Increased frequency of micronuclei in B and T8 lymphocytes from smokers

The frequency of micronuclei was measured in peripheral B lymphocytes and some T lymphocyte subpopulations from 5 medium-tar cigarette smokers, and 5 non-smokers with no regular exposure to tobacco smoke. The peripheral lymphocytes were stimulated in vitro with phytohemagglutinin and B lymphocytes and the various T lymphocyte subsets were classified by a recently developed MAC (Morphology, Antibody, Chromosomes) method which allows the immunologic identification of different cell lineages. An increased frequency of micronuclei was observed in B and especially in the suppressor/cytotoxic T8 lymphocytes from smokers, as compared with non-smoker values. In non-smoker cultures, no differences in the frequency of micronuclei were observed among the different T lymphocyte subsets. In these cultures, B cells tended to have a higher frequency of micronuclei than did T cells. The proportions of B cells and the various T subpopulations among mitotic and interphasic lymphocytes from smokers and non-smokers were also determined. The proportions of B cells and T cell subsets among all mitotic lymphocytes were similar in smokers and non-smokers. Contrarily, a significant decrease in the proportion of T8 lymphocytes among all interphasic lymphocytes was observed in cultures derived from smokers.     

In vitro response of subpopulations of human lymphocytes. II. DNA synthesis induced by anti-immunoglobulin antibodies.

A significant and constant increase in DNA synthesis was observed in human lymphocytes cultured in the presence of purified anti-immunoglobulin antibodies specific for human IgG, IgA, and IgM. This has been found in cultures of lymphocytes isolated from blood, tonsils, spleen, and lymph nodes. The optimal culture conditions for blood and tonsil lymphocytes were determined. As a rule 6-day cultures containing 2 x 10(6) cells/ml and 100 mug/ml of antibody yielded the highest 3H-thymidine uptake. Purified T cell cultures could not be stimulated, whereas a low response could be observed in most of the purified B cell cultures. Optimal culture conditions were the same for the B and total tonsil lymphocytes. However, when the purified B cells were totally depleted of T cells, no response was observed. A T and B cell synergy has been demonstrated by supplementing B cell cultures with purified T cells, whether treated or not with mitomycin. These experiments indicated a permissive and potentiating effect of T cells on the B cell response. Cultures containing mitomycin-treated B cells and purified T cells (mB + T) could be stimulated by a-Ig, thus indicating a T cell proliferation. In keeping with this finding was the observation of an increased response of total lymphocytes supplemented with T cells but not with B cells. Adherent cells are necessary for an optimal response to a-Ig; they enhanced the B cell proliferation observed in (Tm + B) cultures and suppressed the response of T cells in (T + Bm) cultures.     

Immunostimulation by cytomegalovirus (CMV): helper T cell-dependent activation of immunoglobulin production in vitro by lymphocytes from CMV-immune donors

Cytomegalovirus (CMV) is the cause of a number of different diseases ranging from self-limited benign infections in healthy adults to life threatening illnesses among immunocompromised hosts and newborns. Suppression of cell-mediated immunity is often found in cases of acute CMV infection, and in addition, the virus may also be a potent stimulant of lymphoid cells in vivo. We studied cellular proliferation and immunoglobulin (Ig) production induced by CMV to determine its effect on human lymphocytes in vitro. The CMV that was added to cultures of lymphocytes from CMV-seronegative donors failed to induce either significant cellular proliferation or Ig production. By contrast, CMV-stimulated cultures from CMV-seropositive donors induced both prominent cellular proliferation and Ig production. B cell differentiation into Ig-secreting cells required the presence of T cells, and this T cell help was sensitive to irradiation with 2000 rad and to treatment with cyclosporin A. When T cells were depleted of OKT4+ cells with monoclonal antibody and complement, the co-cultured B cells failed to produce Ig, whereas the depletion of OKT8+ cells had no effect on the Ig-secreting cell response. Inactivation of CMV before culture did not result in a reduction of either cellular proliferation or Ig production. Thus, infection of target cells is not required for in vitro lymphocyte activation by CMV. These results demonstrate that CMV is a potent activator of B cells inducing Ig production in vitro, and that this process requires the presence of virus-specific memory T cells.     

Cell lines derived from human autorosette-forming cells with suppressive activity

Concanavalin-A-stimulated human T lymphocytes from healthy donors and from patients suffering from diverse immune disorders were fractionated into rosette-forming (R) and nonrosette-forming (NR) cells. The separation method is based upon the ability of the lymphocytes to bind autologous erythrocytes and form autorosettes. Long-term cultures of the R and NR subpopulations were established. The activity of the culture supernatants on the T cell proliferation of normal human phytohemagglutinin (PHA)-induced lymphocytes and of a murine, interleukin-2 (IL-2)-dependent cytotoxic T cell line (CTLL) was investigated. Only the R cell line-derived supernatants from almost all patients tested evinced potent suppressor activity, those from healthy donors less so. The suppressive function was demonstrated not to be due to a cytotoxic effect since preincubation of the PHA-induced lymphocytes and CTLL cells with the factor did not diminish their proliferative capacity. Our study indicates the existence of a competitive relationship between the suppressor factor and IL-2. We found that inhibition of the proliferation decreased with the addition of increasing quantities of exogenous IL-2. We also observed that preincubating the CTLL cells with IL-2 prior to exposing them to the suppressive factor precludes inhibition of their proliferation. Phenotypic analysis of the suppressor cell line revealed that they were comprised of a T cell population which included OKT4+ and OKT8+ cells and that 99% of the cells formed autorosettes. Preliminary purification of the suppressive factor was performed by ultrafiltration and maximal suppression was exhibited by the fraction of less than 10,000 daltons. The development of suppressor cell lines from the unique population of autologous rosette-forming cells may be very helpful in studying the immunoregulatory properties of these cells and their suppressor activity.     

Isolation of human mononuclear cell subsets by counterflow centrifugal elutriation (CCE). I. Characterization of B-lymphocyte-, T-lymphocyte-, and monocyte-enriched fractions by flow cytometric analysis

Rapid separation of large numbers of human peripheral blood mononuclear cells into fractions enriched for B lymphocytes, T lymphocytes, or monocytes was accomplished by counterflow centrifugal elutriation (CCE). The first fraction contained 98% of the platelets. Ten additional fractions containing subpopulations of mononuclear cells were collected by sequential increases in the flow rate while maintaining a constant centrifuge speed. Analysis of the fractions using monoclonal antibodies revealed that fraction 2, which was free of esterase-positive monocytes, was highly enriched for B cells. T lymphocytes (OKT3+) were the predominant cell type found in fraction 4. No enrichment for T-lymphocyte-helper (OKT4+) or -suppressor (OKT8+) subpopulations was observed in the lymphocyte containing fractions. Three fractions (7-9), highly enriched for esterase-positive cells, were predominantly OKM1+ monocytes with no evidence of selective separation of monocyte subpopulations. Thus, cell fractions enriched for B cells, T cells, and monocytes could be obtained, by utilizing CCE, in large enough quantities to enable analysis of their functional properties. Of particular interest was the ability to separate small, resting B lymphocytes from monocytes.     

Micronucleus formation in different lymphocyte subpopulations in peplomycin-treated and control cultures

Spontaneous micronucleus formation and micronucleus induction by peplomycin in B and T lymphocytes was studied by a recently developed MAC (Morphology, Antibody, Chromosomes) method allowing the immunologic identification of different cell lineages. Blood samples from 3 healthy donors were cultured in the presence of phytohaemagglutinin and pokeweed mitogen. An increased frequency of micronuclei was observed in peplomycin cultures compared with controls. B cells were found to be more sensitive to peplomycin induction than were T lymphocytes. In control cultures, pokeweed mitogen yielded a higher frequency of micronuclei than did phytohaemagglutinin. In both pokeweed- and phytohaemagglutinin-stimulated cultures, B cells showed a higher frequency of micronuclei than did T cells. The relative proportion of mitotic B cells was equal in pokeweed and phytohaemagglutinin cultures. In peplomycin cultures, the proportion of B cells decreased as compared with control cultures.     

Effects of Pulsed Electromagnetic Fields on the Proliferation of Lymphocytes from Aids Patients,HIV-Seropositive Subjects,and Seronegative Drug Users

The effect of the exposure of mitogen-stimulated lymphocytes from subjects infected by human immunodeficiency virus to extremely low frequency pulsed electromagnetic fields (PEMFs) was studied, by evaluating the incorporation of tritiated thymidine, the expression of IL-2 receptor, and the amount of activated T lymphocytes. Four groups of subjects were considered patients with acquired immunodeficiency syndrome (AIDS), asymptomatic seropositive subjects, seronegative drug users, and young healthy controls. PEMFs increased cell proliferation only in the group of healthy controls, as measured at the 72nd hour of culture, but an increase in the number of activated T lymphocytes was observed by cytofluorimetric analysis after 18 hrs of PEMF exposure in cultures from AIDS patients.     

Promotion of hematopoietic stem cell differentiation in vitro by a soluble mediator,allogeneic effect factor

This study was designed to investigate the effects of allogeneic effect factor (AEF), a soluble mediator derived from short-term mixed lymphocyte cultures (MLC) of in vitro alloantigen-primed T cells, on cultures of murine bone marrow cells. Cultures established under suboptimal conditions namely, in the absence of a pre-established adherent cell layer as required in conventional Dextertype cultures–declined and lost their stem cell activity rapidly. In contrast, supplementation of these cultures, at initiation and thereafter, with AEF, but not with T cell growth factor (TCGF), induced cell growth and proliferation for several weeks. Such AEF-supplemented cultures exhibited cellular heterogeneity and stem cell activity for significantly longer periods than the control cultures. Even in conventional Dexter cultures, established under optimal conditions, AEF had a beneficial effect on cellular growth and proliferation and myeloid progenitor cell (CFU-C) activity. Furthermore, cells capable of synergizing with suboptimal numbers of mature T cells in con A-induced mitogenic responses, shown by others to be pre-T cells, were detected in the AEF-supplemented cultures for several weeks.     

A VH11V kappa 9 B cell antigen receptor drives generation of CD5+ B cells both in vivo and in vitro

B lymphocytes can be divided into different subpopulations, some with distinctive activation requirements and probably mediating specialized functions, based on surface phenotype and/or anatomical location, but the origins of most of these populations remain poorly understood. B cells constrained by transgenesis to produce an Ag receptor derived from a conventional (B-2) type cell develop a B-2 phenotype, whereas cells from mice carrying a B-1-derived receptor acquire the B-1 phenotype. In this study transgenic enforced expression of a B cell receptor (mu/kappa) originally isolated from a CD5+ (B-1a) B cell generates B-1 phenotype cells in bone marrow cultures that show a distinctive B-1 function, survival in culture. Despite their autoreactivity, we find no evidence for receptor editing or that the paucity of B-2 cells is the result of tolerance-induced selection. Finally, Ca2+ mobilization studies reveal a difference between transgenic B-1 cells in spleen and peritoneal cavity, with cells in spleen much more responsive to anti-B cell receptor cross-linking. We discuss these results in terms of specificity vs lineage models for generation of distinctive B cell subpopulations.     

Replication of herpes simplex virus and cytomegalovirus in human leukocytes.

Human peripheral blood leukocytes, lymphocyte subpopulations, and hemic cell lines were examined for their ability to supprot HSV and CMV replication. Mitogen-stimulated mononuclear leukocytes, B lymphocytes, and T lymphcytes supported the replication of HSV to high titers over 3 to 5 days of infection. HSV replicated in unstimulated mononuclear leukocyte cultures of one of five donors, and to a limited degree in untreated B lymphocytes of three of five donors; HSV replication was not detected in unstimulated T lymphocytes (five donors). There was no evidence of enhanced uptake of 3H-thymidine in the untreated donor cells that replicated HSV. CMV replication was not detected during 9 to 10 days of infection in untreated or mitogen-treated mononuclear leukocytes and lymphocyte subpopulations from the same adult donors or in neonatal cord blood leukocytes. The ability of the cells to support HSV or CMV replication did not correlate with the presence of specific antiviral antibodies in the donor serum. HSV replication in B, T, and myeloid cell lines to high titers over 5 days of infection, whereas CMV failed to replicate in any of the hemic cell lines. A persistent HSV infection has been established in a T cell line (CEM) with high titers of infectious virus being produced concurrently with growth of the cells over the first 11 weeks of infection.     

T cell-mediated immunoregulation of Epstein Barr virus- (EBV) induced B lymphocyte activation in EBV-seropositive and EBV-seronegative individuals

Infection with Epstein Barr virus (EBV) is accompanied by seroconversion and life-long persistence of the virus in B lymphocytes. During acute EBV-induced infectious mononucleosis, suppressor T cells become activated, which may provide an additional mechanism of host defense against the causative agent. When cultures of lymphocytes from normal adults seropositive for EBV were stimulated with the B95-8 strain of EBV, purified B cells produced increasingly higher numbers of immunoglobulin- (Ig) secreting cells, whereas in co-cultures of autologous B and T cells a profound suppressor T cell activity inhibited further B cell activation after 10 to 12 days in culture. No such T cell-mediated inhibitory effect was seen in cultures of lymphocytes obtained from normal adults seronegative for EBV, indicating a correlation between the suppressor effect with evidence of prior immunity to this virus. The T cell-mediated suppression in patients with infectious mononucleosis is characterized by an early-acting inhibitory effect on B cell differentiation that is not specific in that all polyclonal B cell activators are inhibited, whereas in EBV-seropositive normal subjects suppression is delayed in time and affects only EBV-activated cultures. These data indicate that after infection with EBV, immunoregulatory T cells are generated that are capable of inhibiting further EBV-induced activation of autologous B cells and thus may provide an additional unique mechanism of host defense against persisting EBV-infected B cells.     

Direct measurement of T cell subset kinetics in vivo in elderly men and women

The age-associated decline in immunocompetence is paralleled by changes in the proportions of PBL subpopulations. In turn, the size and composition of the peripheral lymphocyte pool is determined by input from the thymus and bone marrow and by the balance of proliferation and death in each lymphocyte subpopulation. We compared the kinetics of lymphocyte subtypes in young (seven of eight CMV seronegative) and healthy elderly human subjects (six of seven CMV seropositive), using deuterated glucose DNA labeling in vivo to measure rates of T cell proliferation and disappearance. For CD45RO(+) cells of both CD4(+) and CD8(+) subtypes and for CD4(+)CD45RA(+) cells the kinetics of proliferation and disappearance were remarkably similar between elderly and young subjects. In the young, the kinetics of CD8(+)CD45RA(+) cells with a naive phenotype resembled those of CD4(+)CD45RA(+) cells. However, CD8(+)CD45RA(+) T cells from the elderly exhibited a predominantly primed phenotype, and for this subset, although the proliferation rate was similar to that of other CD45RA(+) cells, the disappearance rate of labeled cells was greatly decreased compared with that of all other T cell subsets. Our data provide a direct demonstration that there are no substantial changes in in vivo kinetics for most T cell populations in healthy elderly compared with young subjects. However, primed CD8(+)CD45RA(+) cells show unusual kinetic properties, indicating the persistence of these cells in the blood and dissociation of proliferation from disappearance.     

Spontaneous immortalisation of Schwann cells in culture: short-term cultured Schwann cells secrete growth inhibitory activity.

In the developing peripheral nerve, Schwann cells proliferate rapidly and then become quiescent, an essential step in control of Schwann cell differentiation. Cell proliferation is controlled by growth factors that can exert positive or inhibitory influences on DNA synthesis. It has been well established that neonatal Schwann cells divide very slowly in culture when separated from neurons but here we show that when culture was continued for several months some cells began to proliferate rapidly and non-clonal lines of immortalised Schwann cells were established which could be passaged for over two years. These cells had a similar molecular phenotype to short-term cultured Schwann cells, except that they expressed intracellular and cell surface fibronectin. The difference in proliferation rates between short- and long-term cultured Schwann cells appeared to be due in part to the secretion by short-term cultured Schwann cells of growth inhibitory activity since DNA synthesis of long-term, immortalised Schwann cells was inhibited by conditioned medium from short-term cultures. This conditioned medium also inhibited DNA synthesis in short-term Schwann cells stimulated to divide by glial growth factor or elevation of intracellular cAMP. The growth inhibitory activity was not detected in the medium of long-term immortalised Schwann cells, epineurial fibroblasts, a Schwannoma (33B), astrocytes or a fibroblast-like cell-line (3T3) and it did not inhibit serum-induced DNA synthesis in epineurial fibroblasts, 33B cells or 3T3 cells. The activity was apparently distinct from transforming growth factor-beta, activin, IL6, epidermal growth factor, atrial natriuretic peptide and gamma-interferon and was heat and acid stable, resistant to collagenase and destroyed by trypsin treatment. We raise the possibility that loss of an inhibitory autocrine loop may contribute to the rapid proliferation of long-term cultured Schwann cells and that an autocrine growth inhibitor may have a role in the cessation of Schwann cell division that precedes differentiation in peripheral nerve development.     

Production of IL 2 and IFN-gamma by T cells from malaria patients in response to Plasmodium falciparum or erythrocyte antigens in vitro

T cells from patients acutely infected with malaria exhibit a disease-related stimulation of DNA synthesis in response to Plasmodium falciparum antigen in vitro. This response is weak and short-lived, suggestive of induction of suppressor mechanisms. Exogenous T cell growth factor (IL 2) that was added to antigen-stimulated T cell cultures enhanced proliferation in antigen-responsive cultures, indicating that the lymphocytes expressed IL 2 receptors. In contrast, the addition of IL 2 to cultures that did not respond to antigen had no effect. Antigen-responsive cultures contained endogenous IL 2 as well, and the antigen-induced lymphocyte proliferation was correlated with IL 2 production. However, the results suggested that IL 2 production by the patients' T cells was insufficient or actively shut off, and that this was responsible for the premature cessation of their DNA synthesis. Supernatants from 60% of the T cell cultures treated with malaria antigen and from 30% treated with RBC ghost antigen contained interferon-gamma (IFN-gamma), as determined by a cytopathic effect inhibition assay combined with acid treatment and antibody neutralization or by an IFN-gamma-specific ELISA. There was no obvious correlation between antigen-induced lymphocyte proliferation and the presence of IFN-gamma in the culture supernatants. A high IFN-gamma activity was also seen in antigen-treated cultures from P. falciparum-immune donors living in highly endemic malaria areas. In contrast, no IFN-gamma was found in supernatants of antigen-treated T cells from healthy donors or patients with Plasmodium vivax malaria. Thus, the IFN-gamma activity of these cultures appears to reflect the presence of antigen-reactive T cells and may be useful as a sensitive indicator of cellular immunity in P. falciparum malaria.     

Inhibition of the in vitro outgrowth of Epstein-Barr virus-infected lymphocytes by TG lymphocytes.

Human T lymphocytes subpopulations from subjects not acutely infected with EBV have been selected and examined for ability to inhibit the outgrowth of autologous B lymphocytes that have undergone in vitro infection with EB virus. The results show that only TG lymphocytes are inhibitory. TG lymphocytes from subjects who are EBV antibody-negative inhibit as well as those from subjects who have EBV antibody. TG lymphocyte populations, as well as other T cell fractions obtained from neonatal subjects, fail to inhibit the outgrowth of infected, autologous lymphocytes under the conditions tested. We propose that NK cells are responsible for the inhibitory effects described in this report.     

B-lymphocyte dysfunction in chronic HIV-1 infection does not prevent cross-clade neutralization breadth

Aberrant expression of regulatory receptors programmed death-1 (PD-1) and B- and T-lymphocyte attenuator (BTLA) is linked with dysregulation and exhaustion of T lymphocytes during chronic human immunodeficiency virus type 1 (HIV-1) infection; however, less is known about whether a similar process impacts B-lymphocyte function during HIV-1 infection. We reasoned that disruption of the peripheral B cell compartment might be associated with decreased neutralizing antibody activity. Expression of markers that indicate dysregulation (BTLA and PD-1), immune activation (CD95), and proliferation (Ki-67) was evaluated in B cells from HIV-1-infected viremic and aviremic subjects and healthy subjects, in conjunction with immunoglobulin production and CD4 T cell count. Viral load and cross-clade neutralizing activity in plasma from viremic subjects were also assessed. Dysregulation of B lymphocytes was indicated by a marked disruption of peripheral B cell subsets, increased levels of PD-1 expression, and decreased levels of BTLA expression in viremic subjects compared to aviremic subjects and healthy controls. PD-1 and BTLA were correlated in a divergent fashion with immune activation, CD4 T cell count, and the total plasma IgG level, a functional correlate of B cell dysfunction. Within viremic subjects, the total IgG level correlated directly with cross-clade neutralizing activity in plasma. The findings demonstrate that even in chronically infected subjects in which B lymphocytes display multiple indications of dysfunction, antibodies that mediate cross-clade neutralization breadth continue to circulate in plasma.     

Impaired regulation of Epstein-Barr virus-induced lymphocyte proliferation in rheumatoid arthritis is due to a T cell defect

The rate of outgrowth of EBV-infected B lymphocytes is regulated by normal T lymphocytes. Removal of T cells from normal whole lymphoid populations (PBM) markedly shortens the outgrowth time of the remaining B lymphocytes. There is little difference in the much more rapid outgrowth of rheumatoid PBM after the removal of T cells, which suggests that RA lymphoid cells are unable to regulate this process. To determine whether RA T cells are defective, or EBV-infected RA B cells are unresponsive to regulatory signals, EBV-induced outgrowth in autologous and allogeneic mixtures of RA and normal B and T cells was evaluated, employing morphologic criteria and 3H-thymidine incorporation. The difference in outgrowth between RA and normal PBM was reproduced by reconstitution of EBV-infected B cells with mitomycin-treated autologous T cells. In cell-mixing experiments, normal T cells appropriately regulated both normal and RA B cells similarly, whereas RA T cells were defective in regulating either B cell population. Thus, the rapid outgrowth of EBV-infected rheumatoid lymphoid cells is due to defective T cell regulation. Moreover, normal regulation does not require cell proliferation.     

Study of Response of Thymic and Submaxillary Lymph Node Lymphocytes to Administration of Lead by Different Routes

A number of studies have reported that heavy metals are not only toxic for the organism but they may modulate immune responses. In the current study, the effect of 4-week administration of 200 ppm of PbAc₂, using different routes of administration (orally and intraperitoneal injection), on lymphatic organs was evaluated. In the thymus, the number of lymphocyte cells and the cellularity diminished significantly for both routes of treatment. Regarding the submaxillary lymph nodes, no significant variations took place. Cell-mediated immune response is commonly evaluated by cell proliferation assays. Mitogens are known to induce a vigorous proliferative response in lymphoid cells from mammals. An increase in the proliferation of T lymphocytes stimulated by concanavalin A and the proliferation of B lymphocytes stimulated with lipopolysaccharides was found in thymus for both routes of administration, whereas in the lymph nodes, there was a decrease in proliferation of T lymphocytes. Furthermore, lead administration by intraperitoneal route caused an effect on B and T lymphocyte subpopulations. Thus, there was an increase in B+ cells and a decrease in T+ cells. Regarding CD4+ and CD8+ T cells, there were only variations, concretely a drop in both subpopulations, in lymph nodes when lead was administered intraperitoneally. It is important to emphasize that an increase in apoptosis was found in this tissue. At the histological level, evident alterations were described in thymus both for the oral and for the intraperitoneal route. Therefore, it is possible to show that lead administered by both routes generated effects on an immunological level.     

Ecto-5'-nucleotidase activity in T and B lymphocytes from normal subjects and patients with congenital X-linked agammaglobulinemia.

Ecto-5'-nucleotidase activity was measured in lymphocyte subpopulations isolated from normal subjects and patients with congenital X-linked agammaglobulinemia. B lymphocytes from normal subjects have at least three times more ecto-5'-nucleotidase activity than T lymphocytes. Patients with X-linked agammaglobulinemia have 56% of normal activity in their T cells, and lack a lymphocyte subpopulation high in nucleotidase activity. High activity of ecto-5'-nucleotidase may be a biochemical marker for mature surface immunoglobulin-bearing B cells.     

Activation of T cell antigen receptor alpha- and beta-chain genes in the thymus: implications for the lineages of developing cortical thymocytes

Mammalian T lymphocytes mature in the thymus through a series of differentiation events that involve both rapid proliferation and extensive cell death. The mechanisms that govern these processes are currently unknown; however, both mitogenesis and death affect particular subpopulations of cells, suggesting the selective amplification and destruction of specific T cell clones. In mature peripheral T cells, proliferation is most commonly triggered by the recognition of antigen through the T cell antigen receptor complex. If antigen recognition also controls proliferation in the thymus, the differential expression of antigen receptor genes during maturation could play some role in determining the fate of developing T cells. In this study, we examined the expression of the alpha- and beta-chain genes of the T cell antigen receptor in different subpopulations of adult thymocytes. We compared two postmitotic populations--one that appears committed to die and one that appears mature--and several blast cell populations that are enriched for precursors of one or another presumptive lineage. We have found that Lyt-2-, L3T4- precursor thymocytes express much lower levels of both alpha- and beta-chain mRNA than the cells likely to be their immediate descendents. Furthermore, our results show that the cells of the major cortical lineage, which have at least a 95% probability of death, nevertheless express high levels of mature mRNA encoding both the alpha- and the beta-chains of the T cell antigen receptor. These results have important implications for the mechanisms involved in the overproduction and elimination of this major class of T lymphocyte.