Selectively impaired development of intestinal T cell receptor gamma delta+ cells and liver CD4+ NK1+ T cell receptor alpha beta+ cells in T cell factor-1-deficient mice

From inception to output, Tcf1 and Lef1 safeguard development of T cells and innate immune cells

Transcription factors have recurring roles during T cell development and activation. Tcf1 and Lef1 are known to be essential for early stages of thymocyte maturation. Recent research has revealed several novel aspects of their functionality. Tcf1 is induced at the very earliest step of specifying hematopoietic progenitors to the T cell lineage as a key target gene downstream of Notch activation. In addition to promoting maturation of T-lineage-committed thymocytes, Tcf1 functions as a tumor suppressor in developing thymocytes, and this is mediated, paradoxically, by restraining Lef1 expression. After positive selection, Tcf1 and Lef1 act together to direct CD4(+)CD8(+) double positive thymocytes to a CD4(+) T cell fate. Although not required for CD8(+) T cell differentiation, Tcf1 and Lef1 cooperate with Runx factors to achieve stable silencing of the Cd4 gene in CD8(+) T cells. Tcf1 is also found to have versatile roles in innate immune cells, which partly mirror its functions in mature T helper cells. Discrepancy in requirements of Tcf1/Lef1 and β-catenin in T cells has been a long-standing enigma. We will review other protein factors interacting with Tcf1 and Lef1 and discuss their regulatory roles independent of β-catenin.

Transcriptional regulation of innate and adaptive lymphocyte lineages

The lymphocyte family has expanded significantly in recent years to include not only the adaptive lymphocytes (T cells, B cells) and NK cells, but also several additional innate lymphoid cell (ILC) types. ILCs lack clonally distributed antigen receptors characteristic of adaptive lymphocytes and instead respond exclusively to signaling via germline-encoded receptors. ILCs resemble T cells more closely than any other leukocyte lineage at the transcriptome level and express many elements of the core T cell transcriptional program, including Notch, Gata3, Tcf7, and Bcl11b. We present our current understanding of the shared and distinct transcriptional regulatory mechanisms involved in the development of adaptive T lymphocytes and closely related ILCs. We discuss the possibility that a core set of transcriptional regulators common to ILCs and T cells establish enhancers that enable implementation of closely aligned effector pathways. Studies of the transcriptional regulation of lymphopoiesis will support the development of novel therapeutic approaches to correct early lymphoid developmental defects and aberrant lymphocyte function.

T cell factor 1 is required for group 2 innate lymphoid cell generation

Group 2 innate lymphoid cells (ILC2) are innate lymphocytes that confer protective type 2 immunity during helminth infection and are also involved in allergic airway inflammation. Here we report that ILC2 development required T cell factor 1 (TCF-1, the product of the Tcf7 gene), a transcription factor also implicated in T cell lineage specification. Tcf7(-/-) mice lack ILC2, and were unable to mount ILC2-mediated innate type 2 immune responses. Forced expression of TCF-1 in bone marrow progenitors partially bypassed the requirement for Notch signaling in the generation of ILC2 in vivo. TCF-1 acted through both GATA-3-dependent and GATA-3-independent pathways to promote the generation of ILC2. These results are reminiscent of the critical roles of TCF-1 in early T cell development. Hence, transcription factors that underlie early steps of T cell development are also implicated in the development of innate lymphoid cells.

SMAD regulatory networks construct a balanced immune system

A balanced immune response requires combating infectious assaults while striving to maintain quiescence towards the self. One of the central players in this process is the pleiotropic cytokine transforming growth factor-β (TGF-β), whose deficiency results in spontaneous systemic autoimmunity in mice. The dominant function of TGF-β is to regulate the peripheral immune homeostasis, particularly in the microbe-rich and antigen-rich environment of the gut. To maintain intestinal integrity, the epithelial cells, myeloid cells and lymphocytes that inhabit the gut secrete TGF-β, which acts in both paracrine and autocrine fashions to activate its signal transducers, the SMAD transcription factors. The SMAD pathway regulates the production of IgA by B cells, maintains the protective mucosal barrier and promotes the balanced differentiation of CD4(+) T cells into inflammatory T helper type 17 cells and suppressive FOXP3(+) T regulatory cells. While encounters with pathogenic microbes activate SMAD proteins to evoke a protective inflammatory immune response, SMAD activation and synergism with immunoregulatory factors such as the vitamin A metabolite retinoic acid enforce immunosuppression toward commensal microbes and innocuous food antigens. Such complementary context-dependent functions of TGF-β are achieved by the co-operation of SMAD proteins with distinct dominant transcription activators and accessory chromatin modifiers. This review highlights recent advances in unravelling the molecular basis for the multi-faceted functions of TGF-β in the gut that are dictacted by fluid orchestrations of SMADs and their myriad partners.

Decision checkpoints in the thymus

The development of T cells in the thymus involves several differentiation and proliferation events, during which hematopoietic precursors give rise to T cells ready to respond to antigen stimulation and undergo effector differentiation. This review addresses signaling and transcriptional checkpoints that control the intrathymic journey of T cell precursors. We focus on the divergence of alphabeta and gammadelta lineage cells and the elaboration of the alphabeta T cell repertoire, with special emphasis on the emergence of transcriptional programs that direct lineage decisions.

Cutting edge: Gammadelta intraepithelial lymphocytes of the small intestine are not biased toward thymic antigens

gammadelta Tau cells, together with alphabeta Tau cells, are abundantly present in the epithelial layer of the small intestine (IEL) and are essential for the host's first line of defense. Whether or not gammadelta IELs, like alphabeta IELs, are derived from thymocytes that encounter self-Ags in the thymus is unclear. In this study, we report that a natural population of gammadelta T cells that are specific for the nonclassical MHC class I molecules T10 and T22 are present in the IEL compartment of mice that do not express T10/T22. Furthermore, the small intestinal homing receptor CCR9 is preferentially expressed on gammadelta thymocytes that have yet to encounter a ligand, and gammadelta thymocytes with high affinity for self-ligand are CCR9(low). These observations suggest that the Ag-specific repertoire of gammadelta IELs is not biased toward thymic Ags. Instead, gammadelta IELs appear suited to respond to novel Ags revealed in pathological settings.

Molecular mechanisms that control mouse and human TCR-alphabeta and TCR-gammadelta T cell development

Following specification of hematopoietic precursor cells into the T cell lineage, several developmental options remain available to the immature thymocytes. The paradigm is that the outcome of the T cell receptor rearrangements and the corresponding T cell receptor signaling events will be predominant to determine the first of these choices: the alphabeta versus gammadelta T cell pathways. Here, we review the thymus-derived environmental signals, the transcriptional mediators, and other molecular mechanisms that are also involved in this decision in both the mouse and human. We discuss the differences in cellular events between the alphabeta and gammadelta developmental pathways and try to correlate these with a corresponding complexity of the molecular mechanisms that support them.

The thymus chapter in the life of gut-specific intra epithelial lymphocytes

The intestinal intraepithelial lymphocytes (IEL) represent multi-lineage T cell populations. In addition to a major gammadeltaTCR(+) T cell subset, many IEL express alphabetaTCRs and they can be separated into alphabeta sublineages. Some TCRalphabeta(+)IEL have characteristics in common with conventional TCRalphabeta(+)T cells whereas others share an unconventional phenotype with their TCRgammadelta(+) counterparts. Because the latter are enriched for autoreactive TCRs and can be generated in the absence of a thymus, it has long been postulated that some IEL subsets develop locally in the intestine. Several new data however, indicate that under physiological conditions, IEL require a thymic education that directs lineage commitment and functional differentiation. This review will discuss the contributions of the thymus in shaping the various intestinal IEL sublineages.

The Thymus as an Inductive Site for T Lymphopoiesis

Like all hematopoietic cells, T lymphocytes are derived from bone-marrow-resident stem cells. However, whereas most blood lineages are generated within the marrow, the majority of T cell development occurs in a specialized organ, the thymus. This distinction underscores the unique capacity of the thymic microenvironment to support T lineage restriction and differentiation. Although the identity of many of the contributing thymus-derived signals is well established and rooted in highly conserved pathways involving Notch, morphogenetic, and protein tyrosine kinase signals, the manner in which the ensuing cascades are integrated to orchestrate the underlying processes of T cell development remains under investigation. This review focuses on the current definition of the early stages of T cell lymphopoiesis, with an emphasis on the nature of thymus-derived signals delivered to T cell progenitors that support the commitment and differentiation of T cells toward the alphabeta and gammadelta T cell lineages.

Does the developmental status of Valpha14i NKT cells play a role in disease?

CD1d-restricted natural killer T (NKT) cells that express an invariant Valpha14 T-cell receptor (TCR) represent a subset of T cells implicated in the regulation of several immune responses, including autoimmunity, infectious diseases, and cancer. Their immunoregulatory functions are defined by their ability to rapidly and abundantly produce cytokines when activated. Unlike conventional T cells, Valpha14i NKT cells appear unique in their tendency to simultaneously produce both Th1 and Th2 cytokines, and whereas they enhance immunity in some disease models, they are reported to suppress immunity in others. This makes their effect on immune responses unpredictable. We reported recently that several important changes in gene expression occur in the course of Valpha14i NKT cell development. Immature and mature Valpha14i NKT cells differ in their expression of cytokines and chemokines, their cytotoxicity, and their expression of diverse chemokine receptors important for their migration. These results suggest that functionally distinct and developmentally linked subsets of Valpha14i NKT cells exist. Although mature NKT cells make up the majority of the peripheral NKT cells, a steady and sizable number of immature NKT cells migrate from the thymus into the periphery each day. These immature NKT cells, contrary to their name, are functional but are likely to behave quite differently from their mature counterparts. To what extent the developmental status of Valpha14i NKT cells plays a role in the outcome of any given immune response remains to be determined. Here we review the current knowledge of Valpha14i NKT cell development and propose that different developmental intermediates might be responsible for the various effects that have been observed in the many models where Valpha14i NKT cells have been implicated.

A retrospective on the requirements for gammadelta T-cell development

Since the discovery of gammadelta T cells two decades ago, considerable effort has been made to understand their developmental program, their antigen specificity, and their contribution to the immune response. In this review, we focus on what is known about gammadelta T-cell development and on the advances that have been made in determining which genes are required. In addition, we compare the genetic requirements for alphabeta and gammadelta T-cell development with the hope of gaining a better picture of the signaling pathways that govern the development of gammadelta lineage cells.

Integrated morphogen signal inputs in gammadelta versus alphabeta T-cell differentiation

Morphogens, a class of secreted proteins that regulate gene expression in a concentration-dependent manner, are responsible for directing nearly all lineage fate choices during embryogenesis. In the thymus, morphogen signal pathways consisting of WNT, Hedgehog, and the transforming growth factor-beta superfamily are active and have been implicated in various developmental processes including proliferation, survival, and differentiation of maturing thymocytes. Intriguingly, it has been inferred that some of these morphogen signal pathways differentially affect gammadelta and alphabeta T-cell development or maintenance, but their role in T-cell lineage commitment has not been directly probed. We have recently identified a modulator of morphogen signaling that significantly influences binary gammadelta versus alphabeta T-cell lineage diversification. In this review, we summarize functions of morphogens in the thymus and provide a highly speculative model of integrated morphogen signals, potentially directing the gammadelta versus alphabeta T-cell fate determination process.

Molecular events that regulate alphabeta versus gammadelta T cell lineage commitment: old suspects, new players and different game plans

The divergence of alphabeta and gammadelta T cells from a common precursor in the thymus is regulated by multiple cell-intrinsic and cell-extrinsic factors, most of which are not well defined. Recent studies have provided crucial data regarding the precise timing of lineage commitment and some clarification on the extent of the involvement of Notch and T-cell receptor signaling in this process. Combined with new insights into the differential regulation of molecular pathways active in alphabeta and gammadelta precursors, these data have led to the generation of a revised model of lineage commitment.

Developmental program of mouse Valpha14i NKT cells

Natural killer T (NKT) cells are a distinct lymphocyte lineage that regulates immune responses. During their development in the thymus, immature uncommitted double-positive CD4+CD8+ thymocytes that rearrange the semi-invariant T-cell receptor found on mature NKT cells are positively selected by the non-classical MHC class I molecule CD1d, which is expressed at the surface of cortical thymocytes. At this stage, the positively selected cells branch off from the conventional T-cell developmental program and start to acquire activated and/or memory markers and several 'bona fide' NK cell attributes. Recent work has started to reveal the specific developmental requirements for this divergent pathway of differentiation. These include several signal transduction molecules, transcription factors and cytokines, including T-bet, members of the NF-kappaB family, Fyn and IL-15.

Positive and negative selection of T cell repertoires during differentiation in allogeneic bone marrow chimeras

T cells acquire immune functions during expansion and differentiation in the thymus. Mature T cells respond to peptide antigens (Ag) derived from foreign proteins when these peptide Ag are presented on the self major histocompatibility complex (MHC) molecules but not on allo-MHC. This is termed self-MHC restriction. On the other hand, T cells do not induce aggressive responses to self Ag (self-tolerance). Self-MHC restriction and self-tolerance are not genetically determined but acquired a posteriori by positive and negative selection in the thymus in harmony with the functional maturation. Allogeneic bone marrow (BM) chimera systems have been a useful strategy to elucidate mechanisms underlying positive and negative selection. In this communication, the contribution of BM chimera systems to the investigation of the world of T-ology is discussed.

Biology of natural killer cells in cancer and infection

NK cells are the important cells of the immune system derived from stem cells in the marrow. Their physiology is tightly regulated to control proliferation, cytotoxicity and cytokine production. In cancer, NK cells may be abnormal due to the cancer itself or possibly related to its therapy. The finding of class I recognizing inhibitory receptors may play a role in stem cell transplant rejection, immune surveillance and cancer immunotherapy. NK cells should no longer be thought of as direct cytotoxic killers alone, as they clearly play a critical role in cytokine production which may be important to control cancer and infection. Understanding NK cell function and homing may lead to novel therapeutic strategies for the treatment of human disease.

A critical role for the T cell receptor alpha-chain connecting peptide domain in positive selection of CD1-independent NKT cells

Natural killer T (NKT) cells are a subset of mature alpha beta TCR(+) cells that co-express NK lineage markers. Whereas most NKT cells express a canonical Valpha14/Vbeta8.2 TCR and are selected by CD1d, a minority of NKT cells express a diverse TCR repertoire and develop independently of CD1d. Little is known about the selection requirements of CD1d-independent NKT cells. We show here that NKT cells develop in RAG-deficient mice expressing an MHC class II-restricted transgenic TCR (Valpha2/Vbeta8.1) but only under conditions that lead to negative selection of conventional T cells. Moreover development of NKT cells in these mice is absolutely dependent upon an intact TCR alpha-chain connecting peptide domain, which is required for positive selection of conventional T cells via recruitment of the ERK signaling pathway. Collectively our data demonstrate that NKT cells can develop as a result of high avidity TCR/MHC class II interactions and suggest that common signaling pathways are involved in the positive selection of CD1d-independent NKT cells and conventional T cells.

T-cell factor-1 expression during human natural killer cell development and in circulating CD56(+) bright natural killer cells

Transcription factors are essential to govern differentiation along the lymphoid lineage from uncommitted hematopoietic stem cells. Although many of these transcription factors have putative roles based on murine knockout experiments, their function in human lymphoid development is less known and was studied further. Transcription factor expression in fresh and cultured adult human bone marrow and umbilical cord blood progenitors was evaluated. We found that fresh CD34(+)Lin(-) cells that are human leukocyte antigen (HLA)-DR(-) or CD38(-) constitutively express GATA-3 but not T-cell factor-1 (TCF-1) or Id-3. Culture with the murine fetal liver cell line AFT024 and defined cytokines was capable of inducing TCF-1 mRNA. However, no T-cell receptor gene rearrangement was identified in cultured progeny. Id-3, a basic helix loop helix factor with dominant negative function for T-cell differentiation transcription factors, also was upregulated and may explain unsuccessful T-cell maturation. To better understand the developmental link between natural killer (NK) cells derived from progenitors, we studied NK cell subsets circulating in blood. CD56(+bright), but not CD56(+dim), NK cells constitutively express TCF-1 by reverse transcriptase polymerase chain reaction and Western blot analysis. The TCF-1 isoform found in CD56(+bright) cells, which express lectin but not immunoglobulin class I recognizing inhibitory receptors, was identical to that induced in NK cell differentiation culture and was distinctly different from isoforms in T cells. These results suggest that TCF-1 does not target human killer immunoglobulin receptor genes, TCF-1 is uniquely expressed in circulating CD56(+bright) NK cells, and specific TCF-1 isoforms may play an important role in regulating NK differentiation from a common NK/T-cell progenitor.

Defective development of NK1.1+ T-cell antigen receptor alphabeta+ cells in zeta-associated protein 70 null mice with an accumulation of NK1.1+ CD3- NK-like cells in the thymus

Development of natural killer 1.1+ (NK1.1+) CD3+ (NK1.1+ T) cells was analyzed in zeta-associated protein 70 (ZAP-70) null ((-/-)) mice. Both NK1.1+ TCRalphabeta+ and NK1.1+ TCRgammadelta+ cell populations were absent in the thymus and spleen. By contrast, the number of NK1.1+ CD3- cells was increased in these tissues. The NK1.1+ CD3- thymocytes in ZAP-70(-/-) mice had surface phenotypes in common with NK or NK1.1+ T cells. However, some of them were discordant either with NK cells or with NK1.1+ T cells. The NK1.1+ CD3- cells produced interferon-gamma upon stimulation with NK1.1 cross-linking in the presence of interleukin-2 and exhibited a substantial cytotoxicity against YAC-1 cells. Moreover, the generation of NK1.1+ T cells with invariant Valpha14Jalpha281 chains was induced from the NK1.1+ CD3- thymocytes following stimulation with phorbol myristate acetate and ionomycin in a neonatal thymic organ culture. An introduction of TCRalpha and beta transgenes to the ZAP-70(-/-) mice resulted in generation of an NK1.1+ TCRalphabeta(dim) population, whereas no substantial CD4+ CD8- or CD4- CD8+ population that expressed the introduced TCRalphabeta was generated in the mainstream T lineage. These findings demonstrate that ZAP-70 kinase is indispensable for the development of NK1.1+ T cells and that the unique NK1.1+ CD3- thymocytes in ZAP-70(-/-) mice contain immediate precursors of NK1.1+ T cells.

NKT cells are phenotypically and functionally diverse

NK1.1(+)alpha betaTCR(+) (NKT) cells have several important roles including tumor rejection and prevention of autoimmune disease. Although both CD4(+) and CD4(-)CD8(-) double-negative (DN) subsets of NKT cells have been identified, they are usually described as one population. Here, we show that NKT cells are phenotypically, functionally and developmentally heterogeneous, and that three distinct subsets (CD4(+), DN and CD8(+)) are differentially distributed in a tissue-specific fashion. CD8(+) NKT cells are present in all tissues but the thymus, and are highly enriched for CD8alpha(+)beta(-) cells. These subsets differ in their expression of a range of cell surface molecules (Vbeta8, DX5, CD69, CD45RB, Ly6C) and in their ability to produce IL-4 and IFN-gamma, with splenic NKT cell subsets producing lower levels than thymic NKT cells. Developmentally, most CD4(+) and DN NKT cells are thymus dependent, in contrast to CD8(+) NKT cells, and are also present amongst recent thymic emigrants in spleen and liver. TCR Jalpha281-deficient mice show a dramatic deficiency in thymic NKT cells, whereas a significant NKT cell population (enriched for the DN and CD8(+) subsets) is still present in the periphery. Taken together, this study reveals a far greater level of complexity within the NKT cell population than previously recognized.

Clonal acquisition of the Ly49A NK cell receptor is dependent on the trans-acting factor TCF-1

Families of clonally expressed major histocompatibility complex (MHC) class I-specific receptors provide specificity to and regulate the function of natural killer (NK) cells. One of these receptors, mouse Ly49A, is expressed by 20% of NK cells and inhibits the killing of H-2D(d) but not D(b)-expressing target cells. Here, we show that the trans-acting factor TCF-1 binds to two sites in the Ly49A promoter and regulates its activity. Moreover, we find that TCF-1 determines the size of the Ly49A NK cell subset in vivo in a dosage-dependent manner. We propose that clonal Ly49A acquisition during NK cell development is regulated by TCF-1.

A Novel Role for HEB Downstream or Parallel to the Pre-TCR Signaling Pathway During αβ Thymopoiesis

TCR gene rearrangement and expression are central to the development of clonal T lymphocytes. The pre-TCR complex provides the first signal instructing differentiation and proliferation events during the transition from CD4−CD8−TCR− double negative (DN) stage to CD4+CD8+ double positive (DP) stage. How the pre-TCR signal leads to downstream gene expression is not known. HeLa E-box binding protein (HEB), a basic helix-loop-helix transcription factor, is abundantly detected in thymocytes and is thought to regulate E-box sites present in many T cell-specific gene enhancers, including TCR-α, TCR-β, and CD4. Targeted disruption of HEB results in a 5- to 10-fold reduction in thymic cellularity that can be accounted for by a developmental block at the DN to DP stage transition. Specifically, a dramatic increase in the CD4low/−CD8+CD5lowHSA+TCRlow/− immature single positive population and a concomitant decrease in the subsequent DP population are observed. Adoptive transfer test shows that this defect is cell-autonomous and restricted to the αβ T cell lineage. Introduction of an αβ TCR transgene into the HEBko/ko background is not sufficient to rescue the developmental delay. In vivo CD3 cross-linking analysis of thymocytes indicates that TCR signaling pathway in the HEBko/ko mice appears intact. These findings suggest an essential function of HEB in early T cell development, downstream or parallel to the pre-TCR signaling pathway.

Selection of phenotypically distinct NK1.1+ T cells upon antigen expression in the thymus or in the liver

Unlike the main TCR alphabeta T cell lineage in which deletion occurs at the CD4+ CD8+ double-positive (DP) stage upon TCR engagement by antigen in the thymus, some T cells appear to require such engagement for their selection, either in the thymus or extrathymically. We used a transgenic TCR (tgTCR) model which, as we previously showed, led to selection upon expression of the corresponding antigen H-2Kb (Kb) in the thymus, of tgTCR/CD3(lo) CD4- CD8- double-negative (DN) thymocytes that expressed the NK1.1 marker (NK T cells) (Curnow, S. J., et al., Immunity 1995. 3: 427). We now report that antigen expression on medullary epithelial cells of the thymus failed to select the NK T cells, whereas its expression on thymocytes did, although tgTCR DP thymocyte development was affected under both conditions. Antigen expression on hepatocytes (Alb-Kb mice) did not perturb tgTCR DP thymocyte development. No enrichment in tgTCR NK T cells was detected in the periphery, except for the liver of the Alb-Kb/tgTCR mice. When reconstitution of thymectomized and irradiated H-2k hosts expressing or not Kb was performed with bone marrow from tgTCR H-2k mice, an enrichment in tgTCR+ NK T cells was found in the liver, but not in the spleen, of the hosts which expressed Kb, either selectively on hepatocytes or ubiquitously. Surprisingly, the majority of the hepatic tgTCR+ NK T cells also expressed the CD8 alpha/beta heterodimer. These results indicate that thymus-independent NK T cells with unique phenotypic characteristics can be selected upon antigen encounter in the liver.

Critical Involvement of Tcf-1 in Expansion of Thymocytes

T cell maturation in Tcf-1−/− mice deteriorates progressively and halts completely around 6 mo of age. During fetal development thymocyte subpopulations seem normal, although total cell numbers are lower. By 4 to 6 wk of age, obvious blockades in the differentiation of CD4−8− thymocytes are observed at two distinct stages (CD44+25+ and CD44−25−), both of which are normally characterized by extensive proliferation. This lack of thymocyte expansion and/or differentiation was also observed when Tcf-1−/− progenitor cells from the aorta-gonad-mesonephros region (embryonic day 11.5), fetal liver (embryonic day 12.5/14.5), and fetal bone marrow (embryonic day 18.5) were allowed to differentiate in normal thymic lobes (fetal thymic organ cultures) or were injected intrathymically into normal recipients. Despite these apparent defects in thymocyte differentiation and expansion, adult Tcf-1−/− mice are immunocompetent, as they generate virus neutralizing Abs at normal titers. Furthermore, their peripheral T cells have an activated phenotype (increased CD44 and decreased CD62L expression) and proliferate normally in response to Ag or mitogen, suggesting that these cells may have arisen from the early wave of development during embryogenesis and are either long lived or have subsequently been maintained by peripheral expansion. As Tcf-1 is a critical component in the Wnt/β-catenin signaling pathway, these data suggest that Wnt-like factors play a role in the expansion of double-negative thymocytes.

Transcriptional control during T-cell development

During the past few years, the essential role of distinct transcription factors in specifying cell-fate decisions in a stepwise fashion during T-cell differentiation has been revealed. One striking feature is that a single factor can act at several sites throughout T-cell development, possibly through interactions with different partners. The challenge is now to understand how these interactions can account for the co-ordination of complex extracellular signals and gene expression programs, such as those involved in T-cell receptor gene recombination and expression.

Redundant regulation of T cell differentiation and TCRalpha gene expression by the transcription factors LEF-1 and TCF-1

Lymphoid enhancer factor 1 (LEF-1) and T cell factor 1 (TCF-1) are closely related transcription factors that are both expressed during murine T cell differentiation and that regulate the T cell receptor alpha (TCRalpha) enhancer in transfection assays. Targeted gene disruption of either the Tcf1 or Lef1 gene in mice did not affect TCRalpha gene expression and resulted in an incomplete defect or no defect in thymocyte differentiation. Here, we examine a potential redundancy of these transcription factors by analyzing double-mutant mice. In fetal thymic organ cultures from Lef1-/- Tcf1-/- mice, alpha/beta T cell differentiation is completely arrested at the immature CD8+ single-positive (CD8+ ISP) stage and is markedly impaired at an earlier stage. In addition, we find that sorted CD8+ ISP cells from Lef1-/- Tcf1-/- mice express TCRbeta but show a severely reduced level of TCRalpha gene transcription. Together, these data show that LEF-1 and TCF-1 are redundant in the regulation of T cell differentiation and gene expression.

Complexity of the mouse gut T cell immune system: identification of two distinct natural killer T cell intraepithelial lineages

Gut thymo-dependent (CD8 alpha + beta + or CD4+) or -independent (CD8 alpha + beta -) intraepithelial lymphocytes (IEL) mediate cytotoxicity following T cell receptor (TCR)-CD3 signaling, but only TCR gamma delta + and alpha beta + thymo-independent IEL show cytotoxicity of natural killer (NK) and antibody-dependent cell-mediated cytotoxicity types. Moreover, TCR alpha beta + and gamma delta + thymo-independent IEL express NK receptors, and may therefore be referred to as NK-TIEL. NK-TIEL cytotoxicity is mediated through perforin, Fas, or both pathways. In contrast to that of other NK cells, this cytotoxicity is not negatively regulated by signals delivered through the recognition of major histocompatibility complex class I molecules. Thus, gut IEL include T cell subsets with unique specificities and functions, ontogenically distinct from other T cell lineages, which may increase the antigenic repertoire diversity of the immune system participating in the defense of the epithelial barrier.