RA directly influences the development and effector functions of various immune cell types [2
]. Emerging evidence suggest that RA plays a significant role in regulating the functions of APCs in the intestinal immune system. RA is produced by many subsets of intestinal APCs, and facilitates the induction of T regulatory cells by these APCs. Here we will review the emerging data on the importance of specific DC and macrophage subsets in the intestine that express RALDH enzymes, and facilitate the induction of T regulatory cells.
It is now clear that there are several phenotypically distinct subsets APCs in the intestine, and the evidence suggests that they may differentially regulate Th1/Th2/Th17/T regulatory responses [2
]. These subsets are situated in three major locations: the lamina propria (LP), Peyer’s patches (PP) and mesenteric lymph nodes (MLN) [2
]. Several years ago, Iwata and colleagues described that DCs in the mesenteric lymph nodes and Peyer’s patches expressed RA producing enzymes, and were capable of producing RA from retinol [44
]. Subsequently, much effort has gone into identifying the specific subsets of APCs in the intestine that produce these enzymes, and their roles in inducing T regulatory cells.
In the lamina propria (LP), there are two major subsets of DCs: CD11c+
DCs (). Lamina propria DCs can also be classified, based on the expression of the chemokine receptor CX3CR1 (the receptor of CX3CL1, fractalkine) [45
]and the α– integrin CD103 [43
]. The correlation between CX3CR1, CD103 and the “traditionally defined” subsets is at present murky, but our recent work suggests that CD11c+
DCs in the LP are CX3CR1+
and a major fraction of this subset also expresses CD103 [52
]. In addition, a proportion of the CD11c+
DCs in the LP are CX3CR1−
]. The functional properties of various subsets of DCs in the LP are only now beginning to be appreciated.
Subsets of intestinal antigen presenting cells produce RA and induce T regulatory cells
Recent work by Sun et al., suggests that DCs in the LP (some of which express CD103) induce T regulatory cells via a mechanism dependent on RA [51
]. The conversion of naïve T cells to T regulatory cells could be impaired by adding inhibitors of retinal dehydrogenases, indicating that the RA produced by the DCs facilitated the conversion. Importantly RA alone could not induce the conversion of naïve T cells to T regulatory cells, but enhanced the conversion in the presence of TGF-β. Furthermore, both the CD103+
DC subsets in the lamina propria seemed capable of converting naïve T cells to T regulatory cells. In the mesenteric lymph node (MLN), it is only the CD103+
DCs, (and not the CD103−
DCs) which express aldh1a2
, a retinal dehydrogenase involved in the conversion of retinal into RA [49
], and, induce T regulatory cells, in the presence of TGF-β.
In addition to DCs, the gastrointestinal tract contains large number of macrophages (10–20% of all mononuclear cells). Mucosal macrophages are derived from unique blood monocytes precursors and are recruited to the lamina propria by the non-inflamed tissues [53
]. Recent work from our laboratory has identified a population of CD11b+
macrophages in the lamina propria of the small intestine () [52
]. In the “steady-state” condition, these macrophage subsets are functionally different from splenic macrophages. For example, unlike splenic macrophages, intestinal macrophages constitutively produce high levels of the anti-inflammatory cytokine IL-10 [52
], and produce lower amounts of pro-inflammatory cytokine upon stimulation with TLR ligands. Furthermore, these macrophages express higher levels of aldh1a1
, relative to their splenic counterparts. Importantly, these macrophages convert naïve CD4+
T cells to Foxp3+
T cells in vitro, in the presence of exogenous TGF-beta, and via a mechanism dependent on both IL-10 and aldh1a1
. The question of whether this particular subset of macrophage is restricted to the lamina propria, or whether equivalent cells can also be found in the mesenteric lymph nodes, and whether they bear any relation to the aldh1a2+
DCs found in the lamina propria deserves further study. Furthermore, since naïve T cells are not thought to be present in the lamina propria, the question of whether these macrophages can present antigens to activated or memory T cells in situ to induce tolerance remains to be determined. Indeed, under non-inflammed conditions, these cells may play a role in the induction and maintenance of T cell tolerance to food antigens and normal flora.
However, not all lamina propria APC subsets induce T regulatory cells. For example, our recent work also suggests that the CD11c+
subset of DCs in the lamina propria of the small intestine, induces robust Th17 responses[52
]; this is consistent with recent data demonstrating the potent capacity of these CD11c+
lamina propria DCs, to promote the differentiation of Th17 cells[56
]. Intriguingly, this latter study also showed a concentration dependent effect of RA in promoting Th17 responses – with low doses (1 nM) stimulating Th17 responses, and higher doses (10 μM) suppressing both Th17 and Th1 responses[56
]. What could explain the apparent differences between the studies demonstrating potent T regulatory versus Th17 induction by DCs in the lamina propria? One likely reason is the nature of the particular subset of DC being studied; alternatively, the differences could reflect distinct isolation procedures involved.
In contrast to the gut DCs, DCs in the periphery are not though to express aldh1a1 and aldh1a2,
but do constitutively express different isoforms of ADH and hence, they lack the ability to convert vitamin A to RA[44
]. Although current studies have shown that RA is constitutively produced by several subsets of DCs and macrophages in the intestine several questions remain unanswered. For example, is this dependent on innate signals from commensals? If so, is it dependent on stimulation of APCs via TLRs, or other non-TLRs such as NOD like receptors, and C-type lectins. In this context, the question of whether RA synthesizing enzymes can be induced in APCs in other tissues, and if so, the mechanisms of their regulation are currently not known. Indeed, vitamin A is stored in other organs such liver, lungs and bone marrow[27
]. So, it is not know whether DCs present in these organs constitutively express vitamin A metabolizing enzymes or it can be induced under certain conditions. It is interesting to note that a recent report suggests that IL-4 induces aldh1a2
in MLN DCs in vitro[58
]. Further research will be required to see the how these enzymes are regulated in different subpopulation of APCs during infection or in other inflammatory disease conditions.