Food chemistry and Technology laboratory, Agricultural and food engineering department, Indian institute of technology, Kharagpur, India
Navneet Singh Deora, Food chemistry and Technology laboratory, Agricultural and food engineering department, Indian institute of technology, Kharagpur, India, Email: email@example.com
Celiac disease (CD) is an immune-mediated enteropathy triggered by the ingestion of gluten containing grains (including wheat, rye and barley) in genetically susceptible individuals. The only treatment currently available is strict adherence to a gluten-free diet for life. Dietary compliance to meet the gluten free diet is difficult and in future better alternatives are needed to mitigate the problems caused due to celiac disease. Current advances in the pathogenesis of celiac disease have directed towards new pharmacologic treatments like oral enzyme supplementation, tissue transglutaminase inhibition, blockage of HLA-DQ presentation, and silencing of gluten-reactive T cells using cytokines or other methods. Currently all of these proposed solutions are in the experimental phase and they have a long way to go in terms of clearing clinical studies. This article addresses the insight about these alternative therapies and future direction.
Celiac disease (CD) is an immune-mediated enteropathy triggered by the ingestion of gluten containing grains (including wheat, rye and barley) in genetically susceptible individuals. Epidemiological studies conducted during the past decade revealed that CD is one of the most common lifelong disorders worldwide [1,2]. CD can manifest itself with a previously unappreciated range of clinical presentations, including the typical malabsorption syndrome and a spectrum of symptoms potentially affecting any organ system. Since CD often presents in a typical or even silent manner, many cases remain undiagnosed and carry the risk of long-term complications, including anemia, osteoporosis, infertility or cancer .
In celiac disease a gluten-induced small-bowel mucosal lesion develops gradually in genetically susceptible persons. Evidence shows that clinically pertinent celiac disease exists despite a normal small-bowel villous architecture. The characteristic villous atrophy with crypt hyperplasia, a flat lesion, may become evident only after decades of gluten ingestion, representing the end stage of the mucosal response to the environmental trigger, gluten. Positive serum celiac auto antibodies in individuals with normal histology indicate early developing celiac disease. When these auto antibodies are demonstrated in vivo in small-intestinal mucosa – where they are in fact produced – it is possible to find even more reliably patients having celiac disease in its early stages. It is foreseen that the demonstration of enteropathy will no longer be the gold standard in the diagnosis of celiac disease, and the diagnostic criteria are extended to include ‘genetic gluten intolerance’
Advancement in Treatment
In the current situation, the treatment available for celiac disease is a strictly gluten free diet for life. Under administration of a gluten free diet, taxonomically similar species such as rye and barley are also avoided. The International Food Authority demands complete absence of gluten for qualifying to be gluten free, whereas Codex Alimentations allows a limit of 200 ppm of gluten per food. However, addressing the need to produce a completely gluten free diet makes way for other challenges.
Gluten is, however, a common ingredient in the human diet, presenting a big challenge for celiac disease patients since it is normally unlabeled in many countries. Also another concern area is the dietary compliance especially in adolescents and adults. In context with above mentioned views, it is important need to develop safe and effective therapeutic alternatives. The quality of life of celiac disease patients would improve if there was a treatment that would allow some gluten to be consumed over a short period of time, for instance during social events or travel. Documentation of the long-term safety for such an agent would probably be less necessary because the indication would be for intermittent treatment. This type of therapy could then pave the way for long-term or permanent treatment. One goal for a new therapeutic agent would be to enhance the return of full intestinal function in patients who show incomplete recovery in response to a gluten-free diet. This agent might also allow moderate quantities (1–5 g/day) of gluten to be tolerated. Although this is less gluten than is consumed as part of a typical Western diet (~20 g/day), it could improve quality of life by protecting patients from most forms of ‘hidden gluten’. Finally, there is a particular need for treatment alternatives for refractory sprue, which, although rare, is currently treated only with harsh immunosuppressive drugs. Figure 1 shows the current and alternative therapy for celiac disease.
Alternative Options for Celiac Disease
Gluten is one of the distinctive proteins due to the fact that it contains approximately 15% proline and 35% glutamine residues. Gluten’s markedly high proline content causes a complex structure with regions which are largely inaccessible to human endoproteases, allowing large, proline-rich gluten fragments to reach the small intestine intact. These large molecules, the products of gluten digestion, are up to 33 amino acid in length remain. These are found out to be toxic to patients suffering from celiac disease. Therefore, proline- and glutamine- specific endoproteases, often referred to as glutenases, are proposed as therapeutic agents for celiac sprue on account of their ability to detoxify proteolytically resistant gluten epitopes [4,5].
Prolyl endopeptidases (PEPs) are endoproteolytic enzymes. In contrast to human gastrointestinal proteases, PEPs can readily cleave proline-rich immunostimulatory gluten peptides. The potential to use PEPs as a treatment for celiac disease is appealing because their specificity complements gastrointestinal proteolytic processes. Not only does every PEP-catalyzed cleavage generate one new amino and carboxyl terminus, but it also truncates the long-peptide end products of gastric and duodenal gluten metabolism. Both of these outcomes provide residual smaller peptides that are suitable substrates for the intestinal brush-border aminopeptidases and carboxypeptidases.
Currently most of the advancement is intended for enzyme therapy utilizing the F. meningosepticum PEP . This enzyme fulfills most of the requirement for a therapeutically effective PEP, with the possible exception of sluggish kinetics against a few T-cell epitopes in gluten. However it suffers from the disadvantage of higher costs of production due to its relatively poor level of heterologous expression. A homologous PEP from Myxococcus xanthus seems to be comparable to the F. meningosepticum enzyme with respect to gluten detoxification, but can be expressed at much higher levels in Escherichia coli . In addition, Lactobacillus helveticus has a zinc-dependent PEP that can also cleave long substrates with relatively broad subsite specificity. Further studies should help to clarify the relative pros and cons of the available PEPs and show which should be used for human clinical trials. A pharmacologically useful oral PEP formulation might also include one or more complementary enzymes.
It is to be noted that we need to evaluate the long terms safety of these enzymes in animals followed by in Humans as well. Initial screening must ensure that the exogenous enzyme is non-allergenic, and is not assimilated intact into the bloodstream in appreciable quantities. Other potential risks include structural damage to intestinal mucosa and the consequent loss of nutrient absorptive capacity or alterations in regulation of gut hormones. In this regard, it must be noted that some proteases of pharmacological interest (e.g. barley EPB) have been components of the human diet for a long time, and are therefore unlikely to present a health risk. Proteases can also detoxify gluten-containing products before they are ingested. This is particularly relevant for products that have minor, but significant, gluten content. Analogously, proteases of certain lactobacilli present in sourdough are able to proteolyze proline-rich gluten peptides, and a challenge experiment indicates that celiac disease patients, at least in the short term, tolerate carefully prepared sourdough bread.
In coming time, Glutenases seems to be a promising class of therapeutic agents for celiac sprue . Early proof-of-concept experiments focused on the use of PEPs for gluten detoxification in the duodenum. PEPs were selected because of their ready commercial availability, although it was recognized that clinical implementation of these agents would require advanced drug delivery methods to promptly release high enzyme doses in the duodenum. More recent efforts have led to the identification of gastric glutenases that do not require sophisticated formulation and can detoxify dietary gluten before its release in the affected organ. At least 2 such purified glutenases, EP-B2 and a Aspergillus niger PEP, have been tested in the laboratory  and there is considerable evidence for the existence of other promising gastrically active glutenases in germinating cereals.
Tissue Transglutaminase Inhibitors
Tissue transglutaminase has a significant role in CD pathogenesis. This is largely attributed to its specific auto antibodies and gluten-specific T-cell response . This enzyme can also cross-link gluten peptides with matrix proteins, thereby maintaining gluten in the tissue and generating complexes that induce an immune response to additional auto antigens [12,13]. For example, Cystamine and monodansylcadaverine are the competitive inhibitors of tTG2. They act in the transamidation reaction by interfering with the natural amine substrates. The proliferative capacity of gluten-responsive T cells can be blocked in ex vivo small intestinal biopsies by co-treatment with one of these products.
However, impact of TG2 inhibiters it is still unclear in terms of efficacy and side effects. A few gluten T-cell epitopes are recognized without being modified by TG2. Activation of such cells might be sufficient to drive the gut inflammation. Although TG2 knockout mice have no overt spontaneous abnormalities, they develop splenomegaly, autoantibodies and immune complex glomerulonephritis by systemic triggering of apoptosis. Thus, the optimal TG2 inhibitor should have activity limited to the gut mucosa. Local side effects related to extracellular matrix formation and wound healing can be envisaged.
Blocking of HLA-DQ peptide presentation
It is known that CD is strongly associated with HLA-DQ2/DQ8 molecules. These molecules are involved in disease pathogenesis by presenting the gluten peptides to T cells. The crucial role of the HLA in celiac disease development makes it an apparent target for therapeutic options. Blocking the binding sites would prevent the presentation of disease-inducing gluten peptides. It is to be noted that this concept was initially developed for the treatment of type 1 diabetes and rheumatoid arthritis. However there was limited success using the treatment. The lack of success was due partly to difficulties in obtaining effective drug delivery. This should be less of a problem in celiac disease because the blocking compound can be administered before or in parallel with the antigen (i.e. gluten). Going forward, HLA blockade as a therapy for celiac disease should be pursued by the researchers across the world. Side effects, such as immunosuppression, are unlikely due to the fact that several healthy individuals are homozygous for HLA alleles. The recently solved X-ray crystal structure of HLA-DQ2 complexed with a deamidated gluten peptide provides important information for the development of an HLA-DQ2-blocking compound . A blocking compound should prevent presentation of gluten peptides to T cells and be unrecognizable by any T cell, to avoid hypersensitivity responses to the blocking compound itself. This might be achieved by making the compound either small enough to avoid forming interactions with the T-cell receptor, or so big that no T-cell receptor can dock onto HLA-DQ2 with the bound blocker.
Silencing of Gluten-Reactive T-Cells
Celiac disease is exceptional in that the disease can be put in remission and induced in a controlled fashion. This could be utilized for therapeutic purposes . Possibly, gluten-reactive T cells could be eliminated or made unresponsive by oral gluten challenge concomitant with the administration of agents that alter the outcome of the T-cell activation. For example, Antibodies to CD321 and CD154 (CD40L),  can induce T-cell silencing. However they are found to produce unwanted side effects such as toxic cytokine syndrome (anti-CD3) and thromboembolic events (anti-CD154). As another options, gluten-reactive T cells might be silenced by soluble dimers of HLA–peptide complexes, because such dimers induce the apoptosis of antigen-specific T cells as a result of inappropriate stimulation . This approach is convoluted by an escalating number of characterized gluten epitopes. Induction of tolerance by intranasal administration of gluten or gluten T-cell epitopes has also been suggested as a possible treatment modality . Moreover, tolerance can be achieved by targeting gluten epitopes to dendritic cells that induce T-cell tolerance but this approach is further complicated by the presence of multiple T-cell epitopes. In addition, it is unclear which marker should be used to target tolerogenic dentritic cells in the intestinal mucosa.
In the mucosa of patients with active CD, gluten-reactive CD4+ T cells produce several pro-inflammatory cytokines, with interferon-g being dominant. These trigger various effector mechanisms including increased secretion of tissue-damaging metalloproteinases and cytotoxicity of intraepithelial lymphocytes against enterocytes with increased apoptosis and villous flattening. Various cytokine therapies are being developed for the treatment of chronic inflammatory diseases .
Interferon (IFN)-γ is the dominant cytokine produced by gluten-reactive T cells, and antibodies neutralizing IFN-γ should have a good chance of curtailing the inflammatory effects of the anti-gluten T-cell response. Antibodies of this kind are being tested in Phase I/II trials for Crohn’s disease. Depending on the results of these trials, anti-IFN-γ agents might become candidates for testing in celiac disease. IL-15 is thought to be the central mediator of the innate effects of gluten in celiac disease, making IL-15 neutralizing agents therapeutic candidates. Two such compounds are in clinical trials; a humanized anti-IL-15 antibody, HuMax-IL-15, is in Phase II trials for rheumatoid arthritis and possibly other inflammatory conditions, and an IL-15/Fc chimeric protein, CRB-15, is in preclinical testing . Earlier studies suggest that HuMax-IL-15 has acceptable side effects, and might therefore be a candidate for testing in celiac disease.
Selective Adhesion Molecule Inhibition
Another novel class of therapeutic agents under development for the treatment of chronic inflammatory diseases acts by selective inhibition of leukocyte adhesion [22,23]. This inhibition prevents leukocytes from migrating into inflamed tissues. For example, a humanized antibody against INTEGRIN-α4, natalizumab, is being used for the treatment of multiple sclerosis, and is under evaluation for inflammatory bowel disease (IBD). A small-molecule integrin-α4 antagonist, T0047, has also been developed and is undergoing clinical testing. Preliminary reports indicate that natalizumab has beneficial effects in IBD with moderate side effects. Another humanized monoclonal antibody, MLN02, which targets the adhesion molecule integrin-α4β7 that is expressed by gut T cells, is being tested in Phase II clinical trials for the treatment of IBD. This agent is intriguing with respect to celiac disease, because it aims to prevent migration of T cells to the lamina propria. Conceivably, this agent could interfere with the action of the HLA-DQ-restricted T cells in the laminapropria-recognizing gluten peptides. Possible side effects include increased susceptibility to gastrointestinal infections and a potential for disturbances in oral tolerance to food proteins.
The need for alternatives to the gluten exclusion diet for the treatment of celiac disease has been made even more urgent by the increasing number of patients diagnosed with this disease. Fundamental studies have revealed several attractive targets for therapy, some of which are already showing promise in the context of other medical conditions. It will be interesting to see whether any of these will become reality in the coming years.
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