Profiling studies have revealed considerable phenotypic heterogeneity in cancer-associated fibroblasts (CAFs) present within the tumour microenvironment, however, functional characterisation of different CAF subsets is hampered by the lack of specific markers defining these populations. Here we show that genetic deletion of the Endo180 (MRC2) receptor, predominantly expressed by a population of matrix-remodelling CAFs, profoundly limits tumour growth and metastasis; effects that can be recapitulated in 3D co-culture assays. This impairment results from a CAF-intrinsic contractility defect and reduced CAF viability, which coupled with the lack of phenotype in the normal mouse, demonstrates that upregulated Endo180 expression by a specific, potentially targetable CAF subset is required to generate a supportive tumour microenvironment. Further, characterisation of a tumour subline selected via serial in vivo passage for its ability to overcome these stromal defects provides important insight into, how tumour cells adapt to a non-activated stroma in the early stages of metastatic colonisation.

Although the 5-year survival rates for sarcoma patients have improved, the proportion of patients relapsing after first line treatment remains high and survival of patients with metastatic disease is dismal. Moreover, the extensive molecular heterogeneity of the multiple different sarcoma subtypes poses a substantial challenge to developing more personalized treatment strategies. From immunohistochemical staining of a large set of 625 human soft tissue sarcomas we demonstrate strong tumor cell staining of the Endo180 (MRC2) receptor in a high proportion of samples, findings echoed in gene expression datasets showing a significantly increased expression in both soft tissue and bone sarcomas compared to normal tissue. Endo180 is a constitutively recycling transmembrane receptor and therefore an ideal target for an antibody-drug conjugate (ADC). An anti-Endo180 monoclonal antibody conjugated to the anti-mitotic agent, MMAE via a cleavable linker, is rapidly internalized into target cells and trafficked to the lysosome for degradation, causing cell death specifically in Endo180 expressing sarcoma cell lines. In a sarcoma tumor xenograft model, the Endo180-vc-MMAE ADC, but not an Isotype-vc-MMAE control or the unconjugated Endo180 antibody, drives on target cytotoxicity resulting in tumor regression and a significant impairment of metastatic colonization of the lungs, liver and lymph nodes. These data, together with the lack of a phenotype in mice with an Mrc2 genetic deletion, provide pre-clinical proof-of-principle evidence for the future development of an Endo180-ADC as a therapeutic strategy in a broad range of sarcoma subtypes and, importantly, with potential impact both on the primary tumor and in metastatic disease.

<p>Endo180 protein and gene expression in healthy human tissue compared to sarcoma subtypes.</p>