Factor requirements for activation and proliferation steps of human CD2+CD3-CD4-CD8- early thymocytes

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.

CD3-CD8+ intestinal intraepithelial lymphocytes (IEL) and the extrathymic development of IEL

Present evidence suggests that a majority of murine CD3+ intraepithelial intestinal lymphocytes (IEL) are extrathymically derived T cells and that these extrathymically derived IEL phenotypically express the CD8 homodimer (CD8 alpha alpha). Recently, CD3- IEL have been reported to express the recombination activating gene (RAG-1), suggesting that precursors to extrathymically derived CD3+CD8+ alpha alpha IEL exist on the intestinal epithelium. To study in detail whether these CD3-IEL can develop into CD3+CD8+ alpha alpha IEL, we analyzed the CD3-IEL subset and found that it can be separated into two subsets, namely CD3-CD8- and CD3-CD8+ IEL. We show that (1) CD3-CD8- IEL are mostly small, non-granular and phenotypically Pgp-1+ IL-2R+ B220-, while CD3-CD8+ IEL are mostly large, granular and phenotypically Pgp-1- IL-2R+ B220+, (2) CD(3-)-CD8+ IEL express the RAG-1 gene, and (3) CD3-CD8-, CD3-CD8+ and CD3+CD8+ alpha alpha IEL, respectively, appear sequentially in normal ontogeny and in bone marrow-reconstituted thymectomized radiation chimeras. In the latter, virtually all CD3+CD8+ alpha alpha IEL expressed the gamma delta T cell receptor (TCR), but not the alpha beta TCR. From this and what is presently known about T cell development, we propose that CD3-CD8+ IEL are an intermediate in extrathymic IEL development and that the development of extrathymically derived IEL occurs at the intestinal epithelium from CD3-CD8- to CD3-CD8+ to CD3+(gamma delta TCR)CD8+ alpha alpha.

Human T-cell leukemia virus type I-induced proliferation of human immature CD2+CD3- thymocytes

The mitogenic activity of human T-cell leukemia virus type I (HTLV-I) is triggering the proliferation of human resting T lymphocytes through the induction of the interleukin-2 (IL-2)/IL-2 receptor autocrine loop. This HTLV-I-induced proliferation was found to be mainly mediated by the CD2 T-cell antigen, which is first expressed on double-negative lymphoid precursors after colonization of the thymus. Thus, immature thymocytes express the CD2 antigen before that of the CD3-TCR complex. We therefore investigated the responsiveness of these CD2+CD3- immature thymocytes and compared it with that of unseparated thymocytes, containing a majority of the CD2+CD3+ mature thymocytes, and that of the CD2-CD3- prothymocytes. Both immature and unseparated thymocytes were incorporating [3H]thymidine in response to the virus, provided that they were cultivated in the presence of submitogenic doses of phytohemagglutinin. In contrast, the prothymocytes did not proliferate. Downmodulation of the CD2 molecule by incubating unseparated and immature thymocytes with a single anti-CD2 monoclonal antibody inhibited the proliferative response to HTLV-I. These results clearly underline that the expression of the CD2 molecule is exclusively required in mediating the proliferative response to the synergistic effect of phytohemagglutinin and HTLV-I. Immature thymocytes treated with a pair of anti-CD2 monoclonal antibodies were shown to proliferate in response to HTLV-I, even in the absence of exogenous IL-2. We further verified that the proliferation of human thymocytes is consecutive to the expression of IL-2 receptors and the synthesis of IL-2. These observations provide evidence that the mitogenic stimulus delivered by HTLV-I is more efficient than that provided by other conventional mitogenic stimuli, which are unable to trigger the synthesis of endogenous IL-2. Collectively, these results show that the mitogenic activity of HTLV-I is able to trigger the proliferation of cells which are at an early stage of T-cell development. They might therefore represent target cells in which HTLV-I infection could favor the initiation of the multistep lymphoproliferative process leading to adult T-cell leukemia.

Defective signal transduction by the CD2 molecule in immature T-cell receptor/CD3- thymocytes

The CD2 accessory molecule mediates an activation pathway in mature T cells, transducing signals similar to those observed following stimulation of the T-cell receptor/CD3 (TCR/CD3) complex. CD2 is also one of the earliest cell surface markers to appear during thymic ontogeny and has been proposed to be a stimulatory pathway for immature thymocytes that have not yet expressed TCRs on their surface (TCR/CD3-). To examine this hypothesis highly purified TCR/CD3- human thymocytes were stimulated using mitogenic combinations of anti-CD2 monoclonal antibodies or individual biotinylated anti-CD2 monoclonal antibodies crosslinked with avidin. TCR/CD3+ thymocytes responded readily to either stimulus as determined by anti-phosphotyrosine immunoblotting, and the pattern of tyrosine phosphorylated substrates was similar to that of mature T cells. In contrast, TCR/CD3- thymocytes responded weakly and with a distinct substrate pattern. In addition, the altered signal transduced by CD2 in TCR/CD3- thymocytes did not lead to a rise in intracellular calcium, failed to induce interleukin 2 receptor expression, and did not serve as a comitogen with phorbol ester or interleukin 2, functions that were all intact in TCR/CD3+ thymocytes. Failure of TCR/CD3- thymocytes to respond to CD2 stimulation was not due to an intrinsic defect in these cells as they responded normally to phorbol ester plus calcium ionophore. In TCR/CD3- thymocytes, CD2 stimulation also failed to affect steady-state mRNA levels of the recombination-activating genes RAG1 and RAG2, whereas in TCR/CD3+ cells activation of the CD2 pathway terminated their expression. Together, these data support the concept that CD2 engagement does not deliver a stimulus to TCR/CD3- thymocytes and suggests that this molecule may not directly participate in the earliest stages of thymic development.

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).

Thymic epithelial cell-derived supernatants sustain the maturation of human prothymocytes: involvement of interleukin 1 and CD23

During their development, human CD7+ lymphoid stem cells migrate into the thymus where, following intimate contact with thymic tissue, they proliferate and differentiate into functionally mature T lymphocytes. In this study, we investigated the effect of thymic epithelial cell-derived supernatants (TEC-SN) on early CD7+CD2-CD3- thymocytes. Our results indicate that TEC-SN are able to promote CD2 and CD3/TcR alpha/beta expression by CD7+ precursors. This activity correlated with soluble CD23 and interleukin 1 levels in TEC-SN. Furthermore, monoclonal antibodies to these cytokines decreased in vitro maturation of prothymocytes. Thus, in addition to cell-cell interactions, human TEC produce cytokines able to support early steps of thymocyte differentiation.

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.

Activation of a natural killer clone upon target cell binding via CD2

The functional relevance of CD2-mediated signal transduction in triggering T cell receptor/CD3-independent natural killer cell cytotoxicity was investigated by introducing wild-type CD2 or signaling-deficient CD2 with truncated cytoplasmic region into a murine natural killer-like large granular lymphocyte clone lacking CD2 and T cell receptor/CD3. The introduction of wild-type CD2 led to CD2-dependent binding and cytotoxicity against P815 target cells. In contrast, the introduction of mutant CD2 with truncated cytoplasmic region led to comparable P815 binding but reduced cytotoxicity as compared with the wild type. These results provide a direct evidence for CD2-mediated triggering of NK cell cytotoxicity upon target cell binding.

In vitro expansion of CD3/TCR- human thymocyte populations that selectively lack CD3 delta gene expression: a phenotypic and functional analysis

Highly purified CD1-3-4-8- human thymocytes were obtained by panning techniques combined with cell depletion with antibody-coated magnetic beads. Most of these cells expressed cytoplasmic CD3 antigen, as assessed by mAbs known to react with the CD3 epsilon chain. After culture with low doses of PMA (0.5 ng/ml) and subsequent addition (at 24 h) of recombinant interleukin 2 (rIL-2; 100 U/ml) cells underwent extensive proliferation (40-60-fold of the initial cell input after 2 wk). The majority of the proliferating cells were CD3-TCR-. The remaining cells (5-40%) were represented by CD3+ TCR gamma/delta+ (BB3- A13+) cells. Further removal of CD3+ TCR-gamma/delta+ cells resulted in highly purified CD3- populations that further proliferated in culture with no substantial phenotypic changes. When CD3+ thymocytes were cultured under the same experimental conditions, only CD3+ TCR-alpha/beta+ cells could be detected, thus indicating that PMA did not affect the surface expression of the CD3/TCR complex, but rather induced preferential growth of CD3- thymocytes. Surface marker analysis of cultured CD3- thymocytes showed that they were homogeneously CD7+, whereas low proportions of cells expressed CD2 and CD8 antigens. Among the natural killer (NK) cell markers, CD56 was highly expressed by all cells, whereas CD16, CD57, CD11b, NKH2, and GL183 were absent. Importantly, these cells were different from peripheral NK cells, as 80-95% of them expressed cytoplasmic CD3 antigen. Functional analysis revealed a strong cytolytic activity against both NK-sensitive (K562) and NK-resistant (M14, Daudi) human target cells. In a redirected killing assay against the Fc gamma R+ P815 cells, mAbs specific for triggering molecules including CD3, CD2, and CD16 failed to augment target cell lysis, while a strong cytolytic effect was induced by PHA. In addition, PHA alone or in combination with PMA induced tumor necrosis factor alpha (TNF-alpha) and interferon gamma (IFN-gamma) (but not IL-2) production by CD3- thymocytes. Cloning of fresh CD1-3-4-8-thymocytes in the presence of PMA and rIL-2 resulted in CD3-CD56+ clones that displayed a pattern of cytolytic activity and lymphokine production similar to that of the polyclonal populations. Northern blot analysis of transcripts coding for CD3/TCR molecules revealed the presence of CD3 zeta, epsilon, and gamma transcripts, while CD3 delta was undetectable. Mature transcripts for both gamma and delta TCR chains could be detected, whereas no TCR-alpha mRNA and only a truncated (1.0 kb) form of TCR-beta mRNA were revealed.(ABSTRACT TRUNCATED AT 400 WORDS)

Stage specific phosphoinositides turnover capacity of human intrathymic T cells following CD2-triggering

Triggering of distinct CD2 epitopes on human T lymphocytes increases their phosphatidylinositol (PI) cycle-related metabolism. In this work, we investigated the relationship between this signal transduction pathway following surface CD2 antigen triggering and intrathymic T cell development. Therefore, various thymocyte subsets were incubated with co-mitogenic CD2I+III mAb. The cells were then tested for their various phosphoinositides levels as well as their ability to proliferate in response to recombinant interleukin-2 (rIL-2). Our results indicate that immature CD4- CD8- cells have high PI metabolism while more mature CD4+CD8+ and unfractionated thymocytes display significantly lower PI-turnover. Mature CD4+CD8- and CD4-CD8+ thymocytes regain this transduction capacity. Thus, PI-turnover following CD2- triggering is linked to the developmental fate of thymocyte subclasses.