In situ detection of individual classical MHC-I gene products in cancer.

Tumor-specific HLA class I expression is required for cytotoxic T-cell elimination of cancer cells expressing tumor-associated or neo-antigens. Cancers downregulate antigen presentation to avoid adaptive immunity. The highly polymorphic nature of the genes encoding these proteins, coupled with quaternary-structure changes after formalin fixation, complicate detection by immunohistochemistry. In this study we determined recognition of 16 specific HLA-A, B and C alleles by 15 antibodies commercially available for immunohistochemical use, identifying and validating pan and specific HLA-A, B, and C antibodies, providing a validated method that can be applied to investigate HLA-A, B and C molecule-specific loss in cancer. We applied this approach to a series of breast cancers as a proof of utility, identifying differential HLA-A, B and C loss, with a higher incidence of HLA-A and B loss in hormone-driven breast cancers, HLA-B loss in HER2+ cancers, and an equal loss of all three molecules in triple-negative disease. Additionally, we found that at the protein level, HLA-A and B loss were early events prevalent in premalignant lesions, while HLA-C loss was less common throughout tumor evolution. Effective response to immunotherapies such as checkpoint inhibitors and MHC-I-targeted cancer vaccines, which hinge on the carriage of specific allele groups, require MHC-I expression on tumor cells. These findings have implications for the success of checkpoint inhibitors and vaccine strategies.