Hedgehog-Hippo pathway interactions promote T cell exclusion from the tumor microenvironment in basal cell carcinoma
Basal cell carcinoma (BCC) is the most common non-melanoma skin cancer driven primarily by genetic activation of the Hedgehog/GLI (HH/GLI) signaling pathway. BCC also displays a high mutational burden, with frequent co-occurrence of mutations in the TP53, NOTCH, N- MYC, and Hippo/YAP pathways, though the impact of these mutations on HH/GLI-driven tumorigenesis remains poorly understood. This study examines the interaction between the HH/GLI and Hippo/YAP signaling pathways in the pathogenesis of BCC and the establishment of an immunosuppressive tumor microenvironment (TME). The study used 3D human organotypic skin models and humanized mouse models to demonstrate that co-activation of HH/GLI and Hippo/YAP in epidermal cells suppresses T cell chemotaxis. Transcriptome analysis revealed a significant downregulation of chemotactic and inflammatory genes, particularly under combined GLI-YAP activation. Analysis of primary human BCC biopsies confirmed these findings and demonstrated a marked suppression of T cell chemo-attractants, including CCL22 and CCL27. Spatial profiling of immune cells in BCC tissues revealed preferential exclusion of CD8+ T cells from the TME, which correlated with high HH/GLI and Hippo/YAP activity. Further, functional assays in humanized mice showed that the GLI-YAP interaction controls immune cell distribution by inhibiting T cell recruitment and migration into the skin. These findings suggest that cooperation between oncogenic HH/GLI and Hippo/YAP signaling contributes to the development of an immunosuppressive TME in BCC by disrupting local chemokine signaling. Understanding these mechanisms provides a basis for the development of targeted combination therapies to enhance the efficacy of existing treatments, such as Smoothened (SMO) inhibitors and immune checkpoint blockers, by promoting T cell infiltration and activation within the TME.