Loss of tumor suppressors promotes a non-cell-autonomous inflammatory tumor microenvironment and enhances LAG3+T cell mediated immune suppression [RNA_seq_Cas9_PT]

Low response rate, treatment relapse, and resistance remain key challenges in the application of immune checkpoint blockade (ICB). Here we report that loss of specific tumor suppressors (TSs) induces an inflammatory response and promotes an immune suppressive tumor microenvironment. Importantly, low expression of these TSs is associated with a higher expression of immune checkpoint inhibitory mediators. Using CRIPSR/Cas9 in vivo loss-of-function screening, we identified NF1, TSC1, and TGF-b RII as TSs that regulate immune composition. Loss of each of these three TSs leads to alterations in chromatin accessibility and enhances IL6-JAK3-STAT3/6 inflammatory pathways. This results in an immune suppressive landscape characterized by increased LAG3+ CD8 and CD4 T cells. ICB targeting LAG3 in combination with PD-L1 decreased metastatic progression in preclinical triple negative breast cancer (TNBC) mouse models of NF1-, TSC1- or TGF-b RII- deficient tumors. Together, our studies identify LAG3 as a new ICB target for patients whose cancer displays inactivation of these TSs. In addition, our studies reveal an unexpected role for TSs in the regulation of metastatic spread via non-cell-autonomous modulation of the immune compartment.