Immune Checkpoint B7x Promotes Immune Evasion and Resistance to PD-1/PD-L1 Blockade in Bladder Cancer
Cancer cells adopt multiple strategies to avoid detection and destruction by the immune system, including exploiting immune checkpoint pathways. B7x (B7-H4, B7S1, or VTCN1), a member of the B7/CD28 family, is frequently expressed in advanced bladder cancer (BC), yet its role in BC progression and resistance to therapy remains poorly understood. Resistance to PD-1/PD-L1 immune checkpoint blockade immunotherapy significantly limits durable responses, with only 20%–25% of patients with muscle-invasive BC achieving long-term benefits. Here, we demonstrated that B7x mRNA and protein expression were associated with poor survival outcomes in muscle-invasive BC patients and mouse models of BC. Stable expression of B7x in immune-competent BC mouse models resulted in enhanced tumor growth and splenomegaly, driven by the exclusion and suppression of tumor-infiltrating anti-tumor immune cells and the enrichment of pro-tumor and immunosuppressive cells. Consistently, in the IMvigor210 clinical trial, high B7x mRNA expression was correlated with poorer survival in muscle-invasive BC patients treated with PD-L1 blockade. Notably, combination therapy targeting B7x alongside PD-1/PD-L1 or CTLA-4 blockade reduced tumor burden and overcame resistance to monotherapy. These findings establish B7x as a substantial driver of immune evasion in BC and highlight its potential as a therapeutic target to improve immune checkpoint blockade efficacy in muscle-invasive BC. Overall design: Given the significant tumor growth-enhancing effect of B7x observed in BBN963 and, to a lesser extent, UPPL1541 tumors, but not in MB49, we conducted bulk RNA-seq on BBN963, UPPL1541, and MB49 cell lines (n = 5) to identify immunomodulatory factors contributing to their sensitivity to B7x.