| Summary |
Immune checkpoint blockade has shown tremendous anti-tumor potential in the clinic. However, these therapies are only effective in a subset of patients, so identification of additional immunomodulatory molecules that enhance the anti-tumor activity of these treatments may expand their clinical utility. In particular, identifying small molecules that complement existing immunotherapies has been relatively unexplored, so we performed a small molecule screen to identify compounds that can enhance co-inhibitory molecule blockade, to improve the anti-tumor adaptive immune response. Our unbiased screen identified inhibitors of cyclin-dependent kinase 4 and 6 (CDK4/6), including the FDA-approved palbociclib, as a class of small molecule compounds that exhibited significant immunostimulatory activity in vitro. In accordance with our in vitro finding of enhanced NFAT signaling, single-cell RNA-sequencing confirmed that in vivo exposure to CDK4/6 inhibitors enhanced NFAT signaling in tumor infiltrating T cells. Moreover, our results revealed that CDK4/6 inhibition up-regulated activation molecules and down-regulated suppressive molecules in these cells. CDK4/6 inhibition also increased the number of T cells with activated TCR (T cell receptor) signaling, as well as factors that are important for signal transduction downstream of TCR signaling. In summary, the impact of CDK4/6i on cell cycle progression and T cell proliferation are balanced favorably towards increased T cell recruitment and enhanced effector cell function, mediated in part by activation of the NFAT family of transcription factors. Further, our results demonstrate that CDK4/6i enhances PD-1 blockade through increased T-cell effector function and inhibition of immune suppressive cytokine production. While prolonged CDK4/6i treatment could be immunosuppressive due to adverse effects on lymphocyte proliferation, properly timed/sequenced CDK4/6i may potentiate the clinical impact of anti-PD-1/PD-L1 antibodies. As palbociclib is FDA-approved and multiple other CDK4/6 inhibitors are in clinical trials, we expect that this hypothesis will undergo rapid testing in humans.
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