Veli‐Matti Kähäri

Cutaneous squamous cell carcinoma (cSCC) is the most common metastatic skin cancer and the metastatic disease is associated with poor prognosis. Cancer‐associated fibroblasts (CAFs) promote progression of cancer, but their role in cSCC is largely unknown. We examined the potential of CAF markers in the assessment of metastasis risk and prognosis of primary cSCC. We utilized multiplexed fluorescence immunohistochemistry for profiling CAF landscape in metastatic and non‐metastatic primary human cSCCs, in metastases, and in premalignant epidermal lesions. Quantitative high‐resolution image analysis was performed with two separate panels of antibodies for CAF markers and results were correlated with clinical and histopathological parameters including disease‐specific mortality. Increased stromal expression of fibroblast activation protein (FAP), α‐smooth muscle actin, and secreted protein acidic and rich in cysteine (SPARC) were associated with progression to invasive cSCC. Elevation of FAP and platelet‐derived growth factor receptor‐β (PDGFRβ) expression was associated with metastasis risk of primary cSCCs. High expression of PDGFRβ and periostin correlated with poor prognosis. Multimarker combination defined CAF subset, PDGFRα−/PDGFRβ+/FAP+, was associated with invasion and metastasis, and independently predicted poor disease‐specific survival. These results identify high PDGFRβ expression alone and multimarker combination PDGFRα−/PDGFRβ+/FAP+ by CAFs as potential biomarkers for risk of metastasis and poor prognosis.