Piezo1 facilitates optimal T cell activation during tumor challenge

Functional effector T cells in the tumor microenvironment (TME) are critical for successful anti-tumor responses. T cell anti-tumor function is dependent on their ability to differentiate from a naïve state, infiltrate into the tumor site, and exert cytotoxic functions. The factors dictating whether a particular T cell can successfully undergo these processes during tumor challenge are not yet completely understood. Piezo1 is a mechanosensitive cation channel with high expression on both CD4+ and CD8+ T cells. Previous studies have demonstrated that Piezo1 optimizes T cell activation and restrains the CD4+ regulatory T cell (Treg) pool in vitro and under inflammatory conditions in vivo. However, little is known about the role Piezo1 plays on CD4+ and CD8+ T cells in cancer. We hypothesized that disruption of Piezo1 on T cells impairs anti-tumor immunity in vivo by hindering inflammatory T cell responses. We challenged mice with T cell Piezo1 deletion (P1KO) with tumor models dependent on T cells for immune rejection. P1KO mice had the more aggressive tumors, higher tumor growth rates and were unresponsive to immune-mediated therapeutic interventions. We observed a decreased CD4:CD8 ratio in both the secondary lymphoid organs and TME of P1KO mice that correlated inversely with tumor size. Poor CD4+ helper T cell responses underpinned the immunodeficient phenotype of P1KO mice. Wild type CD8+ T cells are sub-optimally activated in vivo with P1KO CD4+ T cells, taking on a CD25loPD-1hi phenotype. Together, our results suggest that Piezo1 optimizes T cell activation in the context of a tumor response.

Detailing the mechanism of CDK5-mediated PD-L1 regulation in Medulloblastoma

Novel therapies for the pediatric brain tumor medulloblastoma (MB) are hindered by limited knowledge of the roles played by host-derived cells at the local tumor site. Immune resistance in a murine model of MB (MM1) is associated with enhanced tumor expression of programmed death ligand-1 (PD-L1) in response to anti-tumor cytokine interferon-gamma (IFNγ) signaling. Disrupting this pathway via knockdown of cyclin dependent kinase 5 (CDK5), an essential transducer of the IFNγ signal, leads to an inflammatory tumor microenvironment (TME) and enhanced tumor rejection in vivo. In this study, we sought to detail the mechanism of CDK5-mediated PD-L1 regulation.

In response to IFNγ, PD-L1 expression is thought to be regulated at the promoter level by competition between the transcriptional activator IRF1 and repressor IRF2. We hypothesized that CDK5-IFNγ signaling inhibits IRF2 activity, resulting in unleashed PD-L1 promoter activity. We assessed IRF1/IRF2-DNA interactions using chromatin immunoprecipitation (ChIP) and gene reporter assays, with complementary analysis of PD-L1 transcription and surface expression with IRF1/IRF2 depletion.

ChIP-qPCR revealed a similar, IFNγ-induced enrichment of both IRF1 and IRF2 at the PD-L1 promoter. This is a surprising result, given the assumption that they compete for the same binding site and exert opposing effects. Additionally, inducible PD-L1 promoter activity was found to be decreased in tumors with IRF2 depletion. Together, these results suggest that IRF2 may be necessary for full activity of the PD-L1 promoter in MB. IRF1/IRF2 also bind to other cis-regulatory sites at genes that contribute to tumor immunogenicity, necessitating genome-wide characterization of their interactions.