Immune checkpoint blockade therapies constitute one of the major advances in cancer treatment. Despite the long-lasting responses observed in a wide range of tumor types, the majority of patients do not respond or relapse shortly after. In order to increase response rates, we need to better predict patients that would benefit from the treatment as well as identify resistance mechanisms to find novel treatment strategies. However, the interplay between the immune system, the cancer cells and the tumor microenvironment is complex and dynamic, rendering the understanding of resistant mechanisms particularly challenging. Lack of immune cell infiltration constitutes one of the main mechanisms of primary resistance to checkpoint blockade. The absence of T cells on the tumor margin leaves the therapeutic targeting of immune checkpoints ineffective. Interestingly, cancer cell-intrinsic mechanisms could actively participate in the process of immune evasion and importantly, could be pharmacologically targeted to reverse immune exclusion. The finding of actionable molecules that increases T cell infiltration in the tumor could be used in combination with PD-1 blockade, in order to reverse adaptive immune resistance. Here, we characterized the tumors of melanoma patients treated with PD-1 blockade. Our analyses further validated lack of immune cell infiltration as one mechanism of resistance. Importantly, we identified PAK4 as a novel and actionable target that is enriched in biopsies with poor immune infiltration and lack of response to PD-1 blockade. We show how genetically and pharmacologically that inhibition of PAK4 increases immune cell infiltration and improves checkpoint blockade therapy in several in vivo mouse models. We further characterized the impact of inhibiting PAK4 expression. The transcriptomic profiles of PAK4 KO tumors revealed the profound effect that has on the tumor microenvironment, particularly in the genes related to the extracellular matrix and blood vessel formation. This work serves as an example of how direct changes in cancer cells impact the tumor microenvironment and influence the anti-tumor immune response. Importantly, this work also provides the scientific rationale for a novel treatment strategy to combine PAK4 inhibitor with checkpoint blockade
