Reversible Downregulation of HLA Class I in Adenoid Cystic Carcinoma

Purpose Adenoid cystic carcinoma (ACC), a rare and lethal cancer, has shown low response rates to systemic therapies, such as cytotoxic chemotherapy and immune-checkpoint inhibitors (ICIs). Despite numerous clinical trials, some employing aggressive ICI combinations, no effective treatments for patients with recurrent or metastatic adenoid cystic carcinoma have emerged, and ACC mortality rates remain stagnant. Therefore, we aimed to characterize the ACC immune landscape to understand the poor response rates to ICIs. Experimental Design We leveraged automated multiplex immunofluorescence (mIF), RNA in-situ hybridization, and scRNAseq Gene Expression analysis to identify pathways supporting the cold ACC immune environment and molecularly characterize ACC tumors, adjacent normal tissues, and normal tissues from regions where ACCs arise. In vitro, we treated freshly resected ACCs with interferon-gamma or a STING agonist. Results mIF demonstrated that ACC tumors are immunologically cold, with few tumor-infiltrating T-lymphocytes (TILs) and low PD-L1 expression. The most striking finding, however, was a very low HLA/B2M class I expression in almost all ACCs, which was reversible through treatment with interferon-gamma, or a STING agonist. mIF and RNAseq analyses of normal tissues revealed a p63+, NFIB+, basal duct cell population with similarly low HLA/B2M class I expression. Conclusions Low/absent HLA/B2M expression may explain the immunologically cold status of ACC tumors and their lack of response to ICIs. Our findings suggest that the normal cell of ACC origin exists in an HLA-low state, and that pharmacologic manipulation with immune activators, such as STING agonists, can restore HLA/B2M in ACCs, creating a path to urgently needed, effective immunotherapies.