Background Inhibition of transforming growth factor-β (TGF-β) activity is an attractive strategy to augment the response of human tumors to immune checkpoint blockade (ICB). We previously demonstrated that dual inhibition of integrins α_vβ₈ and α_vβ1, two cell-surface proteins that activate latent TGF-β, significantly improves the response to ICB in multiple pre-clinical tumor models. While upregulation of integrin α_vβ₈ in solid tumors has previously been shown to associate with poor patient outcome, expression of integrin α_vβ1 in solid tumors has not previously been characterized. Here, we evaluated α_vβ1 protein expression across a diverse set of human tumor tissues; and investigated the role of integrin α_vβ1 in primary human cancer associated fibroblasts (CAFs).

Methods α_vβ1 protein expression in human tumor tissues was evaluated via immunohistochemistry. α_vβ1 protein levels in primary human CAFs (isolated from pancreatic, lung, and ovarian serous carcinoma) were quantified via custom electrochemiluminescence assay. CAF adhesion to latency associated peptide (LAP) was quantified with or without PLN-101095 (α_vβ8/α_vβ1 selective dual inhibitor) and PLN-78146 (α_vβ1 selective inhibitor) by high-content imaging in vitro. Analysis of PLN-101095 treated human breast cancer tissues for fibrotic markers α-smooth muscle actin (αSMA) and fibroblast activating protein (FAP) was performed via immunofluorescent staining.

Results α_vβ1 protein expression was confirmed by IHC in both tumor cell and CAF-rich areas of several human cancer types including breast and pancreatic tumors. When compared to normal human lung fibroblasts, the expression of α_vβ1 protein was upregulated approximately 2-fold in primary human CAFs, with pancreatic CAFs showing highest upregulation (2.8-fold). Inhibition of α_vβ1 with either PLN-101095 or PLN-78146 blocked the binding of CAFs to LAP, the integrin-binding region of latent TGF-β, in a significant (up to 90%, p<0.001) and dose-dependent manner, demonstrating an α_vβ1-specific interaction with latent TGF-β. Furthermore, human breast tumor tissues treated with PLN-101095 ex vivo showed reduced expression of fibrotic markers αSMA (2.9-fold) and FAP (2.0-fold) compared to vehicle treated tissues, indicating a reduced TGF-β activity within the stromal regions of the tumor microenvironment.

Conclusions Here, we demonstrate that the TGF-β activating integrin α_vβ1 is expressed by multiple human cancer types, is present on primary human CAFs, and mediates CAF interaction with latent TGF-β. A first-in-human clinical trial investigating the combination of PLN-101095 with ICB is currently underway.