The human thymic microenvironment

CD90 Marks a Mesenchymal Program in Human Thymic Epithelial Cells In Vitro and In Vivo

Thymic epithelium is critical for the structural integrity of the thymus and for T cell development. Within the fully formed thymus, large numbers of hematopoietic cells shape the thymic epithelium into a scaffold-like structure which bears little similarity to classical epithelial layers, such as those observed in the skin, intestine or pancreas. Here, we show that human thymic epithelial cells (TECs) possess an epithelial identity that also incorporates the expression of mesenchymal cell associated genes, whose expression levels vary between medullary and cortical TECs (m/cTECs). Using pluripotent stem cell (PSC) differentiation systems, we identified a unique population of cells that co-expressed the master TEC transcription factor FOXN1, as well as the epithelial associated marker EPCAM and the mesenchymal associated gene CD90. Using the same serum free culture conditions, we also observed co-expression of EPCAM and CD90 on cultured TECs derived from neonatal human thymus in vitro. Single cell RNA-sequencing revealed these cultured TECs possessed an immature mTEC phenotype and expressed epithelial and mesenchymal associated genes, such as EPCAM, CLDN4, CD90 and COL1A1. Importantly, flow cytometry and single cell RNA-sequencing analysis further confirmed the presence of an EPCAM+CD90+ population in the CD45- fraction of neonatal human thymic stromal cells in vivo. Using the human thymus cell atlas, we found that cTECs displayed more pronounced mesenchymal characteristics than mTECs during embryonic development. Collectively, these results suggest human TECs possess a hybrid gene expression program comprising both epithelial and mesenchymal elements, and provide a basis for the further exploration of thymus development from primary tissues and from the in vitro differentiation of PSCs.

Anatomical structures, cell types and biomarkers of the Human Reference Atlas

The Human Reference Atlas (HRA) aims to map all of the cells of the human body to advance biomedical research and clinical practice. This Perspective presents collaborative work by members of 16 international consortia on two essential and interlinked parts of the HRA: (1) three-dimensional representations of anatomy that are linked to (2) tables that name and interlink major anatomical structures, cell types, plus biomarkers (ASCT+B). We discuss four examples that demonstrate the practical utility of the HRA.

The fibroblast: An emerging key player in thymic T cell selection

Fibroblasts have recently attracted attention as a key stromal component that controls the immune responses in lymphoid tissues. The thymus has a unique microenvironment comprised of a variety of stromal cells, including fibroblasts and thymic epithelial cells (TECs), the latter of which is known to be important for T cell development because of their ability to express self‐antigens. Thymic fibroblasts contribute to thymus organogenesis during embryogenesis and form the capsule and medullary reticular network in the adult thymus. However, the immunological significance of thymic fibroblasts has thus far only been poorly elucidated. In this review, we will summarize the current views on the development and functions of thymic fibroblasts as revealed by new technologies such as multicolor flow cytometry and single cell–based transcriptome profiling. Furthermore, the recently discovered role of medullary fibroblasts in the establishment of T cell tolerance by producing a unique set of self‐antigens will be highlighted.

Fetal Lymphoid Organ Immune Responses to Transient and Persistent Infection with Bovine Viral Diarrhea Virus

Bovine Viral Diarrhea Virus (BVDV) fetal infections occur in two forms; persistent infection (PI) or transient infection (TI), depending on what stage of gestation the fetus is infected. Examination of lymphoid organs from both PI and TI fetuses reveals drastically different fetal responses, dependent upon the developmental stage of the fetal immune system. Total RNA was extracted from the thymuses and spleens of uninfected control, PI, and TI fetuses collected on day 190 of gestation to test the hypothesis that BVDV infection impairs the innate and adaptive immune response in the fetal thymus and spleen of both infection types. Transcripts of genes representing the innate immune response and adaptive immune response genes were assayed by Reverse Transcription quatitative PCR (RT-qPCR) (2^−ΔΔCq; fold change). Genes of the innate immune response, interferon (IFN) inducible genes, antigen presentation to lymphocytes, and activation of B cells were downregulated in day 190 fetal PI thymuses compared to controls. In contrast, innate immune response genes were upregulated in TI fetal thymuses compared to controls and tended to be upregulated in TI fetal spleens. Genes associated with the innate immune system were not different in PI fetal spleens; however, adaptive immune system genes were downregulated, indicating that PI fetal BVDV infection has profound inhibitory effects on the expression of genes involved in the innate and adaptive immune response. The downregulation of these genes in lymphocytes and antigen-presenting cells in the developing thymus and spleen may explain the incomplete clearance of BVDV and the persistence of the virus in PI animals while the upregulation of the TI innate immune response indicates a more mature immune system, able to clear the virus.

Survivin expression and localization in different organs of yaks (Bos grunniens)

Yaks (Bos grunniens) have special physiological structures that help them adapt to high-altitude environments. Survivin is actively studied in cancer tissues, but less in normal tissues. Therefore, the aim of the present study was to analysis the relationship between survivin expression and apoptosis rate in yaks. A partial gene sequence of survivin was cloned and characterized using bioinformatics. The expression of survivin was investigated using real-time quantitative PCR (RT-qPCR) and western blot (WB) analysis and localized using immunohistochemistry (IHC). The results revealed that in normal physiological organs, survivin is mainly expressed in cytoplasm and its expression was up-regulated with age. Its expression in heart and liver was higher than in other organs, such as spleen, lung, brain, kidney, and testis. It is noteworthy that the expression of survivin in spleen is differed from that in other organs. Therefore, we selected immune organs (lymph node, thymus and spleen) to investigate the relationship between survivin expression and apoptosis. Caspase-3 was used as a reference. Within the same age group, the expression of survivin was the highest in the spleen, but that of caspase-3 was the highest in the lymph node (P < 0.01). Furthermore, the IHC analysis revealed that survivin and caspase-3 are expressed in the same location (mainly in the cytoplasm, Hassall's corpuscles, the medulla of the lymph node, the red pulp and marginal zone of the spleen. More importantly, survivin expression was down-regulated with age in immune organs, and the opposite trend was observed for caspase-3 expression (P < 0.01). The results proved that the expression of survivin and caspase-3 is down- and up-regulated with age, respectively, suggesting that survivin and caspase-3 might coordinating and participating in slowing down the rate of apoptosis rate in immune organs of healthy yak.

Role of the Perivascular Epithelium in the Histogenesis of Hassall's Corpuscles

Thirteen human thymuses and one thymoma were morphologically and immunohistologically investigated to define the histogenesis of Hassall's corpuscles (HCs). The following monoclonal antibodies: antisquamous cytokeratin on paraffin sections and TE-4 and TE-8 on frozen sections, were used to show the distribution of the epithelial components; PAL-E on frozen and anti-CD31 and anti-CD34 on paraffin sections detected the endothelial cell distribution. In the thymoma, epithelial onion-like structures, looking like true HCs, were found to originate from the perivascular epithelium lining dilatated spaces and some of them partially obliterated the space where the blood capillary showed thickened wall and endothelial regressive changes. Antisquamous cytokeratin stained: (1) in the thymus: subcapsular, medullary, and HC epithelial cells; (2) in the thymoma: epithelial cells lining the perivascular spaces and forming HCs. TE-4 stained: (1) in the thymus: the subcapsular and medullary epithelium; (2) in the thymoma: the epithelium lining the perivascular spaces and epithelial cells forming HCs. TE-8 stained: (1) in the thymus: HCs only; (2) in the thymoma: HCs and perivascular epithelial cells. PAL-E, CD31, and CD34, which specifically react with endothelial cells, stained remnants of capillary structures in the core of some HCs. The results indicate that: (1) corpuscular structures in thymoma originate from perivascular epithelium; (2) thymus medullary epithelial cells stained by cytokeratin and TE-4 correspond to perivascular epithelial cells whose staining is well documented in thymoma; (3) the subcapsular-perivascular epithelium and HCs represent different steps of differentiation of a single ectodermal cell lineage; (4) the PAL-E-, CD31-, and CD34-positive reaction in the core of some HCs suggests that the perivascular epithelium would be stimulated to transform into HCs as a consequence of endothelial changes with fragmentation of the capillary included in the perivascular space.

Characterization of the expression of cytokeratins 5, 8, and 14 in mouse thymic epithelial cells during thymus regeneration following acute thymic involution

The thymus is a central lymphoid organ for T cell development. Thymic epithelial cells (TECs) constitute a major component of the thymic stroma, which provides a specialized microenvironment for survival, proliferation, and differentiation of immature T cells. In this study, subsets of TECs were examined immunohistochemically to investigate their cytokeratin (CK) expression patterns during thymus regeneration following thymic involution induced by cyclophosphamide treatment. The results demonstrated that both normal and regenerating mouse thymuses showed a similar CK expression pattern. The major medullary TECs (mTEC) subset, which is stellate in appearance, exhibited CK5 and CK14 staining, and the minor mTEC subset, which is globular in appearance, exhibited CK8 staining, whereas the vast majority of cortical TECs (cTECs) expressed CK8 during thymus regeneration. Remarkably, the levels of CK5 and CK14 expression were enhanced in mTECs, and CK8 expression was upregulated in cTECs during mouse thymus regeneration after cyclophosphamide-induced acute thymic involution. Of special interest, a relatively high number of CK5⁺CK8⁺ TEC progenitors occurred in the thymic cortex during thymus regeneration. Taken together, these findings shed more light on the role of CK5, CK8, and CK14 in the physiology of TECs during mouse thymus regeneration, and on the characterization of TEC progenitors for restoration of the epithelial network and for concomitant regeneration of the adult thymus.

Evaluation of patulin toxicity in the thymus of growing male rats

Patulin is a mycotoxin produced by several Penicillium, Aspergillus, and Byssachlamys species growing on food products. In this study, we investigated the effects of patulin on the thymus of growing male rats aged five to six weeks. The rats were receiving it orally at a dose of 0.1 mg kg-1 bw a day for either 60 or 90 days. At the end of the experiment, the thymus was examined for histopathology by light microscopy and for epidermal growth factor (EGF) and its receptor (EGFR) by immunolocalisation. For morphometry we used the Bs200prop program to analyse images obtained with the Olympus BX51 light microscope. Cell ultrastructure was studied by electron microscopy. In rats treated with patulin, the thymus showed haemorrhage, plasma cell hyperplasia, a dilation and fibrosis in the cortex, enlarged interstitial tissue between the thymic lobules, enlarged fat tissue, thinning of the cortex, and blurring of the cortico-medullary demarcation. Electron microscopy showed signs of cell destruction, abnormalities of the nucleus and organelles, and loss of mitochondrial cristae. However, no differences were observed in thymus EGF and EGFR immunoreactivity between treated and control rats.

The expression of retinoic acid receptors in thymus of young children and the effect of all-transretinoic acid on the development of T cells in thymus

To explore the effect of retinoic acid on the development of T cells in thymus, we examined the expression and distribution of retinoic acid receptors mRNA in thymus of young children (< or =5 years of age) and investigated the affection of all-transretinoic acid on the differentiation and maturation of the T cells in thymus and cultured in vitro. Twenty thymus samples were collected. By in situ hybridization, we found that four retinoic acid receptor genes expressed in thymus, and the levels of mRNA for retinoic acid receptor genes (detected by reverse transcriptase, fluorescent quantitative PCR), were changed with age. In in vitro culture, all-transretinoic acid promoted the maturation from CD4+CD8+ cells to CD4+ cells but inhibited the differentiation from CD4+CD8+ cells to CD8+ cells. Thus, retinoic acid likely plays important roles in T cell development in thymus and perhaps by affecting the relative expression of retinoic acid receptors.

Leptin selectively augments thymopoiesis in leptin deficiency and lipopolysaccharide-induced thymic atrophy

The thymus is a lymphoid organ that selects T cells for release to the peripheral immune system. Unfortunately, thymopoiesis is highly susceptible to damage by physiologic stressors and can contribute to immune deficiencies that occur in a variety of clinical settings. No treatment is currently available to protect the thymus from stress-induced involution. Leptin-deficient (ob/ob) mice have severe thymic atrophy and this finding suggests that this hormone is required for normal thymopoiesis. In this study, the ability of leptin to promote thymopoiesis in wild-type C57BL/6 and BALB/c mice, as well as in leptin-deficient (ob/ob) and endotoxin-stressed (Escherichia coli LPS) mice, was determined. Leptin administration induced weight loss and stimulated thymopoiesis in ob/ob mice, but did not stimulate thymopoiesis in wild-type C57BL/6 nor BALB/c mice. In endotoxin-stressed mice, however, leptin prevented LPS-induced thymus weight loss and stimulated TCRalpha gene rearrangement. Coadministration of leptin with LPS blunted endotoxin-induced systemic corticosterone response and production of proinflammatory cytokines. Thus, leptin has a selective thymostimulatory role in settings of leptin deficiency and endotoxin administration, and may be useful for protecting the thymus from damage and augmenting T cell reconstitution in these clinical states.

Reversible disruption of thymic function by steroid treatment

The effect of steroid treatment on the thymic output of T cells was examined in an avian model. Recent thymic emigrants in chickens transiently express the chicken T cell Ag 1 thymocyte marker, and thymic function can be monitored indirectly by measuring the levels of TCR gene rearrangement excision circles in peripheral T cells. Both parameters were used to show that intensive steroid treatment induces thymic involution and a profound reduction in the supply of naive T cells to the periphery. Conversely, resident T cells in the peripheral lymphocyte pool were relatively spared. Thymopoiesis immediately recovered following cessation of steroid treatment, concurrent with restoration of the thymic output of newly formed T cells. Repopulation of the peripheral T cell pool recapitulated the ontogenetic pattern of gamma delta T cell replenishment before alpha beta T cell reseeding, thereby indicating the complete recovery of thymic function after a course of steroid treatment.

Ultrastructural study of the FVB/N mouse thymus: presence of an immature epithelial cell in the medulla and premature involution

FVB/N inbred mice have been widely used to generate a variety of transgenic lines, but their physiology and especially their immunological characteristics are poorly documented. We therefore studied the ultrastructure of the thymus and the distribution of thymocyte subpopulations in FVB/N mice at several ages. In young FVB/N mice the stromal microenvironment exhibits the three types of epithelial cells and the two types of bone-marrow derived cells (macrophages and interdigitated cells) previously described in other strains of mice. Moreover, in the thymic medulla of young FVB/N mice, a fourth cell type with the morphological characteristics of an immature epithelial cell was present in relatively high number. Furthermore, thymocyte subpopulations distribution shows an earlier thymocyte maturation than in other strains. Finally, changes associated with thymic involution were observed about 5 months earlier than in many other mouse strains. Our results demonstrated that the FVB/N strain has a specific immunological status.

True epithelial hyperplasia in the thymus of early-onset myasthenia gravis patients: implications for immunopathogenesis

The early-onset myasthenia gravis (EOMG) thymus shows characteristic medullary epithelial bands (MEB), greatly expanded perivascular infiltrates and fenestrations of the intervening basement membranes. We now compare epithelial expression of epidermal growth factor receptor (EGFR) and many integrins in EOMG and control samples. The main differences are striking/consistent thickening (in MEB) of what is normally a monolayer of perivascular epithelium, with focal protrusion into the infiltrates. This evidently hyperplastic epithelial subpopulation also strongly expresses EGFR and certain integrins. We suggest that its enhanced interactions with the locally increased extracellular matrix protein deposits may play an important role in autosensitization.

Expression of CD146 adhesion molecules (MUC18 or MCAM) in the thymic microenvironment

The thymic microenvironment is thought to play a critical role in T-lymphocyte development, providing signals both via cell surface molecules such as adhesion molecules and soluble molecules. The present investigation is focused on immunoelectron microscopical analysis of distribution patterns of CD146 adhesion molecules (MUC18 or MCAM) in the microenvironment of normal human thymus, using the pre-embedding indirect immunoperoxidase technique. The anti-CD146 monoclonal antibody (mAb) revealed strong membraneous labelling of immature thymocytes at both the light and electron microscopical level. Proliferating thymocytes, most of the epithelial cells, macrophages, endothelial cells and smooth musle cells of small vessels and capillaries showed both membraneous and cytoplasmic labelling with anti-CD146 mAb as was demonstrated by electron microscopy. In contrast, these cells displayed a strong cytoplasmic immunoreactivity at the light microscopical level. The extracellular matrix was also stained with the anti-CD146 mAb. No labelling was observed in interdigitating cells. Interestingly, the CD146 molecule was strongly expressed on apical and lateral membranes of endothelial cells as was demonstrated electron microscopically. This selective CD146 labelling of capillary endothelium mainly localized at the cortico-medullary junction may be manifestations of lymphocyte transmembrane migration and lymphocyte homing. In conclusion, the present study suggests that CD146 is expressed by most elements of the microenvironment of normal human thymus. Therefore, it may be a pan-antigen which is essential for the maintenance of thymic architecture and function.

Evidence of alloreactive T lymphocytes in fetal liver: implications for fetal hematopoietic stem cell transplantation

The use of hematopoietic stem cells for in utero transplantation to create permanent hematochimerism represents a new concept in fetal therapy, although this approach has provided heterogeneous results. In this paper we have undertaken molecular, phenotypic and functional studies aimed at identifying the presence of fully competent T lymphocytes in samples of fetal livers and cord blood. We found mature VDJ TCR beta chain transcripts in fetal liver cells taken from 7 to 16 weeks of gestation and a similar pattern was detected in cord blood cells sampled from 13.5 to 20.5 weeks of gestation. A Vbeta8 gene sequence comparable to that detected in adult PBMC was found in fetal liver samples at 9 or 17 weeks gestation. PreTalpha message was detected in all samples and its expression decreased in fetal blood samples with increasing gestational age while Calpha message appeared at 9.4 weeks and its expression increased during gestational age. T cell clones obtained from fetal liver cells showed a mature TCR alphabeta+, CD8+ phenotype and displayed strong alloreactivity against allo-MHC class I molecules. The presence of alloreactive T lymphocytes may explain the failure to engraft in fetuses older than 13 to 16 weeks and may provide insights into fetal liver transplantation. Bone Marrow Transplantation (2000) 25, 135-141.

Quantification of acetylcholinesterase-positive structures in human thymus during development and aging

Acetylcholinesterase (AChE) localization in the human thymus has been studied by biochemical and morphological methods during development and aging. The occurrence, the amount and the distribution of acetylcholinesterase and the changes with age were examined in 24 human thymuses. The whole human thymus was removed during autopsies in males of the following age-groups: prenatal of six months, new-born, infant, young, adult and elderly. The thymuses were weighed, measured and dissected: the microanatomical details were stained with Eosin-orange, nervous structures were identified by means of Bodian's method. Protein content was determined with biochemical methods. Histoenzymatical and biochemical demonstration of acetylcholinesterase was performed. The morphological results obtained were submitted to quantitative image analysis. Our results show that the thymic microenvironment changes with age; moreover, an increase of acetylcholinesterase-positive structures can be observed with age. Biochemical results are in agreement with morphological results and both are confirmed by the outcome of quantitative analysis of images. Acetylcholinesterase activity in human thymus may play a key role in thymic functions.

Occurrence of gamma-aminobutyric acid-transaminase activity in nerve fibers of human thymus

The specific localization of gamma-aminobutyric acid-transaminase (GABA-t) in the thymus of young and elderly men was studied. Our results show a specific vascular localization of GABA-t in the human thymus, and deal with the amount and distribution of GABA-t and its changes with age. Samples of human thymus were harvested throughout of 12 autopsies in infants (n = 3), as well as young (n = 3), adult (n = 3) and elderly (n = 3) men. Histologic staining of the human thymus was performed with eosin-orange, while histologic staining of nerve fibers was performed with the Bodian method. Histochemical and biochemical demonstration of GABA-t, including protein dosage, was performed by the methods of Van Gelder and Jung, respectively. Finally, quantitative analysis of images was performed. Staining with eosin-orange reveals the micro-anatomical details of the thymic micro-environment. The Bodian method shows the nerve fibers and neurofibrils. Histochemical staining for GABA-t shows an increase of this enzyme with age and a marked localization in the nerve fibers of the thymus in infant, young, adult, and elderly men, as well as specific vascular localization of this enzyme. These biochemical data are in accordance with the histoenzymatic results and confirm all of our previous observations. Finally, quantitative analysis of images performed on slices let us confirm all the morphological changes induced by age. We can conclude that GABA is an inhibitory neurotransmitter of the human thymus, while GABA-t plays an important role in GABA metabolism.

HCA, an Immunoglobulin-Like Adhesion Molecule Present on the Earliest Human Hematopoietic Precursor Cells, Is Also Expressed by Stromal Cells in Blood-Forming Tissues

We have previously shown that the HCA/ALCAM (CD166) glycoprotein, a member of the immunoglobulin family that mediates both homophilic and heterophilic cell-cell adhesion, via the CD6 ligand, is expressed at the surface of all of the most primitive CD38−/lo, Thy-1+, rho123lo, CD34+hematopoietic cells in human fetal liver and fetal and adult bone marrow. In the present report we show that HCA is also expressed by subsets of stromal cells in the primary hematopoietic sites that sequentially develop in the human embryo and fetus, ie, the paraaortic mesoderm, liver, thymus, and bone marrow. Adult bone marrow stromal cells established in vitro, including those derived from Stro-1+ progenitors and cells from immortalized cell lines, express HCA. In contrast, no HCA expression could be detected in peripheral lymphoid tissues, fetal spleen, and lymph nodes. HCA membrane molecules purified from marrow stromal cells interact with intact marrow stromal cells, CD34+ CD38−hematopoietic precursors, and CD3+ CD6+peripheral blood lymphocytes. Finally, low but significant levels of CD6 are here for the first time detected at the surface of CD34+ rho123med/lo progenitors in the bone marrow and in mobilized blood from healthy individuals. Altogether, these results indicate that the HCA/ALCAM surface molecule is involved in homophilic or heterophilic (with CD6) adhesive interactions between early hematopoietic progenitors and associated stromal cells in primary blood-forming organs.

HCA, an Immunoglobulin-Like Adhesion Molecule Present on the Earliest Human Hematopoietic Precursor Cells, Is Also Expressed by Stromal Cells in Blood-Forming Tissues

We have previously shown that the HCA/ALCAM (CD166) glycoprotein, a member of the immunoglobulin family that mediates both homophilic and heterophilic cell-cell adhesion, via the CD6 ligand, is expressed at the surface of all of the most primitive CD38−/lo, Thy-1+, rho123lo, CD34+hematopoietic cells in human fetal liver and fetal and adult bone marrow. In the present report we show that HCA is also expressed by subsets of stromal cells in the primary hematopoietic sites that sequentially develop in the human embryo and fetus, ie, the paraaortic mesoderm, liver, thymus, and bone marrow. Adult bone marrow stromal cells established in vitro, including those derived from Stro-1+ progenitors and cells from immortalized cell lines, express HCA. In contrast, no HCA expression could be detected in peripheral lymphoid tissues, fetal spleen, and lymph nodes. HCA membrane molecules purified from marrow stromal cells interact with intact marrow stromal cells, CD34+ CD38−hematopoietic precursors, and CD3+ CD6+peripheral blood lymphocytes. Finally, low but significant levels of CD6 are here for the first time detected at the surface of CD34+ rho123med/lo progenitors in the bone marrow and in mobilized blood from healthy individuals. Altogether, these results indicate that the HCA/ALCAM surface molecule is involved in homophilic or heterophilic (with CD6) adhesive interactions between early hematopoietic progenitors and associated stromal cells in primary blood-forming organs.

Cellular and molecular aspects of thymic T-cell education in neuroendocrine self principles. Implications for autoimmunity

Thymic epithelial and nurse cells from different species express a repertoire of neuroendocrine polypeptide precursors. This repertoire exerts a dual role in T-lymphocyte selection according to their status either as cryptocrine signals or as neuroendocrine self-antigens of the peptide sequences that are processed from those precursors then presented to pre-T cells. Thymic neuroendocrine self-antigens correspond to peptide sequences highly conserved throughout evolution of their family. Though thymic MHC class I molecules are involved in the processing of thymic neuroendocrine self-antigens, preliminary data show that their presentation to pre-T cells is not allelically restricted. Thymic T-cell education in neuroendocrine families also implies that the structure of a given family may be presented to pre-T cells. Our studies have evidenced the homology between thymic neuroendocrine-related self-antigens and dominant T-cell epitopes of peripheral neuroendocrine signals (neuroendocrine autoantigens). The biochemical difference between neuroendocrine autoantigens and homologous thymic self-antigens might explain the opposite immune responses evoked by those two types of antigens (activation and memory induction vs. tolerogenic effect). Altogether, these studies support the therapeutic use of thymic neuroendocrine self-antigens in reprogramming the immunological self-tolerance that is broken in autoimmune endocrine diseases like insulin-dependent diabetes type I. As recently stated by P. M. Allen in an important review, the fate of developing T lymphocytes in the thymus is influenced by the numerous types of peptidic interactions within the thymic cellular environment. To define the precise nature of thymic cells and naturally occurring biochemical peptide signals involved in positive and negative selection of immature T cells has become a prominent objective for the future research efforts in thymic physiology. This paper will try to show how thymic neuroendocrine-related peptides synthesized and processed within the thymic microenvironment indeed can play a role both in the development of the peripheral T-cell repertoire and in the death of randomly rearranged, self-reactive T cells.

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.

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

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

Cytokine production by human thymic epithelial cells: control by the immune recognition of the neurohypophysial self-antigen

Oxytocin (OT) has been shown to be the dominant peptide of the neurohypophysial family expressed by thymic epithelial and nurse cells (TEC/TNC) in various species. Thymic OT is not secreted but, after translocation of a hybrid neurophysin/MHC class I protein, is integrated within the plasma membrane of TEC, thus allowing its presentation to pre-T cells. In order to further demonstrate that thymic OT behaves like a membrane antigen, we assessed the effect of mAbs to OT on cytokine productions by cultures enriched in human TEC. 75-85% pure TEC cultures were prepared from human thymic fragments. Using immunofluorescence and confocal microscopy, ir-OT, ir-interleukin-1 beta (IL-1 beta), ir-interleukin-6 (IL-6) and ir-leukemia inhibitory factor (LIF) could be detected in these TEC cultures. ir-OT was restricted to TEC, while some ir-IL-6 and ir-LIF were also seen in occasional fibroblasts. In basal conditions, ir-IL-6 and ir-LIF (but not ir-OT and ir-IL-1 beta) were detected in the supernatants of human TEC cultures. MAbs to OT induced a marked increase of ir-IL-6 and ir-LIF secretion in TEC cultures. No significant effect was observed using mAbs against vasopressin, mouse immunoglobulins, or control ascitic fluid controls. These data show that OT is fully processed and recognized by specific mAbs at the outer surface of TEC plasma membrane. They further support that thymic OT behaves as the self-antigen of the neurohypophysial family.

Human thymic epithelial cells express functional HLA-DP molecules

HLA-DP molecules function as restriction elements in the presentation of foreign antigens to T cells by antigen presenting cells and certain HLA-DP molecules confer susceptibility to autoimmune disease. Because HLA molecules play an essential role in thymic selection and elimination of autoreactive T lymphocytes, we examined whether human thymic epithelial cells (TEC) expressed HLA-DP molecules. We present evidence that TEC obtained from short time culture express low but significant levels of HLA-DP molecules. The expression of HLA-DP molecules was comparable to or higher than the expression of HLA-DQ but lower than that of HLA-DR. Upon IFN-gamma treatment, HLA-DP expression was strongly upregulated. Since HLA-DQ and DR expression was upregulated in parallel, the hierarchy between MHC class II isotypes remained unchanged following interferon treatment. TEC elicited significant proliferation of HLA-DP allospecific primed lymphocyte typing (PLT) CD4 T cell lines. IFN-gamma treatment strongly upregulated the HLA-DP allospecific PLT responses whereas other PLT responses remained largely unchanged. In conclusion, these data indicate that human thymus epithelial cells express significant levels of functional HLA-DP molecules.

Characterization of an antigen shared by human thymic epithelium and human T cell leukemia virus p19 Gag protein

The molecular basis for cross-reactive antibody binding to human T cell leukemia virus type I (HTLV-I) p19 core protein and human thymic epithelium has been defined with two monoclonal antibodies (mAbs), 12/1-2 and 13B12, raised to HTLV-I p19. The mAb 12/1-2 has previously been shown to react with HTLV-I p19, HTLV-II p22, and antigens of normal human thymic epithelium, placenta, and foreskin, whereas mAb 13B12 binds only to the carboxyl terminus of HTLV-I p19. In the present study, mAb 12/1-2 bound to a subset of Triton X-100-insoluble intermediate filaments in human thymic epithelium also recognized by antikeratin antibodies AE1 and AE3. The mAb 12/1-2 also reacted in Western blot assays with proteins of 54, 46, and 40 kDa present in extracts of human thymic epithelium and with hexameric peptides containing overlapping sequences of HTLV-I p19 with the amino acids IPP (amino acids 117-119). In contrast, the HTLV-I-specific mAb 13B12 did not bind to human thymic epithelium and reacted with a single hexameric peptide containing the carboxy-terminal HTLV-I p19 sequence IPPPYV (amino acids 117-122). Binding of mAb 12/1-2 to thymic epithelium could be inhibited by adsorption with peptide SP-79 containing a C-terminal sequence (amino acids 112-125) of p19. The crossreactive IPP site is within a region of p19 that has been previously shown to be highly immunogenic in HTLV-I-infected individuals and that is also encoded by genes or mRNA of human cytokeratin 17, keratin 4, epidermal cytokeratin 2, and 50-kDa type I epidermal keratin. Thus, our studies define the sequence of a cross-reactive antigen on HTLV-I p19 that is also associated with keratin intermediate filaments from human thymic epithelium and other normal human tissues and that could serve as a focus of an autoimmune response during HTLV-I infection.

Presence of eosinophilic precursors in the human thymus: evidence for intra-thymic differentiation of cells in eosinophilic lineage

The distribution of myeloid cells in the human thymus was investigated by light and electron microscopy, immunohistochemistry, and/or flow cytometry. A series of 74 thymic samples, from newborn to 37 year old patients, were studied. By light microscopy, aggregates of mononuclear cells were frequently present in intralobular septa and outer medulla. Among those cells, eosinophilic precursors (promyelocyte, myelocytes and metamyelocytes) were readily identified. These immature granular cells were present in all pre-involutional thymi, and were particularly frequent in the thymi of patients who were younger than 5 years of age. The cells made up 30-50% of the total eosinophilic population and were frequently observed as a group of cells at various stages of differentiation, suggesting that they differentiate from pre-existing precursors in the thymus. These eosinophilic precursors were mostly located in the intralobular septa and fibroreticular network at the corticomedullary junction, while mature eosinophils were scattered throughout the thymus. Flow cytometric analyses, using stem cell-enriched preparations, showed that cells expressing CD33 or CD34 constituted on average 2.55% and 3.33% (0.09% and 0.12% of the total cells), respectively. CD33+/CD34+ coexpressors were also identified, and they constituted 0.36% of the analyzed cells (0.01% of the total cells). No statistical difference in the proportions of CD33+ and/or 34+ cells was noted between any age groups. It is concluded that eosinophilic precursors present in the thymus differentiate into cells in the eosinophilic lineage in particular areas such as the intralobular septa and fibroreticular network of the outer medulla in preinvolutional human thymi.

Immunohistochemical localization and biological significance of the phylogenically conserved thymus-brain antigen (UB-13 antigen) in skate, rat and human

A monoclonal antibody (UB-13) originally raised against the brain of the skate (Raja kenojei, a cartilaginous-fish) was found to react with lymphoid and brain tissues from many species when examined immunohistochemically. In rat and human thymus, UB-13 antigen was observed to be closely associated with reticular tissue in the medulla and cortex. Interestingly, a few or several thymocytes were encircled by the UB-13-reactive reticular tissue. At 14 days gestation, rat thymus consisted mainly of reticular epithelial tissue, after which strong thymocyte production started. At this stage, some of the reticular tissue was heavily stained with UB-13. In the thymus tissues of the irradiated and recovering rats, where reduction and massive reproduction of thymocytes were observed, extensive UB-13 antigen expression localized on the reticular epithelial tissue, an observation which may support the thymocyte re-population. These findings suggest that the antigen recognized by UB-13 may be important for thymocyte proliferation and maturation. UB-13 antigen was found in the fibrous structure of the molecular and granular layer of the human cerebellum. Some glial cells were also stained strongly with UB-13 in the human cerebellar or cerebral grey and white matter. In rat, glial cells, especially astroglias, and the endothelial structure of blood vessels were stained strongly with UB-13. These findings suggest that UB-13 may be a useful monoclonal antibody for analysis of brain-lymphoid antigen in many species.

The ER-TR4 monoclonal antibody recognizes murine thymic epithelial cells (type 1) and inhibits their capacity to interact with immature thymocytes: immuno-electron microscopic and functional studies

The thymic stroma is heterogeneous with regard to cellular morphology and cellular function. In this study, we employed the monoclonal antibody ER-TR4 to characterize stromal cells at the ultrastructural level. To identify the labelled cell type, we used two techniques: immunogold labelling on ultrathin frozen sections and immunoperoxidase staining on thick "vibratome" sections. ER-TR4 reacted with thymic Type 1 epithelial cells (according to our classification). A dense labelling appears in the cytoplasm of cortical cells using the two techniques. Immunogold labelling identified small cytoplasmic vesicles whereas the cytoplasm and the cell membrane seem to be labelled with the immunoperoxidase technique. ER-TR4 also identified isolated thymic nurse cells (TNC), and was observed in vitro to inhibit the capacity of some type 1 epithelial cells to establish interactions with immature thymocytes. This finding supports the hypothesis that the factor is involved in the formation of lymphoepithelial interactions within thymic nurse cells, and thus in the relations that immature thymocytes establish with the thymic microenvironment.

A mathematical model of the effect of aging on bone marrow cells colonizing the thymus

The process of T cell generation in the thymus involves complex cell-cell interactions between the various types of thymic stromal cells, thymocyte progenitors, thymocytes at different stages of differentiation and external factors. We applied the tool of mathematical modelling to analyze hypotheses and direct experiments concerning mechanisms underlying the observed developmental inferiority of bone-marrow thymocyte progenitors from old mice. Previous experimental data showed that lower cell numbers were obtained from old bone marrow-derived thymocyte progenitors, compared to young bone marrow-derived progenitors, when colonizing simultaneously the same fetal thymus. In this study, simulations based on the mathematical model indicate that the developmental inferiority of old bone marrow-derived progenitors cannot be explained by a change in a single parameter, such as the observed differences in progenitor frequency, an increase in cell cycle duration, a reduction in the fraction of proliferating cells in old age, and/or an increase in the rate of cell death. We have performed experimental measurements of the fractions of cycling cells. No significant difference was found between these fractions in young and old bone marrow-derived thymocytes. The difference in developmental patterns of young and old bone marrow-derived thymocytes may be due to a combination of more than one mechanism, possibly including interactions between competing thymocytes of old and young bone marrow origin.

A study of the chick thymus microenvironment during development: analysis by monoclonal antibodies against thymic epithelium

The process of T-lymphocyte differentiation within the thymus involves a series of molecular interactions. In this work we have carried out an analysis of the chick thymus microenvironment in order to evaluate its heterogeneity during development. We have produced 11 monoclonal antibodies whose staining patterns detected by the immunoperoxidase technique allowed us to divide them into five groups. A first group (E19-E2, P0-E5, and P15-T1) binds to thymic medullary stroma showing a reticular pattern on medullary epithelial cells and whose significance would be related to thymic stromal secretion. The second group of monoclonal antibodies (P15-T3) stains thymic corpuscles of 10- and 15-day chicks. The third group of antibodies includes P0-E1, P0-E3, P5-A6, and P15-T2 whose staining pattern is both medullary and cortical. The fourth group (P10-HB1 and P10-HB2) binds to thymic stromal and cortical thymocytes, and the fifth group (P5-A1) is characterized by the staining of medullary vessels of 5-day chicks. These five groups of monoclonal antibodies corroborate the existence of an antigenic diversity of the chick thymus microenvironment. Their possible relationships with T-cell differentiation and stromal-thymocyte interactions are discussed.

Putative prethymic T cell precursors within the early human embryonic liver: a molecular and functional analysis

Hematopoietic cells present in the liver in early human fetal life were characterized by phenotypic analysis using a broad panel of monoclonal antibodies. Expression of very late antigen 4 and leukocyte function-associated antigen 3 cell adhesion receptors and 4F2 cell activation molecules was found in all fetal liver hematopoietic cells before acquisition of T cell-, B cell-, or myeloid-specific surface markers, and before the time of intrathymic colonization. Molecular studies showed that expression of the interleukin 2 receptor beta (IL-2R beta) also occurred in the embryonic liver at this early ontogenic stage. In contrast, no expression of IL-2R alpha or IL-2 transcripts was found in fetal liver cells, whereas transcription of the IL-4 gene was detected in a small fetal liver cell subset. Putative T cell precursors were identified among the hematopoietic fetal liver cells by the expression of genes encoding the gamma, delta, epsilon, and zeta invariant chains of the CD3-T cell receptor (TCR) complex. However, no transcription of the polymorphic alpha and beta TCR genes was detected. Functional in vitro assays further demonstrated that fetal liver hematopoietic cells from those early embryos were capable of proliferating in response to T cell growth factors, including IL-4 and IL-2. However, whereas IL-4-induced proliferation paralleled the appearance in vitro of CD45+CD7-CD4dull cells expressing the CD14 myeloid antigen, as well as of CD34+ primitive hematopoietic progenitors, differentiation into CD45+CD7+CD8+CD3- immature T cells was observed when using IL-2. Moreover, coculture with thymic epithelial cell monolayers provided additional evidence that early fetal liver hematopoietic cells may include very primitive T cell precursors, which were able to differentiate in vitro into TCR alpha/beta+ mature T cells. Therefore, our results indicate that, after triggering of the T cell-specific maturation program in primitive fetal liver hematopoietic progenitors, specific signals provided intrathymically by epithelial cells may fulfill the requirements to drive terminal differentiation of prethymically committed T cell precursors.

Cell adhesion molecules: a unifying approach to topographic biology

Cell adhesion molecules are pivotal to the development and maintenance of tissue structure in metazoan organisms. In mammals, several families of proteins are involved in cell-cell and cell-matrix adhesion. The cadherins are homophilic, primary CAMs, involved in the establishment of boundaries between cell collectives early in embryogenesis. The Ig gene superfamily have diversified widely, with homophilic and heterophilic CAMs and antigen recognition molecules amongst the members. The Integrin family play an important role in binding to extracellular matrix, as well as counter-receptors on the surface of other cells. The Selectin family and HCAM are carbohydrate-binding proteins, and play a prominent role in the circulation of lymphocytes and neoplastic cells. CAMs are fundamental to development of tissue structure in metazoan organisms. Cellular differentiation dictates adherence to a specific microenvironment, through the pattern of surface CAM expression. Conversely, CAM binding can affect gene expression within the cell itself. Cell differentiation and cell adhesion are interdependent processes. In the adult, CAM are crucial to tissue maintenance. Cells frequently change their adhesive properties in response to physiological or pathological processes. The integrity of the vascular system is maintained by circulating platelets which are capable of rapid upregulation of cell adhesion and profound changes in metabolism, on contact with subendothelial matrix. Both endothelial cells and neutrophils undergo changes in CAM expression in response to inflammatory mediators, permitting rapid and appropriate recruitment of phagocytes to damaged tissue. Tissue repair is dependent on phenotypic changes in normally static cells, allowing increased motility and replication. The immune system requires constitutive cells to undergo multiple complex adhesion and detachment events over short periods of time, and is capable of discriminating normal self from aberrant-self or non-self, through antigen specific recognition and adhesion molecules. The pathophysiology of processes such as infection and neoplasia are profoundly affected by cellular CAM expression. CAMs and related molecules are fundamental to the development, maintenance and surveillance of tissue structure.

Regulation of cytokine production in the human thymus: epidermal growth factor and transforming growth factor alpha regulate mRNA levels of interleukin 1 alpha (IL-1 alpha), IL-1 beta, and IL-6 in human thymic epithelial cells at a post-transcriptional level

Human thymic epithelial (TE) cells produce interleukin 1 alpha (IL-1 alpha), IL-1 beta, and IL-6, cytokines that are important for thymocyte proliferation. The mRNAs for these cytokines are short-lived and are inducible by multiple stimuli. Thus, the steady-state levels for IL-1 and IL-6 mRNAs are critical in establishing the final cytokine protein levels. In this study we have evaluated the effect of epidermal growth factor (EGF), a growth factor for TE cells, and its homologue transforming growth factor alpha (TGF-alpha), on primary cultures of normal human TE cells for the levels of IL-1 alpha, IL-1 beta, IL-6, and TGF-alpha mRNA. We showed that TE cells expressed EGF receptors (EGF-R) in vitro and in vivo, and that treatment of TE cells with EGF or TGF-alpha increased IL-1 and IL-6 biological activity and mRNA levels for IL-1 alpha, IL-1 beta, and IL-6. Neither EGF nor TGF-alpha increased transcription rates of IL-1 alpha, IL-1 beta, and IL-6 genes, but rather both EGF and TGF-alpha increased cytokine mRNA stability. By indirect immunofluorescence assay, TGF-alpha was localized in medullary TE cells and thymic Hassall's bodies while EGF-R was localized to TE cells throughout the thymus. Thus, TGF-alpha and EGF are critical regulatory molecules for production of TE cell-derived cytokines within the thymus and may function as key modulators of human T cell development in vivo.

Thymic epithelial cell-derived supernatants sustain the maturation of human prothymocytes: involvement of interleukin 1 and CD23

During their development, human CD7+ lymphoid stem cells migrate into the thymus where, following intimate contact with thymic tissue, they proliferate and differentiate into functionally mature T lymphocytes. In this study, we investigated the effect of thymic epithelial cell-derived supernatants (TEC-SN) on early CD7+CD2-CD3- thymocytes. Our results indicate that TEC-SN are able to promote CD2 and CD3/TcR alpha/beta expression by CD7+ precursors. This activity correlated with soluble CD23 and interleukin 1 levels in TEC-SN. Furthermore, monoclonal antibodies to these cytokines decreased in vitro maturation of prothymocytes. Thus, in addition to cell-cell interactions, human TEC produce cytokines able to support early steps of thymocyte differentiation.

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.

Loss of medullary dendritic cells in the thymus after cyclosporine and irradiation

Cyclosporine (CsA) induces a paradoxical graft-vs-host-like disease (GVHD) in syngeneic rat chimeras, providing the rat has a thymus and receives mediastinal irradiation. Here we evaluate the effect of CsA and irradiation on the relative abundance of thymic dendritic cells (DC). DC were identified with an immunoperoxidase stain for ED1 and were quantified by computerized planimetry. Normal young rats have scattered DC in the cortex and numerous DC in the medulla with a concentration at the cortico-medullary junction. Short-term CsA induces a marked loss of medullary DC (798 +/- 126 cells/mm to 88 +/- 103, P less than 0.001) and a modest loss of cortical DC (543 +/- 55 to 330 +/- 130, P less than 0.01). While medullary DC normally recover promptly post-CsA, rats receiving mediastinal irradiation demonstrated minimal recovery post-CsA (P less than 0.0003). The prolonged deficiency of medullary DC could represent an essential early step for loss of self-tolerance and sGVHD.

Functional histology of the human thymus

The thymus develops from a paired epithelial anlage in the neck. This review considers how ectoderm (vesicula cervicalis) and endoderm (third pharyngeal pouch) contribute to the epithelial stroma of the thymus. Stromal elements of mesodermal origin are capillaries, septae and perivascular spaces and single invading cells. These elements separate the thymus into pseudolobuli. The thymus epithelial space and the perivascular spaces are always separated from each other by a closed, flat epithelial cell layer, with a basal lamina which contributes to the blood-thymus barrier. From the 9th gestational week, prethymic precursor cells from hemopoietic centers, begin to invade the thymus anlage. There they finally mature to committed post-thymic T cells. The thymus microenvironment of postnatal thymus is composed of six different types of epithelial cells and several stromal cells of mesodermal origin. The location of these diverse stationary cells is described, and their functional significance is discussed. Obviously these stromal cell types have a special function in providing the proper environment for T-cell maturation. The function of the thymus includes the maturation and/or selection of antigen specific T-cells. The main issue of intra-thymic T-cell differentiation is the development and expression of T-cell-antigen receptors. The great diversity of these receptors is generated by a rearrangement of the T-cell-receptor-genes in order to furnish the host with a mature T-cell repertoire that is capable of recognizing the world of extrinsic antigens. In a synopsis the manyfold interrelationships between the thymus microenvironment and the developing thymocytes are summarised.

Successful engraftment of human postnatal thymus in severe combined immune deficient (SCID) mice: differential engraftment of thymic components with irradiation versus anti-asialo GM-1 immunosuppressive regimens

To develop a model of human thymus growth in vivo, we have implanted postnatal human thymus under the renal capsule of severe combined immune deficient (SCID) mice and assayed for graft survival and graft characteristics 1-3 mo after engraftment. Three groups of SCID mice were engrafted with postnatal human thymus: untreated SCID mice, SCID mice pretreated with 400 cGy of gamma irradiation 1-5 d before engraftment, and SCID mice treated with intraperitoneal anti-asialo GM-1 antiserum every 4-5 d during engraftment. In the untreated group of SCID mice, only 37% of grafts survived and consisted of human thymic microenvironment components and human immature thymocytes. Irradiation of SCID mice before engraftment improved survival of human thymic grafts to 83%, but these grafts were largely devoid of thymocytes and contained only thymic microenvironment components with large numbers of thymic macrophages. Treatment of SCID mice with anti-asialo GM-1 antiserum throughout the engraftment period also promoted human thymus engraftment (70%) and induced SCID B cell Ig production (SCID[Ig+]) in 38% of animals. In SCID(Ig-) anti-asialo GM-1-treated mice, the human thymic grafts were similar in content to those in untreated SCID mice. However, in anti-asialo GM-1-treated animals with grafts that became SCID(Ig+), all animals were found to have mouse-human chimeric grafts in that the human thymic microenvironment (human fibroblasts, thymic epithelium, vessels) was colonized by murine T cells. These data demonstrate that human postnatal thymus will grow as xenografts in SCID mice, and that the components of human thymus that engraft are dependent on the immunosuppressive regimen used in recipient mice. A striking finding in this study was the induction of T and B lymphopoiesis in SCID mice by abrogation of NK cell activity with in vivo anti-asialo GM-1 treatment. These data strongly suggest that asialo GM-1+ NK cells and/or macrophages play a role in mediation of suppression of lymphopoiesis in SCID mice.

Immunophenotype of thymoma-associated lymphoid cell component of T-cell type. A new analytic procedure in keeping with structural heterogeneities

The phenotype of the lymphoid cell component of 35 thymomas was investigated by analyzing cryostat sections and lymphocyte suspensions. The morphology in each case was determined by examining multiple tissue samples from different parts of the tumor. Structural heterogeneity was shown in 14 thymomas, and a homogeneous morphology of cortical or medullary or mixed types in the others. To assess whether this heterogeneity was correlated with differences in the lymphoid phenotype, we analyzed both lymphocyte suspensions and frozen sections from the same samples. Phenotypical differences in the suspensions of each thymoma in the heterogeneous group were noted and similar differences were also observed in the cryostat sections. Phenotypical abnormalities were found in some thymomas. They consisted of the simultaneous expression of cortical and medullary markers, which was most marked in the heterogenous mixed-type thymomas invading the lung. Furthermore, the global phenotype was tested on a pool of lymphocyte suspensions in all thymomas. This procedure distinguished cortical, medullary and intermediate cortico-medullary immunophenotype models which closely correlated with the tumor histology. It was concluded that, due to the frequent structural and immunological heterogeneity of thymomas, correct assessment of their lymphoid component requires a two-step analysis. This comprises: 1) individual suspensions from samples taken from different areas of the same thymoma, and 2) a pool of these suspensions. The first step will reveal the different immunological characteristics. In the second, the lymphocyte phenotype, which may vary widely throughout the tumor, will be represented in its totality. These findings may be of great help in predicting clinical patterns, especially possible malignant evolution.

Epithelial cells and macrophages in myasthenia gravis thymus culture

Monolayer culture of thymic nonlymphoid cells derived from female patients with myasthenia gravis (MG) and individuals who underwent heart surgery was established to investigate the cellular composition of the thymic microenvironment and the interaction of nonlymphoid cells with autologous thymocytes. Thymic epithelial cells were identified by immunoperoxidase staining using monoclonal antibodies (mAbs) specific for cytokeratin and MR6 and MR19 antigens expressed on cortical and medullary epithelial cells, respectively. Macrophages were characterized by determination of alpha-naphthyl acetate esterase activity and detection of M1 antigen by mAb. It was demonstrated that in MG thymus cultures the number of cortical MR6+ epithelial cells is significantly reduced, and the ability of the remaining MR6+ cells to bind autologous thymocytes is markedly affected. On the other hand, the number of macrophages and the interaction of those cells with thymocytes were similar in MG and control thymus cultures. Since MR6+ epithelial cells are numerically and functionally affected in MG, maturational events of T cells occurring in the inner cortex may be altered. The mechanisms underlying the induction and expansion of T helper clones in MG are discussed.

Adhesion receptors of the immune system

The adhesive interactions of cells with other cells and with the extracellular matrix are crucial to all developmental processes, but have a central role in the functions of the immune system throughout life. Three families of cell-surface molecules regulate the migration of lymphocytes and the interactions of activated cells during immune responses.