Advanced
Lineage re-commitment of CD4CD8αα intraepithelial lymphocytes in the gut
Lineage re-commitment of CD4CD8αα intraepithelial lymphocytes in the gut
BMB Reports. 2016. Feb, 49(1): 11-17
Copyright © 2016, Korean Society for Biochemistry and Molecular Biology
This is an open-access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/4.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.
  • Received : October 25, 2015
  • Published : February 29, 2016
Download
PDF
e-PUB
PubReader
PPT
Export by style
Share
Article
Author
Metrics
Cited by
TagCloud
About the Authors
Yunji Park
Division of Integrative Biosciences and Biotechnology
yunji@postech.ac.kr
Sook-Jin Moon
Division of Integrative Biosciences and Biotechnology
Seung-Woo Lee
Department of Life Sciences, Pohang University of Science and Technology, Pohang 37673, Korea
yunji@postech.ac.kr

Abstract
The gastrointestinal tract forms the largest surface in our body with constantly being exposed to various antigens, which provides unique microenvironment for the immune system in the intestine. Accordingly, the gut epithelium harbors the most T lymphocytes in the body as i ntraepithelial l ymphocytes (IELs), which are phenotypically and functionally heterogeneous populations, distinct from the conventional mature T cells in the periphery. IELs arise either from pre-committed thymic precursors (natural IELs) or from conventional CD4 or CD8αβ T cells in response to peripheral antigens (induced IELs), both of which commonly express CD8α homodimers (CD8αα). Although lineage commitment to either conventional CD4 T helper (Th) or cytotoxic CD8αβ T cells as well as their respective co-receptor expression are mutually exclusive and irreversible process, CD4 T cells can be redirected to the CD8 IELs with high cytolytic activity upon migration to the gut epithelium. Recent reports show that master transcription factors for CD4 and CD8 T cells, ThPOK (Th-inducing BTB/POZ-Kruppel-like factor) and Runx3 (Runt related transcription factor 3), respectively, are the key regulators for re-programming of CD4 T cells to CD8 lineage in the intestinal epithelium. This review will focus on the unique differentiation process of IELs, particularly lineage re-commitment of CD4 IELs. [BMB Reports 2016; 49(1): 11-17]
Keywords
INTRODUCTION
The intestine is the biggest immunological organ in our body with its huge surface that is constantly exposed to foreign matters, such as food and pathogens (1) . There is a delicate balance between protective immunity against detrimental pathogens and tolerance to food and beneficial bacteria. To keep the homeostatic balance, various immune cells are located in the gut epithelial lining. Among the immune cells, lymphocytes located within the gut epithelium are intestinal i ntraepithelial l ymphocytes (IELs), which represent the largest heterogeneous T cell population in the organisms. Due to the constant exposure of antigens at the mucosal barrier, IELs have unique and antigen-experienced activated phenotypes, constitutively expressing CD8α homodimer, CD8αα, and CD103 (αE integrin), distinct from the conventional T cells in the periphery. Further, they are suggested to play a pivotal role in providing protective immunity as the first line of immune defense (reviewed in Ref 2-4).
In this review, we discuss the development, subtypes, and functions of IELs, especially focusing on the unusual CD4 IELs expressing CD8α homodimer, CD4CD8αα double positive (DP) IELs. There are decent reports recently revealed that conventional CD4 T cells post-thymically reprogram to cytotoxic T lymphocyte (CTL)-like DP IELs expressing CTL-lineage associated genes in the gut microenvironment (5 , 6) . We focus on how conventional CD4 T cells in the periphery reprogram their cellular fate from the T helper lineage to the CTL-like DP IELs in the gut environment.
SUBSETS OF IELS
IELs can be classified into “natural” and “induced” categories based on their developmental origin and co-receptor expressions (2 - 4) . Natural IELs (nIELs), previously classified as “type b” IELs, differentiate unconventionally from their own precursors and are activated during selection process in response to self-antigens in the thymus. They are barely detectable in peripheral mainstream lymphoid tissues and express either αβ T cell receptor (TCR) or γδ TCR with a unique co-receptor, CD8αα, without conventional CD4 or CD8αβ co-receptors (CD8αα TCRαβ IELs or CD8αα TCRγδ IELs). Induced IELs (iIELs), previously “type a IELs”, are derived from the conventional CD4 or CD8αβ T cells in the periphery and postthymically differentiated in the intestine. They are αβ TCR-bearing CD4 or CD8αβ T cells with a predominance of CD8 T cells, which have much in common with conventional CD4 and CD8αβ T cells found in the spleen and other lymphoid tissues. These conventional iIELs also co-express CD8αα together with CD4 or CD8αβ, thus the expression of CD8αα is a hallmark of IEL populations in the gut epithelium (2 - 4) .
UNIQUE DEVELOPMENT OF IELS
Originally, nIELs are thought to differentiate thymus-independently. Early experiment described that thymectomy has little effects on the generation of IELs in the intestine (7) , and CD8αα IELs are present in neonatally thymectomized mice reconstituted with bone marrow or fetal liver (8 - 10) . Studies show that cryptopatches (CP) in the small intestine contain IEL precursor cells (11 , 12) , and that CP cells from nude mice are able to reconstitute IELs (12) , further supporting the idea that nIELs are derived extra-thymically from CP precursors. However, increasing evidence supports the importance of thymus and thymic progenitors for nIEL development. It has been reportedly observed that the number of CD8αα IELs is extremely reduced in athymic mice (9 , 13 , 14) , indicating that thymus is functional for nIEL development. Moreover, a genetic fate mapping study showed direct evidence that all CD8αα-bearing αβ TCR IELs originate from CD4 + CD8 + DP thymocytes rather than CP precursors (15) . The thymic development of nIELs is further clarified by the identification of thymic precursors of CD8αα IELs. Cheroutre and co-workers discovered that CD4 + CD8αβ + CD8αα + triple positive (TP) cells in the thymus, distinct from classical CD4 + CD8αβ + DP thymocytes, are unique precursors for CD8αα IELs, which are selected and matured in a “alternative“ way by strong interactions with self-antigens, termed “agonist selection” (16) ( Fig. 1 ). As a result, they become mature, memory-like CD4 CD8αβ αβ or γδ TCR-expressing T cells and acquire gut-homing capacity in the thymus, thereby directly migrating to the intestinal epithelium (16 , 17) .
PPT Slide
Lager Image
Development and subsets of IELs. Natural CD8αα IELs (nIELs) differentiate from unique thymic precursors, CD4+CD8αβ+CD8αα+ triple positive (TP) cells, which go through a self antigen-based thymic selection process (“agonist selection”). On the other hand, induced IELs (iIELs) are derived from the circulating conventional CD4 or CD8αβ T cells, which further undergo post-thymic differentiation process.
Contrary to nIELs, iIELs are the progeny of conventional MHC class I-restricted CD8αβ or MHC class II-restricted CD4 naïve T cells that further undergo post-thymic differentiation process in the intestine (reviewed in ref 2-4). Naïve T cells are activated in response to antigens in the periphery, particularly gut-associated lymphoid tissue (GALT), including Peyer’s patches (PPs) and mesenteric lymph nodes (MLN), and then migrate into the intestinal epithelium to become iIELs. It is well known that CD103 + dendritic cells (DCs) in the GALT with their ability to produce the vitamin A metabolite, retinoic acid (RA), confer the gut-homing capacity to naïve T cells they prime (18 , 19) . RA induces gut-homing receptors, α 4 β 7 and CCR9 on T cells, which bind to mucosal adhesion molecule 1 (MAdCAM-1) and CCL25, respectively (20) . These are constitutively expressed on small intestinal endothelium and epithelium, thus directing T cells to traffic into the intestine (21 - 23) . In the intestine, T cells subsequently down-regulate α 4 subunit of α 4 β 7 and gradually acquire α E (CD103) expression to form integrin α E β 7 (24 , 25) , which is induced by transforming growth factor (TGF)-β in the gut environment (26) . α E β 7 is a major adhesion molecule, which mediates selective localization or retention of IELs to the epithelial compartment of the intestine by interacting with E-cadherin expressed on the basolateral surface of enterocytes (27) .
FUNCTIONS OF IELS
The precise physiological functions of IELs are not still clear. Yet, their strategic location at the front line of defense and concomitant expression of effector and regulatory molecules allows us to predict that these cells are of importance for controlling pathogen infection and preserving the integrity of epithelial barrier in the gut. Although CD8αα TCRαβ IELs are self-reactive and cytotoxic effectors with high granzyme B expression (3) , they are not self-destructive, and instead they have been shown to mediate immune regulation by expressing several regulatory molecules including interleukin-10 (IL-10), TGF-β and lymphocyte activation gene 3 (LAG 3) (28 , 29) . In a mouse model of induced colitis, self-specific CD8αα TCRαβ IELs exert a regulatory function by controlling the migration and activity of pathogenic CD4 T cells in the intestine (28) . Nevertheless, it is still possible that these self-reactive CD8αα TCRαβ IELs may drive autoimmune pathology under inflammatory or damaged conditions (4) . TCRγδ IELs also contribute to protective immunity, secreting TNF and IFN-γ (30) and keep gut integrity through producing TGF-β1, TGF-β3 and keratinocyte growth factor (KGF) to repair epithelial damage (31) .
CD8αβTCRαβ IELs are reported to serve as long-residing effector memory cells within the epithelium to provide protection against various invading pathogens, including simian immunodeficiency virus (SIV) (32 , 33) , lymphocyte choriomeningtis virus (LCMV) (34) , and rotavirus (35) . Studies suggest that CD4 IELs, though a minor subset compared to natural or CD8αβTCRαβ IELs, may contribute to the prevention of immunopathology in the intestine by exerting their protective and regulatory functions (36 , 37) .
CD4CD8αα IELS
In general, CD4 T cells are known as helper cells in terms of their ability to modulate the functions of other immune cells, such as cytotoxic T cells, B cells and antigen presenting cells. However, their effector functions as CTLs and the existence of CD8-expressing CD4 T cells have continuously been reported over the decades, particularly during virus infection (38 - 40) . Nevertheless, the cytotoxic activity of CD4 T cells has been considered an in vitro artifacts or a functional variant of T helper 1 subset in vivo , and there is little convincing evidence to support this subset of CD4 T cells as an independent subtype of CD4 T cells (41) .
Two important reports recently identified CD4 CTLs as a distinct subset of effector CD4 T cells, which express co-receptor CD8αα and CTL lineage genes distinguishing them from any other conventional CD4 T helper subsets or regulatory T cells (5 , 6) . At steady state, these cytotoxic CD4 T cells are mostly found in the small intestine as CD4CD8αα double positive (DP) IELs (42) and can be experimentally generated in the T cell transfer model of colitis, in which naïve T cells are transferred into lymphopenic hosts (5 , 6) . Remarkably, these reports show that the cytotoxic DP IELs lack ThPOK, the master transcription factor for CD4 Th lineage commitment, of which expression is maintained highly in the peripheral conventional CD4 T cells and critical for their helper functions. DP IELs are initially originated from conventional CD4 + ThPOK + T cells, which lose ThPOK expression post-thymically upon migration to the gut (5) . The loss of ThPOK is induced by Runx3-mediated silencing, the master transcription factor for the CD8 CTL lineage program, thus CD4 + ThPOK T cells reprogram to MHC-II-restricted cytotoxic CD4 T cells (6) .
ThPOK-Runx3: MASTER TRANSCRIPTIONAL REGULATORS FOR CD4CD8αα IEL DEVELOPMENT
The co-receptor expression of either CD4 or CD8αβ is precisely correlated with T cell function as helper or killer T cells, respectively. Therefore, this lineage decision process from CD4 + CD8 + DP thymocytes to either lineage is crucial in T cell biology and extensively studied over the decades. With the identification of the ThPOK transcription factor (43 , 44) and its counteraction with Runx3 (45) , it is now widely accepted that this key developmental decision is made at the transcriptional level.
ThPOK is a key transcription regulator, which drives CD4 + CD8 + thymocytes to become mature CD4 T cells, and Runx3 is required for CD8 T cell development (46 , 47) . The fate decision of helper or CTL is driven by the antagonistic interaction between these two transcription factors, as ThPOK is highly expressed and maintained in mature CD4 T cells in the periphery, preventing Runx3-mediated CTL program. On the other hand, Runx3 expression is preferentially up-regulated during CD8 lineage differentiation in the thymus and remains high in mature CD8αβ T cells, which represses ThPOK expression by binding to silencer of ThPOK gene in immature CD8αβ thymocytes (46 , 47) . In addition to silencing CD4 by repressing ThPOK, Runx3 also reactivates CD8 by directly binding to Cd8 enhancer, E8I, of the CD8 gene locus (48) .
It has long been thought that CD4-CD8 lineage commitment is mutually exclusive and irreversible. Although mature CD4 T cells are able to differentiate into separate subtypes of helper T cells in response to peripheral antigens, such Th1, Th2, Th17, and regulatory T cells, they maintain their ThPOK expression and helper lineage gene profiles (5 , 49) Therefore, little progress has been made in the study of CD8-expressing cytotoxic CD4 T cells in terms of lineage reprogramming despite their existence in the gut and other tissues during viral infection. Recently, two groundbreaking studies bring a new concept of plasticity of mature CD4 T cell development, revealing that CD4 T cells can reprogram to cytotoxic CD8 lineage-like cells, CD4CD8αα T cells, in the small intestine in response to cognate antigens by losing their ThPOK expression, which is mediated by Runx3 up-regulation in the specific gut environment (5 , 6)
Mucida et al. (5) found that CD4CD8αα DP IELs, the most observed cytotoxic CD4 T cells, lack ThPOK expression, which is normally expressed high in the conventional CD4 T cells in the periphery. Concomitantly, DP IELs display cytotoxic phenotypes like CD8αβ CTLs, including high expression of granzyme B and the activation-induced degranulation marker CD107a. By using well-designed fate-mapping mouse model, they showed that these cells are derived from ThPOK + CD4 T cells, which lose their ThPOK expression after activation in the periphery and migration into the gut. In line with this, the development of cytotoxic CD4CD8αα IELs are significantly hampered in mice with germline deletion of ThPOK silencer or mice deficient for the zinc-finger transcription factor MAZR that activates the ThPOK silencer. This indicates that reactivation of ThPOK silencer thereby terminating ThPOK post-thymically is essential for the differentiation of DP IELs and their CTL program.
Reis et al. (6) further revealed that loss of ThPOK is initiated by Runx3 expression. By using knock-in reporter mice for Th-POK and Runx3, they observed that expression of CD8αα on CD4 IELs is directly associated with the high expression of Runx3, normally repressed in conventional CD4 T cells in the periphery. Runx3 hi CD4 IELs have a very low level of ThPOK expression and other T helper genes, but have increased expression of CTL lineage genes. TGF-β and RA are the environmental cues in the intestine to induce Runx3 and CD8αα while down-regulating ThPOK, as evidenced by in vitro and in vivo experimental settings using mice deficient for TGF-β and RA signaling. Commensal microorganisms in the intestine are also directly or indirectly involved in cytotoxic DP IEL development, since DP ILEs are absent in the germ-free animals (5) . Remarkably, the presence of Runx3 hi ThPOK hi CD4 T cells, but not Runx3 lo ThPOK lo cells, indicates that Runx3 induction precedes the down-regulation of ThPOK (6) . Indeed, mice with a conditional deletion of Runx3 in T cells fail to down-regulate ThPOK expression in CD4 IELs, thereby exhibiting reduced frequency of CD4 IELs expressing CD8αα, CD103 and CTL genes. Overall, upon migration of CD4 T cells to the intestine, Runx3 is up-regulated within the gut environment, which in turn down-regulates ThPOK, promoting cytotoxic CD4CD8αα DP IEL development.
T-bet AS A SWITCH FOR CD4CD8αα IEL DEVELOMENT
Subsequently, Reis et al. (50) also reported that transcription factor, T-bet (T-box expressed in T cells), is a critical upstream regulator of Runx3 and ThPOK in the development of cytotoxic CD4CD8αα DP IELs. T-bet is a well-known transcription factor for Th1 differentiation and CD8 T cells for their IFN-γ production, whereas inhibiting the development of other T helper subtypes. They found that the expression of Tbx21 encoding T-bet is up-regulated in ThPOK lo IELs, and DP IEL differentiation is impaired in T-bet-deficient CD4 T cells, comparable to Runx3 −/− CD4 T cells. In the presence of gut microenvironmental cues, such as TGF-β and RA, and T-bet inducing cytokines in the intestinal milieu, such as IFN-γ or IL-27, preferentially induce cytotoxic CD4CD8αα DP IEL differentiation by inducing Runx3 and suppressing ThPOK expression. Notably, these combinations suppress TGF-β/RA-mediated Foxp3 induction and T-bet-mediated Th1 differentiation, suggesting a distinct role of T-bet in the CD4CD8αα DP IEL development.
Interestingly, overexpression of T-bet in the absence of Runx3 suppresses Th17 differentiation, and T-bet prevents Foxp3 induction regardless of Runx3 expression, suggesting that T-bet inhibits other CD4 T cell subsets in a Runx3-independent manner. A series of experiments using genetically modified CD4 T cells as well as chromatin immunoprecipitation (ChIP) analysis indicate that T-bet directly binds to the -39 kb and the -17 kb Runx3 regulatory elements and induces Runx3 expression. T-bet also binds to ThPOK regulatory binding sites to down-modulate ThPOK expression in a Runx3-dependent manner. Collectively, this study suggests that T-bet that is induced by gut environmental factors serves as an upstream regulator for Runx3 induction thereby promoting CD4CD8αα DP IEL development. Yet once Runx3 expression is induced, T-bet and Runx3 have complementary roles in ThPOK down-regulation and cytotoxic DP IEL generation ( Fig. 2 ).
PPT Slide
Lager Image
Transcriptional regulation in the development of CD4CD8αα DP IEL. Runx3, known as a key transcription factor for CD8 T cell lineage, which are normally repressed in conventional CD4 T cells, can be reactivated in peripheral CD4 T cells by gut environmental factors including TGF-β and RA and T-bet. Runx3 repressess ThPOK, a transcription factor for CD4 T cell lineage, through direct binding to ThPOK silencer elements, thereby promoting CD8αα and CD103 expression and CTL-associated gene program in the CD4 T cells.
FUNCTIONS OF CD4CD8αα IELS
Although DP IELs are equipped with cytotoxic properties since their development, DP IELs do not exert CTL function immediately and instead stay immunologically quiescent at steady state. Antigen-specific CD4CD8αα DP IELs, as generated by transferring OVA-specific OT-II cells into immunodeficient recipients following OVA-feeding, remain immunologically inactive even in the continuous presence of cognate dietary antigens, though they possess cytotoxic potential (5) . This indicates that DP IELs are “activated yet resting” like other IEL subsets, which is confirmed by increased uptake of thymidine analog BrdU and week signals of cell-cycle marker Ki67 (5) .
Upon exposure to inflammatory IL-15 with cognate antigens, however, diet-induced DP IELs become immunologically active, producing great amounts of IFN-γ and TNF and up-regulation of CD107a expression, implying the pathogenic potential of cytotoxic DP IELs (5) . On the other hand, Reis et al. showed the plasticity of CD4CD8αα T cells in their functional aspects (6) . In a T cell-transfer model of colitis, ThPOK deletion in CD4 T cells, which enhance the generation of CD4CD8αα T cells, coincides with reduced intestinal inflammation, suggesting non-pathogenic function of DP IELs. In line with this, mice receiving Runx3-deficient CD4 T cells accelerate colitis, which is associated with reduced CD4CD8αα T cells, but increased Th17 differentiation. Mice deficient for CD4-specific Runx3 also develop more pathogenic Th17 cells with increased inflammation and tissue damage compared to control mice after Citrobacter rodentium infection; however, they are more efficient at clearing C. rodentium than the wild-type control (6) . These results suggest that two facet of inflammation, protection and tissue damage, can be modulated at the transcriptional level by Runx3 and ThPOK. Further studies are needed to define the roles of CD4CD8αα T cells during infection and disease progress.
CONCLUDING REMARKS
Cytotoxic CD4 T cells have not been recognized for a long time since there is a lack of distinct functions, developmental mechanisms, and physiological relevance. Nevertheless, cytotoxic phenotypes of CD4 T cells have been repeatedly reported in human and rodents, mostly CD4CD8αα DP IELs in the intestine. Furthermore, there is accumulating evidence showing the presence of cytotoxic CD4 T cell subsets during anti-viral (38 - 40) and anti-tumor immune responses (51 , 52) . Finally, recent elegant studies provide us unequivocal answers for the cytotoxic CD4 T cells as a distinct, independent subtype of effector CD4 T cells, which are derived by the unique modulation of transcription factors that drive their development and functional differentiation (5 - 7) . Although the precise role of CD4CD8αα DP IELs and the relevance of their cytolytic phenotypes in vivo are not understood yet, the identification of a new CD4 subset and novel developmental program will improve our understanding on the functional differentiation of effector T cells. New insights gained from these studies will contribute to the identification of new targets for the design of new therapies to prevent and cure infection and diseases.
Acknowledgements
This work was supported by a grant of the Korean Health Technology R&D project (HI14C2171) and Korean Health Technology R&D project through the Korean Health Industry Development Institute (KHIDI) (HI14C2640), funded by the Ministry of Health & Welfare, Korea, and a BK21 Plus grant (grant number: 10Z20130012243) funded by the Ministry of Education, Korea. S-J Moon was supported by National Research Foundation of Korea (NRF) funded by the Korean Government (NRF-2015-Fostering Core Leaders of the Future Basic Science Program/Global Ph.D. Fellowship Program).
References
Hooper LV , Macpherson AJ (2010) Immune adaptations that maintain homeostasis with the intestinal microbiota. Nat Rev Immunol 10 159 - 169    DOI : 10.1038/nri2710
Chetroutre H (2004) Starting at the beginning; new perspectives on the biology of mucosal T cells. Annu Rev Immunol 22 217 - 246    DOI : 10.1146/annurev.immunol.22.012703.104522
Cheroutre H (2005) IELs: enforcing law and order in the court of the intestinal epithelium. Immunol Rev 206 114 - 131    DOI : 10.1111/j.0105-2896.2005.00284.x
Cheroutre H , Lambolez F , Mucida D (2011) The light and dark sides of intestinal intraepithelial lymphocytes. Nat Rev Immunol 11 445 - 456    DOI : 10.1038/nri3007
Mucida D , Husain MM , Muroi S (2013) Transcriptional reprogramming of mature CD4+ helper T cells generates distinct MHC class II-restricted cytotoxic T lymphocytes. Nat Immunol 14 281 - 289    DOI : 10.1038/ni.2523
Reis BS , Rogoz A , Costa-Pinto FA , Taniuchi I , Mucida D (2013) Mutual expression of the transcription factors Runx3 and ThPOK regulates intestinal CD4+ T cell immunity. Nat Immunol 14 271 - 280    DOI : 10.1038/ni.2518
Ferguson A , Parrott DM (1972) The effect of antigen deprivation on thymus-dependent and thymus-independent lymphocytes in the small intestine of the mouse. Clin Exp Immunol 12 477 - 488
Guy-Grand D , Cerf-Bensussan N , Malissen B (1991) Two gut intraepithelial CD8+ lymphocyte populations with different T cell receptors: a role for the gut epithelium in T cell differentiation. J Exp Med 173 471 - 481    DOI : 10.1084/jem.173.2.471
Badeira A , Itohara S , Bonneville M (1991) Extrathymic origin of intestinal intraepithelial lymphocytes bearing T-cell antigen receptor gamma delta. Proc Natl Acd Sci U S A 88 43 - 47    DOI : 10.1073/pnas.88.1.43
Rocha B , Vassalli P , Guy-Grand D (1994) Thymic and extrathymic origins of gut intraepithelial lymphocyte populations in mice. J Exp Med 180 681 - 686    DOI : 10.1084/jem.180.2.681
Kanamori Y , Ishimaru K , Nanno M (1996) Identification of novel lymphoid tissues in murine intestinal mucosa where clusters of c-kit+IL-7R+Thy1+lympho-hemopoietic progenitors develop. J Exp Med 184 1449 - 1459    DOI : 10.1084/jem.184.4.1449
Saito H , Kanamori Y , Takemori T (1998) Generation of intestinal T cells from progenitors residing in gut cryptopatches. Science 280 275 - 278    DOI : 10.1126/science.280.5361.275
De Geus B , Van den Enden M , Coolen C (1990) Phenotype of intraepithelial lymphocytes in euthymic and athymic mice: implications for differentiation of cells bearing a CD3-associatedγδT cell receptor. Eur J Immunol 20 291 - 298    DOI : 10.1002/eji.1830200210
Naito T , Shiohara T , Hibi T (2008) RORγt is dispensable for the development of intestinal mucosal T cells. Mucosal Immunol 1 198 - 207    DOI : 10.1038/mi.2008.4
Eberl G , Littman D (2004) Thymic origin of intestinal alpha beta T cells revealed by fate-mapping of RoR-gamma-t+cells. Science 305 248 - 251    DOI : 10.1126/science.1096472
Gangadharan D , Lambolez F , Attinger A (2006) Identification of pre- and postselection TCRαβ+intraepithelial lymphocyte precursors in the thymus. Immunity 25 631 - 641    DOI : 10.1016/j.immuni.2006.08.018
Lambolez F , Kronenberg M , Cheroutre H (2007) Thymic differentiation of TCRαβ+CD8αα IELs. Immunol Rev 215 178 - 188    DOI : 10.1111/j.1600-065X.2006.00488.x
Iwata M , Hirakiyama A , Eshima Y (2004) Retinoic acid imprints gut-homing specificity on T cells. Immunity 21 527 - 538    DOI : 10.1016/j.immuni.2004.08.011
Scott CL , Aumeunier AM , Mowat AM (2011) Intestinal CD103+dendritic cells: master regulators of tolerance? Trends Immunol 32 412 - 419    DOI : 10.1016/j.it.2011.06.003
Johansson-Lindbom B , Agace WW (2007) Generation of gut-homing T cells and their localization to the small intestinal mucosa. Immunol Rev 215 226 - 242    DOI : 10.1111/j.1600-065X.2006.00482.x
Hamann A , Andrew DP , Jablonski-Westrich D (1994) Role of α4-integrins in lymphocyte homing to mucosal tissues in vivo. J Immunol 152 3282 - 3293
Kantele A , Zivny J , Hakkinen M (1999) Differential homing commitments of antigen-specific T cells after oral or parenteral immunization in humans. J Immunol 162 5173 - 5177
Svensson M , Marsal J , Ericsson A (2002) CCL25 mediates the localization of recently activated CD8αβ+lymphocytes to the small-intestinal mucosa. J Clin Invest 110 1113 - 1121    DOI : 10.1172/JCI0215988
Andrew DP , Rott LS , Kilshaw PJ (1996) Distribution of α4β7 and αEβ7 integrins on thymocytes, intestinal epithelial lymphocytes and peripheral lymphocytes. Eur J Immunol 26 897 - 905    DOI : 10.1002/eji.1830260427
Ericsson A , Svensson M , Arya A , Agace WW (2004) CCL25/CCR9 promotes the induction and function of CD103 on intestinal intraepithelial lymphocytes. Eur J Immunol 34 2720 - 729    DOI : 10.1002/eji.200425125
El-Asady R , Yuan R , Liu K (2005) TGF-β-dependent CD103 expression by CD8+T cells promotes selective destruction of the host intestinal epithelium during graft-versus-host disease. J Exp Med 201 1647 - 1657    DOI : 10.1084/jem.20041044
Cepek KL , Shaw SK , Parker CM (1994) Adhesion between epithelial cells and T lymphocytes mediated by E-cadherin and the alpha E beta 7 integrin. Nature 372 190 - 193    DOI : 10.1038/372190a0
Poussier P , Ning T , Banerjee D (2002) A unique subset of self-specific intraintestinal T cells maintains gut integrity. J Exp Med 195 1491 - 1497    DOI : 10.1084/jem.20011793
Denning TL , Granger SW , Mucida D (2007) Mouse TCRαβ+CD8αα intraepithelial lymphocytes express genes that down-regulate their antigen reactivity and suppress immune responses. J Immunol 178 4230 - 4239    DOI : 10.4049/jimmunol.178.7.4230
Shires J , Theodoridis E , Hayday AC (2001) Biological insights into TCRγδ+ and TCRαβ+intraepithelial lymphocytes provided by serial analysis of gene expression (SAGE). Immunity 15 419 - 434    DOI : 10.1016/S1074-7613(01)00192-3
Yang H , Antony PA , Wildhaber BE , Teitelbaum DH (2004) Intestinal intraepithelial lymphocyte gamma delta-T cell derived keratinocyte growth factor modulates epithelial growth in the mouse. J Immunol 172 4151 - 4158    DOI : 10.4049/jimmunol.172.7.4151
Hansen SG , Vieville C , Whizin N (2009) Effector memory T cell responses are associated with protection of rhesus monkeys from mucosal simian immunodeficiency virus challenge. Nat Med 15 293 - 299    DOI : 10.1038/nm.1935
Hansen SG , Ford JC , Lewis MS (2011) Profound early control of highly pathogenic SIV by an effector memory T-cell vaccine. Nature 473 523 - 527    DOI : 10.1038/nature10003
Mueller S , Buhler-Jungo M , Mueller C (2011) Intestinal intraepithelial lymphocytes exert their potent protective cytotoxic activity during acute virus infection. J Immunol 164 1986 - 1994    DOI : 10.4049/jimmunol.164.4.1986
Dharakul T , Labbe M , Cohen J (1991) Immunization with baculovirus-expressed recombinant rotavirus proteins VP1, VP4, VP6, and VP7 induces CD8+T lymphocytes that mediate clearance of chronic rotavirus infection in SCID mice. J Virol 65 5928 - 5932
Sugimoto K , Ogawa A , Mizoguchi E (2008) IL-22 ameliorates intestinal inflammation in a mouse model of ulcerative colitis. J Clin Invest 118 534 - 544
McGeachy MJ , Bak-Jensen KS , Chen Y (2007) TGF-βand IL-6 drive the production of IL-17 and IL-10 by T cells and restrain TH-17 cell-mediated pathology. Nat Immunol 8 1390 - 1397    DOI : 10.1038/ni1539
Appay V , Zaunders JJ , Papagno L (2002) Characterization of CD4+CTLsex vivo. J Immunol 168 5954 - 5958    DOI : 10.4049/jimmunol.168.11.5954
Appay V (2004) The physiological role of cytotoxic CD4+T-cells: the holy grail? Clin Exp Immunol 138 10 - 13    DOI : 10.1111/j.1365-2249.2004.02605.x
Brown DM (2010) Cytolytic CD4 cells: Direct mediators in infectious disease and malignancy. Cell Immunol 262 89 - 95    DOI : 10.1016/j.cellimm.2010.02.008
Cheroutre H , Husain MM (2013) CD4 CTL: Living up to the challenge. Semin Immunol 25 273 - 281    DOI : 10.1016/j.smim.2013.10.022
Sasahara T , Tamauchi H , Ikewaki N (1994) Unique properties of a cytotoxic CD4+CD8+intraepithelial T-cell line established from the mouse intestinal epithelium. Microbiol Immunol 38 191 - 199    DOI : 10.1111/j.1348-0421.1994.tb01764.x
He X , He X , Dave VP (2005) The zinc finger transcription factor Th-POK regulates CD4 versus CD8 T-cell lineage commitment. Nature 433 826 - 833    DOI : 10.1038/nature03338
Sun G , Liu X , Mercado P (2005) The zinc finger protein cKrox directs CD4 lineage differentiation during intrathymic T cell positive selection. Nat Immunol 6 373 - 381    DOI : 10.1038/ni1183
Setoguchi R , Tachibana M , Naoe Y (2008) Repression of the transcription factor Th-POK by Runx complexes in cytotoxic T cell development. Science 319 822 - 825    DOI : 10.1126/science.1151844
He X , Park K , Kappes DJ (2010) The role of ThPOK in control of CD4/CD8 lineage commitment. Annu Rev Immunol 28 295 - 320    DOI : 10.1146/annurev.immunol.25.022106.141715
Collins A , Littman DR , Taniuchi I (2009) RUNX proteins in transcription factor networks that regulate T-cell lineage choice. Nat Rev Immunol 9 106 - 115    DOI : 10.1038/nri2489
Sato T , Ohno S , Hayashi T (2005) Dual functions of Runx proteins for reactivating CD8 and silencing CD4 at the commitment process into CD8 thymocytes. Immunity 22 317 - 328    DOI : 10.1016/j.immuni.2005.01.012
Wang L , Wildt KF , Castro E (2008) The zinc finger transcription factor Zbtb7b represses CD8-lineage gene expression in peripheral CD4+ T cells. Immunity 29 876 - 887    DOI : 10.1016/j.immuni.2008.09.019
Reis BS , Hoytema van Konijnenburg DP , Grivennikov SI (2014) Transcription Factor T-bet Regulates Intraepithelial Lymphocyte Functional Maturation. Immunity 41 244 - 256    DOI : 10.1016/j.immuni.2014.06.017
Quezada SA , Simpson TR , Peggs KS (2010) Tumorreactive CD4+T cells develop cytotoxic activity and eradicate large established melanoma after transfer into lymphopenic hosts. J Exp Med 207 637 - 650    DOI : 10.1084/jem.20091918
Xie Y , Akpinarli A , Maris C (2010) Naive tumor-specific CD4+ T cells differentiated in vivo eradicate established melanoma. J Exp Med 207 651 - 667    DOI : 10.1084/jem.20091921