RAS/MEK/PI3K pathway inhibition augments response to CD40 agonism by targeting CD11b+ Bregs thereby overcoming melanoma PD1-resistance

Development of effective second-line treatment options for patients with BRAFwtNRASwt or BRAFwtNRASmut melanoma resistant to immune checkpoint blockade (ICB) is crucial. While systemic delivery of agonist CD40 (aCD40) plus anti-PD1 (αPD1) showed activity in patients with ICB-resistant melanoma, the objective response rate was modest (15%), in part due to induction of B regulatory cells (Bregs) which suppress CD8+ effector T cell responses. We previously reported that RAS/RAF/PI3K-inhibition elevates CD40 expression in melanoma cells and sensitizes tumors to ICB. Here, we show that combined treatment with a RAS/PI3K/AKT-pathway inhibitor rigosertib (RGS), and/or a MEK1/2 inhibitor trametinib (T), plus aCD40, overcomes the ICB resistance of BRAFwtNRASwt and BRAFwtNRASmut melanoma tumors growing in C57BL/6 mice. In addition, overexpression of CD40 in these melanoma cells effectively reverses ICB-resistance and aCD40 + αPD1 treatment induces tumor regression. Mechanistically, RGS + T suppress aCD40-associated CD11b+PD-L1+ Bregs, promoting CD8+ T-cell mediated killing in melanoma. scRNA-Seq analyses confirm CD40-associated CD11b+ Bregs across cancer types in patients. Our data demonstrate that addition of RAS/PI3K/AKT and MEK inhibitors to aCD40 resolves the issue of aCD40 induction of CD11b+PD-L1+ Bregs and provides alternative therapeutic options for ICB-resistant BRAFwtNRASwt or BRAFwtNRASmut metastatic melanoma. Induction of CD11b-positive regulatory B cells and low expression of CD40 in melanoma cells have been associated with resistance to agonist CD40 (aCD40) and immune checkpoint blockade (ICB). Here the authors show that the addition of RAS/MEK/PI3K inhibitors to aCD40 abrogates these effects and reverses ICB resistance in preclinical melanoma models.