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 report 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. Comparison between PAK4 wild type and KO anti-PD-1 treated tumors revealed how PAK4 deletion sensitizes tumors to ICB from a transcriptomic perspective. 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 inhibition 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.