Cellular Origins and Pathogenesis of Gastrointestinal NK- and T-Cell Lymphoproliferative Disorders

Simple Summary Intestinal T- and NK-cell lymphoproliferative disorders are a group of rare gastrointestinal disorders that arise from immune cells in the intestinal mucosa that are also relatively unknown. Diseases such as indolent T-cell lymphoproliferative disorders of the gastrointestinal tract do not even require treatment, whereas others, such as monomorphic epitheliotropic intestinal T-cell lymphoma, will generally cause death within a year. No effective treatment is currently available, as little is known about how these tumours form or even what cells they arise from. This article summarizes the current state of knowledge about the main types of immune cells in the gastrointestinal mucosa and the processes by which they may transform into neoplasms. The clinical behaviour, pathological appearances and the molecular alterations that underlie these diseases are also discussed. Abstract The intestinal immune system, which must ensure appropriate immune responses to both pathogens and commensal microflora, comprises innate lymphoid cells and various T-cell subsets, including intra-epithelial lymphocytes (IELs). An example of innate lymphoid cells is natural killer cells, which may be classified into tissue-resident, CD56bright NK-cells that serve a regulatory function and more mature, circulating CD56dim NK-cells with effector cytolytic properties. CD56bright NK-cells in the gastrointestinal tract give rise to indolent NK-cell enteropathy and lymphomatoid gastropathy, as well as the aggressive extranodal NK/T cell lymphoma, the latter following activation by EBV infection and neoplastic transformation. Conventional CD4+ TCRαβ+ and CD8αβ+ TCRαβ+ T-cells are located in the lamina propria and the intraepithelial compartment of intestinal mucosa as type ‘a’ IELs. They are the putative cells of origin for CD4+ and CD8+ indolent T-cell lymphoproliferative disorders of the gastrointestinal tract and intestinal T-cell lymphoma, NOS. In addition to such conventional T-cells, there are non-conventional T-cells in the intra-epithelial compartment that express CD8αα and innate lymphoid cells that lack TCRs. The central feature of type ‘b’ IELs is the expression of CD8αα homodimers, seen in monomorphic epitheliotropic intestinal T-cell lymphoma (MEITL), which primarily arises from both CD8αα+ TCRαβ+ and CD8αα+ TCRγδ+ IELs. EATL is the other epitheliotropic T-cell lymphoma in the GI tract, a subset of which arises from the expansion and reprograming of intracytoplasmic CD3+ innate lymphoid cells, driven by IL15 and mutations of the JAK-STAT pathway.


Introduction
Various subsets of natural killer (NK) and T-cell populations exist in the normal gastrointestinal (GI) mucosa, with distinct developmental pathways, phenotypes and The innate arm of the immune system is the first line of defence against invading pathogens, which are recognized by the pattern recognition receptors and Toll-like receptors expressed in macrophages, neutrophils and dendritic cells. At the same time, gastrointestinal immune cells have an immunoregulatory role in limiting inflammatory responses to commensal organisms and dietary antigens, ensuring intestinal epithelial integrity and immune homeostasis.
The intestinal immune system comprises T-cells, B-cells, plasma cells, dendritic cells and a group of innate lymphoid cells that are variously located in mesenteric lymph nodes and intestinal mucosa within Peyer's patches, cryptopatches and isolated lymphoid follicles, are diffusely distributed in the lamina propria, or as intra-epithelial lymphocytes (IELs). The latter may be subdivided into CD4+ TCRαβ+ and CD8αβ+ TCRαβ+ (induced or 'type a') IELs, CD8αα+ TCRαβ+ and CD8αα+ TCRγδ+ (natural or 'type b') IELs and TCR− innate lymphoid cell-like IELs, a subset of which displays intracytoplasmic CD3 expression (iCD3+ innate IELs). In this review, we will focus on innate lymphoid cells (particularly NK-cells) and T-cells in the gastrointestinal immune system, as well as the lymphoproliferative disorders that may arise from them.

Innate Lymphoid Cells (ILCs)
Innate lymphoid cells (ILCs) such as natural killer (NK) cells in the gastrointestinal tract are important effectors of the innate immune system with a central role in the early phase of the immune response, in tissue repair and remodelling, containment of commensal microbes and maintenance of epithelial cell integrity. Located in the lamina propria and between epithelial cells, they are defined by the lack of antigen-specific receptors, lack of myeloid and dendritic cell markers and having lymphocyte morphology.

CD4 and CD8 Expression in NK-Cells
Although most NK-cells are CD4− CD8−, CD8αα homodimers may be expressed in NK-cells, as they do in subsets of T-cells. CD8αα enhances the interaction of MHC class I molecules with KIR3DL1, an inhibitory receptor of NK-cells, leading to inhibition of NK-cell activation [41]. NK-cells resident in lymphoid tissue also express higher levels of CD4 compared to those in peripheral blood [42]. CD4 in activated NK-cells acts as a chemokine receptor and increases cytokine production.

NK-cells in the GI Tract
The role of NK-cells in the human GI tract has been reviewed by Poggi et al. [47]. In keeping with CD56 bright NK-cells, they do not express CD16 and lack strong cytolytic activity. They are mainly located in the intestinal epithelium as intraepithelial lymphocytes (IELs) but may be seen in the lamina propria and express IFNγ in response to IL12 [47,48]. NK-cells express T-bet and produce Th1-like cytokines such as IFNγ and TNFα [49] during inflammatory responses.

T-Cell Populations in the Gastrointestinal Tract
T-cells within intestinal mucosa comprise conventional CD4+ helper and CD8+ cytotoxic T-cells that primarily populate the lamina propria, as well as unconventional T-cells mainly located in the intraepithelial compartment that display regulatory rather than effector properties.

Conventional T-Lymphocytes
Conventional T-cells derive from developing thymocytes that have undergone positive and negative selection, migrate into gut-associated lymphoid tissue (GALT), e.g., Peyer's patches, where they undergo antigen-driven maturation to become effector memory cells that home to intestinal mucosa. Conventional T-cells are TCRαβ+ and express CD4 or CD8αβ, which serve as co-receptors for TCR. Responding to environmental cues, conventional CD4+ T-cells may differentiate into Th1, Th2, Th17 and induced regulatory T (iTreg) cells. Conventional T-cells comprise the bulk of the T-cell population in the intestinal mucosa and may be located both in the intraepithelial compartment and in the lamina propria [50].

Unconventional T-Lymphocytes
Instead of proceeding to positive selection, CD4+ CD8+ thymocytes undergo agonist selection, downregulate CD4 and CD8, and develop into self-reactive precursors of CD8αα T-cells that escape negative selection [51,52]. Unconventional T-cells derived from such double-negative thymocytes express CD8αα, along with TCRγδ or TCRαβ and home directly to the gastrointestinal tract [50,53,54]. Such unconventional T-cells include CD8αα+ TCRαβ+ and CD8αα+ TCRγδ+ subsets. CD8αα acts as a TCR repressor, negatively regulates T-cell activation and assumes a regulatory role.
CD8αβ+ TCRαβ+ circulating T-cells are different from IELs with the same phenotype. The latter's development may be influenced by the high levels of TGFβ in the gut microenvironment, which induces expression of CD103 and CD8αα. Compared with circulating CD8αβ+ TCRαβ+ T-cells, CD8αβ+ TCRαβ+ IELs express granzyme B, CD103, CD69 and produce lower levels of IFNγ and TNFα [66].

Innate Lymphoid Cells (ILCs)
Finally, ILCs that rearrange but do not express TCR genes form the third group of IELs, which may be prominent in ileal grafts. They show expression of CD103, CD56, RORγt, AhR [23,25], NKp46, NKp44 and a subset that lacks surface CD3 but expresses cytoplasmic CD3ε and CD3γ [25,54,57]. The latter is referred to as innate CD3+ (iCD3+) IELs and may be either CD8αα+ or CD8αα−. Whilst the development of ILCs depend on Id2, iCD3+ IELs arise from Notch1-activated common lymphoid precursors under the influence of IL15. Granzyme B induced by IL15 cleaves Notch1 into an inactive peptide, thereby silencing downstream target genes that are essential for T-cell differentiation [25]. They differentiate in the absence of Id2 but require Notch1, TL and IL15 signals for development [27].
The CD8 molecule exists as a dimer of two isoforms, CD8α and CD8β. Conventional CD8+ T-cells express CD8αβ, which acts as a co-receptor for TCR and is expressed on MHC class I-restricted T-cells. In contrast, CD8αα decreases antigenic sensitivity of the TCR and when co-expressed with CD8αβ, CD8αα downmodulates the functional avidity of the CD8αβ-TCR:Ag-MHC activation complex [73,74]. Transient or permanent expression of CD8αα homodimers in T-cells can be induced, regardless of MHC restriction or the presence of CD4 or CD8αβ, and may be seen in all groups of IELs [75].

Lymphomas and Lymphoproliferative Disorders Derived from NK-Cells
NK-cell is the putative cell of origin for indolent lymphomatoid gastropathy/NK-cell enteropathy and aggressive NK-cell leukaemia. Both NK-cells and CD56+ T-cells may give rise to extranodal NK/T cell lymphoma and chronic active EBV infection. Apart from aggressive NK-cell leukaemia, the other entities may involve the gastrointestinal tract.

. Incidence and Prevalence
This is a rare disease, and only 36 patients with either LG or NKCE have been reported as of 2019 [76]. Figures on global incidence are not available, and lymphomatoid gastropathy was initially recognized based on a review of only 10 cases of CD56+ atypical lymphoid infiltrates in the stomach between 1998 and 2009 [77].

Clinical Features
This is a benign, localized lymphoproliferative disorder of NK-cells in the gastrointestinal tract. Many Japanese cases were reported in the stomach (median age: 56, M = F) [77][78][79][80], but this may relate to a national surveillance program for gastric cancer detection in the country. They tend to present incidentally and regress spontaneously with a relapse rate of 38%.

Cellular Origin from CD56 bright Subset of NK-cells
The consistent lack of TCR expression, a polyclonal pattern by T-cell clonality studies and expression of CD56, all point to the origin of this disease from NK-cells within the gastrointestinal mucosa, rather than T-cells that express NK-like features. Takata K et al. reported two cases of lymphomatoid gastropathy that express CD8 by immunohistochemistry [86]. However, this is not inconsistent with the NK-lineage of this entity, as the commonly used antibodies against CD8 in FFPE material are directed against CD8α, Figure 1. NK cell gastropathy. The lamina propria is expanded by a dense infiltrate of mediumsized lymphocytes with round nuclei, slightly dispersed chromatin, occasional nucleoli and ample cytoplasm. H&E; original magnification, ×200 (A), ×600 (B). The lymphoid cell population stains negative for both TCRβ (C) and TCRγ (D). They stain positive for CD2 (E), CD3 (F), CD7 (H) but not CD5 (G). Neoplastic lymphocytes may display CD8αα phenotype, being positive for CD8α (I) but not CD8β (J). They display a cytotoxic phenotype with TIA1 expression (K), but in situ hybridization for EBER is negative (L). There is strong expression of CD56 (M), and the proliferation fraction is high at 70% with Ki67 staining (N).

Cellular Origin from CD56 bright Subset of NK Cells
The consistent lack of TCR expression, a polyclonal pattern by T-cell clonality studies and expression of CD56, all point to the origin of this disease from NK-cells within the gastrointestinal mucosa, rather than T-cells that express NK-like features. Takata K et al. . Neoplastic lymphocytes may display CD8αα phenotype, being positive for CD8α (I) but not CD8β (J). They display a cytotoxic phenotype with TIA1 expression (K), but in situ hybridization for EBER is negative (L). There is strong expression of CD56 (M), and the proliferation fraction is high at 70% with Ki67 staining (N).
All cases of lymphomatoid gastropathy/NK-cell enteropathy are CD56+, and flow cytometry performed in one case showed 'aberrantly bright' CD56 expression [84]. Apart from an occasional case that is CD16+ (but with reduction of staining compared to CD56) [79], most cases display dim or negative expression of CD16 [77,85]. The CD2+ CD5− CD7+ phenotype seen in most cases [77,85] is also typical of origin from CD56 bright NK-cells ( Table 1). Expression of CD335/NKp46 is reported in a case [89], and another case expressed CD94/NKG2 [84], whilst staining for CD103 is negative in a case so tested [76]. This phenotype suggests that lymphomatoid gastropathy/NK-cell enteropathy may arise from the CD56 bright subset of NK-cells resident within the lamina propria.

Pathogenetic Mechanisms
The aetiology and pathogenetic mechanisms are unknown. No specific mutations have been described in this entity, but H. pylori infection was often reported in lymphomatoid gastropathy [77,79,80]. A case of NK-cell enteropathy associated with high titres of antigliadin antibody showed partial regression following gluten withdrawal, which suggests that the disease may be driven by an abnormal immune response [84]. In addition, clonality has not been demonstrated in this entity (although clonality analysis is difficult to perform in NK-cell neoplasms), and a reactive aetiology has been proposed [84]. However, recurrent mutations of JAK3 have been reported [90], and targeted mutational analysis in a case similarly detected somatic variants of AXL and JAK3 [88], which suggests a neoplastic rather than a reactive process.

Incidence and Prevalence
ENKTL constitutes 3-10% of non-Hodgkin lymphoma in Asia and South America but is uncommon in western populations, where it comprises less than 1% of cases [91]. Gastrointestinal ENKTL is seen in 2.7% of ENKTL and 3.1% of intestinal non-Hodgkin lymphoma cases [92,93]. Primarily seen in Asians and native South Americans, there is an association between certain HLA class II molecules (HLA-DPB1, HLA-DRB1, IL18RAP) and increased risk for this EBV+ tumour [94,95].

Clinical Features
This is an aggressive neoplasm of NK-and cytotoxic T-cells primarily involving the upper aerodigestive tract, skin, testes and gastrointestinal tract.

Pathological Findings
Macroscopically, the disease more often presents as mucosal ulcers or erosions [93,97,98,101] rather than fungating masses [98]. They feature a diffuse population of mainly small/medium-sized cells showing twisted, irregular nuclei, condensed chromatin; with widespread necrosis, karyorrhexis and numerous reactive inflammatory cells [93,101,102]. Occasional cases feature large cells with conspicuous nucleoli [101].

Cellular Origin from CD56+ T-Cells
ENKTL was previously considered a neoplasm of NK-cells. However, it was highlighted by Harabuchi Y [112] that clonal TCR rearrangement can be demonstrated in this tumour. Nagata et al. had created a cell line of nasal ENKTL of TCRγδ lineage [113], and a study in Thailand showed examples of both NK-and T-cell lineages with cases showing expression of TCRαβ, TCRγδ or both. One case was also shown to have clonal rearrangement of TCRB gene, in keeping with TCRαβ lineage [114].
Apart from expression of TCR, ENKTL of T-cell lineage was more frequently PDL1+ but less often CD56+ and lacks CXCL13 expression compared to those of NK lineage [114]. CD5 expression has been reported [114][115][116][117] but was seen only in TCRαβ+ cases in one study [114]. In a large study of primary intestinal extranodal NK/T cell lymphoma, the majority (52.9%) was of T-cell lineage with expression of CD4 and CD8 in 14% and 2.4% of cases, respectively. All the CD4+ cases tested showed clonal TCR rearrangements and displayed typical CD56+ EBER+ cytotoxic phenotypes [93].
The cellular origins of ENKTL of T-cell lineage are unclear, but suspicion naturally falls on various subsets of CD56+ T-cells. CD1d-restricted invariant natural killer T-cells (iNKT) produce cytokines within minutes of antigen stimulation, as with NK-cells. Similar to ENKTL, human NKT cells express CD69, CD94/NKG2, CD158/KIR and may be CD4+ CD8+ or CD4− CD8− [118,119]. However, with rare exceptions, NKT cells lack expression of CD335/NKp46 that is typically seen in ENKTL [120]. Distinct from NKT cells is another population of CD56+ mature T-cells in the gut lamina propria that may be of either the CD4+ or CD8+ subset. They express CD45RO, are non-proliferating, and express Th1-like cytokines such as IFNγ and TNFα [121]. Nevertheless, the cellular origin of T-lineage ENKTL remains a matter of debate.
Recently, Xiong et al. described three molecular subtypes of ENKTL with distinct biological characteristics, EBV signatures, clinical outcomes and potential targetable vulnerabilities [140]. The TSIM subtype originates from NK-cells and displays JAK-STAT activation and PD-L1 overexpression. Overexpression of c-myc, EBV latency program 1 and poor outcome are characteristics of MB subtype, whilst the HEA subtype originates from T-cells and features histone acetylation and activation of NF-kB. Mutations of JAK3, KCNB2 and KCNH8 have also been identified in gastrointestinal ENKT, although it is unclear if the mutational landscape is different in other locations.

Lymphomas and Lymphoproliferative Disorders Derived from T-Cells
Intestinal T-cells give rise to an indolent T-cell lymphoproliferative disorder and three aggressive lymphoma entities. Of the aggressive intestinal T-cell lymphomas, two are characterized by epitheliotropism (probably arising from intraepithelial lymphocytes) and one is non-epitheliotropic (probably arising from a precursor cell in the lamina propria).
6.1. Indolent T-Cell Lymphoproliferative Disorder of the GI Tract 6.1.1. Incidence and Prevalence Based on a recent review, fewer than 80 cases of this uncommon condition have been reported as of early 2021, mostly as case reports and in small series [141].

Pathological Findings
The disease presents as thickened mucosa, nodules, polyps or erosions anywhere in the gastrointestinal tract but mainly in the small intestine and colon [145,147,152,156,157]. Histology shows a dense infiltrate of small/medium-sized lymphocytes confined to the lamina propria and submucosa (Figure 2), although focal (but not diffuse) epitheliotropism has been reported, particularly in CD4+ cases, along with varying degrees of villous atrophy [142,144,145,148,[152][153][154]. Invasion of the muscularis propria is not a feature [142,144,145], but eosinophils, granulomas and lymphoid follicles may be seen [149,156].

Cellular Origin of Indolent T-Cell LPD of GI Tract
As all cases display clonal rearrangement of TCR genes with TCRαβ+ phenotype, innate lymphoid cell origin can be excluded. The lack of CD56 and CD335/NKp46 expression in most cases does not support an origin from unconventional T-cell subsets with NK-like features. Given that cases may be CD4+ or CD8+ and mostly CD103−, this appears to be a heterogeneous disease arising from conventional T-cells [141,152] in the lamina propria.
CD4+ Indolent T-cell LPD CD4+ cases lack the phenotype of TFH cells (as in primary cutaneous CD4+ small/ medium-sized pleomorphic T-cell lymphomas) and Treg cells (as in adult T-cell leukaemia/ lymphoma), which lends no support to an origin from these cellular subsets [144]. The disease appears to be a heterogeneous group with cases featuring Th1 (T-bet+), Th2 (GATA3+) and hybrid Th1/2 (T-bet+/GATA3+) differentiation [152]. This suggests that CD4+ indolent T-cell LPD may arise from conventional CD4+ TCRαβ+ T-cells that show variable differentiation towards Th1, Th2 and hybrid Th1/2 types. CD8+ Indolent T-cell LPD CD8+ cases display a non-activated cytotoxic phenotype and are located in the lamina propria, in keeping with the origin from conventional CD8αβ+ TCRαβ+ T-cells [160].

Celiac Disease
Although this review is focused on intestinal NK and T-cell neoplasms, celiac disease is briefly discussed, as it lends understanding towards the cellular origins of EATL, which is a sequela of this disease. Celiac disease is an autoimmune, chronic intestinal disease due to sensitivity to gluten in genetically susceptible individuals, leading to symptoms and signs of malabsorption. Patients will initially respond to a gluten-free diet, but a subset of patients progresses into a refractory celiac disease (RCD) phase that is resistant to gluten withdrawal.

Epidemiology
Celiac disease is more common in western populations, with a prevalence of 0.4% in South America, 0.5% in Africa and North America, 0.6% in Asia, and 0.8% in Europe and Oceania. The global pooled prevalence is 1.4% based on serology and 0.7% based on biopsy results [162].

Clinical Features
The clinical presentation is variable and age-dependent. The typical presentation in children is that of failure to thrive, malabsorption, diarrhoea, abdominal pain and distension, but symptoms may be nonspecific in older patients [163]. Endoscopic features include reduced and scalloped duodenal folds, atypical submucosal vascular pattern (referred to as 'mosaicism'), as well as mucosal grooves and fissures [164].

Refractory Celiac Disease (RCD)
When celiac disease does not respond histologically despite 12 months of a gluten-free diet, this is referred to as refractory celiac disease [172]. The absence and presence of phenotypically atypical IELs divides RCD into two phases, RCD1 and RCD2, respectively. About half of those with RCD2 will develop EATL [173][174][175], whilst progression to lymphoma in patients with RCD1 is exceptional [173,176,177]. The histological features of RCD are similar to that of celiac disease, but ulcerative jejunitis and lymphocytic gastritis tend to be more frequent in RCD2 [174].
The IELs in RCD are morphologically normal, but the immunophenotype is atypical in RCD2, being sCD3− but intracytoplasmic CD3+, CD8− and TCRαβ− [178][179][180]. Given that development of EATL is seen almost exclusively in cases with >20% atypical T-cells by flow cytometry [181], the cut-off criterion between RCD1 and RCD2 is defined as >20% of phenotypically atypical (sCD3-, CD3ε+ CD8−) T-cells by flow cytometry [181] and >50% of such T-cells by immunohistochemistry [182,183]. Clonal rearrangement of TCR gene is generally negative in RCD1 but is seen in about 75% of RCD2 cases [184,185]. The latter includes clonal rearrangements of TCRD and/or TCRG, but with either incomplete or no rearrangement of TCRB genes, as well as cases that display complete rearrangements of TCRB, and TCRG and/or TCRD [185].

Enteropathy
Associated T-Cell Lymphoma 6.4.1. Epidemiology EATL comprised 4.2% of T-cell lymphomas in one study [186], with an incidence of 0.10/100,000 in the Netherlands [187] and 0.016/100,000 in the United States [188]. In Asia, EATL is rare and comprises only 1.4% (7/490 cases) of T-cell lymphoma cases in a multi-centre Asian study [189]. About half of the cases of EATL are preceded by a history of celiac disease, and the remainder appears de novo [190], although celiac disease may be underdiagnosed.

Clinical Findings
This neoplasm is strongly associated with celiac disease (52% of cases) [190][191][192] and geographical regions with higher frequency of the HLA-DQ2 and DQ8 haplotypes, which are rare in Asia [193]. The disease comprises 5.4% of NK-and T-cell neoplasms [190] and presents in older adults. Usual presentations include abdominal pain, weight loss, diarrhoea or bowel perforation [190,194,195]. Disease spread to lymph nodes, lung, liver and bone marrow occurs in 10-20% of cases, and the median survival is poor at <10 months [157,190,194].

Pathological Findings
Typically, there are multiple ulcerated plaques and constricting tumour masses in the small intestine (especially jejunum) and regional lymph nodes [190,191,194]. Microscopic examination features sheets of medium sized to large pleomorphic, immunoblastic or anaplastic lymphocytes showing significant nuclear pleomorphism and necrosis, accompanied by a pronounced inflammatory infiltrate of small lymphocytes, eosinophils, histiocytes and plasma cells (Figure 3). The adjacent intestinal mucosa often shows histological changes of celiac disease such as villous atrophy, crypt hyperplasia and increased IELs [196]. the small intestine (especially jejunum) and regional lymph nodes [190,191,194]. Microscopic examination features sheets of medium sized to large pleomorphic, immunoblastic or anaplastic lymphocytes showing significant nuclear pleomorphism and necrosis, accompanied by a pronounced inflammatory infiltrate of small lymphocytes, eosinophils, histiocytes and plasma cells (Figure 3). The adjacent intestinal mucosa often shows histological changes of celiac disease such as villous atrophy, crypt hyperplasia and increased IELs [196].

Immunophenotype
Neoplastic lymphocytes are typically CD3ε+ CD4− CD8− TIA1+ granzyme B+ perforin+ CD5− CD7+ CD56− and CD103+, with a high proliferation fraction [180,190,191,194,197,198]. CD56 was detected by flow cytometry in some cases of RCD2 but was absent by immunohistochemistry in the corresponding EATL. Similarly, CD103 expression in EATL may be lost despite CD103+ IELs in the corresponding RCD2 [194]. Expression of TCR is lacking in RCD2 and in most cases of EATL [179,180,191,198], but a case preceded by RCD1 has been reported to arise from TCRγδ+ IELs [199]. There are usually CD30+ large cells within the tumour [194], and there is expression of CD335/NKp46 [159] whilst EBV is negative [194,199]. CD8 is usually negative in EATL, and in one study, it was present only in cases associated with non-clonal enteropathy (celiac disease and RCD1) [194].

Cellular Origins
There are significant differences between RCD1 and RCD2, which may even suggest different disease entities [197] and cellular origins. The prognosis is different with significantly inferior prognosis in RCD2 [177], whilst progression of RCD1 to RCD2 [177] and from RCD1 to EATL [174] is exceptional.

Cellular Origin of EATLs Arising from RCD2
Most cases of EATL display a similar phenotype as RCD2, such as CD4− CD8− TCR− CD3ε+ but sCD3−. The phenotype is in keeping with an origin from iCD3+ IELs, which is the group of ILCs that is Id2 independent and rearranges but does not express TCR genes. They are sCD3− and express intracytoplasmic CD3.

Cellular Origins of EATL Arising from RCD1 and CD
Whilst EATL with typical CD8− TCR− phenotype usually develops from RCD2, cases associated with RCD1 and celiac disease have been reported that express CD8 and TCR [194,199]. The cellular origin of such exceptional cases is unclear but based on the phenotype, it is reasonable to suggest that they may originate from CD8αβ+ TCRαβ+ (type 'a') or CD8αα TCRαβ+/CD8αα+ TCRγδ+ (type 'b') IELs.

Pathogenetic Mechanisms in Celiac Disease, RCD and EATL
Gluten molecules penetrate intestinal epithelium by transcytosis or paracellular routes and undergo deamidation by tissue transglutaminase. Deamidated gliadin peptides are presented by dendritic cells in association with HLA DQ2 or DQ8 molecules to CD4+ helper T-cells. Homozygosity for HLA DQ2/8 increases presentation of gliadin peptides [200,201] and portends a higher risk of EATL [202]. Gliadin-specific CD4+ TCRαβ+ IELs mount a Th1 response and produce pro-inflammatory cytokines such as IFNγ, IL2 and IL21 that damage intestinal epithelial cells (IECs) [203].
IL15 produced by intestinal epithelial and dendritic cells also enhances the cytotoxicity of IELs through activation of perforin/granzyme B [208]. IL15 upregulates production of IL21 by epithelial cells and CD4+ T-cells [209], further increasing the numbers and cytotoxicity of IELs [210,211].
In early celiac disease, there is an initial increase in CD8αβ+ TCRαβ+ and TCRγδ+ IELs, at the expense of CD7+ TCR-iCD3+ IELs, which normally comprise a minute population of human IELs. However, IL15 suppresses apoptosis of lymphocytes in the intraepithelial compartment, resulting in accumulation of IELs [208,212]. Sustained cytokine stimulation also induces genotoxic stress [213] and acquisition of mutations (JAK1, STAT3, STAT5B and SOCS1) that increase sensitivity of IELs to IL15. The result is an expansion and reprogramming towards iCD3+ IELs [25,208,212] that outcompete CD8αβ+ TCRαβ+ IELs [25,212], the result being that iCD3+ IELs become the predominant IEL population in RCD2 and EATL.

. Incidence and Prevalence
In the International Peripheral T-cell Lymphoma Project, 5.4% of T-cell lymphomas were either EATL or MEITL with 66% EATL and 34% MEITL cases [190]. In Asia, MEITL appears to be more numerous as a proportion of intestinal T-cell lymphoma, with figures as high as 83% in Japan [218], 80% in Korea [219] and 82% in Hong Kong [220]. A recent Asian study found intestinal T-cell lymphomas to comprise 3.4% of T-cell lymphomas composed of EATL and MEITL in about equal proportions [189]. However, this was not a population-based study and included only cases from selected institutions.

Pathological Findings
Endoscopic findings include oedematous and finely granular mucosa, erythematous erosions and shallow ulcers [236][237][238][239][240]. Macroscopically, the tumour presents as a transmural, ulcerative mass, often with perforation. The lymphomatous infiltrate comprises relatively monotonous, medium-sized lymphocytes with scarcity of reactive inflammatory cells and necrosis [191,220,222,225,226,228] (Figure 4). The intestinal mucosa adjacent to the invasive tumour features increased IELs, which may be atypical in morphology and phenotype, whilst IELs appear normal further away from the tumour [220,225], the latter mimicking celiac disease or lymphocytic colitis [228,236,238,239].

Cellular Origin from Non-Conventional CD8αα+ T-Cells
In addition to CD8 expression, MEITL displays a cytotoxic phenotype (TIA1+, granzyme B+) and expresses NK markers such as CD335/NKp46 and CD56, but unlike EATL, most cases express TCR of either TCRαβ or TCRγδ lineages, in keeping with T-cell origin. There is phenotypic heterogeneity in MEITL, with most cases being CD8αα+TCRαβ+ or CD8αα+TCRγδ+, but there are also cases that are CD8αβ+ TCRαβ+ and a subset that is CD8αα+ but TCR-silent [225]. The central feature is CD8αα expression, which is seen in MEITLs that may be TCRαβ+, TCRγδ+ or TCR-silent [225]. The CD8αα+ TCR+ phenotype in most cases of MEITL is therefore in keeping with the population of type 'b' (natural) IELs that are CD8αα+ TCRγδ+ or CD8αα+ TCRαβ+. As for the <25% of MEITLs that are CD8αβ+ TCRαβ+ [225], co-expression of CD8αα cannot be excluded by immunohistochemistry, given that conventional CD8αβ+ TCRαβ+ T-cells can be reprogrammed to express CD8αα. Finally, there is a subset of MEITL that is TCR-silent but displays rearrangement of TCR genes [244] and are mostly CD8αα+ [225]. This raises the possibility that a minor subset of MEITL originates from iCD3+ IELs that may be either CD8αα-or CD8αα+ (iCD8α IELs).
The aetiology is unknown, but there is no association with celiac disease, even in western populations [223]. However, the pathogenesis of MEITL may share overlapping features with that of EATL. Environmental factors such as dietary and microbial stimuli in the intestinal lumen may injure intestinal epithelial cells, leading to production of IL15 as with celiac disease. IL15 triggers T-bet and promotes proliferation, maturation and survival of CD8αα+ natural IELs. However, as with iCD3+ IELs in RCD2 and EATL, CD8αα+ T-cells constitute a small proportion of normal human IELs. Mutations of the JAK-STAT pathway and other genetic alterations may facilitate the outgrowth of these rare IEL populations during lymphomagenesis. Similarly, TGFβ, retinoic acid and IFN-γ or IL-27 in the intestinal environment may activate T-bet on CD8αβ+ TCR+ IELs, leading to CD8αα and CD103 expression [8,59,65].

6.6.
Intestinal T-Cell Lymphoma, NOS 6.6.1. Incidence and Prevalence As a rare disease and newly recognized entity, accurate incidence and prevalence figures are not available. However, a multi-centre Asian study reported that intestinal T-cell lymphoma, NOS comprises only 0.4% of mature T-cell lymphomas [189].

Clinical Features
By definition, the diagnosis of this intestinal T-cell lymphoma requires the exclusion of other well-defined entities, such as EATL, MEITL, anaplastic large cell lymphoma and ENKTL [192,221]. This neoplasm is more frequently reported in Asia and primarily affects adult males [253]. As this is a diagnosis of exclusion, the aetiology is unknown but is likely to be diverse. Most cases are non-epitheliotropic, but this diagnostic category also includes cases associated with autoimmune enteropathy that shows EATL-like histological features [254].
As with other gastrointestinal tumours, typical presenting complaints include abdominal pain, bleeding and acute abdomen due to bowel perforation. The disease usually involves the small and large intestines but may also arise in the stomach, where hematemesis is a common presentation. Dissemination to regional lymph nodes and distant sites may occur [255][256][257], and the prognosis is poor, but the median survival (35 months) is better than that of EATL and MEITL [92,257]. Large cell morphology is associated with poorer prognosis [256].

Pathological Findings
The tumour may be protruding and obstructive or ulcerative and constricting. Neoplastic cells are medium to large lymphocytes that display a greater degree of pleomorphism than that in MEITL ( Figure 5). Unlike EATL and MEITL, epitheliotropism is not prominent but may be seen in 15% of cases [256,257]. Neoplastic lymphocytes express CD2 and CD3, with most cases being either CD4+ or CD4− CD8−. A subset displays aberrant expression of CD56, in contrast with indolent CD4+ T-cell LPD of the GI tract. Many cases have a cytotoxic phenotype, being TIA1+, but with variable expression of granzyme B and CD30 [255][256][257]. TCR expression may be either of TCRαβ type or is silent [255,256]. The tumour may be protruding and obstructive or ulcerative and constricting. Neoplastic cells are medium to large lymphocytes that display a greater degree of pleomorphism than that in MEITL ( Figure 5). Unlike EATL and MEITL, epitheliotropism is not prominent but may be seen in 15% of cases [256,257]. Neoplastic lymphocytes express CD2 and CD3, with most cases being either CD4+ or CD4− CD8−. A subset displays aberrant expression of CD56, in contrast with indolent CD4+ T-cell LPD of the GI tract. Many cases have a cytotoxic phenotype, being TIA1+, but with variable expression of granzyme B and CD30 [255][256][257]. TCR expression may be either of TCRαβ type or is silent [255,256].

Cellular Origins from Conventional and Non-Conventional T-Cells
As this is a relatively new entity, little data are available to postulate its cellular origins. In addition, as with diagnoses of exclusion, the cellular origins are likely heterogeneous as well.
Most cases are non-epitheliotropic and display expression of TCRαβ+, in keeping with derivation from CD4+TCRαβ+ conventional T-cells in the lamina propria. However, cases that are double negative (CD4−CD8−) and TCR-silent raise the question if they may also arise from innate lymphoid cells in the lamina propria. Cases associated with autoimmune enteropathy that resemble EATL and express TCRαβ may have arisen from conventional type 'a' IELs.

Pathogenetic Mechanisms
As with the aetiology, the pathogenesis is unclear. However, mutations of the JAK/STAT and MAPK pathways have been identified [258].

Current and Novel Treatment in EATL, MEITL and ITCL, NOS
The usual treatment for EATL is surgery, followed by anthracycline-containing chemotherapy with or without radiotherapy. In recent years, the Newcastle regimen comprising ifosfamide, etoposide, and epirubicin/methotrexate has shown better outcomes [195,285]. CD30 is often expressed in EATL, and a Phase 2 study found that brentuximab vedotin was efficacious in this tumour [286]. There is an ongoing trial (EATL-001) of the BV-CHP (brentuximab vedotin, cyclophosphamide, doxorubicin, prednisone) regimen followed by consolidation with high dose therapy or transplant (NCT03217643).

Conclusions
Lymphoma classification is based on identification of distinct clinico-pathological entities and are often named after the putative cells of origin. The gastrointestinal tract is a unique immunological site where lymphocytes need to serve the dual, conflicting functions of inflammation and tolerance. Complex, specialized subsets of immune cells in the gastrointestinal mucosa have developed unique properties to mount a protective immune reaction against invading microbes yet possess immunomodulatory mechanisms to prevent unwanted tissue damage. Indolent and aggressive lymphoid neoplasms may arise from such immune cells, with unique phenotypes that resemble those of their putative cells of origin ( Figure 6).
The two intestinal T-cell lymphomas that demonstrate prominent epitheliotropism, namely EATL and MEITL, are now considered separate entities based on association with celiac disease, morphological features and immunophenotype. Their cellular origins are also different. EATL cases that complicate RCD2 arise from iCD3+ ILCs (and thus not strictly a T-cell lymphoma), whilst MEITL arises from CD8αα+ IELs. However, both iCD3+ ILCs and CD8αα+ IELs (whether CD8αα+ TCRαβ+ T-cells, CD8αα+ TCRγδ+ T-cells or iCD8a+ ILCs) depend on IL15 and T-bet for development and display significant overlap in mutational profiles. At the level of cellular origins and pathogenesis, EATL and MEITL may share more similarities than differences. Future therapeutic strategies may be directed at the microenvironment of the intraepithelial compartment, such as inhibition of IL15 [318]. An understanding of the types and biology of gastrointestinal lymphocytes may hold the key to better diagnosis and treatment of gastrointestinal lymphomas.   The two intestinal T-cell lymphomas that demonstrate prominent epitheliotropism, namely EATL and MEITL, are now considered separate entities based on association with

Acknowledgments:
The authors thank Gareth Turner of Oxford University Hospitals NHS Foundation Trust for the images in Figure 3.