Production of retinoic acid by antigen presenting cells in the healthy and inflamed human intestine.

Abstract

Murine small intestinal CD103+ dendritic cells (DCs) produce retinoic acid (RA) through retinaldehyde dehydrogenase (RALDH) activity, thereby inducing ‘gut-homing’ α4β7 and CCR9 on T cells they activate, enhancing TGF-β-mediated induction of Foxp3+ regulatory T cells and suppressing induction of pro-inflammatory TH17 cells. RALDH activity of CD103+ DCs is reduced in mouse models of inflammatory bowel disease (IBD) but the role of RALDH activity in human intestinal DCs in the pathogenesis of IBD is undefined. This project aimed to determine the influence of inflammation on RALDH activity of antigen presenting cell (APC) subsets including CD103+ DCs within human distal intestinal mucosa. RALDH activity was identified by Aldefluor assay in intestinal DCs (CD103+ and CD103- subsets) alongside ALDH1A2 expression in healthy controls. In contrast with mouse models, RALDH activity was not reduced in CD103+ DCs from IBD patients. An increased frequency of CD14+ macrophages (MФ) of IBD patients displayed ALDH1A1-associated RALDH activity compared with healthy controls. Blood CD14+ monocytes, putative precursors of intestinal CD14+ MФ, of healthy controls and IBD patients displayed ALDH1A1-associated RALDH activity indicating RALDH is systemically acquired by monocytes and upregulated within the mucosa of IBD patients, or alternatively that RALDH+ monocytes are selectively recruited in IBD. In vitro, inhibition of RA receptor-α signalling blocked GM-CSF-mediated differentiation of TNFα-producing pro-inflammatory RALDH+ CD14+ MФ from monocytes, consistent with enhanced RALDH activity of intestinal CD14+ MФ in IBD supporting a pro-inflammatory phenotype. Soluble intestinal mediators including prostaglandin E2 suppressed RALDH activity of MoDCs in vitro, whilst mediators from inflamed IBD mucosa conditioned MoDCs to imprint enhanced levels of α4β7 expression on naive CD4+ T cells independent of RALDH activity. This study provides the first systematic analysis of RALDH activity in human intestinal APCs and indicates important distinctions between mouse models and human IBD