Oral tolerance and TGF-beta-producing cells.

Multiple mechanisms have been proposed to explain the immune hyporesponsiveness to fed antigens, a phenomenon named oral tolerance. Low doses of orally administered antigen are reported to favor active suppression with the generation of regulatory cells, whereas high doses would favor clonal anergy/deletion. A major conceptual advance in oral tolerance has been the demonstration that TGF-beta plays a central role in oral tolerance as a mediator secreted by Th3 cells. In addition, recent pieces of evidence suggest that TGF-beta may be a primary link between distinct populations of regulatory T cells that are induced by feeding. Conversion of CD4+CD25- into CD4+CD25+ T cells by the expression of FoxP3 involves TGF-beta. A membrane-bound form of TGF-beta (containing latency-associated peptide - LAP) has also been described and LAP+ CD4+ T cells mediate suppression in the gut by a TGF-beta-dependent mechanism. Most of these regulatory T cells are anergic cells indicating that anergy may be also related to Treg induction. Moreover, deletional events taking place in the gut mucosa induce TGF-beta production by either macrophages that phagocyte apoptotic cells or by the dying T cells. Thus, it appears that TGF-beta-producing cells are not only crucial for oral tolerance, but they may be master regulators of most of the mechanisms triggered by antigen feeding.