CDK5 Promotes Tumor Immune Evasion via PD-L1 Upregulation

Cyclin-dependent kinase 5 (CDK5) is a serine–threonine kinase that plays a key role in central nervous system (CNS) development and has been implicated in angiogenesis, apoptosis, myogenesis, and senescence, suggesting it as a potential therapeutic target in cancer. Dorand and colleagues investigated the role of CDK5 in medulloblastoma, which is a malignant pediatric CNS tumor. CDK5 was expressed in medulloblastoma cell lines and clinical specimens. Depletion did not affect medulloblastoma cell proliferation in vitro, but increased tumor-free survival in medulloblastoma xenografts, and tumors that developed were smaller than in wild-type mice. Further, in human medulloblastomas, CDK5 depletion was associated with increased T-cell infiltration, altogether suggesting that CDK5 deficiency may promote T cell–dependent tumor rejection. In mouse CDK5-deficient medulloblastoma xenografts, depletion of CD4⁺ T cells reversed the effects of CDK5 depletion, resulting in accelerated tumor growth and tumor development in all mice. Additionally, the majority of mice that rejected CDK5-deficient tumors remained tumor free even after rechallenge with medulloblastoma cells, altogether supporting a CD4⁺ T cell–mediated rejection of CDK5-deficient tumors and the generation of antitumor immune memory. Interferon-γ (IFNγ) is a CD4⁺ T-cell effector cytokine that induces PD-L1, and it was highly expressed in CDK5-deficient tumors. Consistent with these findings, CDK5-deficient tumors exhibited decreased PD-L1 expression and reduced upregulation of PD-L1 in response to IFNγ. In wild-type cells, IFNγ stimulation downregulated IRF2 and IRF2BP2, both of which are transcriptional repressors of PD-L1. In contrast, in CDK5-deficient cells, IRF2BP2 hyperphosphorylation led to increased abundance of IRF2 and IRF2BP2. In vivo, orthotopic injection of CDK5-deficient medulloblastoma cells resulted in increased PD-L1 expression and a 50% tumor incidence, compared with 100% incidence in CDK5-proficient tumors. Collectively, these findings reveal a role for CDK5 in immune checkpoint regulation and suggest that CDK5-deficiency may enhance CD4⁺ T cell–mediated tumor cell killing by reducing PD-L1 expression.

Dorand RD, Nthale J, Myers JT, Barkauskas DS, Avril S, Chirieleison SM, et al. Cdk5 disruption attenuates tumor PD-L1 expression and promotes antitumor immunity. Science 2016;353:399–403.

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