The development of the human thymus microenvironment

Eph/Ephrins-Mediated Thymocyte-Thymic Epithelial Cell Interactions Control Numerous Processes of Thymus Biology

Numerous studies emphasize the relevance of thymocyte-thymic epithelial cell (TECs) interactions for the functional maturation of intrathymic T lymphocytes. The tyrosine kinase receptors, Ephs (erythropoietin-producing hepatocyte kinases) and their ligands, ephrins (Eph receptor interaction proteins), are molecules known to be involved in the regulation of numerous biological systems in which cell-to-cell interactions are particularly relevant. In the last years, we and other authors have demonstrated the importance of these molecules in the thymic functions and the T-cell development. In the present report, we review data on the effects of Ephs and ephrins in the functional maturation of both thymic epithelial microenvironment and thymocyte maturation as well as on their role in the lymphoid progenitor recruitment into the thymus.

Sonographic measurements of the thymus in male and female fetuses

OBJECTIVE

The purpose of this study was to determine whether the size of the thymus is different in male and female fetuses.

METHODS

In this prospective study, the transverse diameter and perimeter of the thymus were measured in healthy fetuses between 24 and 37 weeks' gestation. The means of the study variables from male and female fetuses were compared by the Student t test, and the relationships between the transverse diameter and perimeter of the thymus and gestational age and other common fetal biometric parameters were determined by linear regression modeling.

RESULTS

No differences were noted between male and female fetuses for the means of the study variables. After the relationship between the transverse diameter and perimeter of the thymus and gestational age was confirmed (R(2) = 0.8 and 0.75, respectively; both P < .01), the 95% confidence interval-predicted changes were calculated, and the scatterplots of the measurements suggested that sex did not affect the size of the thymus. Bland-Altman plots were used to analyze intraobserver variability and showed good agreement for both of these thymic measurements for male and female fetuses.

CONCLUSIONS

These results suggest that fetal sex does not affect the size of the thymus and, together with previous reports, support the hypothesis that a quantitative reduction in fetal thymus size could serve as an indirect marker of abnormal thymopoiesis and congenital thymic insufficiency.

Thymic reticulo-epithelial cells

The reticulo-epithelial (RE) cellular network of the thymic stromal cellular microenvironment plays a vital role in neuroendocrine regulation and lymphoid cell homing and development. Transmission electronmicroscopic observations have confirmed that there are four functional subtypes of medullar RE cells: undifferentiated; squamous; villous; and cystic. Immunocytochemical observations have shown that the secreted thymic hormones, thymosin alpha1 and thymopoietin (and its short form, thymopentin or TP5), are both produced by RE cells. Thymic RE cells also produce numerous cytokines, including IL-1 and -6, G-CSF, macrophage-CSF and GM-CSF that likely are important during the various stages of thymocyte activation and differentiation. The coexistence of pituitary hormone and neuropeptide secretion, such as growth hormone, prolactin, adrenocorticotopic hormone and thyroid-stimulating hormone, among many others, and the production of a number of interleukins and growth factors, as well as the expression of receptors for all, by the same RE cell, is an unique molecular biological phenomenon. The thymic RE cell network represents an important cellular and humoral microenvironment in the neuroendocrine homeopathic regulatory mechanisms of the multicellular organism.

A developmental look at thymus organogenesis: where do the non-hematopoietic cells in the thymus come from?

The origins of the non-hematopoietic cell types that comprise the thymic stroma remain a topic of considerable controversy. Three recent studies, using lineage analysis and other methods to determine the developmental potential of specific cell types within the thymus, have provided strong evidence of a single endodermal origin for all thymic epithelial cells. Together with other investigations that merge immunological and developmental biology approaches, these studies have suggested a new model of thymus organogenesis, and have begun to uncover the molecular pathways that control this process.

The development of the fetal thymus: an in utero sonographic evaluation

OBJECTIVES

To establish a nomogram for fetal thymus size during gestation.

METHODS

The study is a prospective, cross sectional evaluation of 403 male and female fetuses between 14 and 38 weeks of normal singleton pregnancies. Measurements of fetal thymus size were performed by high resolution transvaginal ultrasonography between 14 and 17 weeks' gestation, and by transabdominal ultrasonography after 18 weeks' gestation.

RESULTS

Adequate thymus size measurements were obtained in all 403 fetuses. Thymus size as a function of gestational age was expressed by the regression equation: (square root) thymus size (mm)= -39.39+4.41 x gestational age (weeks). The correlation coefficient, r=0.965, was found to be highly statistically significant (p<0.0001). The normal mean and the 90% prediction limits were defined.

CONCLUSION

The present data offer the normal range of fetal thymus size from early stages of gestation that may allow intrauterine assessment of its development. It may be helpful in the prenatal diagnosis of thymic pathologies.

Partial blockade of T-cell differentiation during ontogeny and marked alterations of the thymic microenvironment in transgenic mice with impaired glucocorticoid receptor function

Glucocorticoids (GCs) are widely known to be potent modulators of the immune system. The role of GCs in thymopoiesis as well as the integration of the thymus with the neuroendocrine system is, however, poorly understood. In the present work, we have studied, in transgenic mice with an impaired GC function, the alterations which occur in both T-cell differentiation and thymic stroma maturation, throughout ontogeny as well as in adult condition, analyzing their possible rebounding on the status of adult splenic T lymphocyte populations. These transgenic mice have been described to present a significant decrease (60-70%) of thymic and splenic GC receptor binding capacity but maintain normal their basal plasma ACTH and corticosterone levels. The animals showed a partial blockade of T-cell differentiation and decreased percentages of apoptotic cells during fetal development but not in adult life, when thymic cellularity was significantly increased although thymocyte apoptosis response was not affected. In contrast, thymic stroma was profoundly altered from early fetal stages and large epithelium-free areas appeared in adult thymus. On the other hand, our study revealed a reduction of the splenic TcRalphabeta population accompanied by an increase in the CD4/CD8 ratio. The analysis of different adhesion molecules as well as activation markers demonstrated that most of them (CD5, CD11a, CD11b, CD69 and MHC Class II) were normally expressed in transgenic lymphocytes, whereas CD44 and CD62L expression was altered indicating the existence of an increased proportion of primed T-cells in these animals. In view of the mutual interdependence of thymic stroma and thymocyte maturation, the partial blockade of T-cell differentiation during ontogeny and the profound alterations of the stromal cell compartment in transgenic mice with impaired GR function suggest a key role for GCs in coordinating the physiological dialogue between the developing thymocytes and their microenvironment.

Post-Thymic Maturation of Migrating Human Thymic Single-Positive T Cells: Thymic CD1a− CD4+ T Cells Are More Susceptible to Anergy Induction by Toxic Shock Syndrome Toxin-1 than Cord Blood CD4+ T Cells

To determine whether human CD4+ T cells undergo post-thymic maturation, we compared the susceptibility to anergy induction in human thymic CD1a− CD4+ single-positive (CD4+), cord blood (CB) CD4+, and adult peripheral blood (APB) CD4+ T cells by stimulation with toxic shock syndrome toxin-1 (TSST-1). Most TSST-1-induced T cell blasts derived from either T cell preparation expressed TCR Vβ2, which determines the potential reactivity to TSST-1. Most thymic CD4+ T cell blast preparations exhibited little or no production of IL-2 and IL-4 after restimulation with TSST-1 and only marginal responses after stimulation with rIL-2 or a combination of PMA and calcium ionophore, while the APB CD4+ T cell blasts showed high responses to these stimuli. The responses of CB CD4+ T cell blasts to these stimuli varied, ranging from minimal to relatively high. Studies of DNA fragmentation showed that there was no significant cell death of thymic CD4+ T cell blasts. Most thymic CD1a− CD4+ and CB CD4+ T cells were CD38 positive. APB CD4+ T cell blasts derived from the CD38+ fraction and from the CD38− fraction exhibited equally high responses to restimulation with TSST-1. These results indicate that thymic CD1a− CD4+ and CB CD4+ T cells are inherently highly susceptible to anergy induction by bacterial superantigens and that thymic CD1a− CD4+ T cells are less mature than CB CD4+ T cells, suggesting that post-thymic maturation in thymic T cells migrating to the periphery is required for acquisition of full reactivity to antigenic stimulation.

Immuno-electron microscopy of the thymic epithelial microenvironment

Normal T cell development depends upon interactions between progenitor cells and the thymic microenvironment. Monoclonal antibodies (Mabs) have been used to define subtypes of thymic epithelium by light microscopy (clusters of thymic epithelial staining [CTES]). We have now used a range of these Mabs together with gold-coupled reagents in immuno-electron microscopy to study the fine cellular distribution of the molecules to which the antibodies bind. Anti-cytokeratin antibodies were used to identify all thymic epithelial cells, while the distribution of MHC class II molecules was revealed with Mabs to shared nonpolymorphic determinants. MR6, a CTES III Mab, shows strong surface labelling of cortical epithelial cells and thymic nurse cells and very weak surface staining of thymocytes, medullary macrophages, and interdigitating cells. Mab 8.18 (CTES V) also labels a cell surface molecule; this is present on Hassall's corpuscles and associated medullary epithelial cells. The molecules detected by Mabs MR6 and 8.18 are therefore located in a position where they are available to interact with external cellular and soluble signals within the thymus. In contrast, Mabs MR10 and MR19 (CTES II) recognise intracellular molecules within subcapsular, perivascular, and medullary epithelium. A similar distribution was seen with Mab 4beta, directed against the thymic hormone thymulin, although, in addition to the expected intracellular epithelial staining, large lymphoblasts in the subcapsular zone showed surface positivity, indicating the presence of thymulin bound to surface receptors on these early lymphoid cells. As expected, MHC class II molecules were expressed on some medullary and essentially all cortical epithelial cells. However, although most subcapsular epithelium was class II-negative, some cells did express these MHC molecules on their apical surface and on the surface of their cytoplasmic extensions into the cortex. Interestingly, some cortical epithelial cells surrounding capillaries were positive for both MR6 (CTES III) and for MR10, MR19, and 4beta (CTES II). Double-labelling experiments, using MR6 and MR19 simultaneously, revealed a double-positive perivascular epithelial cell population in the thymic cortex. The possibility that these cells represent a thymic epithelial progenitor population is discussed.

Thymic microvascular system

Morphological studies of the microcirculatory system in the thymus were reviewed in regards to methodology and structural organization of blood and lymphatic vessels. The blood capillaries and postcapillary venules (PCVs) in the thymus are characterized by a double-walled structure. These vessels are surrounded more or less by perivascular spaces (PVSs) containing many lymphocytes. This space is delimited on the one side by abluminal surface of the vascular endothelium and on the other side by cytoplasmic processes of epithelial reticular cells. There are interruptions or gaps on the outer epithelial reticular layer. The lymphatic vessels can be distinguished histochemically from blood vessels based on strong 5'-nucleotidase (5'-Nase) activity. The 5'-Nase-positive lymphatic vessels were seen predominantly in the capsule and interlobular connective tissue but sometimes in the immediate vicinity of the PVS around the PCV, when a discrete opening in the lymphatic wall next to the PVS was found. Thus, it may be regarded as an initial part of lymphatics closely associated with the PVS, suggesting a possible route for lymphocyte efflux into the lymphatic vessel from the PVS. The endothelial cells of lymphatic vessels as well as PCVs are often infiltrated by lymphocytes, particularly more heavily during acute involution of the thymus. These images represent the migration of lymphocytes into the blood or lymphatic microcirculation.

Thymic microenvironment at the light microscopic level

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.

Heterogeneity of epithelial cells in the rat thymus

The morphological heterogeneity of the thymic epithelium has been well documented both at the light and electron microscopic level. Immunohistochemistry has revealed four broad classes of epithelial cells (EC): subcapsule/perivascular, cortical, medullary EC, and medullary Hassall's corpuscles. Ultrastructural analysis has revealed further heterogeneity. In the cortex, four EC subtypes have been described ultrastructurally: subcapsular/perivascular, "pale," "intermediate," and "dark" EC. These subtypes are also present in the medulla. Two additional EC subtypes are restricted to the medulla: an undifferentiated subtype, and a subtype displaying signs of high metabolic activity. Based on the morphological features of the epithelium, it has been hypothetized that the thymic EC subtypes represent a process of differentiation.

Cellular and molecular analysis of lymphoid development using Rag-deficient mice

The establishment of a functional immune system with diverse antigen receptors is dependent on the V(D)J recombination activating gene products Rag-1 and Rag-2. These two proteins constitute the key lymphoid components required for the activation of antigen receptor rearrangement. Both Rag-1 and Rag-2 are required for the catalysis of the initial stages of V(D)J recombination. Thus, functional disruption of either the Rag-1 or Rag-2 genes by homologous recombination, leads to immunodeficiency due to lymphoid arrest at a stage prior to the recombination of the antigen receptor loci. In Rag-deficient mice, both B- and T-cell differentiation is eliminated due to the absence of antigen receptors. Lymphoid development can be restored by the introduction of rearranged antigen receptor transgenes that give rise to monoclonal populations of fully mature B- or T-cells. The absence of the major conventional populations of B- and T-cells from the Rag-deficient mice provided an excellent background for studying the molecular and cellular mechanisms of lymphoid differentiation. The Rag-deficient background has been used as a system for: the functional analysis of Rag-1 and Rag-2; studying the developmental functions of antigen receptors and other molecules of the immune system; the molecular analysis of the early stages of the B- and T-cell lineages; the co-development of lymphocytes with stroma cells; the identification of minor subpopulations of the developing immune system; the involvement of lymphoid populations in the onset of pathogenesis. In addition, the development of the "blastocyst complementation assay" methodology, based on the phenotype of the Rag-/- mice, allowed the functional analysis of numerous lymphoid specific components.

Evaluation of a histogenetic classification for thymic epithelial tumours

We reviewed 87 thymic epithelial tumours from Chinese patients and typed them according to the Marino and Müller-Hermelink classification as updated by Kirschner and Müller-Hermelink in 1989. Related categories were grouped for statistical analyses: group 1, medullary thymoma and mixed thymoma; group 2, cortical predominant thymoma; group 3, cortical thymoma and well-differentiated thymic carcinoma; group 4, other thymic carcinomas; and group 5, unclassified. Group 3 tumours were more frequently associated with the myasthenia gravis syndrome compared with group 1 tumours (P = 0.001). They also presented at a more advanced stage. Groups 1 and 2 showed an excellent prognosis (100% survival at 10 years). The 10-year survival for groups 3 and 4 patients was 40% and 30% respectively. Pure medullary thymoma made up a higher proportion of our cases (10.3%) than those of a similar Caucasian study (5.3%). The eight thymic carcinomas (group 4) included two thymic lymphoepitheliomas. We conclude that the histogenetic classification evaluated shows a clear correlation with prognosis and clinical features, even when tested on separate geographic groups, where pathogenetic factors may be different. A common approach to classification of thymic epithelial tumours would greatly facilitate future studies on these possible differences.

Thymic pathology in primary and secondary immunodeficiencies

For the sake of clarity and in agreement with the World Health Organization immunodeficiency classification, it is important to distinguish the congenital, inherited malformative lesions called generically 'thymic dysplasia' from the secondary, acquired changes, designated under the broad term of 'severe thymic atrophy'. Thymic dysplasia represents the archetype of thymic changes in cellular immunodeficiency, since there is no example of a thymic dysplasia associated with a normal T-cell function. Thymic dysplasia is observed in several inherited diseases, the most frequent of them being severe combined immunodeficiency. More than the depletion of lymphoid cells, the lack of differentiation of the thymic epithelium, responsible for the absence of Hassal's corpuscles, is the main and constant feature of this condition. Thymic dysplasia underscores the crucial role of the thymic epithelium in the normal differentiation of the T-cell population. Severe thymic atrophy is secondary to various causes, including prolonged protein malnutrition and immunosuppressive or cytotoxic drugs, graft versus host reaction and, chiefly today, chronic viral infection, especially with HIV-1. The morphological changes are similar and are characterized by a partial lymphoid depletion, involving mainly the CD1+ population, necrosis and calcification of epithelial cells, the frequent presence of plasma cells and, more significantly, fibrohyaline changes of the basement membrane of the vessels and thymic epithelium. The severity of the atrophic changes and the immunodeficiency-related manifestations depend on the duration of the aetiological factors and, more significantly, with their early occurrence, within the first months of life. The mechanisms underlying thymic atrophy are poorly understood. A primary impairment of lymphoid cells seems at present to be the most likely hypothesis.

Towards an integrated view of thymopoiesis

One prediction from the complex series of steps in intrathymic T-cell differentiation is that to regulate it the stroma controlling the process must be equally complex: the attraction of precursors, commitment to the T-cell lineage, induction of T-cell receptor (TCR) gene rearrangement, accessory molecule expression, repertoire expansion, major histocompatibility complex (MHC) molecule-based selection (positive and negative), acquisition of functional maturity and migratory capacity must all be controlled. In this review, Richard Boyd and Patrice Hugo combine knowledge of T-cell differentiation with thymic stromal cell heterogeneity to offer an integrated view of thymopoiesis within the thymic microenvironment.

Prominence of gamma delta T cells in the ruminant immune system

The lymphoid systems of sheep and cattle contain a large number of gamma delta T cells, in striking contrast to the lymphoid systems of humans and mice. In neonatal animals particularly, these cells comprise the predominant fraction of T cells in the blood. Here Wayne Hein and Charles Mackay discuss what is currently known about the ontogeny, phenotype, tissue distribution and function of gamma delta T cells in ruminants. There are a number of interesting molecular features that characterize ruminant gamma delta T cells, but these do not entirely explain the high frequency of use of the gamma delta T-cell receptor in these animals. Studies on sheep, cattle or other animals that preferentially use gamma delta T cells should provide insights into the biological significance of the existence of two distinct forms of the T-cell receptor.

Acute thymus involution in infancy and childhood: a reliable marker for duration of acute illness

To evaluate the relationship between histologic parameters and clinical data, we studied thymus histology in 234 fetuses and young children who died after a short period of acute illness. Thymus weight and volume percentages of interstitium, cortex, and medulla were significantly related to prenatal or postnatal status and age of the patient. Thymus weight was related to the duration of acute illness only in prenatal patients. The histology, categorized in five grades according to appearance of macrophages (with a starry-sky aspect) in the cortex, increase of interlobular interstitium, and lymphodepletion of the cortex, correlated significantly with the duration of acute illness and not with any other clinical parameter. This finding enables the pathologist to estimate the duration of acute disease before death.

The study of mammalian organogenesis by mosaic pattern analysis

Chimeras are animals derived from more than one zygote and composed of two cell lineages which are distinguishable in some way at the cellular level. Spontaneous mosaic animals are also composed of distinguishable cell lineages but are monozygotic. The tissues of both mono- and multizygotic animals of this type are mosaic arrays in which aggregates of like cells form patches, the size and distribution of which can be useful in the analysis of diverse problems in developmental biology. Both biochemical and in situ methods have been applied to the elucidation of mosaic pattern. Both forms of mosaicism have proven useful in establishing theoretic constructs of the formation and maintenance of mammalian organs. A number of these constructs are discussed: cell fusion as related to myotube formation; mechanisms of coat pigmentation and the cellular origin of melanocytes; and pattern analyses of the retinal pigmented epithelium, the intestine, liver, adrenal cortex and thymus. Pathologic alterations in such animals have also been studied utilizing mosaic pattern analysis. In particular, neoplastic tumors and their associated preneoplastic lesions have been shown to be clonal.

Immunohistological patterns of non-neoplastic changes in the thymus in Myasthenia gravis

Non-neoplastic thymuses from 20 patients with myasthenia gravis (MG) have been studied by routine stains on paraffin sections and by immunohistological methods on frozen sections using a panel of monoclonal antibodies against thymic epithelial cells, macrophages/reticulum cells, lymphoid cells and myoid cells. Three types of thymic histology in MG were distinguished: (1) thymitis with lymphoid follicular hyperplasia (11 cases), (2) thymitis with diffuse B-cell infiltration (5 cases) and (3) thymic atrophy (4 cases). Thymitis was more common in younger females and thymic atrophy in older patients. Both types of thymitis were associated with conspicuous structural disturbance of the thymic perivascular space (PVS) and medulla, characterized by a distinct enlargement of the PVS and disruption of the epithelium and reticulin fibre network at the medullary boundary, leading to fusion of the two compartments. The PVS and medulla contained a striking B-cell infiltration. Large well-developed germinal centers (GCs), showing the same cellular organization as in the peripheral lymphatic system, occurred in thymitis with lymphoid follicular hyperplasia, whereas thymitis with diffuse B-cell infiltration merely exhibited a few tiny lymphoid follicles, which could be demonstrated only by immunostaining of dendritic reticulum cells. In thymic atrophy a diffuse B-cell infiltration of the PVS and the medulla was also observed, but only minor alterations of the epithelial framework were seen. There was an increased number of interdigitating reticulum cells with variable expression of the T-6 antigen in all the thymuses examined, indicating an immune stimulation of the intrathymic T-cells. Myoid cells, the supposed target of the intrathymic immune reaction in MG, were found to be less frequent in thymic atrophy than in thymitis. This variable number of myoid cells may explain the different grades of immune stimulation and different types of histology seen in the thymus in MG.