Mucosal tolerance prevents pathological reactions against environmental and food antigens, and its failure results in exacerbated inflammation typical of allergies and asthma. One of the proposed mechanisms of oral tolerance is the induction of Tregs. Using a mouse model of hyper-IgE and asthma, we found that oral tolerance could be effectively induced in the absence of naturally occurring thymus-derived Tregs. Oral antigen administration prior to i.p. immunization prevented effector/memory Th2 cell development, germinal center formation, class switching to IgE, and lung inflammation. Oral exposure to antigen induced development of antigen-specific CD4CD25Foxp3CD45RB cells that were anergic and displayed suppressive activity in vivo and in vitro. Oral tolerance to the Th2 allergic response was in large part dependent on TGF-beta and independent of IL-10. Interestingly, Tregs were also induced by single i.p. immunization with antigen and adjuvant. However, unlike oral administration of antigen, which induced Tregs but not effector T cells, i.p. immunization led to the simultaneous induction of Tregs and effector Th2 cells displaying the same antigen specificity.

Mice deficient in interleukin (IL)-2 production or the IL-2 receptor α or β chains develop a lethal autoimmune syndrome. CD4+ regulatory T cells have been shown to prevent autoimmune diseases, allograft rejection, and to down-regulate antibody responses against foreign antigens. To assess the role of IL-2 in the generation and function of regulatory T cells, we transferred CD4+ T cells from mice genetically deficient in IL-2 or IL-2Rα (CD25) expression. A small number of splenic or thymic CD4+ T cells from IL-2 knockout mice can protect mice from spontaneous experimental autoimmune encephalomyelitis (EAE). In contrast, splenic or thymic CD4+ T cells from CD25 knockout donor mice conferred little or no protection. We conclude that T cells with regulatory potential can develop, undergo thymic selection, and migrate to the peripheral lymphoid organs in the absence of IL-2, and do not protect from disease by means of IL-2 secretion. However, IL-2 signaling in regulatory T cells is essential for their protective function. Altogether, our results favor a model whereby IL-2 induces regulatory T cell activity.

Adaptive Foxp3(+) regulatory T (Treg) cells develop during induction of mucosal tolerance and after immunization. Large numbers of Foxp3(+) T cells have been found in inflamed tissues. We investigated the role of adaptive Foxp3(+) Treg cells in mucosal tolerance and in chronic allergic lung inflammation. We used two strains of mice that are devoid of naturally occurring Treg cells; one is capable of generating adaptive Foxp3(+) Treg cells upon exposure to antigen, whereas the other is deficient in both naturally occurring and adaptive Foxp3(+) Treg cells. We found that adaptive Foxp3(+) Treg cells were essential for establishing mucosal tolerance and for suppressing IL-4 production and lymphoid neogenesis in chronic inflammation, whereas IL-5 production and eosinophilia could be controlled by Foxp3-independent, IFN-gamma-dependent mechanisms. Thus, whereas adaptive Foxp3(+) Treg cells regulate sensitization to allergens and the severity of chronic inflammation, IFN-gamma-producing cells can play a beneficial role in inflammatory conditions involving eosinophils.

A key event in the pathogenesis of asthma and allergies is the production of IgE antibodies. We show here that IgE(+) cells were exceptional because they were largely found outside germinal centers and expressed, from very early on, a genetic program of plasma cells. In spite of their extragerminal center localization, IgE(+) cells showed signs of somatic hypermutation and affinity maturation. We demonstrated that high-affinity IgE(+) cells could be generated through a unique differentiation program that involved two phases: a pre-IgE phase in which somatic hypermutation and affinity maturation take place in IgG1(+) cells, and a post-IgE-switching phase in which IgE cells differentiate swiftly into plasma cells. Our results have implications for the understanding of IgE memory responses in allergy.

Naturally occurring CD4+ regulatory T cells are generally identified through their expression of CD25. However, in several experimental systems considerable Treg activity has been observed in the CD4+CD25− fraction. Upon adoptive transfer, the expression of CD25 in donor-derived cells is not stable, with CD4+CD25+ cells appearing in CD4+CD25− T cell-injected animals and vice versa. We show in this study that CD25+ cells arising from donor CD25− cells upon homeostatic proliferation in recipient mice express markers of freshly isolated Treg cells, display an anergic state, and suppress the proliferation of other cells in vitro. The maintenance of CD25 expression by CD4+CD25+ cells depends on IL-2 secreted by cotransferred CD4+CD25− or by Ag-stimulated T cells in peripheral lymphoid organs.