TGF-beta-activated cancer-associated fibroblasts limit cetuximab efficacy in preclinical models of head and neck cancer

Most head and neck cancer (HNC) patients are resistant to cetuximab, an antibody against the epithelial growth factor receptor. Such therapy resistance is known to be mediated, in part, by stromal cells surrounding the tumor cells; however, the mechanisms underlying such a resistance phenotype remain unclear. To identify the mechanisms underlying cetuximab resistance in an unbiased manner, RNA-sequencing (RNA-seq) of HNC patient-derived xenografts (PDXs) was performed. Comparing the gene expression of HNC-PDXs before and after treatment with cetuximab indicated that the TGF-beta signaling pathway was upregulated in the stromal cells of PDXs that progressed to cetuximab (CetuximabProg-PDX). However, in PDXs that were extremely sensitive to cetuximab, and when tumors shrunk following cetuximab treatment (CetuximabSen-PDX), the TGF-beta pathway was downregulated in the stromal compartment. Histopathological analysis of PDXs showed that in CetuximabProg-PDX, TGF-beta-activation was detected in cancer-associated fibroblasts (CAFs). These TGF-beta-activated CAFs were sufficient to limit cetuximab efficacy in vitro and in vivo. Moreover, blocking the TGF-beta pathway using the SMAD3 inhibitor, SIS3, enhanced cetuximab efficacy and prevented the progression of CetuximabProg-PDX. Altogether, our findings indicate, for the first time, that TGF-beta-activated CAFs play a role in limiting cetuximab efficacy in HNC. Three tumors from each group (vehicle and cetuximab, a total of 6 samples for each PDX) were subjected to sequencing.