Interleukin 7 receptor-deficient mice lack gammadelta T cells

A role for pref-1 and HES-1 in thymocyte development

T lymphocyte development requires a series of interactions between developing thymocytes and thymic epithelial (TE) cells. In this paper we show that TE cells in the developing thymus express Pref-1, a Delta-like cell-surface molecule. In fetal thymus organ cultures (FTOC), thymocyte cellularity was increased by the exogenous dimeric Pref-1 fusion protein, but was reduced by the soluble Pref-1 monomer or anti-Pref-1 Ab. Dimeric Pref-1 in FTOC also increased thymocyte expression of the HES-1 transcription factor. Thymocyte cellularity was increased in FTOC repopulated with immature thymocytes overexpressing HES-1, whereas FTOC from HES-1-deficient mice were hypocellular and unresponsive to the Pref-1 dimer. We detected no effects of either Pref-1 or HES-1 on developmental choice among thymocyte lineages. These results indicate that Pref-1 expressed by TE cells and HES-1 expressed by thymocytes are critically involved in supporting thymocyte cellularity.

PIM1 reconstitutes thymus cellularity in interleukin 7- and common gamma chain-mutant mice and permits thymocyte maturation in Rag- but not CD3gamma-deficient mice

The majority of lymphomas induced in Rag-deficient mice by Moloney murine leukemia virus (MoMuLV) infection express the CD4 and/or CD8 markers, indicating that proviral insertions cause activation of genes affecting the development from CD4(-)8(-) pro-T cells into CD4(+)8(+) pre-T cells. Similar to MoMuLV wild-type tumors, 50% of CD4(+)8(+) Rag-deficient tumors carry a provirus near the Pim1 protooncogene. To study the function of PIM proteins in T cell development in a more controlled setting, a Pim1 transgene was crossed into mice deficient in either cytokine or T cell receptor (TCR) signal transduction pathways. Pim1 reconstitutes thymic cellularity in interleukin (IL)-7- and common gamma chain-deficient mice. In Pim1-transgenic Rag-deficient mice but notably not in CD3gamma-deficient mice, we observed slow expansion of the CD4(+)8(+) thymic compartment to almost normal size. Based on these results, we propose that PIM1 functions as an efficient effector of the IL-7 pathway, thereby enabling Rag-deficient pro-T cells to bypass the pre-TCR-controlled checkpoint in T cell development.

Defective development of gamma/delta T cells in interleukin 7 receptor-deficient mice is due to impaired expression of T cell receptor gamma genes

Mice lacking the interleukin 7 receptor (IL-7R) generate alpha/beta T cells at a detectable but greatly reduced rate, but gamma/delta T cells are completely absent. The special role of IL-7R signaling in gamma/delta T cell development has remained unclear. IL-7Ralpha(-/-) mice exhibit a paucity of gamma gene rearrangements. This striking observation can be explained by a defect in T cell receptor (TCR)-gamma gene rearrangement, a defect in TCR-gamma gene transcription leading to death of gamma/delta lineage cells, and/or a requirement for IL-7R in commitment of cells to the gamma/delta lineage. To determine the role of IL-7R signaling in gamma/delta T cell development, we examined transcription of a prerearranged TCR-gamma transgene in IL-7Ralpha(-/-) mice, as well as the effects of IL-7 on transcription of endogenous, rearranged TCR-gamma genes in alpha/beta lineage cells. The results demonstrate that IL-7R-mediated signals are necessary for the normal expression of rearranged TCR-gamma genes. Equally significant, the results show that the poor expression of TCR-gamma genes in IL-7Ralpha(-/-) mice is responsible for the selective deficit in gamma/delta cells in these mice, since a high copy TCR-gamma transgene exhibited sufficient residual expression in IL-7Ralpha(-/-) mice to drive gamma/delta cell development. The results indicate that the absence of gamma/delta T cells in IL-7Ralpha(-/-) mice is due to insufficient TCR-gamma gene expression.

Induction of germline transcription in the TCRgamma locus by Stat5: implications for accessibility control by the IL-7 receptor

IL-7 receptor (IL-7R) plays critical roles in lymphocyte development by promoting survival and proliferation and by inducing V(D)J recombination in TCR and Ig loci. Here, we demonstrate that IL-7R-activated Stat5 binds to consensus motifs in the 5' regions of Jgamma segments and induces germline transcripts. We also show that a constitutively active form of Stat5 restores V-J recombination of TCRgamma genes and partially rescues T cell development from IL-7R(-/-) T cell precursors, especially in favor of gammadelta T cells. Therefore, this study reveals a potential role of Stat5 in T cell development and also implies that IL-7R may control the accessibility of the TCRgamma locus through Stat5-induced germline transcription.

Cytolytic and IFN-gamma-producing activities of gamma delta T cells in the mouse intestinal epithelium are T cell receptor-beta-chain dependent

We analyzed the cytolytic activity of intraepithelial T cells (IEL) isolated from the small intestines of 2- to 3-month-old mutant mice rendered deficient in different gene(s) in which the number of IEL expressing either T cell receptor (TCR)-alpha beta (alpha beta-IEL) or TCR-gamma delta (gamma delta-IEL) were absent or markedly diminished. When compared with wild-type littermates, cytolytic activity of gamma delta-IEL was sharply attenuated in TCR-beta mutant mice but remained unaltered in TCR-alpha mutant mice in which a minor population of dull TCR-beta+ (betadim)-IEL was also present. Cytolytic activity of gamma delta-IEL was maintained in mice doubly homozygous for beta2-microglobulin and transporter associated with antigen processing 1 gene mutations in which a conspicuous decrease was noted in absolute numbers of alpha beta-IEL. In contrast, both TCR-delta and IL-7 receptor-alpha gene mutations that lead to lack of gamma delta-IEL generation did not affect the development or cytolytic activity of the remaining alpha beta-IEL. The anti-CD3 and anti-TCR-gamma delta mAb-induced IFN-gamma production of gamma delta-IEL showed the same TCR-alpha and TCR-beta mutation-dependent variability. These results indicate that cytolytic and IFN-gamma-producing activities of gamma delta T cells in mouse intestinal epithelium are TCR-beta-chain-dependent.

Interleukin-15 production at the early stage after oral infection with Listeria monocytogenes in mice

We previously reported that exogenous interleukin-15 (IL-15) induces proliferation and activation of intestinal intraepithelial lymphocytes (i-IEL) in naive mice. To investigate the ability of endogenous IL-15 to stimulate i-IEL in vivo, we monitored i-IEL and intestinal epithelial cells (i-EC) in mice after an oral infection with Listeria monocytogenes. Although the populations of alphabeta and gammadelta i-IEL were not significantly changed after the oral infection, the expression level of interferon-gamma (IFN-gamma) was increased both at transcriptional and protein levels, and a conversely marked decrease in interleukin-4 (IL-4) was detected in the i-IEL on day 1 after infection as compared with before infection. The T helper 1 (Th1)-biased response of i-IEL coincided with a peak response of IL-15 production in the i-EC after oral infection. These results suggested that IL-15 produced from i-EC may be at least partly involved in the stimulation of i-IEL to produce IFN-gamma after oral infection with L. monocytogenes.

IL-7 receptor alpha+ CD3(-) cells in the embryonic intestine induces the organizing center of Peyer's patches

Peyer's patch (PP) organogenesis proceeds through three histologically distinct steps: formation of organizing centers expressing VCAM-1 and ICAM-1 in segregated regions of the intestine at 15.5 days post-coitus (d.p.c.) (step I), accumulation of blood cells expressing different sets of surface markers to this region at 16.5-17.0 d.p.c. (step II), and entry of CD3+ and B220+ lymphocytes just before birth (step III). PP formation of both Il7ra-/- and Lta-/- mice is impaired from step I, suggesting involvement of the two molecules at the same timing in PP organogenesis. Expression of lymphotoxin (LT) alpha and LTbeta in IL-7 receptor (IL-7R) alpha+ cells in the intestine indicates that defects of Il7ra-/- and Lta-/- mice are due to functional inability of IL-7Ralpha+ cells in the induction of PP anlage. Blocking of IL-7Ralpha function by a single injection of the antagonistic mAb in 15.5 d.p.c. embryos suppressed appearance of VCAM-1(+) spots and expression of LTalpha and LTbeta in the intestine, which eventually resulted in mice without PP but are otherwise normal. Intestinal IL-7Ralpha+ cells are lymphoid in morphology but CD3(-) and functional in both nu/nu and Rag2-/- mice. These results implicate IL-7Ralpha+ CD3(-) cells as the direct inducer of the organizing center of PP.

Interleukin-7: physiological roles and mechanisms of action

Interleukin-7 (IL-7), a product of stromal cells, provides critical signals to lymphoid cells at early stages in their development. Two types of cellular responses to IL-7 have been identified in lymphoid progenitors: (1) a trophic effect and (2) an effect supporting V(D)J recombination. The IL-7 receptor is comprised of two chains, IL-7R alpha and gamma(c). Following receptor crosslinking, rapid activation of several classes of kinases occurs, including members of the Janus and Src families and PI3-kinase. A number of transcription factors are subsequently activated including STATs, c-myc, NFAT and AP-1. However, it remains to be determined which, if any, previously identified pathway leads to the trophic or V(D)J endpoints. The trophic response to IL-7 involves protecting lymphoid progenitors from a death process that resembles apoptosis. This protection is partly mediated by IL-7 induction of Bcl-2, however other IL-7-induced events are probably also involved in the trophic response. The V(D)J response to IL-7 is partly mediated through increased production of Rag proteins (which cleave the target locus) and partly by increasing the accessibility of a target locus to cleavage through chromatin remodeling.

Antigen-driven clonal accumulation of peritoneal gammadelta T cells in vivo

How the clonality of gammadelta T cells changes in response to exogenous antigens is uncertain. Here we analyzed kinetics of Vgamma1.1 and Vgamma2 T cell clonality after intraperitoneal injection of purified protein derivatives (PPD) by the heterogeneity of the third complementarity determining region (CDR3) length in Vgamma1.1-Jgamma4-Cgamma4 and Vgamma2-Jgamma1-Cgamma1 junctions. The V-J junctions were analyzed in intrahepatic lymphocytes (IHL), spleen cells, and peritoneal exudate cells (PEC) by polyacrylamide gel electrophoresis. Gammadelta T cells expressing Vgamma1.1 and Vgamma2 genes were heterogeneous in normal mice. Accumulation of specific Vgamma1.1 T cell clones was transiently detected 7 days after the injection in PEC, but no accumulation was observed in IHL and spleen cells. The accumulated clones disappeared by 4 weeks. Transient accumulation of Vgamma2 T cell clones was also observed in PEC at the early phase after the injection. These results suggest that gammadelta T cells with specific TCR respond to PPD and temporary accumulate in the peritoneal cavity, but not in liver and spleen.

Multiple gamma c-dependent cytokines regulate T-cell development

Mutations in the common gamma chain (gamma c) of cytokine receptors account for human X-linked severe combined immunodeficiency disease. gamma c contributes to ligand binding and signaling as a component of five cytokine receptors: interleukin-2-receptor (IL-2R), IL-4R, IL-7R, IL-9R and IL-15R. Here, Thomas Malek and colleagues discuss the contribution of individual gamma c-dependent cytokines in both conventional and intraepithelial T-cell development.

Distinct Effects of Jak3 Signaling on αβ and γδ Thymocyte Development

Janus kinase 3 (Jak3) plays a central role in the transduction of signals mediated by the IL-2 family of cytokine receptors. Targeted deletion of the murine Jak3 gene results in severe reduction of αβ and complete elimination of γδ lineage thymocytes and NK cells. The developmental blockade appears to be imposed on early thymocyte differentiation and/or expansion. In this study, we show that bcl-2 expression and in vivo survival of immature thymocytes are greatly compromised in Jak3−/− mice. There is no gross deficiency in rearrangements of the TCRδ and certain γ loci in pre-T cells, and a functional γδ TCR transgene cannot rescue γδ lineage differentiation in Jak3−/− mice. In contrast, a TCRβ transgene is partially able to restore αβ thymocyte development. These data suggest that the signals mediated by Jak3 are critical for survival of all thymocyte precursors particularly during TCRβ-chain gene rearrangement, and are continuously required in the γδ lineage. The results also emphasize the fundamentally different requirements for differentiation of the αβ and γδ T cell lineages.

The common cytokine receptor gamma chain and the pre-T cell receptor provide independent but critically overlapping signals in early alpha/beta T cell development

Intracellular signals emanating from cytokine and antigen receptors are integrated during the process of intrathymic development. Still, the relative contributions of cytokine receptor signaling to pre-T cell receptor (TCR) and TCR-mediated differentiation remain undefined. Interleukin (IL)-7 interactions with its cognate receptor complex (IL-7Ralpha coupled to the common cytokine receptor gamma chain, gammac) play a dominant role in early thymopoiesis. However, alpha/beta T cell development in IL-7-, IL-7Ralpha-, and gammac-deficient mice is only partially compromised, suggesting that additional pathways can rescue alpha/beta T lineage cells in these mice. We have investigated the potential interdependence of gammac- and pre-TCR-dependent pathways during intrathymic alpha/beta T cell differentiation. We demonstrate that gammac-dependent cytokines do not appear to be required for normal pre-TCR function, and that the rate-limiting step in alpha/beta T cell development in gammac- mice does not involve TCR-beta chain rearrangements, but rather results from poor maintenance of early thymocytes. Moreover, mice double mutant for both gammac and pre-Talpha show vastly reduced thymic cellularity and a complete arrest of thymocyte differentiation at the CD44(+)CD25(+) cell stage. These observations demonstrate that the pre-TCR provides the gammac-independent signal which allows alpha/beta T cell development in gammac- mice. Thus, a series of overlapping signals derived from cytokine and T cell receptors guide the process of alpha/beta thymocyte development.

Role of Bcl-2 in αβ T Cell Development in Mice Deficient in the Common Cytokine Receptor γ-Chain: The Requirement for Bcl-2 Differs Depending on the TCR/MHC Affinity

Mice lacking the common cytokine receptor γ-chain (γc) exhibit severely compromised T cell development, with diminished Bcl-2 expression in mature (CD4+ or CD8+) thymocytes and peripheral T cells. Enforced expression of Bcl-2 in these mice partially rescued αβ T cell development but not γδ T cell development. Transgenic expression of the OVA-specific DO11.10 (DO10) TCR also could modestly increase thymocyte numbers, and T cells expressing the transgenic TCR (KJ1-26+ T cells) were found in the periphery. Interestingly, the presence of KJ1-26+ T cells was dependent on the MHC background and was seen in the moderate affinity H-2d/d background but not in the higher affinity H-2d/b background in γc-deficient mice. In contrast, KJ1-26+ T cells exist in the periphery in both the H-2d/d and H-2d/b backgrounds in DO10 transgenic γc wild-type mice. These results suggest that the importance of γc-dependent signals for T cell development differs depending on the affinity of TCR for MHC. Moreover, enforced expression of Bcl-2 had a much greater effect on the development of γc-deficient T cells expressing the DO10 TCR in the high affinity H-2d/b background than in the H-2d/d background, suggesting that γc-dependent Bcl-2 expression influences T cell development in a TCR/MHC-dependent manner.

Specificity of signaling by hematopoietic cytokine receptors: instructive versus permissive effects

The helical cytokines constitute a family of proteins with a common three-dimensional structure. They exert a wide variety of biological effects with a preference for the hematopoietic system. The effects of helical cytokines are mediated by cell surface receptors, which belong to the cytokine receptor superfamily and signal by activating cytoplasmic tyrosine kinases of the Janus kinase (Jak) family and other downstream signaling pathways. The relevance of each of these pathways for eliciting a specific cellular response remains to be determined. This review will focus on cytokine receptors which play a role in the regulation of hematopoiesis and summarize data the address the question how specificity of signaling is achieved.

Characterization of CD4+ CD8alphaalpha+ and CD4-CD8alphaalpha+ intestinal intraepithelial lymphocytes in rats

Intestinal intraepithelial lymphocytes (i-IEL) of aged rats comprise CD4+CD8alphaalpha+ and CD4-CD8alphaalpha+ T cells expressing TCR alphabeta. In the present study, we compared characteristics between CD4+CD8alphaalpha+ and CD4-CD8alphaalpha+ i-IEL, which were purified by a cell sorter from the i-IEL of 6-month-old Lewis rats. Most of the CD4+CD8alphaalpha+ i-IEL were of the CD44(hlgh) phenotype, while CD4-CD8alphabeta+ i-IEL were CD44(low). Vbeta usage in the CD4-CD8alphaalpha+ i-IEL was much diversified, while CD4+CD8alphaalpha+ i-IEL showed a skewed Vbeta repertoire. The CD4+CD8alphaalpha+ i-IEL but not the CD4-CD8alphaalpha+ i-IEL proliferated in response to syngeneic spleen cells, which was partially inhibited by addition of anti-MHC class I mAb. The CD4+CD8alphaalpha+ i-IEL produced IFN-gamma and IL-2 but no IL-4 or transforming growth factor (TGF)-beta in response to syngeneic spleen cells, while CD4-CD8alphaalpha+ i-IEL produced abundant levels of TGF-beta but no IL-2, IFN-gamma or IL-4. CD4+CD8alphaalpha+ i-IEL proliferated in response to exogenous IL-2 but not to IL-15, while CD4-CD8alphaalpha+ i-IEL could respond to IL-15 as well as IL-2. These results suggest that a significant fraction of CD4+CD8alphaalpha+ i-IEL belongs to Th1-type T cells capable of responding to self-MHC class I, while CD4-CD8alphaalpha+ i-IEL are a unique population with a diversified Vbeta repertoire that respond to IL-15 in rats.

The interaction of intestinal epithelial cells and intraepithelial lymphocytes in host defense

Intestinal intraepithelial lymphocytes (i-IEL) are located at the basolateral surfaces of intestinal epithelial cells (i-EC) and play important roles in the homeostasis of intestinal microenvironment. i-IEL comprise unique T cell populations including CD4-CD8alphaalpha+ T cells expressing T cell receptor (TCR)alphabeta or TCRgammadelta and CD4+ CD8alphaalpha+ T cells expressing TCR alphabeta. We show here that CD4+ CD8alphaalpha+ i-IEL belongs to Th1 type T cells capable of responding to self-MHC class I on i-EC and that a significant fraction of i-IEL expressed Fas ligand (Fas-L) and induced apoptosis in the i-EC via Fas-dependent pathway. i-IEL may recognize and eliminate the effete i-EC for homeostatic regulation of intestinal epithelia. The interaction of i-EC and i-IEL through E-cadherin/alphaEbeta7 integrin is important for homing and maintenance of i-IEL in intestine. Listeria monocytogenes are also known to interact with E-cadherin on i-EC and invade into the epithelial cells. Invasion of L. monocytogenes into i-EC activated NFkappa-B and subsequently up-regulated the expression of IL-15 gene, which has a NFkappa-B binding site at the promoter region. i-IEL, especially gammadelta T cells, were significantly activated to produce Th1 type cytokines at the early stage after oral infection with L. monocytogenes in mice and rats. The activation of i-IEL coincided with a peak response of IL-15 production by i-EC after infection. Taken together, mutual interaction of i-IEL and i-EC may be important not only for homeostatic regulation but also host defense against microbial infection in intestine.

Interleukin 7 receptor control of T cell receptor gamma gene rearrangement: role of receptor-associated chains and locus accessibility

VDJ recombination of T cell receptor and immunoglobulin loci occurs in immature lymphoid cells. Although the molecular mechanisms of DNA cleavage and ligation have become more clear, it is not understood what controls which target loci undergo rearrangement. In interleukin 7 receptor (IL-7R)alpha-/- murine thymocytes, it has been shown that rearrangement of the T cell receptor (TCR)-gamma locus is virtually abrogated, whereas other rearranging loci are less severely affected. By examining different strains of mice with targeted mutations, we now observe that the signaling pathway leading from IL-7Ralpha to rearrangement of the TCR-gamma locus requires the gammac receptor chain and the gammac-associated Janus kinase Jak3. Production of sterile transcripts from the TCR-gamma locus, a process that generally precedes rearrangement of a locus, was greatly repressed in IL-7Ralpha-/- thymocytes. The repressed transcription was not due to a lack in transcription factors since the three transcription factors known to regulate this locus were readily detected in IL-7Ralpha-/- thymocytes. Instead, the TCR-gamma locus was shown to be methylated in IL-7Ralpha-/- thymocytes. Treatment of IL-7Ralpha-/- precursor T cells with the specific histone deacetylase inhibitor trichostatin A released the block of TCR-gamma gene rearrangement. This data supports the model that IL-7R promotes TCR-gamma gene rearrangement by regulating accessibility of the locus via demethylation and histone acetylation of the locus.

alpha4 and alpha5 integrins costimulate the CD3-dependent proliferation of fetal thymocytes

Although integrin receptors have been shown to function as costimulatory molecules on mature thymocytes and T cells, it is not known whether these receptors can function as costimulatory molecules on immature thymocytes. Previous studies have shown that the expression of alpha4 and alpha5 integrins were significantly higher on immature, adult CD4(-)CD8(-) thymocytes than on either mature thymocytes or T cells, suggesting that these receptors are involved in early thymocyte development. In this study, we show that day 16 fetal thymocytes express levels of alpha4 and alpha5 equivalent to those of adult CD4(-)CD8(-) thymocytes. Immobilized fibronectin, a ligand for alpha4 and alpha5 integrins, was found to enhance the CD3-dependent proliferation of these fetal thymocytes. In the presence of IL-7, the magnitude of the proliferative response increased with time of incubation, resulting in a dramatic increase in the percentage of gammadelta thymocytes. The enhancement of proliferation by fibronectin was abrogated by soluble antibodies against alpha4 and alpha5, whereas immobilized mAb to alpha4 and alpha5 substituted for fibronectin in enhancing CD3-dependent proliferation, demonstrating that alpha4 and alpha5 integrins were responsible for the enhanced proliferation by fibronectin. Anti-alpha4 mAb enhanced proliferation of fetal thymocytes by 100%, whereas anti-alpha5 mAb and anti-CD28 mAb enhanced proliferation by 25%. Other costimulatory molecules, such as CD2, FcRgamma, and Thy-1, had no effect on the CD3-dependent proliferation of day 16 fetal thymocytes. This study demonstrates that alpha4 and alpha5 integrins are capable of costimulating fetal thymocytes.

Natural killer and T cells of innate and adaptive immunity: lymphoid compartments with different requirements for common gamma chain-dependent cytokines

A group of cytokines, including interleukin-2, -4, -7, -9 and -15, are related through the usage of a shared receptor subunit, the common cytokine receptor gamma chain, gamma c. gamma c-dependent cytokines critically affect the development and maintenance of the lymphoid system. This review will highlight our current knowledge on the gamma c-dependent cytokine network and on the non-redundant roles that these cytokines play in the development and homeostasis of T and natural killer cells involved in innate and adaptive immunity.

IL-7 promotes the survival and maturation but not differentiation of human post-thymic CD4+ T cells

This study examines the influence of IL-7 on post-thymic CD4+ T cells using cord blood as a model system. Survival of naive cord blood T cells in the presence of IL-7 alone was significantly prolonged by up-regulating bcl-2, thereby preventing apoptosis while maintaining maximal cell viability. Cultures without IL-7 showed high rates of apoptosis resulting in 50% cell death by day 5 of culture. Upon phorbol 12-myristate 13-acetate + ionomycin stimulation, accumulation of cytoplasmic IL-2 was similar to that observed in freshly isolated cells, but no IL-4- or IFN-gamma-positive cells were detected. IL-7 maintained the naive T cells in a quiescent state expressing the CD45RA antigen. A significant finding was the loss of CD38 antigen expression on the naive cord blood T cells to levels similar to that observed on adult naive T cells. In contrast to the reduced proliferative response of fresh cord blood T cells to anti-CD2 + CD28 stimulation, the proliferative response of IL-7-treated cells was similar to that of adult naive T cells. This study shows that as well as maintaining the naive T cell pool by enhancing cell survival and up-regulating bcl-2 expression, IL-7 also functions as a maturation factor for post-thymic naive T cells.

Intestinal intraepithelial T lymphocytes. Our T cell horizons are expanding

The alimentary tract is an essential structure for the ingesting of nutrients from the outside, and even most primitive animals have a straight tract that runs from the mouth to the anus. We come into contact with the outside world through our skin and mucous membranes. The surface area of the enteric mucous membrane, which absorbs nutrients, is enlarge through its ciliary structure, and the enteric cavity creates by far the largest external world that we come into contact with. For instance, the enteric mucosal surface of the human gastrointestinal tract covered by a single layer of epithelial cells corresponds to the size of one-and-a-half tennis courts, and the innumerable number of epithelial cells covering this mucous surface are entirely replaced by new epithelial cells in the space of just several days. Simultaneously, the fact that 60-70% of peripheral lymphocytes are congregating in the gastrointestinal tract supports the notion that the enteric mucous membrane represents an extremely dangerous locale, where numerous harmless/precarious external antigens come in through the wide array of food we injest on a daily basis, and the literally infinite amounts of normal intestinal flora intermingled from time to time with life-threatening microbes surge across. Surprisingly, approximately one out of the five cells in the intestinal epithelium are lymphocytes, most of which are ill-defined T cells having unusual, but distinctive characteristics and situated apparently so close to external antigens in the entire body. This article deals with the information that has been accumulated mainly in the past decade concerning the development, phenotypes, and possible function of these yet unacknowledged mucosal T cells that lurk in the anatomical front of the intestine.

The Role of IL-7 in Thymic and Extrathymic Development of TCRγδ Cells

IL-7-deficient (IL-7−/−) mice have reduced numbers of B and TCRαβ cells, but lack mature TCRγδ cells. Although most T cell development occurs in the thymus, some intestinal intraepithelial lymphocytes (IEL), including TCRγδ cells, can develop extrathymically. Epithelial cells in both thymus and intestine synthesize IL-7, suggesting that TCRγδ cell development could occur in either site. To evaluate the role of thymic IL-7 in development of TCRγδ cells, newborn TCRβ-deficient (TCRβ−/−) thymi were grafted to IL-7−/− mice. Donor- and host-derived TCRγδ cells were recovered from thymus grafts, spleen, and IEL. However, when IL-7−/− thymi were grafted to TCRβ−/− mice, no development of graft-derived TCRγδ cells occurred, indicating that extrathymic IL-7 did not support TCRγδ IEL generation from newborn thymic precursors. In contrast, TCRγδ IEL development occurred efficiently in adult, thymectomized, irradiated C57BL/6J mice reconstituted with IL-7−/− bone marrow. This demonstrated that extrathymic development of TCRγδ IEL required extrathymic IL-7 production. Thus, intrathymic IL-7 was required for development of thymic TCRγδ cells, while peripheral IL-7 was sufficient for development of extrathymic TCRγδ IEL.

The Trophic Action of IL-7 on Pro-T Cells: Inhibition of Apoptosis of Pro-T1, -T2, and -T3 Cells Correlates with Bcl-2 and Bax Levels and Is Independent of Fas and p53 Pathways

Signals from the IL-7R are essential for normal thymocyte development. We isolated thymocytes from early developmental stages and observed that suspensions of pro-T1, -T2, and -T3 cells rapidly died in culture. Addition of IL-7 promoted their survival, but did not induce cell division. Pro-T4 cells did not undergo rapid cell death, and their survival was therefore independent of IL-7. Death in the absence of IL-7 showed the hallmarks of apoptosis, including DNA fragmentation and annexin V binding; however, caspase inhibitors blocked DNA fragmentation, but did not block cell death. The trophic effect of IL-7 was partially inhibited by blocking protein synthesis. The p53 pathway was not involved in this death pathway, since pro-T cells from p53−/− mice also underwent cell death in the absence of IL-7. The Fas/Fas ligand pathway was not involved in cell death, since Fas-deficient pro-T cells died normally in the absence of IL-7, anti-Fas Abs did not protect cells from death in the absence of IL-7, and Fas expression was undetectable on cells at these stages. The IL-7 trophic affect correlated with increased intracellular levels of Bcl-2 and decreased levels of Bax, whereas no Bcl-XL, Bcl-w, or Bad was detectable. Thus, maintaining a favorable Bcl-2/Bax ratio may account for the trophic action of IL-7.

Distinct roles of the interleukin-7 receptor alpha chain in fetal and adult thymocyte development revealed by analysis of interleukin-7 receptor alpha-deficient mice

Mouse mutants lacking expression of the IL-7 receptor (IL-7R) alpha chain are defective in thymopoiesis. The adult thymus has multiple defects, including reduced cell numbers and proportions of the more mature thymocyte subsets, a complete absence of CD25+ cells and a reduced level of RAG1 and RAG2 expression. We show here that, in contrast to the profound developmental arrest observed in the adult thymus, fetal thymocytes from IL-7Ralpha-/- mice have normal proportions of all of the major thymocyte subpopulations, including CD25+ thymocytes and the most mature single-positive subsets. Moreover, normal levels of RAG1 and RAG2 were observed. Total thymocyte numbers, however, remained reduced. These data suggest that the IL-7Ralpha chain is a key regulator of both survival and proliferation during thymocyte development but that it is not essential for the production of T cells during fetal thymopoiesis.

Requirement of the IL-2 receptor beta chain for the development of Vgamma3 dendritic epidermal T cells

Vgamma3 TCR cells develop in the fetal thymus and migrate to the skin as dendritic epidermal T cells (DETC). Fetal Vgamma3 thymocytes differentiate from immature heat stable antigen (HSA)high cells to mature HSAlow cells and the latter subset predominantly expresses IL-2 receptor beta chain (IL-2Rbeta). In this study, the role of IL-2Rbeta in the development of Vgamma3 cells was determined in IL-2Rbeta-deficient mice. There was a moderate reduction of mature HSAlow Vgamma3 thymocytes in IL-2Rbeta-deficient mice. Small numbers of Vgamma3 DETC were detected in the fetal skin of IL-2Rbeta-deficient mice, but they were absent in newborn and adult mice. These results suggest that IL-2Rbeta may transduce the crucial signal for survival and/or expansion of Vgama3 cells in the fetal thymus and in the fetal skin. In normal mice, IL-15 but not IL-2 mRNA was expressed in the fetal epidermis and exogenous addition of low concentration of IL-15 to fetal skin organ culture induced proliferation of Vgamma3 DETC. The dependence of fetal Vgamma3 DETC on the expression of IL-2Rbeta and the presence of IL-15 mRNA in the fetal epidermis imply an essential role of IL-15 signaling through IL-2Rbeta in the selective localization of this gammadelta T cell subpopulation in the skin.

Immunologic abnormalities exhibited in IL-7 transgenic mice with dermatitis

Interleukin (IL)-7 transgenic mice, which we established previously, developed severe dermatitis characterized by massive infiltration of gammadelta T cells in the dermal lesion. To fully understand the pathology of this intriguing skin disease, we examined several immunologic features of dermis infiltrating lymphocytes from the lesional skin of IL-7 transgenic mice. We observed a moderate response to mitogens, a poor response to alloantigens, and the absence of cytotoxic activities to several tumor cell lines and skin derived cells regardless of the presence of IL-2 or IL-7. On the other hand, dermis infiltrating lymphocytes could proliferate with exogenous IL-2 and IL-7. Moreover, reverse transcriptase polymerase chain reaction and fluorescence activated cell sorter analysis revealed that dermis infiltrating lymphocytes expressed various cytokines including IL-4 and IL-7, and several activation markers for T cells (CD44, CD69, IL-2R alpha), in addition to IL-7R alpha. In the sera of the affected mice, hyper epsilon-globulinemia was observed. These findings suggested that dermis infiltrating lymphocytes proliferated in an activated state in the skin lesion in an autocrine and/or paracrine manner and produced Th2 type cytokines that might evoke immunologic abnormalities. This study and previous findings suggest that IL-7 transgenic mouse with dermatitis offer the potential of serving as a useful tool for investigating the immunologic role of cutaneous gammadelta T cells, especially their participation in IgE production in vivo.

Gene disruption in mice: models of development and disease

Gene targeting technology in mice by homologous recombination has become an important method to generate loss-of-function of genes in a predetermined locus. Although the inactivation is limited to irreversible alteration of chromosomal DNA and a surprising variety of genes have given unexpected and disappointing results, modification of the basic technology now provides additional choices for a more specific and variety of manipulations of the mouse genome. This includes conditional cell-type specific gene targeting, knockin technique and the induction of the specific balanced chromosomal translocations. In the past decade this technique not only generated a wealth of knowledge concerning the roles of growth factors, oncogenes, hormone receptors and Hox genes but also helped to produce animal models for several human genetic disorders. In the future it may provide more powerful and necessary tools to dissect the psychiatric disorders, understanding the complex central nervous system and to correct the inherited disorders.

In vivo roles of receptor tyrosine kinases and cytokine receptors in early thymocyte development

The early phases of T-cell development require both cell-cell interactions and soluble factors provided by stromal cells within the thymic microenvironment. Still, the precise nature of the signals delivered in vivo by cytokines (resulting in survival, proliferation or differentiation) remains unclear. Recent studies using mice deficient in cytokines or in their receptors have helped to identify essential signaling pathways required for the development of intrathymic precursors to mature alpha beta and gamma delta T cells. In addition, cytokine requirements for the development of natural killer cells were revealed in such mutants. The results obtained demonstrate that the development of all classes of lymphocytes (natural killer, gamma delta T cells and alpha beta T cells) is cytokine dependent, but the specific requirements differ for each lineage.

Interleukin-7 (IL-7) knockout mice. Implications for lymphopoiesis and organ-specific immunity

Interleukin-7 (IL-7) is produced by both immune and non-immune cells including stromal cell lines, B-cells, monocytes/macrophages, follicular dendritic cells, keratinocytes, and gut epithelial cells. The development of IL-7 knockout mice aided to elucidate the role of this multifaceted cytokine in lymphopoiesis. Additionally, IL-7 gene-deleted mice may represent an excellent model in order to define the functional role of locally secreted IL-7 in organ-specific immunity and in anti-microbial responses as well. For instance, analysis of IL-7 gene-deleted mice revealed reduced numbers of total T-lymphocytes with preservation of the CD4/CD8 ratio and increased ratio of alpha beta + T-cells compared to gamma delta + T-cells. Transition of pro-T-cells to pre-T-cells was impaired. Cell marker analysis of thymocytes in IL-7 -/- mice suggested that IL-7 may induce expression of as yet unidentified cytokine receptors, and that IL-7 may also be critically involved in T-cell differentiation. However, there are clear differences in the requirements of alpha beta or gamma delta T-cells for IL-7. In general, IL-7 appears to serve as the major growth and differentiation factor for gamma delta T-cells. IL-7 -/- mice are characterized by a block of maturation of V gamma 3low, CD24+ T-cells to V gamma 3high, CD24low T-cells. Thus, IL-7 does not only represent a 'maintenance factor', but rather a cytokine required for successful thymic and extrathymic development and maturation of gamma delta T-cells. gamma delta + intestinal intraepithelial lymphocytes (iIEL) are absent in IL-7 -/- animals. In contrast, alpha beta + iIEL can be detected in IL-7 gene-deleted animals, but not in gamma c, or in JAK-3 deficient mice suggesting that alternative cytokines may be involved in development of iIEL alpha beta + T-cells, but not necessarily for gamma delta T-cells. To this end, IL-7 has predominantly been studied in the context of B- and T-cell development. With the availability of IL-7 gene-deleted mice, the paracrine effects of IL-7, which may be secreted in vivo by non-immune cells including keratinocytes or gut epithelial cells, can now be critically examined.

Interleukin-7, a non-redundant potent cytokine whose over-expression massively perturbs B-lymphopoiesis

Interleukin-7, originally described as a factor controlling the survival of B-cell progenitors, has been shown by gene knock-out technology to be a non-redundant cytokine. Of all single cytokine knock-out mice, those in which the IL-7 gene has been ablated show a profound defect in lymphocyte development. Likewise, mice in which signals emanating from the corresponding receptor, whether it be by ablation of the unique alpha or common gamma chain of the receptor, or by interference with downstream signalling elements generated by this receptor complex, also show profound defects in lymphocyte differentiation. Transgenic mice over-expressing the IL-7 gene also show profound changes in lymphocyte development which, in some instances can result in the development of lymphoid tumours. Here, we review some of these aspects of IL-7 biology with particular reference to an IL-7 over-expressing transgenic mouse line in which the IL-7 transgene is controlled by the mouse MHC class II promoter.

Lymphoid Hyperplasia, Autoimmunity, and Compromised Intestinal Intraepithelial Lymphocyte Development in Colitis-Free Gnotobiotic IL-2-Deficient Mice

IL-2-deficient (IL-2−/−) mice develop disorders of the hemopoietic and immune systems characterized by anemia, lymphocytic hyperplasia, and colitis. The mechanisms responsible for these abnormalities remain unclear. To investigate the underlying basis of autoimmunity, the particular role of commensal gut flora in the initiation of colitis, and the role of IL-2 in the development of intestinal intraepithelial lymphocytes (iIEL), we evaluated IL-2−/− mice reared and maintained under gnotobiotic (germfree) conditions. By 8 wk of age, 80% (20 of 25) of germfree IL-2−/− mice show signs of disease, including anemia, disturbances in bone marrow hemopoietic cells, lymphocytic hyperplasia, and generalized autoimmunity, similar to those seen in specific pathogen-free (SPF) IL-2−/− mice. In striking contrast to SPF IL-2−/− mice, germfree IL-2−/− mice do not develop colitis. However, the numbers of γδ+ and TCRαβ+CD8αα+ iIELs are reduced, and in lethally irradiated SPF IL-2+/+ mice, reconstituted with IL-2−/− bone marrow TCRγδ+ iIELs fail to develop, consistent with an important role of IL-2/IL-2R signaling in the development of γδ iIELs. Consequently, our findings demonstrate that the colitis seen in SPF IL-2−/− mice depends upon the presence of intestinal bacterial flora and that environmental Ags are not responsible for the anemia and extraintestinal lymphoid hyperplasia that occur in IL-2−/− mice. Thus, germfree IL-2−/− mice represent a unique system in which the role of IL-2 deficiency in hemopoietic and immune system disorders can be investigated in dissociation from complications that may arise due to colitis.

Interleukin-7 treatment promotes the differentiation pathway of T-cell-receptor-alpha beta cells selectively to the CD8+ cell lineage

In this report we have studied the influence of interleukin-7 (IL-7) on thymocyte differentiation by evaluating the effects of IL-7 on the generation of T-cell receptor-alpha beta (TCR-alpha beta) and TCR-gamma delta thymocyte subpopulations in rat fetal thymus organ culture. IL-7 enhanced the differentiation pathway of TCR alpha beta thymocytes, first increasing the numbers of immature CD8+ cells, and later those of both CD4+ CD8+ and mature thymocytes. The kinetics of thymocyte migration out of thymic lobes was also accelerated, and the average number of mature TCR-alpha beta phi emigrants per day was increased in the presence of IL-7. Moreover, mature CD4- CD8+ thymocytes were preferentially generated after IL-7 administration. This TCR-alpha beta hi cell population was not actively dividing, indicating that IL-7-promoted thymocyte differentiation was selective to the CD8 cell lineage. Distribution of some TCR-V alpha and TCR-V beta segments among mature thymocytes was also modified in IL-7-treated thymic lobes. On the contrary, the maturation of TCR-gamma delta was not affected by IL-7 addition during the first days of culture, but their numbers sharply increased by day 6 of culture. These results were confirmed with IL-7-treated cultures for 24 hr, showing that IL-7 responsiveness was acquired by TCR-gamma delta cells late in thymus ontogeny. The present results thus indicate a key role for IL-7 in the maturation of TCR-alpha beta thymocytes and the expansion of thymic TCR-gamma delta cells.

Bacterial invasion induces interleukin-7 receptor expression in colonic epithelial cell line, T84

The intestinal epithelial layer forms the interface between the external and the internal environments of a host. Since the interleukin-7 (IL-7) and IL-7 receptor (IL-7R) signaling pathway has been shown to play an important role in the mucosal immune system, we studied the expression of IL-7R in T84, a colonic epithelial cell line, after cells were infected with several types of enteropathogenic bacteria, including Salmonella typhimurium, enteropathogenic Escherichia coli and enteroinvasive Escherichia coli. Bacterial invasion induced IL-7R expression in T84 assessed by a semi-quantitative reverse transcription-polymerase chain reaction technique and flow cytometry. The inhibition of bacterial invasion by cytochalasin D, a specific inhibitor of actin polymerization, led to a reduction in the expression of IL-7R. These data indicate that bacterial invasion into intestinal epithelial cells is likely to be an essential process in the induction of IL-7R. The communication between the epithelium and mucosal lymphocytes which is mediated via IL-7 and IL-7R may be involved in the modulation of the mucosal inflammation which occurs in bacterial infection.

Interleukin-15 preferentially promotes the growth of intestinal intraepithelial lymphocytes bearing gamma delta T cell receptor in mice

Several cytokines including stem cell factor (SCF) and interleukin (IL)-7 are known to be required for development of gamma delta T cell receptor (TCR) intestinal intraepithelial lymphocytes (i-IEL) in mice. We show here the effects of IL-15 on the proliferation and maintenance of murine gamma delta i-IEL in vitro. gamma delta i-IEL constitutively expressed a high level of IL-15 receptor alpha mRNA and proliferated in response to IL-15 more vigorously than alpha beta i-IEL. V gamma/delta repertoire analysis revealed that IL-15, like IL-2, induced polyclonal expansion of gamma delta i-IEL, whereas gamma delta i-IEL responding to IL-7 showed a V gamma/delta repertoire skewed towards V gamma 1/V delta 4, V delta 5. IL-15 efficiently prevented gamma delta i-IEL from apoptosis induced by growth factor deprivation. This rescue was accompanied by up-regulation of Bcl-2 expression. These results suggest that IL-15 plays important roles in proliferation and maintenance of gamma delta i-IEL.

The common cytokine receptor gamma chain controls survival of gamma/delta T cells

We have investigated the role of common gamma chain (gamma c)-signaling pathways for the development of T cell receptor for antigen (TCR)-gamma/delta T cells. TCR-gamma/delta-bearing cells were absent from the adult thymus, spleen, and skin of gamma c-deficient (gamma c-) mice, whereas small numbers of thymocytes expressing low levels of TCR-gamma/delta were detected during fetal life. Recent reports have suggested that signaling via interleukin (IL)-7 plays a major role in facilitating TCR-gamma/delta development through induction of V-J (variable-joining) rearrangements at the TCR-gamma locus. In contrast, we detected clearly TCR-gamma rearrangements in fetal thymi from gamma c- mice (which fail to signal in response to IL-7) and reduced TCR-gamma rearrangements in adult gamma c thymi. No gross defects in TCR-delta or TCR-beta rearrangements were observed in gamma c- mice of any age. Introduction of productively rearranged TCR V gamma 1 or TCR V gamma 1/V delta 6 transgenes onto mice bearing the gamma c mutation did not restore TCR-gamma/delta development to normal levels suggesting that gamma c-dependent pathways provide additional signals to developing gamma/delta T cells other than for the recombination process. Bcl-2 levels in transgenic thymocytes from gamma c- mice were dramatically reduced compared to gamma c+ transgenic littermates. We favor the concept that gamma c-dependent receptors are required for the maintenance of TCR-gamma/delta cells and contribute to the completion of TCR-gamma rearrangements primarily by promoting survival of cells committed to the TCR-gamma/delta lineage.

Ontogeny of the immune system: gamma/delta and alpha/beta T cells migrate from thymus to the periphery in alternating waves

The embryonic thymus is colonized by the influx of hemopoietic progenitors in waves. To characterize the T cell progeny of the initial colonization waves, we used intravenous adoptive transfer of bone marrow progenitors into congenic embryos. The experiments were performed in birds because intravenous cell infusions can be performed more efficiently in avian than in mammalian embryos. Progenitor cells, which entered the vascularized thymus via interlobular venules in the capsular region and capillaries located at the corticomedullary junction, homed to the outer cortex to begin thymocyte differentiation. The kinetics of differentiation and emigration of the T cell progeny were analyzed for the first three waves of progenitors. Each progenitor wave gave rise to gamma/delta T cells 3 d earlier than alpha/beta T cells. Although the flow of T cell migration from the thymus was uninterrupted, distinct colonization and differentiation kinetics defined three successive waves of gamma/delta and alpha/beta T cells that depart sequentially the thymus en route to the periphery. Each wave of precursors rearranged all three TCR Vgamma gene families, but displayed a variable repertoire. The data indicate a complex pattern of repertoire diversification by the progeny of founder thymocyte progenitors.

Bacterial pneumonia causes augmented expression of the secretory leukoprotease inhibitor gene in the murine lung

The cDNA of murine secretory leukoprotease inhibitor (SLPI) was cloned from a mouse lung cDNA library. The amino acid sequence deduced from the cDNA showed 58 and 51% homology with those of human and porcine SLPI, respectively. A two-domain structure with similar amino acid sequences, four intradomain disulfide bonds, and high proline content, which are characteristics common to human and porcine SLPI, was also found in the mouse protein. The amino acid residues for the signal sequence and active site are also conserved in mouse SLPI. RNase protection assay showed the expression of the SLPI gene in liver, intestine, spleen, and epididymis, suggesting the distribution of SLPI in tissues other than lung and seminal vesicles. In the lung infected with Streptococcus pneumoniae strain FP1284, 10 h after inoculation of bacteria the number of SLPI mRNA transcripts was three times higher than baseline. The increased level of expression remained constant for at least 48 h. This result clearly contrasts to that obtained for spleen, in which the SLPI mRNA transcript level was mostly unchanged during the course of pneumonia. These facts suggested the local regulation of the SLPI gene expression in vivo in response to inflammatory stimuli at the site of inflammation.

An interleukin-7 internet for intestinal intraepithelial T cell development: knockout of ligand or receptor reveal differences in the immunodeficient state

Both interleukin-7 (IL-7) and IL-7 receptor (R) gene knockout (IL-7-/- and IL-7R-/-) mice were employed in order to directly investigate the importance of the IL-7 and IL-7R signaling pathway for the development of intestinal intraepithelial lymphocytes (IEL). Loss of the IL-7R-specific gene resulted in complete deficiency of the gamma delta T cell lineage with lack of V gamma 4- and V gamma 7-specific messages in the epithelium of the gastrointestinal (GI) tract in comparison to control mice of the same genetic background (approximately 40%). Disruption of the IL-7-specific gene resulted in marked, but not complete depletion of gamma delta T cells (2-3%) in IEL. Furthermore, mRNA for both V gamma 4 and V gamma 7 genes were detected in the gamma delta IEL subset of IL-7-/- mice. The subtle differences between IL-7-/- and IL-7R-/- mice suggest that although IL-7 controls most of the expansion and/or development of gamma delta IEL, another ligand binding to the IL-7R also plays a discernable role. Furthermore, alpha beta IEL developed more slowly in IL-7R-/- mice when compared with ligand knockouts; however, the frequency of IEL T cells subsequently increased with age and normal levels of CD3+ T cells expressing the alpha beta TCR were detected by 2 and 3 months of age in IL-7-/- and IL-7R-/- mice, respectively. The direct comparison of IL-7-/- and IL-7R-/- mice clearly supports the hypothesis that both IL-7 and another IL-7R binding molecule can influence the development of gamma delta T cells in the intestinal epithelium.

Increased B-lymphopoiesis by interleukin 7 induces bone loss in mice with intact ovarian function: similarity to estrogen deficiency

Estrogen deficiency caused by ovariectomy (OVX) results in a marked bone loss due to stimulated bone resorption by osteoclasts. During our investigations of the pathogenesis of bone loss in estrogen deficiency, we found that OVX selectively stimulates B-lymphopoiesis which results in marked accumulation of B220-positive pre-B cells in mouse bone marrow. To examine the possible correlation between stimulated B-lymphopoiesis and bone loss, 8-week-old female mice were treated with interleukin (IL) 7, which stimulates B-lymphopoiesis in bone marrow. We also examined bone mass in IL-7 receptor-knockout mice that exhibit marked suppression of B-lymphopoiesis in the bone marrow. The increased B-lymphopoiesis induced by IL-7 administration resulted in marked bone loss by stimulation of osteoclastic bone resorption in mice with intact ovarian function. The changes in both B-lymphopoiesis and bone mass in IL-7-treated female mice were similar to those in age-matched OVX mice. In contrast, the trabecular bone volume of the femur was greatly increased in both female and male IL-7 receptor-knockout mice when compared with the respective wild-type and heterozygous littermates. These results show that the perturbation of B-lymphopoiesis in the bone marrow is closely linked to the change in bone mass. We propose here that the increased B-lymphopoiesis due to estrogen deficiency is involved in the mechanism of stimulated bone resorption.

The role of Jak3 in lymphoid development, activation, and signaling

Mutations in a number of lymphoid signaling molecules lead to immunodeficiencies in mice and humans. Among these, one very pleiotropic syndrome results from deficiencies in an array of cytokine signaling pathways utilizing a cytokine receptor common gamma chain, gammac, and the tyrosine kinase Jak3. Recent advances in our understanding of the role of gammac and Jak3 in lymphocyte development and function highlight the importance of cytokine receptor signaling pathways in regulating lymphoid homeostasis and responsiveness.

Intrathymic delta selection events in gammadelta cell development

The major pathway of gammadelta cell development is shown to be regulated by in-frame rearrangements at the T cell receptor (TCR) delta locus. Such "delta selection" occurs at or around the same point in thymocyte development as selection for in-frame rearrangements at the TCRbeta locus. However, there are at least two major differences with beta selection: first, delta selection commonly involves selection on the cognate TCR chain, gamma, suggesting that there is no "preTgamma" chain of major biological significance; second, most gammadelta-selected thymocytes differentiate rather than proliferate. Nonetheless, some delta selection events seemingly facilitate thymocyte expansion, similar to alphabeta T cell development. In these cases, TCRgamma selection is less obvious. Furthermore, the capacity of individual gamma chains to facilitate gammadelta selection is shown to vary with developmental age. The results further clarify early T cell development at the beta selection/delta selection stage and place clear constraints on models of cell fate determination.

Bcl-2 rescues T lymphopoiesis, but not B or NK cell development, in common gamma chain-deficient mice

The common cytokine receptor gamma chain (gamma(c)) is an indispensable subunit for the formation of lymphoid-related cytokine receptors, including IL-7 and IL-15 receptors, that mediate nonredundant or critical signals for the differentiation of T and B cells and natural killer (NK) cells, respectively. We introduced the bcl-2 transgene driven by E mu or H-2K promoters into gamma(c)-deficient mice that lack all three lymphoid subclasses. The forced expression of Bcl-2 restored all stages of T lymphopoiesis, but not B or NK cell development, indicating that a primary function of gamma(c)-mediated signals in the T lineage might be to maintain cell survival. Therefore, the development of T, B, and NK cells may be influenced by distinct intracytoplasmic signaling cascades that are activated by coupling of gamma(c)-related receptors.