This week on TIBDI, we see a plethora of interesting articles including a parallel publication about the necessity of IL-10 conditioning for gut macrophages and the ways that DNA methylation influences colon Treg proliferation.
Intestinal Macrophages Need IL-10 Conditioning: Parallel Publication
Macrophages are an important immune cell of the intestines. For instance, CX3CR1hi macrophages capture antigens from the lumen by extending dendrites up through the epithelial layer and into the mucus to interact with passing bacteria. In the latest set of publications by the journal Immunity, parallel articles examine the relationship between macrophages and the anti-inflammatory cytokine interleukin (IL)-10. Using two different approaches, Dr. Ehud Zigmond of the Weizmann Institute of Science in Israel and Dr. Dror S. Shouval of Harvard Medical School in the United States were able to make similar conclusions.
Dr. Zigmond, using macrophage-restricted Il-10-/- and Il-10ra-/- mice, determined that macrophages with deficient IL-10 secretion were not nearly so harmful to the gut as macrophages not being able to respond to IL-10. Losing the ability to be conditioned by IL-10 made the macrophages more pro-inflammatory and led to spontaneous colitis. Dr. Shouval approached his research by creating bone-marrow chimeras with Rag2-/-Il-10rb-/- bone marrow and using the T cell transfer model of colitis. He found that loss of IL-10 signaling in innate immune cells led to colitis development. His work, unlike that of Dr. Zigmond, revealed that IL-10 conditioned macrophages are needed for proper regulatory T cell (Treg) development, and mucosal immune tolerance. He also found that pediatric inflammatory bowel disease (IBD) patients with mutations in their IL-10 receptors also had more pro-inflammatory macrophages. This work may lead to insights about why IBD develops.
Colonic Treg Proliferation Needs Uhrf1
Finding the ways that epigenetic mechanisms control T cell function and numbers is an exciting new field of research. One of the latest Nature Immunology articles adds fuel to the fire by publishing the work of Dr. Yuuki Obata of the University of Tokyo in Japan. She found that the DNA-methylation adaptor Uhrf1 was needed for Treg proliferation in the colon. This was determined by profiling genes activated in proliferating Treg after colonization with bacteria. This was then confirmed using T cell-specific Uhrf1-/- mice. Loss of Uhrf1 led to hypomethylation of a cell-cycle gene and a loss of Treg division in the colon. As a consequence of the low Treg numbers, Uhrf1-/- mice developed spontaneous colitis. It will be interesting to see if the same results can be found in the human setting.
This week on TIBDI! Gene expression signatures of anti-TNFα non-responders are investigated, breast milk oligosaccharides regulate developing immune responses, and an anti-CD3 antibody offers hope for T cell regulation in the gut.
Inflammatory Signatures of Anti-TNFα Non-Responders
Even though anti-TNFα therapy for Crohn’s disease (CD) patients is very effective, up to 40% of patients are or become non-responders. To find out if there were differences in gene expression between these groups of patients, Dr. Raquel Franco Leal of the Hospital Clinic in Barcelona Spain examined mRNA levels of inflammatory genes in these two populations. She found that treatment with anti-TNFα effectively regulated many cytokines and chemokine genes despite the clinical outcome. However, those that achieved a clinical remission also had a number of changes in many other genes including IL1B, S100A8 and CXCL1. In contrast, refractory patients continued to have deregulated genes associated with pathways inducing IL17A. Besides introducing new drugs targets, these results reemphasize the importance of IL-17 pathways in CD.
Developing Immune Systems Need Milk
The complex immunoregulatory mechanisms needed to protect and control the human gut are developed early after birth, and are catalyzed by the colonization of the intestinal tract with bacteria. Suspecting that breast milk may protect the early intestinal tract from unwanted inflammatory responses, Dr. Y. He and colleagues investigated human milk oligosaccharides from colostrum (cHMOSs). Using human fetal intestine explants, they were able to determine that cHMOSs significantly altered immune gene expression. Their model suggests that cHMOSs attenuate pathogen-associated receptor signaling, simultaneously lowering immune cell activation and enhancing pathways needed for clearance, regulation and tissue repair.
T cells likely play an important role in inflammatory bowel disease (IBD) by maintaining inflammatory responses. Finding a way to specifically reduce or deactivate these cells in IBD patients could be a possible therapy. Dr. Anna Vossenkämper, together her colleagues, experimented with this idea using a special anti-CD3 antibody called otelixizumab, which is known to induce tolerance. Using mucosal biopsies from IBD patients, she was able to determine that otelixizumab could decrease pro-inflammatory cytokine production and lower the activity of multiple immune pathways. The antibody’s effects were determined to be dependent on IL-10 expression.
This week on TIBDI: IL-35-secreting B cells inhibit immune responses; lactate interferes with inflammasome activation; and mucus and microbiota link nature and nurture.
New Inhibitory B Cells
B cells, known more for their antibody producing potential, also have a regulatory function when they secrete the anti-inflammatory cytokine interleukin (IL)-10. In a recent publication of Nature, Ping Shen and Toralf Roch of the German Rheumatology Research Center (DRFZ) in Germany discovered that IL-35-secreting B cells also play a similar role. They found that triggering co-stimulatory receptors on B cells induced IL-35 production and that IL-35-deficient B cells both hindered the recovery from a model of autoimmune disease (multiple sclerosis) and increased the immune response to an intestinal pathogen (Salmonella). Given the widespread influence of IL-35 producing B cells during infection and inflammatory disease, it will be interesting to see if they also are important for inflammatory bowel disease (IBD).
Lactate Slows Down Inflammasomes
Previous literature has indicated that the NLRP3 inflammasome is associated with Crohn’s disease (CD), and may be needed to induce protective immune responses against invading bacteria. Inflammasomes in macrophages are activated, in part, by danger signals. While danger signals mainly induce pro-inflammatory cytokine production, they also stimulate metabolic pathways, and one product that is produced is lactate. According to results produced by Rafaz Hoque of Yale University, lactate can function as a negative regulator of inflammasome activation. The team at Yale found that stimulation of the lactate receptor GPR81 could modify Toll-like receptor 4 signaling and lower subsequent NLRP3 activation. In vivo, lactate was effective at reducing acute organ injury in models with potent inflammasome activation, such as hepatitis and pancreatitis. This could mean that lactate modulates NLRP3 responses in Crohn’s disease as well.
Nature, Nurture and Mucus Production
Intestinal mucus has the important function of preventing bacterial contact with the epithelial surface. In fact, TMF-/- mice lacking a specific Golgi-associated protein (TMF/ARA160), which produce thick mucus, are generally protected from experimental colitis. However, Shai Bel of the Bar Ilan University in Israel has found that the protection is not derived from mucus alone. The intestinal microbiota are also important. The team found that the microbiota of TMF-/- mice is different from that of wild-types, and has larger populations of bacteria from the Firmicutes phylum. Even more importantly, transfer of these populations to normal mice, by co-housing, also transferred the colitis protection. This underscores the potential role of microbiota manipulation in lowering IBD susceptibility despite genetic predisposition.
Q: What’s your opinion about microbiota manipulation for IBD prevention? Feel free to contribute your thoughts here or on the LinkedIn discussion group.
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.