Fibroblast activation protein (FAP)-mediated promotion of metastasis via the FN1-TGFβ axis and immune suppression in aggressive thyroid cancer.

[BACKGROUND] Treatment options of aggressive thyroid carcinoma (TC) is limited due to its aggressiveness. Therefore, increased knowledge of the role of tumor intrinsic immune escape mechanisms and the microenvironment contributing to metastases formation is essential to improve the patients' outcome. These include extracellular matrix (ECM) components, cancer-associated fibroblasts (CAFs) and the expression of the fibroblast activation protein (FAP) known to be involved in immune evasion, disease progression and a worse prognosis of thyroid carcinoma (TC) patients.

[METHODS] The Cancer Genome Atlas (TCGA) and Human Protein Atlas datasets of TC specimens as well as tissue microarrays (TMA) consisting of 187 TC samples were analyzed for marker expression and immune cell infiltration. TC cell lines treated with recombinant FAP, a FAP inhibitor, transfected with siRNA-FAP plasmids and/or co-cultured with CAFs were subjected to qPCR, Western blot analyses and flow cytometry, cell supernatants to ELISA for cytokine secretion, the TMAs to conventional immunohistochemistry (IHC).

[RESULTS] Immunohistochemical staining and bioinformatics analyses of publicly available mRNA and protein expression profiles revealed a significant positive correlation between high FAP and fibronectin 1 (FN1) expression, metastatic parameters and CD8 T cell infiltration in TC lesions compared to adjacent normal tissues. FAP directly cleaved FN1 associated with lower adhesion, a high invasive TC phenotype and inhibition of transforming growth factor beta (TGF-β). This is accompanied by reduced programmed death ligand 1 (PD-L1) and increased human leukocyte antigen (HLA)-ABC surface levels. Co-culture of TC cells with CAFs demonstrated an influence of FAP on the immune suppressive CAF subtypes.

[CONCLUSIONS] FAP-mediated alterations of ECM integrity, TC cells and CAFs underscore its potential as a powerful marker for tumor immunogenicity, immune infiltration and metastasis rendering it a promising therapeutic target for the treatment of aggressive ATC.