Anti-activin treatment increases T cell infiltration in breast and pancreatic tumours and promotes survival in a SMAD4-null mouse pancreatic cancer model

Activin A and B share the same downstream signalling pathway (activation of SMAD2/3) as TGF-β and consequently elicit many of the same functional responses as TGF-β, including immune suppression, activation of cancer-associated fibroblasts (CAFs) and extracellular matrix production and remodelling. However, activin’s role in tumourigenesis has been relatively overlooked compared to TGF-β’s. We generated and characterized a dual specificity human antibody that recognizes both activin A and B and compared its activity in syngeneic mouse models of breast cancer and pancreatic ductal adenocarcinoma (PDAC) with an activin A-specific antibody. We demonstrate that activin A and B are central to the function of CAFs and therapeutic inhibition of activin results in a reduction of collagen rich desmoplastic barriers, enabling the infiltration of cytotoxic T cells. This is correlated with an upregulation of the T cell chemoattractant CXCL10, which is normally repressed by activin signalling. Interestingly, despite greater T cell infiltration, activin A inhibition resulted in poorer survival in the KPC mouse model of PDAC and slightly larger tumours in the breast cancer model, indicating a tumour suppressive role of activin A-rich CAFs. Strikingly, however, treatment with the same anti-activin A antibody of PDAC tumours where SMAD4 is deleted in the tumour cells, resulted in increased survival, which was potentiated with additional treatment with immune checkpoint inhibitors. These results suggest that anti-activin therapy has potential for the cohort of PDAC patients exhibiting inactivation of SMAD4.