BMI1 promotes cholangiocarcinoma progression and correlates with antitumor immunity in an exosome-dependent manner.

Background: Cholangiocarcinoma (CCA) is a class of malignant tumors originating from bile duct epithelial cells. Due to difficult early diagnosis and limited treatment, the prognosis of CCA is extremely poor. BMI1 is dysregulated in many human malignancies. However, the prognostic significance and oncogenic role of BMI1 in cholangiocarcinoma (CCA) are not well elucidated.

Methods: In the present study, we investigated its clinical importance and the potential mechanisms in the progression of CCA. We detected BMI1 expression in a large CCA cohort. We demonstrated that BMI1 was substantially upregulated in CCA tissues and was identified as an independent prognostic biomarker of CCA. Moreover, overexpression of BMI1 promoted CCA proliferation, migration, and invasion. And BMI1 knockdown could inhibit proliferation and metastases of CCA in vitro and in vitro/vivo validation. Interestingly, we found that CCA-derived exosomes contain BMI1 proteins, which can transfer BMI1 between CCA cells. The unique BMI1-containing exosomes promote CCA proliferation and metastasis through autocrine/paracrine mechanisms. In addition, we demonstrated that BMI1 inhibits CD8T cell-recruiting chemokines by promoting repressive H2A ubiquitination in CCA cells.

Conclusions: BMI1 is an unfavorable prognostic biomarker of CCA. Our data depict a novel function of BMI1 in CCA tumorigenesis and metastasis mediated by exosomes. Besides, BMI1 inhibition may augment immune checkpoint blockade to inhibit tumor progression by activating cell-intrinsic immunity of CCA.