The thymus microenvironment in regulating thymocyte differentiation

The thymus plays a crucial role in the development of T lymphocytes providing an inductive microenvironment in which committed progenitors undergo proliferation, T-cell receptor gene rearrangements and thymocyte differentiation into mature T-cells. The thymus microenvironment forms a complex network of interaction that comprises non lymphoid cells (e.g., thymic epithelial cells, TEC), cytokines, chemokines, extracellular matrix elements (ECM), matrix metalloproteinases and other soluble proteins. The thymic epithelial meshwork is the major component of thymic microenvironment, both morphologically and phenotypically limiting heterogeneous regions in thymic lobules and fulfilling an important role during specific stages of T-cell maturation. The process starts when bone marrow–derived lymphocyte precursors arrive at the outer cortical region of the thymic gland and begin to mature into functional T lymphocytes that will finally exit the thymus and populate the peripheral lymphoid organs. During their journey inside the thymus, thymocytes must interact with stromal cells (and their soluble products) and extracellular matrix proteins to receive appropriate signals for survival, proliferation and differentiation. The crucial components of the thymus microenvironment and their complex interactions during the T-cell maturation process with the objective of contributing to a better understanding of the function of the thymus as well as assist in the search for new therapeutic approaches to improve the immune response in various pathological conditions are summarized here.     

Amplification of the proliferative response to alloantigen by a factor present in an extract of syngeneic thymic lymphoid cells: demonstration of optimal conditions and target cell for factor activity.

A factor extracted from syngeneic thymic lymphoid cells (thymocytes) is shown to amplify the proliferative (MLC) response of syngeneic lymphoid cells to alloantigen in vitro. The optimal conditions for an effect of the thymus factor are quantitatively defined by kinetic and dose-response studies. Other variables that could potentially influence the activity of the thymus factor, such as the presence of 2-mercaptoethanol and the source of alloantigen, are identified. Factor activity can be recovered from semi-allogeneic thymocytes, as well as syngeneic thymocytes. The factor appears to predominantly effect the proliferative response of T cells localized in peripheral lymphoid organs. As such, this factor appears to be distinct from the variety of previously described factors derived from thymic reticuloepithelial elements that are thought to primarily induce the differentiation of T cell precursors found predominantly in bone marrow. Several possible mechanisms of action of this thymocyte-derived factor are considered.     

Influence of thymectomy on the lymphoid regeneration of hematopoietic bone marrow after sublethal irradiation]

In C57BL mice, bone marrow lymphoid regeneration after a sublethal irradiation is modified by a graft of normal marrow cells. This effect is suppressed in thymectomized mice since a lymphoid peak is observed after a 350 R irradiation; its composition is heterogeneous: small lymphocytes, lymphoblasts and peculier cells named "X cells". The same phenomenon is observed in mice where all the thymocytes and thymus derived and peripheral lymphocytes are destroyed. These results exclude that bone marrow lymphoid regeneration after irradiation is due to a migration of lymphoid cells of thymic origin to the marrow. They could be explained by the effect of a humoral thymic factor on marrow lymphopoiesis.     

Fate of thymocytes: studies with 125I-iododeoxyuridine and 3H-thymidine in mice.

Cortical thymocytes of young adult mice were labeled in situ with radioactive DNA precursors. As a result of cell emigration and cell death, total thymic radioactivity decreased within 8 days to 10% or less of that present on day 1. Accumulation of thymic migrants in peripheral lymphoid organs was estimated by computing the net thymus-derived radioactivity in these tissues. Thymic cell death was assessed by comparing values obtained with 125I-UdR to those acquired with 3H-TdR; The results indicate that cortical thymocytes migrate to the spleen, mesenteric lymph node, femurs and intestine; nevertheless, only a small fraction of the activity originally present in the thymus was recovered in these organs; the vast majority of newly formed cortical thymocytes apparently die after a relatively short life span. Exclusive of the fraction which dies in situ, evidence for thymocyte death is seen in bone marrow; however, most migrants appear to terminate in the intestine.     

The thymus microenvironment in regulating thymocyte differentiation

The thymus plays a crucial role in the development of T lymphocytes by providing an inductive microenvironment in which committed progenitors undergo proliferation, T-cell receptor gene rearrangements and thymocyte differentiate into mature T cells. The thymus microenvironment forms a complex network of interaction that comprises non lymphoid cells (e.g., thymic epithelial cells, TEC), cytokines, chemokines, extracellular matrix elements (ECM), matrix metalloproteinases and other soluble proteins. The thymic epithelial meshwork is the major component of the thymic microenvironment, both morphologically and phenotypically limiting heterogeneous regions in thymic lobules and fulfilling an important role during specific stages of T-cell maturation. The process starts when bone marrow-derived lymphocyte precursors arrive at the outer cortical region of the thymic gland and begin to mature into functional T lymphocytes that will finally exit the thymus and populate the peripheral lymphoid organs. During their journey inside the thymus, thymocytes must interact with stromal cells (and their soluble products) and extracellular matrix proteins to receive appropriate signals for survival, proliferation and differentiation. The crucial components of the thymus microenvironment, and their complex interactions during the T-cell maturation process are summarized here with the objective of contributing to a better understanding of the function of the thymus, as well as assisting in the search for new therapeutic approaches to improve the immune response in various pathological conditions.Key words: thymus, T-cell maturation, thymic microenvironment, thymocyte differantiation, chemokines, extracellular matrix, thymic nurse cells, metalloproteinases     

FATE OF THYMOCYTES: STUDIES WITH 125I-IODODEOXYURIDINE AND 3H-THYMIDINE IN MICE

Cortical thymocytes of young adult mice were labeled in situ with radioactive DNA precursors. As a result of cell emigration and cell death, total thymic radioactivity decreased within 8 days to 10% or less of that present on day 1. Accumulation of thymic migrants in peripheral lymphoid organs was estimated by computing the net thymus-derived radioactivity in these tissues. Thymic cell death was assessed by comparing values obtained with ¹²⁵I-UdR to those acquired with ³H-TdR. The results indicate that cortical thymocytes migrate to the spleen, mesenteric lymph node, femurs and intestine; nevertheless, only a small fraction of the activity originally present in the thymus was recovered in these organs; the vast majority of newly formed cortical thymocytes apparently die after a relatively short life span. Exclusive of the fraction which dies in situ, evidence for thymocyte death is seen in bone marrow; however, most migrants appear to terminate in the intestine.     

The role of the thymus in the ontogeny of the immune system

The proper development of the organs of the immune system is dependent on at least three factors: (1) the development of anlagen with the capacity to trap antigens and support the proliferation of lymphoid and plasma cell precursors; (2) the production by the bone marrow of lymphoid and plasma cell precursors which seed in the lymphoid organs; and (3) the thymus, which seeds reactive cells to the lymphoid organs and produces a humoral factor stimulating antigen-triggered proliferation of primitive lymphoid and plasma cells. Studies on cell population changes in the lymph nodes following thymectomy in mice confirm earlier evidence that most cells produced in the thymus do not seed to the lymphoid organs, but die locally in the thymus.     

Peripheral T cell lymphopenia and concomitant enrichment in naturally arising regulatory T cells: the case of the pre-Talpha gene-deleted mouse

In pre-Talpha (pTalpha) gene-deleted mice, the positively selectable CD4+ CD8+ double-positive thymocyte pool is only 1% that in wild-type mice. Consequently, their peripheral T cell compartment is severely lymphopenic with a concomitant increase in proportion of CD25+ FoxP3+ regulatory T cells. Using mixed bone marrow chimeras, where thymic output was 1% normal, the pTalpha(-/-) peripheral T cell phenotype could be reproduced with normal cells. In the pTalpha(-/-) thymus and peripheral lymphoid organs, FoxP3+ CD4+ cells were enriched. Parabiosis experiments showed that many pTalpha(-/-) CD4+ single-positive thymocytes represented recirculating peripheral T cells. Therefore, the enrichment of FoxP3+ CD4+ single-positive thymocytes was not solely due to increased thymic production. Thus, the pTalpha(-/-) mouse serves as a model system with which to study the consequences of chronic decreased thymic T cell production on the physiology of the peripheral T cell compartment.     

Thymic T-cell tolerance of neuroendocrine functions: physiology and pathophysiology.

Intimate interactions between the two major systems of cell-to-cell communication, the neuroendocrine and immune systems, play a pivotal role in homeostasis and developmental biology. During phylogeny as well as during ontogeny, the molecular foundations of the neuroendocrine system emerge before the generation of diversity within the system of immune defenses. Before reacting against non-self infectious agents, the immune system has to be educated in order to tolerate the host molecular structure (self). The induction of self-tolerance is a multistep process that begins in the thymus during fetal ontogeny (central tolerance) and also involves anergizing mechanisms outside the thymus (peripheral tolerance). The thymus is the primary lymphoid organ implicated in the development of competent and self-tolerant T-cells. During ontogeny, T-cell progenitors originating from hemopoietic tissues (yolk sac, fetal liver, then bone marrow) enter the thymus and undergo a program of proliferation, T-cell receptor (TCR) gene rearrangement, maturation and selection. Intrathymic T-cell maturation proceeds through discrete stages that can be traced by analysis of their cluster differentiation (CD) surface antigens. It is well established that close interactions between thymocytes (pre-T-cells) and the thymic cellular environment are crucial both for T-cell development and for induction of central self-tolerance. Particular interest has focused on the ability of thymic stromal cells to synthesize polypeptides belonging to various neuroendocrine families. The thymic repertoire of neuroendocrine-related precursors recapitulates at the molecular level the dual role of the thymus in T-cell negative and positive selection. Thymic precursors not only constitute a source of growth factors for cryptocrine signaling between thymic stromal cells and pre-T-cells, but are also processed in a way that leads to the presentation of self-antigens by (or in association with) thymic major histocompatibility complex (MHC) proteins. Thymic neuroendocrine self-antigens usually correspond to peptide sequences highly conserved during the evolution of their corresponding family. The thymic presentation of some neuroendocrine self-antigens does not seem to be restricted by MHC alleles. Through the presentation of neuroendocrine self-antigens by thymic MHC proteins, the T-cell system might be educated to tolerate main hormone families. More and more recent experiments support the concept that a defect in thymic tolerogenic function is implicated as an important factor in the pathophysiology of autoimmunity.     

Cellular events during radiation-induced thymic leukemogenesis in mice: abnormal T cell differentiation in the thymus and defect of thymocyte precursors in the bone marrow after split-dose irradiation

Cellular events during the development of thymic lymphomas in young B10.BR mice given leukemogenic split-dose irradiation were studied by examining the differentiation of functional T lymphocyte precursors in the regenerating thymus. It was found that leukemogenic radiation treatment resulted in a sustained depression of the level of thymic cytotoxic T lymphocyte precursors (CTLp) and of mixed lymphocyte reactivity of thymus cells when assessed between 1 and 4 mo after irradiation, in spite of the fact that the total number of thymocytes was restored to the normal level within 2 mo and continued to increase thereafter. In vitro mixing studies of normal thymocytes with thymus cells from split-dose irradiated mice provided no evidence for active suppression as a mechanism for this depressed activity. The ability of bone marrow cells from split-dose irradiated mice to regenerate the thymus and to differentiate into functional CTLp was examined by use of supralethally irradiated Thy-1 congenic recipients. Reconstitution of supralethally irradiated B10.BR Thy-1.2 mice with normal bone marrow from B10.BR Thy-1.1 mice resulted in the complete repopulation of host-thymus with donor-derived cells when assessed at 4 wk after reconstitution. Lymphocytes from the regenerating thymus of these animals were shown to contain high levels of CTLp which were donor-derived. On the other hand, when the recipient mice were reconstituted with bone marrow cells from donor mice which had been split-dose irradiated 1 mo earlier, regeneration of the recipient thymus was severely depressed when assessed at 4 wk to 3 mo after reconstitution. Although variable but small numbers of donor-derived Thy-1+ cells were detected, CTL activity for alloantigen could not be induced in these donor-derived cells. The results suggest that T cell precursors derived from split-dose irradiated donor mice were unable to undergo active proliferation and differentiation into functional CTLp. The significance of these findings on radiation-induced thymic leukemogenesis is discussed.     

Developmental regulation of the intrathymic T cell precursor population

The maturation potential of CD4-8- thymocytes purified from mice of different developmental ages was examined in vivo after intrathymic injection. As previously reported, 14-day fetal CD4-8- thymocytes produced fewer CD4+ than CD8+ progeny in peripheral lymphoid tissues, resulting in a CD4+:CD8+ ratio of less than or equal to 1.0. In contrast, adult CD4-8- thymocytes generated CD4+ or CD8+ peripheral progeny in the proportions found in the normal adult animal (CD4+:CD8+ = 2 to 3). Here we have shown that CD4-8- precursor cells from the 17-day fetal thymus also produced peripheral lymphocytes with low CD4+:CD8+ ratios. Precursors from full term fetuses produced slightly higher CD4+:CD8+ ratios (1.1-1.6) and precursors from animals three to 4 days post-birth achieved CD4+:CD8+ ratios intermediate between those produced by fetal and adult CD4-8- thymocytes. Parallel changes in the production of alpha beta TCR+ peripheral progeny were observed. Fetal CD4-8- thymocytes generated fewer alpha beta TCR+ progeny than did adult CD4-8- thymocytes. However, peripheral lymphocytes arising from either fetal or adult thymic precursors showed similar proportions of gamma delta TCR+ cells. The same pattern of progeny was observed when fetal CD4-8- thymocytes matured in an adult or in a fetal thymic stromal environment. In contrast to fetal thymic precursors, fetal liver T cell precursors resembled adult CD4-8- thymocytes by all parameters measured. These results suggest that fetal thymic precursors are intrinsically different from both adult CD4-8- thymocytes and fetal liver T cell precursors. Moreover, they lead to the hypothesis that the composition of the peripheral T cell compartment is developmentally regulated by the types of precursors found in the thymus. A model is proposed in which migration of adult-like precursors from the fetal liver to the thymus approximately at birth triggers a transition from the fetal to the adult stages of intrathymic T cell differentiation.     

Bone marrow-derived hemopoietic precursors commit to the T cell lineage only after arrival in the thymic microenvironment

T lymphocytes develop in the thymus from hemopoietic precursors that commit to the T cell lineage under the influence of Notch signals. In this study, we show by single cell analyses that the most immature hemopoietic precursors in the adult mouse thymus are uncommitted and specify to the T cell lineage only after their arrival in the thymus. These precursors express high levels of surface Notch receptors and rapidly lose B cell potential upon the provision of Notch signals. Using a novel culture system with complexed, soluble Notch ligands that allows the titration of T cell lineage commitment, we find that these precursors are highly sensitive to both Delta and Jagged ligands. In contrast, their phenotypical and functional counterparts in the bone marrow are resistant to Notch signals that efficiently induce T cell lineage commitment in thymic precursors. Mechanistically, this is not due to differences in receptor expression, because early T lineage precursors, bone marrow lineage marker-negative, Sca-1-positive, c-Kit-positive and common lymphoid progenitor cells, express comparable amounts of surface Notch receptors. Our data demonstrate that the sensitivity to Notch-mediated T lineage commitment is stage-dependent and argue against the bone marrow as the site of T cell lineage commitment.     

c-fos expression interferes with thymus development in transgenic mice

To study the function of the proto-oncogene c-fos in hematopoietic tissues, transgenic mice were generated that express c-fos from the H2-Kb promoter in several organs. These H2-c-fos mice have enlarged spleens and hyperplastic thymuses containing an increased number of thymic epithelial cells. The exogenous c-fos expression specifically affects T cell development in the thymus, thereby increasing the fraction of mature thymocytes. Results obtained with bone marrow radiation chimeras suggest that the altered distribution of T cell subsets is not a direct effect of c-fos expression within the T cell lineage. No changes in the proportion of hematopoietic cell lineages are seen in the spleen, and these mice do not develop lymphoid malignancies. B and T cell function, however, is impaired, and H2-c-fos mice are immune deficient. It appears that c-fos specifically stimulates the proliferation of thymic epithelial cells, and may thus indirectly affect T cell development.     

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

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

The functional capabilities of cells leaving the thymus

There has been a controversy for many years over the functional status of cells that leave the thymus (thymus migrants) to populate the peripheral lymphoid organs. Are they immunoincompetent like cortical thymocytes and so probably derived from them, or are they functionally mature like medullary thymocytes? Until recently the techniques used to assess putative thymus migrants have been indirect, but it is now possible to measure the function of recent thymus migrants directly. We used intrathymic injection of a solution of fluorescein isothiocyanate to label thymocytes, and used electronic cell sorting to purify the fluorescent cells that accumulate in the periphery over the following 3 to 4 hr. The migrants have been enriched from an original frequency of about 1:1000 in lymph nodes and spleen, to greater than 98% purity. These cells have been compared with normal peripheral T cells for proliferative and cytotoxic precursor activity in a high cloning efficiency, lectin-induced, limit dilution culture system and in an allospecific limit dilution system. The frequency of precursors of proliferative lymphocytes and cytotoxic lymphocytes and the size of the clones produced is the same for recent migrants and peripheral T cells. Thus by the criteria of proliferation and cytotoxic responses to mitogens and generation of allospecific CTL, thymus migrants, a few hours after their emigration from the thymus, are fully immunocompetent; we therefore see no evidence of a post-thymic precursor-type cell that requires major maturation steps after leaving the thymus.     

The stem cell antigens Sca-1 and Sca-2 subdivide thymic and peripheral T lymphocytes into unique subsets

Stem cell Ag 1 and 2 (Sca-1 and Sca-2), so named due to their expression by mouse bone marrow stem cells, were evaluated for expression by populations of cells within the thymus. Immunohistochemical analysis demonstrated that Sca-1 was expressed by cells in the thymic medulla and by some subcapsular blast cells, as well as by the thymic blood vessels and capsule. Sca-2 expression, which was limited to the thymic cortex, could be associated with large cycling thymic blast cells. Both Sca-1 and Sca-2 were expressed on a sub-population of CD4-CD8- thymocytes, and this subpopulation was entirely contained within the Ly-1lo progenitor fraction of cells. Sca-1 expression by a phenotypically mature subset of CD4+CD8- thymocytes was also noted. Conversely, Sca-2 expression was observed on a phenotypically immature or nonmature subpopulation of CD4-CD8- thymocytes. MEL-14, an antibody that defines functional expression of a lymphocyte homing molecule, identified a small population of thymocytes that contained all four major thymic subsets. Sca-2 split the MEL-14hi thymocyte subset into two Sca-2+ non-mature/immature phenotype fractions and two Sca-2- mature phenotype fractions. In peripheral lymphoid organs, Sca-1 identified a sub-population of mature T lymphocytes that is predominantly CD4+CD8-, in agreement with the thymic distribution of Sca-1. Peripheral T cells of the CD4-CD8+ phenotype were predominantly Sca-1-. In contrast, Sca-2 did not appear to stain peripheral T lymphocytes, but recognized only a subset of B lymphocytes which could be localized by immunohistochemistry to germinal centers. Thus, expression of Sca-1 is observed throughout T cell ontogeny, whereas Sca-2 is expressed by some subsets of thymocytes, including at least one half of thymic blasts, but not by mature peripheral T lymphocytes.     

DKK1 Mediated Inhibition of Wnt Signaling in Postnatal Mice Leads to Loss of TEC Progenitors and Thymic Degeneration

Background

Thymic epithelial cell (TEC) microenvironments are essential for the recruitment of T cell precursors from the bone marrow, as well as the subsequent expansion and selection of thymocytes resulting in a mature self-tolerant T cell repertoire. The molecular mechanisms, which control both the initial development and subsequent maintenance of these critical microenvironments, are poorly defined. Wnt signaling has been shown to be important to the development of several epithelial tissues and organs. Regulation of Wnt signaling has also been shown to impact both early thymocyte and thymic epithelial development. However, early blocks in thymic organogenesis or death of the mice have prevented analysis of a role of canonical Wnt signaling in the maintenance of TECs in the postnatal thymus.

Methodology/Principal Findings

Here we demonstrate that tetracycline-regulated expression of the canonical Wnt inhibitor DKK1 in TECs localized in both the cortex and medulla of adult mice, results in rapid thymic degeneration characterized by a loss of ΔNP63⁺ Foxn1⁺ and Aire⁺ TECs, loss of K5K8DP TECs thought to represent or contain an immature TEC progenitor, decreased TEC proliferation and the development of cystic structures, similar to an aged thymus. Removal of DKK1 from DKK1-involuted mice results in full recovery, suggesting that canonical Wnt signaling is required for the differentiation or proliferation of TEC populations needed for maintenance of properly organized adult thymic epithelial microenvironments.

Conclusions/Significance

Taken together, the results of this study demonstrate that canonical Wnt signaling within TECs is required for the maintenance of epithelial microenvironments in the postnatal thymus, possibly through effects on TEC progenitor/stem cell populations. Downstream targets of Wnt signaling, which are responsible for maintenance of these TEC progenitors may provide useful targets for therapies aimed at counteracting age associated thymic involution or the premature thymic degeneration associated with cancer therapy and bone marrow transplants.     

Neonatal tolerance induction in the thymus to MHC-class II-associated antigens. III. Significance of hemopoietic stem cells for induction and maintenance of Mls tolerance by continuous supply of tolerance-inducing nonlymphocytes

The role of hemopoietic stem cells and other cell types in the induction and maintenance of immunologic tolerance in the thymus was investigated by intravenous injection of Mls-semi-allogeneic cells into newborn mice less than 24 hr after birth. Mls-specific tolerance was induced by inoculation of peritoneal cells and thymus cells, and the tolerant state was compared with that induced by bone marrow cells which had hemopoietic stem cell activity and were able to create a stable chimera in both central and peripheral lymphoid organs. When peritoneal or thymus cells were injected, the level of tolerance attained was proportional to the number of cells injected, though peritoneal cells were 20 times as effective as thymus cells. In vivo functions of tolerance-inducing cells and their immediate precursors were radiosensitive and belonged to a Thy-1-, nylon-wool-nonadherent (probably non-B), weakly Sephadex G-10-adherent cell population. Tolerance induced by peritoneal cell injections was transient, starting to terminate within the first 2 weeks of life, while tolerance caused by bone marrow cell injections persisted through more than 6 weeks. Such transient tolerance induced by the former became long-lasting when followed by an additional injection of bone marrow cells, which did not cause thymic lymphocyte chimerism. All data indicated that bone marrow stem cells were engaged in tolerance induction and maintenance by continuously supplying tolerance-inducing nonlymphocytes.     

Characterization of thymic progenitors in adult mouse bone marrow

Thymic cellularity is maintained throughout life by progenitor cells originating in the bone marrow. In this study, we describe adult mouse bone cells that exhibit several features characteristic of prothymocytes. These include 1) rapid thymic engraftment kinetics following i.v. transplantation, 2) dramatic expansion of thymic progeny, and 3) limited production of hemopoietic progeny other than thymocytes. The adult mouse bone marrow population that is depleted of cells expressing any of a panel of lineage-specific Ags, stem cell Ag-1 positive, and not expressing the Thy1.1 Ag (Thy1.1(-)) (Thy1.1(-) progenitors) can repopulate the thymus 9 days more rapidly than can hemopoietic stem cells, a rate of thymic repopulation approaching that observed with transplanted thymocytes. Additionally, Thy1.1(-) progenitors expand prolifically to generate thymocyte progeny comparable in absolute numbers to those observed from parallel hemopoietic stem cell transplants, and provide a source of progenitors that spans multiple waves of thymic seeding. Nevertheless, the Thy1.1(-) population yields relatively few B cells and rare myeloid progeny posttransplant. These observations describe the phenotype of an adult mouse bone marrow population highly enriched for rapidly engrafting, long-term thymocyte progenitors. Furthermore, they note disparity in B and T cell expansion from this lymphoid progenitor population and suggest that it contains the progenitor primarily responsible for seeding the thymus throughout life.     

Impaired development of T lymphoid precursors from pluripotent hematopoietic stem cells in New Zealand Black mice.

Bone marrow cells from autoimmune-prone New Zealand Black (NZB) mice are less efficient at colonizing fetal thymic lobes than cells from normal strains. This study demonstrates that the reduced capacity of NZB bone marrow cells to repopulate the thymus does not result from their inability to migrate to or enter the thymus. Rather, the T lymphopoietic defect of NZB mice is due to an impaired ability of pluripotent hematopoietic stem cells (PHSCs) to generate more committed lymphoid progeny, which could include common lymphoid precursors and/or other T cell-committed progenitors. Although PHSCs from NZB mice were not as efficient at thymic repopulation as comparable numbers of PHSCs from control strains, the ability of common lymphoid precursors from NZB mice to repopulate the thymus was not defective. Similarly, more differentiated NZB T cell precursors included in the intrathymic pool of CD4(-)CD8(-) cells also exhibited normal T lymphopoietic potential. Taken together, the results identify an unappreciated defect in NZB mice and provide further evidence that generation of lymphoid progeny from the PHSCs is a regulated event.