This week on TIBDI: Macrophages and dendritic cells work together to collect antigens in the gut, diet could influence inflammatory bowel disease development, and interleukin-7 maintains natural killer T cells.
Capturing Antigen Takes Two in the Gut
Oral tolerance, immune tolerance against ingested antigens, may influence the chronic nature of inflammatory bowel disease (IBD). Finding out how antigen is picked up in the gut and presented to T cells to initiate regulatory mechanisms could lead to new therapies. Now Elisa Mazzini of the European Institute of Oncology in Italy has found that in the small intestine antigen uptake leading to oral tolerance requires teamwork between macrophages and dendritic cells. Macrophages expressing the chemokine receptor CX3CR1 were exceptional in collecting soluble antigens by sticking dendrites between epithelial cells towards the lumen. They then transferred their antigen loads to neighboring dendritic cells expressing CD103, which have the abilities to initiate oral tolerance. Dr. Mazzini found that the transfer required the expression of the gap junction protein connexin 43, which allowed appeared to allow the transfer of entire peptide-MHC complexes.
Diet in IBD
The populations of microorganisms in the human gut are known to change during IBD. However, it’s unknown if there are specific changes that could initiate IBD development. A study by Dr. Lawrence A. David from Harvard University shows how easily the intestinal microbiota is changed by diet. Volunteers on an animal-based diet had significant changes in their populations as soon as one day after the diet reached the gut. Moreover the gene expression and activity of the intestinal residents were also altered. Those with an animal-based diet had increases in Bilophila species, which is associated with the development of intestinal inflammation in mice. The authors suggest that animal-based diets could contribute to the development of IBD.
IL-7 and NKT Cell Survival
Interleukin (IL)-7 is a cytokine that helps the long-term survival of Th17 cells and innate lymphoid cells that express the transcription factor RORγt. It is suspected to be important for maintaining populations of T cells that induce and propagate IBD. Dr. Kylie Webster from the Garvan Institute in Australia has now increased IL-7’s circle of influence. She found that IL-7 also maintains natural killer T (NKT) cells that produce IL-17. IL-7, using the PI3K/AKT/mTOR pathway. IL-7 caused the NKT cells to proliferate in vivo. Because more IL-7 is produced in response to bacterial triggers. Dr. Webster speculated that NKT cells may use IL-7 to quickly respond to bacterial pathogens.
This week on TIBDI: A gene behind very early onset inflammatory bowel disease (IBD) is uncovered, the receptor needed for colon regulatory T cell development is found and siblings of Crohn’s disease (CD) patients have signs of nascent intestinal inflammation.
Gene Behind Early Onset IBD
While many IBD cases are diagnosed during young adulthood and middle age, there is subset of patients that develop the disease before the age of six. To determine if there were certain mutated genes behind this early disease presentation, scientists from a multitude of institutions examined the DNA of children with very early onset IBD. They found that the children had loss of function mutations in the gene for tetratricopeptide repeat domain 7 (TTC7A). TTC7A is involved with phosphatidylinositol-4 kinase signaling. When the researchers specifically knocked down this gene in intestinal cell lines, they found that the cells lost adhesion and had increased apoptosis. This supported the clinical presentation in the children studied, which was marked by apoptotic enterocolitis.
Regulatory T cell development with GPR109A
Butyrate, a short chain fatty acid (SCFA) produced by intestinal bacteria, was recently shown to induce regulatory T cells in the colon. Scientists from Georgia Regents University have now clarified this effect even further by discovering the butyrate receptor behind increased regulatory T cells. This receptor is GPR109A, and it is also the receptor for the B vitamin niacin. They found that both butyrate and niacin gave anti-inflammatory properties to dendritic cells and macrophages via GPR109A, which encouraged Foxp3 and IL-10 expression in T cells. In colitis models, loss of the receptor led to severe disease. Studies using germ-free mice suggested that regulatory T cell defects caused by the loss of microbiota-derived butyrate could potentially be replaced by pharmacological doses of niacin.
CD Siblings As CD Models
Sisters and brothers of CD patients have an increased risk of developing disease. Initial studies of these potential patients show that they have some signs of intestinal inflammation like increased fecal calprotectin and intestinal permeability. Scientists from the United Kingdom have now investigated further and found that CD patients and their siblings have other striking similarities, such as abnormal changes in the intestinal microbiota and T cell phenotypes. The siblings were significantly different from control healthy populations, making them an unique “at risk” group. The researchers feel that studies of patient siblings could lead to new insights about the immune processes that lead to full blown CD.
Creeping fat, the unusual increase of mesenteric fat deposition in Crohn’s disease (CD), is a common characteristic in CD patients. Even though it was described in the 1930s, scientists have only just begun study it. This is due to increased knowledge about obesity and the realization that it adipose tissue can influence inflammation. While scientists have yet to decipher its true role in CD pathogenesis, there are some interesting hints.
Adipose Tissue Is an Organ with Immune Functions
Early on biomedical scientists assumed that adipose tissue (AT) was passive, however, research in the 1990s changed this concept. It became clear that AT was composed of different cell types: adipocytes, preadipocytes, macrophages, endothelial cells, fibroblasts and leukocytes. These cells functioned together to exert metabolic and immune functions, and secreted various factors such as cytokines, chemokines and hormone factors called adipokines.
Crohn’s Disease with Creeping Fat Is not Obesity
At first glance, it’s obvious that obesity and CD with creeping fat are not the same. The clinical outlook of the two diseases is enormously different. CD is a problem dominated by severe inflammation in the intestines. Those with obesity have only minor issues with inflammation and are, instead, plagued by metabolic problems, such as diabetes and high cholesterol.
Creeping fat is found in more than half of the CD cases, but it doesn’t contribute greatly to body mass index (BMI). Obesity in CD patients is actually rare. This is because creeping fat is actually normal mesenteric fat that is behaving differently, extending from the mesenterium (see figure) to the small or large intestine. In extreme cases, it wraps around more than 50% of the intestinal circumference. It is correlated with increased severity including changes in tissue structure and cellular infiltration.
AT in obesity and creeping fat is strikingly dissimilar. In obesity, adipocytes grow large and stay the same in number (hypertrophy), while in creeping fat adipocytes have increased proliferation (hyperplasia). The size of creeping fat adipocytes is actually 75% smaller than normal adipocytes.
Obese Adipose Tissue is Inflammatory
While obese AT is not obviously associated with inflammation like creeping fat, it is obesity that introduced AT an immune organ. Studies show that obese patients suffer from what appears to be a low-grade inflammation and their fat has a high proportion of macrophages. This stimulated research into the functions of adipocytes beyond simple fat storage. Many of these findings can still be applied to creeping fat.
Researchers found that adipocytes (fat cells) have functions similar to macrophages. They are able to react to danger signals and secrete immune factors in response, encouraging inflammation and migration of immune cells into AT. Not only that, adipocyte precursors, called preadipocytes, have the ability to convert into macrophages.
Investigations uncovered a variety of factors secreted by adipocytes, which is together called the secretome. The adipocyte secretome included specialized factors like the adipokines leptin and adiponectin; typical pro-inflammatory cytokines like tumor necrosis factor alpha (TNFα), interleukin (IL)-6 and IL-8; chemokines such as CCL2 and CCL8; acute-phase proteins and even antimicrobial peptides.
The adipokine leptin initially was believed to be involved only in appetite suppression. However, later research revealed that it encourages the expression of important pattern-recognition receptors (PRR) that can recognize pathogens. Recent work with leptin has revealed that it can encourage dendritic cell migration to lymphoid tissues to stimulate adaptive immune responses. It also increases macrophage cytokine production and modulates CD4+ T cell polarization. Leptin secretion is proportional to total fat mass. Therefore, obese people secrete more of it and this likely supports their low-grade inflammation.
Another important player in adipocyte-controlled inflammation is adiponectin, which has anti-inflammatory properties. Its secretion is inversely proportional to total fat mass, meaning that lean individuals secrete more of it and obese individuals have less. It exerts its function by downregulating pro-inflammatory pathways initiated by PRRs and encouraging IL-10 secretion.
Interestingly, secreted factors were not the only reasons for mild inflammation in obese patients. It was found that those with the highest levels of pro-inflammatory cytokines in the blood also had high amounts of free fatty acids (FFA). It is believed that free fatty acids may trigger PRRs, like Toll-like receptor 4, and set off inflammatory pathways.
One suspicious observation by obesity researchers is the importance of intra-abdominal fat. They found that the body mass index (BMI) was not nearly so important for determining the risk of inflammation, as was the amount of intra-abdominal fat. Apparently, there’s something special about this area that promotes inflammation.
Creeping Fat and IBD Inflammation
Like obese AT, mesenteric fat of CD patients secretes similar mediators such as pro-inflammatory cytokines, chemokines and acute phase proteins. One striking difference between mesenteric fat from obese individuals and CD patients is the secretion of adiponectin; it is higher in CD patients. Considering its anti-inflammatory function this could suggest that creeping fat may actually be trying to control inflammation instead of making it worse. In fact, a recent study suggests that it may mediate some of functions by supporting IL-10-secreting macrophages.
However, the situation is far too complex for snap hypotheses. Leptin is produced by creeping fat, and it is known from animal models that it causes severe colitis. Creeping fat adipocytes are also a major source of TNFα. Anti-TNF therapies are extremely helpful for lowering CD inflammation, and therefore, it is highly unlikely that creeping fat-derived TNFα would be desired.
PRRs appear to tether inflammation with obesity, and this may be the same for creeping fat. PRRs of the mesenteric fat tissue could be triggered either by FFA or pathogen associated molecules, such as bacterial cell wall components. One well-known gene associated with CD is NOD2. NOD2 recognizes bacterial products, and it is often non-functional in CD patients. Some scientists speculate that the loss of NOD2 function could allow bacteria leaking from the gut to reside in the mesenteric fat surrounding the intestines. This bacteria reservoir hypothesis would suggest that creeping fat develops secondary to an intestinal insult that allows bacteria to translocate from the intestinal lumen to the mesenteric AT depots.
Scientists have determined that, at least, in normal situations, bacterial translocation would lead to inflammatory reactions in the mesenteric fat. Using an animal model, they were able to measure increases in C-reactive protein after bacterial translocation. Furthermore, there is evidence that some immune reactivity is lost in CD patient mesenteric AT. Creeping fat from CD patients was non-responsive towards bacterial pathogens in an ex vivo test. However, there isn’t a complete loss of functionality. Peroxisome proliferator-activated response-gamma (PPAR-γ) is expressed after PRR triggering and it encourages adipocyte proliferation and differentiation. This factor is over expressed in the mesenteric fat of CD patients. Therefore, there may be continual signals from bacteria encouraging the proliferation of adipocytes without proper signals leading to their elimination.
Another hypothesis involves the brain-fat-gut axis. Studies into neuropeptides have given us interesting insights into the relationship between creeping fat and intestinal inflammation. Neuropeptides can be released in CD patients via the central or enteric nervous system. Neuropeptides can instruct adipocytes to release pro-inflammatory cytokines and increase neuropeptide receptor expression (leading to a positive feedback loop). This could increase local inflammation in the neighboring intestines and keep disease active. Moreover, neuropeptides are known to change fat depot physiology by increasing proliferation and reducing apoptosis, which might explain the large adipocyte numbers and unusual fat wrapping seen in creeping fat.
Creeping fat is an unusual characteristic of Crohn’s disease. New studies looking at the inflammatory potential of AT in obese individuals has set the stage for new interest in this phenomenon. Scientists need to find out if it’s a true participant in maintaining chronic inflammation, an initiator or a simple bystander. The most interesting avenues of research involve figuring out the factors that set the pro-inflammatory activities of AT in motion. Are they FFAs, bacteria, neuropeptides or a combination? It will also be interesting to determine if mesenteric AT can be an initiator of disease in some circumstances or if creeping fat can induce CD flares.
This week, we learn more about how mesenchymal stem cells affect the immune system, why CX3CR1 is needed to clear a yeast infection and how the body stops helminths in their tracks.
Stem Cells and Immunosuppression
A recent study on mesenchymal stem cell (MSC) therapy has uncovered a novel mechanism of immunosuppression in the colon. The French group was using MSC therapy to counter gastrointestinal complications caused during tumor radiotherapy. They found that the treatment reduced colon epithelial damage, but also reduced the relative proportion of infiltrating CD4+ and CD8+ T cells. There was also a marked reduction of T cell activation and proliferation. This downregulation was parallel with significant increases of corticosterone secretion and subsequent interleukin-10 expression. Experiments with a glucocorticoid receptor blocker pointed towards an impaired TCR signal transduction initiated by the MSC treatment. It will be interesting to see if this finding can help improve MSC treatment of inflammatory bowel disease (IBD).
CX3CR1 and Candida
Crohn’s disease (CD) is often associated with harmful Candida albicans infections. Unlocking the secrets behind how the body controls Candida infections could be beneficial for CD patients. In this study, performed by the NIH, it was determined that the chemokine receptor CX3CR1 is crucial for macrophage survival in the kidney, the preferred hideout for Candida. During an infection, it was found that CX3CR1 was upregulated in the kidney macrophages. Mice deficient in CX3CR1 had a high mortality and were found to have uncontrolled Candida growth in the kidneys. The loss of the receptor prevented macrophages from accumulating in the kidney and engulfing the yeast. Given the growing role CX3CR1 in the pathogenesis of IBD, it will be interesting to consider if this receptor forms a link between CD and associated Candidiasis.
Helminth Induced Immune Reactions
As intestinal worms are becoming a new therapeutic for IBD, it’s interesting to learn about the immune responses that are induced upon their introduction to the body. A Japanese study in the Journal of Experimental Medicine has now shown that the skin does its best to prevent reinfection of Nippostrongylus brasiliensis. This species is similar to Necator americanus, which is used in IBD therapy. These worms enter through the skin. The scientists found that during the first exposure the skin had a limited response. However during a second application, there was a huge immune reaction, which was characterized by cellular infiltrates that trapped the larvae. The key immune cell mediating this trapping was the basophil, which detected worm antigens (bound to antibodies) using FcεRI and elicited the help of macrophages. It will be useful to determine if this process hinders worm treatment in IBD patients who have already had helminth infections.