Introduction All-trans retinoic acid (RA) has emerged as an important immunoregulatory molecule with specific functions in the intestine. RA is generated enzymatically by the sequential action of alcohol dehydrogenase (ADH) and retinaldehyde dehydrogenase (RALDH) enzymes. In the mouse, RALDH expression is confined to CD103+ intestinal dendritic cells (DC) giving this subset the unique ability to mediate RA-dependent functions such as the imprinting of gut tropism on T cells. RALDH activity is downregulated in mouse models of inflammatory bowel disease (IBD) but little is known about its role in human intestine. The aim of this study was to assess RA-dependent immune regulation by human intestinal antigen presenting cells (APC).

Methods Lamina propria mononuclear cells (LPMC) were extracted from intestinal biopsies by enzymatic digestion and analysed by multicolour flow cytometry. Purified subsets of intestinal APC were obtained by immunomagnetic or FACS sorting. Expression of ADH and RALDH enzymes was quantified by quantitative RT-PCR and RALDH activity assessed using the Aldefluor assay. The ability to induce the gut homing receptor α4β7 on T cells was assessed by flow cytometry following stimulation of CFSE-labelled naïve CD4+ T cells.

Results Induction of the gut homing receptor α4β7 on human T cells was RA dependent. Intestinal myeloid APC were potent stimulators of naïve CD4+ T cells and induced high levels of α4β7. Both of these activities were attributed to the CD103+ APC fraction. However, analysis of ex vivo intestinal populations revealed RALDH activity in all myeloid APC populations studied: CD103+ and CD103− DC identified as HLA−DR+CD11c+Lin− (Lin=anti-CD3,14,16,19,34) as well as HLA−DR+CD11c+Lin+ monocyte-like cells. Lymphocytes had little or no Aldefluor activity. Comparison of intestinal DC from healthy controls and IBD patients showed similar RALDH activity in inflamed and non-inflamed tissue. RALDH activity was equivalent in CD103+ and CD103− DC. In contrast, the ADH RDH10 was expressed at levels 4.5-fold higher in CD103+ DC.

Conclusion As in the mouse, RA-mediated induction of α4β7 is a property of the CD103+ subset of intestinal DC. However in divergence from murine data, this property is associated with restricted expression of the ADH enzyme RDH10 rather than “downstream” RALDH enzymes. These data imply that RA availability is regulated differently in mice and man, with expression of RDH10 providing an important control point in humans.

Competing interests None declared.