Emergence of a mixed CAF population by FAP-CD3 T cell engager limits therapeutic efficacy

To unveil the complex changes that occur in the tumor microenvironment after FAP+ fibroblast depletion, we generated comprehensive single cell RNA-sequencing analysis of FAP+ CAFs depletion within the TME to understand the key population and genetic modulations in all cell subtypes.

Methods

A CD3 T cell engager directed against FAP (FAP TcE) was utilized to deplete FAP+ fibroblasts in a preclinical murine model of pancreatic cancer. To understand complex population dynamics upon FAP TcE, we performed single cell RNA-sequencing of treated vs. untreated tumors to unveil population and genetic changes. Results: Administration of FAP TcE resulted in tumor growth control in vivo that was not dependent on T cell priming and egress via draining lymph nodes.

After FAP TcE, T cell exhaustion was prevalent with increased T cell exhaustive state (Tex) and the emergence of a T cell progenitor exhausted state (Tpex), yet addition of anti-PD1 failed to enhance tumor efficacy. Within fibroblast populations, FAP TcE depleted FAP+ cancer associated fibroblasts (CAFs); however, depletion is compensated by an emergence of a “mixed CAF” population, potentially limiting the efficacy of FAP TcE.

Conclusion: This study highlights the complex and plastic fibroblast changes occurring with stroma targeted therapies that may limit therapeutic efficacy.