Persistence of residual basal cell carcinoma (BCC) cells in vismodegib-treated Ptch1+/-/SKH-1 mice is associated with treg accumulation

Summary: Hedgehog (Hh)-targeted therapies effectively reduce BCC burden in patients with Gorlin syndrome (GS). However, a major challenge with targeted Hh inhibitors (HHi) is their inability to fully eliminate tumor cells, thus driving significant tumor recurrence. Preclinical and clinical studies suggest that residual tumor cells refractory to HHi persist, underscoring the need for developing novel therapeutic strategies to achieve sustained remission. Prior research has revealed a complex immune landscape within the BCC tumor microenvironment in both humans and Ptch1^+/-/SKH-1 mice, a murine model that mimics multiple features of GS-associated BCC, suggesting that an altered immunogenic landscape may enhance tumor progression and inhibit treatment response. To further investigate BCC-associated immune cell composition, we conducted high-dimensional spectral flow cytometry using an optimized immune profiling panel. Compared to non-tumor-bearing Ptch1^+/-/SKH-1 skin, BCC-bearing skin exhibited substantial T cell infiltration, including CD8+ T cells, conventional CD4+ T cells, CD4+ regulatory T cells (Tregs), and γδT cells. Notably, immune profiling of Ptch1^+/- skin harboring residual microscopic BCCs revealed pronounced accumulation of CD4+ Tregs within the skin, whereas Treg levels in the draining lymph nodes were unchanged. Further analysis identified two distinct Treg subclusters enriched for activation markers including ICOS, CD39, and KLRG1, with substantial increases in ICOS+FOXP3+ cells in vismodegib-treated Ptch1^+/-/SKH-1 mice with residual BCCs. These findings highlight specific immune correlates that support the persistence of residual BCCs, pointing to Treg-mediated immunosuppression as a potential mechanism of HHi resistance. Our results suggest that combinatorial targeting of Tregs and the Hh pathway may enhance anti-tumor immunity, reduce residual disease, and enhance therapeutic GS efficacy. Arianna L. Kim¹, Casey R. Ager², Yucui Zhu¹, Collin Jugler², Mohammad Athar³, Benjamin Izar¹, David R. Bickers¹ 1. Columbia University, New York, NY, United States. 2. Mayo Clinic Arizona, Scottsdale, AZ, United States. 3. University of Alabama at Birmingham, Birmingham, AL, United States. UV Biology/Injury and Non-melanoma Cancers