The Effects of All-Trans Retinoic Acid on the Induction of Oral Tolerance in a Murine Model of Bronchial Asthma.

BACKGROUND Active suppression induced by regulatory T (Treg) cells is reported to be one of the mechanisms involved in oral tolerance. All-trans retinoic acid (ATRA) has been reported to affect Treg cell differentiation. The present study examined the effects of ATRA on the induction of oral tolerance in a murine model of bronchial asthma. METHODS BALB/c mice were sensitized to and challenged with ovalbumin (OVA) through feeding followed by OVA challenges. In some study groups ATRA was orally administered concomitantly with OVA feeding either in the presence or absence of the retinoic acid receptor antagonist LE135. Lung CD4+ T cells were isolated from mice exposed to ATRA and/or OVA, and transferred to control mice. Airway hyperresponsiveness (AHR), cell counts and cytokine levels in bronchoalveolar lavage (BAL) fluid, and lung histology were assessed. RESULTS Concomitant administration of ATRA with OVA ameliorated AHR, airway eosinophilia, elevation of cytokines in BAL fluid and goblet cell metaplasia. The proportion of Treg cells in the lungs was increased in mice treated with OVA and ATRA, as compared to those treated with OVA only. Transfer of lung CD4+ T cells from mice treated with OVA and ATRA induced suppression of AHR and airway inflammation. LE135 completely reversed the effects of ATRA on AHR, airway allergic inflammation and the number of Treg cells in the lungs. CONCLUSION These data suggested that oral administration of ATRA with OVA had the potential to enhance oral tolerance in this murine model of bronchial asthma. These effects were mediated, at least in part, by Treg cell expansion.