Normal Induction of Oral Tolerance in the Absence of a Functional IL-12-Dependent IFN-γ Signaling Pathway

There is considerable evidence that regulatory cytokines play an important role in mediating the systemic tolerance found after oral administration of protein Ags. Although most existing work has focused on cytokines such as IL-4, IL-10, and TGF-β, recent evidence from TCR transgenic systems suggests that the induction of oral tolerance is accompanied by priming of Ag-specific IFN-γ production. IFN-γ has also been implicated as a mediator of T cell tolerance in other models in vivo and in vitro, including that induced by aerosol administration of protein. We show here that feeding tolerogenic doses of OVA primes for IFN-γ production in the spleen of mice with a normal T cell repertoire. However, depleting IFN-γ at the time of feeding OVA had no effect on the induction of tolerance. In addition, tolerance was induced normally in both IFN-γ receptor knockout (IFN-γR−/−) and IL-12 p40 knockout (IL-12−/−) mice. This was the case for all components of the systemic immune response and also with a variety of feeding protocols, including those believed to induce distinct regulatory mechanisms. We conclude that IL-12-dependent IFN-γ-mediated regulation does not play an essential role in oral tolerance.

Kinetic Analysis of Oral Tolerance: Memory Lymphocytes Are Refractory to Oral Tolerance

Oral administration of soluble Ag before immunization induces peripheral tolerance and is effective in suppressing animal models of autoimmune diseases. Although tolerance induction in primed animals is more clinically relevant, it is not well studied. Therefore, this study was designed to examine the feeding effects on different phases of the immune response. We observed that feeding a single high dose (250 mg) of OVA to OVA-primed BALB/c mice could induce OVA-specific suppression in the Ab production and T cell proliferation only at the naive and the activation phases of the immune response, whereas multiple high doses (100 mg/feed for 10 days) were effective at the effector phase. OVA-specific IL-4 production in culture supernatant was also suppressed in the tolerized groups. However, when the mice had resting memory lymphocytes, even multiple feeding regimens were not effective in tolerance induction, although multiple low doses (1 mg/feed for 10 days) partially suppressed Ab production. This phenomenon was confirmed by adoptive transfer study. Nevertheless, the reactivated memory response was suppressed partially by multiple high doses. Our findings have an important implication for understanding the mechanism of oral tolerance and for the therapeutic applications of oral tolerance to autoimmune diseases.

Oral Administration of Myelin Basic Protein Is Superior to Myelin in Suppressing Established Relapsing Experimental Autoimmune Encephalomyelitis

Oral administration of a myelin component, myelin basic protein (MBP), induces immunological unresponsiveness to CNS Ags and ameliorates murine relapsing experimental autoimmune encephalomyelitis (REAE). However, a recent clinical trial in which multiple sclerosis patients were treated with repeated doses of oral myelin was unsuccessful in reducing disease exacerbations. Therefore, we directly compared the tolerizing capacity of myelin vs MBP during REAE in B10.PL mice. Oral administration of high doses of myelin, either before disease induction or during REAE, did not provide protection from disease or decrease in vitro T cell responses. In contrast, repeated oral administration of high doses of MBP suppressed established disease and MBP-specific T cell proliferation and cytokine responses. The frequency of IL-2-, IFN-γ-, and IL-5-secreting MBP-specific T cells declined with MBP feeding, implicating anergy and/or deletion as the mechanism(s) of oral tolerance after high Ag doses. We have previously shown that the dosage and timing of Ag administration are critical parameters in oral tolerance induction. Studies presented here demonstrate that Ag homogeneity is also important, i.e., homogeneous Ag (MBP) is more effective at inducing oral tolerance than heterogeneous Ag (myelin).

Orally Induced Peripheral Nonresponsiveness Is Maintained in the Absence of Functional Th1 or Th2 Cells

Intragastric administration of soluble protein Ags results in peripheral tolerance to the fed Ag. To examine the role of cytokine regulation in the induction of oral tolerance, we fed OVA to mice deficient in Th1 (Stat 4−/−) and Th2 (Stat 6−/−) cells and compared their response to that of normal BALB/c controls. We found that, in spite of these deficiencies, OVA-specific peripheral cell-mediated and humoral nonresponsiveness was maintained in both Stat 4−/− and Stat 6−/− mice. In the mucosa, both Peyer’s patch T cell proliferative responses and OVA-specific fecal IgA were reduced in Stat 4−/− and Stat 6−/− mice fed OVA but not in normal BALB/c controls. Mucosal, but not peripheral, nonresponsiveness was abrogated by the inclusion of a neutralizing Ab to TGF-β in the culture medium. Our results show that, in the periphery, tolerance to oral Ag can be induced in both a Th1- or Th2-deficient environment. In the mucosa, however, the absence of Th1 and Th2 cytokines can markedly affect this response, perhaps by regulation of TGF-β-secreting cells.

Mucosally Induced Systemic T Cell Unresponsiveness to Ovalbumin Requires CD40 Ligand-CD40 Interactions

CD40 ligand (CD40L) gene-disrupted (CD40L−/−) mice were employed to examine the role of costimulatory signals via CD40L-CD40 interactions in mucosally induced tolerance. CD40L−/− and control (CD40L+/+) mice of the same C57BL/6 × 129/J background were immunized orally with 25 mg of OVA before systemic challenge with OVA in CFA. While CD40L+/+ mice showed reductions in Ag-specific T cell responses including delayed-type hypersensitivity (DTH) and proliferative responses, CD40L−/− mice underwent normal T cell responses. Further, cytokine analysis of splenic CD4+ T cells showed that both Th1-type (e.g., IFN-γ and IL-2) and Th2-type (e.g., IL-4, IL-5, IL-6, and IL-10) responses were maintained in CD40L−/− mice orally immunized with OVA, whereas these cytokine responses in CD40L+/+ mice were significantly reduced. In addition, splenic CD4+ T cells from CD40L−/− mice orally immunized with OVA provided B cell help in Ag-specific Ab-forming cells when the cells were cultured with naive B cells in the presence of Ag and CD40L-transfected cell lines. In contrast, an identical culture condition containing splenic CD4+ T cells from orally tolerized CD40L+/+ mice did not exhibit helper activity. Taken together, these findings indicate that CD40L and CD40 interactions are essential for the induction of systemic T cell unresponsiveness to orally administered Ag.

Antibodies Against IL-12 Prevent Superantigen-Induced and Spontaneous Relapses of Experimental Autoimmune Encephalomyelitis

Immunization of (PL/J × SJL/J)F1 mice with myelin basic protein (MBP) induces relapsing experimental autoimmune encephalomyelitis (EAE). Relapses occur 7 to 10 days after recovery from the initial paralysis. Staphylococcal enterotoxins (SE) A or B, administered after recovery from the initial paralysis, induce immediate relapses. IL-12 is involved in the induction of EAE. Here, we show that SEA and SEB induce IL-12 in splenocytes from (PL/J × SJL/J)F1 mice in vitro and increase the level of IL-12 in the sera of mice treated with these superantigens. IL-12 administration mimics SE in inducing spontaneous relapses and in enhancing the severity and frequency of spontaneous relapses. IL-12 neutralization blocks SE-induced and subsequent relapses of EAE, and, when instituted after recovery from the initial attack, prevents spontaneous relapse. This is the first report of prevention of relapses of EAE with anti-IL-12 Ab, an approach which may prove useful in the prevention of exacerbations in multiple sclerosis.

IL-12, But Not IFN-γ, Plays a Major Role in Sustaining the Chronic Phase of Colitis in IL-10-Deficient Mice

IL-10-deficient (IL-10−/−) mice develop chronic enterocolitis mediated by CD4+ Th1 cells producing IFN-γ. Because IL-12 can promote Th1 development and IFN-γ production, the ability of neutralizing anti-IL-12 mAb to modulate colitis in IL-10−/− mice was investigated. Anti-IL-12 mAb treatment completely prevented disease development in young IL-10−/− mice. Treatment of adult mice resulted in significant amelioration of established disease accompanied by reduced numbers of mesenteric lymph node and colonic CD4+ T cells and of mesenteric lymph node T cells spontaneously producing IFN-γ. In contrast, anti-IFN-γ mAb had minimal effect on disease reversal, despite a significant preventative effect in young mice. These findings suggested that IL-12 sustains colitis by supporting the expansion of differentiated Th1 cells that mediate disease independently of their IFN-γ production. This conclusion was supported by the finding that anti-IL-12 mAb greatly diminished the ability of a limited number of CD4+ T cells expressing high levels of CD45RB from diseased IL-10−/− mice to expand and cause colitis in recombination-activating gene-2−/− recipients, while anti-IFN-γ mAb had no effect. Furthermore, IL-12 could support pathogenic IL-10−/− T cells stimulated in vitro in the absence of IL-2. While these studies show that IL-12 plays an important role in sustaining activated Th1 cells during the chronic phase of disease, the inability of anti-IL-12 mAb to abolish established colitis or completely prevent disease transfer by Th1 cells suggests that additional factors contribute to disease maintenance.

Factors Involved in the Differentiation of TGF-β-Producing Cells from Naive CD4+ T Cells: IL-4 and IFN-γ Have Opposing Effects, While TGF-β Positively Regulates Its Own Production

TGF-β has been shown to play a central role in regulating inflammatory responses; thus, understanding the factors involved in the generation of TGF-β-producing cells could lead to interventions that are useful in effecting disease progression. In initial studies, the capacity of naive CD4+ T cells from TCR transgenic (Tg) mice to produce TGF-β following primary and secondary stimulation was assessed. TGF-β, IL-4, or IFN-γ production could not be detected from highly purified naive CD4+/lymphocyte endothelial cell adhesion molecule (LECAM)-1high cells following primary stimulation for 36 h with plate-bound anti-CD3, anti-CD28, and IL-2. This population was subsequently used to study the differentiation of TGF-β-producing CD4+ T cells. In further studies, naive CD4+/LECAM-1high cells from TCR transgenic mice of both the BALB/c and B10.A backgrounds were stimulated with T-depleted spleen cells (TDS) and specific peptide in the presence of various cytokines and/or cytokine antagonists for 5 days, restimulated, and TGF-β, IL-4, and IFN-γ production were measured. Priming conditions favoring high IL-4 production and/or low IFN-γ production greatly enhanced TGF-β production in secondary cultures. Furthermore, the presence of IL-10 in cultures was associated with an increase in TGF-β production following restimulation. The importance of IL-4 and IFN-γ in regulating TGF-β production was confirmed in studies showing that cells from IFN-γ−/− mice produced more TGF-β, while cells from IL-4−/− mice produced less TGF-β compared with wild-type controls. Finally, the addition of exogenous TGF-β to priming cultures significantly enhanced the production of TGF-β upon restimulation, demonstrating that TGF-β has a role in self-regulating its own production.

In Situ Immune Response in Gut-Associated Lymphoid Tissue (GALT) Following Oral Antigen in TCR-Transgenic Mice

Oral administration of Ag results in systemic hyporesponsiveness termed oral tolerance. The regulatory cells induced by oral Ag are effective in the suppression of Th1-type autoimmune diseases. We examined the cytokine microenvironment in gut-associated lymphoid tissue in response to orally administered OVA in OVA TCR-transgenic mice. Mice were fed a low (0.5 mg) or high (500 mg) dose of OVA one time or five times. Immunohistochemical analysis demonstrated increased IL-4, IL-10, and TGF-β in the gut within 6 h of a low-dose feeding and after five low-dose or high-dose feedings. IFN-γ and IL-2 either decreased or showed no change, with the exception of a small transient increase in IL-2 at 6 h after a low dose. Increases in IL-4 and IL-10 were found in the dome of the Peyer’s patch, and increases in TGF-β were observed in the interfollicular region and the villi. IL-10 was also substantially increased in the villi. IL-4 and IL-10 were produced predominately by CD4+ T cells. TGF-β was found predominately in macrophages and CD4+ T cells. Peyer’s patches had a marked up-regulation of TGF-β mRNA as measured by RT-PCR. These results demonstrate the differential activation of cytokine production in discrete regions of gut-associated lymphoid tissue. The induction of cytokines known to inhibit autoimmune disease at the site of Ag absorption indicates an important role for the mucosal immune system in the establishment of oral tolerance.

A Helminth-Induced Mucosal Th2 Response Alters Nonresponsiveness to Oral Administration of a Soluble Antigen

A fascinating feature of the intestinal mucosal immune system is its ability to guard against invasion by pathogens while avoiding a response to the many potential Ags present in food. The phenomenon of systemic tolerance after oral administration of protein Ags is well documented, but the cellular and molecular basis for the observed nonresponsiveness is not fully understood. To gain insight into the role of the mucosal microenvironment in the induction of orally induced nonresponsiveness, we attempted to induce tolerance to OVA in mice primed for a Th2-biased mucosal immune response by infection with the nematode parasite Heligmosomoides polygyrus. We found that oral tolerance for Th1-type responses to OVA is maintained when OVA is fed during the peak of the mucosal immune response to H. polygyrus. Tolerance for Th2 cytokine responses or a Th2-dependent isotype of IgG is not induced in this Th2-biased mucosal environment. Treatment of infected mice with rIL-12 to reverse the Th2 polarity of the parasite-specific immune response restores tolerance of both Th1 and Th2 responses to OVA. We conclude that the polarized Th2 response induced by this enteric infection plays a central role in determining whether or not systemic tolerance is induced. Our results imply that attempts to use oral administration of Ag to suppress systemic immune responses will be influenced strongly by the presence of mucosal infection.

Lack of Local Suppression in Orally Tolerant CD8-Deficient Mice Reveals a Critical Regulatory Role of CD8+ T Cells in the Normal Gut Mucosa

We found that feeding keyhole limpet hemocyanin (KLH) to CD8-deficient (CD8−/−) mice induced oral tolerance that was comparable in both magnitude and quality to that induced in wild-type (wt) mice. The tolerance was dose dependent, and only higher doses of KLH caused significant reduction in specific Ab and T cell responses. Both Th1 and Th2 CD4+ T cell functions were affected. Feeding KLH together with cholera toxin (CT) adjuvant, however, abrogated the induction of oral tolerance equally well in CD8−/− and wt mice. On the contrary, CT adjuvant was unable to abrogate already established oral tolerance in both CD8−/− and wt mice. Most importantly, whereas Ag feeding induced hyporesponsiveness in systemic as well as in local gut IgA responses in wt mice, a lack of local suppression was evident in orally tolerant CD8−/− mice following oral immunizations. Thus, contrary to the situation in wt mice, Ag feeding induces systemic, but not local, gut IgA hyporesponsiveness in CD8−/− mice, suggesting that CD8+ T cells in the normal gut mucosa exert an important down-regulatory function. In wt mice the local suppression extended to an unrelated Ag, OVA, given together with KLH and CT adjuvant, i.e., bystander suppression. Based on these results we propose that tolerance induced by feeding Ag is highly compartmentalized, requiring CD8+ T cells for local suppression of IgA responses, whereas systemic tolerance may affect CD4+ T cells of both Th1 and Th2 types independently of CD8+ T cells. Finally, the adjuvant effect of CT abrogates induction, but not established, oral tolerance through a mechanism that does not require CD8+ T cells.