Phenotypic and functional stages in the intrathymic development of alpha beta T cells

Novel insight into the role of the vitamin D receptor in the development and function of the immune system

Immune cells express the vitamin D receptor (VDR) and are therefore vitamin D targets. The Vdr protein can be readily measured in the kidney using antibodies to the Vdr and western blot. It is much more difficult to measure Vdr protein in the spleen because of the low level of VDR expression in resting immune cells. In order to more sensitively measure VDR expression, the Cre enzyme was inserted in the 3rd exon of the VDR gene and a reporter mouse that irreversibly expresses tdTomato was made. Mice that express one copy of the VDRCre gene were confirmed to be VDR +/- and mice that express two copies were confirmed to be VDR -/-. Initial characterization of the immune cells from the VDR +/-/VDR^tdTomato+ mice, compared to VDR+/+ wildtype (WT) littermates, showed no effect of being hemizygous for the VDR on immune cell frequencies. High tdTomato expression was shown to be present in the bone marrow (BM) and thymus immune cell precursors. In the periphery, monocytes, neutrophils and macrophages had very high tdTomato+ (88-98%) expression while lymphocytes ranged from 60% to 70% tdTomato+. Tissue resident innate lymphoid cell (ILC) 1 and 3 cells were about 60-80% tdTomoto+, while ILC2 cells had very low tdTomato expression. Stimulation of VDR^tdTomato+ splenocytes showed that the tdTomato- CD4+ and CD8+ T cells proliferated more than their tdTomato+ counterparts. T cells were sorted for tdTomato+ and tdTomato- and then activated for 72 h. Sorted tdTomato+ T cells expressed the VDR protein only after 72 h post-activation. The sorted tdTomato- T cells proliferated more than the sorted tdTomato+ T cells. Interestingly, activation of the tdTomato- T cells failed to induce new tdTomato expression. The data suggest that an early immune precursor expresses the VDR. In the periphery, neutrophils and monocytes are almost all tdTomato+, while some immune cells (ILC2 and some T cells) may never express the VDR.

Calorie restriction induces reversible lymphopenia and lymphoid organ atrophy due to cell redistribution

Calorie restriction (CR) without malnutrition increases life span and health span in multiple model organisms. In non-human and human primates, CR causes changes that protect against several age-related pathologies, reduces inflammation, and preserves or improves cell-mediated immunity. However, CR has also been shown to exhibit adverse effects on certain organs and systems, including the immune system, and to impact genetically different organisms of the same species differentially. Alternately, short periods of fasting followed by refeeding may result in the proliferation of bone marrow stem cells, suggesting a potential rejuvenation effect that could impact the hematopoietic compartment. However, the global consequences of CR followed by refeeding on the immune system have not been carefully investigated. Here, we show that individuals practicing long-term CR with adequate nutrition have markedly lower circulating levels of total leukocytes, neutrophils, lymphocytes, and monocytes. In 10-month-old mice, short-term CR lowered lymphocyte cellularity in multiple lymphoid tissues, but not in bone marrow, which appears to be a site of influx, or a "safe haven" for B, NK, and T cells during CR. Cellular loss and redistribution was reversed within the first week of refeeding. Based on BrdU incorporation and Ki67 expression assays, repopulating T cells exhibited high proliferation in the refeeding group following CR. Finally, we demonstrated that the thymus was not essential for T cell repopulation following refeeding. These findings are of potential relevance to strategies to rejuvenate the immune system in mammals and warrant further investigation.

Reversible left recurrent laryngeal nerve palsy in pediatric Graves' disease

Vocal cord paralysis associated with goiter usually indicates the presence of a malignancy. Pediatric patients retain significant thymic tissue that regresses only later in life. This thymic tissue can develop significant hyperplasia during an acute autoimmune process. We describe a case of a 17-year-old girl who presented with a goiter secondary to severe Graves' disease and a 2-month history of hoarseness, choking on liquid intake, and small-volume vomiting especially after eating. She demonstrated a left vocal cord paralysis probably secondary to a unilateral left recurrent laryngeal nerve palsy. A marked enlargement of the thymus was discovered on thoracic imaging. Treatment was initiated with methimazole, with near complete remission of her vocal cord paralysis within 3 months. Given the immunomodulatory effects of methimazole, a potential mechanism of the left recurrent laryngeal nerve palsy was autoimmune hyperstimulation of the thymus and consequent hyperplasia, resulting in distension of the nerve. Attenuation of the hyperactive immune process with methimazole may have resulted in regression of the mass effect of the thymus and associated reduction of the nerve distension. This case illustrates the unique risk of left recurrent laryngeal nerve palsy in pediatric patients with an acute immune stimulation and hyperplasia of the thymus and the reversibility in the context of mitigation of the immune hyperactivity. Methimazole may be an optimal initial treatment choice in pediatric patients with Graves' disease and left recurrent laryngeal nerve palsy.

Vascularized tissue engineering mouse chamber model supports thymopoiesis of ectopic thymus tissue grafts

We have previously established a chamber model of tissue engineering that promotes de novo angiogenesis and vascularization of engrafted cells and tissues when combined with an extracellular matrix. Here we demonstrate that the mouse chamber (MC) model can sustain ectopic grafts of murine fetal thymus lobes and, to a limited degree, human pediatric thymus tissue, resulting in de novo T-cell production. Silicone chambers containing Matrigel((R)) and thymus tissues were placed around exposed epigastric vessels and the ends sealed with bone wax, before implantation into the inguinal fat pad of athymic Balb/c(nu/nu) (nude) mice. Murine, embryonic day 15 (E15) thymus grafts were found to be well vascularized and viable within the MC upon harvest at week 11. In contrast, engraftment of both adult murine and pediatric human thymus tissue was limited, with only one out of the seven human thymus grafts sustaining mature, murine-derived T-cell development. Increased CD4(+) and CD8(+) T-cell numbers were observed in the peripheral blood of nude mice within 2 weeks after E15 thymus-MC grafts (n = 8), compared with nude control mice. Peripheral blood T-cell percentage and subset distribution were comparable to mice receiving conventional thymus kidney capsule grafts. T-cell function of both kidney capsule- and MC-E15 thymus grafts was established via successful rejection of major histocompatibility complex (MHC)-mismatched skin grafts. Sustained growth of fetal thymus tissue in the MC provides an alternative model for the study of thymopoiesis and related applications of T-cell-mediated immunity.

TGFbeta signalling in control of T-cell-mediated self-reactivity

In the immune system, transforming growth factor-beta (TGFbeta) affects multiple cell lineages by either promoting or opposing their differentiation, survival and proliferation. Understanding the cellular mechanisms of TGFbeta-mediated regulation is complicated due to a broad distribution of TGFbeta receptors on the surface of different immune-cell types. Recent studies using in vivo genetic approaches revealed a critical role for TGFbeta signalling in T cells in restraining fatal autoimmune lesions. Here, we review recent advances in our understanding of a role for TGFbeta signalling in the regulation of T-cell differentiation in the thymus and in the periphery, with a particular emphasis on TGFbeta-mediated control of self-reactive T cells.

TGF-β type II receptor–deficient thymocytes develop normally but demonstrate increased CD8+ proliferation in vivo

We have taken advantage of the Cre/lox system to generate a mouse model with inducible deficiency of transforming growth factor β receptor II (TβRII). Using this approach, transforming growth factor β (TGF-β) signaling deficiency can be restricted to the hematopoietic system by bone marrow transplantation. Mice that received transplants with TβRII-/- bone marrow develop a lethal inflammatory disorder closely resembling that of TGF-β1-null mice. Previous in vitro studies have suggested multiple roles for TGF-β in T-cell development, including proliferation, apoptosis, and differentiation. We used our transplantation model to ask whether T-cell development is normal in the absence of TGF-β signaling. The findings show for the first time in vivo and in fetal thymus organ culture (FTOC) that TGF-β is not required for thymocytes to differentiate along the entire pathway of thymic T-cell development, as defined by the expression patterns of CD4, CD8, CD25, and CD44. In contrast to previous investigations, no increase of thymocyte apoptosis was observed. However, TβRII-deficient CD8+ thymocytes displayed a 2-fold increase in proliferation rate, as determined by bromodeoxyuridine (BrdU) incorporation in vivo. These results reinforce the importance of TGF-β as an immune regulator critical for T-cell function.

Reduced primary T lymphopoiesis in 3-month-old lurcher mice: sign of premature ageing of thymus?

OBJECTIVE

The nervous, endocrine and immune systems are functionally interconnected so that they may form a complex metasystem. Abnormalities within the neuroendocrine compartment can thus affect immune mechanisms and vice versa. The Lurcher-type mutation in mice has a profound impact on brain development in both homozygous and heterozygous individuals, which is followed by immune system changes. We investigated whether macroscopic changes in the thymus size were associated with an altered thymocyte development or with changes in peripheral T cell subset distribution.

METHODS

CD3, CD4 and CD8 expressions on thymocytes and peripheral T cells were compared with those in wild-type and Lurcher heterozygous C3H mice using surface immunophenotyping and flow cytometry. Galanthus nivalis agglutinin binding to thymocytes was measured at the same time.

RESULTS

While no differences between experimental groups were observed in 1-month-old mice, a critical reduction of numbers of thymocytes and namely double-positive cells occurred before the age of 3 months in Lurcher mice, which was accompanied by thymus involution. Interestingly, this was not accompanied by significant differences in major T subset proportions in the peripheral lymphatic tissue.

CONCLUSIONS

We interpret our observations as hallmarks of premature thymus ageing in heterozygous Lurcher mice with only a marginal effect in the periphery in early adulthood.

Is tolerance a prospective for clinical research?

Tolerance is an emotive issue in transplantation. It is the promised land for which we all strive and which we all hope we shall live to see. In such circumstances, tolerance must always be a prospective for clinical research! The question is, therefore, better posed in a more optimistic fashion and with a small act of faith: do we, in 1991, have that crucial combination of basic scientific knowledge and creative imagination to make it possible?

Entrapment of recent thymic emigrants in lymphoid tissues from HIV-infected patients: association with HIV cellular viral load

OBJECTIVE(S)

Depletion of thymus derived naive T-cells is a feature of HIV infection. Here the impact of HIV infection on the compartmentalization of recent thymic emigrants of (RTE) and naive T-cells was examined.

METHODS

Peripheral blood mononuclear cells (PBMC) and lymphoid tissue (LT) from 43 HIV-infected patients and 12 controls were examined for RTE distribution by measuring coding joint T-cell receptor excisional circles (cjTREC) by PCR and naive and memory T-cell subsets and adhesion molecules (L-selection, LFA-1) by flow cytometry.

RESULTS

In HIV-infected patients, the RTE as quantified by cjTRECs in CD4 LT cells were significantly higher than in PBMC. Their values, however, were less than in control subjects, in both the LT and PBMC compartments. This was associated with an increase in L-selectin and LFA-1 expression on LT derived T cells. In PBMC, a significant positive relationship between TREC and naive CD4 cells and an inverse relationship between TREC and cellular viral load (CVL) was observed. Whereas in LT, there was a positive relationship between cjTREC and both naive CD4 cell percentage and CVL.

CONCLUSIONS

Collectively, the data suggests that LT is a significant reservoir for RTE. The RTE appeared to be entrapped in LT from HIV-infected subjects. Such entrapment is probably a response to the high viral load in these tissues. These observations may partially explain the decline in RTE observed in the peripheral blood of HIV-infected patients, and the delay in recovery of naive cells in blood after initiation of HAART.

The ETS protein MEF plays a critical role in perforin gene expression and the development of natural killer and NK-T cells

We utilized gene targeting by homologous recombination to define the role that MEF, a transcriptional activating member of the ETS family of transcription factors, plays in lymphopoiesis. MEF-/- mice have a profound reduction in the number of NK-T and NK cells. Purified MEF-/- NK cells cannot lyse tumor cell targets and secrete only minimal amounts of IFNgamma. Perforin protein expression is severely impaired in MEF-deficient NK cells, likely accounting for the lack of tumor cell cytotoxicity. Promoter studies and chromatin immunoprecipitation analyses demonstrate that MEF and not ETS-1 directly regulates transcription of the perforin gene in NK cells. Our results uncover a specific role of MEF in the development and function of NK cells and in innate immunity.

Essential role for the lymphostromal plasma membrane Ly-6 superfamily molecule thymic shared antigen 1 in development of the embryonic adrenal gland

Thymic shared antigen 1 (TSA-1) is a plasma membrane protein of the Ly-6 superfamily expressed on thymocytes, thymic stromal cells, and other cells of the hematopoietic system. TSA-1 is also expressed in other nonhematopoietic tissues, in particular, embryonic and adult adrenal glands. To address the function of TSA-1, we generated mutant mice in which TSA-1 expression was inactivated by gene targeting. Here we show that deletion of both TSA-1 alleles results in abnormal adrenal gland development and midgestational lethality due to cardiac abnormalities. We also report that TSA-1-deficient adrenal glands have significantly reduced levels of the catecholamines noradrenaline and adrenaline. We conclude that TSA-1 is required for normal embryonic development but that deletion of its expression does not obviously impair lymphoid development.

Binding of the Galanthus nivalis agglutinin to thymocytes reveals alterations in surface glycosylation during T-cell development

Surface binding of the Galanthus nivalis agglutinin (GNA) to thymocyte subsets has been studied in pigs and rodents by multicolour flow cytometry. In all the species examined, analogous staining profiles have been recorded. Counter-staining with anti-CD3epsilon, anti-CD4 and anti-CD8 monoclonal antibodies (MoAb) revealed that a significant increase of the GNA targets on the cell surface occurred during early thymocyte differentiation and reached its maximum at the level of the CD3loCD4+CD8+ small cortical thymocyte. This was followed by a decrease in the GNA binding capacity upon terminal maturation to the single positive thymocytes. PAGE analysis has revealed a dominant GNA-binding glycoprotein (molar mass approx. 90 kDa) present on thymocyte plasma membranes and absent on the surface of splenic lymphocytes, although both the whole cell lysates from both organs contained GNA ligands of the same size. Our findings are in agreement with previous data showing that immature thymocytes differ from their mature counterparts and peripheral T lymphocytes in the surface glycosylation pattern, and support the hypothesis that lectin-glycoprotein interaction plays a significant role in the cell-to-cell crosstalk in the thymic cortex.

Extrathymic deletion of CD8+ alloreactive T cells in a transgenic T cell receptor model of neonatal tolerance

BACKGROUND

Immunological tolerance to foreign antigen is most easily achieved during the neonatal period. Although deletion of T cells has been demonstrated in neonatal tolerance models in which donor and recipient express different MHC class II molecules, the requirement for deletion in MHC class I-disparate models is less clear. To address this issue, we used as recipient the T cell receptor (TCR) transgenic mouse (TgM) strain 2C in which the majority of CD8+ T cells express a single alpha/beta TCR alloreactive to H-2Ld, thus facilitating direct monitoring of the class I alloreactive population.

METHODS

Newborn (less than 24 hr of age) 2C TgM received injections i.v.with syngeneic C57BL/6J (H-2b) (B6) or semiallogeneic (B6xDBA)F1 (H-2bd; H-2Ld+) splenocytes. Adults were subsequently analyzed in terms of tolerance, deletion of 2C+ T cells, and chimerism.

RESULTS

The results showed that semiallogeneic-, but not syngeneic-, injected neonates were unresponsive as adults to H-2Ld-expressing target cells in vitro and the majority of these mice accepted H-2Ld+ skin grafts. Delaying the injection to 72 hr after birth or reducing the number of cells injected essentially abolished in vivo unresponsiveness in 2C recipients. Thus, the 2C TCR Tg model demonstrates the characteristics typical of neonatal tolerance. Injection of 2C neonates within 24 hr of birth with semiallogeneic versus syngeneic cells led to more than a 12-fold reduction of CD8+ 2C+ T cells in adult spleen and LNCs. In contrast, deletion of CD8+ 2C+ cells in adult thymus was not consistently observed. Based on MHC class II expression to distinguish donor (I-E+) and recipient (I-E-) cells, semiallogeneic-injected mice were chimeric in spleens and lymph nodes (LNs).

CONCLUSIONS

These results demonstrate that neonatal MHC class I tolerance in the adult is associated with low level hematopoietic chimerism and extrathymic deletion of alloreactive CD8+ T cells.

Dysregulated expression of pre-Talpha reveals the opposite effects of pre-TCR at successive stages of T cell development

The pre-TCR complex (TCRbeta-pre-TCRalpha chain (pTalpha)), first expressed in a fraction of CD8(-)4(-)CD44(-)25(+) (DN3) cells, is believed to facilitate or enable an efficient transition from the CD8(-)4(-) double-negative (DN) to the CD8(+)4(+) double-positive (DP) developmental stage. Subsequent to pre-TCR expression, DN3 thymocytes receive survival, proliferation, and differentiation signals, although it is still unclear which of these outcomes are directly induced by the pre-TCR. To address this issue, we generated mice bearing a range of pTalpha transgene copy number under the transcriptional control of the p56(lck) proximal promoter. All lines exhibited increased DN3 cycling, accelerated DN3/4 transition, and improved DN4 survival. However, the high copy number lines also showed a selective reduction in thymic cellularity due to increased apoptosis of DP thymocytes, which could be reversed by the ectopic expression of Bcl-2. Our results suggest that transgenic pTalpha likely caused apoptosis of DP thymocytes due to competitive decrease in surface TCRalphabeta formation. These results highlight the critical importance of precise temporal and stoichiometric regulation of pre-TCR and TCR component expression.

Characterization of the immunophenotype and the metastatic properties of a murine T-lymphoma cell line. Unexpected expression of cytoplasmatic CD4

We report the first characterization of a mouse T-lymphoma cell line that surprisingly expresses cytoplasmatic (cy) yCD4. Phenotypically, LBC cells are CD5+, CD8+, CD16+, CD24+, CD25+, CD2-/dim, CD3-/dim, TCRbeta-/dim, TCRgammadelta, CD154 , CD40-, and CD45R. Coexpress cyTCRbeta, cyCD3, cyCD4, and yet lack surface CD4 expression. Transplantation of LBC cells into mice resulted in an aggressive T-lymphoblastic lymphoma that infiltrated lymph nodes, thymus, spleen, liver, ovary, and uterus but not peripheral blood or bone marrow. LBC cells display a modal chromosome number of 39 and a near-diploid karyotype. Based on the characterization data, we demonstrated that the LBC cell line was derived from an early T-cell lymphocyte precursor. We propose that the malignant cell transformation of LBC cells could coincide with the transition stage from late double-negative, DN3 (CD4- CD8 CD44-/low, CD25+) or DN4 (CD4-low, CD8-/low, CD44-, CD25-) to double-positive (DP: CD4+CD8+) stage of T-cell development. LBC cells provide a T-lymphoblastic lymphoma model derived from a malignant early T-lymphocyte that can be potentially useful as a model to study both cellular regulation and differentiation of T-cells. In addition, LBC tumor provides a short latency neoplasm to study cellular regulation and to perform preclinical trials of lymphoma-relatel clisorders.

Premature TCR alpha beta expression and signaling in early thymocytes impair thymocyte expansion and partially block their development

In thymocyte ontogeny, Tcr-a genes rearrange after Tcr-b genes. TCR alpha beta transgenic (Tg) mice have no such delay, consequently expressing rearranged TCR alpha beta proteins early in the ontogeny. Such mice exhibit reduced thymic cellularity and accumulate mature, nonprecursor TCR(+)CD8(-)4(-) thymocytes, believed to be caused by premature Tg TCR alpha beta expression via unknown mechanism(s). Here, we show that premature expression of TCR alpha beta on early thymocytes curtails thymocyte expansion and impairs the CD8(-)4(-) --> CD8(+)4(+) transition. This effect is accomplished by two distinct mechanisms. First, the early formation of TCR alpha beta appears to impair the formation and function of pre-TCR, consistent with recently published results. Second, the premature TCR alpha beta contact with intrathymic MHC molecules further pronounces the block in proliferation and differentiation. These results suggest that the benefit of asynchronous Tcr-a and Tcr-b rearrangement is not only to minimize waste during thymopoiesis, but also to simultaneously allow proper expression/function of the pre-TCR and to shield CD8(-)4(-) thymocytes from TCR alpha beta signals that impair thymocyte proliferation and CD8(-)4(-) --> CD8(+)4(+) transition.

Death the Fas way: regulation and pathophysiology of CD95 and its ligand

Apoptotic cell death mediated by the members of the tumor necrosis factor receptor family is an essential process involved in the regulation of cellular homeostasis during development, differentiation, and pathophysiological conditions. Among the cell death receptors comprising the tumor necrosis factor receptor superfamily, CD95/APO-1 (Fas) is the best characterized. The specific interaction of Fas with its cognate ligand, Fas ligand (FasL), elicits the activation of a death-inducing caspase (cysteine aspartic acid proteases) cascade, occurring in a transcription-independent manner. Caspase activation executes the apoptosis process by cleaving various intracellular substrates, leading to genomic DNA fragmentation, cell membrane blebbing, and the exposure of phagocytosis signaling molecules on the cell surface. Recent studies have shown that the Fas/FasL pathway plays an important role in regulating the life and death of the immune system through activation-induced cell death. In addition, these molecules have been implicated in aging, human immunodeficiency virus infection, drug abuse, stress, and cancer development. In this review, we will focus on the mechanisms that regulate Fas and FasL expression, and how their deregulation leads to diseases.

Differential effects of chronic propranolol treatment on the phenotypic profile of thymocytes from immature and adult rats

To elucidate a putative role of beta-adrenoceptors in the modulation of intrathymic T-cell maturation, the expression of major differentiational antigens (CD4/CD8 and TCR alphabeta) on the thymocytes from both immature (aged 21 day at the beginning of the treatment) and adult (aged 75 days at the beginning of treatment) male rats subjected to a 15-day-long propranolol treatment (0.40 mg/100 g/day, s.c.) was analyzed by two- and one-color flow cytometry, respectively. Rats of matched age injected with saline served as controls. The propranolol treatment in immature but not adult rats caused a significant reduction in both the relative thymus weight and total thymocyte yield. In addition, a significant increase in the percentage of CD4+ 8+ double-positive cells, with a proportional decrease in the relative proportion of CD4+ 8- single positive cells, was found in immature rats. In contrast, a slight but significant decrease in the percentage of CD4+ 8+ cells with a parallel increase in the relative proportion of CD4+ 8- cells was found in adult rats. In both groups of rats, the percentage of TCR alphabeta(total) thymocytes was increased: in immature rats this was due to an increase in the percentage of TCR alphabeta(low) thymocytes, while in the adult rats it reflected a rise in the relative proportion of TCR alphabeta(high) cells. In conclusion, the study revealed that propranolol treatment in both immature and adult rats alters the relative proportion of CD4+ 8+ and CD4+ 8- thymocytes, but in opposite fashion, and the data suggest that this treatment affects distinct fractions within the population of CD4+ 8+ thymocytes with respect to expression of TCR alphabeta. The results also indicate that, regardless of rat sexual maturity, the development of thymocytes towards CD4- 8+ T-cells is relatively insensitive to long-lasting beta-adrenoceptor blockade.

Failure of Rearranged TCR Transgenes to Prevent Age-Associated Thymic Involution

After puberty, the thymus undergoes a dramatic loss in volume, in weight and in the number of thymocytes, a phenomenon termed age-associated thymic involution. Recently, it was reported that age-associated thymic involution did not occur in mice expressing a rearranged transgenic (Tg) TCRαβ receptor. This finding implied that an age-associated defect in TCR rearrangement was the major, if not the only, cause for thymic involution. Here, we examined thymic involution in three other widely used MHC class I-restricted TCRαβ Tg mouse strains and compared it with that in non-Tg mice. In all three TCRαβ Tg strains, as in control mice, thymocyte numbers were reduced by ∼90% between 2 and 24 mo of age. The presence or absence of the selecting MHC molecules did not alter this age-associated cell loss. Our results indicate that the expression of a rearranged TCR alone cannot, by itself, prevent thymic involution. Consequently, other presently unknown factors must also contribute to this phenomenon.

Identification of a New Type of Invariant Vα14+ T Cells and Responsiveness to a Superantigen, Yersinia pseudotuberculosis- Derived Mitogen

We examined the expression of the H4 T cell activation marker in thymic T cell subpopulations and found that TCR-αβ+ CD4+ thymic T cells are segregated into three subpopulations based upon H4 levels. Thymic T cells with either no or low H4 expression differentiate via the mainstream differentiation pathway in the thymus. H4int thymic T cells, which express a skewed Vβ repertoire of Vβ2, -7, and -8 in their TCRs, show the phenotype of NKT cells: CD44high, Ly6Chigh, NK1.1+, and TCR-αβlow. H4high thymic T cells also show a skewed Vβ repertoire, Vβ2, -7, and -8, and predominantly express an invariant Vα14-Jα281+ α-chain in their TCRs but constitute a distinct population in that they are CD44int, Ly6C−, NK1.1−, and TCR-αβhigh. Thus, invariant Vα14+ thymic T cells consist of ordinary NKT cells and a new type of T cell population. Vβ7+ and Vβ8.1+ invariant Vα14+ thymic T cells are present in DBA/2 mice, which carry mammary tumor virus-7-encoded superantigens, in comparable levels to those in BALB/c mice. Furthermore, Vβ7+ invariant Vα14+ thymic T cells in DBA/2 mice are in the immunologically responsive state, and Yersinia pseudotuberculosis-derived mitogen-induced Vβ7+ invariant Vα14+ thymic T cell blasts from DBA/2 and BALB/c mice exhibited equally enhanced responses upon restimulation with Y. pseudotuberculosis-derived mitogen. Thus, invariant Vα14+ thymic T cells that escape negative selection in DBA/2 mice contain T cells as functionally mature as those in BALB/c mice.

Enhancement of activation-induced cell death by fibronectin in murine CD4+ CD8+ thymocytes

Development of T cells in the thymus is achieved through the interactions of thymocytes with their microenvironments. This study focused on the function of fibronectin (FN), a major extracellular matrix molecule in the thymus, in the cell death induced by activation via the T-cell antigen receptor. FN alone did not increase cell death in murine thymocytes above the baseline level, but it significantly enhanced the cell death induced by fixed anti-CD3 monoclonal antibody (mAb), especially when a high concentration of anti-CD3 mAb was used. DNA fragmentation increased in parallel with cell death, indicating that cell death was a result of the apoptosis. Fluorescence-activated cell sorter (FACS) analysis revealed that the activation-induced cell death (AICD) caused by anti-CD3 mAb alone, or by a combination of anti-CD3 mAb and FN, occurred selectively in CD4+ CD8+ thymocytes. Very late activation antigen (VLA)-4 and VLA-5 are two major ligands to FN on thymocytes. The expression of both ligands was investigated at different stages of thymocyte development. VLA-4 was predominantly expressed at the CD4- CD8- stage, and thereafter the expression was reduced, whereas VLA-5 was constantly expressed during maturation. Furthermore, the enhancing effect by FN was inhibited in the presence of the Gly-Arg-Gly-Asp-Ser-Pro (GRGDSP) peptide but not in the presence of the connecting segment-1 (CS-1) peptide, suggesting that enhancement of AICD observed in CD4+ CD8+ thymocytes is mediated through VLA-5.

Lineage commitment and differentiation of T and natural killer lymphocytes in the fetal mouse

T cells and natural killer (NK) cells are presumed to share a common intrathymic precursor. The development of conventional alpha beta T lymphocytes begins within the early fetal thymus, after the colonization of multipotent CD117+ precursors. Irrevocable commitment to the T lineage is marked by thymus-induced expression of CD25. However, the contribution of the fetal thymus to NK lineage commitment and differentiation remains largely unappreciated. Recently, we demonstrated that the development of functional mouse NK cells occurs first in the fetal thymus. Moreover, the appearance of mature fetal thymic NK cells (NK1.1+/CD117-) is preceded by a thymus-induced developmental stage (NK1.1+/CD117+) that marks lineage commitment of multipotent hematopoietic precursors to the T and NK-cell fates. Commitment to the T/NK bipotent stage is induced by fetal thymic stroma, but is not thymus dependent. Recent data indicate that CD90+/CD117lo fetal blood prothymocytes exhibit NK lineage potential and are phenotypically and functionally identical to fetal thymic NK1.1+/CD117+ progenitors. This finding also indicates that full commitment of circulating precursors to the T-cell lineage occurs after thymus colonization. In this review, we discuss recent insights into the cellular and molecular events involved in fetal mouse T and NK lineage commitment and differentiation to unipotent progenitors.

Requirement for the thymus in alphabeta T lymphocyte lineage commitment

We recently identified a fetal thymic developmental stage (NK1.1+/CD117(lo)) that characterizes committed T/NK progenitors. We now report the existence of phenotypically and functionally identical T/NK progenitors in mouse fetal blood and spleen but not in fetal liver. These precursors are indistinguishable from previously characterized fetal blood "prothymocytes" (CD90+/CD117(lo)), with the exception that they express NK1.1, lack markers associated with T lineage commitment, maintain a germline TCRbeta locus, and can give rise to both T and NK cells. Moreover, NK1.1+/CD90+/CD117(lo) fetal blood precursors are present in athymic nude mice. These results suggest that the T/NK lineage commitment pathway is thymus-independent. In contrast, full commitment to the alphabeta T lineage does not precede thymus colonization.

Skewed TCRV beta repertoire in human thymus persists after thymic emigration: influence of genomic imposition, thymic maturation and environmental challenge on human TCRV beta usage in vivo

In order to investigate the mechanisms involved in originating a diverse TCR repertoire in human peripheral blood we analyzed TCRV beta surface expression in different T cell subsets of unrelated individuals. The relative frequencies of 11 distinct V beta chains were determined for immature double positive (DP) as well as for mature CD4 single positive (4SP) and CD8 single positive (8SP) thymocytes, respectively. By comparing these data with expression in peripheral blood T lymphocytes of the same donors we were able to show that usage of TCRV beta in peripheral T cells is significantly (p < 0.001) depending on the pattern in mature SP thymocytes whereas the frequency of TCRV beta families in immature DP thymocytes has no impact (p > 0.2). No association with distinct HLA-haplotypes was observed. Preferential usage of V beta-families in either CD4- or CD8-positive peripheral T cells also correlates with the status in mature thymic precursors (p < 0.001). Altogether, this first combined study of TCR frequencies within different stages of human T cell ontogeny indicates that TCRV beta repertoire is determined mainly through selectional processes within the thymus. Since neither genomically imposed expression nor modulating events in the periphery seem to have strong influence on the relative expression of TCRV beta chains these findings have to be considered in future studies of human diseases.

Regulation of immune responses by TGF-beta

The transforming growth factor beta (TGF-beta) family of proteins are a set of pleiotropic secreted signaling molecules with unique and potent immunoregulatory properties. TGF-beta 1 is produced by every leukocyte lineage, including lymphocytes, macrophages, and dendritic cells, and its expression serves in both autocrine and paracrine modes to control the differentiation, proliferation, and state of activation of these immune cells. TGF-beta can modulate expression of adhesion molecules, provide a chemotactic gradient for leukocytes and other cells participating in an inflammatory response, and inhibit them once they have become activated. Increased production and activation of latent TGF-beta have been linked to immune defects associated with malignancy and autoimmune disorders, to susceptibility to opportunistic infection, and to the fibrotic complications associated with chronic inflammatory conditions. In addition to these roles in disease pathogenesis, TGF-beta is now established as a principal mediator of oral tolerance and can be recognized as the sine qua non of a unique subset of effector cells that are induced in this process. The accumulated knowledge gained through extensive in vitro functional analyses and from in vivo animal models, including newly established TGF-beta gene knockout and transgenic mice, supports the concept that clinical therapies based on modulation of this cytokine represent an important new approach to the treatment of disorders of immune function.

Epithelial V-like antigen (EVA), a novel member of the immunoglobulin superfamily, expressed in embryonic epithelia with a potential role as homotypic adhesion molecule in thymus histogenesis

Thymus development depends on a complex series of interactions between thymocytes and the stromal component of the organ. To identify regulated genes during this codependent developmental relationship, we have applied an RNA fingerprinting technique to the analysis of thymus expansion and maturation induced in recombinase-deficient mice injected with anti-CD3 antibodies. This approach led us to the identification of a gene encoding a new member of the immunoglobulin superfamily, named epithelial V-like antigen (EVA), which is expressed in thymus epithelium and strongly downregulated by thymocyte developmental progression. This gene is expressed in the thymus and in several epithelial structures early in embryogenesis. EVA is highly homologous to the myelin protein zero and, in thymus-derived epithelial cell lines, is poorly soluble in nonionic detergents, strongly suggesting an association to the cytoskeleton. Its capacity to mediate cell adhesion through a homophilic interaction and its selective regulation by T cell maturation might imply the participation of EVA in the earliest phases of thymus organogenesis.

Flt3 Ligand Plus IL-7 Supports the Expansion of Murine Thymic B Cell Progenitors That Can Mature Intrathymically

Flt3 ligand (flt3L) has potent effects on hemopoietic progenitors, dendritic cells, and B lymphopoiesis. We have investigated the effects of flt3L on intrathymic precursors. The addition of flt3L + IL-7 to lobe submersion cultures of murine fetal thymic lobes resulted in the expansion of an immature population of Thy-1low, CD44high, HSAhigh cells. This population contained cells with precursor activity, as determined by their capacity to repopulate deoxyguanosine-treated fetal thymic lobes. Upon reentry to the thymic lobe, flt3L + IL-7-cultured Thy-1low, CD44high, HSAhigh cells underwent expansion and differentiation into B cells. Two weeks after fetal thymic organ culture following thymic lobe reconstitution, intrathymic cells were Thy-1−, B220+, and a subset was sIgM+. The intrathymic B cells shared features of adult thymic B cells, including CD5 expression and proliferative responses to IL-4 + IL-5 + CD40 ligand, but not to LPS or soluble anti-IgM. Ig production was noted upon stimulation with IL-4 + IL-5 + LPS and IL-4 + IL-5 + CD40 ligand. In conclusion, we have demonstrated that flt3L + IL-7 supports the expansion of a subset of progenitors present in the fetal thymus. The cultured progenitors can repopulate a fetal thymic lobe and develop into mature functional B cells, demonstrating that the fetal thymus is able to support B cell as well as T cell development.

Thymotrophic effect of ether lipid 1-O-octadecyl-2-O-methoxy-rac-glicero-3-phosphocholine in the mouse

1-O-octadecyl-2-O-methoxy-rac-glicero-3-phosphocholine (ET-18-OCH3) is a synthetic derivate of 2-lysophosphatidyl-choline, endowed with some immunomodulatory and anticancer effects. In the present work we report that a chronic (1 microgram/g b.w. for 2 weeks) or acute single dose injection of ET-18-OCH3 produced a recovery of thymus weight and thymocytes cellularity in two different strains of mice, C57BL6 and Swiss mice, undergoing thymus age-dependent involution. This effect was significant when the thymus weight was reduced at 50% and it was without effect on thymus lacking age dependent involution, such as young mice. The ability of ET-18-OCH3 to produce thymus weight and thymocyte cellularity recovery was also demonstrated in adult mice showing hypotrophy of thymus induced by chronic corticosterone treatment, suggesting that this compound could be effective against thymus hypotrophy induced by external stimuli. This thymotrophic effect of ET-18-OCH3 was not dependent on direct action on thymocyte proliferation, but probably it was dependent on its action on thymic epithelial cells to produce hormone thymulin, which level was found significantly increased in the plasma. These results provide further immunomodulatory propriety of ET-18-OCH3 and open the possibility to use this compound to counteract thymus hypotrophy.

Characterization of CD4+CD8+ thymocytes observed in SCID-bg mice

Previously, we reported that a strain of severe combined immunodeficient (SCID) mice, namely SCID-bg, spontaneously develop CD4+CD8+ double positive (DP) thymocytes despite their scid mutation. In the present study, we intend to further characterize the DP thymocyte population in SCID-bg mice with Southern hybridization and flow cytometry. Southern hybridization analyses of sorted DP thymocytes in SCID-bg mice showed rearrangement of T cell receptor (TCR) beta and TCR gamma gene segments, although the expression of TCR beta and gamma delta TCR molecules was absent. The phenotype of the DP thymocytes in SCID-bg mice was CD44- heat-stable antigen (HSA)+CD25-, and they did not express CD69. Interestingly, the expression of thymus leukaemia (TL) antigen was observed in the DP thymocytes of SCID-bg mice but not in their CD4-CD8- double negative (DN) fraction. Fas expression was higher and bcl-2 expression was down-regulated in the DP thymocytes as compared to the DN thymocytes in SCID-bg mice, suggesting that they are not immortalized cells having escaped from apoptosis. Taken together, these results show that the phenotype of the DP thymocytes in SCID-bg mice is similar to that of the earliest phase of the DP thymocytes in normal mice, although the expression of the molecules in the pre-TCR complex is absent.

Immunohistochemical analysis of murine CD95/Fas/Apo-1 receptor and its ultrastructural distribution in the thymus

CD95/Fas/Apo-1 is a cell surface receptor that, upon contact with its ligand, induces cells to die by apoptosis. In view of the importance of Fas receptor (FasR) in immunologic tolerance, an immunohistochemical analysis of FasR expression was performed in the lymphoid and certain parenchymal tissues of normal and mutant MRL/lpr mice using a rabbit polyclonal anti-Fas receptor antibody. FasR was expressed by immunoperoxidase (IP) in the cortex and at the corticomedullary junction of the thymus of normal mice. By immunoelectron microscopy FasR was detected on the cell membrane of normal thymocytes. In MRL/lpr mice, FasR protein expression could not be clearly detected. FasR protein expression was not detected in the heart, liver or ovary by IP, presumably reflecting the low number of receptors in these tissues.

Phenotypic and immunohistological analyses of the human adult thymus: evidence for an active thymus during adult life

We analyzed cellular content of thymic samples from 26 human healthy donors, ranging from 1 week postnatal to 49 years old. Our results showed that there was an overall decrease in cellular density, beginning early during life, but with two peaks of cellular density, at 9 months and 10 years of age. Histological and immunohistological analyses showed that variations in cellular density were correlated with the morphological changes observed during thymic involution, namely the enlargement of interlobular trabeculae and the development of adipocytic tissue. However, the adult thymus still contained thymocytes, up to 49 years. Phenotypic analysis showed no significant variations according to the age of donors in the distribution of the main thymocyte subsets, both precursors and more mature cells. These results suggest that the human thymus remains active during adult life.

Differential effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin, bis(tri-n-butyltin) oxide and cyclosporine on thymus histophysiology

Recent advances in the histophysiology of the normal thymus have revealed its complex architecture, showing distinct microenvironments at the light and electron microscopic level. The epithelium comprising the major component of the thymic stroma is not only involved in the positive selection of thymocytes, but also in their negative selection. Dendritic cells, however, are more efficient than epithelial cells in mediating negative selection. Thymocytes are dependent on the epithelium for normal development. Conversely, epithelial cells need the presence of thymocytes to maintain their integrity. The thymus rapidly responds to immunotoxic injury. Both the thymocytes and the nonlymphoid compartment of the organ can be targets of exposure. Disturbance of positive and negative thymocyte selection may have a major impact on the immunological function of the thymus. Suppression of peripheral T-cell-dependent immunity as a consequence of thymus toxicity is primarily seen after perinatal exposure when the thymus is most active. Autoimmunity may be another manifestation of chemically mediated thymus toxicity. Although the regenerative capacity of thymus structure is remarkable, it remains to be clarified whether this also applies to thymus function. In-depth mechanistic studies on chemical-induced dysfunction of the thymus have been conducted with the environmental contaminants 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and bis(tri-n-butyltin)oxide (TBTO) as well as the pharmaceutical immunosuppressant cyclosporine (CsA). Each of these compounds exerts a differential effect on the morphology of the thymus, depending on the cellular targets for toxicity. TCDD and TBTO exposure results in cortical lymphodepletion, albeit by different mechanisms. An important feature of TCDD-mediated thymus toxicity is the disruption of epithelial cells in the cortex. TBTO primarily induces cortical thymocyte cell death. In contrast CsA administration results in major alterations in the medulla, the cortex remaining largely intact. Medullary epithelial cells and dendritic cells are particularly sensitive to CsA. The differential effects of these three immunotoxicants suggest unique susceptibilities of the various cell types and regions that make up the thymus.

Susceptibility of different subsets of immature thymocytes to apoptosis

In the present study the susceptibility of different subsets of immature rat thymocytes to undergo apoptosis was examined. Unfractionated rat thymocytes were negatively enriched into immature double positive (CD4+ CD8+), immature single positive (CD4- CD8+ CD3-) and triple negative (CD4- CD8- CD3-) thymocytes. These enriched subsets of immature thymocytes were then exposed to various apoptotic stimuli such as dexamethasone, etoposide and thapsigargin which readily induced apoptosis in unfractionated rat thymocytes. We found that the double positive thymocytes and their precursor cells, i.e. the single positive immature thymocytes, were equally sensitive to apoptosis after treatment with the apoptotic stimuli. In sharp contrast, the early migrants or precursor-containing thymocytes which are triple negative have a lower spontaneous apoptosis rate and were relatively resistant to all the apoptotic stimuli. These findings showed a breakpoint in thymocyte sensitivity to apoptosis which occurs after the onset of CD8 expression, suggesting that susceptibility of thymocytes to apoptosis is developmentally regulated.

Effects of neonatal dexamethasone treatment on hypothalamo-pituitary adrenal axis and immune system of the rat

Synthetic glucocorticoids (GC) are widely used drugs, also in the prevention of diseases that occur in the preterm newborn. Previously we have found that GC treatment of pregnant rats resulted in a persistent increase in the ratio of AVP over CRH in the mediobasal hypothalamus, and in an increased CD4/CD8 ratio in the thymus of the newborn. The objective of the present study was to investigate whether such effects were also seen after neonatal GC exposure, given in clinically-relevant doses. Dexamethasone-21-phosphate (DEX; 1.2 microg/g BW, i.p.) was given at day 5 and day 7 after birth. At day 18, 33, and 48 after birth effects of GC on the HPA-axis, and on CD4+ and CD8+ T cells in thymus and spleen were examined. Neonatal DEX treatment temporarily increased (p < 0.01) AVP stores in the external zone of the median eminence (ZEME) on day 18 after birth, and did not affect CRH stores. Resting plasma levels of ACTH and corticosterone remained unchanged after neonatal DEX treatment at any time interval studied. In the thymus and spleen, neonatal DEX treatment decreased (p < 0.0001) T cell numbers on day 18 after birth. Furthermore, neonatal DEX treatment increased (p < 0.01) the ratio of mature CD4-CD8+ over CD4-CD8+ thymocytes on day 18 after birth, but not on day 33 and day 48 after birth. In conclusion, neonatal DEX treatment has temporary effects on peptide expression in hypothalamic CRH neurons, and on thymocyte maturation. Apparently, neonatal exposure to GC affects potentially sensitive targets within the endocrine system and immune system but these alterations are reversible and readjusted during development.

Specific demethylation of the CD4 gene during CD4 T lymphocyte differentiation

The expression of CD4 during T cell development is a highly regulated process. Numerous regulatory elements have been identified including a promoter, two distinct enhancers and a silencer. Here we report a methylation site in the first intron of the CD4 gene that is specifically demethylated in cells which have previously, or are currently expressing CD4. In addition, this site becomes progressively demethylated as T lymphocytes differentiate from double-negative to double-positive to CD4 single-positive thymocytes, and finally to CD4 single-positive peripheral T lymphocytes. This specific and progressive demethylation suggests that this site represents another potential control region for the regulation of CD4.

Isolation of the most immature population of murine fetal thymocytes that includes progenitors capable of generating T, B, and myeloid cells

Thymus cells of murine fetuses at day 12 of gestation are exclusively of the CD3-CD4-CD8-CD44+CD25- phenotype, which is known as a hallmark of the most immature subset of thymus cells. In the present study, we show that day 12 fetal thymus (FT) cells express Fc gamma RII/ III (FcR) at a broad range of levels on their surface. The FcR+ FT cells seem to represent T lineage cells, because a large majority of them express the T lineage specific transcription factors TCF-1 and GATA-3 as well as CD3 epsilon in the cytoplasm. Also shown is that the FcR- population contains progenitors capable of developing into not only T cells but also B and myeloid cells, whereas FcR+ progenitors are mostly committed to the T lineage. These findings indicate that thymic T lineage cells express FcR on their surface at the earliest stage of differentiation, and thus FcR is a useful marker in isolating the most immature population of murine FT cells.

Naltrexone prevents ethanol-induced changes in rat thymus

Ethanol is known to suppress the immune response, but the underlying mechanism accounting for the immunosuppression is not clearly elucidated yet. The purpose of this study was to investigate the effect of a single dose of ethanol on relative proportion of the four major rat thymocyte subsets and possible mechanism of its action. To this end, adult female AO rats were treated with: a) ethanol (2 or 4 g/kg, i.p.), b) naltrexone (5 mg/kg, i.p.) followed 45 min later by ethanol (2 or 4 g/kg, i.p.), c) naltrexone (5 mg/kg, i.p.), or d) only saline. Twenty hours later the rats were sacrificed and the proportion of the four major thymocyte populations defined by expression of CD4 and CD8 molecules was analyzed. Flow cytometric analysis revealed that ethanol evoked a decrease in the percentage of double-positive CD4+CD8+ thymocytes followed by a proportional increase in the percentage of single-positive CD4+CD8- cells. Naltrexone pretreatment prevented the ethanol-induced alterations in thymocyte subsets. The results clearly indicate that ethanol affects the process of intrathymic T-cell maturation. It seems that this effect might be mediated by an opioid-dependent mechanism.

Estrogen blocks early T cell development in the thymus

PROBLEM

Pregnancy and estrogen are known to suppress B lymphopoiesis as well as lead to thymic involution in the mouse. Additionally, estrogen deficiency by oophorectomy reportedly causes a selective increase in the B220+ B cells in the murine bone marrow. The purpose of this study was to determine if estrogens played a regulatory role in T cell development.

METHODS

The first experimental group consisted of 5-6-week-old Balb/c mice that received subcutaneous pellets of placebo, estriol, estradiol, or progesterone. The thymus glands were examined 2-4 weeks after treatment. The second group consisted of 6-week-old Balb/c mice who underwent either bilateral oophorectomy or a sham procedure. Two weeks after the surgery, extensive phenotypic characterization of the thymus and spleen cells was performed by flow cytometry using monoclonal antibodies to surface markers of T cell subsets.

RESULTS

Estrogen treatment causes a dramatic reduction of thymic size and cellularity. All defined T cell subsets of CD4 and CD8 were reduced, with a disproportionate loss of CD4+CD8+ double positive cells. Examination of the triple negative (CD3-CD4-CD8-) subset revealed a striking loss of TN developmental progression of the early precursor cells. Based on the expression of CD44 (pgp-1) and CD25 (IL-2R alpha) markers, the TN thymic compartment was composed almost entirely of the earliest population (CD44+, CD25-), with the remaining maturational stages (CD44+, CD25+; CD44-, CD25+; CD44-, CD25-) depleted. In contrast, all T cell developmental stages in the thymus were found to be in normal proportions in the oophorectomized mice, with no differences in the splenic T and B cell subsets.

CONCLUSIONS

The study demonstrates that estrogen but not progesterone blocks T cell development in the thymus. However, contrary to our expectation, estrogen deprivation by oophorectomy does not enhance T cell development.

Involvement of transcription factors TCF-1 and GATA-3 in the initiation of the earliest step of T cell development in the thymus

Flow cytometric and immunocytochemical analyses of murine fetal thymus (FT) cells with antibodies to various surface markers and transcription factors reveal that the synthesis of TCF-1 and GATA-3 protein begins simultaneously in a fraction of the most immature population of FT cells, which have the phenotype of CD4-CD8-CD44+CD25-. No TCF-1-producing cells is found in the fetal liver (FL). In CD44+CD25- FT cells, the production of TCF-1 is immediately followed by intracellular expression of CD3 epsilon. It is also found that the T cell development from FL, but not FT, progenitors in the FT organ culture system is severely inhibited by the addition of antisense oligonucleotides for either TCF-1 or GATA-3. These results strongly suggest that TCF-1 and GATA-3 play essential roles in the initiation of the earliest steps of T cell development in the thymus.

Both activated and nonactivated leukocytes from the periphery continuously enter the thymic medulla of adult rats: phenotypes, sources and magnitude of traffic

Although the thymus is primarily noted for the export of T cells to the periphery, a small influx of cells has also been observed. It is still a matter of debate whether entry into the thymus depends on prior activation. The phenotypes, sources and degree of immigration are largely unknown. We monitored by quantitative immunohistochemistry the entry of cells from the periphery into the rat thymus in three experimental models. We injected i.v. recirculating, small, nonactivated CD4+ T cell subsets, often referred to as naive (CD45RC+) and memory or antigen-experienced (CD45RC-) cells, purified from thoracic duct lymph of allotype-marked donors, allotype-marked leukocytes released from spleen or lung transplants, or leukocytes labeled in the periphery for 12 weeks during the S-phase of the cell cycle by oral application of 5-bromo-2-deoxyuridine (BrdUrd). Early after i.v. injection (0.5 h), significantly more antigen-experienced (CD45RC-) CD4+ T cells entered the thymus, and by 24 h four times as many cells from the CD45RC- subset as from the CD45RC+ subset had entered the thymus and localized to the medulla. None of the thymic entrants expressed the interleukin (IL)-2 receptor. Following spleen transplantation approximately 40% of donor cells entering the thymic medulla were T cells and approximately 55% were B cells. In contrast, from a lung transplant, approximately 85% of peripheral immigrants were T cells and approximately 10% were B cells. After both procedures, a small number of NK cells and monocytes/macrophages were found among the immigrants (< 5%). Rats were fed BrdUrd continuously for 12 weeks, a procedure which labeled approximately 30% of peripheral lymphocytes but not cortical thymocytes. BrdUrd-labeled cells were localized almost exclusively to the thymic medulla and represented approximately 10% of medullary cells. Of the thymic immigrants approximately 50% were T cells, approximately 30% were B cells (including approximately 15% IgD+ cells), approximately 15% were NK cells and the remainder (approximately 5%) were monocytes/macrophages. Only a quarter of BrdUrd-labeled cells expressed the IL-2 receptor. The thymus is continuously infiltrated by both activated and nonactivated leukocytes from the periphery, including T cells, B cells, NK cells and monocytes. These immigrants are supplied by lymphoid and nonlymphoid organs in a characteristic subset composition. Their entry is facilitated by prior antigen experience or activation. Thus, the participation of the thymic medulla in general leukocyte traffic suggests a mechanism by which the T cell repertoire could potentially be modulated by the peripheral tissues.

Effect of pregnancy on thymic T cell development

PROBLEM

The thymus gland decreases in size during pregnancy. The significance of this alteration is not known.

METHOD

In this report, we examined thymic function by evaluating the development of T lymphocytes in the thymus of pregnant Balb/c mice at 15 and 20 days gestation using multi-color flow cytometry. Comparative analysis was made with non-pregnant mice, post-partum lactating mice, and postpartum non-lactating mice.

RESULTS

Progressive reduction of thymic size and cellularity during pregnancy was observed. All of the CD4 and CD8 defined subsets were reduced, with a disproportionate loss of CD4+, CD8+ double positive cells. Examination of the CD4-, CD8- double negative compartment revealed a predominance of TCR alpha, beta+ double negative cells, and a striking loss of precursor cells. The CD3-, CD4-, CD8- triple negative thymic subset was composed almost entirely of the earliest population (CD44+, CD25-), with the remaining maturational stages (CD44+, CD25+; CD44-, CD25+; and CD44-, CD25-) depleted. At 2 weeks postpartum, the subset ratios normalized, and the total cell count showed recovery.

CONCLUSION

T cell development is blocked at the precursor level during the mouse pregnancy. These effects are transient, and gradual recovery is observed in the postpartum period.

Differential effects of gonadectomy on the thymocyte phenotypic profile in male and female rats

As an organ responsible for generation of T-cell repertoire the thymus occupies a central position in establishment of mature immune response. To assess the potential role of the gonadal steroids in development and maintenance of immunological sexual dimorphism, the effects of gonadectomy pre- and postpuberty on the thymocyte profile of male and female rats were examined. Rats aged 30 days or 75 days were gonadectomized; 30 days later the thymic cellularity was estimated and the expression of the cell surface antigens (CD4 and CD8) and the T-cell receptor (TCR) alpha beta was analyzed by flow cytometry. Regardless of age at surgery, the thymus weight and total thymocyte yield were greater in sham-operated males than females; this sexual dimorphism in thymic cellularity persisted after gonadectomy. Sexual dimorphism in the composition of thymocyte subsets was also evident in sham-operated rats, with males expressing a higher percentage of CD4-8- cells, and remained after gonadectomy of adult rats. In male rats, gonadectomy at day 75 increased the percentage of CD4+8- single-positive and TCR alpha beta + cells. In contrast, in females, ovariectomy decreased the percentages of CD4+8- single-positive, CD4-CD8- double-negative, and TCR alpha beta + cells and increased the percentage of CD4+CD8+ double-positive cells. In the immature rats gonadectomy increased the percentages of CD4+CD8- single-positive and TCR alpha beta + thymocytes and decreased the percentages of double-positive and double-negative cells in males, while in the female it increased the percentage of CD4+8- single-positive thymocytes. Gonadectomy at that age abolished the sexual dimorphism in the expression of accessory molecules (i.e., CD4/CD8), but facilitated gender-specific expression of TCR alpha beta. In conclusion, the results suggest that the gonadal steroids are more important for the development than for the maintenance of the sexual dimorphism in the thymocyte composition.

Morphological study of thymus stromal cells (TEL-2 cell) which play a role in the elimination of double positive immature thymocytes by phagocytosis

BACKGROUND

The process of the selective elimination of immature double positive thymocytes, during culture with TEL-2 cells (an epithelial cell line from the thymus stroma), is initiated by the contact between thymocytes and TEL-2 cells (Nakashima et al. 1990 Eur. J. Immunol., 20: 47-53; Hirata et al. 1991 Anat. Rec., 230: 524-530).

METHODS

Our approach was to follow the process of thymocyte internalization as a sequelae of this interaction with scanning electron microscopy (SEM), transmission electron microscopy (TEM), and electron microscopic cytochemistry for the hydrolytic enzyme acid phosphatase (Ac Pase).

RESULTS

With SEM, numerous thymocytes lay underneath the TEL-2 cells. An enhanced activity of the TEL-2 cell membrane, consisting of several thin thread-like projections either with or without an expanded termination, was involved with contacting the thymocytes. With TEM, close appositions were noted either at adjacent segments of the cellular membrane or between thick plump local protrusions of the plasma membrane. Blunt pseudopodia and deep invaginations on indented portions of the TEL-2 surface engulfed one or more thymocytes. With Ac Pase histochemistry: 1) the incorporation of thymocytes into a phagosome was indicated by the Ac Pase negative thymocytes enclosed in a membrane beneath the protuberance of a TEL-2 cell surface without fusion to the TEL-2 lysosomes; 2) most of these thymocytes were morphologically intact, whereas the rest were already damaged, having changes similar to apoptosis; and 3) a few Ac Pase positive dense bodies of the TEL-2 cell, mostly with the morphology of secondary lysosomes, fused with the thymocyte-enclosing membrane after which the digestion of several thymocytes proceeded.

CONCLUSIONS

With the prominent activity of the plasma membrane involving the initial attachment and subsequent phagocytosis, a murine thymic epithelial cell line TEL-2 plays a demonstrating role in the mechanism of the elimination of double positive thymocytes.

Regulation of AP-1 and NFAT transcription factors during thymic selection of T cells

The ability of thymocytes to express cytokine genes changes during the different stages of thymic development. Although CD4- CD8- thymocytes are able to produce a wide spectrum of cytokines in response to a T-cell receptor (TcR)-independent stimulus, as they approach the double-positive (DP) CD4+ CD8+ stage, they lose the ability to produce cytokine. After the DP stage, thymocytes become single-positive CD4+ or CD8+ thymocytes which reacquire the ability to secrete cytokines. In an attempt to understand the molecular basis of this specific regulatin, we use AP-1-luciferase and newly generated NFAT-luciferase transgenic mice to analyze the transcriptional and DNA-binding activities of these two transcription factors that are involved in the regulation of cytokine gene expression. Here, we show that both AP-1 and NFAT transcriptional activities are not inducible in the majority of DP cells but that during the differentiation of DP cells to the mature single-positive stage, thymocytes regain this inducibility. Subpopulation analysis demonstrates that this inducibility is reacquired at the DP stage before the down-modulation of one of the coreceptors. Indeed AP-1 inducibility, just like the ability to express the interleukin-2 gene, is reacquired during the differentiation of DP TcRlow CD69low heat-stable antigen (HSA)high thymocytes to DP TcRhigh CD69high HSAhigh cells, which is considered to be the consequence of the first signal that initiates positive selection. We therefore propose that the inability of DP thymocytes to induce AP-1 and NFAT activities is one of the causes for the lack of cytokine gene expression at this stage and that this inducibility is reacquired at the latest stage of DP differentiation as a consequence of positive selection. This could be a mechanism to prevent the activation of DP thymocytes before selection has taken place.

DN4: a novel cell surface molecule on cortical epithelial cells of the human thymus

A novel cell surface molecule, DN4, defined by an mAb raised against human thymic epithelial cells, showed a specificity for epithelial cells of the thymic cortex. This antigen was not expressed at detectable levels on any other types of tissues in the human body except for the thymus and bone marrow. Immunohistochemical analysis revealed that the reactivity of anti-DN4 mAb was restricted to the thymic cortex, and the antigen-expressing cells were arranged in a reticular network with long processes between thymocytes. The cellular nature of DN4-positive cells was identified as cortical epithelial cells, as DN4 was expressed in a subpopulation of freshly prepared thymic stromal cells which contain a large amount of keratin and expression of DN4 was strictly confined to the cortical area within the thymus on immunohistochemical analysis of frozen tissues. Immunofluorescence and flow cytometric analysis revealed that a subpopulation of bone marrow cells was also positive for DN4 (20%). The large blasts of normal bone marrow cells were clearly labeled with anti-DN4 mAb, in contrast to small-sized bone marrow cells. This finding suggests that DN4 seems to be transiently expressed in certain blastic stages during the differentiation of bone marrow cells. Immunoprecipitation of 125I-labeled cell lysates from THP-1 and U937 cell lines with anti-DN4 mAb yielded a single chain glycoprotein with an approximate size of 80-85 kd. There was a reduction in apparent molecular weight of approximately 40 kd in the immunoprecipitation of cell lysates after endoglycosidase F treatment. Thus, DN4 seemed to have a considerable amount of carbohydrate group. DN4 appears to be a novel cortical epithelial cell antigen of the human thymus, and although the role of this molecule has not been well established experimentally, the possibility can be suggested that the DN4 molecule might be involved in the positive selection of thymocytes which occurs predominantly in the thymic cortical area.

T cell receptor delta gene rearrangement and T early alpha (TEA) expression in immature alpha beta lineage thymocytes: implications for alpha beta/gamma delta lineage commitment

Mature T cells comprise two mutually exclusive lineages expressing heterodimeric alpha beta or gamma delta antigen receptors. During development, beta, gamma, and delta genes rearrange before alpha, and mature gamma delta cells arise in the thymus prior to alpha beta cells. The mechanism underlying commitment of immature T cells to the alpha beta or gamma delta lineage is controversial. Since the delta locus is located within the alpha locus, rearrangement of alpha genes leads to deletion of delta. We have examined the rearrangement status of the delta locus immediately prior to alpha rearrangement. We find that many thymic precursors of alpha beta cells undergo VDJ delta rearrangements. Furthermore, the same cells frequently coexpress sterile T early alpha (TEA) transcripts originating 3' of C delta and 5' of the most upstream J alpha, thus implying that individual alpha beta lineage cells undergo sequential VDJ delta and VJ alpha rearrangements. Finally, VDJ delta rearrangements in immature alpha beta cells appear to be random, supporting models in which alpha beta lineage commitment is determined independently of the rearrangement status at the TCR delta locus.

The pattern of T lymphocyte differentiation is altered during thymic involution

Thymic involution is likely to be a significant factor in the alteration of peripheral T lymphocyte function with age. The process of thymic involution involves the progressive loss of normal organ architecture and cellular composition, and a significant reduction in the output of mature T lymphocytes. The present study assesses the impact of thymic involution on the T cell differentiation process by quantitating the number and percent representation of various phenotypically distinguishable T cell developmental intermediates in C57BL/6 mice of various ages. The results suggest that several distinct sites in the developmental sequence are impacted by aging. By middle-age (14-17 months), significant perturbations in the frequencies of several CD4-CD8- (DN) subpopulations have occurred. These include a shift towards an increased percentage of Pgp-1+ IL-2R- DN cells, the earliest thymic progenitors, and a decreased percentage and total number of Pgp-1- IL-2R+ DN cells. Furthermore there is a threefold increase in the percentage of DN cells which express CD3 (from 16.6% to 45.5%) which occurs between 4 and 14 months of age. By 24-27 months of age, the percentage of the total DN population increases two- to threefold over that of young (2-3 months) animals, while the fraction of CD4+CD8+ (DP) is significantly reduced. These alterations are consistent with the possibility that thymic involution results in one or more 'developmental' blocks which limit key differentiative transitions within the DN population, and furthermore, the marked increase in the frequency of DN cells displaying CD3 argues that an alternative T cell differentiation pathway plays an increasingly significant role with advancing age.