Efeitos do envelhecimento na tolerância oral e na alergia alimentar

Abstract

Oral tolerance has been classically defined as a state of hyporesponsiveness of cellular and/or humoral response to an antigen by prior administration of the same antigen by oral route. Studies from our group have shown that susceptibility to oral tolerance decreases with age. Since oral tolerance is critical to maintain gut homeostasis, its decline with aging may result in increased susceptibility to develop gut inflammatory reactions such as food allergy in aged animals. Thus, the aim of this study was to evaluate the effects of aging in oral tolerance and food allergy induction. We used BALB/c mice at 8, 28, 53 and 92 weeks of age and analyzed oral tolerance induction by either gavage or continuous feeding of ovalbumin (Ova). We observed that aging is followed by a decreased in induction of both oral tolerance induction by gavage and immune responses (Ig, IgG1, S-IgA and IgE) to Ova. Production of specific IL-4 and IL-10, cytokines with proinflammatory/regulatory activity, by spleen cells stimulated with Ova was reduced in aged mice. The effects of aging were more accentuated in spleen than in mucosal sites (mesenteric lymph nodes and Peyers patches). However, numbers of spleen and mesenteric lymph node CD4+ T cells with regulatory phenotypes (CD25+FoxP3+, CD25+LAP+, CD25-LAP+) increased in tolerant aged mice. This result is in concert with our finding that some regulatory elements might be preserved in aged mice since they could still be rendered tolerant by a continuous feeding protocol. Since aged mice were less susceptible to oral tolerance, we next study whether they were prone to develop gut inflammatory reactions. To test the impact of aging in gut inflammation, we used a model of Ova-induced food allergy. First, the general increased availability of immune elements (activated T cells and cytokines) in aged mice was associated with a reduced efficiency in mounting specific immune responses of both classes, inflammatory and regulatory. Serum anti-ovalbumin (OVA) IgG, IgA and IgE levels were reduced in allergic aged mice (53- to 92-week-old). Oral tolerance induction to these parameters also declined with age. Regarding the local antibody response, production of total secretory IgA (S-IgA) in non-manipulated mice was unaffected by aging as we have previously reported. However, anti-OVA SIgA levels in allergic mice diminished with aging. It seems that the accumulation of already experienced and committed cells in the immune system of aged animals lead to a reduced ability to deal with novel antigens. Second, the reduced ability of aged mice to mount an immune response could be kinetically monitored in the food allergy model. Young (8-week-old) allergic mice produced high levels of specific serum IgE and IgG1 as well as specific S-IgA in the intestinal mucus; they had elevated levels of specific IL-4, IL-5, IL-10 and TGF-beta produced by spleen and MLN cells in vitro and a significant reduction in the consumption of OVA solution (aversion). All these parameters were progressively increased after primary immunization, boost and peaked after oral challenge in young mice. Suppression of all parameters was obtained by OVA feeding. Allergic aged (53-week-old) mice, however, had lower levels of specific serum IgG1 and IgE, and showed less accentuated aversion. Interestingly, they showed no suppression of allergic parameters when fed OVA by gavage. Specific cytokine production (IL-4, IL-5, IL-10 and TGF-beta) by spleen and MLN cells showed a plateau with no variation throughout time points of the experiment (sensitization and boost) and we could only observed a small peak in cytokine production after 7 days of oral challenge with egg white solution. To examine putative variations in regulatory and activated T cells during oral tolerance and food allergy induction, we used DO.11.10 (Ova-TCR transgenic) mice. We observed that CD4+CD25+FoxP3+ and CD4+CD25+LAP+ as well as CD4+CD44+ T cells were reduced in mesenteric lymph nodes and spleens of tolerant mice. Therefore, we concluded that both susceptibility to oral tolerance and induction of inflammatory immune responses declined with aging. Maintenance of some regulatory elements (namely IL-10 and some populations of regulatory T cells) in the gut lymphoid tissues of aged mice might have the effect of preserving mucosal homeostasis and oral tolerance induction by natural regimens such as continuous feeding.