New thymus transplant experiments reveal that in the absence of competing bone marrow progenitors, existing thymocytes can self-renew, guaranteeing thymus cellularity and the rapid reconstitution of the peripheral T cell pools.
Thymus transplants can correct deficiencies of the thymus epithelium caused by the complete DiGeorge syndrome or FOXN1 mutations. However, thymus transplants were never used to correct T cell–intrinsic deficiencies because it is generally believed that thymocytes have short intrinsic lifespans. This notion is based on thymus transplantation experiments where it was shown that thymus-resident cells were rapidly replaced by progenitors originating in the bone marrow. In contrast, here we show that neonatal thymi transplanted into interleukin 7 receptor–deficient hosts harbor populations with extensive capacity to self-renew, and maintain continuous thymocyte generation and export. These thymus transplants reconstitute the full diversity of peripheral T cell repertoires one month after surgery, which is the earliest time point studied. Moreover, transplantation experiments performed across major histocompatibility barriers show that allogeneic transplanted thymi are not rejected, and allogeneic cells do not induce graft-versus-host disease; transplants induced partial or total protection to infection. These results challenge the current dogma that thymocytes cannot self-renew, and indicate a potential use of neonatal thymus transplants to correct T cell–intrinsic deficiencies. Finally, as found with mature T cells, they show that thymocyte survival is determined by the competition between incoming progenitors and resident cells.
T cell differentiation in the adult thymus depends on sequential interactions between lymphoid progenitors and stromal cells found in distinct regions of the cortex and medulla. Therefore, migration of T cell progenitors through distinct stromal environments seems to be a crucial process regulating differentiation and homeostasis inside the thymus.
Here we show that CCR7-deficient mice are distinguished by a disturbed thymic architecture, impaired T cell development, and decreased numbers of the thymocytes. Analysis of developing double negative (CD4−CD8−) pool of wild-type thymus reveals that CCR7 expression is restricted to a CD25intCD44+ subpopulation. Correspondingly, CCR7 deficiency results in an accumulation of this population in mutant thymus. Furthermore, immunohistology shows that in CCR7-deficient mice CD25+CD44+ cells accumulate at the cortico-medullary junction, suggesting that CCR7 signaling regulates the migration of early progenitors toward the outer thymic cortex, thereby continuing differentiation. Results obtained from mixed bone marrow chimeras support this view, since the development of CCR7-deficient thymocytes is also disturbed in a morphologically intact thymus. Thus, our findings establish an essential role for CCR7 in intrathymic migration and proper T cell development.
chemokines; T cell development; cell migration; thymus; progenitor
The thymus is a complex organ with an epithelium formed by two main cell types, the cortical thymic epithelial (cTECs) and medullary thymic epithelial cells (mTECs), referred to as stroma. Immature thymocytes arising from the bone marrow, macrophages and dendritic cells also populate the thymus. Thymocytes evolve to mature T cells featuring cell differentiation antigens (CDs), which characterize the phenotypically distinct stages, defined as double-negative (DN), double positive (DP) and single positive (SP), based on expression of the coreceptors CD4 and CD8. The thymus is therefore implicated in T cell differentiation and during development into T cells thymocytes are in close association with the stroma. Recent evidence showed that mTECs express a diverse set of genes coding for parenchymal organ specific proteins. This phenomenon has been termed promiscuous gene expression (PGE) and has led to the reconsideration of the role of the thymus in central T cell tolerance to self-antigens, which prevents autoimmunity. The evidence of PGE is causing a reanalysis in the scope of central tolerance understanding. We summarize the evidence of PGE in the thymus, focusing particularly the use of cDNA microarray technology for the broad characterization of gene expression and demarcation of PGE emergence during thymus ontogeny.
A quantitative assay for the hematopoietic precursor of thymocytes has been developed. Using this assay the kinetics of appearance of the progeny of transfused bone marrow and spleen cells in the thymus of irradiated (760 R) mice has been studied. Precursor cells are seven to eightfold more common in bone marrow than in spleen and are absent from peripheral lymph nodes. They decline in number as the animals age. When hematopoietic cells are injected immediately after lethal irradiation only a small number of cells actually enter the gland. Their progeny are not detectable in the thymus for 8-12 days. The time of their detection depends both upon the size of the residual endogenous thymocyte population and the number of progenitor cells injected. Evidence has been presented that excludes thymic injury as the basis for the delay in the appearance of donor type cells and indicates that neither the production of a "homing" signal in the irradiated animal nor the development of precursor cells are limiting factors in the rate of thymic repopulation. These studies indicate that only an exceedingly small number (less than 100) of prothymocytes are required to repopulate the thymus of an irradiated mouse. This restricted number of progenitors must produce the entire repertory of T-cell immunologic responsiveness seen in the first weeks after repopulation.
To be added
Thymus function is thought to depend on a steady supply of T cell progenitors from the bone marrow. The notion that the thymus lacks progenitors with self-renewal capacity is based on thymus transplantation experiments in which host-derived thymocytes replaced thymus-resident cells within 4 wk. Thymus grafting into T cell–deficient mice resulted in a wave of T cell export from the thymus, followed by colonization of the thymus by host-derived progenitors, and cessation of T cell development. Compound Rag2−/−γc−/−KitW/Wv mutants lack competitive hematopoietic stem cells (HSCs) and are devoid of T cell progenitors. In this study, using this strain as recipients for wild-type thymus grafts, we noticed thymus-autonomous T cell development lasting several months. However, we found no evidence for export of donor HSCs from thymus to bone marrow. A diverse T cell antigen receptor repertoire in progenitor-deprived thymus grafts implied that many thymocytes were capable of self-renewal. Although the process was most efficient in Rag2−/−γc−/−KitW/Wv hosts, γc-mediated signals alone played a key role in the competition between thymus-resident and bone marrow–derived progenitors. Hence, the turnover of each generation of thymocytes is not only based on short life span but is also driven via expulsion of resident thymocytes by fresh progenitors entering the thymus.
Bone marrow progenitors migrate to the thymus, where they proliferate and differentiate into immunologically competent T cells. In this report we show that mice transgenic for SV40 T and t antigens under the control of the L-pyruvate kinase promoter develop, in a first step, thymic hyperplasia of both thymocytes and epithelial cells. Morphological studies (histology, immunohistolabeling and electron microscopy) revealed modifications of the thymic microenvironment and gradual expansion of medullary epithelial cells in 1 month-old mice, taking over the cortical region. Then, a thymic carcinoma develops. Two-color labeling of frozen sections identified the transgene in medullary epithelial cells. Flow cytometry analysis demonstrated a marked increase in mature CD4+ and CD8+ thymocytes in adult mice (39±10×106 in transgenic mice and 12±5×106 in age-matched controls). Furthermore, thymocyte export was disturbed.
The chicken thymic microenvironment, as it developed in an embryonic thymus organ
culture system, was phenotypically mapped using a panel of mAb defining both
epithelial and nonepithelial stromal cell antigens. We have previously reported that
thymocyte proliferation and differentiation will proceed for up to 6–8 days in thymus
organ culture, hence demonstrating the functional integrity of the thymic
microenvironment in vitro. During this time, the stromal component reflected that of the
normal embryo with cortical and medullary epithelial areas readily identifiable by both
morphology and surface-antigen expression. An abundance of subcapsular and cortical
epithelial antigens was detected in the cultured thymus, particularly those normally
expressed by the epithelium lining the capsule, trabeculae, and vascular regions (type
epithelium) in the adult and embryonic thymus. Medullary epithelial antigens
developed in organ culture, although were present in lower frequency than observed in
the age-matched embryonic thymus. MHC class II expression by both epithelial and
nonepithelial cells was maintained at high levels throughout the culture period. With
increasing time in culture, the ratio of epithelial to nonepithelial cells decreased,
concurrent with a decrease in thymocyte frequency and suggestive of a bidirectional
interaction between these two cell types. Thus, a functionally intact thymic
microenvironment appears to be maintained in embryonic thymus organ culture, a
model that is currently being exploited to assess the role of stromal antigens, as defined
by our mAb, in the process of thymopoiesis.
Chicken embryonic thymus; thymic stromal cells; thymus organ culture
The seeding and colonization of the thymus by bone marrow stem cells and the maturation of these cells into mature T lymphocytes are dependent on cell-surface recognition events between different cell lineages within the thymic microenvironment. Positive and negative selection processes within the thymus produce a peripheral T-cell repertoire capable of recognizing peptides derived from foreign antigen bound to self MHC molecules. In addition to the TCR/ MHC-peptide interaction, many other cell-surface molecules act in concert to regulate the
kinetics of cellular interactions and intracellular signaling events during thymopoiesis. We have investigated the complexity of the thymic stroma by using monoclonal antibodies to clone cellmembrane molecules of thymic stromal cells. Thymic-shared antigen-1 (TSA-1) is a molecule of interest because it is expressed by both immature thymocytes and stromal cells. We report herein the structural and evolutionary relationships between TSA-1 and molecules of the Ly-6 superfamily (Ly-6SF), and present evidence that TSA-1 functions as a cell-surface receptor by
binding a cognate cell target molecule on the surface of a subset of thymocytes.
Membrane protein; Ly-6 superfamily; cell adhesion; thymocyte
Marrow cells and thymocytes of unprimed donor mice were transplanted separately into X-irradiated syngeneic hosts, with or without sheep erythrocytes (SRBC). Antigen-dependent changes in number or function of potentially immunocompetent cells were assessed by retransplantation of thymus-derived cells with fresh bone marrow cells and SRBC; of marrow-derived cells with fresh thymocytes and SRBC; and of thymus-derived with marrow-derived cells and SRBC. Plaque-forming cells (PFC) of the direct (IgM) and indirect (IgG) classes were enumerated in spleens of secondary host mice at the time of peak responses. By using this two-step design, it was shown (a) that thymus, but not bone marrow, contained antigen-reactive cells (ARC) capable of initiating the immune response to SRBC (first step), and (b) that the same antigen complex that activated thymic ARC was required for the subsequent interaction between thymus-derived and marrow cells and/or for PFC production (second step). Thymic ARC separated from marrow cells but exposed to SRBC proliferated and generated specific inducer cells. These were the cells that interacted with marrow precursors of PFC to form the elementary units for plaque responses to SRBC, i.e. the class- and specificity-restricted antigen-sensitive units. It was estimated that each ARC generated 80–800 inducer cells in 4 days by way of a minimum of 6–10 cell divisions. On the basis of the available evidence, a simple model was outlined for cellular events in the immune response to SRBC.
The thymus is a central lymphoid organ, in which bone marrow-derived T cell precursors undergo a complex process of maturation. Developing thymocytes interact with thymic microenvironment in a defined spatial order. A component of thymic microenvironment, the thymic epithelial cells, is crucial for the maturation of T-lymphocytes through cell-cell contact, cell matrix interactions and secretory of cytokines/chemokines. There is evidence that extracellular matrix molecules play a fundamental role in guiding differentiating thymocytes in both cortical and medullary regions of the thymic lobules. The interaction between the integrin α5β1 (CD49e/CD29; VLA-5) and fibronectin is relevant for thymocyte adhesion and migration within the thymic tissue. Our previous results have shown that adhesion of thymocytes to cultured TEC line is enhanced in the presence of fibronectin, and can be blocked with anti-VLA-5 antibody.
Herein, we studied the role of CD49e expressed by the human thymic epithelium. For this purpose we knocked down the CD49e by means of RNA interference. This procedure resulted in the modulation of more than 100 genes, some of them coding for other proteins also involved in adhesion of thymocytes; others related to signaling pathways triggered after integrin activation, or even involved in the control of F-actin stress fiber formation. Functionally, we demonstrated that disruption of VLA-5 in human TEC by CD49e-siRNA-induced gene knockdown decreased the ability of TEC to promote thymocyte adhesion. Such a decrease comprised all CD4/CD8-defined thymocyte subsets.
Conceptually, our findings unravel the complexity of gene regulation, as regards key genes involved in the heterocellular cell adhesion between developing thymocytes and the major component of the thymic microenvironment, an interaction that is a mandatory event for proper intrathymic T cell differentiation.
Kir1.1 channels are important in maintaining K+ homeostasis in the kidney. Intracellular acidification reversibly closes the Kir1.1 channel and thus decreases K+ secretion. In this study, we used Foster resonance energy transfer (FRET) to determine whether the conformation of the cytoplasmic pore changes in response to intracellular pH (pHi)-gating in Kir1.1 channels fused with enhanced cyan fluorescent protein (ECFP) and enhanced yellow fluorescent protein (EYFP) (ECFP-Kir1.1-EYFP). Because the fluorescence intensities of ECFP and EYFP were affected at pHi < 7.4 where pHi-gating occurs in the ECFP-Kir1.1-EYFP construct, we examined the FRET efficiencies of an ECFP-S219R-EYFP mutant, which is completed closed at pHi 7.4 and open at pHi 10.0. FRET efficiency was increased from 25% to 40% when the pHi was decreased from 10.0 to 7.4. These results suggest that the conformation of the cytoplasmic pore in the Kir1.1 channel changes in response to pHi gating such that the N- and C-termini move apart from each other at pHi 7.4, when the channel is open.
We have generated rats bearing an oxytocin (OXT)-enhanced cyan fluorescent protein (eCFP) fusion transgene designed from a murine construct previously shown to be faithfully expressed in transgenic mice. In situ hybridisation histochemistry revealed that the OXT-eCFP fusion gene was expressed in the supraoptic (SON) and the paraventricular nuclei (PVN) in these rats. The fluorescence emanating from eCFP was observed only in the SON, the PVN, the internal layer of the median eminence (ME) and the posterior pituitary (PP). In in vitro preparations, freshly dissociated cells from the SON and axon terminals showed clear eCFP fluorescence. Immunohistochemistry for OXT and arginine vasopressin (AVP) revealed that the eCFP fluorescence co-localises with OXT-immunofluorescence, but not with AVP-immunofluorescence in the SON and the PVN. Although the expression levels of the OXT-eCFP fusion gene in the SON and the PVN showed a wide range of variation in transgenic rats, eCFP fluorescence was markedly increased in the SON and the PVN, but decreased in the PP after chronic salt loading. The expression of the OXT gene was significantly increased in the SON and the PVN after chronic salt loading in both non-transgenic and transgenic rats. Compared to wild-type animals, euhydrated and salt-loaded male and female transgenic rats showed no significant differences in plasma osmolality, sodium concentration, OXT and AVP levels, suggesting that the fusion gene expression did not disturb any physiological processes. These results suggest that our new transgenic rat is a valuable new tool to identify OXT-producing neurones and their terminals.
hypothalamus; neuropeptides; neurohypophysis; oxytocin; vasopressin; osmolarity; salt loading; transgenic rats; fluorescent proteins; CFP
The role of interleukin-2 (IL-2) in thymic development is uncertain. Not surprisingly, IL-2 knockout (KO) mice have been used to address this question. However, as we report here, such mice are chimeric, containing both IL-2 KO cells and IL-2-expressing cells transferred in utero from their heterozygous mothers. These cells produce IL-2 in amounts detectable by conventional means, and their presence in lymphoid tissues confounds efforts to define the true IL-2 KO phenotype. To minimize the amount of IL-2 available to the thymus, we subjected recombinase activating gene – 1 KO mice to bone marrow transplantation using IL-2 KO donors, and then followed the reconstitution of the thymus. The thymuses of these mice became increasingly aberrant over time, including abnormalities in both stromal cells and thymocytes. These results demonstrate that IL-2 is critical to several aspects of thymic function, a finding previously obscured by the presence of IL-2 in IL-2 KO mice.
interleukin 2; thymus; knock out; chimerism; murine
T cell ontogeny is a sophisticated process, which takes place within the thymus through a series of well-defined discrete stages. The process requires a proper lympho-stromal interaction. In particular, cortical and medullary thymic epithelial cells (cTECs, mTECs) drive T cell differentiation, education, and selection processes, while the thymocyte-dependent signals allow thymic epithelial cells (TECs) to maturate and provide an appropriate thymic microenvironment. Alterations in genes implicated in thymus organogenesis, including Tbx1, Pax1, Pax3, Pax9, Hoxa3, Eya1, and Six1, affect this well-orchestrated process, leading to disruption of thymic architecture. Of note, in both human and mice, the primordial TECs are yet unable to fully support T cell development and only after the transcriptional activation of the Forkhead-box n1 (FOXN1) gene in the thymic epithelium this essential function is acquired. FOXN1 is a master regulator in the TEC lineage specification in that it down-stream promotes transcription of genes, which, in turn, regulate TECs differentiation. In particular, FOXN1 mainly regulates TEC patterning in the fetal stage and TEC homeostasis in the post-natal thymus. An inborn null mutation in FOXN1 leads to Nude/severe combined immunodeficiency (SCID) phenotype in mouse, rat, and humans. In Foxn1−/− nude animals, initial formation of the primordial organ is arrested and the primordium is not colonized by hematopoietic precursors, causing a severe primary T cell immunodeficiency. In humans, the Nude/SCID phenotype is characterized by congenital alopecia of the scalp, eyebrows, and eyelashes, nail dystrophy, and a severe T cell immunodeficiency, inherited as an autosomal recessive disorder. Aim of this review is to summarize all the scientific information so far available to better characterize the pivotal role of the master regulator FOXN1 transcription factor in the TEC lineage specifications and functionality.
Foxn1 gene; TECs; thymus gland; immunodeficiency; Nude/SCID
AKR/J thymocytes derived from fetal liver cells do not produce virus when they differentiate in lethally irradiated B10.K mice, whereas spleen and bone marrow cells are virus producers. In contrast, B10.K thymocytes that differentiate in lethally irradiated AKR mice become virus producers. These results suggest that infection of the thymus in AKR mice is initiated in thymic stromal cells.
Mtg16/Eto2 is a transcriptional corepressor that is disrupted by t(16;21) in acute myeloid leukemia. Using mice lacking Mtg16, we found that Mtg16 is a critical regulator of T-cell development. Deletion of Mtg16 led to reduced thymocyte development in vivo, and after competitive bone marrow transplantation, there was a nearly complete failure of Mtg16−/− cells to contribute to thymocyte development. This defect was recapitulated in vitro as Mtg16−/− Lineage−/Sca1+/c-Kit+ (LSK) cells of the bone marrow or DN1 cells of the thymus failed to produce CD4+/CD8+ cells in response to a Notch signal. Complementation of these defects by reexpressing Mtg16 showed that 3 highly conserved domains were somewhat dispensable for T-cell development but required the capacity of Mtg16 to suppress E2A-dependent transcriptional activation and to bind to the Notch intracellular domain. Thus, Mtg16 integrates the activities of signaling pathways and nuclear factors in the establishment of T-cell fate specification.
We previously demonstrated prolonged, profound CD4+ T-lymphopenia in rheumatoid arthritis (RA) patients following lymphocyte-depleting therapy. Poor reconstitution could result either from reduced de novo T-cell production through the thymus or from poor peripheral expansion of residual T-cells. Interleukin-7 (IL-7) is known to stimulate the thymus to produce new T-cells and to allow circulating mature T-cells to expand, thereby playing a critical role in T-cell homeostasis. In the present study we demonstrated reduced levels of circulating IL-7 in a cross-section of RA patients. IL-7 production by bone marrow stromal cell cultures was also compromised in RA. To investigate whether such an IL-7 deficiency could account for the prolonged lymphopenia observed in RA following therapeutic lymphodepletion, we compared RA patients and patients with solid cancers treated with high-dose chemotherapy and autologous progenitor cell rescue. Chemotherapy rendered all patients similarly lymphopenic, but this was sustained in RA patients at 12 months, as compared with the reconstitution that occurred in cancer patients by 3–4 months. Both cohorts produced naïve T-cells containing T-cell receptor excision circles. The main distinguishing feature between the groups was a failure to expand peripheral T-cells in RA, particularly memory cells during the first 3 months after treatment. Most importantly, there was no increase in serum IL-7 levels in RA, as compared with a fourfold rise in non-RA control individuals at the time of lymphopenia. Our data therefore suggest that RA patients are relatively IL-7 deficient and that this deficiency is likely to be an important contributing factor to poor early T-cell reconstitution in RA following therapeutic lymphodepletion. Furthermore, in RA patients with stable, well controlled disease, IL-7 levels were positively correlated with the T-cell receptor excision circle content of CD4+ T-cells, demonstrating a direct effect of IL-7 on thymic activity in this cohort.
immune reconstitution; interleukin-7; T-cell differentiation; therapeutic lymphodepletion
Livers of the adult mice contain c-kit+ stem cells that can reconstitute thymocytes, multiple lineage cells, and bone marrow (BM) stem cells. Transfer of 1 x 10(7) hepatic mononuclear cells (MNC) and 5 x 10(4) hepatic c-kit+ cells of BALB/c mice induced DP thymocytes within a week in four Gy-irradiated CB17/-SCID mice, but 2 wk were required for BM cells or BM c-kit+ cells to produce DP thymocytes. Moreover, B cell-depleted BM cells or liver MNC of SCID mice that had been rescued by hepatic MNC of BALB/c mice again reconstituted thymus and B cells of other irradiated SCID mice. CD3- IL-2R beta- populations of both BM cells and hepatic MNC of C57BL/6 (B6) mice could generate T cells with intermediate TCR (mostly NK1.1-) in the liver of irradiated B6 SCID mice before thymic reconstitution (extrathymic T cells). Furthermore, transfer of liver c-kit+ cells of B6-Ly 5.1 mice into irradiated B6 SCID (Ly5.2) mice revealed that liver c-kit+ cells can reconstitute myeloid and erythroid lineage cells. These results strongly suggest that the liver contains pluripotent stem cells and serves an important hematopoietic organ even into adulthood.
Bone marrow prothymocytes from me/me and mev/mev mutant mice fail to generate thymocytes in irradiated (600 rad) +/+ wild-type recipients after intravenous injection. However, these same prothymocytes readily generate thymocytes after intrathymic injection. The results of the present study demonstrate that this apparent defect in the thymus- homing capacity of mev/mev prothymocytes can be corrected by mixing irradiated wild-type bone marrow cells with mev/mev bone marrow cells before intravenous injection. However, this defect is not corrected by passage of mev/mev bone marrow cells through the bone marrow of irradiated wild-type recipients. One interpretation of these results is that the maturation of prothymocytes is reversibly arrested in mev/mev mice by a defect in the radiosensitive compartment of the bone marrow microenvironment.
We developed a novel experimental strategy to study T cell regeneration after bone marrow transplantation. We assessed the fraction of competent precursors required to repopulate the thymus and quantified the relationship between the size of the different T cell compartments during T cell maturation in the thymus. The contribution of the thymus to the establishment and maintenance of the peripheral T cell pools was also quantified. We found that the degree of thymus restoration is determined by the availability of competent precursors and that the number of double-positive thymus cells is not under homeostatic control. In contrast, the sizes of the peripheral CD4 and CD8 T cell pools are largely independent of the number of precursors and of the number of thymus cells. Peripheral “homeostatic” proliferation and increased export and/or survival of recent thymus emigrants compensate for reduced T cell production in the thymus. In spite of these reparatory processes, mice with a reduced number of mature T cells in the thymus have an increased probability of peripheral T cell deficiency, mainly in the naive compartment.
CD4 T cells; CD8 T cells; homeostasis; thymus regeneration; thymus export
While most hematopoietic lineages develop in the bone marrow (BM), T cells uniquely complete their development in the specialized environment of the thymus. Hematopoietic stem cells with long-term self-renewal capacity are not present in the thymus. As a result, continuous T cell development requires that BM-derived progenitors be imported into the thymus throughout adult life. The process of thymic homing begins with the mobilization of progenitors out of the bone marrow, continues with their circulation in the bloodstream, and concludes with their settling in the thymus. This review will discuss each of these steps as they occur in the unirradiated and post-irradiation scenarios, focusing on the molecular mechanisms of regulation. Improved knowledge about these early steps in T cell generation may accelerate the development of new therapeutic options in patients with impaired T cell number or function.
thymus; bone marrow; T cells; progenitor; hematopoesis; Hematopoietic stem cells
Although both the T and B cells of the Lewis rat have immunoglobulin receptors for basic protein (BP) of myelin, and both cell types are required for antibody production to BP, the present results demonstrate that the T cells are the only cells required for the induction of experimental allergic encephalomyelitis (EAE). Both EAE and anti-BP were readily induced in thymectomized, irradiated Lewis rats reconstituted with normal thymus and bone marrow cells and challenged with BP in complete Freund's adjuvant. If the thymus cells were first treated with BP heavily labeled with 125I so as to eliminate (sucide) specific T cells, the recipients neither develop EAE nor produce antibody to BP. On the other hand, if the thymus cells were untreated and the specific B cells of bone marrow were eliminated by treatment with 125I-BP, EAE was not inhibited, although no antibody was produced. These results strongly suggest that the T cell is responsible for the induction of EAE although both the T and B cells are competent to respond to BP. Evidence was presented which suggests that neither suppressor T cells nor circulating antibody are involved in the inhibition of EAE by injection of Lewis rats with nonencephalitogenic preparations of BP. The immune status of T and B cells of the Lewis rat to BP was compared with the immune status of these cells in other species to thyroglobulin, where only the B cells appear to be competent. In this context, Brown Norway rats, which are resistant to the induction of EAE, also appear to lack T cells reactive to BP, although competent B cells are present.
A method has been developed for the enrichment of the hematopoietic precursors of thymocytes from spleen and bone marrow cells. Up to 40- fold enrichments were obtained resulting in preparations in which as few as 10(5) cells produced prompt repopulation of the thymus of an irradiated mouse. Precursor cells from bone marrow appear to contain the enzyme terminal deoxyribonucleotidyl transferase (Tdt), an agent suggested as a potential somatic mutator. This enzyme (Tdt) was not detectable in any spleen cell preparation examined, including one in which a 40-fold enrichment of thymocyte precursors had been produced. This is the first difference reported between the splenic and bone marrow precursors of thymocytes.
Seeding of distinct intrathymic microenvironments defined by direct thymocyte-stromal cell interactions was correlated with T cell development in situ using radiation and nonradiation chimeras of Thy- 1.1/1.2 congenic mice. The results identify associations of thymocytes with I-A- macrophages in the cortex as the earliest discernible cell- cell interactions during thymopoiesis. After a significant delay, this recognition stage is followed by concomitant interactions of T cells with I-A+ epithelial cells in the cortex and bone marrow-derived I-A+ dendritic cells in the medulla. All three types of T cell-stromal cell interactions occur after seeding of the intrathymic precursor cell subset and before development of mature medullary-type T cells. The seeding kinetics imply that recognition of cortical epithelial cells by thymocytes in situ represents a relatively late stage of cortical T cell development, whereas thymocyte-dendritic cell interactions denote a very early stage of T cell development in the medulla. The relative positioning of these cell-cell recognition stages during the course of T cell maturation pertains to a putative role of these microenvironments in selection and tolerization of the T cell repertoire.
Reconstitution of the T-cell compartment after bone marrow transplantation depends on
successful colonization of the thymus by bone-marrow-derived progenitor cells. Recent studies
compared the development of syngeneic and allogeneic bone-marrow-derived cells in cocultures
with lymphoid-depleted fetal thymus explants, leading to the discovery of MHC-linked
syngeneic developmental preference (SDP) in the thymus. To determine the nature of cell
interactions among the bone marrow and thymic elements that might underlie SDP, we analyzed
this phenomenon by mathematical modeling. The results indicate that syngeneic mature T cells,
responsible for inducing this preference, probably interfere both with the seeding of allogeneic
bone-marrow-derived thymocyte progenitors in the thymic stroma and with their subsequent
proliferation. In addition, the possibility of augmented death among the developing allogeneic
thymocytes cannot be ruled out.
Thymus; T-cell development; mathematical model; MHC; simulations; syngeneic preference