This week on TIBDI: Impressive results in Nature show how gene mutations cause reduced autophagy, and mutations in the XIAP gene lead to early-onset CD in male patients.
From an Autophagy Gene to Crohn’s Disease
Genome wide association scans confirmed the importance of ATG16L1 mutations in Crohn’s disease (CD), especially a variant consisting of an Alanine to Threonine exchange at the 300th amino acid. Despite an abundance of literature, the precise mechanism linking the mutation to reduced autophagy was unknown. Dr. Aditya Murthy from Genentech, Inc. has now found an answer. The mutation is located in a cleavage site for the enzyme caspase-3, and it makes the protein more susceptible to cleavage. Caspase-3 is well known for its role in initiating apoptosis during cellular trauma, for instance during metabolic stress or intestinal infection. Without proper autophagy, macrophages are unable to neither regulate their energy consumption nor properly eliminate pathogens, and have an heightened inflammatory response.
XIAP Mutations in Early Onset CD
Variants in the gene encoding for the X-linked inhibitor of apoptosis protein (XIAP) can sometimes lead to intestinal inflammation. XIAP is involved with a multitude of processes including NOD signaling, apoptosis and NKT cell development. To investigate XIAP’s possible role in CD, Dr. Yvonne Zeissig and her colleagues at the University Medical Center Schleswig-Holstein in Germany, looked at CD patient samples to find if there were clear associations between XIAP and immune cell function. She found that approximately 4% of male early-onset CD patients had unique mutations in their XIAP genes. Experiments with patient primary cells revealed that loss of XIAP function caused defects in NOD1/2 signaling.
This week on TIBDI: Human stem cell transplantation redefines T cell repertoires, SIRT1 blocks the development of induced regulatory T cells, and stress sets the stage for intestinal inflammation.
Stem Cell Transplantation Wipes CD4+ T cell Memory
Human stem cell transplantation (HSCT) is a potential treatment for severe cases of inflammatory bowel disease (IBD). One way that HSCT works is by resetting the adaptive immune system. However, few studies have looked in depth at changes in the T cell repertoires. Dr. Paolo Muraro from the Imperial College of London has now addressed this question. In a HSCT trial for multiple sclerosis (MS) patients, he and his team used high-throughput sequencing to assess T cell receptor changes in 24 patients. They found that CD4+ and CD8+ T cells responded differently to HSCT. The patients’ CD4+ T cells were redefined and had a new repertoire of clones, while the CD8+ T cells reflected pre-HSCT clones. Resetting CD4+ T cells could be one reason why that HSCT is also successful for IBD.
SIRT1 Suppresses Suppressor Induction
Regulatory T cells (Tregs) are known to be important in IBD, and work from animal models shows that they can regulate the severity of symptoms. Previous work by Dr. Tatiana Akimova and her colleagues at the Children’s Hospital of Philadelphia demonstrated a connection between SIRT1 and Tregs. To investigate this more in induced Tregs, they used SIRT1 deficient cells in the T cell transfer model of colitis. Loss of SIRT1 increased the induction of Tregs and effectively attenuated colitis development. This result was mirrored in dextran sodium sulfate colitis using an inhibitor of SIRT1 (EX-527). It will be interesting in the future to see if targeting SIRT1 will work in a therapeutic setting.
Stress Hinders Regulatory T Cells
Most IBD patients are quite aware that stress plays a role in their disease progression. However, the connection between stress and IBD remains shaky. Dr. Wei Wu of Tongji University considered that the missing link could be Treg function. To test this concept, they stressed mice and investigated the Tregs both in vitro and in vivo. Tregs from stressed mice were unable to function as normal, and some expressed IL-17 and TNFα. Prolactin, a stress mediator, mediated this change via dendritic cells. Stressed mice were highly susceptible to colitis, however, blocking prolactin reduced colitis. The authors feel that stress and prolactin set the stage for IBD development by the conversion of Tregs from effective suppressors to harmful pro-inflammatory T cells.
This week on TIBDI: Interleukin-13-producing NKT cells may be behind ulcerative colitis, and nanoparticle transport of CD98 small interfering RNAs may offer new treatment options for IBD.
An NKT Cell Antigen for Ulcerative Colitis
Ulcerative colitis (UC) is well known for its association with Th2 responses. More recently, it was found that IL-13-producing, Type II NKT cells accumulate in a rodent model of UC. Furthermore, these studies showed that IL-13 is cytotoxic for the epithelial cells and increases the killing activities of NKT cells. Dr. Ivan J Fuss of the National Institutes of Health continued this work in human UC patients. He found that the NKT cells accumulate in the lamina propria and when exposed to the self-antigen lyso-sulfate begin to secrete large amounts of IL-13 and upregulate IL-13Rα2 (IL-13 receptor). The authors speculate that these cells could be the main drivers of UC inflammation and that a key pathologic event in UC may be the abnormal expression of lyso-sulfate by gut epithelial cells in response to signals from microbiota.
Nanoparticles for the Treatment of IBD
Small interfering RNAs (siRNAs) offer the possibility of treating IBD by silencing disease-related genes. However, carriers are needed to bring these molecules to the cytoplasm of the correct cells. Dr. Bo Xiao of Georgia State University found success delivering CD98 siRNA to CD98+ gut cells by using orally administered nanoparticles coated with a targeting antibody and polyethylene glycol, which allows the particles to enter the mucus layer and reach the immune cells below. CD98 is part of the amino acid transporter LAT1, which amplifies integrin signaling and supports immune cell activation. The research team was able to successfully use their system to reduce the symptoms of two colitis models.
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.
This week on TIBDI: A new review is published on the gut microbiome, IBD patients have less butyrate-producing bacteria, and IL-10 deficient mice are inflamed by inflammasomes.
Healthy Gut Microbiome in the Spotlight
The state of the intestinal microbiome, in essence the microbiota genome, is proving to be an important factor during disease development and progression. However before in depth studies are done to define disease-related microbiome profiles, it’s essential to also have an idea of what profiles define a healthy state. Dr. Emily B. Hollister of the Baylor College of Medicine and Texas Children’s Hospital reviewed the current literature. In general, the gut microbiome has approximately more than 10 million non-redundant genes, and a more diverse microbiome is healthier than less diverse one. Not everyone has the same types of populations of bacteria; most healthy microbiomes can be classified into three basic enterotypes. The influence of the microbiome extends to the immune system, cellular nutrition, cellular protection, metabolic processes and the functioning of the nervous system.
Lost Faecalibacteria in IBD
Many researchers are searching for the right probiotics to treat inflammatory bowel disease (IBD). To support this kind of search, simultaneous research investigating the microbiota in IBD patients as compared to healthy ones is also necessary. Wei Wang of Wuhan University recently published evidence that some major changes in the IBD microbiota are an increase in Bifidobacteria and Lactobacilli along with a loss of Faecalibacterium prausnitzii. The loss of F. prausnitzii was especially considerable in patients with active Crohn’s disease (CD). F. prausnitzii is known to produce butyrate, which is especially important for the formation of regulatory T cells in the colon. The author suggests that instead of focusing on common lactic acid producing probiotics in IBD, patients may be better served by looking at butyrate-producing probiotic species.
IL-10 Deficient Mice Inflamed by Inflammasomes
An interesting model of IBD is the interleukin (IL)-10 deficient mouse, which develops spontaneous colitis. Dr. J. Zhang of the Medical University of South Carolina found evidence that inflammasomes play a role in this model by promoting chronic intestinal inflammation. He found that loss of IL-10 increased the levels of NLRP3 and contributed to more inflammasome activity. This caused higher amounts of active IL-1β to be produced in gut tissues, which also led to increased colitogenic Th17. Blocking inflammsome activation successfully improved the colitis of the IL-10 deficient mice, suggesting that similar strategies could be useful in IBD.