Reversing myeloid-derived suppressor cells mediated immunosuppression via p38α inhibition enhances immunotherapy efficacy in triple negative breast cancer

Infiltration of myeloid-derived suppressor cells (MDSCs) leads to Immunosuppressive tumor microenvironment (TME), which is one of the major causes for low objective response rates of immune checkpoint blockade (ICB) therapy. Here, we report that chemical inhibition of p38 reverses this MDSC-induced immunosuppressive TME and improves the immunotherapy efficacy in triple negative breast cancer (TNBC). Firstly, by combining the tumor immunological phenotype (TIP) gene signature and high throughput sequencing based high throughput screening (HTS2), we identified that ponatinib significantly inhibits the expression of "cold" tumor associated chemokines CXCL1 and CXCL2 in cancer cells. This inhibition decreases the infiltration of MDSCs and consequently increased the accumulation of "hot" tumor associated T cells and NK cells and thus reverses the immunosuppressive TME. Then, by multiple preclinical models, we found that ponatinib significantly inhibits tumor growth in a TME-dependent manner and enhances the efficacy of anti-PD-L1 immunotherapy on TNBC in vivo. Notably, ponatinib exhibits no significant inhibition on immune cells in mouse spleens. Mechanistically, ponatinib directly inhibits the kinase activity of p38, which results in the reduction of the phosphorylation of STAT1 at Ser727, and thus the decreased expression of CXCL1 and CXCL2 in cancer cells. Our study provided the therapeutic potential of combining p38 inhibition with ICB for the treatment of TNBC.