T Cell-Intrinsic CDK6 Is Dispensable for Anti-Viral and Anti-Tumor Responses In Vivo

Abstract

The cyclin-dependent kinase 6 (CDK6) regulates the transition through the G1-phase of the cell cycle, but also acts as a transcriptional regulator. As such CDK6 regulates cell survival or cytokine secretion together with STATs, AP-1 or NF-κB. In the hematopoietic system, CDK6 regulates T cell development and promotes leukemia and lymphoma. CDK4/6 kinase inhibitors are FDA approved for treatment of breast cancer patients and have been reported to enhance T cell-mediated anti-tumor immunity. The involvement of CDK6 in T cell functions remains enigmatic. We here investigated the role of CDK6 in CD8+ T cells, using previously generated CDK6 knockout (Cdk6-/-) and kinase-dead mutant CDK6 (Cdk6K43M) knock-in mice. RNA-seq analysis indicated a role of CDK6 in T cell metabolism and interferon (IFN) signaling. To investigate whether these CDK6 functions are T cell-intrinsic, we generated a T cell-specific CDK6 knockout mouse model (Cdk6fl/fl CD4-Cre). T cell-intrinsic loss of CDK6 enhanced mitochondrial respiration in CD8+ T cells, but did not impact on cytotoxicity and production of the effector cytokines IFN-γ and TNF-α by CD8+ T cells in vitro. Loss of CDK6 in peripheral T cells did not affect tumor surveillance of MC38 tumors in vivo. Similarly, while we observed an impaired induction of early responses to type I IFN in CDK6-deficient CD8+ T cells, we failed to observe any differences in the response to LCMV infection upon T cell-intrinsic loss of CDK6 in vivo. This apparent contradiction might at least partially be explained by the reduced expression of Socs1, a negative regulator of IFN signaling, in CDK6-deficient CD8+ T cells. Therefore, our data are in line with a dual role of CDK6 in IFN signaling; while CDK6 promotes early IFN responses, it is also involved in the induction of a negative feedback loop. These data assign CDK6 a role in the fine-tuning of cytokine responses.

Targeting protein tyrosine phosphatases for CDK6-induced immunotherapy resistance

“…This suggests that specific depletion of CDK6 in the TME triggers different mechanisms from dual CDK4/6 inhibition to regulate immune cell subpopulations and their activities to augment the anti-tumor response. While it has been reported that CDK6 depletion in T cells alone using the CD4-CRE system is not sufficient to suppress tumor growth, 50 our study showed that CDK6 depletion in all types of cells in the TME inhibited tumor growth via enhancing T cell activity and memory T cell differentiation. CDK6-depleted T cells may need priming from other types of CDK6-depleted immune cells to efficiently suppress tumor growth.…”

“…Intrinsic CDK6 deletion in T cells using the CD4-CRE system does not inhibit tumor growth. 50 Lack of costimulatory signaling may lead to immunological ignorance, as evidenced by several T cell receptor (TCR) transgenic models, while the anti-tumor effects are significantly enhanced when transgenic T cells are activated in vitro before cells are adoptively transferred into the host. 51 – 54 Thus, to determine whether CDK6 depletion in TCR-activated T cells enhances tumor suppression efficacy after adoptive transfer, we generated CDK6 KO OT-I mice by crossing OT-I transgenic mice with CDK6 KO mice.…”

“…It is known that TCR activation leads to memory T cell differentiation, 74 , 75 which may explain our finding that targeting the CDK6-PTP1B/TCPTP-CD3ζ axis not only increases T cell activity but also promotes memory T cell differentiation. One study, in which phorbol myristate acetate (PMA)/ionomycin was used to activate naive T cells, 50 showed that CDK6 depletion does not regulate protein kinase C (PKC)-stimulated T cell activation. PMA/ionomycin stimulation bypasses early T cell activation steps (i.e., the TCR-CD3 complex and CD3ζ phosphorylation) to directly stimulate a downstream regulator, PKC, to activate T cells.…”

STAT3/LKB1 controls metastatic prostate cancer by regulating mTORC1/CREB pathway

“…Cells were washed once before being equilibrated in a non-CO 2 incubator. A Mito Stress test was performed as previously described [ 73 ]. The drugs were used in the following final concentrations: oligomycin, 1.5 μM; carbonyl cyanide-p-trifluoromethoxyphenylhydrazone (FCCP), 2 μM; rotenone + antimycin A, 1 μM.…”

Cell Cycle Entry Control in Naïve and Memory CD8+ T Cells

“…Treatment of CD8 + T cells with CDK4/6 inhibitors enhances NFAT activity, increasing expression of CD25 and production of IL-2 and granzyme B, indicating that CDK6 may have a role in limiting T cell effector differentiation (Deng et al, 2018). Indeed, loss of CDK6 in T cells led to an increase in IL-2 production, and was also seen to impair type I interferon signalling events and increased metabolic processes resulting in enhanced ATP production and maximal respiration in addition to affecting proliferation (Klein et al, 2021).…”