The AvB6 integrin is up-regulated on epithelial malignancies and has been implicated in various aspects of cancer progression. Immunohistochemical analysis of AvB6 expression in 10 human tumor types showed increased expression relative to normal tissues. Squamous carcinomas of the cervix, skin, esophagus, and head and neck exhibited the highest frequency of expression, with positive immunostaining in 92%(n= 46), 84%(n= 49), 68%(n= 56), and 64%(n= 100) of cases, respectively. We studied the role of AvB6 in Detroit 562 human pharyngeal carcinoma cells in vitro and in vivo. Prominent AvB6 expression was detected on tumor xenografts at the tumor-stroma interface resembling the expression on human head and neck carcinomas. Nonetheless, coculturing cells in vitro with matrix proteins did not up-regulate AvB6 expression. Detroit 562 cells showed AvB6-dependent adhesion and activation of transforming growth factor-B (TGF-B) that was inhibited> 90% with an AvB6 blocking antibody, 6.3 G9. Although both recombinant soluble TGF-B receptor type-II (rsTGF-BRII-Fc) and 6.3 G9 inhibited TGF-B–mediated Smad2/3 phosphorylation in vitro, there was no effect on proliferation. Conversely, in vivo, 6.3 G9 and rsTGF-BRII-Fc inhibited xenograft tumor growth by 50%(n= 10, P< 0.05) and> 90%(n= 10, P< 0.001), respectively, suggesting a role for the microenvironment in this response. However, stromal collagen and smooth muscle actin content in xenograft sections were unchanged with treatments. Although further studies are required to consolidate in vitro and in vivo results and define the mechanisms of tumor inhibition by AvB6 antibodies, our findings support a role for AvB6 in human cancer and underscore the therapeutic potential of function blocking AvB6 antibodies.[Cancer Res 2008; 68 (2): 561–70]