Reduced MHC class II expression in medullary thyroid cancer identifies patients with poor prognosis.

Increasing body of recent studies determining the expression of tumor-specific major histocompatibility complex (MHC) class II protein supports its potential role in several malignancies, but little is known in human medullary thyroid cancer (MTC). Here, we report the expression of MHC-II and its clinicopathologic and prognostic relevance in MTC patients. Immunohistochemistry staining revealed a significant reduction in tumor cell-specific MHC-II expression in a higher AJCC stage and its poor prognostic correlation with human MTC development. Further statistical analysis identified the low MHC-II expression as a significant and independent risk factor for MTC recurrence and patient survival. Moreover, in vitro studies showed that the MHC-II expression was remarkably increased by RET inhibitors, which were prescribed to treat advanced MTC. Similarly, inhibitors blocking the MAPK/ERK and AKT/mTOR pathways also augmented MHC-II expression, suggesting their implications in RET-MHC-II signaling axis. Importantly, in vitro assays manifested enhanced peripheral blood leukocytes-mediated cytotoxicity in MTC cells treated with RET inhibitors, which were partially alleviated by HLA knock-down. Together, our study demonstrates that low MHC-II expression levels may serve as a prognostic biomarker for aggressive diseases in MTC patients and indicates that RET activation may promote MTC immune escape through downregulating MHC-II expression.