Tag Archives: IFNg

New Insights about NOD2 and Th17 Differentiation

microRNA Mir210
This little piece of RNA has the power to influence Th17 differentiation.
This week on TIBDI: NOD2 and IFNγ work together to recruit cells to the small intestine, and a microRNA offers an interesting way to control Th17 differentiation.

NOD2 Behind Intestinal T Cell Recruitment

One of the most important receptors involved with Crohn’s disease (CD) is NOD2, a pattern recognition receptor that recognizes bacterial cell walls. Dr. Xingxin Wu of the Yale University School of Medicine investigated its involvement in an acute intestinal disease model induced by systemic anti-CD3. His results provide unique insight into infiltration dynamics of the characteristic CD8+ T cells found in the small intestine of this model. He discovered that NOD2 stimulation was needed for optimal infiltration. Without these signals, chemokines, specifically CXCR3-ligands, were not secreted by macrophages, dendritic cells and stromal cells. This prevented CD8+ T cells from leaving the circulation and entering the intestinal lamina propria. Moreover, the loss of CD8+ T cells in the small intestine led to reduced IFNγ, which also plays a role in stimulating immune cell chemotaxis.

Unexpected MicroRNA Control of Th17

During low oxygen conditions, immune cells upregulate transcription factors that turn on genes that help them cope with the hypoxia. One of these transcription factors, HIF-1α, also contributes to the differentiation of Th17 cells, which are important in the pathogenesis of inflammatory bowel disease (IBD). In an extremely interesting Nature Immunology publication, Dr. Haopeng Wang of the University of California in San Francisco described how the microRNA Mir210 inhibited HIF-1α expression and Th17 differentiation. MicroRNAs are small RNAs that prevent gene expression. By controlling the abundance of Mir210, he was also able to influence the numbers of Th17 T cells differentiated in vitro. Using the T cell transfer model of colitis with genetically manipulated T cells, which lacked Mir210 expression, he found that Mir210-deficient T cells caused increased numbers of Th17 and worsened symptoms. The authors suggest that drugs that function similarly to Mir210 could be interesting therapeutics.

References

  • Wang, H., Flach, H., Onizawa, M., Wei, L., McManus, M. T., & Weiss, A. (2014). Negative regulation of Hif1a expression and T. Nature Immunology, 1–10. doi:10.1038/ni.2846
  • Wu, X., Lahiri, A., Haines, G. K., Flavell, R. A., & Abraham, C. (2014). NOD2 Regulates CXCR3-Dependent CD8+ T Cell Accumulation in Intestinal Tissues with Acute Injury. The Journal of Immunology. doi:10.4049/jimmunol.1302436

The Dark Side of Retinoic Acid and Interleukin-22

Salmonella species growing on XLD agar - Showing H2S production
IL-22 secretion during intestinal inflammation gives Salmonella a competitive advantage.
This week on TIBDI: Th1 cells can activate macrophages with innate signals alone, retinoic acid is no hero in Crohn’s disease, and interleukin-22 allows some pathogens to thrive.

TCRs Are Not Always Needed

Macrophages and T cells play are important in inflammatory bowel disease (IBD). Learning about how these cells interact could lead to more insight about how IBD progresses. Hope O’Donnell of the University of Minnesota has now gleaned new insights about their interactions. She looked into the mechanisms behind non-cognate stimulation of Th1 cells (non-TCR stimulation) and their ability to secrete macrophage-activating IFNγ. Using genetically manipulated mice and a Salmonella infection model, her results show that Th1 (and CD8+) cells produce plenty of IFNγ as long as they are exposed to Toll-like receptor ligands and products of activated inflammasomes like interleukin (IL)-18 and IL-33. This study underscores the flexibility and strength of the adaptive immune response.

The Pitfalls of Retinoic Acid

Retinoic acid is the current darling of those studying anti-inflammatory responses as it has been shown that retinoic acid can lead to regulatory T cell development. To determine if retinoic acid was actually lowered during Crohn’s disease (CD), Dr. Theodore J. Sanders of the Blizard Institute in London measured retinaldehyde dehydrogenase (RALDH) activity in cell samples collected from CD patients and controls. In all of the dendritic cells and macrophages tested, the RALDH activity (ability to produce retinoic acid) was increased in CD patients compared to controls. Surprisingly, blocking retinoic acid signaling actually decreased the ability of monocytes to differentiate into TNFα-producing macrophages in in vitro tests. This would suggest that retinoic acid is less helpful in CD than what one would expect.

Salmonella Exploits Interleukin-22

Interleukin-22 is a cytokine that is designed to boost immune defenses at the gut-lumen interface. It induces antimicrobial peptide release along with factors that sequester essential metal ions (like iron) that bacteria need to grow. Dr. Judith Behnsen of the University of California has now discovered that these processes can be exploited by certain pathogens, like Salmonella. She found that IL-22 deficient mice were much less susceptible to Salmonella overgrowth. The reason was that Salmonella has the ability to compensate for the loss of ambient metal ions, while this is not the situation for many commensals. This allows Salmonella to create for a rather large niche for itself, while IL-22-induced processes decimate the competition.

References

The Curious Case of Toxoplasma gondii in Intestinal Disorders

T. gondii, a protozoan carried by cats, is known to reside in humans as well. At least 1/3 of the human population carries the cysts without even knowing it. Recent studies have shown that many more IBD patients than healthy individuals carry antibodies recognizing this pathogen, suggesting a possible association in some way. Moreover, current studies in mice show that T. gondii can wreak havoc in the intestines during initial exposure to the parasite leading to the loss of Paneth cells.  As Paneth cells are crucial for keeping intestinal bacteria under control, their loss also leads to an overgrowth.

Main points:

  • T. gondii antibodies are found in IBD patients.
  • T. gondii via TLR11 causes T cells to release huge amounts of IFNγ that leads to the death of Paneth cells in mice.
  • The loss of Paneth cells leads to changes in the microbiota and intestinal inflammation.
  • Humans do not express TLR11, but they could recognize T. gondii via TLR7 or TLR9.

Take home message: The possibility exists that T. gondii infections could complicate or even help initiate IBD in certain individuals.

A recent Journal of Autoimmunity article found that antibodies directed towards T. gondii were more prevalent in IBD patients than in controls. This suggests that T. gondii, a primitive protozoan, could be involved in the pathology of IBD. As up to 1/3 of the human population is often latently infected with T. gondii, this sparked my interest and a quick look at the recent literature about T. gondii revealed that this protozoan isn’t going unnoticed in the intestines (at least in mice that is…).

TIBDI post 4 finalT. gondii is well known for its ability to be spread through cat feces. Indeed, cats are its primary host and is the only mammal in which it reproduces (produces oocysts). T. gondii’s lifecycle consists of first being excreted as an oocyst in cat feces, which are, through various means, ingested by other mammals. In mammals, other than cats, T. gondii can survive as cysts within nervous or muscle tissue, often without the host noticing anything. Mice, which are considered secondary hosts, do not secrete the oocysts in their feces, but are great spreaders of T. gondii because the cysts they carry in their bodies can infect cats when they are caught and eaten.

Mice are efficient at keeping the latent T. gondii under control and show no signs of the infection. However, during acute toxoplasmosis, they do develop an acute immune response that, interestingly enough, includes an intestinal dysbiosis, especially in the area of the ileum.

It is this intestinal pathology that was the focus of a recent article found in Nature Immunology. The authors were interested in finding out more about the immune processes associated with the development of the intestinal disorder. They discovered that the pathology was related to the production of IFNγ by T cells, particularly CD4+ T cells, which would lead to the destruction Paneth cells.

Paneth cells are particularly important cells found in the crypt regions of the intestines. They produce anti-microbial peptides, which keep the crypts free of bacteria. If they die, then intestinal bacteria are free to enter the deep folds of the intestinal surface. This can lead to unwanted interactions between bacteria and bacterial sensing receptors (such as Toll-like receptors (TLRs)) and can, ultimately, cause inflammation.

In the case of the T. gondii-infected mice, it appears that this is happening, and it is a part of the reason that the intestinal problems occur. Their results indicated that recognition of T. gondii through a MyD88 (an adaptor molecule for TLR signaling) dependent cascade in T cells caused a huge release of IFNγ that lead to mitochondrial-damage-induced death of Paneth cells. The loss of these Paneth cells then caused the loss of crucial anti-microbial peptide production and subsequent bacterial over growth. In this environment, they detected that the intestinal flora composition changed and was predominated by the Enterobacteriaceae family of Gram-negative bacteria. These bacteria were not bystanders and their presence was necessary for the intestinal symptoms including the loss of Paneth cells.

This story is fascinating, but has a chicken and the egg problem. If Enterobacteriaceae bacteria are necessary for the loss of Paneth cells, how can it be necessary that the loss of Paneth cells is needed for their abnormal overgrowth? It must be that the combination of T. gondii infection plus normal interactions with Enterobacteriaceae is causing the problem.

The authors do address this somewhat in the discussion and say that it must be a combination of activation of TLR11 and other TLRs that is doing the trick. TLR2, which recognizes Gram-negative bacteria, would be such a candidate. They do find that TLR11-/- mice are partially protected from the intestinal problems, suggesting that TLR11 plays a predominant role. However, they do not check to see if TLR11 is actually the main TLR being triggered on the T cells or even investigate its expression on different immune cells.

Incidentally, TLR11 is not even produced in humans, which would make one wonder how relevant this study actually is for humans. In fact, a recent study in Cell Host & Microbe explains that two alternative TLRs in mice, 7 and 9, recognize RNA and DNA of T. gondii, respectively. They also found that in human cells, this method of T. gondii recognition via nucleic acids is particularly potent.

It is interesting to consider that T. gondii may play a role in IBD. However, intestinal problems are not the most common symptom for human infected with T. gondii. Though the number of IBD patients producing antibodies that recognize T. gondii is significantly higher than healthy controls, the number is still very low (8% of patients react to T. gondii as opposed to 1% in the normal population). Still, it could be that those 8% have just the right genetic background that when exposed to T. gondii produces just the right conditions for the disease to develop. Only further research will answer that question.

Disclaimer: This article is not intended to provide medical advice, diagnosis or treatment.

Sources

Andrade, W. A., do Carmo Souza, M., Ramos-Martinez, E., Nagpal, K., Dutra, M. S., Melo, M. B., et al. (2012). Combined Action of Nucleic Acid-Sensing Toll-like Receptors and TLR11/TLR12 Heterodimers Imparts Resistance to Toxoplasma gondii in Mice. Cell Host & Microbe, 1–12. doi:10.1016/j.chom.2012.12.003

Raetz, M., Hwang, S.-H., Wilhelm, C. L., Kirkland, D., Benson, A., Sturge, C. R., et al. (2012). Parasite-induced TH1 cells and intestinal dysbiosis cooperate in IFN-gamma-dependent elimination of Paneth cells. Nature Immunology, 1–9. doi:10.1038/ni.2508

Shapira, Y., Agmon-Levin, N., Selmi, C., Petríková, J., Barzilai, O., Ram, M., et al. (2012). Prevalence of anti-toxoplasma antibodies in patients with autoimmune diseases. Journal of Autoimmunity, 39(1-2), 112–116. doi:10.1016/j.jaut.2012.01.001

 

One T Cell to Rule Them All

Ly6B staining during colitis
Inflamed Colon with Stained Neutrophils

In the last post, we discussed a bit about how some stem cells are able foster memory T cells, which contribute to the chronicity of colitis. This time, I look at an article that sheds even more light on the role of the bone marrow during inflammation. It was found that under conditions of inflammation, stem cell number increase. During colitis, these cells proliferate and, ultimately, produce more neutrophils and macrophages.

Main points:

  • Hematopoietic stem cells and progenitor cells can be found outside the bone marrow during inflammation in the colon.
  • This appeared to be mediated by T cell-derived cytokines.
  • This process increased the amounts neutrophils and macrophages at the sites of inflammation.

Take home message: In light of my last post, T cells can profoundly influence the innate immune system through stem cells. This strengths their potential role in chronic inflammation.

Neutrophils and macrophages are directly involved in causing damage in the colon during inflammation. Neutrophils and macrophages are the tanks of the immune system and each has their own weapon. Neutrophils are capable of releasing granules containing harmful reactive oxygen species and enzymes. They also release microbial webs (à la Spiderman). Macrophages engulf harmful bacteria and other pathogens and then secrete cytokines (chemical messengers) that alert other immune cells.

The authors used mainly the T cell transfer colitis model, induced by transferring CD4+CD25-CD45RBhi T cells (naïve T cells) to Rag1-/- mice (mice lacking T cells). They found that the inflammatory environment created by T cell transfer colitis had an effect on hematopoietic stem cells (HSC) in the bone marrow (BM), causing them to proliferate. HSC are the stem cells that produce immune cells. The developmental process involves first the production of multipotent progenitor cells. These HSC daughters have a limited life span, but are capable of further differentiating into two types of progenitors, the myeloerythroid progenitors (CMPs) or common lymphoid progenitors (CLPs). CMPs can further differentiate into either megakaryocyte-erythroid progenitors (MEPs) or granulocyte-monocyte progenitors (GMPs). The authors found that under conditions of inflammation, the GMPs would begin to dominate the total myeloid progenitor population. Moreover, HSC and GMP were also found outside of the bone marrow in the spleen and even in the inflamed colon.

IFNg and GM-CSF were considered possible candidates for the induction of these changes. IFNg is produced mainly by T cells and is important in supporting a Th1-associated immunity and is also known to directly affect HSC function. When the authors neutralized IFNg in vivo with antibodies, they found that the proliferation of the HSC decreased considerably. GM-CSF, on the other hand, is produced by stromal cells in the bone marrow (e.g. fibroblasts) and can also be produced by Th17 cells.  GM-CSF is known to be involved with the differentiation of granulocytes. When this cytokine was neutralized, there was little effect on the HSCs, but instead an effect of the downstream GMPs. They were less dominant and loss of GM-CSF reduced their ability to populate the spleen and the colon during colitis.

One point that the authors wished to make was that IL-23 was involved in their findings. This is because IL-23 has such a prominent role in IBD pathogenesis. Current studies looking at genetic association in IBD show that many genes involved in IBD have something to do with the IL-23 pathway. To make this point, they emphasized that in the T cell transfer model many of the transferred T cells eventually differentiate into Th17 cells that require IL-23 for maintenance. They also looked at a form of colitis induced by H. hepaticus infection of 129SvEv.Rag2-/- mice (mice lacking T cells) that is mediated via IL-23. They found that this model also displayed inflammation-associated changes in HSCs; abundant and proliferating HSCs were evident and neutralizing IFNg reversed the phenotype. They also looked at T cells isolated from both wildtype and IL-23R-/- mice and found that Th17-derived GM-CSF was at least partially dependent on IL-23R expression on T cells. However, the most convincing evidence was that IL-23-/- mice with T cell transfer colitis showed reduced GMP accumulations.

I find the IL-23 point in this article not particularly exciting. What I do like is the concept of T cell-derived GM-CSF and IFNg being so important in encouraging the production of excess GMPs during inflammation, though the authors didn’t actually prove this. Still, if you consider these findings together with my last post, which discussed how IL-7 is necessary for T cell transfer colitis by creating a niche for colitogenic memory T cells, it would almost seem that the MSCs (via IL-7) of the bone marrow are controlling inflammation by supporting memory T cells that can provide crucial information to the HSCs. The T cells “communicate” to the HSCs via IFNg that supports of HSC proliferation and mobilization to peripheral regions. GM-CSF would then provide an additional signal to encourage the differentiation of GMPs and, ultimately, the increase of neutrophils and monocytes.

In light of the last post, it seems that there is an intimate relationship between the stem cells of the bone marrow and T cells during colitis development. Naturally, the question is, which cells are more important in generating inflammation in the colon, the T cells or the stem cells? In other words, which cell really does rule them all?

Disclaimer: This article is not intended to provide medical advice, diagnosis or treatment.

Sources:

Griseri T, McKenzie BS, Schiering C, Powrie F. Dysregulated hematopoietic stem and progenitor cell activity promotes interleukin-23-driven chronic intestinal inflammation. Immunity. 2012 Dec 14;37(6):1116-29. doi: 10.1016/j.immuni.2012.08.025. Epub 2012 Nov 29.