Retained or altered expression of major histocompatibility complex class I in patient-derived xenograft models in breast cancer.

The expression of major histocompatibility complex class I (MHC I) in tumor cells is regulated by interferon signaling, and it is an important factor in the efficacy of cytotoxic T cell-dependent immunotherapy. To determine the impact of immune cells in MHC I expression on tumor cells, we compared the expression of MHC I in tumor cells derived from primary breast cancers and patient-derived xenograft (PDX) models. MHC I and myxovirus resistance gene A (MxA) expression were analyzed using immunohistochemistry in 23 cases of tumor tissue and corresponding primary and secondary PDXs. The median H score of MHC I was 210 (0-300) in patient tumor tissues, 197.5 (0-300) in primary PDX tumors, and 157.5 (5-300) in secondary PDX tumors. Cases were divided into four groups based on the difference in MHC I expression between the patient tumor tissues and secondary PDXs. Eleven cases constituted the high MHC I group, four constituted the low MHC I group, six comprised the decreased MHC I group, and two comprised the increased MHC I group. MHC I and MxA expressions in each tumor were weakly correlated within patients' tumors, while strongly correlated within PDX models. Retained or altered expression of MHC I in breast cancer PDXs reveals the presence of intrinsic and extrinsic interferon signaling pathways in tumor cells. Thus, considering MHC I expression in PDX is important when using PDX models to evaluate the efficacy of cancer immunotherapy in a preclinical setting.