Every once in a while, there’s an article that’s so groundbreaking that opens up whole new avenues of study. Today’s post talks about the chemokine receptor, GPR15, which is needed for regulatory T cells to travel to the large intestine.
Science is a very respected journal and it’s not often that an article appears that so relevant is for inflammatory bowel disease research. So when something does appear, I assume that it’s definitely worth checking out. This latest offering concerns a G- protein coupled receptor called, GPR15, or by some simply as “Bob.” This receptor is known to be a chemokine receptor, and it wasn’t discovered as a consequence of research looking at cell homing, but during a search for co-receptors necessary for HIV and SIV viruses. While it was known that it was expressed highly in gut tissues and lymphoid organs, until now, little else was known about it. This changed when the team of Kim et al. decided to make a GPR15 green-fluorescent protein (GFP) reporter mice as well as GPR15 knockout mice.
Initial studies of the GFP+ CD4+ T cell revealed that they looked a lot like Foxp3+ regulatory T cells (Treg), and that the brightest cells were located in the colon lamina propria. The knockouts, on the other had, showed huge losses of Treg in the colon as compared to the small intestine. By crossing T cell receptor (TCR) transgenic mice with their special mice, they were able to conclude that even during an antigen-specific T cell response (induced by feeding the mice antigens recognizable by the transgenic TCRs), loss of Gpr15 still primarily affected the Treg and not the non-regulatory T cells.
The experiments that the group performed to prove that GPR15 was needed for Treg homing to the large intestine were extensive. They performed transfer experiments, blocked the receptor signalling using drugs and even mutated the receptors signalling motif.
They found that the expression of GPR15 was the most induced by TGFβ1, and not by the other darling of the anti-inflammatory mediators, retinoic acid. They wondered if GPR15 was also influenced by the presence of microbiota and looked at their reporter mice after treatment with antibiotics. They found that loss of microbiota led to loss of Gpr15 expression, however, GPR15 expressing Treg were still able to find their way to the colon even in mice treated with antibiotics or housed in germ-free conditions. This suggests that while microbiota can change GPR15 expression, it doesn’t seem to affect the expression of its ligand(s), which is a very interesting observation.
The team then investigated three different types of colitis models with their knockout mice: Rag2-/- mice with anti-CD40, Citrobacter rodentium infection, and Helicobacter hepaticus infection/T cell transfer model. In the purely innate immune model induced with Rag2-/- mice, which lack T and B cells, there were more cytokines produced in the colon tissues. In the Citrobacter model, the differences were more astounding. Unlike the wild type mice, which could resolve their colitis, the knockout animals were unable. They ended up losing a lot of weight and dying. Bringing home the point, again, how important Treg are for resolving inflammation in the gut. In the last model, they only showed that the transfer of naïve T cells from both Gpr15 wild type and knockout in Helicobacter infected mice led to the same severity of disease. This result does make me suspect that there were some problems with experimental design, the isolation of naïve T cells removes established Treg, and that is what leads to the colitis in the first place.
To cap off their study, they looked at GPR15 mRNA expression in human tissues. They found that the highest expression was in the lymphocytes of the large intestine, however, the expression was not preferentially in the Treg-enriched CD25+ T cells that they isolated. It was actually higher in the CD25- cells. My own studies with human Treg have taught me that, at least in this regard, mice and humans are not the same. I am curious what will happen when they try isolation using additional human Treg markers.
The authors state that GPR15 influences both the thymic and induced populations of Treg. A very recent letter to Nature described that the majority of the Treg in the colon, however, is of thymic origin and are not induced Treg. Seeing that thymic Treg do need to travel to the colon, a chemokine receptor, like GPR15, would be of utmost importance. Another line of research would be to see if GPR15 is necessary for retention of thymic Treg in the colon as opposed to just homing. It would be very interesting to see more on this particular aspect.
Finding such a marker is like opening Pandora’s box. One is left with a million questions. In one of my last posts, there was about the possibility of defective Tregs in Crohn’s disease. Could GPR15 be defective in IBD? Levels should be checked on patient Treg.
I was also intrigued by the concept that microbiota is able to influence the expression of GPR15 on Treg. The intestinal microbiota is known to induce TGFβ secretion by intestinal epithelial cells, which according to this study would promote GPR15 expression. This TGFβ in turn encourages TGFβ production by dendritic cells, which can, in turn, lead to induced Treg that produce even more of the cytokine. Clearly under normal conditions, there is no loss of TGFβ in the intestines under steady state conditions. Could it be that under inflammatory conditions, TGFβ expression is lowered enough so that Treg are no longer retained and migrate away from the colon?
This was a fascinating article, and, in my opinion, it has brought the field of mucosal immunity one step closer to understanding what’s going on in inflammatory bowel disease. Maybe the next step for treatment, is finding a way to increase GPR15 expression on Treg in IBD patients or to increase the expression of its ligand.
Cebula, A., Seweryn, M., Rempala, G. A., Pabla, S. S., McIndoe, R. A., Denning, T. L., et al. (2013). Thymus-derived regulatory T cells contribute to tolerance to commensal microbiota. Nature, 497(7448), 258–262.
Kim, S. V., Xiang, W. V., Kwak, C., Yang, Y., Lin, X. W., Ota, M., et al. (2013). GPR15-Mediated Homing Controls Immune Homeostasis in the Large Intestine Mucosa. Science.
Maynard, C. L., Elson, C. O., Hatton, R. D., & Weaver, C. T. (2012). Reciprocal interactions of the intestinal microbiota and immune system. Nature, 489(7415), 231–241.