Inflammatory bowel disease (IBD) is an umbrella term for Crohn's disease and Ulcerative Colitis (UC), which are thought to be caused by disruption of the intestinal epithelial cell barrier, leading to changes in the intestinal flora and a consequent aberrant activation of the mucosal immune system. The resulting chronic intestinal inflammation is highly dependent on different subsets of CD4+ T helper (Th) cells. For example, correlative data suggest that Crohn's disease is driven by exaggerated Th1 and Th17 cell responses, since inflamed lesions contain increased levels of cytokines associated with these cells including IFNγ, IL-12, and IL-17 and IL-18. In contrast, although Ulcerative Colitis is in the same family of diseases, it is typically associated with Th2 cells since patients have high levels of IL-13 in the intestinal mucosa compared to patients with Crohn's disease or healthy individuals. Recent evidence implicates the newly emerging Th9 subset of Th cells in the pathology of UC. The development and function of Th cells with pathogenic potential is kept in check by another subset of CD4+ T cells, which is known as the regulatory T cell, or Treg. In healthy individuals, the intestinal lamina propria has a large proportion of Tregs, and evidence that simply lacking Tregs leads to IBD, suggests Tregs have a critical role in controlling intestinal homeostasis. Indeed, work in animal models and early phase clinical trials have shown that restoration of Treg function can ameliorate IBD. These data suggest that effective therapies need to promote the function of Tregs. Hence in order to understand the mechanism of action of new therapeutics it is critical to assess their impact on the balance between effector and regulatory CD4+ T cells.

Homing of effector and regulatory T cells to the intestine is controlled by a variety of integrins and chemokine receptors, with evidence that expression of alpha4beta7-integrin molecules (α4β7) on T cells has a key role in this process. Since Vedolizumab (Entyvio) specifically blocks the interaction between α4β7 and its ligands, which are expressed in mucosal tissue, its therapeutic effects in IBD are presumed to be related to a reduction in T cell trafficking to the intestine. However, how this biologic agent specifically affects the homing of Tregs versus effector T cells is unknown. In humans, Tregs in the peripheral blood are reported to express lower levels of α4β7 compared to effector T cells, but the relative expression on different subsets (i.e. Th1, vs. Th2, vs. Th17) of CD4+ T cells is unknown. Interestingly, in the intestine there is also a subset of Tregs that produces IL-17 and expresses high levels of α4β7, but the functional relevance of these cells in IBD is unknown. In addition, in mice, expression of α4β7 on CD4+ T cells is unstable under inflammatory conditions, suggesting that studies with circulating T cells in healthy individuals may not accurately reflect integrin expression in states of inflammation. Understanding how Vedolizumab affects the localization of circulating versus tissue-localized subsets of CD4+ T cells is key to understanding how this therapy works.

Also unknown is how signaling by α4β7 affects the development and/or function of different subsets of CD4+ T cells. In T cells, integrins not only mediate homing, but they also provide tissue-specific signals. For example, they can act as costimulatory molecules 10-12 and influence cytokine production. The molecular basis for the effects on T cells has not been well characterized, but in other cells integrins activate the PI3K pathway, providing a pro-survival signal 16. Since we have shown that activation of the PI3K pathway regulates the balance of effector versus regulatory T cells 17, it is possible that blockade of α4β7 may reduce PI3K signaling and favor the development of Tregs. In support of this possibility, we have shown that fibronectin, which is a ligand for α4β7, inhibits the development of Tregs.

This is an observational study to determine the expression pattern and function of α4β7 on effector and regulatory CD4+ T cells, and to define how treatment with Vedolizumab affects the homing and function of these cells. We hypothesize that that treatment with Vedolizumab will shift the balance of effector and regulatory T cells through two mechanisms: 1) altered migration of different subsets of CD4+ T cells to the intestine; and 2) promoting Treg stability as a consequence of reduced PI3K signaling downstream of α4β7.