Blockade of αvβ6 and αvβ8 integrins with a chromogranin A-derived peptide inhibits TGFβ activation in tumors and suppresses tumor growth

BackgroundThe alpha v beta 6- and alpha v beta 8-integrins, two cell-adhesion receptors upregulated in many solid tumors, can promote the activation of transforming growth factor-beta (TGF beta), a potent immunosuppressive cytokine, by interacting with the RGD sequence of the latency-associated peptide (LAP)/TGF beta complex. We have previously described a chromogranin A-derived peptide, called "peptide 5a", which recognizes the RGD-binding site of both alpha v beta 6 and alpha v beta 8 with high affinity and selectivity, and efficiently accumulates in alpha v beta 6- or alpha v beta 8-positive tumors. This study aims to demonstrate that peptide 5a can inhibit TGF beta activation in tumors and suppress tumor growth.MethodsPeptide 5a was chemically coupled to human serum albumin (HSA) to prolong its plasma half-life. The integrin recognition properties of this conjugate (called 5a-HSA) and its capability to block TGF beta activation by alpha v beta 6+ and/or alpha v beta 8+ cancer cells or by regulatory T cells (Tregs) were tested in vitro. The in vivo anti-tumor effects of 5a-HSA, alone and in combination with S-NGR-TNF (a vessel-targeted derivative of tumor necrosis factor-a), were investigated in various murine tumor models, including pancreatic ductal adenocarcinoma, fibrosarcoma, prostate cancer, and mammary adenocarcinoma.ResultsIn vitro assays showed that peptide 5a coupled to HSA maintains its capability of recognizing alpha v beta 6 and alpha v beta 8 with high affinity and selectivity and inhibits TGF beta activation mediated by alpha v beta 6+ and/or alpha v beta 8+ cancer cells, as well as by alpha v beta 8+ Tregs. In vivo studies showed that systemic administration of 5a-HSA to tumor-bearing mice can reduce TGF beta signaling in neoplastic tissues and promote CD8-dependent anti-tumor responses. Combination therapy studies showed that 5a-HSA can enhance the anti-tumor activity of S-NGR-TNF, leading to tumor eradication.ConclusionPeptide 5a is an efficient tumor-homing inhibitor of alpha v beta 6- and alpha v beta 8-integrin that after coupling to HSA, can be used as a drug to block integrin-dependent TGF beta activation in tumors and promote immunotherapeutic responses.Graphical AbstractThe 5a-HSA conjugate, a compound consisting of the chromogranin A-derived peptide 5a coupled to human serum albumin (HSA), can bind the RGD binding site of alpha v beta 6 and alpha v beta 8 integrins expressed by tumor cells and tumor-infiltrating regulatory T cells (Tregs) and inhibits alpha v beta 6- and/or alpha v beta 8-mediated activation of TGF beta, thereby reducing its immunosuppressive effects and promoting anti-tumor immune responses