We investigated the impact of vitamin A status on the phenotype and function of FoxP3+ T cells in regulation of intestinal inflammation. We found that both high and low vitamin A therapies induce distinct subsets of FoxP3+ T cells with potent regulatory activities. A population of FoxP3+ T cells termed “Hi-A FoxP3+ T cells” was greatly increased by retinoic acid under excessive vitamin A availability. On the other hand, a population of FoxP3+ T cells termed “Low-A FoxP3+ T cells” was greatly increased under limited vitamin A availability in the absence RARα occupancy. Interestingly, both the Hi-A and Low-A FoxP3+ T cells were significantly more efficient in suppression of intestinal inflammation than control FoxP3+ T cells in vivo.
It has been reported that retinoic acid can enhance TGF-β1-mediated induction of FoxP3 and CD103 in vitro.17-22
Indeed, we found that ~70% of the FoxP3+
T cells induced in the intestine but not other organs of Mid-A or HI-A mice had this phenotype (). This organ specificity is thought to be due to the fact that dendritic cells and epithelial cells of the intestine have the ability to produce retinoic acid.21
T cells were increased in HI-A mice but decreased in the intestine of Low-A mice. Importantly, Hi-A FoxP3+
T cells were highly effective in reversing intestinal inflammation in SCID mice. Thus, the pathway of induction of FoxP3+
T cells by retinoic acid operates in vivo in a physiological setting. We, unexpectedly, found in this study that a novel subset of FoxP3+
T cells enriched with CD103+
cells is highly induced in limited vitamin A availability in most tissues including the intestine.
It has been reported that CD103+
Tregs isolated from lymphoid tissues were highly suppressive in regulation of inflammation.34
While they share the CD103 expression, the relationship between these CD103+
Tregs of normal mice and the Low-A FoxP3+
T cells is unclear at the moment. CD103+
Tregs highly express the Treg-associated molecules such as GITR, 4-1BB and LAG3 compared to control Tregs. Perhaps the most intriguing phenotype of the Low-A FoxP3+
T cells is their homing receptor expression pattern. The Low-A Tregs preferentially express CD103, CCR7 and CCR5 but not the gut homing receptors CCR9 and α4β7, and they have better circulating ability in blood, spleen and marrow compared to control Tregs. A rather surprising fact is that, despite the deficiency of CCR9 and α4β7, Low-A FoxP3+
T cells are found at high frequencies in the small and large intestine. Based on the fact that the Low-A FoxP3+
T cells do not migrate well to the small intestine (), it is likely that these cells are the result of induction or expansion rather than migration to the intestine.
We propose that unavailability of RARα ligands in the Low-A condition, which makes the RAR-RXR heterodimer receptor inactive (“apo state”),35
is responsible for the increased induction of the specialized FoxP3+
T cells. This is supported by the increased conversion of naïve CD4+
T cells into CD103+
T cells by antigen presenting cells of both thymus and peripheral lymphoid tissues in the presence of a RARα antagonist. In this regard, the frequency of CD103+
T cells was significantly increased in the thymus and most peripheral tissues of Low-A mice (), suggesting that both the thymic output and peripheral induction of the specialized FoxP3+
T cells are greatly increased in the Low-A mice.
In this study, we found that Th17 cells were highly enriched in the gut of Hi-A mice. This was surprising given the fact that retinoic acid can suppress the induction of Th17 cells in vitro.17, 21, 22
This may be due to the fact that retinoic acid production in vivo is tightly regulated in space and time that the retinoid-induced suppression of Th17 cells does not happen widely in the body within the physiological concentration range of retinoic acid (~10 nM). Another possibility is that even the migration and retention of Th17 cells in the gut would require the retinoic acid signal in a manner similar to other effector T cells. Indeed, vitamin A is required also for the optimal population of Th1 and Th2 in the small intestine. Th17 cells were also increased in SCID mice after transfer of Low-A or HI-A FoxP3+
T cells, and this is consistent with others’ finding that FoxP3+
T cells could promote the generation of Th17 cells.36
We observed a negative correlation between frequencies of Th17 cells and intestinal inflammation and a positive correlation between Th1 and intestinal inflammation. The negative correlation between Th17 cells and intestinal inflammation is in line with the protective role of IL-17 and Th17 cells in this model.37
In Low-A SAMP/YP mice, however, there were decreases in both Th17 cells and intestinal inflammation. In this case, CD103+
cells were increased and Th1 cells were decreased. Thus, it appears that the intestinal pathology in the Low-A and Hi-A SAMP1/YP mice and in the SCID mice would be regulated by the balance between protective (Tregs and perhaps Th17) and inflammatory (Th1) cells.
It has long been observed in animals and humans that, vitamin A deficiency leads to defective immunity and increased susceptibility to infection.38
There are several explanations available for this phenomenon such as defective maturation of neutrophils and IgA deficiency. The results of this study provide another potential explanation. The greatly increased frequencies of highly suppressive Low-A FoxP3+
regulatory T cells and concomitantly decreased effector T cells could induce the typical immunodeficiency associated with vitamin A deficiency. Taken together, our results provide novel therapeutic strategies to suppress inflammation in the intestine and potentially other tissue sites by regulating vitamin A availability. It is expected also that the Hi-A and Low-A FoxP3+
T cells would have the potential to be used as effective cellular therapeutics in suppression of inflammatory diseases.