Background Patients with basal cell nevus syndrome (BCNS) - a rare genetic disease characterized by widespread hedgehog signaling-driven basal cell carcinomas (BCCs) – have high rates of recurrence following treatment with the Smoothened (SMO) inhibitor vismodegib. We sought to identify and evaluate the combination of vismodegib and immunotherapy as an enhanced targeted option for BCNS patients with excessive tumor burden.

Methods We leveraged a clinically relevant genetically engineered murine model of UVB-induced BCNS (Ptch1^+/- /SKH-1) to examine the therapeutic effects of vismodegib, anti-PD-1 blockade, and the combination regimen. Mice were UV-irradiated three times weekly for 18 weeks to induce BCCs, followed by three weeks of therapy with vismodegib and/or anti-PD-1. Following treatment, we evaluated tumor burden in dorsal skin specimens by both histological appearance and positive staining for β-galactosidase. We additionally isolated skin-infiltrating immune populations for immunophenotyping using a 37-parameter spectral flow cytometry panel. We utilized an established high dimensional analytical workflow to comprehensively map the immune contexture of UVB-induced BCCs in Ptch1^+/- /SKH-1 mice at baseline and in response to each therapy.

Results We found that UVB-induced BCC tumorigenesis is associated with immune infiltration into tumor-associated skin and upregulation of PD-L1 on skin myeloid populations, providing rationale for use of PD-(L)1 blockade in BCNS. Both vismodegib and anti-PD-1 exhibited single agent therapeutic efficacy against established UVB-induced BCCs, and we identified a trend toward additive therapeutic efficacy following the combined use of these agents. Mechanistically, we observed disparate impacts on the immune microenvironment by vismodegib and anti-PD-1. Single agent vismodegib expanded regulatory T cell (Treg) frequencies in tumor-associated skin, while anti-PD-1 elicited increases in skin γδT cell frequencies. When combined, we observed a distinct effect compared to monotherapies characterized by significant expansion in tumor-associated CD8 T cells enriched in classical CD103⁺CD69⁺ resident memory (T_RM) phenotypes.

Conclusions Our results support the concept that combinatorial targeted therapy and immunotherapy enhance anti-tumor efficacy in patients with BCNS and warrant further investigation for this approach in both pre-clinical and clinical settings.