Molecular and cellular biology of dendritic epidermal T cells

The function and specificity of gamma delta T cells remain enigmatic. Dendritic epidermal T cells (DETC) expressing an invariant gamma delta TCR represent a well established model system to investigate these key issues. Accumulating evidence supports the initial observation that recognition of a keratinocyte self-antigen by DETC proceeds without a requirement for MHC gene products. In addition, recent data have identified bioactive polypeptides expressed by DETC but not by most other T cells. For example, keratinocyte growth factor appears to be exclusively produced by activated DETC and intestinal intraepithelial gamma delta cells. These findings suggest that DETC may recognize antigen in novel ways as well as perform specialized functions complementary to those normally attributed to T cells.     

The role of IL-7 in thymic and extrathymic development of TCR gamma delta cells

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

T cell receptor alpha gene rearrangement and transcription in adult thymic gamma delta cells

T cells belong to two separate lineages based on surface expression of alpha beta or gamma delta T cell receptors (TCR). Since during thymus development TCR beta, gamma, and delta genes rearrange before alpha genes, and gamma delta cells appear earlier than alpha beta cells, it has been assumed that gamma delta cells are devoid of TCR alpha rearrangements. We show here that this is not the case, since mature adult, but not fetal, thymic gamma delta cells undergo VJ alpha rearrangements more frequently than immature alpha beta lineage thymic precursors. Sequence analysis shows VJ alpha rearrangements in gamma delta cells to be mostly (70%) nonproductive. Furthermore, VJ alpha rearrangements in gamma delta cells are transcribed normally and, as shown by analysis of TCR beta-/- mice, occur independently of productive VDJ beta rearrangements. These data are interpreted in the context of a model in which precursors of alpha beta and gamma delta cells differ in their ability to express a functional pre-TCR complex.     

A TCR alpha chain transgene induces maturation of CD4- CD8- alpha beta+ T cells from gamma delta T cell precursors

The proportion of CD4- CD8- double-negative (DN) alpha beta T cells is increased both in the thymus and in peripheral lymphoid organs of TCR alpha chain-transgenic mice. In this report we have characterized this T cell population to elucidate its relationship to alpha beta and gamma delta T cells. We show that the transgenic DN cells are phenotypically similar to gamma delta T cells but distinct from DN NK T cells. The precursors of DN cells have neither rearranged endogenous TCR alpha genes nor been negatively selected by the MIsa antigen, suggesting that they originate from a differentiation stage before the onset of TCR alpha chain rearrangements and CD4/CD8 gene expression. Neither in-frame V delta D delta J delta nor V gamma J gamma rearrangements are over-represented in this population. However, since peripheral gamma delta T cells with functional TCR beta gene rearrangements have been depleted in the transgenics, we propose that the transgenic DN population, at least partially, originates from the precursors of those cells. The present data lend support to the view that maturation signals to gamma delta lineage-committed precursors can be delivered via TCR alpha beta heterodimers.     

Isolation and characterization of a carbonic anhydrase homologue from the zebrafish (Danio rerio)

Lymphomas with T-cell phenotype represent a heterogeneous group of diseases differing in histopathology, tumour site, and cell origin. They include peripheral T-cell lymphomas (PTCLs) derived from alpha beta cells, but also some recently recognized entities such as gamma delta, hepatosplenic lymphomas and natural killer (NK) cell lymphomas. Only a few studies have investigated the possibility that at least some PTCLs could be derived from lymphocytes with cytotoxic potential. In order to investigate this possibility, 60 cases of PTCL, including 27 cases expressing the alpha beta T-cell receptor (TCR alpha beta), 15 TCR gamma delta cases and 18 cases expressing neither TCR (TCR silent), as well as 14 cases of NK-cell lymphomas, were studied by immunohistochemistry for the expression of TIA-1, perforin, and granzyme B proteins. Expression of TIA-1 is characteristic of cytotoxic cells regardless of their activation status, whereas expression of perforin and granzymes is highly increased in activated cytotoxic cells and correlates with the induction of cytolytic activity. All NK-cell lymphomas (11 sinonasal, three systemic cases) expressed TIA-1, perforin, and granzyme B in most tumour cells. All gamma delta PTCLs (15 cases) expressed TIA-1 protein in most tumour cells, with a different cytotoxic antigen profile in hepatosplenic gamma delta PTCL (TIA-1+, perforin-, granzyme B-) and in non-hepatosplenic gamma delta PTCLs (three nasal, one skin, one lung), the latter expressing the three cytotoxic proteins. Of the 45 cases of alpha beta and TCR silent PTCL, 15 (33 per cent) were considered to be derived from cytotoxic lymphocytes with expression of at least one cytotoxic protein (TIA-1, 15/45; perforin, 10/41; granzyme B, 14/38) in tumour cells. This cytotoxic protein expression appeared to be related to the site of localization, since 7/13 (54 per cent) extranodal and only 8/32 (25 per cent) nodal alpha beta and TCR silent PTCLs expressed TIA-1, and to histology, since this pattern was observed in a proportion of anaplastic (6/8, 75 per cent) and pleomorphic (8/17, 47 per cent) lymphomas, but not in AILD-type NHL (0/16). Taken together, our data suggest that NK-cell lymphomas and non-hepatosplenic gamma delta PTCLs represent tumours of activated cytotoxic NK cells and gamma delta T cells, respectively; that hepatosplenic gamma delta PTCLs represent tumours of non-activated cytotoxic gamma delta T cells; and that a small proportion of alpha beta and TCR silent PTCLs, mostly extranodal cases, or nodal anaplastic lymphomas, represent tumours of cytotoxic T cells.     

The enigmatic specificity of gamma delta T cells

In most scientific investigations, the study of mechanism follows the study of function. For example, alpha beta T cells were shown to be important mediators of immunity before the interaction between the T cell receptor (TCR) and peptide-MHC complexes was understood. However, sometimes the study of function follows from the study of mechanism. Research of gamma delta T cell receptors falls into this category. The gamma chain of the TCR was first cloned in 1984, which then led to the discovery of gamma delta T cells in 1985. Since then, research has focused on understanding ligands of the gamma delta TCR with the hope of better understanding the function of gamma delta T cells. An initial assumption was that gamma delta T cells, like alpha beta T cells, recognize peptides bound to MHC molecules; however, recent data indicate that gamma delta T cells are not biased towards MHC recognition in the same way as alpha beta T cells. Although there are intriguing new insights, the specificity and function of gamma delta T cells remains a mystery.     

Mouse dendritic epidermal T cells exhibit chemotactic migration toward PAM 212 keratinocyte culture supernatants

Dendritic epidermal T cells (DETCs) are Thy-1+, CD45+, CD3+, CD4-, CD8-, and T-cell receptor-V gamma 3/V delta 1+ leukocytes that reside normally in adult mouse skin. We have demonstrated previously that keratinocytes serve as adhesion substrates for DETCs, and that interleukin 7 (IL-7), which is produced by keratinocytes, serves as a growth factor for DETCs. The present study was conducted to address the mechanisms by which DETCs migrate into the epidermis, reasoning that keratinocytes may also be a source of chemotactic activity. Short-term DETC lines were 35S-labeled and tested for migration toward Pam 212 keratinocyte culture supernatants using a modified Boyden chamber method; cell movement from upper chambers toward test samples in lower chambers was traced by counting radioactivity. DETC displayed rapid (within 60 min) and marked (> 50%) migration toward keratinocyte supernatants. The majority of cells that had migrated into keratinocyte supernatants expressed the V gamma 3 T-cell receptor, thus verifying that the migrating cells were DETCs. Addition of keratinocyte supernatants to the upper chambers completely blocked migration, suggesting its chemotactic nature. By contrast, no DETC migration was observed toward 3T3 fibroblast supernatants. Chemotactic activities were 1) produced by Pam 212 cells even in the absence of serum; 2) greater than 12 kD in size; 3) heat and pH labile; 4) trypsin sensitive; and 5) precipitated by 60-100% ammonium sulfate. Several cytokines (e.g., IL-1 alpha and IL-8) failed to mediate DETC migration when added to the lower chambers. Likewise, the same cytokines, when added to the upper chambers, failed to inhibit DETC migration toward Pam 212 supernatants. These results support our hypothesis that keratinocytes facilitate the residence of DETC in epidermis by secreting unique chemotactic factors, by providing adhesion substrates, and by elaborating specific growth factors.     

Bone marrow-derived dendritic epidermal T cells express T cell receptor-alpha beta/CD3 and CD8. Evidence for their extrathymic maturation

The majority of murine Thy-1+ dendritic epidermal T cells (DETC) express virtually identical TCR encoded by V gamma 5 and V delta 1 genes and are derived from early fetal thymic precursors. However, not consistent with this notion is an early finding that DETC arise continuously from bone marrow (BM) precursors by a thymus-independent mechanism. To address this issue, donor-type DETC were characterized in lethally irradiated mice that were reconstituted by Thy-1-disparate BM cells with or without a thymus. The BM-derived DETC, unlike their normal TCR-gamma delta counterparts, were found to express the TCR-alpha beta/CD3 complex and CD8, and their migration to the epidermis dermis occurred independently of the thymus. The numbers of the BM-derived DETC increased with time and reached a plateau 6 mo after BM transfer, at which time the TCR-alpha beta/CD3 complex was expressed on a small fraction of the DETC in athymic BM chimeras. Although no further increase in the number was observed at later times, at 1 yr after transfer most of the BM-derived DETC came to express the TCR-alpha beta/CD3 complex in the absence of thymic influence. By contrast, most of BM-derived T cells in other lymphoid organs from athymic BM chimeras still failed to express the TCR-alpha beta/CD3 complex even at 1 yr after transfer. These results suggest that extrathymic differentiation of BM-derived DETC could occur with the epidermal microenvironment.     

Thymic medulla epithelial cells acquire specific markers by post-mitotic maturation

We recently showed that secretion of non-chimeric disulfide-linked human gamma delta TCR ('soluble' TCR, sTCR) comprising V gamma 9 and V delta 2 regions could be achieved by simply introducing translational termination codons upstream from the sequences encoding TCR transmembrane region. Here we extended these findings by demonstrating efficient secretion and heterodimerization of gamma delta sTCR comprising V gamma 8, V delta 1 and V delta 3 regions, obtained via the same strategy. After immunization against immunoaffinity-purified soluble TCR, several hundreds of TCR-specific monoclonal antibodies (mAb) were generated, which fell in at least seven groups. One set of mAb was directed against a V gamma 8-specific epitope. Strikingly, despite the high degree of sequence homology between V gamma 8 and other V gamma I domains, none of these mAb were crossreactive with other members of the V gamma I family. Three other sets of mAbs were shown to recognize delta chains comprising V delta 1, V delta 2 and V delta 3 regions respectively, regardless of their junctional sequence or of the gamma chain to which they were paired. Among the V delta 1-specific mAb, some specifically recognized V delta 1D delta J delta C delta chains while others reacted with both V delta 1 D delta J delta C delta and V delta 1J alpha C alpha chains, which suggested V domain conformational alterations induced by the C region. Moreover, reactivity of one V delta 1-specific mAb (#R6.11) was affected by a polymorphic residue located on the predicted CDR4 loop of the V delta region. Two delta chain-specific mAb (#178 and #515) showed a highly unusual reactivity, which was negatively affected by particular V delta and J delta sequences: (i) mAb #515 and #178 recognized all TCR delta chains except those comprising V delta 1 or V delta 2 regions, respectively and (ii) within TCR delta chains carrying 'permissive' V delta regions, none of those comprising the J delta 2 region were recognized by #515 and/or #178 mAbs, which suggested a highly specific conformation adopted by this particular J delta sequence. Apart from its usefulness in TCR structural studies, this novel set of mAb represents an important tool for the characterization and isolation of gamma delta T cells expressing particular combinations of V gamma/V delta regions and for analysis of V alpha/V delta usage by alpha beta T cells. Moreover, since our present data strongly suggest that gamma delta TCR are easier to obtain in a soluble form than alpha beta TCR, an efficient strategy for the generation of V alpha region-specific mAb might be to immunize with chimeric gamma delta sTCR comprising particular V alpha regions.     

Murine gamma/delta-expressing T cells affect alloengraftment via the recognition of nonclassical major histocompatibility complex class Ib antigens

T cells with antidonor specificities have been isolated from human recipients experiencing graft rejection after allogeneic bone marrow transplantation (BMT). Partial T-cell depletion of unrelated BM grafts with an anti- T-cell receptor (TCR) monoclonal antibody (MoAb) directed against the TCR alpha/beta heterodimer have shown that the incidence of graft-versus-host disease is low and that the incidence of durable engraftment is high. These studies suggest either that the number of residual TCR alpha/beta+ cells was sufficient to permit alloengraftment or that the preservation of cells other than TCR alpha/beta+ cells was beneficial for engraftment. With respect to the latter, one such candidate cell is the TCR gamma/delta+ T cell. Because no studies have specifically examined whether TCR gamma/delta+ cells might be capable of eliminating BM-derived hematopoietic cells, we established a new graft rejection model system in which transgenic (Tg) H-2d mice (termed G8), known to express gamma/delta heterodimers on high proportion of peripheral T cells, were used as BMT recipients. These Tg TCR gamma/delta+ cells respond vigorously to target cells that express the nonclassical major histocompatibility complex (MHC) class lb region gene products encoded in H-2T region of H-2T(b)+ strains. G8 Tg mice were used as recipients for C57BL/6 (B6: H-2(b); H-2T(b)) T-cell-depleted (TCD) donor BM. We show that G8 Tg (H-2(d), H-2T(d)) mice are potent mediators of B6 BM graft rejection and that the rejection process was inhibited by anti-TCR gamma/delta MoAbs. In contrast, BM from a B6 congenic strain that expresses the H-2T(a) allele, B6.A-Tl(a)/BoyEg, was readily accepted, suggesting that H-2T antigens on repopulating donor BM cells are the targets of host graft rejecting T cells that express the TCR gamma/delta heterodimer. PB chimerism studies were performed at > or = 1.5 months post-BMT using TCD BM from severe combined immunodeficient allogeneic donors, which is highly susceptible to rejection by the host. The addition of donor G8 TCR gamma/delta+ cells to TCD donor BM was shown to significantly increase alloengraftment in B6 recipients. These results show that (1) host TCR gamma/delta+ cells can reject repopulating donor cells, presumably by responding to nonclassical MHC class lb gene products expressed on BM-derived hematopoietic progenitor cells; and (2) donor TCR gamma/delta+ cells can facilitate the alloengraftment of rigorously TCD donor BM.     

Peripheral lymphoid development and function in TCR mutant mice

We describe the development and function of the peripheral lymphoid system of mutant mice rendered deficient in either alpha beta or gamma delta T cells via targeting of TCR genes in embryonic stem cells. In the spleen of alpha beta T cell-deficient mice, gamma delta T cells do not compensate in numbers for the lack of alpha beta T cells, but B cells do. alpha beta T cell-deficient mice are unable to mount an antibody response to ovalbumin and do not reject skin allografts. Natural killer cell function is not impaired in any of the mutant mice. TCR mutant mice will prove useful in dissecting differential functions of alpha beta and gamma delta T cells in vivo.     

The role of the clinical psychologist in gynecological cancer

To assess the gamma delta TCR T cells in the control of the timing of the mucosal response to enteric parasitic infections, we used C57BL mice, orally infected with 200 viable T. spiralis larvae. The small intestine, spleens and Peyer's patches (PP) were excised on 1, 4, 7, 14, 21 and 29 postinfection days (p.i.) for immunophenotyping and histological studies. Uninfected mice served as control. Characterization of isolated lymphocytes of C57BL control mice, confirmed that T cell immunophenotype differs in spleen, PP and i-IEL. Practically all i-IEL were CD3+ cells (83%). In addition, most of the i-IEL expressed Ly-2 (65%). Among the i-IEL, the level of gamma delta TCR+ cells was significantly higher (29%) than that found in spleen (3%) and PP (3%). The expression was high on CD3+ and Ly-2+ (26 and 21%, respectively) and low on L3T4+ i-IEL (< 1%). During T. spiralis infection alpha beta TCR+ CD3+, gamma delta TCR+ CD3+ and gamma delta TCR+ Ly-2+ i-IEL increased on day 4 and 7. However, infected mice displayed a reduction in i-IEL number from 14 to 29 p.i. day. At the same time the proportion of gamma delta TCR on spleen Ly-2+ and on PP CD3+ and Ly-2+ cells increased on 14 and 21 p.i. day. Adult worms were expelled from the gut by day 14. Thus, the kinetics of gamma delta TCR+ i-IEL, but not spleen and PP gamma delta TCR, corresponded to the kinetics of worm expulsion in C57BL mice. Most murine i-IEL of the gamma delta T cell lineage tend to be cytolytic when activated. We speculated that gamma delta T cells of i-IEL during the early stages of infection recognize and eliminate damaged epithelial cells generated by parasite antigens, simultaneously accelerating the worm expulsion.     

IL-7 overexpression in transgenic mouse keratinocytes causes a lymphoproliferative skin disease dominated by intermediate TCR cells: evidence for a hierarchy in IL-7 responsiveness among cutaneous T cells

IL-7 is a keratinocyte-derived lymphocyte growth factor critical for the development of gammadelta T cells including murine dendritic epidermal T cells (DETC). We derived transgenic mice that overexpress IL-7 in basal keratinocytes under the control of the human K14 promoter. These K14/IL-7 mice develop dermal and epidermal T cell infiltrates associated with alopecia. This lymphoproliferative skin disease is substantially more severe in mice homozygous for the K14/IL-7 transgene. Conventional DETC expressing a Vgamma5 Vdelta1 TCR are rare or absent among the cutaneous T cells in these mice. The T cells in the skin infiltrates of young K14/IL-7 mice are predominantly gammadelta T cells that express intermediate levels of TCR, are negative for E-cadherin, often lack expression of CD2, and include cells that coexpress NK1.1. T cells expressing intermediate levels of a TCR-alphabeta are also present in transgenic skin, and progressively increase in number as the mice age. Phenotypically similar intermediate gammadelta and alphabeta T cell subsets also constitute the major lymphocyte populations recovered from organ culture of normal mouse skin in the presence of IL-7, suggesting that the T cells that accumulate in the epidermis of K14/IL-7 mice are derived from precursors normally resident in skin. We conclude that intermediate TCR cells, some of which coexpress NK1.1, can be selectively expanded in skin under the influence of IL-7 produced locally. Our results also suggest that features of the epidermal microenvironment besides keratinocyte-derived IL-7 account for the normal predominance of Vgamma5 Vdelta1 DETC in mouse epidermis.     

Assembly of productive T cell receptor delta variable region genes exhibits allelic inclusion

The generation of a productive "in-frame" T cell receptor beta (TCR beta), immunoglobulin (Ig) heavy (H) or Ig light (L) chain variable region gene can result in the cessation of rearrangement of the alternate allele, a process referred to as allelic exclusion. This process ensures that most alphabeta T cells express a single TCR beta chain and most B cells express single IgH and IgL chains. Assembly of TCR alpha and TCR gamma chain variable region genes exhibit allelic inclusion and alphabeta and gammadelta T cells can express two TCR alpha or TCR gamma chains, respectively. However, it was not known whether assembly of TCR delta variable regions genes is regulated in the context of allelic exclusion. To address this issue, we have analyzed TCR delta rearrangements in a panel of mouse splenic gammadelta T cell hybridomas. We find that, similar to TCR alpha and gamma variable region genes, assembly of TCR delta variable region genes exhibits properties of allelic inclusion. These findings are discussed in the context of gammadelta T cell development and regulation of rearrangement of TCR delta genes.     

Gamma/delta T cells from tolerized alpha/beta-TCR-deficient mice antigen specifically inhibit contact sensitivity in vivo and IFN-gamma production in vitro

Contact sensitivity (CS) responses to reactive hapten antigens (Ag), such as picryl chloride, are classical examples of T-cell-mediated immune responses in vivo. There is also abundant evidence that T cells exposed in vivo to high intravenous doses of Ag can downregulate CS (high-dose Ag tolerance). To clarify cell types that effect CS and mediate its downregulation, we have studied CS in mice congenitally deficient in alpha/beta T cells (alpha-/- mice). We show that alpha-/- mice cannot mount CS, implicating alpha/beta T cells as critical CS effector cells. However, after high-dose Ag tolerization, these alpha-/- mice can downregulate alpha/beta CS effector cells adoptively transferred to them. The active cells in tolerized alpha-/- mice are gamma/delta TCR+ cells which downregulate CS effector alpha/beta T cells Ag-specifically upon adoptive cell transfer. Moreover, gamma/delta cells can Ag-specifically downregulate IFN-gamma production by CS effector cells in vitro. These findings establish that gamma/delta T cells are not CS effector cells but downregulate CS, in agreement with recent reports that gamma/delta T cells downregulate IgE responses.     

Diminution of the number of gamma delta T lymphocytes in hepatocellular carcinoma patients treated with transcatheter arterial embolization

gamma delta T lymphocytes, which are CD3+ lymphocytes that express gamma delta chains of the T-cell antigen receptor (TCR) on their surface, are functionally distinct from alpha beta T lymphocytes, which express alpha beta chains of the TCR. gamma delta T lymphocytes are thought to differentiate in mouse hepatic sinusoids, to play a role in antitumor action, and to act as natural killer cells. The purpose of this study was to examine whether gamma delta T lymphocytes in the peripheral blood are suppressed when hepatic sinusoids are damaged during transcatheter arterial embolization (TAE). The numbers of alpha beta T lymphocytes and gamma delta T lymphocytes in the peripheral blood were examined with monoclonal antibodies and flow cytometry before and after TAE in 32 patients (from 46 to 78 years of age) with liver cirrhosis and hepatocellular carcinoma. The number of alpha beta T lymphocytes before and after TAE was unchanged. However, the number of gamma delta T lymphocytes and the ratio of gamma delta T lymphocytes to CD3+ lymphocytes were significantly decreased for 3 weeks after TAE treatment. This decrease suggests that TAE suppresses the supply of gamma delta T lymphocytes to the peripheral blood. In addition, TAE may weaken a patient's antitumor immunity, because gamma delta T lymphocytes that have antitumor activity decrease after TAE.     

Early preferential stimulation of gamma delta T cells by TNF-alpha

Although recent findings indicate that gamma delta T cells influence both early innate and Ag-specific adaptive host responses, it has remained unclear what triggers gamma delta T cell reactivity. Investigating very early T cell activation in mouse and human models of bacterial infection, we measured CD69 expression as an indicator of early cellular activation. Both murine alpha beta and gamma delta T cells responded polyclonally to systemic bacterial infections, and to LPS. However, gamma delta T cells responded more strongly to the bacteria and to LPS. In vitro LPS-stimulated human T cells showed a similar differential response pattern. We identified TNF-alpha as mediator of the early differential T cell activation, and of differential proliferative responses. The stronger response of gamma delta T cells to TNF-alpha was correlated with higher inducible expression levels of TNF-Rp75. Among unstimulated splenocytes, more gamma delta T cells than alpha beta T cells expressed CD44 at high levels. The data suggest that TNF-Rp75 determines the differential T cell reactivity, and that most gamma delta T cells in the normal spleen are present in a presensitized state. As TNF-alpha stimulates activated T cells, it may early preferentially connect gamma delta T cell functions with those of cells that produce this cytokine, including activated innate effector cells and Ag-stimulated T lymphocytes.     

Lineage relationships of the fetal thymocyte subset that expresses the beta chain of the interleukin-2 receptor

The beta chain (p75) of the interleukin-2 (IL-2) receptor (IL-2R) is expressed on up to 5-7% of fetal thymocytes on day 16 of gestation, declining thereafter to a minute proportion of less than 1% around birth, and of 1-2% of adult thymocytes. A significant part of fetal IL-2R beta+ thymocytes are gamma delta cells. The precursor-progeny relationships of fetal IL-2R beta+ thymocytes to the alpha beta T cell lineage have not been previously studied, nor has their position within the developmental sequence been determined. Here we show that IL-2R beta is expressed on a subset of very immature cells, along with high amounts of Pgp1 and Fc gamma RII/III, partially preceding the expression of intracellular CD3 epsilon. IL-2-R beta disappears before expression of IL-2R alpha. IL-2R beta+ cells, purified by sorting on day 15 of gestation, efficiently reconstituted fetal thymic lobes depleted of lymphoid cells by treatment with desoxyguanosine. They developed into T cell receptor (TCR) alpha beta+, TCR gamma delta+, and CD4/CD8 double- and single-positive cells in similar proportions as did sorted IL-2R alpha+ day 15 fetal thymocytes. These data suggest that IL-2R beta expression marks a short period of very early thymocyte development, perhaps immediately after entry into the thymus.     

Separately expressed T cell receptor alpha and beta chain transgenes exert opposite effects on T cell differentiation and neoplastic transformation

Two aspects of T cell differentiation in T cell receptor (TCR)-transgenic mice, the generation of an unusual population of CD4-CD8-TCR+ thymocytes and the absence of gamma delta cells, have been the focus of extensive investigation. To examine the basis for these phenomena, we investigated the effects of separate expression of a transgenic TCR alpha chain and a transgenic TCR beta chain on thymocyte differentiation. Our data indicate that expression of a transgenic TCR alpha chain causes thymocytes to differentiate into a CD4-CD8-TCR+ lineage at an early developmental stage, depleting the number of thymocytes that differentiate into the alpha beta lineage. Surprisingly, expression of the TCR alpha chain transgene is also associated with the development of T cell lymphosarcoma. In contrast, expression of the transgenic TCR beta chain causes immature T cells to accelerate differentiation into the alpha beta lineage and thus inhibits the generation of gamma delta cells. Our observations provide a model for understanding T cell differentiation in TCR-transgenic mice.