FGFR3 alterations in bladder cancer stimulate serine synthesis to induce immune-inert macrophages that suppress T-cell recruitment and activation.

Fibroblast growth factor receptor 3 (FGFR3) alterations are common in patients with bladder cancer (BCa). While the FGFR tyrosine kinase inhibitor erdafitinib has been approved as a targeted therapy for FGFR3-altered (aFGFR3) BCa patients, the response rate remains suboptimal, prompting development of strategies to improve treatment response. Here, we observed an immune-desert tumor microenvironment (TME) phenotype in human aFGFR3 BCa and demonstrated that mutant FGFR3 indirectly induces a "cold" TME in mouse BCa models. Single-cell RNA sequencing (scRNA-seq) revealed the central role of macrophages in inducing the cold TME of aFGFR3 tumors. Macrophages in aFGFR3 tumors exhibited reduced T cell recruitment and antigen presentation capabilities. Increased serine synthesis in BCa cells that was induced by mutant FGFR3 activated the PI3K/Akt pathway in macrophages, shifting them to an immune-inert phenotype. Targeting PI3K in aFGFR3 tumors with duvelisib achieved promising efficacy by reversing the macrophage phenotype, and combination therapy with duvelisib and erdafitinib demonstrated increased antitumor activity. Overall, these findings reveal the critical role of enhanced serine synthesis efflux from cancer cells with mutant FGFR3 in shifting macrophages to an immune-inert phenotype. Reversing the macrophage phenotype holds promise for enhancing erdafitinib efficacy.