Knockdown of CDK5 down-regulates PD-L1 via the ubiquitination-proteasome pathway and improves antitumor immunity in lung adenocarcinoma

• Knockdown of CDK5 down-regulates PD-L1 in lung adenocarcinoma and improves tumor immunity. • Interference of CDK5 leads to ubiquitination and degradation of PD-L1 protein. • TRIM21 mediates the ubiquitination and degradation process of PD-L1. • Combination of CDK5 disruption and anti-PD-L1 therapy has a stronger effect on inhibiting tumor formation, compared with CDK5 knockdown alone. Although immunotherapy (anti-PD-1/PD-L1 antibodies) has been approved for clinical treatment of lung cancer, only a small proportion of patients respond to monotherapy. Hence, understanding the regulatory mechanism of PD-L1 is particularly important to identify optimal combinations. In this study, we found that inhibition of CDK5 induced by shRNA or CDK5 inhibitor leads to reduced expression of PD-L1 protein in human lung adenocarcinoma cells, while the mRNA level is not substantially altered. The PD-L1 protein degradation is mediated by E3 ligase TRIM21 via ubiquitination-proteasome pathway. Subsequently, we studied the function of CDK5/PD-L1 axis in LUAD. In vitro , the absence of CDK5 in mouse Lewis lung cancer cell (LLC) has no effect on cell proliferation. However, the attenuation of CDK5 or combined with anti-PD-L1 greatly suppresses tumor growth in LLC implanted mouse models in vivo . Disruption of CDK5 elicits a higher level of CD3 + , CD4 + and CD8 + T cells in spleens and lower PD-1 expression in CD4 + and CD8 + T cells. Our findings highlight a role for CDK5 in promoting antitumor immunity, which provide a potential therapeutic target for combined immunotherapy in LUAD.