Because TGF-β can induce expression of CD103 in some cells, 14 a potential explanation for the reduced ability of Itgb8 (CD11c-Cre) mice to induce iTregs is that lower CD103+ DC numbers are present in these mice owing to reduced TGF-β activation. However, we found that Itgb8 (CD11c-Cre) mice had comparable numbers of CD103+ DCs in all gut-associated SB431542 lymphoid tissue examined ( Figure 6C). Taken together with our in vitro data, these results strongly indicate that αvβ8-mediated TGF-β activation by specialized intestinal
CD103+ DCs is essential for the induction of tolerogenic Foxp3+ iTregs in the gut. Intestinal CD103+ DCs have emerged as key cells in maintaining gut tolerance, with recent data showing that these cells have the enhanced ability to induce gut-homing receptors on responding T cells15 and convert naïve T cells to immune-suppressive Foxp3+ iTregs.6 and 7 These important functions appear to be due to high expression of the retinal dehydrogenase aldh1a2 in CD103+ intestinal DCs, suggesting they have the capacity to metabolize retinal acid to RA. 6 However, our data now show that CD103+ gut DCs have an enhanced ability to induce iTregs that is independent of RA but completely learn more dependent on TGF-β function. These results strongly suggest that the enhanced ability of CD103+
intestinal DCs to induce iTregs is linked to an increased ability of these cells to produce active TGF-β. Indeed, we directly show for the first time that CD103+ intestinal
DCs are specialized to activate latent TGF-β and that elevated expression of the TGF-β–activating integrin αvβ8 by CD103+ intestinal DCs is responsible for the enhanced ability of these cells to activate latent TGF-β. Importantly, elevated integrin αvβ8-mediated TGF-β activation by CD103+ intestinal DCs is responsible for their increased ability to induce Foxp3+ Tregs both in vitro and in vivo. We have therefore identified a novel molecular pathway by which a specialized gut DC subset activates TGF-β to promote a tolerogenic environment via induction of Foxp3+ iTregs. Many different immune cells produce TGF-β (predominately the isoform TGF-β116) Farnesyltransferase but always noncovalently bound to an N-terminal propeptide (LAP), preventing TGF-β binding to its receptor.8 Hence, TGF-β function is exquisitely regulated at the level of TGF-β activation. Strong evidence in vivo now supports a critical role for integrin receptors in activating latent TGF-β1 via interaction with an RGD integrin binding motif present in the LAP region of the latent complex.17 Our finding that the TGF-β–activating integrin αvβ8 is highly expressed and functionally important on specialized tolerogenic DCs in the intestine correlates with our previous findings that Itgb8 (CD11c-Cre) mice develop severe colitis associated with reduced levels of total Foxp3+ Tregs in the colonic lamina propria.