Control of the thymic microenvironment by growth hormone/insulin-like growth factor-I-mediated circuits

The thymus gland is a central lymphoid organ in which bone marrow-derived T cell precursors undergo maturation, eventually leading to the migration of positively selected thymocytes to the T-dependent areas of peripheral lymphoid organs. This process occurs under the influence of the thymic microenvironment, by means of secretory polypeptides and cell-cell contacts. The thymic microenvironment is a tridimensional cellular network composed of epithelial cells (its major component), macrophages, dendritic cells, fibroblasts and extracellular matrix elements. The epithelial reticulum is a heterogeneous tissue, in which a particular lymphoepithelial structure has been isolated in vitro: the thymic nurse cell complex, which possibly creates particular microenvironmental conditions for thymocyte differentiation. Additionally, thymic nurse cells are useful tools to study mechanisms involved in intrathymic T cell migration, including neuroendocrine influences. Previous data showed that thymic hormonal function can be modulated by hormones and neuropeptides, including growth hormone. Interestingly, GH acts pleiotropically on the thymic epithelium increasing cell growth and expression of extracellular matrix ligands and receptors, the latter resulting in an enhancement of thymocyte adhesion to the epithelial cells and thymocyte release from thymic nurse cells. The role of GH on thymus development is further stressed by the findings obtained with GH-deficient dwarf mice. Besides the precocious decline in serum thymulin found in these animals, a progressive thymic hypoplasia occurs, with decreased numbers of CD4+CD8+thymocytes, both defects being largely restored by long-term GH treatment. The effects of GH in the thymus are apparently mediated by IGF-1. Enhancement of thymulin secretion induced by GH, as well as the stimulation of thymocyte adhesion to thymic epithelial cells can be prevented in vitro by treatment with antibodies for IGF-I or IGF-I receptor. Moreover, in both systems IGF-I alone can yield similar effects. Also, the enhanced concanavalin-A mitogenic response and IL-6 production by thymocytes observed in GH-treated mice can be detected in animals treated with IGF-I. Lastly, mouse substrains selected for high or low IGF-I circulating levels exhibited differential thymus developmental patterns correlating with IGF-I levels. A further conceptual aspect concerning the GH-IGF-I-mediated control of thymus physiology is the recent demonstration of an intrathymic production of these molecules, leading to the hypothesis that, in addition to the classical endocrine pathway, GH-IGF-I-mediated paracrine and autocrine pathways may also be implicated in the control of thymus physiology. In any case, such control is exerted pleiotropically, with modulation in the expression of several genes in different cell types of the organ. In this respect, it is exciting to imagine a role of GH-IGF-I loops in shaping the intrathymically generated T cell repertoire.     

Human immunodeficiency virus infection of the human thymus and disruption of the thymic microenvironment in the SCID-hu mouse

Infection with the human immunodeficiency virus (HIV) results in immunosuppression and depletion of circulating CD4+ T cells. Since the thymus is the primary organ in which T cells mature it is of interest to examine the effects of HIV infection in this tissue. HIV infection has been demonstrated in the thymuses of infected individuals and thymocytes have been previously demonstrated to be susceptible to HIV infection both in vivo, using the SCID-hu mouse, and in vitro. The present study sought to determine which subsets of thymocytes were infected in the SCID-hu mouse model and to evaluate HIV-related alterations in the thymic microenvironment. Using two different primary HIV isolates, infection was found in CD4+/CD8+ double positive thymocytes as well as in both the CD4+ and CD8+ single positive subsets of thymocytes. The kinetics of infection and resulting viral burden differed among the three thymocyte subsets and depended on which HIV isolate was used for infection. Thymic epithelial (TE) cells were also shown to endocytose virus and to often contain copious amounts of viral RNA in the cytoplasm by in situ hybridization, although productive infection of these cells could not be definitively shown. Furthermore, degenerating TE cells were observed even without detection of HIV in the degenerating cells. Two striking morphologic patterns of infection were seen, involving either predominantly thymocyte infection and depletion, or TE cell involvement with detectable cytoplasmic viral RNA and/or TE cell toxicity. Thus, a variety of cells in the human thymus is susceptible to HIV infection, and infection with HIV results in a marked disruption of the thymic microenvironment leading to depletion of thymocytes and degeneration of TE cells.     

Case report: non-extraction treatment with functional and mechanical therapy of a Class II division 1 malocclusion

The cardinal sign of acute stress is thymic involution, which subsequently attenuates the activity of immunocompetent cells, notably T-lymphocytes, macrophages and NK cells. Sodium diethyldithiocarbamate (DTC), a low molecular weight sulphur compound, may function as a thymic hormone to induce precursor cells to become functionally mature T-lymphocytes. The studies were conducted on Balb/c mice exposed to restraint stress twice for 12 h at 24 h intervals. DTC at a dose of 20 mg/kg or calf thymus extract (TFX) at a dose of 10 mg/kg were injected i.p. four times at 24 h intervals prior to the exposure. It has been found that restraint stress markedly reduces the number of thymocytes which is concomitant with reduction in the weight of the thymus. In our study the changes sustained for 10 days of the observation. Besides, alterations in proliferative response of the thymocytes stimulated in vitro with concanavalin A (Con A) and phytohemagglutinin (PHA) were observed. The proliferative response of thymocytes to Con A was reduced 24 h after the exposure to restraint stress, but between days 4 and 7 it was found at increased levels, which decreased again on day 10. In contrast, the proliferative response of thymocytes to PHA was depressed for the entire 10 day period of the observation. It has been found that DTC and TFX administered to mice prior to restraint stress successfully counteract stress-induced immunosuppression, albeit TFX exerts stronger protective and regenerating impact on the thymus than DTC. TFX totally inhibits the suppressive effect of stress on proliferative activity of the thymocytes stimulated in vitro with Con A and PHA, stimulates restoration of thymic cells and increases the weight of the thymus. In contrast, DTC is not able to counteract the decrease in proliferative response of thymocytes to PHA.     

Epithelioid granulomas in Hodgkin''s disease--prognostic significance

Liposome-entrapped dichloromethylene diphosphonate (Cl2MDP) was injected locally into the thymus of adult rats. This treatment, which has been found to deplete resident macrophages in other organs, also reduced the number of thymic macrophages. The depletion was evident in the cortex and the corticomedullary zone at 5, 13 and 23 days after Cl2MDP treatment, while the decrease in medullary macrophages only became significant 13 and 23 days after injection. However, the maximal reduction of macrophages was evident 13 days after this treatment. Thirty-five days after Cl2MDP treatment, macrophages repopulated the thymus. Our results suggest that the cortical and cortico-medullary macrophages have a high phagocytic activity consistent with their predominant role as scavengers of the large numbers of thymocytes that die under physiological conditions. It is concluded that liposome-mediated depletion of thymic macrophages can serve as an experimental model to study the roles of these cells.     

Cardiac risk in surgery. A review and guidelines for risk evaluation and reduction of cardiac risk in general surgery

We report herein the phenotypic and functional analysis of human bone marrow and thymus derived early T cells. Commitment to T cell lineage is acquired during CD7 antigen expression by CD34+ precursors in human bone marrow and before thymus colonization. Early thymocytes show similar phenotypic characteristics as bone marrow T cells. They rapidly acquire CD4 before the dual expression of CD4 and CD8. Their expansion and differentiation is regulated by two major factors: thymic stroma and cytokines produced by these stroma cells or by thymocytes themselves. Among cytokines, IL1 and sCD23 produced by thymic epithelial cells support in vitro early T cell development.     

Phenotypic mapping of the chicken embryonic thymic microenvironment developing within an organ culture system

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 with 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 I 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.     

Tolerance induction by clonal deletion of CD4+8+ thymocytes in vitro does not require dedicated antigen-presenting cells

The cellular requirements of T cell tolerance induction in the thymus by clonal deletion was investigated by using an in vitro assay: thymocytes from mice expressing a transgenic TcR specific for lymphocytic choriomeningitis virus (LCMV) and H-2Db were co-cultured with various H-2b cell types as antigen-presenting cells in the presence of the antigenic LCMV peptide. The results revealed that all cell lines examined including embryonic and transformed fibroblasts, melanoma cells, cortical thymic epithelial cells, lymphomas and neuronal cells induced an antigen dose-dependent deletion of CD4+8+ thymocytes. Similarly, highly enriched accessory cell populations from thymus and spleen (macrophages, dendritic and cortical epithelial cells, i.e. thymic nurse cells) could induce antigen-specific depletion of immature CD4+8+ thymocytes. Depending on the cell type examined micromolar to picomolar concentration of LCMV peptide were required to induce deletion. The effectiveness of deletion by the different cell types did not correlate with their major histocompatibility class I expression level; it was, however, influenced by the presence of ICAM-1 adhesion molecules.     

Endometrial effects during hormone replacement therapy with a sequential oestradiol valerate/cyproterone acetate preparation

Different stages of thymus morphogenesis and thymocyte differentiation have been studied at the ultrastructural level in the lizard, Chalcides ocellatus. On stage 36 of embryonic development, the thymus primordium was composed principally of undifferentiated epithelial cells and some lymphoid stem-cells. From stage 37 to 38, the lymphoid stem-cells differentiate into lymphoblasts and then transform into typical lymphocytes. A clasmotosis phenomenon seems to be involved in this transformation. In the developing cortical regions, lymphoblasts accumulated rapidly, stretching the epithelial cells which become stellate in shape. From stage 39 to 40, a phase of intense proliferation occurs and numerous lymphocytes die in the thymic tissue and are phagocytosed by macrophages. On stage 41, the presence of interdigitating cells in the medullary area completed cortico-medullary differentiation. On neonatal and juvenile lizards, small cortical thymocytes differentiated and the thymus possessed all characteristic of an adult thymus. Thus, at birth, the histogenesis of the lizard thymus was achieved and the only further modification consisted in a gain of weight.     

Disaggregating parental depression and family stress in assessing families of children with developmental disabilities: a multisample analysis

Thymic nurse cells are defined in vitro as multicellular complexes of epithelial cells and thymocytes. Although these structures have been implicated in the intrathymic differentiation of thymocytes, little is known about the biology of this cell complex and about the occurrence of the cells in the thymus in situ. Therefore, to clarify the matter, in this review we have presented characteristics of epithelial cells capable of forming complexes with thymocytes, in light of the literature data and the experience of the authors. The structure of cells within the complexes allowed us to distinguish three types of thymic nurse cells. Three-dimensional reconstruction of the thymus and observations employing TEM and SEM demonstrated the presence of distinct types of complexes in various topographic regions of the thymus. Where possible, the functional relevance of the morphological data was analyzed.     

Thymic dendritic cells are primary targets for the oncogenic virus SL3-3

The murine retrovirus SL3-3 causes malignant transformation of thymocytes and thymic lymphoma in mice of the AKR and NFS strains when they are inoculated neonatally. The objective of the present study was to identify the primary target cells for the virus in the thymuses of these mice. Immunohistochemical studies of the thymus after neonatal inoculation of the SL3-3 virus showed that cells expressing the viral envelope glycoprotein (gp70(+) cells) were first seen at 2 weeks of age. These virus-expressing cells were found in the cortex and at the corticomedullary junction in both mouse strains. The gp70(+) cells had the morphology and immunophenotype of dendritic cells. They lacked macrophage-specific antigens. Cell separation studies showed that bright gp70(+) cells were detected in a fraction enriched for dendritic cells. At 3 weeks of age, macrophages also expressed gp70. At that time, both gp70(+) dendritic cells and macrophages were found at the corticomedullary junction and in foci in the thymic cortex. At no time during this 3-week period was the virus expressed in cortical and medullary epithelial cells or in thymic lymphoid cells. Infectious cell center assays indicated that cells expressing infectious virus were present in small numbers at 2 weeks after inoculation but increased at 5 weeks of age by several orders of magnitude, indicating virus spread to the thymic lymphoid cells. Thus, at 2 weeks after neonatal inoculation of SL3-3, thymic dendritic cells are the first cells to express the virus. At 3 weeks of age, macrophages also express the virus. In subsequent weeks, the virus spreads to the thymocytes. This pathway of virus expression in the thymus allows the inevitable provirus integration in a thymocyte that results in a clonal lymphoma.     

Fas ligand expression is restricted to nonlymphoid thymic components in situ

The cell surface receptor Fas (Apo-1/CD95) and its ligand (FasL) are mediators of apoptosis that have been shown to be implicated in activation-induced death of mature T cells and in killing mediated by cytolytic T cells. The role of the Fas pathway in apoptosis associated with thymic selection events is, however, controversial. Although Fas and FasL are known to be expressed in the thymus, the nature and in vivo localization of FasL-expressing cells have not been determined. Using recently developed anti-FasL Abs in combination with in situ hybridization on tissue sections, we show in this work that FasL-expressing cells are present in the thymus, particularly within the medulla. FasL mRNA was detected readily in thymic stromal cell extracts, but not in isolated thymocytes. Moreover, immunohistochemical analysis of serial tissue sections stained with Abs against FasL in conjunction with epithelial and dendritic cell markers indicated that both thymic epithelial and dendritic cells express FasL in situ. The coexistence of FasL-expressing stromal cells and Fas-expressing thymocytes may have important implications for the role of the Fas pathway in apoptosis associated with thymic selection events.     

Thymic overexpression of CD40 ligand disrupts normal thymic epithelial organization

We characterized the distribution of CD40 and CD40 ligand (CD40-L) in the adult and developing murine thymus. Before birth, CD40 was almost exclusively localized to scattered foci of medullary cells. By birth there was a dramatic upregulation of CD40 expression by cortical epithelial cells, which was accompanied by a consolidation of medullary epithelial foci. CD40-L+ thymocytes displayed a medullary location. Analysis of mice deficient in CD40-L expression indicated that CD40-L/CD40 interactions were not required for development of the medullary compartment. Overexpression of CD40-L targeted to thymocytes altered thymic architecture, as reflected by a dramatic loss of cortical epithelial cells, expansion of the medullary compartment, and extensive infiltration of the capsule with a mixture of CD3+ cells, B-cells, and macrophages/dendritic cells. Reconstitution of lethally irradiated normal mice with lck CD40-L bone marrow cells also resulted in loss of cortical epithelium and expansion of the medullary compartment. Disruption of the normal pattern of thymic architecture and epithelial differentiation as a consequence of increased intrathymic levels of CD40-L expression points to a role for CD40-L/CD40 interactions in the normal pattern of epithelial compartmentalization/differentiation within the thymic environment.     

Contact with thymic epithelial cells as a prerequisite for cytokine-enhanced human immunodeficiency virus type 1 replication in thymocytes

We report here that human immunodeficiency virus type 1 (HIV-1)-infected human thymocytes, in the absence of any exogenous stimulus but cocultivated with autologous thymic epithelial cells (TEC), obtained shortly (3 days) after thymus excision produce a high and sustained level of HIV-1 particles. The levels and kinetics of HIV-1 replication were similar for seven distinct viral strains irrespective of their phenotypes and genotypes. Contact of thymocytes with TEC is a critical requirement for optimal viral replication. Rather than an inductive signal resulting from the contact itself, soluble factors produced in the mixed culture are responsible for this effect. Specifically, the synergistic effects of tumor necrosis factor, interleukin-1 (IL-1), IL-6, and granulocyte-macrophage colony-stimulating factor may account by themselves for the high level of HIV-1 replication in thymocytes observed in mixed cultures. In conclusion, the microenvironment generated by TEC-thymocyte interaction might greatly favor optimal HIV-1 replication in the thymus.     

In vitro study of urinary calculi fragmentation with 4 different systems of lithofragmentation

The thymus is a primary lymphoid organ that serves the immune system by providing an optimal microenvironment for developing T cells to rearrange the genes encoding the T-cell receptor and to undergo positive and negative selection in shaping the peripheral T-cell repertoire. The microenvironment of the organ is peculiar among lymphoid organs, as the supporting stroma consists of reticular epithelial cells. Bone marrow-derived interdigitating cells and macrophages are the main accessory cell populations. The epithelium, interdigitating cells, and macrophages each contribute to the T-cell selection process. During the last decade knowledge has been gathered that these cell populations show a considerable heterogeneity, as documented for subcellular features and immunologic phenotype. This heterogeneity may reflect various stages in differentiation, but may otherwise be linked to the functional activity of the cells. The authors survey the major cell populations, i.e., epithelial cells and lymphocytes. Macrophages and interdigitating cells are briefly discussed. Emphasis is given to functional aspects of histologic/ cytologic features.     

Accelerated maturation of the thymic stroma in the progeny of adrenalectomized pregnant rats

The possible role played by glucocorticoids (GCs) in the development of thymic stromal cell components has been studied in the progeny of adrenalectomized pregnant rats (FAdx), an experimental model which ensures the absence of GCs until the establishment of the fetal hypothalamus-pituitary gland-adrenal gland axis. As previously demonstrated for thymocytes, the lack of GCs early in ontogeny results in an accelerated maturation of the thymic stromal elements. Early expression of specific cell markers for thymic epithelial cell subsets and appearance of a well-established cytokeratin-positive epithelial cytoreticulum confirmed the ultrastructural evidence of a faster maturation of the thymic epithelium in FAdx than in FSham. A similar faster and stronger pattern of both class I and class II molecule expression on the epithelial cells occurred in the former fetuses than in control ones. Changes in the pattern of expression of laminin, but not that of fibronectin, throughout thymic maturation also reflected accelerated maturation. Immunohistochemically identified thymic macrophages appeared late in both FSham and FAdx but in higher numbers in these latter indirectly demonstrating their faster development. Finally, the maturation and turnover of thymic dendritic cells showed a remarkable acceleration in the FAdx. In 15- to 16-day-old FAdx thymuses there was a high number of dendritic cells which sharply decreased in the following days suggesting a massive migration to the periphery and/or in situ cell death. In parallel a new wave of dendritic cell progenitors began to differentiate in the FAdx thymuses but not in the FSham ones. The results are discussed from the view of close relationships known to occur between thymocytes and the stromal components, although a direct effect of GCs cannot be discarded.     

Early intrathymic precursor cells acquire a CD4(low) phenotype

C4Dlow cells are a population of lymphoid lineage-restricted progenitor cells representing the earliest precursors present in the adult thymus. Paradoxically, thymic progenitors with a similar phenotype in fetal mice and adult RAG-2-deficient (RAG-2-/-) mice lack this characteristic low-level expression of CD4. We now show that radiation-induced differentiation of CD4+ CD8+ double positive thymocytes in RAG-2-/- mice results in the appearance of low levels of CD4 on thymocytes that are phenotypically identical to C4Dlow progenitor cells present in the normal adult thymus. This suggests that CD4 surface expression can be passively transferred from double positive cells to early progenitor thymocytes. Analysis of mixed bone marrow chimeras, reconstituted with hematopoietic stem cells from both CD4-/- (CD45.2) and CD4wt (CD45.1) congenic mice, revealed a CD4low phenotype on cells derived from CD4-/- bone marrow cells. Furthermore, these CD4-/- -derived "C4Dlow" progenitors were capable of reconstituting lymphocyte-depleted fetal thymi, with all thymocytes displaying a CD4-/- phenotype. This directly demonstrates that genetically CD4-deficient thymic progenitor cells can passively acquire a C4Dlow phenotype. Moreover, CD4 expression on C4Dlow progenitor thymocytes is sensitive to mild acid treatment, indicating that CD4 may not exist as an integral cell surface molecule on this thymocyte population. Our findings demonstrate that low-level CD4 surface expression can be passively acquired by intrathymic progenitor cells from the surrounding thymic microenvironment, suggesting that other cell surface molecules expressed at low levels may also result from an acquired phenotype.     

Growth hormone and its receptor are expressed in human thymic cells

GH has been shown to modulate various functions of the thymus. We now demonstrate the production of human GH (hGH) by human thymic cells, and the expression of GH receptors in thymic epithelial cells (TEC) and in thymocytes at different stages of differentiation. The presence of hGH messenger RNA was shown by RT-PCR in both human thymocytes and in primary cultures of TEC. Moreover, immunoreactive hGH material was detected in culture media of thymocytes and TEC with the use of a sensitive immunoradiometric assay. GH receptor gene expression was shown in TEC in primary cultures and in fetal and postnatal TEC lines as well as in thymocytes. By immunocytochemistry, the presence of GH receptors in the various TEC preparations was confirmed. In cytofluorometric studies with the use of a biotinylated anti-GH receptor monoclonal antibody, we could show that GH receptors are predominantly expressed by immature thymocytes: over 90% of CD3- CD4- CD8- CD19- CD34+ CD2- cells (a phenotype characterizing the most immature T cell progenitors in the thymus) were GH receptor positive. Our results provide a molecular basis for an autocrine/paracrine mode of action of GH in the human thymus.     

Modulation of cytokine gene expression by thymic lympho-stromal cell to cell interaction: effect of retinoic acid

We have examined the expression of a panel of cytokines in thymic epithelial cells and CD4-CD8- (DN) thymocytes following cell to cell lymphostromal interaction, in an experimental model which enhances in vitro thymocyte maturation. Since retinoic acid (RA) has been previously shown to be an inhibitor of thymocyte maturation process in this model, we wanted to analyse cytokine expression in DN thymocytes and thymic epithelial cells following the RA-induced impairment of in vitro thymocyte maturation. Cell to cell lymphostromal interaction results in increased IL2 and decreased IL7 expression in thymocytes while the expression of IL1 beta and IL7 increased and decreased, respectively, in thymic epithelial cells. Addition of RA to lympho-stromal cell co-culture results in the enhancement of IL4 and IL7 expression in thymocytes while in thymic epithelial cells IL1 alpha decreased and IL6 and IL7 increased. These data indicate that discrete patterns of cytokine expression are present in thymocyte precursors and in thymic epithelial cells during in vitro T-cell development. They furthermore suggest that specific cytokine modulation might contribute to the RA-induced impairment of thymocyte differentiation.     

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.