No evidence for proliferation in the blood CD4+ T-cell pool during HIV-1 infection and triple combination therapy

OBJECTIVE

To evaluate the role of cell proliferation in peripheral blood lymphocyte (PBL) dynamics during HIV infection and potent antiretroviral therapy including protease inhibitors.

DESIGN

Transverse study of 150 patients at different stages of infection. Longitudinal study of 50 patients on triple combination antiretroviral therapy with 9-month follow-up.

METHODS

Ex vivo incubation of fresh PBL with the DNA biosynthetic marker bromodeoxyuridine (BrdU). Flow cytometric analysis of cell phenotypes and BrdU incorporation. Parallel determination of plasma virus load and CD4+ cell counts.

RESULTS

Percentages of BrdU+ B and T lymphocytes found in patients with asymptomatic HIV infection were not different from the low values found in HIV-seronegative controls, and were not correlated with the CD4+ cell count. DNA synthesis increased significantly only during acute opportunistic infections occurring in patients with high plasma viral load and fewer than 100 x 10(6) CD4+ cells/l. Triple combination therapy induced a decrease of plasma virus load and a rise of CD4+ cell counts, whereas BrdU incorporation remained low or decreased.

CONCLUSION

Proliferation of peripheral blood T cells observed at late stages of HIV infection corresponds to a response to opportunistic infections. Apart from these particular cases, proliferation in this compartment does not appear as a critical parameter of CD4+ cell kinetics during chronic HIV infection and potent therapy.

T cell deletion induced by chronic infection with mouse mammary tumor virus spares a CD25-positive, IL-10-producing T cell population with infectious capacity

We found that T cells recognizing viral superantigen (vSAG) can be subdivided into two distinct functional subsets based on IL-2R alpha (CD25) expression. CD4+Vbeta6+CD25- and CD4+Vbeta6+CD25+ T cells were sensitive to vSAG activation. When obtained from BALB/c(SW) mice, both subsets were infected and capable to induce the tolerance process when transferred into noninfected recipients. However, in contrast to CD4+Vbeta6+CD25- cells, which were gradually deleted in MMTV(SW)-infected mice, the pool of CD4+Vbeta6+CD25+ lymphocytes was constant even at the end of the deletion process, and maintained a limited reactivity to vSAG-induced activation. The constant number of Vbeta6+CD25+ observed in infected mice could not be explained by their rapid turnover (deletion and renewal), as their proliferative rate measured by BrdU incorporation was similar in infected and naive mice, as well as in virus-nonspecific (Vbeta8.2+) cells. Neither was the Vbeta6+CD25+ subset dependent on vSAG activation since it was also present in MMTV-free mice and was not generated from Vbeta6+CD25- cells upon in vivo vSAG stimulation. Vbeta6+CD25+ T cells constitutively expressed IL-4 and IL-10 mRNA. IL-10 has been shown to be associated with viral, bacterial, and parasitic infections. This permanent CD25+ subpopulation may play a role in the control of viral infection and tolerance induction via vSAG recognition and IL-10 production.

T cell deletion induced by chronic infection with mouse mammary tumor virus spares a CD25-positive, IL-10-producing T cell population with infectious capacity

We found that T cells recognizing viral superantigen (vSAG) can be subdivided into two distinct functional subsets based on IL-2R alpha (CD25) expression. CD4+Vbeta6+CD25- and CD4+Vbeta6+CD25+ T cells were sensitive to vSAG activation. When obtained from BALB/c(SW) mice, both subsets were infected and capable to induce the tolerance process when transferred into noninfected recipients. However, in contrast to CD4+Vbeta6+CD25- cells, which were gradually deleted in MMTV(SW)-infected mice, the pool of CD4+Vbeta6+CD25+ lymphocytes was constant even at the end of the deletion process, and maintained a limited reactivity to vSAG-induced activation. The constant number of Vbeta6+CD25+ observed in infected mice could not be explained by their rapid turnover (deletion and renewal), as their proliferative rate measured by BrdU incorporation was similar in infected and naive mice, as well as in virus-nonspecific (Vbeta8.2+) cells. Neither was the Vbeta6+CD25+ subset dependent on vSAG activation since it was also present in MMTV-free mice and was not generated from Vbeta6+CD25- cells upon in vivo vSAG stimulation. Vbeta6+CD25+ T cells constitutively expressed IL-4 and IL-10 mRNA. IL-10 has been shown to be associated with viral, bacterial, and parasitic infections. This permanent CD25+ subpopulation may play a role in the control of viral infection and tolerance induction via vSAG recognition and IL-10 production.

In vivo T cell response to viral superantigen. Selective migration rather than proliferation

Superantigens induce T cell activation and proliferation in vitro, and some also induce cell activation in vivo. MMTV(SW) is an infectious mouse mammary tumor virus (MMTV) encoding a superantigen with the same Vbeta specificity as MIs-1a (Mtv-7), which induces a strong local response in vivo. injection of MMTV(SW) into mouse footpads leads to accumulation of superantigen-reactive T cells (Vbeta6+CD4+) and B cells in the draining lymph nodes (LN). We investigated the kinetics of this cell accumulation by measuring cell activation (blastogenesis, CD25 and CD69 expression), cell migration (using syngenic FITC-labeled CD4+ cells and L-selectin detection), and cell proliferation (using in vivo labeling with bromodeoxyuridine). Specific T cells selectively migrated to the draining LN. Accumulating Vbeta6+CD4+ T cells were large CD69+ cells, but remained CD25 negative and showed down-regulated L-selectin expression. Their DNA synthesis rate, studied by pulse labeling and continuous administration of bromodeoxyuridine, was increased, but remained too low to explain the draining LN hyperplasia. These data show that the local T cell response to MMTV(SW) mainly consists of selective migration followed by local activation of reactive T cells, and that cell proliferation is only a minor component of the response. By contrast, the optimal dose of staphylococcal enterotoxin B that, nevertheless, leads to a lower reactive T cell accumulation in the draining LN induces a very high proliferation rate.

In vivo T cell response to viral superantigen. Selective migration rather than proliferation

Superantigens induce T cell activation and proliferation in vitro, and some also induce cell activation in vivo. MMTV(SW) is an infectious mouse mammary tumor virus (MMTV) encoding a superantigen with the same Vbeta specificity as MIs-1a (Mtv-7), which induces a strong local response in vivo. injection of MMTV(SW) into mouse footpads leads to accumulation of superantigen-reactive T cells (Vbeta6+CD4+) and B cells in the draining lymph nodes (LN). We investigated the kinetics of this cell accumulation by measuring cell activation (blastogenesis, CD25 and CD69 expression), cell migration (using syngenic FITC-labeled CD4+ cells and L-selectin detection), and cell proliferation (using in vivo labeling with bromodeoxyuridine). Specific T cells selectively migrated to the draining LN. Accumulating Vbeta6+CD4+ T cells were large CD69+ cells, but remained CD25 negative and showed down-regulated L-selectin expression. Their DNA synthesis rate, studied by pulse labeling and continuous administration of bromodeoxyuridine, was increased, but remained too low to explain the draining LN hyperplasia. These data show that the local T cell response to MMTV(SW) mainly consists of selective migration followed by local activation of reactive T cells, and that cell proliferation is only a minor component of the response. By contrast, the optimal dose of staphylococcal enterotoxin B that, nevertheless, leads to a lower reactive T cell accumulation in the draining LN induces a very high proliferation rate.

CD4low TCRint thymocytes do not belong to the CD8 lineage maturation pathway

Thymocytes with a low expression of CD4 and an intermediate density of the TCR (CD4low TCRint) were analyzed for phenotype, MHC dependence, production kinetics, and TCR repertoire to investigate their position in the intrathymic T cell maturation process. Comparison of normal and MHC-deficient mice showed that the CD4low TCRint cell subset was MHC class II dependent, as this subpopulation could not be defined in MHC class II- or double (class I and II)-deficient mice. These thymocytes were heat-stable Aghigh and CD69+, thus immature and recently engaged in a TCR interaction, probably with MHC class II molecules. Their generation kinetics were studied in two systems: development of exogenous bone marrow cells transferred into RAG-2-/- mice, and pulse labeling with bromodeoxyuridine. In both systems, CD4low TCRint cells were produced well before CD4low TCRhigh cells, the direct precursors of CD8 single-positive cells. Their production paralleled that of CD4high TCRint cells, but they were different than these thymocytes in their smaller cell size. Moreover, they had the same V beta 6 frequency in Mls-1a and Mls-1b mice, suggesting that these cells could be undergoing a negative selection process. The data here clearly demonstrate that CD4low TCRint thymocytes do not belong to the CD8 lineage maturation pathway, and suggest that these cells could represent a MHC class II-restricted dead-end subset.

CD4low TCRint thymocytes do not belong to the CD8 lineage maturation pathway

Thymocytes with a low expression of CD4 and an intermediate density of the TCR (CD4low TCRint) were analyzed for phenotype, MHC dependence, production kinetics, and TCR repertoire to investigate their position in the intrathymic T cell maturation process. Comparison of normal and MHC-deficient mice showed that the CD4low TCRint cell subset was MHC class II dependent, as this subpopulation could not be defined in MHC class II- or double (class I and II)-deficient mice. These thymocytes were heat-stable Aghigh and CD69+, thus immature and recently engaged in a TCR interaction, probably with MHC class II molecules. Their generation kinetics were studied in two systems: development of exogenous bone marrow cells transferred into RAG-2-/- mice, and pulse labeling with bromodeoxyuridine. In both systems, CD4low TCRint cells were produced well before CD4low TCRhigh cells, the direct precursors of CD8 single-positive cells. Their production paralleled that of CD4high TCRint cells, but they were different than these thymocytes in their smaller cell size. Moreover, they had the same V beta 6 frequency in Mls-1a and Mls-1b mice, suggesting that these cells could be undergoing a negative selection process. The data here clearly demonstrate that CD4low TCRint thymocytes do not belong to the CD8 lineage maturation pathway, and suggest that these cells could represent a MHC class II-restricted dead-end subset.

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

The development of thymocyte subsets and of the thymic epithelium in SCID and RAG-2/-mice was monitored after normal bone-marrow-cell transfer. The kinetics of thymic reconstitution and their relationships with cell proliferation were investigated by using bromodeoxyuridine to detect DNA-synthesizing cells among lymphoid cells by 3-color flow cytometry, and in epithelial compartments by staining frozen sections. Thymocytes started to express CD8 and CD4 10 days after transfer, simultaneously with extensive proliferation. The first mature CD4+ single-positive cells were generated, from resting CD4+CD8+ cells after day 15. During this day 10-15 period, many epithelial cells positive for cortex-specific or panepithelial markers were labeled with BrdUrd after pulse-injection. Organized medullary epithelium also developed after day 15, that is, synchronously with the appearance of mature thymocytes, but medullary cells were never found BrdUrd+. These results suggest that, in these models, the reconstitution of the thymic epithelial network proceeds through expansion of preexisting cortical or undifferentiated cells and by later maturation (acquisition of specific markers) of medullary cells. This last process is dependent of the presence of mature thymocytes.

Stochastic coreceptor shut-off is restricted to the CD4 lineage maturation pathway

Kinetics of mature T cell generation in the thymus of normal or major histocompatibility complex (MHC) class I- or II-deficient mice were studied by the bromodeoxyuridine pulse labeling method. As previously described, the early activation and final maturation phases were found to be synchronous for the two T cell lineages, but CD4+8- cells were generated faster than CD4-8+ cells in MHC class I- and II-deficient mice, respectively. CD8 downregulation started on day 2 after cell proliferation even in the absence of MHC class II expression. CD8 downregulation thus appears to be stochastic at its beginning. By contrast, CD4 shut-off was found totally instructive, as the generation of CD4lo8+ cells with a high TCR density was not observed in class I-deficient mice. The analysis of the V beta 14 TCR frequencies in CD4/8 subsets in normal and MHC-deficient mice confirmed that CD4 and CD8 generation pathways are not symmetrical. These findings show that commitment towards the CD4+8- or CD4-8+ phenotype is controlled at the CD8lo step for the former and at the CD4+8+ double-positive stage for the latter.

Endogenous granulocyte-macrophage colony-stimulating factor is involved in IL-1- and IL-7-induced murine thymocyte proliferation

We have reported previously that IL-1 induces murine thymocyte proliferation in the absence of artificial comitogens, provided that the cells are cultured at high densities. In the present study, we show that, in these conditions, TdR uptake in response to IL-1 is diminished significantly by anti-granulocyte-macrophage colony-stimulating factor (GM-CSF) Abs. Indeed, a substantial production of this growth factor occurs when thymocytes are cultured in the presence of IL-1. Maximal GM-CSF levels are attained within 3 days of culture, and mRNA expression is detected after a 48-h stimulation. Both GM-CSF production and IL-1-induced thymocyte proliferation are decreased considerably by the depletion of I-A+ Mac-1+ accessory cells. Yet, addition of exogenous GM-CSF to accessory cell-depleted thymocytes does not restore the proliferative response to IL-1 alone, suggesting the implication of another accessory cell-derived mediator. Our data design IL-7 as the endogenous factor required in our culture system because: 1) GM-CSF can reverse the decrease in the proliferation after accessory cell depletion when IL-7 is provided together with IL-1, and 2) the proliferative response to IL-1 plus IL-7 is diminished as much by neutralization of GM-CSF by its specific Abs as by accessory cell removal (approximately 30%). Finally, the cells responding to IL-1 + IL-7 were identified as mature CD4-CD8-TCR+ thymocytes by the use of bromodeoxyuridine (BrdUrd), suggesting that the GM-CSF produced by thymic accessory cells in response to IL-1 participates in IL-7-dependent, intrathymic expansion of the CD4-CD8-TCR+ compartment.

Endogenous granulocyte-macrophage colony-stimulating factor is involved in IL-1- and IL-7-induced murine thymocyte proliferation

We have reported previously that IL-1 induces murine thymocyte proliferation in the absence of artificial comitogens, provided that the cells are cultured at high densities. In the present study, we show that, in these conditions, TdR uptake in response to IL-1 is diminished significantly by anti-granulocyte-macrophage colony-stimulating factor (GM-CSF) Abs. Indeed, a substantial production of this growth factor occurs when thymocytes are cultured in the presence of IL-1. Maximal GM-CSF levels are attained within 3 days of culture, and mRNA expression is detected after a 48-h stimulation. Both GM-CSF production and IL-1-induced thymocyte proliferation are decreased considerably by the depletion of I-A+ Mac-1+ accessory cells. Yet, addition of exogenous GM-CSF to accessory cell-depleted thymocytes does not restore the proliferative response to IL-1 alone, suggesting the implication of another accessory cell-derived mediator. Our data design IL-7 as the endogenous factor required in our culture system because: 1) GM-CSF can reverse the decrease in the proliferation after accessory cell depletion when IL-7 is provided together with IL-1, and 2) the proliferative response to IL-1 plus IL-7 is diminished as much by neutralization of GM-CSF by its specific Abs as by accessory cell removal (approximately 30%). Finally, the cells responding to IL-1 + IL-7 were identified as mature CD4-CD8-TCR+ thymocytes by the use of bromodeoxyuridine (BrdUrd), suggesting that the GM-CSF produced by thymic accessory cells in response to IL-1 participates in IL-7-dependent, intrathymic expansion of the CD4-CD8-TCR+ compartment.

Interleukin 7 induces preferential expansion of V beta 8.2+CD4-8- and V beta 8.2+CD4+8- murine thymocytes positively selected by class I molecules

We analyzed the phenotype and V beta-T cell receptor (TCR) repertoire, together with interleukin 7 receptor (IL-7R) expression in unfractionated thymocytes stimulated in vitro with IL-7. This culture system results in a specific proliferation of mature thymocytes belonging to the CD3+CD4-, CD4+8-, and CD4-8+ subsets. IL-7 induced a preferential expansion of V beta 8.2+CD4-8- and V beta 8.2+CD4-8- thymocytes. This phenomenon is not observed in beta 2-microglobulin-deficient mice, showing that a fraction of CD4+8- thymocytes, enriched in V beta 8.2+ cells, is selected by class I molecules in normal mice, as are a large proportion of CD4-8- alpha beta TCR+ thymocytes. Our findings also establish that IL-7 plays a major role in the expansion of rare thymocyte subsets, which could exert important functions in inflammatory and immune responses.

Production, selection, and maturation of thymocytes with high surface density of TCR

The main steps in intrathymic T cell differentiation have been defined using bromodeoxyuridine as a postmitotic cell tracer. Thymocytes with a high surface expression of the TCR are generated in the first 24 h after DNA synthesis. The phenotype of these TCR(high) cells was studied during 10 days by using pairs of surface markers associated with BrdUrd. During the first 2 days, TCR(high) cells were of the CD4+CD8+HSA(high) phenotype, transiently expressed the early activation marker CD69, and contained a high percentage of cycling cells. This activation step preceded the transition from CD4+CD8+ to CD4+CD8- and then to CD4-CD8+ cells, followed by progressive HSA down regulation and increase in the expression of H-2K, Qa-2, and CD45RB. The phenotypic maturation was completed in 9 days. In Mls-1a mice, negative selection of V beta 6+ cells was observed at the earliest step of TCR(high) cell generation, and positive selection of V beta 8.2+ and V beta 14+ cells took place later and was correlated to the activation step. These data suggest that high TCR expression and cell activation are necessary for positive selection and subsequent T cell maturation.

Production, selection, and maturation of thymocytes with high surface density of TCR

The main steps in intrathymic T cell differentiation have been defined using bromodeoxyuridine as a postmitotic cell tracer. Thymocytes with a high surface expression of the TCR are generated in the first 24 h after DNA synthesis. The phenotype of these TCR(high) cells was studied during 10 days by using pairs of surface markers associated with BrdUrd. During the first 2 days, TCR(high) cells were of the CD4+CD8+HSA(high) phenotype, transiently expressed the early activation marker CD69, and contained a high percentage of cycling cells. This activation step preceded the transition from CD4+CD8+ to CD4+CD8- and then to CD4-CD8+ cells, followed by progressive HSA down regulation and increase in the expression of H-2K, Qa-2, and CD45RB. The phenotypic maturation was completed in 9 days. In Mls-1a mice, negative selection of V beta 6+ cells was observed at the earliest step of TCR(high) cell generation, and positive selection of V beta 8.2+ and V beta 14+ cells took place later and was correlated to the activation step. These data suggest that high TCR expression and cell activation are necessary for positive selection and subsequent T cell maturation.

Normal sequence of phenotypic transitions in one cohort of 5-bromo-2'-deoxyuridine-pulse-labeled thymocytes. Correlation with T cell receptor expression

"In vivo" kinetics of T cell differentiation and TCR expression in the normal murine thymus were re-evaluated using a new technique for simultaneous detection of bromodeoxyuridine and two surface markers. The transition from CD4-8- precursors to CD4+8+ immature cells was directly observed during cell proliferation, and shown to proceed through transitory intermediates expressing no or low amounts of CD4. CD3-TCR expression also started during this transition and resulted in the production of a majority of TCRlo cells but also of a significant number (1 to 2 x 10(6) of TCRhi immature (heat-stable Ag+) thymocytes. After cessation of proliferation, the maturational transition from CD4+8+ to CD4+8- and CD4-8+ (in this order) was restricted to TCRhi cells produced during CD4+8+ cell generation. The acquisition of the single positive phenotype preceded HSA down-regulation, suggesting that maturation of TCRhi thymocytes proceeds in two separate steps. The major TCRloCD4+8+ subset appeared a dead end subset and showed no up-regulation of TCR expression at any time.

Normal sequence of phenotypic transitions in one cohort of 5-bromo-2'-deoxyuridine-pulse-labeled thymocytes. Correlation with T cell receptor expression

"In vivo" kinetics of T cell differentiation and TCR expression in the normal murine thymus were re-evaluated using a new technique for simultaneous detection of bromodeoxyuridine and two surface markers. The transition from CD4-8- precursors to CD4+8+ immature cells was directly observed during cell proliferation, and shown to proceed through transitory intermediates expressing no or low amounts of CD4. CD3-TCR expression also started during this transition and resulted in the production of a majority of TCRlo cells but also of a significant number (1 to 2 x 10(6) of TCRhi immature (heat-stable Ag+) thymocytes. After cessation of proliferation, the maturational transition from CD4+8+ to CD4+8- and CD4-8+ (in this order) was restricted to TCRhi cells produced during CD4+8+ cell generation. The acquisition of the single positive phenotype preceded HSA down-regulation, suggesting that maturation of TCRhi thymocytes proceeds in two separate steps. The major TCRloCD4+8+ subset appeared a dead end subset and showed no up-regulation of TCR expression at any time.

Phenotype analysis of cycling and postcycling thymocytes: evaluation of detection methods for BrdUrd and surface proteins

We present a comparison of two different methods for simultaneous detection of bromodeoxyuridine and cell surface markers. Both methods use enzymatic generation of single-strand DNA with nuclease. The biological system used is the murine thymus, in which in vivo DNA synthetizing cells were labeled by injection of BrdUrd and analyzed at different time points after the nucleoside pulse. The surface proteins detected were CD4 and CD8 differentiation markers and the T-cell receptor. Extraction of DNA-associated proteins with 0.1N HCl and detergent is necessary for the action of EcoR1 and Exonuclease III, but this treatment destroys phycocyanins and induces cell aggregation, as shown using the doublet-discrimination module. For DNAse I action, cells could be treated with paraformaldehyde and a low concentration of Tween 20, and this treatment was adequate for surface staining preservation (even with phycocyanins) and BrdUrd detection. Both methods were adequate for cell cycle studies, but only 7-amino-actinomycin D could be used as total DNA dye after DNAse action, and good results needed long (48-72 h) incubation in the fixative-detergent mixture. The DNAse I method now allows three-color staining (two surface markers and Brd-Urd), analyzed in a one laser-cytometer for the study of the phenotype of cycling cells, and of their progeny, in vivo and in cell cultures. It also allows the quantitative analysis of cell surface receptor densities in conditions similar to fresh cells.

A novel CD45RA+CD4+ transient thymic subpopulation in MRL-lpr/lpr mice: its relation to non-proliferating CD4-CD8-CD45RA+ tumor cells

MRL-lpr/lpr mice have hypertrophied lymph nodes comprising CD4-CD8- T cells. In addition, they contain CD4+CD8- T cells co-expressing the CD45RA marker. The correlation between these two subpopulations has been difficult to assess. We analyzed the expression of CD45RA (with the RA3-2C2 antibody) in various thymic and peripheral T cell subsets, using three-color immunofluorescence. We showed that in lpr mice (i) a transient CD4+CD8- thymic subset co-expresses CD45RA during the course of the disease, and (ii) thymic as well as peripheral CD4-CD8- and CD4+CD8- T cells brightly express CD45RA; furthermore (iii) in the lymph nodes, during lymphadenopathy, CD4+CD8-CD45RA+ T cells show a broad range of the CD4 fluorescence intensity, and (iv) the increase in MHC class II expression is restricted to CD45RA-T cells of the thymus and lymph nodes of lpr mice. Taken together, these data suggest that the CD4+CD8-CD45RA+ population might generate the CD4-CD8- tumor cells. In addition, using the bromodeoxyuridine labeling technique, we demonstrate that these cells are not the result of increased proliferation.

Thy-1 modulation and cell proliferation at early steps of intrathymic bone marrow cell differentiation

Intrathymic (IT) transfer of bone marrow (BM) precursor cells in sublethally irradiated hosts has been widely used to study T cell differentiation and maturation. In this report we have used double congenic mice Ly 5.1 Thy 1.1 (host) and Ly 5.2 Thy 1.2 (donor) and detected cycling Ly 5.2+ BM cells by in vivo bromodeoxyuridine incorporation, before induction of the Thy 1.2 antigen. Until Day 9 post-transfer, some donor type cells express a high level of Thy 1.2 together with macrophage and granulocyte markers. A few days later, a Thy 1.2low population transiently B220+ was detected. Thereafter, donor type cells expressed an intermediate Thy 1.2 brightness; this population then persisted and surpassed the other subsets. Our findings permitted to establish a relationship between cell cycle and Thy 1 fluorescence intensity according to the sequence: Thy 1low resting, Thy 1low cycling, Thy 1high cycling, Thy 1high resting. Moreover, we have shown that cells from the myeloïd and B lineages can, in vivo, transiently express the Thy 1 antigen, develop and differentiate within the thymus microenvironment.

Accumulation of bromodeoxyuridine-labeled cells in central and peripheral lymphoid organs: minimal estimates of production and turnover rates of mature lymphocytes

Daily lymphocyte production in both central and peripheral lymphoid organs was evaluated by associating in vivo incorporation of bromodeoxyuridine (BrdUrd) with cell surface labeling and multi-parameter flow analysis. At least 10% of mature T and B lymphocytes are generated every 24 h. The kinetic behavior of these cell populations differs, however, in that mature B cells are generated predominantly in the precursor compartments of the bone marrow, while most mature T cell generation occurs at the periphery. Therefore, peripheral expansion is the major mechanism of mature T cell production in the adult mouse. By following the accumulation of BrdUrd-labeled cells in peripheral lymphoid organs we found that the progeny of the daily lymphocyte production was sufficient to renew 30%-40% of all peripheral T and B cells every 48 h, demonstrating a high turnover rate of mature lymphocytes. We also examined the conditions of BrdUrd labeling of cycling cells in vivo. We found that while greater than 90% of bone marrow and thymus cells in S phase were labeled with a single injection of BrdUrd, in peripheral lymphoid compartments 70% of T and B cells in S failed to incorporate BrdUrd. Particular schedules of BrdUrd administration were required to overcome the low labeling efficiency of mature cells in vivo. Prolonged BrdUrd administration, however, had toxic effects on resident cells. The low labeling efficiency of BrdUrd incorporation by mature cells, as well as its potential toxicity during prolonged administration, may explain controversial results obtained by the different strategies used to study lymphocyte population dynamics.

Positive selection is an early event in thymocyte differentiation: high TCR expression by cycling immature thymocytes precedes final maturation by several days

T cell antigen receptor expression by cycling and post-cycling thymocytes has been analysed by flow cytometry. Normal mice were pulsed with 5-bromo-2'-deoxyuridine (BrdUrd), a thymidine analogue detectable with a monoclonal antibody. Thymocytes were surface-stained with antibodies against several V beta gene products and against whole alpha beta receptors and detection of BrdUrd in the nuclei was performed after enzymatic generation of single-stranded DNA. A significant (10%) percentage of thymocytes expressing high levels of alpha beta TCR were found in the cycle: these cells were immature, as shown by the CD4+8+ phenotype and by high HSA expression. After division, most alpha beta high BrdUrd+ cells entered a resting state and their number remained constant for 3 days, decreasing in two steps thereafter. This post-mitotic evolution was not modified by injection of an anti-mitotic drug. After day 4, a majority of the studied subset acquired a single positive phenotype. Location of BrdUrd+ V beta 8.2 high cells studied on frozen sections was found cortical at early times and medullary after day 3. V beta 6 expression by cycling and post-cycling thymocytes was analysed in various mouse strains, and early high expression by cycling thymocytes was found to be restricted to MIs 1b strains. These results suggest that high alpha beta TCR expression by cycling immature thymocytes corresponds to positive selection, which must therefore be considered as an early event in intrathymic differentiation.

Cell proliferation and thymocyte subset reconstitution in sublethally irradiated mice: compared kinetics of endogenous and intrathymically transferred progenitors

After sublethal (6 Gy) whole-body irradiation, the C57BL/Ba (Thy-1.1) murine thymus regenerated in two waves, on days 3-10 and 25-32, separated by a severe relapse. The second phase of depletion-reconstitution reproduced the first one, in a less synchronous manner. The depletion affected all cell subsets, but CD4+ CD8- cells decreased later than immature cells. Cell proliferation, measured by BrdUrd incorporation, started on day 3 after irradiation and concerned CD4- CD8-, CD4- CD8+, and CD4+ CD8+ cells, sequentially. CD4+ CD8- cells never represented a significant percentage of cycling cells. When irradiation was immediately followed by an intrathymic injection of 10(5) C57BL/Ka (Thy-1.2) bone marrow cells, the relapse in thymus reconstitution was no longer observed. Detected with anti-Thy-1.2 antibodies, donor cells started cycling on day 14 and showed only one wave of proliferation. In these chimeras, recipient thymocytes behave exactly like thymocytes of solely irradiated mice. Intrathymically transferred CD4- CD8- thymocytes (10(5] showed the same proliferation kinetics as endogenous cells, with a peak in number on day 10 but completely disappeared from the thymus on days 14-21. These data reflect maturational differences between intrathymic and bone marrow precursor cells and suggest different radiosensitivities not linked to proliferative status. The resting state of the thymus immigrants was shown by the absence of Thy-1 acquisition by bone marrow cells continuously labeled for 10 days with BrdUrd in vivo before intrathymic transfer. When such labeled bone marrow cells were injected in the thymus, only the minor BrdUrd- subset gave rise to Thy-1+ cells.

Cell proliferation and differentiation in the fetal and early postnatal mouse thymus

The relationships between cell proliferation and cell differentiation during thymus ontogeny were studied by labeling DNA-synthesizing thymocytes with bromodeoxyuridine and staining with antibodies against CD4, CD8, J11d, phagocytic glycoprotein 1, TCR V beta 8 chain, Thy-1, and IL-2R surface proteins. The development of the thymus was discontinuous, with two well defined growth periods from 13 days to 18 days of fetal life and from 3 days to 6 days after birth, and more progressive growth from day 8 to 2 wk. Cell proliferation started on fetal day 12, 1 day after the arrival of hemopoietic stem cells in the third branchial pouch. These cells were phagocytic glycoprotein 1-positive but IL-2R and Thy-1 negative. Thus, cell proliferation preceded IL-2R expression. Until day 15, CD4-8- thymocytes expanded without differentiation. Then CD4-8+ and CD4+8+ cells appeared; this induction was proliferation dependent and occurred on cells which had already lost IL-2R, but just after maximum expression of this receptor. During several days, the thymus remained of constant size (around 10(7) cells) and behaved like the steady state thymus. On day 3 after birth, expansion started again and was correlated with an increase in CD4-8- proliferation index and IL-2R expression. At the same time, the thymic subset capable of expansion without differentiation was again, transiently, detectable. These results suggest that the inflow of precursor cells into the thymus is permanent but transiently increased at several times during ontogeny. Moreover, the behavior of fetal CD4-8- cells does not appear radically different from that of adult precursors, but the actual difference resides in the variation of the relative proportion of CD4-8- cells at different maturation stages, as revealed by striking variations of IL-2R expression by cycling cells.

Cell proliferation and differentiation in the fetal and early postnatal mouse thymus

The relationships between cell proliferation and cell differentiation during thymus ontogeny were studied by labeling DNA-synthesizing thymocytes with bromodeoxyuridine and staining with antibodies against CD4, CD8, J11d, phagocytic glycoprotein 1, TCR V beta 8 chain, Thy-1, and IL-2R surface proteins. The development of the thymus was discontinuous, with two well defined growth periods from 13 days to 18 days of fetal life and from 3 days to 6 days after birth, and more progressive growth from day 8 to 2 wk. Cell proliferation started on fetal day 12, 1 day after the arrival of hemopoietic stem cells in the third branchial pouch. These cells were phagocytic glycoprotein 1-positive but IL-2R and Thy-1 negative. Thus, cell proliferation preceded IL-2R expression. Until day 15, CD4-8- thymocytes expanded without differentiation. Then CD4-8+ and CD4+8+ cells appeared; this induction was proliferation dependent and occurred on cells which had already lost IL-2R, but just after maximum expression of this receptor. During several days, the thymus remained of constant size (around 10(7) cells) and behaved like the steady state thymus. On day 3 after birth, expansion started again and was correlated with an increase in CD4-8- proliferation index and IL-2R expression. At the same time, the thymic subset capable of expansion without differentiation was again, transiently, detectable. These results suggest that the inflow of precursor cells into the thymus is permanent but transiently increased at several times during ontogeny. Moreover, the behavior of fetal CD4-8- cells does not appear radically different from that of adult precursors, but the actual difference resides in the variation of the relative proportion of CD4-8- cells at different maturation stages, as revealed by striking variations of IL-2R expression by cycling cells.

In vivo dynamics of CD4-8- thymocytes. Proliferation, renewal and differentiation of different cell subsets studied by DNA biosynthetic labeling and surface antigen detection

The proliferative status of CD4-8- thymocyte subsets was determined by in vivo or in vitro labeling with bromodeoxyuridine (BrdUrd), a nonreutilized thymidine analogue detectable with a monoclonal antibody, simultaneously with relevant surface proteins. An actively cycling subset [J11d+, interleukin 2 receptor-positive (IL 2R+)] was defined, besides a relatively resting one (J11d-, Pgp1+, T cell receptor-positive). Continuous per os administration of BrdUrd showed that 85% only of CD4-8- thymocytes were labeled in 6 days confirming the existence of a relatively long-term resting subset. By contrast, CD4+8+ thymocytes were all labeled in 3-4 days. Observation of labeled CD4-8- cells after pulse labeling showed an immediate decrease of their absolute number per thymus, confirming their low autorenewal capacity. However, a small number of labeled cells which were hydroxyurea or colchicine resistant remained CD4-8- for several days and progressively acquired surface expression of IL 2R. IL 2R expression by cycling CD4-8- cells during thymus regeneration after antimitogenic drug treatment was rapid, but very transient. According to these results most CD4-8- thymocytes appear as largely engaged in a proliferation-dependent differentiative process, and do not behave as true stem cells. Consequently, this subset is principally renewed by thymic immigration of exogenously produced resting cells. However, a tenfold expansion of CD4-8- cells was found in the fetal and regenerating thymus, suggesting two proliferative phases during intrathymic CD4-8- cell maturation, the first one yielding to cell expansion and the second to cell differentiation. A tentative evaluation of daily cell immigration is proposed starting with the determination of the number of cells beginning DNA synthesis each day. A global model is finally discussed by confronting our kinetic results with the known reconstitution capacities of CD4-8- thymocyte subsets.

Sequential events in thymocyte differentiation and thymus regeneration revealed by a combination of bromodeoxyuridine DNA labeling and antimitotic drug treatment

The proliferative status of thymocyte cell subsets in vivo was assessed by observing the effect of two antimitotic drugs, hydroxyurea (HU) and demecolcine. Both drugs had the greatest effect on the double-positive (DP) subset followed by the L3T4+ single-positive (SP) subset. However, the decrease in the latter type was delayed by several days, showing that their precursors rather than the cells themselves were killed by HU. Double-negative (DN) cells were less affected, indicating that they contain a resting subset and that they are renewed by emigration rather than by autonomous in situ proliferation. After drug treatment all cycling cells were eliminated from the thymus but, as shown by in vivo and in vitro bromodeoxyuridine incorporation, new cells rapidly reentered in cycle, starting from DN cells and Lyt-2+ SP cells and followed by DP cells. Lyt-2+ SP cycling cells represent an intermediary stage between DN and DP cells, and they are very transient. Injection of HU 24 h after in vivo BrdUrd labeling eliminated most labeled DN cells, but did not prevent the emergence of L3T4+ SP-labeled cells on day 3 as observed in control thymuses. These results suggest that these L3T4+ SP cells are generated from DP cells in the absence of proliferation. Cycling cells in the regenerating thymus were first located at the corticomedullary junction and then in the subcapsular region, suggesting a reverse migration process to that observed after cessation of proliferation. A model is proposed to summarize these sequential events.

Sequential events in thymocyte differentiation and thymus regeneration revealed by a combination of bromodeoxyuridine DNA labeling and antimitotic drug treatment

The proliferative status of thymocyte cell subsets in vivo was assessed by observing the effect of two antimitotic drugs, hydroxyurea (HU) and demecolcine. Both drugs had the greatest effect on the double-positive (DP) subset followed by the L3T4+ single-positive (SP) subset. However, the decrease in the latter type was delayed by several days, showing that their precursors rather than the cells themselves were killed by HU. Double-negative (DN) cells were less affected, indicating that they contain a resting subset and that they are renewed by emigration rather than by autonomous in situ proliferation. After drug treatment all cycling cells were eliminated from the thymus but, as shown by in vivo and in vitro bromodeoxyuridine incorporation, new cells rapidly reentered in cycle, starting from DN cells and Lyt-2+ SP cells and followed by DP cells. Lyt-2+ SP cycling cells represent an intermediary stage between DN and DP cells, and they are very transient. Injection of HU 24 h after in vivo BrdUrd labeling eliminated most labeled DN cells, but did not prevent the emergence of L3T4+ SP-labeled cells on day 3 as observed in control thymuses. These results suggest that these L3T4+ SP cells are generated from DP cells in the absence of proliferation. Cycling cells in the regenerating thymus were first located at the corticomedullary junction and then in the subcapsular region, suggesting a reverse migration process to that observed after cessation of proliferation. A model is proposed to summarize these sequential events.

Localization and phenotype of cycling and post-cycling murine thymocytes studied by simultaneous detection of bromodeoxyuridine and surface antigens

Bromodeoxyuridine (BrdUrd) was incorporated in vivo or in vitro into the DNA of proliferating murine thymocytes. Surface antigens Thy1, Lyt2 (CD8), L3T4 (CD4), interleukin-2 receptor (IL2-R), and the V beta 8 chain of the T-cell receptor were detected using specific monoclonal antibodies with the biotin-avidin system, and cells were then treated for DNA denaturation. Simultaneous detection of BrdUrd and surface markers was performed on cell smears and frozen sections by double-color immunofluorescence. The phenotype of cycling cells, determined in fetal thymus and in the thymus of mice from birth to one year of age, showed relative stability after the initial growth period, despite severe involution of the gland. Phenotypic evolution of cycling cells and their progeny was also studied in colchicine-treated animals and was shown to reproduce sequential events of T-cell differentiation. On sections, the highest frequency of cycling cells was observed in the outer cortex in normal thymus, but the first cells to start proliferation during regeneration were mostly located in the deep cortex and corticomedullary junction. These results show the high potential of this method, as compared to autoradiography of radiolabeled cells.

Control of prothymocyte proliferation by thymic accessory cells

All thymocyte subpopulations derive from intrathymic precursors which are double negative (DN) for Lyt-2 and L3T4 differentiation antigens. Although nearly half of DN cells express a receptor for interleukin 2 (IL 2R), they respond poorly to IL 2. DN cell proliferation can be obtained in the presence of various exogenous stimuli, but the in vivo signal for DN cell response to IL 2 remains unclear. We show in the present report that phagocytic cells of the thymic reticulum are able to induce the proliferation of DN thymocytes in the presence of recombinant IL 2 (rIL 2). Cell-to-cell contact is needed for this effect. Antibodies directed against class I MHC antigens but not against class II can inhibit DN cell proliferation. DNA-synthetizing cells were labeled by incubation with 10 microM bromodeoxyuridine either before or at various times during the culture period. Bromodeoxyuridine was then detected in the DNA of proliferating cells and/or their progeny already stained with anti-Lyt-2 and L3T4 antibodies. During the initial 16 h and independently of culture conditions, 16-25% of the cells expressed surface antigens and 50-65% of them derived from DN cells which were in S phase just before culture; these differentiated cells had a very short life span. In the second culture period, the presence of both rIL 2 and thymic accessory cells was necessary for cell survival. In these conditions, DN cell number and proliferation rate were constant and a low number of Lyt-2+ and/or L3T4+ cells was continuously generated. Thymic accessory cells therefore appear to provide the signal(s) necessary for IL 2-induced proliferation of thymocyte precursors. Implications of these findings for normal in vivo intrathymic proliferation and differentiation are discussed.

In vivo thymocyte maturation. BUdR labeling of cycling thymocytes and phenotypic analysis of their progeny support the single lineage model

Spontaneously cycling thymocytes have been labeled in vitro and in vivo by bromodeoxyuridine (BUdR), a non-reutilized precursor of DNA that is detectable by a monoclonal antibody. Studies of BUdR-labeled cells have included the determination of their anatomical location, size, and nuclear aspects and of their cell surface phenotype. Dividing blasts were initially located in the cortex (mainly but not exclusively in the subcapsular region) and expressed the double-negative (Lyt-2- L3T4-) and double-positive (Lyt-2+ L3T4+) phenotypes. The fate of these cells have been determined in days after BUdR administration, and we observed an initial double-negative to double-positive transition that was followed by the death of the majority of labeled cells in the cortex. As of day 3, the few surviving cells acquired a mature helper phenotype (Lyt-2- L3T4+) and began migrating into the thymic medulla. The exclusive medullary location of blast cell progeny was observed between days 5 and 10 post-BUdR administration. These results suggest a direct precursor-product relationship between dividing cortical cells and mature medullary thymocytes, and therefore support the single lineage model of intrathymic differentiation.

In vivo thymocyte maturation. BUdR labeling of cycling thymocytes and phenotypic analysis of their progeny support the single lineage model

Spontaneously cycling thymocytes have been labeled in vitro and in vivo by bromodeoxyuridine (BUdR), a non-reutilized precursor of DNA that is detectable by a monoclonal antibody. Studies of BUdR-labeled cells have included the determination of their anatomical location, size, and nuclear aspects and of their cell surface phenotype. Dividing blasts were initially located in the cortex (mainly but not exclusively in the subcapsular region) and expressed the double-negative (Lyt-2- L3T4-) and double-positive (Lyt-2+ L3T4+) phenotypes. The fate of these cells have been determined in days after BUdR administration, and we observed an initial double-negative to double-positive transition that was followed by the death of the majority of labeled cells in the cortex. As of day 3, the few surviving cells acquired a mature helper phenotype (Lyt-2- L3T4+) and began migrating into the thymic medulla. The exclusive medullary location of blast cell progeny was observed between days 5 and 10 post-BUdR administration. These results suggest a direct precursor-product relationship between dividing cortical cells and mature medullary thymocytes, and therefore support the single lineage model of intrathymic differentiation.

A trans-acting mechanism represses the expression of the major transplantation antigens in mouse hybrid thymoma cell lines

We have fused an H-2- thymoma (BM5R.9) with an H-2+ thymoma (BW5147) and have found that many of the resulting hybrids exhibit an H-2- phenotype. In several hybrids that were analyzed in detail, this phenotype is related to the absence of steady-state H-2 mRNA and shows some instability, possibly related to the loss of chromosomes in segregants. We conclude from our studies that BM5R.9 cells display a trans-acting mechanism that can repress the expression of H-2 antigens, and that the gene(s) causing the repression are not located on chromosome 17. This mechanism is not sufficient to explain the H-2- phenotype of BM5R.9, for which an additional, cis-acting process, must be postulated. We discuss these results in the context of the regulation of expression of the major class I transplantation antigens.

Regulation of thymocyte proliferation and survival by deoxynucleosides. Deoxycytidine produced by thymic accessory cells protects thymocytes from deoxyguanosine toxicity and stimulates their spontaneous proliferation

Deoxyguanosine (dGuo) inhibits thymic blast DNA synthesis and then induces thymocyte cell death. The dGuo inhibitory action, measured with four different assays (spontaneous thymidine incorporation, immunofluorescent detection of 5-bromodeoxyuridine incorporation, dye exclusion, tetrazolium salt cleavage), was suppressed in the presence of supernatants from cultures containing phagocytic cells. In particular, we studied the anti-dGuo activity in supernatants from thymic reticulum cultures (TRS) and in those from phagocytic cells isolated from TR cultures (P-TR). The anti-dGuo substance was identified as deoxycytidine (dCyd) by high performance liquid chromatography and other physicochemical studies. Secretion of dCyd by P-TR is accompanied by thymidine but not by purine nucleoside secretion. A dual mechanism of thymocyte rescue by dCyd was demonstrated by a study of the dose-dependencies of dCyd-mediated prevention and reversal, respectively, of the dGuo inhibition. In addition to this exogenously added anti-dGuo action, dCyd and dCyd-containing TRS induced significant stimulation of spontaneous thymic blast proliferation, and the kinetics of both effects were identical. These findings might suggest that a major role of thymic phagocytic cells is the supply of pyrimidine nucleosides to thymocytes resulting in the maintenance of proliferation and protection of at least some thymic blasts from the toxic effects of dGTP accumulation. The role of this system in the intrathymic selection process is discussed.

Relationships between terminal transferase expression, stem cell colonization, and thymic maturation in the avian embryo: studies in thymic chimeras resulting from homospecific and heterospecific grafts

Terminal deoxynucleotidyl transferase (TdT) can be detected in 11- to 12-day-old embryonic chick thymuses 5 to 6 days after the first influx of lymphoid stem cells into the thymic rudiment. To identify the main factors of TdT induction, grafting experiments were devised in such a way that the age of the grafted thymus and that of the host were different. Uncolonized embryonic chick thymuses were grafted into chick hosts of different ages. Under these conditions, lymphoid differentiation arose from host lymphoid stem cells (LSC) invading the thymic rudiment. TdT immunofluorescent detection in the first wave of thymocytes showed that the percentages of TdT+ cells were related to the total age of the explant and not to the age of the host (11 to 17 days). Similar results were obtained when the chick thymic rudiment was transplanted into quail embryos, showing that quail LSC have TdT inducibility similar to that of chick LSC while developing in a chick thymic environment. Colonized chick thymuses were also grafted into quail embryos to compare the TdT inducibility of the first lymphoid generation (of chick type) and of the second (of quail origin), taking advantage of the different chromatin structure of quail and chick cells. In these experiments, the majority of chick cells remained TdT negative for as long as 10 days, whereas most lymphocytes of the second generation became TdT+ soon after their arrival in the grafted thymus. Therefore, during embryonic life, most TdT+ cells were derived from the second wave of stem cells, but some early stem cells were also able to acquire the enzyme. In a final series of experiments, early thymic rudiments were cultured in vitro with 14- to 16-day-old bone marrow and then grafted into 3-day-old host embryos. Under these conditions, bone marrow LSC contributed to a variable proportion of the first generation of thymocytes. The percentage of TdT+ cells among the progeny of these bone marrow stem cells was found to be two times higher than that of thymocytes derived from host LSC. These results suggest that, in addition to intrathymic environmental factors, the origin of LSC influences the frequency of TdT expression in their progeny.

Relationships between terminal transferase expression, stem cell colonization, and thymic maturation in the avian embryo: studies in thymic chimeras resulting from homospecific and heterospecific grafts

Terminal deoxynucleotidyl transferase (TdT) can be detected in 11- to 12-day-old embryonic chick thymuses 5 to 6 days after the first influx of lymphoid stem cells into the thymic rudiment. To identify the main factors of TdT induction, grafting experiments were devised in such a way that the age of the grafted thymus and that of the host were different. Uncolonized embryonic chick thymuses were grafted into chick hosts of different ages. Under these conditions, lymphoid differentiation arose from host lymphoid stem cells (LSC) invading the thymic rudiment. TdT immunofluorescent detection in the first wave of thymocytes showed that the percentages of TdT+ cells were related to the total age of the explant and not to the age of the host (11 to 17 days). Similar results were obtained when the chick thymic rudiment was transplanted into quail embryos, showing that quail LSC have TdT inducibility similar to that of chick LSC while developing in a chick thymic environment. Colonized chick thymuses were also grafted into quail embryos to compare the TdT inducibility of the first lymphoid generation (of chick type) and of the second (of quail origin), taking advantage of the different chromatin structure of quail and chick cells. In these experiments, the majority of chick cells remained TdT negative for as long as 10 days, whereas most lymphocytes of the second generation became TdT+ soon after their arrival in the grafted thymus. Therefore, during embryonic life, most TdT+ cells were derived from the second wave of stem cells, but some early stem cells were also able to acquire the enzyme. In a final series of experiments, early thymic rudiments were cultured in vitro with 14- to 16-day-old bone marrow and then grafted into 3-day-old host embryos. Under these conditions, bone marrow LSC contributed to a variable proportion of the first generation of thymocytes. The percentage of TdT+ cells among the progeny of these bone marrow stem cells was found to be two times higher than that of thymocytes derived from host LSC. These results suggest that, in addition to intrathymic environmental factors, the origin of LSC influences the frequency of TdT expression in their progeny.

Terminal deoxynucleotidyl transferase during the development of chicken thymus

Antibodies specific for chicken terminal deoxynucleotidyl transferase (TdT) were used to develop immunoperoxidase and immunofluorescence assays. The cellular distribution and localisation of TdT during the development of chicken thymus were studied. TdT began to appear in the embryonic thymus in the cytoplasm of large cells, between 11 and 12 days of incubation. Thereafter, the proportion of TdT-positive cells increased and TdT was detected in both nucleus and cytoplasm. The first appearance of TdT positive cells, their increasing proportion and the intracellular localisation of TdT will be discussed in correlation with the developmental stages of the thymus.

Subsets of malignant lymphomas in children related to the cell phenotype

We studied the lymphomatous cells of 39 children presenting with the classical features of malignant lymphoma. Twenty-two had T lymphoblasts. We could classify these patients into three subsets: The T lymphoblasts from children group 1 displayed antigen(s) shared by a thymocyte subpopulation, had terminal deoxynucleotidyl transferase (TDT), but no affinity for peanut agglutinin (PNA). The T lymphoblasts from children group 2 lacked the thymocyte antigen(s), had no TDT, but showed affinity for PNA. The T lymphoblasts from children group 3 displayed mature T-cell antigens, had no TDT, and no affinity for PNA. Children from the three groups were similar in terms of clinical presentation, age and sex distribution, and cell morphology; however patients from the three groups might have a different prognosis. Fourteen children had B lymphoblasts that, in half of the cases, had affinity for Helix pomatia agglutinin. Three patients had lymphoblasts lacking specific marker. Two of them had cells displaying an antigen found on common acute lymphoblastic leukemia cells and had TDT.

Human T cell differentiation antigens and correlation of their expression with various markers of T cell maturation

Two sets of differentiation antigens are demonstrated on human T cells by using 11 heterologous anti-human antisera raised against various normal and malignant T cells. The two antigenic determinants from the first set of differentiation antigens are expressed only on thymus cells and on T lymphoblasts, whereas the two antigenic determinants from the second set are expressed on blood T cells, Sezary cells, T.CLL cells, and thymus cells. Four T cell phenotypes are thus defined; two phenotypes are expressed only by T lymphoblasts, whereas the other two phenotypes are expressed both by normal and malignant T cells. Moreover, a clear-cut relationship exists between the four T cell antigenic phenotypes and two other markers of T cell differentiation: terminal deoxynucleotidyl transferase and peanut agglutinin. Two phenotypes are linked with the presence of TdT, one phenotype is linked with the affinity for PNA, and the fourth phenotype is correlated with the absence of both markers.

Acute lymphoblastic leukemia with pre-B-cell characteristics

Blast cells from 6 of 50 patients with acute lymphoblastic leukemia (ALL) displayed intracytoplasmic mu chains in the absence of detectable light chains and surface immunoglobulins. These cells also expressed lalike and common ALL antigens. Terminal deoxynucleotidyltransferase was detectable in 2 of 5 cases tested. These blast cells are probably related to early B-cell precursors (pre-B cells). In 4 of 6 cases the disease had a tumoral presentation; the prognostic significance of this new subgroup, which accounts for 20% of patients with non-T non-B ALL, remains to be established.