(622) FGFR2 Modulates T-Cell Cytotoxicity Through Regulation of PD-L1 Expression in cSCC1 Cell Line

Introduction: Cutaneous squamous cell carcinoma (cSCC) accounts for nearly half of all US skin cancers, with rising costs and incidence necessitating a deeper look into the mechanisms of tumor growth. Our laboratory previously established that epidermal PD-L1 expression is functionally dependent on fibroblast growth factor receptor 2 (FGFR2) signaling.

Methods: We evaluated PD-L1 modulation in cSCC1 cells via western blot following treatment with the selective FGFR2 inhibitor RLY4008. To assess T-cell function, activated splenic T-cells were co-cultured with RLY4008-pretreated cSCC1 cells at a 5:1 effector-to-target ratio. Flow cytometry was used to quantify TNF-α, IFN-γ, and granzyme-B expression in CD4+ and CD8+ populations. Direct cytotoxic potential was further validated using H33342-labeled targets. Complementary in vivo studies utilized a short-term murine UVB-irradiation model to determine the acute impact of FGFR inhibition (AZD4547) on the recruitment and PD-1 expression of tumor-infiltrating immune cells.

Results: RLY4008-mediated FGFR2 inhibition significantly downregulated PD-L1 expression in cSCC1 cells (p = 0.0017). In syngeneic co-cultures, this inhibition led to an increase in IFN-γ (p = 0.004) and granzyme-B (p = 0.01) in CD4+ T-cells, with similar effect observed in the CD8+ cells. These functional changes translated into significantly higher T-cell mediated cytotoxicity (p = 0.007). In our short-term in vivo model, acute UVB exposure triggered a profound expansion of PD-1+ immune cells (~15%); however, pharmacological FGFR2 inhibition using RLY-4008 blunted this recruitment and significantly attenuated the early expression of PD-1.

Conclusions: These findings suggest that FGFR2 activity is a requisite for the PD-L1-mediated immune evasion that characterizes cSCC. By reversing the acute immunosuppressive effects of UVB and enhancing T-cell cytotoxic potential, FGFR2 inhibition represents a potent therapeutic strategy. This data provides a strong mechanistic rationale for targeting FGFR signaling to prevent the progression of UV-damaged skin into invasive, immune-evasive cSCC.