Maturation or differentiation of human thymocyte precursors in vitro?

Leptospira interrogans activation of human peripheral blood mononuclear cells: preferential expansion of TCR gamma delta+ T cells vs TCR alpha beta+ T cells

Innate and adaptive immune responses induced by leptospirosis have not been well characterized. In this study we show that in vitro exposure of naive human PBMC to Leptospira interrogans results in cell proliferation and the production of IFN-gamma, IL-12, and TNF-alpha. Cell proliferation was highest when using high numbers of Leptospira. Optimal cell proliferation occurred at 6-8 days, and the majority of cells contained in these cultures were gamma/delta T cells. These cultures showed a 10- to 50-fold expansion of gamma/delta T cells compared with the initial cellular input. Additionally, these cultures contained elevated numbers of NK cells. In contrast, exposure of PBMC to low numbers of Leptospira failed to induce gammadelta T cell or NK cell expansion, but induced significant alphabeta T cell expansion. Vgamma9/Vdelta2 were expressed on all gamma/delta T cells expanded by exposure of PBMC to Leptorspira: Leptospira stimulation of purified TCRgammadelta(+) T cells, obtained from 8-day cultures of Leptospira-stimulated PBMC, induced high levels of IFN-gamma production, but no cell proliferation, suggesting that such stimulation of gammadelta T cells did not depend on specialized accessory cells or Ag processing. Finally, in patients with acute leptospirosis, there was a significant (4- to 5-fold) increase in the number of peripheral blood TCRgammadelta(+) T cells. These results indicate that Leptospira can activate gammadelta T cells and alphabeta T cells and will guide further investigations into the roles of these T cell populations in host defense and/or the pathology of leptospirosis.

T cell differentiation/maturation of CD34+ stem cells from HIV-seropositive hemophiliacs in cultured thymic epithelial fragments

The clinical manifestations of AIDS are predominantly due to the cellular and humoral immune dysfunction caused by HIV infection, and thymic dysplasia caused by HIV infection probably contributes to the T cell lymphopenia. In the present study, T cell differentiation and/or maturation was assessed when enriched CD34+ stem cells (SCs or SC) purified from bone marrow of HIV-seropositive hemophiliacs were cocultured with allogeneic cultured thymic epithelial fragments (CTEFs). When HIV-seropositive hemophiliacs' enriched CD34+ SC were cocultured with allogeneic CTEFs, acquisition of the T cell phenotypic markers CD7, CD2, CD3, CD4, CD8 and T cell receptor for antigen (TCR) alpha beta was observed from cells harvested from the culture media peaking at approximately 28 days. Origin of the differentiated and matured T cells from the CD34+ SC was confirmed by labeling the SC with 5-(and -6)-(((4-chloromethyl)benzoyl)amino)tetra-methyl-rhodamine (CMTMR), a fluorescent cytoplasmic dye, and detecting fluorescence in the differentiated and matured T cell by flow cytometry. In one experiment, CMTMR labeling was omitted and double positive CD4+CD8+ and triple positive CD3+CD4+CD8+ thymocytes were identified. These studies confirmed that thymocyte differentiation/maturation from SC had occurred. In addition, T cells obtained from the CD34+ SC and CTEF cocultures proliferated to phytohemagglutinin stimulation maximally with stem cell donor antigen-presenting cells (APCs) and also proliferated to pooled B cells in a mixed lymphocyte culture (MLC). Furthermore, the T cells produced were tolerant to thymus donor B cell HLA antigens (p < 0.025); though there was slight MLC reactivity to autologous stem cell donor B cell HLA compared to thymic B cells (p < 0.025). These T cells demonstrated positive self-alloreactivity to stem cell HLA antigens in four of nine persons, though decreased compared to pool B cell alloantigens. Furthermore, in three experiments, responsiveness to stem cell donor B cells subsequently disappeared upon further duration of CD34+ SC-CTEF coculture. These studies suggested that CD34+ SC gave rise to accessory cells populating the thymus that contributed to HLA restriction. To further evaluate this hypothesis, two different donors of CD34+ SC were cultured simultaneously with thymic epithelial fragments and MLC reactivity was then examined toward APC of the stem cell donors. In these experiments, T cells responded to stimulation with HLA antigens of the pool B cells and did not respond to thymus donor B cells. In six of eight experiments, the chimeric SC-CTEF T cells did not respond to stimulation with B cells of either stem cell donor. These studies suggest that HLA restriction and tolerance were induced by cells of the stem cell donor as well as the thymic epithelial cell HLA antigens. In summary, these studies demonstrated that HIV-infected hemophiliac bone marrow-derived nonadherent CD34+ SC were capable of differentiating and/or maturing into T cells when cocultured in a normal allogeneic thymic environment. Furthermore, the T cells derived from derived CD34+ SC were capable of differentiating into T cells when cocultured in a normal allogeneic thymic environment, proliferated maximally with APCs from the stem cell donor and were tolerant of thymic HLA class II antigens, and to a lesser degree to stem cell donor B cell HLA antigens.

Differential effects of interleukin-2 and interleukin-7 on the induction of CD4 and CD8 expression by double-negative human thymocytes

Expression of cell surface CD4 (in the absence of CD3/T cell receptor) characterizes an early stage of intrathymic T cell development. Here, we investigated the appearance of CD4 and CD8 expression on highly purified CD4-8- double-negative human thymocytes in response to interleukin (IL)-7 and IL-2. While IL-7 preferentially promoted the appearance of CD4 single-positive and CD4+8+ double-positive thymocytes, IL-2 primarily induced CD8 single-positive thymocytes. A significant fraction of CD4 single-positive cells generated from double-negative thymocytes via IL-7 lacked cell surface CD3 expression. In contrast, the majority of CD8 single-positive cells generated from double-negative thymocytes via IL-2 coexpressed CD3. We conclude that IL-7 and IL-2 exert differential effects on the differentiation of early human T cell progenitors.

CD34-positive early stages of human T-cell differentiation

Thymus, the main organ for T lymphopoiesis, requires a permanent influx of progenitors from bone marrow (BM) or fetal liver. An essential question relating to early T-cell development is the identification of the progenitor population which actually homes to the thymus. Recent findings have shown that human multipotent progenitor/stem cells expressing CD34 have the capacity to differentiate into T cells when introduced into a thymic environment. More mature CD34+ bone marrow cells coexpressing CD7 and having a poor myeloid differentiation capacity can also efficiently differentiate into T cells in vitro. These lymphoid committed precursors might be the true thymic repopulating cells. In the thymus, cells with a similar CD34+7+ phenotype include the most primitive thymocyte precursors. CD34+ thymocytes have no myeloid differentiation potential, but may include precursors for natural killer (NK) cells. Interleukin-7 (IL7) is a potent in vitro growth factor for CD34+ thymocytes. Whereas current data do not support a crucial role for IL2, patients with IL2 receptor gamma chain (IL2R gamma) deficiency lack T- and NK cells. The recent demonstration that IL2R gamma is part of the receptor for IL7 strongly suggests that this cytokine plays an essential role in in vivo T lymphocyte and NK development.

Differential modulation of human multinegative (CD3-4-8-) thymocyte proliferation by monoclonal antibodies to CD45RA or to CD45

Human multinegative (CD3-4-8-19-; MN) thymocytes proliferate optimally in response to anti-CD2 plus anti-CD28 mAb plus PMA or IL-7. The role of CD45 was assessed by the addition of mAb to a CD45 common determinant, or to CD45RA. MN thymocytes are unresponsive to anti-CD2 mAb. Co-stimulation with anti-CD45RA generated 1.6-5.7-fold enhancement of a proliferative response, with maximal enhancement by cross-linkage of CD45RA molecules. The response to anti-CD2/28 mAb was reproducibly enhanced only by immobilized anti-CD45RA. Cross-linking of CD45RA and CD28 through the use of heteroconjugates of mAb did not enhance the co-stimulation by CD45RA. The most marked enhancement by anti-CD45RA occurred in suboptimal activation conditions. In contrast, the response to anti-CD2 or anti-CD2/28 was inhibited by mAb to CD45 common determinants (anti-CD45) in the presence or absence of PMA or IL-7, with the most profound inhibition (6-8-fold) detected in optimal proliferative conditions. Cross-linking of CD45 and CD28 through heteroconjugates of mAb was required as soluble anti-CD45 or immobilized anti-CD45 were unable to mediate inhibition. This inhibitory effect of (anti-CD45 x 28) was specific to MN thymocytes as no inhibition was detectable when peripheral blood T cells were treated with anti-CD2/28 and the same heteroconjugate. The differential effects of anti-CD45 and anti-CD45RA may reflect either CD45 heterogeneity on MN thymocytes, or the physical modulation of a single CD45 molecules by interactions at different epitopes, and the avidity of the relevant CD45 mAb for thymocyte CD45 isoforms may play a role.(ABSTRACT TRUNCATED AT 250 WORDS)

Precursors of CD3+CD4+CD8+ cells in the human thymus are defined by expression of CD34. Delineation of early events in human thymic development

Studies of the most immature T cell progenitors in the human thymus have been hampered by the lack of markers and assays that define these cells. In this report we used a novel human fetal thymic organ culture system to determine the potential of T cell precursors isolated from human postnatal thymus, to differentiate into CD3+ thymocytes, and to investigate early stages of human T cell development. It was found that thymocytes that lack the markers CD3, CD4, and CD8 (triple negative [TN]) can differentiate in an allogeneic organotypic thymic culture. The capacity of TN thymocytes to differentiate was exclusively confined to the CD34+ population. CD34- TN thymocytes failed to differentiate in this system. In contrast, cloned lines of CD3- thymocytes could only be established from CD34- TN thymocytes. Five subsets of CD3- thymocytes were found with the following phenotype: CD1-TN, CD1+TN, CD1+CD4+CD8-, CD1+CD4+CD8 alpha+ beta-, and CD1+CD4+CD8 alpha beta+. These subpopulations expressed decreasing levels of CD34. The CD1-CD3- population expressed the highest levels of CD34 supporting the notion that this population is the most immature T cell precursor in the thymus, whereas the CD1+CD4+CD8 alpha+ beta+ which did not express CD34 seems to be the most mature of these CD3- populations. This notion is supported by the observations that CD34+ cells isolated from fetal liver, which differentiated into T cells in a FTOC, developed into CD3+ cells via CD1- and CD4+CD8- intermediates. Based on these data, we present a model of early stages in human intrathymic development.

Analysis of thymic stromal cell subpopulations grown in vitro on extracellular matrix in defined medium. IV. Cytokines secreted by human thymic epithelial cells in culture and their activities on murine thymocytes and bone marrow cells

In previous reports we described our approach to the cultivation of murine and human thymic epithelial cells in primary cultures, using defined, serum-free growth factor-supplemented medium and extracellular matrix-coated culture plates. The cells in these cultures displayed high metabolic activity and their supernatant was highly active on thymocytes. In the study reported here we analysed cytokine activities in the supernatant of human thymic epithelial cell cultures (HTES), by using the respective cytokine-dependent cell lines and by neutralization with specific monoclonal antibodies. Three cytokine activities were detected--interleukin-6 (IL-6), granulocyte colony-stimulating factor (G-CSF) and macrophage (M)-CSF. Other cytokine activities tested for [IL-1, IL-2, IL-7, interferon (IFN) and tumour necrosis factor (TNF)] were negative. The effect of HTES on concanavalin A (Con A)-induced proliferation of murine thymocytes could be completely abolished by anti-IL-6 antibodies, but not by antibodies to CSF, whereas enhancement of bone marrow cell proliferation by HTES was partially inhibited by either anti-G-CSF or anti-M-CSF antibodies and completely inhibited by both antibodies, but not at all by anti-IL-6. We can thus distinguish between thymocyte-related cytokines (IL-6) and bone marrow (myeloid/monocyte) related ones (G-CSF, M-CSF) in HTES.

In vivo models of human lymphopoiesis and autoimmunity in severe combined immune deficient mice

The discovery of the SCID mouse mutation has been an important advance for the study of human lymphopoiesis and autoimmunity. Further work in the SCID mouse models described in this review should yield important new information related to transplantation of human hematopoietic stem cells across HLA barriers, characterization of hematopoietic development in vivo, and identification of pathogenic human T cell clones in organ-specific autoimmune diseases. If pluripotent hematopoietic stem cells and pathogenic autoimmune T cells can be defined using SCID mouse recipients, this would pave the way for development of novel strategies for bone marrow transplantation and for interventional immunotherapy of autoimmune diseases targeted at the T cell receptor (99).

Opportunist mycobacteria express ligands that stimulate production of human V gamma 9V delta 2 T lymphocytes

Human gamma delta T cells are known to respond at high frequencies to pathogenic mycobacteria. Here we show that opportunistic strains of mycobacteria share with pathogenic mycobacteria the ability to trigger at high frequencies human V gamma 9V delta 2 T-cell-receptor-positive T lymphocytes. Stimulating ligands were present in part in a low-molecular-weight fraction of lysates from opportunistic mycobacteria, as has been found for pathogenic strains. These results support the view that postnatal exposure to ever-present opportunistic mycobacteria may be a driving force for the numerical expansion of the V gamma 9V delta 2 T-cell subset in adolescence.

The development of functionally responsive T cells

The work reviewed in this article separates T cell development into four phases. First is an expansion phase prior to TCR rearrangement, which appears to be correlated with programming of at least some response genes for inducibility. This phase can occur to some extent outside of the thymus. However, the profound T cell deficit of nude mice indicates that the thymus is by far the most potent site for inducing the expansion per se, even if other sites can induce some response acquisition. Second is a controlled phase of TCR gene rearrangement. The details of the regulatory mechanism that selects particular loci for rearrangement are still not known. It seems that the rearrangement of the TCR gamma loci in the gamma delta lineage may not always take place at a developmental stage strictly equivalent to the rearrangement of TCR beta in the alpha beta lineage, and it is not clear just how early the two lineages diverge. In the TCR alpha beta lineage, however, the final gene rearrangement events are accompanied by rapid proliferation and an interruption in cellular response gene inducibility. The loss of conventional responsiveness is probably caused by alterations at the level of signaling, and may be a manifestation of the physiological state that is a precondition for selection. Third is the complex process of selection. Whereas peripheral T cells can undergo forms of positive selection (by antigen-driven clonal expansion) and negative selection (by abortive stimulation leading to anergy or death), neither is exactly the same phenomenon that occurs in the thymic cortex. Negative selection in the cortex appears to be a suicidal inversion of antigen responsiveness: instead of turning on IL-2 expression, the activated cell destroys its own chromatin. The genes that need to be induced for this response are not yet identified, but it is unquestionably a form of activation. It is interesting that in humans and rats, cortical thymocytes undergoing negative selection can still induce IL-2R alpha expression and even be rescued in vitro, if exogenous IL-2 is provided. Perhaps murine thymocytes are denied this form of rescue because they shut off IL-2R beta chain expression at an earlier stage or because they may be uncommonly Bcl-2 deficient (cf. Sentman et al., 1991; Strasser et al., 1991). Even so, medullary thymocytes remain at least partially susceptible to negative selection even as they continue to mature.(ABSTRACT TRUNCATED AT 400 WORDS)

Antigen-independent pathways of T-cell activation are functional in human immature thymocytes

The signal requirements for proliferation of CD1+CD3- immature thymocytes have been studied in order to define whether this immature cell population could function despite the lack of the CD3/T-cell receptor complex. We found that CD1+CD3- cells proliferate upon stimulation with anti-CD28 monoclonal antibody as well as with a pair of anti-CD2 monoclonal antibodies in the presence of low doses (0.5 ng/ml) of phorbol-13-myristate-12-acetate and/or recombinant interleukin-2. A minor fraction of CD3+ cells (15%-20%) was also present in the proliferating cell population originating from CD1+CD3- thymocytes stimulated with phorbol-13-myristate-12-acetate and recombinant interleukin-2, either in the presence or in the absence of specific monoclonal antibodies. We further observed that the anti-CD3 monoclonal antibody did not induce the proliferation of CD1+CD3- cells, as expected, and efficiently triggered unfractionated or CD1+CD3+ thymocytes only if exogenous recombinant interleukin-2 was provided. Unexpectedly, we noted that highly purified (greater than 99%), CD1+CD3- immature thymocytes could mobilize calcium via CD3, besides CD2 and CD28 surface molecules, suggesting that a minor undetectable fraction (less than 1%) of CD3+ cells was still present in the purified CD3- population. Nevertheless, the preferential expansion of CD3-CD8+ cells (about one-third of proliferating cells) after triggering via CD28, and to a lesser extent via CD2, support the notion that the alternative pathways of T-cell activation are actually functional in CD1+CD3- immature thymocytes.

Involvement of the interleukin 4 pathway in the generation of functional gamma delta T cells from human pro-T cells

We have used the technique of in situ hybridization to investigate the transcription of genes encoding the CD3 complex and the lymphokine interleukin 4 (IL-4) by human pro-T cells--i.e., cells that phenotypically resemble those T-cell precursors that colonize the thymus during early intrathymic development. CD1-2-3-4-7+8-45+ pro-T cells isolated from postnatal thymi via immunoselection with a panel of specific monoclonal antibodies are already committed to the T-cell lineage because most of them transcribe the genes encoding the delta and epsilon chains of the CD3 complex. About half of such pro-T cells synthesize IL-4 mRNA in the absence of any exogenous stimulation. Upon culture with IL-4, pro-T cells extensively proliferate and differentiate into functionally competent, mature gamma delta T cells expressing a T-cell receptor repertoire similar to that of gamma delta T cells that can be found in postnatal thymus. The IL-4 response of pro-T cells is not mediated by induction of the interleukin 2 (IL-2)-IL-2 receptor pathway and, unlike IL-2-driven T-cell differentiation, does not require the presence of stromal cells. Taken altogether, these findings suggest that an autocrine IL-4-mediated pathway might be implicated in early thymocyte differentiation--namely, in the generation of T cells bearing the gamma delta T-cell receptor.

Interleukin 7 receptor engagement stimulates tyrosine phosphorylation, inositol phospholipid turnover, proliferation, and selective differentiation to the CD4 lineage by human fetal thymocytes

The purposes of this study were to elucidate the effects of recombinant human interleukin 7 (rhIL-7) on proliferation as well as differentiation of human fetal thymocytes and to analyze the biochemical nature of the IL-7 receptor-linked transmembrane signal. In the absence of costimulants, rhIL-7 stimulated the in vitro proliferation and colony formation of CD4+CD8+ double-positive immature fetal thymocytes. Furthermore, rhIL-7 promoted partial differentiation of immature thymocytes with a selective advantage for the development of CD4+CD8- single-positive thymocytes. Our observations suggest that IL-7 likely has an important regulatory role during the earliest stages of human T-cell ontogeny. Stimulation of fetal thymocytes with rhIL-7 resulted in enhanced tyrosine phosphorylation of three distinct phosphoproteins with molecular masses of 72, 98, 123, and 190 kDa and induced a rapid and biphasic increase in the production of inositol 1,4,5-trisphosphate, which was inhibitable by the tyrosine protein kinase inhibitor genistein. Thus, the transmembrane signal triggered by engagement of the IL-7 receptor is intimately linked to a functional tyrosine protein kinase pathway and stimulates the inositol phospholipid turnover and proliferation, as well as selective differentiation to the CD4 lineage, by human fetal thymocytes.