Transforming Growth Factor-β Regulates Tissue Resident Memory T cells Retention in the Gut

Tissues such as the skin, lungs, and the gastrointestinal tract act as barriers against invading pathogens. To protect the individual host, invading pathogens must be efficiently and quickly controlled by the immune system. In order to control pathogens with efficiency, one of the hallmarks of the adaptive immune system known as memory provides long-term protection of lymphocytes that easily recognize pathogens to rapidly clear them from the tissues. Cells in the immune system that partake in memory are known as long-lived CD8+ memory T cells, which include central memory (TCM) and effector memory (TEM) cells. TCM cells circulate through secondary lymphoid organs while TEM cells circulate through non-lymphoid tissues. Some of these memory T cells migrate into peripheral tissues, including skin, lung, and gastrointestinal tract, and become resident long-lived memory T cells (TRM) that protect against infection by taking up residence within the peripheral tissues. The underlying mechanisms and signals that regulate the retention of TRM cells in the tissues are not completely understood so scientists in a recent study looked at the effects of the TGF-b signaling on TRM cell migration to the gut tissues as well as TGF-b signaling while TRM cells reside in the gut tissues.  

An important point to know is that the TEM cells develop a gut-homing capacity during local and systemic infections that upregulate a bunch of integrins for the T cells to settle into the gut. The T cells in the spleen upregulate the expression of a4b7 integrin that would allow the T cells to settle into the gut. When the T cells are inside the lamina propria or intraepithelial lymphocyte compartment (both are structural tissues of the gut), the T cells downregulate the integrin a4b7. As a result of residing in the gut, the T cells upregulate CD103 (integrin aE), CD69, and integrin a1 leading the T cells to being TRM cells. The scientists noted that a lack of CD103 (integrin aE) expression leads to less T cells residing in the gut. Given that, the scientists think the 3 proteins’ expression leads to the T cells staying in the gut. The scientists realized TGF-b can increase the expression of CD 103 in the gut on CD8+ T cells to a great extent. TGF-b (Transforming Growth Factor Beta) normally works to differentiate Th17 and Tregs, but it also has many other functions. The scientists believed TGF-b signaling is important for the retention of intestinal TRM cells so they used an experiment to show the regulation of TGF-b on TRM cells.   

Keep the hygiene, lose the inflammation: TGF-β and helminthic therapy

            Incidences of autoimmune and autoinflammatory diseases, such as type 1 diabetes, rheumatoid arthritis, and inflammatory bowel disease, are increasing in the developed world, and the annual costs of treatment amount to billions of dollars in the United States alone (1). There are numerous factors that can account for this rise in prevalence, including genetic differences and environmental factors. One factor that has been gaining support, both correlatively and experimentally, is the hygiene hypothesis: basically, since we have eliminated many of the prominent childhood diseases, children are “too clean,” and thus their immune systems, rather than focusing on pathogens, are stimulated to attack “self” targets instead, or target innocuous factors leading to allergy. In other words, infection with some types of pathogens, particularly parasitic worms such as roundworms, flatworms, and hookworms, can dampen the inflammatory responses that underlie many of these autoimmune/autoinflammatory conditions. We have largely eliminated these infections, which are transmitted through unclean drinking water and soil, in developed nations, thus accounting for some of the rise in autoinflammatory diseases.

            Some suffering patients have taken this idea to the extreme, and have deliberately infected themselves with parasites to try to alleviate their symptoms. Although this treatment is highly experimental and there is not a lot of clinical data demonstrating efficacy, anecdotal evidence suggests that some patients have observed marked reduction in symptoms following “helminthic therapy.” Obviously, there are many problems associated with introducing parasitic organisms into people, so if scientists can determine how helminthic infection tamps down inflammatory responses, it could potentially lead to novel anti-inflammatory treatments that don’t involve the parasites themselves.

            At the core of this issue are two major categories of immune responses, named “Th1” and “Th2” responses for the types of helper T cells that facilitate them. Th1 responses are inflammatory in nature, and involve the activation of macrophages and killer (CD8+) T cells. These responses are elicited by the secreted cytokine interferon-γ (IFN-γ), and typically target intracellular pathogens such as viruses. Th2 responses are mediated largely by B cells, which make antibodies, and are elicited by the cytokine interleukin-4 (IL-4). Th2 responses target extracellular pathogens, such as parasitic worms, and the rationale behind the hygiene hypothesis is that by eliminating many of these Th2 pathogens, the immune system “skews” towards Th1 responses. This, then, leads to increased inflammation and the associated autoinflammatory diseases. However, other responses caused by helminth infection, including the generation of regulatory T cells (T_Regs) and the secretion of immunomodulatory cytokines such as IL-10 and transforming growth factor-β (TGF-β), might also mitigate autoinflammatory disease. A recent paper by Hübner et al attempted to distinguish which of these mechanisms was responsible for the protection from autoimmunity accorded by helminth infection. They found that generation of a Th2 response was not required, but that the production of TGF-β was largely responsible for protection.

Infect Me With Parasites? TGIT(GFbeta)

The use of helminthic therapy (the intentional infection of a patient with parasitic worms) to treat autoimmune diseases has enjoyed recent popularity for treatment of conditions as diverse as Crohn’s disease (1) and multiple sclerosis (2) . The jury is still out on its efficacy, however, and the thought of intentionally infecting patients with parasites for therapeutic purposes seems counterintuitive if not downright crazy. Part of the controversy stems from the conflicting reports as to exactly how helminthic therapy works. Some researchers suggest that helminthic therapy works by an evasive maneuver on the part of the parasitic worms to trick the immune system into producing the wrong kind of response, a type 2 cytokine response. (3) The authors of a recent study used helminthic therapy in a mouse model of non-obese diabetes (an autoimmune disorder) in order to attempt an answer to this question, which will be the focus of this blog post. (4)

The authors first demonstrated, by immunostaining, that mice deficient in IL-4 (a immune signaling molecule critical to the development of a type 2 response) fail to develop a type 2 response when infected with a parasitic worm in contrast to control mice that had normal levels of IL-4 which then developed a type 2 response. Given the role of IL-4 in promoting a type 2 response, this result seems fairly obvious and unnecessary to report. This is vital piece of data, however for the next experiment that these scientists performed.

The scientists then measured the onset of diabetes in the mice they were experimenting upon (remember they are specially designed to be genetically pre-disposed to develop diabetes) by measuring glucose levels in their blood. Surprisingly, mice infected with the parasite, regardless of competency in producing IL-4, did not develop diabetes. This suggests that a shift to a type 2 response is not the critical factor in the protection afforded by helminthic therapy in this model. What then, one might ask, are those wriggly worms doing to fend off diabetes?

The scientists who authored this study thought that it might have to do with regulatory T cells at first, but flow cytometry experiments they performed showed no difference in the numbers of regulatory T cells in any condition, infected/uninfected or IL-4 competent or not. They then hypothesized that either IL-10 or TGFβ, which have been implicated in regulating diabetes in mice could play a role. (5) Further flow cytometry experiments showed that TGFβ production was the critical factor in infected mice that correlated with the resistance to diabetes.

The results of this study suggest that the protection against diabetes afforded by helminthic therapy is not a result of a shift to a type 2 cytokine response, nor by regulatory T cells, but by an increase in the production of TGFβ, which is an immune signaling molecule that serves to tamp down the immune response. This opens up questions as to whether or not infection with parasitic worms is actually needed in order to convey the same protection that administration of TGFβ may be able to provide. This alternative to helminthic therapy must be explored further.

References

1) Summers, R. W., Elliott, D. E., Urban, J. F., Thompson, R., & Weinstock, J. V. (2005). Trichuris suis therapy in Crohn’s disease. Gut, 54(1), 87-90.

2) Benzel, F., Erdur, H., Kohler, S., Frentsch, M., Thiel, A., Harms, L., Wandinger, K.-P., et al. (2011). Immune monitoring of Trichuris suis egg therapy in multiple sclerosis patients. Journal of helminthology, 1-9.

3) Hübner, M. P., Stocker, J. T., & Mitre, E. (2009). Inhibition of type 1 diabetes in filaria-infected non-obese diabetic mice is associated with a T helper type 2 shift and induction of FoxP3+ regulatory T cells. Immunology, 127(4), 512-22.

4) Hubner, M. P., Shi, Y., Torrero, M. N., Mueller, E., Larson, D., Soloviova, K., Gondorf, F., et al. (2011). Helminth Protection against Autoimmune Diabetes in Nonobese Diabetic Mice Is Independent of a Type 2 Immune Shift and Requires TGF- . The Journal of Immunology.

5) Hancock, W. W., Polanski, M., Zhang, J., Blogg, N., & Weiner, H. L. (1995). Suppression of insulitis in non-obese diabetic (NOD) mice by oral insulin administration is associated with selective expression of interleukin-4 and -10, transforming growth factor-beta, and prostaglandin-E. The American journal of pathology, 147(5), 1193-9.