Thymic Differentiation Research Articles

Vitamin E supplementation induces an early recovery of cellular immunity decreased following X-ray irradiation

We have previously reported that vitamin E has an ability to enhance T cell differentiation in rat thymus. The aim of this study is to investigate whether T cell differentiation enhanced by vitamin E supplementation is effective in decreasing cellular immunity after X-ray irradiation in rats. Male Fisher rats, 4-weeks old, were fed control (50 mg vitamin E/kg diet) or high vitamin E diet (585 mg vitamin E/kg diet) for 4 weeks and then irradiated X-ray. On 2, 5 and 9 days after X-ray irradiation, rats were killed under anesthesia and their cellular immune functions were assayed. Vitamin E supplementation did not result in decreased thymic weights or change in the numbers of thymocytes and peripheral blood lymphocytes (PBL) following X-ray irradiation. In addition, proliferation of PBL with T cell mitogens, phytohemagglutinin (PHA) and concanavalin A (ConA), also decreased in both control and high vitamin E groups following X-ray irradiation. On the contrary, proliferation of bone marrow cells (BMC) was maintained much the same as pretreatment of X-ray irradiation in high vitamin E group even after X-ray irradiation compared to a significant decrease in the control group. The proliferation of thymocytes with PHA or ConA also showed an early recovery in high vitamin E, which was associated with not the production of interleukin 2 (IL2), T cell growth factors, but early recovery in the proportion of CD4 +CD8 + T cells in thymocyte. These results suggest that vitamin E supplementation accelerates the recovery of the X-ray irradiation-induced decrease in cellular immunity. The signs of accelerated recovery were enhanced T cell differentiation in thymus and the maintenance of bone marrow cell (BMC) proliferation during X-ray irradiation.

Transcription factors that control development of the thymic microenvironment

Very little is known about genetic determinants of thymic stromal cell differentiation. However, two genes involved in lineage commitment of the thymic stroma have recently been identified, representing a starting point in the genetic dissection of the lineage composition and phenotypic heterogeneity of the thymic microenvironment.

Protein expression during murine thymus differentiation.

Driven by our long-standing interest in identifying proteins of the immune system and in characterizing processes involved in lymphocyte differentiation, we studied protein expression in biosynthetically labeled fetal and newborn thymus by 2D gel electrophoresis. Autoradiographs of the gels were scanned with a densitometer and image analysis was performed using the Kepler system. Calibrated polypeptide spot abundances (volumes) were compared to assesses qualitative and quantitative changes of the spot volumes. Among over 300 proteins evaluated at GD (gestation day) 13,15, and 17, there were sets of proteins that increased and others that decreased in intensity. We could in addition recognize proteins that were completely absent at GD 13 and/or 15 and that appeared thereafter to gradually increase in intensity. Conversely, various polypeptide spots present at early stages (at GD 13 and 15) disappear later (at GD 17 or at birth). Among the proteins that increase in intensity prevail molecules with masses less than 35 kD, whereas a considerable portion of those that decrease in intensity are characterized by masses above 60 kD. Spots reported in this communication were not defined beyond tagging them with numbers, which is a prerequisite to follow them up in the proteinpaedia developed in our laboratory. The next step will be to retrieve the coding sequences from the existing partitioned cDNA library (BW 5147) as well as from thymocyte subtraction libraries. We predict that among those polypeptides with varying intensity, important regulatory proteins in thymus development will be found.

Characterization of the autoreactive T cell repertoire in cyclosporin-induced syngeneic graft-versus-host disease. A highly conserved repertoire mediates autoaggression.

Syngeneic graft-vs-host disease (SGVHD) is a MHC class II-restricted T cell-mediated autoimmune syndrome that occurs following syngeneic bone marrow transplantation and the administration of cyclosporin (CsA). The present studies evaluated the V beta repertoire of T lymphocytes that mediate SGVHD. To facilitate analysis, SGVHD effector cells were adoptively transferred into thymectomized syngeneic recipients reconstituted with T cell-depleted bone marrow to provide an environment that allows for the selective clonal expansion of autoreactive T cells. Analysis of target tissues and PBL by reverse transcriptase PCR using oligonucleotide V beta-specific primers revealed a predominance of V beta 8.5+ T cells and a minor population expressing V beta 10. The majority of infiltrating lymphocytes in target tissues was confirmed to be V beta 8.5+ by in situ hybridization and by immunoperoxidase staining. A small population of V beta 10+ cells could also be detected. Furthermore, SGVHD effector T splenocytes depleted of lymphocytes expressing either the TCR-alpha beta or the V beta 8.5 determinant could not adoptively transfer SGVHD. Depletion of T cells expressing the V beta 10 determinant delayed the onset of this autoaggression syndrome. Subset analysis of the autoreactive T cell compartment revealed that the V beta 8.5 determinant was expressed on both CD4+ and CD8+ lymphocytes whereas the V beta 10 determinant was principally expressed on a minor population of CD4+ autoreactive T cells. These data were confirmed by limiting dilution analysis. Additional studies examining the effect of CsA on thymic differentiation revealed that although V beta 8.5 is not normally clonally deleted, there was a pronounced shift in the expression of this determinant between CD4 and CD8 single positive thymocytes, suggesting that CsA may inhibit normal positive selection processes for MHC class I and class II reactive T cells.

Alternative Splicing Restricts Translation of Rearrangements at theT-Cell Receptor δ/α Locus

Lymphocytes expressing alpha beta or gamma delta T-cell receptors (TCR) represent distinct T-cell populations. Because TCR delta genes lie within the TCR alpha locus, the rearrangement processes, transcription, and translation of TCR delta or TCR alpha variable domain exons require tight regulation. Human precursor B-cell leukemias (eg, the REH cell line) constitute an interesting model to study TCR delta/alpha recombination because they rearrange TCR delta/alpha loci along a hierarchically ordered pathway in which V delta 2D delta 3 segments are joined to the J alpha cluster. We now show for REH cells that chimeric TCR delta/alpha variable domain exons are posttranscriptionally modified by alternative splicing resulting in truncated V delta 2C alpha transcripts. This process also takes place during thymic differentiation. CD7+/CD3- T-cell precursors exhibit V delta 2D delta 3 rearrangements. Further differentiation into CD7+/CD3+ thymocytes is associated with the expression of a truncated V delta 2C alpha RNA species. In contrast, chimeric TCR delta/alpha rearrangements containing a V delta 1 segment (but no D delta sequences) are predominantly expressed as full-length V delta 1J alpha C alpha transcripts. These data suggest that alternative splicing constitutes a mechanism that restricts the production of distinct chimeric TCR alpha chains.

Extrathymic T cell differentiation

In the mouse, the gut mucosa is a major site of extrathymic differentiation of T cells. Recent data in this past year show that this process differs from the main thymic differentiation pathway not only in its location, but also in its use of costimulatory molecules, signal transduction modules, and mechanisms of repertoire selection. The thymus exerts an influence on the expansion of the extrathymically differentiated gut intraepithelial lymphocytes that appears to be varied in nature, including acting as a source of TCR − progenitors. All gut intraepithelial lymphocytes, whatever their extrathymic or thymic site of differentiation, have common features of activated and specialized cytotoxic cells. Other T cells may differentiate extrathymically, in particular in the liver; these later cells appear to have a very restricted, probably autoreactive repertoire, and also display natural killer cell features.

Terminal transferase positive rat thymocytes are resistant to steroid-induced apoptosis.

Apoptosis is a prominent mechanism of programmed cell death in the immune system. In the thymus apoptosis is responsible for the deletion of autoreactive T-cells during thymic differentiation. The typical features of apoptosis are characterized by nuclear and cytoplasmic morphologic changes, along with cleavage of chromatin at regularly spaced sites. Terminal deoxynucleotidyl transferase (TdT) is a DNA polymerizing enzyme found at an early stage of T and B lymphocyte differentiation, which generates diversity in the DNA sequence of immunoglobulin (Ig) or T cell receptor (TCR). The combined evaluations of thymocyte morphological features, immune phenotype and thymic topography associated to TdT expression allow the recognition of three different thymocyte subpopulations, characterized by small-size, intermediate-size and large-size. The results of this study show that dexamethasone (Dx)-treatment induces cell death via apoptosis involving distinct transformations related to differentiation stages of thymic subpopulations. Intermediate and small-size thymocytes that are TdT-negative or weakly positive at nuclear level are Dx sensitive. In contrast the large-size thymocytes, highly TdT positive, corresponding to the undifferentiated cells, do not show significant morphological modifications and TdT positivity to Dx-treatment. Immunocytochemical analysis shows that Dx-treatment does not affect TdT synthesis but morphological changes, occurring during apoptotic process, are responsive to intracellular movement and intranuclear arrangement of the TdT.

Cholinergic Stimulation Modulates Apoptosis and Differentiation of Murine Thymocytes via a Nicotinic Effect on Thymic Epithelium

Effects of cholinergic agonists/antagonists on apoptosis and differentiation of murine thymus cells were investigated in two in vitro models. Treatment with 10−7 M carbachol was found to counteract the effects of a cortical thymus epithelial cell line (TEC 1.4), on apoptosis and the ratio of CD4 CD8 DP/DN cells in cocultured fetal tjymus lobes. A medullary line (TEC 2.3) did not influence apoptosis in fetal thymus lobes. Both TEC lines had the same strong apoptotic effect on thymus cells in suspension, but only the effect of TEC 1.4 was counteracted by carbachol. This cholinergic influence on TEC 1.4 cells is mediated via nicotinic cholinergic receptors, since d-tubocurarine, but not atropine, effectively blocked the effect of carbachol. The results suggest that cholinergic signals to thymic epithelial cells may have regulatory influence on thymic differentiation and selection processes.

Early expression of human CD4 delays thymic differentiation in transgenic mice

CD4 is a cell surface molecule expressed mostly on cells of the T-cell lineage. Studies have shown that this molecule plays an important role in positive and negative selection of T cells in the thymus. It is not surprising therefore, that in T-cell ontogeny, CD4 starts to be expressed on thymocyte subpopulations about to undergo these selection processes. The human CD4 molecule was expressed in mouse thymus ontogeny using a promoter, MMTVD, which targets expression as early as day 14 of ontogeny, prior to expression of endogenous TCR, CD4 and CD8. Thymic ontogeny is delayed in foetal MMTVD-CD4 mice. Human CD4-expressing thymuses show a twofold reduction in cellularity at days 17 and 18 of ontogeny compared with non-transgenic control littermate thymuses, and paradoxically, MMTVD-CD4 thymuses contain more cells in the S and G2/M stages of the cell cycle than control thymuses do. At the cell surface marker level, MMTVD-CD4 thymocytes show a delay in surface expression of CD3, murine CD4 and murine CD8, along with persistent expression of IL2R alpha compared with foetal non-transgenic littermates. Biochemical studies show that, although MMTVD-CD4 thymocytes do not express surface CD3, cytoplasmic CD3 epsilon proteins as well as TCR beta incomplete and complete transcripts are present in foetal day-17 thymocytes. Low levels of surface CD3/TCR expression, however, could partly be due to the low levels of zeta mRNA and proteins detected in these cells. These results suggest that CD4 is not expressed until a certain stage of differentiation not only because it is not yet required for selection processes, but because it can lead to a reversible deregulation of thymocyte development.

Autoimmune gld mutation uncouples suicide and cytokine/proliferation pathways in activated, mature T cells.

Antigen receptor-directed suicide plays an important role in the elimination of potentially autoaggressive immature T cells during thymic differentiation. Here we demonstrated evidence for a second pathway of receptor-directed suicide in mature T cells that is missing in a mutant strain (gld) of mice with an "autoimmune" lymphoproliferative syndrome. The defect is evident within the gld activated T cell and does not require the presence of an antigen-presenting cell for its expression. Receptor-driven suicide is intact in immature T cells of animals with this mutation. These results support the significance of receptor-directed suicide in the mature T cell compartment and suggest that the immune system may use three independent pathways for regulating programmed cell death in shaping and controlling the immune response.

Analysis of the hairless mouse as a model for the effects of aging on the immune system

Studies into the effects of aging on the immune system are hampered by the lack of a suitable animal model that is readily available and cost efficient. The mutant mouse, hairless (hr/hr genotype), has been shown to undergo an accelerated thymic involution with accompanying immunodeficiency. Thus, this strain of mouse has been proposed as a model for studying the interactions of aging and immune function. We have investigated the effects of homozygous hr gene expression over time on the immune function of these mice. It was observed that homozygous hr gene expression had minimal effects on peripheral lymphocyte subset compositions but did appear to result in changes in thymic differentiation. Further, hr/hr mice displayed decreased proliferative responses to IL2 and mitogen stimulation, although cytotoxic responses (both NK and T cell mediated) appeared normal. These defects appear to be attributable to T helper cell dysfunction. Each of the changes found in hr/hr mice were distinct from those seen with age-matched control mice. Thus, the hr/hr inbred strain of mouse does not appear to be a suitable model for use in analyzing the effects of aging on the immune system.

Vitamin E is an important factor in T cell differentiation in thymus of F344 rats.

The effect of vitamin E (dl-alpha-tocopheryl acetate) on T cell differentiation in thymus of F344 rats was examined in this study. The rats were divided into three groups: vitamin E-free, regular and high vitamin E groups and fed a diet containing various levels of vitamin E (0, 50, and 585 mg/kg diet) for 7 weeks. The number of thymocytes was significantly lower in the vitamin E-free group relative to the regular group. Although the proportions of both CD4+CD8- and CD4-CD8+ T cells in thymocytes were significantly greater in the high vitamin E group, the proportion of CD4+CD8- T cells inversely decreased in vitamin E-free group compared to that of the regular group. The ratio of CD4+CD8-/CD4-CD8+ T cells increased in the high vitamin E group (p < 0.01) and significantly decreased in the vitamin E-free group (p < 0.001) compared to that of the regular group. Although the marked changes of T cell subsets were not seen in peripheral blood lymphocytes (PBL), the ratio of CD4+CD8-/CD4-CD8+ T cells was significantly lower in the vitamin E-free group and significantly greater in the high vitamin E group compared to that of the regular group. Production of interleukin (IL) 2 by thymocytes following the stimulation with Con A for 48 h increased about threefold in the high vitamin E group compared to the regular group. Conversely, thymocytes from rats fed the vitamin E-free diet showed a significant decrease of IL2 production compared to that of the regular group. Prostaglandin E2 (PGE2) production from thymocytes was significantly lower in the high vitamin E group compared to that of the regular group, whereas thymocytes of rats fed the vitamin E-free diet showed a significant increase of PGE2 production compared to that of rats fed the regular diet. Furthermore, in vitro addition of indomethacin provided a restoration of IL2 production from thymocytes of rats fed the vitamin E-free diet to the level of rats fed the regular diet. These results suggest that vitamin E plays an important role in T cell differentiation in thymus, which may be related to the action of vitamin E as antioxidant.

Role of CD4 and CD8 in T Cell Activation and Differentiation

Publisher Summary This chapter summarizes the role of CD4 and CD8 in both T cell activation and differentiation. CD4 and CD8 are cell surface glycoproteins expressed on T lymphocytes that have specificity for class II or class I major histocompatibility complex (MHC) proteins, respectively. These proteins play major roles in both the activation of mature peripheral T cells and the thymic differentiation process that leads to the mature T cell repertoire and the expression of CD4 and CD8 on mutually exclusive T cell subsets. The crystal structures of immunoglobulin-like domains of CD4 and CD8 are determined, binding sites for CD8 on class I MHC proteins and CD4 on class II MHC proteins are mapped, and both proteins are shown to be associated via their cytoplasmic tails with the relatively T cell specific, src -family tyrosine kinase p56 lck . The first indication that CD4 and CD8 may play a more active role in T cell receptor (TCR)-mediated cell signaling came from studies demonstrating that anti-CD4 and anti-CD8 monoclonal antibodies inhibited mitogen-induced activation in the absence of relevant MHC ligands.

Negative selection of thymocytes. A novel polymerase chain reaction-based molecular analysis detects requirements for macromolecular synthesis.

Self-tolerance is mainly established through clonal deletion of autoreactive T cells during thymic differentiation. The mechanisms by which deletion is achieved are poorly understood. Here we use a specific polymerase chain reaction-based system to characterize DNA fragmentation and show that after in vivo treatment of neonatal mice with staphylococcus enterotoxin B, selective apoptosis of V beta 8+ thymocytes occurs. This process precedes detectable deletion of V beta 8+ cells as determined by phenotypic analysis. Moreover, in vivo administration of cycloheximide and, to a lesser extent, actinomycin D, inhibits apoptosis of staphylococcus enterotoxin B specific thymocytes. Thus, macromolecular synthesis is a requirement for negative selection.

Flow Cytometric Assessment of Human T-Cell Differentiation in Thymus and Bone Marrow

Abstract Using multidimensional flow cytometry we have defined and quantified the human T-cell differentiation pathway, focusing on those events occurring among the most immature thymocytes and putative bone marrow (BM) T-precursors. Early thymocytes were found to express the CD34 antigen and consisted of a mean 1.2% of cells within human pediatric (n = 9) and 2.0% in fetal thymi (n = 4). All CD34+ thymocytes were atypical blast by morphology, expressed intracytoplasmatic, but not cell surface, CD3, and were cell surface CD2+, CD5+, CD7+, CD38+, CD45+, CD45RA+, CD49d+, and LECAM-1(Leu8)high. CD34high thymocytes lacked surface expression of CD4 and CD8, but as CD34 expression diminished there was a coordinate increase in CD4 levels, followed by the appearance of CD8. The expression of CD1 and CD10 also increased concomitant with the loss of CD34, whereas expression of LECAM-1 diminished with CD34 downregulation. The differential expression of these antigens on early thymocytes (as well as the number of thymocytes displaying these patterns) was highly reproducible among the nine pediatric and four fetal specimens examined, suggesting a precise, stereotyped regulation of early differentiation events. Cell populations with antigen expression patterns suggestive of pluripotent stem cell (CD34high, CD38-), or non-T-lineage committed stem cells (CD34+, CD33+ or CD34+, CD19+) were not identified in either fetal or pediatric thymi (sensitivity = 1/10(4)). The presence of cells with the antigenic profile of the earliest CD34+ thymocytes was explored in human BM. Putative BM T-cell precursors with the appropriate phenotype (CD34+, CD7+, CD5+, CD2+, LECAM-1high) were readily identified in fetal specimens (constituting +/- 2% of the CD34+ population), but could not be reliably detected in adults. In contrast with thymi, only 13% of these cells expressed cytoplasmatic CD3, suggesting the presence of the immediate precursor of the putative prothymocyte population. This was further supported by the detection of CD34bright, CD7+, CD2-, CD5-, LECAM-1moderate cells in fetal specimens. Our results document the flow of cell surface differentiation during T-lymphopoiesis and suggest that T-lineage features are first acquired in the BM. The ability to reproducibly identify and isolate T-cell precursor populations of precisely defined maturational stage in marrow and thymus by multiparameter flow cytometry will facilitate characterization of the molecular events controlling T-lineage differentiation.

Flow cytometric assessment of human T-cell differentiation in thymus and bone marrow

Using multidimensional flow cytometry we have defined and quantified the human T-cell differentiation pathway, focusing on those events occurring among the most immature thymocytes and putative bone marrow (BM) T-precursors. Early thymocytes were found to express the CD34 antigen and consisted of a mean 1.2% of cells within human pediatric (n = 9) and 2.0% in fetal thymi (n = 4). All CD34+ thymocytes were atypical blast by morphology, expressed intracytoplasmatic, but not cell surface, CD3, and were cell surface CD2+, CD5+, CD7+, CD38+, CD45+, CD45RA+, CD49d+, and LECAM-1(Leu8)high. CD34high thymocytes lacked surface expression of CD4 and CD8, but as CD34 expression diminished there was a coordinate increase in CD4 levels, followed by the appearance of CD8. The expression of CD1 and CD10 also increased concomitant with the loss of CD34, whereas expression of LECAM-1 diminished with CD34 downregulation. The differential expression of these antigens on early thymocytes (as well as the number of thymocytes displaying these patterns) was highly reproducible among the nine pediatric and four fetal specimens examined, suggesting a precise, stereotyped regulation of early differentiation events. Cell populations with antigen expression patterns suggestive of pluripotent stem cell (CD34high, CD38-), or non-T-lineage committed stem cells (CD34+, CD33+ or CD34+, CD19+) were not identified in either fetal or pediatric thymi (sensitivity = 1/10(4)). The presence of cells with the antigenic profile of the earliest CD34+ thymocytes was explored in human BM. Putative BM T-cell precursors with the appropriate phenotype (CD34+, CD7+, CD5+, CD2+, LECAM-1high) were readily identified in fetal specimens (constituting +/- 2% of the CD34+ population), but could not be reliably detected in adults. In contrast with thymi, only 13% of these cells expressed cytoplasmatic CD3, suggesting the presence of the immediate precursor of the putative prothymocyte population. This was further supported by the detection of CD34bright, CD7+, CD2-, CD5-, LECAM-1moderate cells in fetal specimens. Our results document the flow of cell surface differentiation during T-lymphopoiesis and suggest that T-lineage features are first acquired in the BM. The ability to reproducibly identify and isolate T-cell precursor populations of precisely defined maturational stage in marrow and thymus by multiparameter flow cytometry will facilitate characterization of the molecular events controlling T-lineage differentiation.

1F7 (CD26): a marker of thymic maturation involved in the differential regulation of the CD3 and CD2 pathways of human thymocyte activation.

We have recently shown that solid-phase immobilization of anti-1F7 recognizing the 110-kDa CD26 Ag is comitogenic for human peripheral blood T cell activation via both the CD3 and CD2 pathways. We have also demonstrated that binding of anti-1F7 leads to the disappearance of CD26 surface expression, and this anti-1F7-induced modulation results in an increase in anti-CD3 or anti-CD2-mediated peripheral blood T cell activation. In this report, we extended these findings by examining the expression and functional relationship of 1F7 on the CD3 and CD2 pathways of activation of human thymocytes. We now demonstrated that most of the anti-1F7 reactivity is found on medullary thymocytes, the population of thymocytes expressing high level of CD3 (CD3H). We have also shown that binding of anti-1F7 can induce a decrease in CD26 surface expression, with no detectable effect on the surface expression of CD3 or CD2. Most importantly, we showed that solid-phase immobilization of anti-1F7 has a comitogenic effect on thymocyte activation induced by anti-CD3 but not anti-CD2. In addition, anti-1F7-induced modulation of CD26 results in an enhancement in CD3-mediated but not CD2-mediated human thymocyte activation. The observed functional effect of CD26 on the CD3/TCR pathway of activation is mainly restricted to mature thymocytes as distinguished by high surface expression of CD5, although CD26 is also functionally associated with the CD3/TCR pathway on cells expressing low level of CD5. Demonstrating that CD26 involvement in the regulation of human thymocyte activation is restricted mainly to the CD3 pathway, unlike its involvement with both the CD3 and CD2 pathways of mature peripheral blood T lymphocyte activation, our data hence suggested that CD26 may play a role in thymic differentiation and maturation via the differential engagement of the CD3 pathway.

Expression of Human Recombination Activating Genes (RAG1 and RAG2) in Neoplastic Lymphoid Cells: Correlation With Cell Differentiation and Antigen Receptor Expression

Regulation of V-(D)-J recombinations that occur in antigen receptor encoding genes remains poorly understood. Recently, two genes, RAG1 and RAG2, that are able to activate rearrangement of synthetic recombination substrates were cloned in mouse and a human gene homologous to RAG1 was described. To define the differentiation stages corresponding to RAG1 and RAG2 RNA expression, we have studied a large number of B- and T-lymphoid neoplasias. First, we show that a human gene homologous to the murine RAG2 is transcribed in humans. Moreover, using a polymerase chain reaction approach, we have shown that RAG are expressed not only in T-cell receptor (TCR)-negative T-cell acute lymphoblastic leukemias (T-ALLs), but also in some cases in which a significant percentage of cells expressed surface TCR. Absence of RAG expression was shown in certain T-ALLs at variable stages of thymic differentiation. Data obtained in B-lineage ALLs show that RAG RNAs are expressed in almost all slg- B-lineage ALLs but are not transcribed in the slg+ B-cell proliferations tested, including Burkitt's ALLs, follicular center cell lymphomas, and chronic leukemias. These findings are consistent with the involvement of RAG in the control of in vivo V-(D)-J recombinations. These findings are also of interest in the delineation of potential regulatory factors acting on RAG transcription and in the understanding of the mechanisms of specific chromosomal abnormalities occurring in immature lymphoid cells.

Expression of human recombination activating genes (RAG1 and RAG2) in neoplastic lymphoid cells: correlation with cell differentiation and antigen receptor expression

Regulation of V-(D)-J recombinations that occur in antigen receptor encoding genes remains poorly understood. Recently, two genes, RAG1 and RAG2, that are able to activate rearrangement of synthetic recombination substrates were cloned in mouse and a human gene homologous to RAG1 was described. To define the differentiation stages corresponding to RAG1 and RAG2 RNA expression, we have studied a large number of B- and T-lymphoid neoplasias. First, we show that a human gene homologous to the murine RAG2 is transcribed in humans. Moreover, using a polymerase chain reaction approach, we have shown that RAG are expressed not only in T-cell receptor (TCR)-negative T-cell acute lymphoblastic leukemias (T-ALLs), but also in some cases in which a significant percentage of cells expressed surface TCR. Absence of RAG expression was shown in certain T-ALLs at variable stages of thymic differentiation. Data obtained in B-lineage ALLs show that RAG RNAs are expressed in almost all slg- B-lineage ALLs but are not transcribed in the slg+ B-cell proliferations tested, including Burkitt's ALLs, follicular center cell lymphomas, and chronic leukemias. These findings are consistent with the involvement of RAG in the control of in vivo V- (D)-J recombinations. These findings are also of interest in the delineation of potential regulatory factors acting on RAG transcription and in the understanding of the mechanisms of specific chromosomal abnormalities occurring in immature lymphoid cells.

Are deficiencies of prostaglandin‐E‐mediated immunoregulation involved in increased IgE synthesis of atopic mononuclear cells in vitro?

We demonstrate that spontaneous in vitro immunoglobulin E synthesis of atopic peripheral blood mononuclear cells could be suppressed by the addition of 10(-6) M to 10(-5) M prostaglandin E1 (PGE1) or PGE2. Impaired suppressor T lymphocyte maturation and function in atopic individuals are explained by an insufficient transmission of prostaglandin E (PGE) signals during thymic lymphocyte differentiation as well as an impaired ability of the atopic immune system to activate suppressor T cells by PGE-mediated feed back mechanisms. Decreased levels of 6-desaturated PGE-precursor fatty acids in plasma, T lymphocytes, monocytes, adipose tissue and breast milk have been observed in atopic individuals. These insights might offer a novel approach to the prevention of atopic disease by substitution of the atopic pregnant and nursing woman and her newborn infant with long-chain omega-6-fatty acids.