enteropathy-associated t-cell lymphoma enteropathy-associated t-cell lymphoma enteropathy-associated t-cell lymphoma (eatl), previously termed enteropathy-associated t-cell lymphoma, type i and at one time termed enteropathy-type t-cell lymphoma (ettl), is a complication of coeliac disease in which a malignant t-cell lymphoma develops in areas of the small intestine affected by the disease's intense inflammation. while a relatively rare disease, it is the most common type of primary gastrointestinal t-cell lymphoma. enteropathy-associated t-cell lymphoma (formerly termed enteropathy-associated t-cell lymphoma, type 1)other namesenteropathy-associated t-cell lymphoma, type imicrograph of enteropathy-associated t cell lymphoma (upper right of image). h&e stain.specialtyoncology, hematology, gastroenterologycomplicationsbowel obstructions, bowel perforationscausescomplication of celiac diseaserisk factorsgenetic predispositionpreventiongluten-free dietprognosisguarded eatl had been defined as a single type of small intestine lymphoma, but in 2008, the world health organization (who) divided the disease into two subtypes: 1) eatl type i, which occurs in individuals with coeliac disease, a chronic immune disorder causing inflammatory responses to dietary gluten primarily in the upper reaches (i.e. jejunum and duodenum) of the small intestine; and 2) eatl type ii, a disorder similar to eatl type i that occurs without coeliac disease. while type i and ii eatl share many similar features, post-2008 studies found some significant differences between the two types. in 2016, the who redefined the two diseases as separate entities, keeping the term enteropathy-associated t-cell lymphoma for the coeliac disease-associated lymphoma and terming type 2 disease as monomorphic epitheliotropic intestinal t cell lymphoma (meitl). eatl is five to ten times more common than is meitl. the who also defined a third type of intestinal t-cell lymphoma that cannot not be classified as eatl or meitl as peripheral t-cell lymphoma not otherwise specified (itcl-nos). eatl arises from the malignant transformation of small-intestinal intraepithelial lymphocytes (iel). iel are a heterogeneous group of principally t-cell lymphocytes residing in epithelial tissues that interface the environment, such as the mucosa of the bronchi, reproductive tract and gastrointestinal tract. at these sites, iel are exposed and regulate immune responses to non-dietary and dietary antigens, pathogenic and non-pathogenic organisms and injured self tissues. gastrointestinal tract iel appear in the epithelium of the small intestine, colon, stomach and esophagus, residing between the epithelial cells that line these organs' lumens. these iel often exhibit natural killer and cytotoxic t-cell cell activation markers, contain various toxic agents (e.g. perforin, granzyme) and therefore are capable, if activated, of causing severe tissue injuries. with coeliac disease, the iel react to the glutelins in dietary gluten by increasing their numbers, becoming pathologically active, producing chronic inflammation that injures intestinal cells, interfering with nutrient absorption and creating an environment conducive to their malignant transformation into eatl. optimal treatment of eatl has used regimens consisting of intensive chemotherapy, hematopoietic stem cell transplantation and, in cases with bulky, obstructive and/or perforated bowel disease, surgical intervention. the disease has a five-year overall survival rate of only ~20%. however, recent studies focusing on the malignant iel in eatl have increased understanding of the disease and suggested newer chemotherapy-based strategies and novel molecular targets that might be attacked therapeutically to improve the disease's prognosis. presentation eatl typically occurs in patients with a history of coeliac disease and who may have been previously diagnosed with type i or ii refractory disease, but in any case, affected individuals present with worsening coeliac disease symptoms of abdominal pain, malabsorption, diarrhea, weight loss, fever or night sweats. eatl diagnosis is more likely if symptoms develop suddenly, or if the serious symptoms of bowel obstruction and/or bowel perforation caused by bulky eatl masses develop. patients with ulcerative jejunitis usually present with more severe symptoms, including more frequent bowel perforations and obstructions. some patients with no history of coeliac disease present with symptoms or signs of a small-intestinal lymphoma but on diagnostic workup are found to have coeliac disease. pathophysiology genetics the cause of eatl, while not fully understood, is by definition related to celiac disease. individuals are genetically predisposed to develop celiac disease because of the specific types of hla-dq proteins expressed by their antigen-presenting cells (apc). hla-dq proteins are on the surface of apc and function to present foreign or self antigens to the t cell receptors (tcr) expressed on the surface of t-cells and thereby to stimulate these cells either to initiate or suppress immune responses to the presented antigens. hla-dq proteins are composed of α and β polypeptide chains encoded by the hla-dqa1 and hla-dqb1 genes, respectively. since there are several different alleles (i.e. gene variants) at these two genetic loci, individuals are usually heterozygous, i.e. have inherited different alleles from each parent at each locus; uncommonly, however, individuals are homozygous at one or both loci because their parents have the same alleles at one or both loci. the hla-dq proteins that predispose individuals to coeliac disease bind and respond specifically to gluten-related antigens presented to them by apc. the genetic predisposition to develop coeliac disease is clinically determined by identifying the serotypes of an individual's apc's hla-dq proteins using serotype-specific antibody preparations and/or by identifying the alleles at an individual's hla-dqa1 and hla-dqb1 genetic loci. studies show that: celiac disease affects ~1% of the population in most parts of the world. ninety to one hundred percent of patients with coeliac disease have inherited genes at the hla-dq locus that encode hla-dq2 and/or hla-dq8 serotype proteins. about 2-3% of individuals who inherit these hla-dq2 and/or hla-dq8 serotypes develop coeliac disease. about 90% of coeliac disease patients are homozygous for (i.e. inherited from both parents) either the hladqa1*0501 or hladqa1*0505 alleles at the hla-dqa1 locus plus either the hladqb1*0201 or hla-dqb1*0202 alleles at the hla-dqb1 locus; <5% of coeliac disease patients are heterozygous (i.e. inherited from just one parent) for these alleles; and 5-10% of coeliac disease patients have inherited hla-dqa1*03 plus hla-dqb1*0302 alleles at the respective hla-dqa1 and daqb1 loci. less than 1% of all individuals with coeliac disease develop eatl. individuals homozygous for the hladqb1*0201 allele at the hla-dqb1 locus develop a particularly severe and tissue-damaging form of coeliac disease and have a greatly increased rate of developing eatl. the vast majority of patients with coeliac disease who develop eatl have either the hladqa1*0501 or hladqb1*0201 allele (see previous paragraphs) at the hla-dqb1 locus. intraepithelial lymphocytes iel are a diverse population of lymphocytes, which unlike most peripheral lymphocytes, do not recirculate through the blood and/or lymphatic system but rather reside permanently in the epithelium of various organs. in the gi tract, iel localize between the epithelial cells lining the colon, small intestine, stomach, and esophagus where they serve to maintain the mucosal barrier, combat infection by pathogens, and regulate immune responses to antigens originating from the diet, pathogens, and damaged tissues. human iel are classified into those which express the tcr (i.e. tcl+iel) and those which do not (i.e. tcr-iel). tcl+iel are further divided into 4 subtypes, tcrαβ+cd4+ iel, tcrαβ+cd4+cd8αα+ iel, tcrαβ+cd8αβ+ iel, and tcrλδ+cd8αα+. these subtypes are based on the expression of alpha (α) and beta (β) chain-containing tcr (i.e. αβtcr); gamma (γ) and delta (δ) chain-containing tcr (i.e. γδtcr); cd4; cd8αβ; and/or cd8αα by individual iel.. a fifth tcl+iel subtype, tcrαβ+cd8αα+, occurs in mice but its presence in the human intestine is disputed. human tcr-iel are also divided into 4 sub-types: their: ilc1-like iel have morphological and functional similarities to normal intestinal epithelial cells and express nkp46; icl3-like iel have morphological similarities to normal epithelial cells and, similar to th1 cells, make interleukin 17 (il-17) and interleukin 22 (il-22) cytokines and express the rorγt transcription factor and nkp44; icd3-iel express icd3; and icd8α-iel express icd3 and cd8α. cd3 designates a protein complex that is attached to the cell surface membrane whereas icd3 refers to a cd3 protein complex in which one or more of its proteins resides abnormally in the cell's cytosol studies suggest that icd3+iel are the principal cell type that becomes malignant in eatl cases that are not classified as de novo (see next section). these cells also express cd103 and, frequently, cd30. acquisition of malignancy coeliac disease patients may be asymptomatic, minimally symptomatic, and/or well-controlled on a gluten-free diet (i.e. a diet free of cereal, rye, wheat, and barley) but nonetheless develop eatl. about 46% of all aetl cases occur in this setting and have had their malignancy described as de novo eatl. the remaining ~54% of eatl cases develop in coeliac disease patients whose disease becomes refractory to dietary control, exhibits increasing symptoms, and progress over ~4–10 years through type i refractor coeliac disease (type 1 rcd) and type ii refractory coeliac disease (type ii rcd) to become eatl. the rates at which non-refractory celiac disease, type i rcd, and type ii rcd progress to de novo or eatl are <1%, 3-14%, and 33-52%, respectively. de novo eatl de novo eatl can occur in individuals whose coeliac disease was undiagnosed until eatl was found or who have mild/well-controlled coeliac disease. the findings in these patients usually differ little from those found in mild/well-controlled cases that do not progress to eatl; their small intestinal mucosa is populated by increased number of iel and exhibits tissue destruction (e.g. small intestinal villus atrophy), nonetheless, their iel are normal-appearing, small cells that on examination are polyclonal (i.e. genetically diverse), express cd3 and cd8, and do not have genetic abnormalities. the mechanism behind the development of eatl in these individuals is not understood. type i refractory coeliac disease type i rcd patients, who constitute 15-23% of all patients with rcd, are refractory to the gluten diet as evidenced by their worsening symptoms, increased tissue destruction, and rising numbers of tcrαβ+cd*αβ+iel in tissue lesions. some type i rcd patients may have failed to respond to the diet from the onset of their disease. if either cases, these patients show no change in the normal appearance and polyclonal nature of their small intestinal iel and these iel show no genetic abnormalities. the cause for these coeliac disease patients progressing to type i rcd, after excluding the very common problem of failure to fully exclude gluten from their diets, is either due to their genetic makeup (see above section on genetics) or is unknown. type ii refractory coeliac disease type i rcd patients may progress to type ii rcd as evidenced by their more severe symptoms, increased intestinal tissue destruction, and expanding numbers of intestinal iel, particularly icd3+iel. their iel typically consist of genetically different subpopulations of cells that have a monoclonal rearrangement of their tcr and therefore are descendant from a single ancestral cell. subpopulations of these iel also have one or more of the following genetic abnormalities: trisomy of chromosome 1's long (or "q") arm at position 22-44 (abbreviated 1-q22-24); genomic alterations around the tp53 tumor suppressor gene at position 13.1 on the short or "p" arm of chromosome 17; genomic alterations around the cdkn2a tumor suppressor and cdkn2b cell proliferation regulator at position p21.3 on chromosome 9 that result in loss of heterozygosity for both genes; and/or activating mutations in jak1 (75% of cases) and stat3 (25% of cases). in type ii rcd, the same types of abnormal ile found in the small intestine may be detected in the colon, stomach, mesenteric lymph nodes, blood, bone marrow, and epithelium of the airways and skin. finally, the small intestinal lesions in type ii rcd contain il-2 and il-21 as well as increased levels of il-15. laboratory studies indicate that these 3 cytokines acting individually or in synergy are potent stimulators of the jak1/stat3 signaling pathway in icd3+iel and thereby promote these cells survival (by blocking apoptosis) and proliferation. the small intestinal lesions also contain a tumor necrosis factor which stimulates icd3+iel survival and proliferation but does so by activating nf-κb, mapk, and/or c-jun n-terminal kinases rather than jak1/stat3 signaling pathway. these data suggest that: a) type ii rcd is a low grade lymphoma; 2) the intense inflammation in type ii rcd, perhaps amplified by the cited cytokines, promotes the proliferation, survival, genome instability, and consequential genetic abnormalities in iel; and 3) one or more of these factors cause the transformation of type ii rd to eatl. as currently understood, the release of il-15 by mucosal epithelial cells, the binding of il-15 to the il-15rβ cell surface receptor on icd3+iel, and the stimulation thereby of these cells appears particularly important in driving type ii rcd to eatl in a significant number of cases. ulcerative jejunitis ulcerative jejunitis