MAdCAM-1 Costimulates T Cells through Integrin α4β7 to Cause Gene Expression Events Resembling Costimulation through CD28

Successful treatment of inflammatory bowel disease (IBD) with the anti-integrin α4β7 mAb vedolizumab suggests that interaction of this integrin with addressin mucosal addressin cell adhesion molecule-1 (MAdCAM-1) is central to IBD pathogenesis. Although this was presumed to be due to an inhibition of lymphocyte trafficking to the gut, as has been observed in animal models, we report no depletion of CD4 T cells from the colonic mucosa as a consequence of vedolizumab treatment in humans, regardless of efficacy. Likewise, no upregulation of alternative trafficking mechanisms was observed as a consequence of therapy to suggest that this homeostasis is maintained in patients by a mechanistic escape from inhibition. Instead, we explore a role for MAdCAM–integrin interaction as a gut-specific costimulatory signal, demonstrating that it can replace CD28 ligation to activate human T cells in vitro. This activation through integrin α4β7 is mediated through the gut-restricted molecule MAdCAM-1, and it cannot be replicated by matrix molecules or proteins that bind other integrins. A detailed analysis of mRNA expression by human T cell subsets following suboptimal TCR stimulation in the presence or absence of CD28 versus MAdCAM-1 costimulation reveals marked similarity in the effect that these two signals have upon T cells, with temporal or quantitative differences detected in the expression of cytokines associated with Th17 cells or pyogenic inflammation. Thus, we describe an alternative costimulatory pathway for T cells in the intestine, through ligation of integrin α4β7 by MAdCAM-1, which may explain the therapeutic efficacy of vedolizumab and have implications concerning the treatment of IBD.