Retinoic acid attenuates ileitis by restoring the balance between T-helper 17 and T regulatory cells

BACKGROUND & AIMS

Retinoic acid (RA), produced by intestinal epithelial cells (IECs) and dendritic cells (DCs) promotes the induction of regulatory T cells (Tregs) and decreases the induction of T-helper (Th)17 cells.

METHODS

We studied the roles of RA in mice that overproduce tumor necrosis factor (TNF) and develop chronic ileitis (TNF_ARE mice). We assessed the frequency and function of CD103+ DCs, Th17 cells, and Tregs by flow cytometry, and we measured expression of cytokines and retinaldehyde dehydrogenase (RALDH) enzymes in ileum samples, DCs, and IECs by real-time polymerase chain reaction. We quantified RA by electrochemical analysis and examined the effect of RA supplementation on TNF-induced ileitis using histologic, coculture, and suppression assays and flow cytometry.

RESULTS

Numbers of CD103+ DCs decreased in the inflamed ilea of mice with chronic disease; RA synthetic machinery (RALDH1,2) was down-regulated. Nevertheless, the proportion of CD4+, CD25+, FoxP3+ Tregs increased, indicating an alternate source for RA. IECs responded to reduced levels of RA by up-regulating RALDH3 in vivo and in vitro. Net tissue levels of RA remained lower in TNF+ARE than wild-type mice, indicating that epithelial up-regulation of RALDH3 could not maintain adequate concentrations of RA, probably because of loss of IEC mass. RA supplementation significantly attenuated disease by increasing the number and function of CD103+ DCs and Tregs and reducing Th17 cells.

CONCLUSIONS

Reduced levels of RA appear to induce IECs to up-regulate synthesis of RA. RA supplementation attenuates ileitis through its effects on CD103+ DCs, Tregs, and Th17 cells. RA supplementation might offer therapeutic benefit in Crohn's disease.

Novel model of TH2-polarized chronic ileitis: the SAMP1 mouse

BACKGROUND

SAMP1/Yit mice develop spontaneous, segmental, transmural ileitis recapitulating many features of Crohn's disease (CD). The ileitic phenotype may have arisen during crosses of SAMP1 mice selected for the presence of skin lesions. We hereby describe that the original SAMP1 strain similarly develops ileitis. Our aim was to characterize the histopathological and immunological features of this model and assess its responsiveness to standard inflammatory bowel disease (IBD) therapy.

METHODS

The time course of histopathological features of ileitis was assessed. Immune compartments were characterized by flow cytometry. Ileal cytokine profiles and transcription factors were determined by real-time reverse-transcription polymerase chain reaction (RT-PCR). Finally, response to corticosteroid therapy and its effect on immune compartments and cellularity was evaluated.

RESULTS

Histological features and time course of disease were conserved, compared to those reported in SAMP1/Yit strains, with similar expansion of CD19+, CD4+, and CD8+ effector (CD44(high) CD62L(low)), and central memory lymphocytes (CD44(high)CD62L(high)). However, different from SAMP1/YitFc mice, analysis of ileal cytokine profiles revealed initial T(H)1 polarization followed by T(H)2-polarized profile accompanied by prominent eosinophilia during late disease. Lastly, corticosteroids attenuated ileitis, resulting in decreased lymphocyte subsets and cellularity of compartments.

CONCLUSIONS

Here we report that the ileitic phenotype of SAMP1-related strains was already present in the original SAMP1 strain. By contrast, the cytokine profile within the terminal ilea of SAMP1 is distinct from the mixed T(H)1/T(H)2 profile of SAMP1/YitFc mice during late disease, as it shows predominant T(H)2 polarization. Dissemination of these strains may advance our understanding of CD pathogenesis, which in 60% of patients involves the terminal ileum.