Remodeling of the Tumor Microenvironment Through PAK4 Inhibition Sensitizes Tumors to Immune Checkpoint Blockade

PAK4 inhibition can sensitize tumors to immune checkpoint blockade (ICB) therapy; however, the underlying mechanisms remain unclear. We re-port that PAK4 inhibition reverses immune cell exclusion by increasing the infiltration of CD8 T cells and CD103+ dendritic cells (DC), a specific type of DCs that excel at cross-presenting tumor antigens and constitute a source of CXCL10. Interestingly, in melanoma clinical datasets, PAK4 expression levels negatively correlate with the presence of CCL21 , the ligand for CCR7 expressed in CD103+ DCs. Furthermore, we extensively characterized the transcriptome of PAK4 knockout (KO) tumors, in vitro and in vivo , and established the importance of PAK4 expression in the regulation of the extracellular matrix, which can facilitate immune cell infiltration. Compar-ison between PAK4 wild type and KO anti-PD-1 treated tumors revealed how PAK4 deletion sensitizes tumors to ICB from a transcriptomic per-spective. In addition, we validated genetically and pharmacologically that inhibition of PAK4 kinase activity is sufficient to improve antitumor efficacy of anti-PD-1 blockade in multiple melanoma mouse models. Therefore, this study provides novel insights into the mechanism of action of PAK4 inhi-bition and provides the foundation for a new treatment strategy that aims to overcome resistance to PD-1 blockade by combining anti-PD-1 with a small-molecule PAK4 kinase inhibitor.Significance: Our findings provide new insights into PAK4 inhibition mechanism of action as well as the scientific foundation for specifically blocking PAK4 kinase activity using a novel and specific PAK4 kinase inhibitor to overcome resistance to PD-1 blockade.