Investigating the role of Cdk5 and p35 in natural killer cell cytotoxicity

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PROJECT SUMMARY/ABSTRACT Natural killer (NK) cells are cytotoxic lymphocytes with important immune functions in killing virally infected cells and cancer cells. NK cells have been explored for cancer immunotherapy and have advantages over T cell- based therapies. However, their cytotoxicity and tumor immunosurveillance functions are often dysfunctional in cancer patients, in large part due to elevated levels of TGF-β, a potent immunosuppressive cytokine. Cyclin- dependent kinase 5 (Cdk5) is a Cdk family proline-directed serine/threonine kinase. Unlike other Cdk members, its kinase activity is primarily dependent on binding of the coactivator p35 or p39, and it has unclear cell cycle roles. Cdk5 was thought to primarily function in neuronal cells, but recent research has discovered new roles for Cdk5 and p35 in other cell types, including cancer and immune cells. For the first time, we have discovered that Cdk5 and p35 protein are both expressed in NK cells and appear to play an important role in regulating NK cell cytotoxicity. Additionally, TGF-β appears to induce p35 expression in NK cells in a dose-dependent manner. Based on our preliminary data, we hypothesize that Cdk5/p35 kinase activity negatively regulates NK cell cytotoxicity and is a key mediator of TGF-β-induced NK cell dysfunction, and we also hypothesize that Cdk5/p35 inhibition can be utilized to enhance NK cell immunotherapy. First, we will explore how the Cdk5/p35 and TGF- β signaling pathways overlap in NK cells. Using genetic tools to knock down p35, as well as the selective Cdk5 inhibitor roscovitine, we will determine whether Cdk5-inhibited NK cells can mitigate the various phenotypic changes caused by TGF-β treatment. We will measure any changes in the expression of NK cell activating/inhibitory receptors, lytic granule cytotoxic enzymes, and cytokine release. We will also determine how TGF-β induces p35 expression in NK cells, then investigate the molecular mechanism of how Cdk5/p35 activity regulates NK cytotoxicity. Whole transcriptome sequencing of p35 knockdown NK cells will be used to reveal differentially expressed pathways downstream of Cdk5 kinase signaling. We also wish to explore the therapeutic potential of Cdk5/p35 inhibition in enhancing NK cell immunotherapy. Using in vitro cytotoxicity assays against cancer cell lines, we will determine whether p35 knockdown will enable NK cells to resist TGF-β-induced suppression of cytotoxicity. Similarly, using established patient-derived xenograft mouse models, we will test whether p35 knockdown NK cells are able to enhance NK cell adoptive therapy against patient-derived B cell acute lymphoblastic leukemia (B-ALL), which is known to cause NK dysfunction through elevated TGF-β secretion. Discoveries from this project would advance our basic understanding of the signaling pathways that regulate NK cytotoxicity and mediate NK dysfunction, potentially leading to improved NK cell-based cancer immunotherapies.