regulator of anti-tumor immune regulatory genes in melanoma tumor cells and upregulation of these pathways leads to enhanced anti-tumor immune response and improved ICB efficacy.