The murine thymic nurse cell: an isolated thymic microenvironment

The importance of the nurse cells and regulatory cells in the control of T lymphocyte responses

T lymphocytes from the immune system are bone marrow-derived cells whose development and activities are carefully supervised by two sets of accessory cells. In the thymus, the immature young T lymphocytes are engulfed by epithelial “nurse cells” and retained in vacuoles, where most of them (95%) are negatively selected and removed when they have an incomplete development or express high affinity autoreactive receptors. The mature T lymphocytes that survive to this selection process leave the thymus and are controlled in the periphery by another subpopulation of accessory cells called “regulatory cells,” which reduce any excessive immune response and the risk of collateral injuries to healthy tissues. By different times and procedures, nurse cells and regulatory cells control both the development and the functions of T lymphocyte subpopulations. Disorders in the T lymphocytes development and migration have been observed in some parasitic diseases, which disrupt the thymic microenvironment of nurse cells. In other cases, parasites stimulate rather than depress the functions of regulatory T cells decreasing T-mediated host damages. This paper is a short review regarding some features of these accessory cells and their main interactions with T immature and mature lymphocytes. The modulatory role that neurotransmitters and hormones play in these interactions is also revised.

[Thymic nurse cells--a specialized thymic microenvironment]

<zakljucak> Vise od dve decenije nakon prvog opisa, TNC su i dalje velika nepoznanica i potrebno je jos mnogo istrazivanja pre nego sto budemo bili u mogucnosti da definisemo preciznu ulogu ovih celija u razvoju T-limfocita. Mnoga od dosadasnjih saznanja ukazuju da se timociti u kontaktu sa TNC nalaze na prekretnici u svom razvojnom putu: ili ce biti uklonjeni indukcijom apoptoze ili ce nastaviti svoj razvoj i dalje sazrevanje. Brojna pitanja su za sada bez odgovora, a medju njima su dva posebno intrigantna. Koja je razlika izmedju timocita koji se vezuju za TNC i onih koji to ne cine? Koja je razlika izmedju populacije adherentnih timocita koji su selektivno internalizovani i onih koji su iskljuceni iz procesa internalizacije? Buduca istrazivanja kretanja timocita ka, unutar i van TNC ce verujemo pruziti dragocene informacije o ovoj fazi u razvoju T-limfocita. Nezavisno od toga sta ce buducnost pokazati o pravoj ulozi TNC, jedinstven kompleks koji ove celije formiraju sa timocitima je vrlo neobican, uzbudljiv i zagonetan bioloski fenomen.

The human and mouse orthologous LIM-only proteins respectively encoded in chromosome 6 and 17 show a different expression pattern

Thymocytes interact with various subpopulations of thymic epithelial cells (TECs) at different stages of their development. To identify new molecules specifically expressed in TECs and/or thymic nurse cells (TNCs), we used representational difference analysis. We identified a LIM protein located on mouse chromosome 17 (m17TLP) and belonging to the family of the LIM-only proteins (LIMo). We found a new splice variant in addition to the two described A and B isoforms. The three alternative species of m17TLP are found strictly in the thymic stroma. This protein is expressed on a subpopulation of TECs and TNCs. Strikingly, we found that the human ortholog of m17TLP, located on chromosome 6 (h6LIMo), is expressed in most tissues, but not in skeletal muscle. We have identified four human splice variants of h6LIMo which differ in their carboxy-terminal regions. The sequence comprising the genomic structure suggests that CRP2 is the closest known relative of m17TLP. Although the human and mouse nucleotide sequences are 88-97% homologous, this homology is reduced to 47% in the promoter regions, which strongly suggests that their differential expression is related to their promoter regulatory activity.

Thymic nurse cells: a microenvironment for thymocyte development and selection

Thymic nurse cells (TNCs) represent a unique microenvironment in the thymus for MHC restriction and T cell repertoire selection composed of a cortical epithelial cell surrounding 20-200 immature thymocytes. TNCs have been isolated from many classes of animals from fish to humans. Studies performed using TNC lines showed that TNCs bind viable alphabetaTCRlow CD4(+)CD8(+)CD69(-) thymocytes. A subset of the bound cells is internalized, proliferates within the TNC, and matures to the alphabetaTCRhigh CD4(+)CD8(+)CD69(+) stage, indicative of positive selection. A subset of the internalized population is released while cells that remain internalized undergo apoptosis and are degraded by lysosomes within the TNC. A TNC-specific monoclonal antibody added to fetal thymic organ cultures resulted in an 80% reduction in the number of thymocytes recovered, with a block at the double positive stage of development. Together these data suggest a critical role for TNC internalization in thymocyte selection as well as the removal and degradation of negatively selected thymocytes. Recent studies have shown that in addition to thymocytes, peripheral circulating macrophages are also found within the TNC complex and can present antigens to the developing thymocytes. These circulating macrophages could provide a source of self-antigens used to ensure a self-tolerant mature T cell repertoire. A reduction in TNC numbers is associated with a variety of autoimmune diseases including thyroiditis and systemic lupus erythematosis.

Questionable thymic nurse cell

Since their discovery in 1980, thymic nurse cells (TNCs) have been controversial. Questions pertaining to the existence of the TNC as a "unit" cell with thymocytes completely enclosed within its cytoplasm were the focus of initial debates. Early skeptics proposed the multicellular complex to be an artifact of the procedures used to isolate TNCs from the thymus. Since that time, TNCs have been found in fish, frogs, tadpoles, chickens, sheep, pigs, rats, mice, and humans. Their evolutionary conservation throughout the animal kingdom relieved most speculations about the existence of TNCs and at the same time demonstrated their apparent importance to the thymus and T-cell development. In this review we will discuss and debate reports that describe (i) the organization or structure of TNCs, (ii) the thymocyte subset(s) found within the cytoplasm of TNCs and their uptake and release, and (iii) the function of this fascinating multicellular interaction that occurs during the process of T-cell development. Discussions about the future of the field and experimental approaches that will lead to answers to remaining questions are also presented.

Is there a glycosaminoglycan-related heterogeneity of the thymic epithelium?

We determined the synthesis and secretion of glycosaminoglycans by three distinct preparations of mouse cultured thymic epithelial cells. These comprised primary cultures of thymic nurse cells (TNCs), which are normally located within the cortex of the thymic lobules, as well as two murine thymic epithelial cells, bearing a mixed, yet distinct, cortico-medullary phenotype. We first identified and measured the relative proportions of the various glycosaminoglycans in the three epithelial cells. Non-sulfated glycosaminoglycans are preponderantly secreted by the TNCs, while the sulfated glycans (particularly heparan sulfate) are relatively more abundant on the cell surface. The three types of epithelial cells differ markedly in their heparan sulfate composition, mainly due to different patterns of N- and O-sulfation. In addition, the cells differ in the synthesis and secretion of other glycosaminoglycans. Thus, TNCs secrete high amounts of dermatan sulfate + chondroitin sulfate to the culture medium. IT-76M1 cells secrete high proportions of heparan sulfate while 2BH4 cells show a more equilibrated proportion of dermatan sulfate/chondroitin sulfate and heparan sulfate. The three epithelial cells also differ in their capacity to produce hyaluronic acid and 2BH4 cells are distinguished by their high rate of synthesis of this glycosaminoglycan. In conclusion, our results show that distinct thymic epithelial cells can synthesize different types of glycosaminoglycans. Although it remains to be definitely determined whether these differences reflect the in vivo situation, our data provide new clues for further understanding of how glycosaminoglycan-mediated interactions behave in the thymus.

Lysosomal-mediated degradation of apoptotic thymocytes within thymic nurse cells

A thymic epithelial cell line (tsTNC-1) that maintains the ability to selectively bind and internalize immature alphabetaTCR(lo)CD4(+)CD8(+) thymocytes in vitro was used in long-term coincubation experiments to determine the ultimate fate of thymocytes that remained within intracytoplasmic vacuoles of thymic nurse cells (TNCs). In an earlier report, a subset of the population released from the TNC interaction was shown to mature to the alphabetaTCR(hi)CD69(hi) stage of development, while thymocytes that bided within the TNC cytoplasm died through the process of apoptosis. Here, we show the presence of both apoptotic and nonapoptotic thymocytes within the cytoplasm of freshly isolated TNCs as well as in tsTNC-1 cells in culture. A microscopic analysis revealed total degradation of the cytoplasmic apoptotic thymocyte population that remained in tsTNC-1 cells after an 8- to 10-h incubation period. A quantitative analysis showed an increase of cytoplasmic thymocyte degradation over time to almost 80% after 9 h of incubation. However, in the presence of bafilomycin A1, which is used to inhibit acidification of lysosomal vesicles, degradation of apoptotic thymocytes never reached 10%. These data suggest that lysosomes within TNCs play a role in the degradation of apoptotic thymocytes. We examined tsTNC-1 cells before the addition of thymocytes to cultures and found lysosomes to be clustered around the nucleus in the cytoplasm of TNCs. Shortly after the internalization event, apoptotic thymocytes move to the area of the cytoplasm containing lysosomes. Using the confocal microscope, we obtained evidence that shows the degradation event to be facilitated through the fusion of lysosomes with the specialized vacuoles within TNCs containing apoptotic cells.

Apoptosis and the thymic microenvironment in murine lupus

The thymus of New Zealand black (NZB) mice undergoes premature involution. In addition, cultured thymic epithelial cells from NZB mice undergo accelerated preprogrammed degeneration. NZB mice also have distinctive and well-defined abnormalities of thymic architecture involving stromal cells, defined by staining with monoclonal antibodies specific for the thymic microenvironment. We took advantage of these findings, as well as our large panel of monoclonal antibodies which recognize thymic stroma, to study the induction of apoptosis in the thymus of murine lupus and including changes of epithelial architecture. We studied NZB, MRL/lpr, BXSB/Yaa, C3H/gld mice and BALB/c and C57BL/6 as control mice. Apoptosis was studied both at basal levels and following induction with either dexamethasone or lipopolysaccharide (LPS). The apoptotic cells were primarily found in the thymic cortex, and the frequency of apoptosis in murine lupus was less than 20% of controls. Moreover, all strains of murine lupus had severe abnormalities of the cortical network. These changes were not accentuated by dexamethasone treatment in cultured thymocytes. However, the thymus in murine lupus was less susceptible to LPS-induced apoptosis than control mice. Finally we note that the number of thymic nurse cells (TNC) was lowest in NZB mice. Our findings demonstrate significant abnormalities in the induction of apoptosis and the formation of TNC-like epithelial cells in SLE mice, and suggest that the abnormalities of the thymic microenvironment have an important role in the pathogenesis of murine lupus.

The conveyor belt hypothesis for thymocyte migration: participation of adhesion and de-adhesion molecules

Thymocyte differentiation is the process by which bone marrow-derived precursors enter the thymus, proliferate, rearrange the genes and express the corresponding T cell receptors, and undergo positive and/or negative selection, ultimately yielding mature T cells that will represent the so-called T cell repertoire. This process occurs in the context of cell migration, whose cellular and molecular basis is still poorly understood. Kinetic studies favor the idea that these cells leave the organ in an ordered pattern, as if they were moving on a conveyor belt. We have recently proposed that extracellular matrix glycoproteins, such as fibronectin, laminin and type IV collagen, among others, produced by non-lymphoid cells both in the cortex and in the medulla, would constitute a macromolecular arrangement allowing differentiating thymocytes to migrate. Here we discuss the participation of both molecules with adhesive and de-adhesive properties in the intrathymic T cell migration. Functional experiments demonstrated that galectin-3, a soluble beta-galactoside-binding lectin secreted by thymic microenvironmental cells, is a likely candidate for de-adhesion proteins by decreasing thymocyte interaction with the thymic microenvironment.

A thymic nurse cell-specific monoclonal antibody

A thymic epithelial cell line (tsTNC-1) that maintains the ability to selectively bind and internalize immature alpha beta TCRloCD4+CD8+ thymocytes in vitro was used in the development of a monoclonal antibody that is specific to the cell surface of thymic nurse cells (TNCs) in the thymus. The rat monoclonal antibody ph91 showed specificity to cells of the subcapsular region of the thymic cortex. Upon mechanical dispersion of the thymus in vitro, ph91 recognized cells displaying the multicellular morphology unique to TNCs. Ph91 staining was not detected on fresh thymocytes, stromal cells of the inner thymic cortex, thymic medullary cells, B cells or fibroblasts. Ph91 recognized a 43-kDa protein on the surface of TNCs. Exposure of tsTNC-1 cells to ph91 in tissue culture significantly reduced the percentage of binding of the alpha beta TCRloCD4+CD8+ thymocyte subset previously shown to target TNCs. In organ culture, ph91 reduced the viability of developing thymocytes by 70%. The largest reduction was found in the alpha beta TCR+CD4+CD8+ thymocyte subset. These results represent the first report of a TNC-specific monoclonal antibody. Further, the antigen to which ph91 binds may play a role in the process of thymocyte binding and their subsequent internalization which is unique to TNCs and important to the T cell developmental process.

Aberrant expression of tissue-specific proteins in the thymus: a hypothesis for the development of central tolerance

Herein we present the case for the existence of a thymic cortical epithelial cell that possesses an unusual gene transcription. It produces tissue-specific proteins that have their usual physiological functions outside the thymus and presents them, as well as household proteins, to the differentiating thymocytes. We suggest that this specialized cell enforces tolerance to most self-proteins by causing release of a signal for programmed cell death to thymocytes that express receptors for these self-antigens.

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.

Thymic nurse cells: their functional ultrastructure

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.

Pituitary hormones modulate cell-cell interactions between thymocytes and thymic epithelial cells

The thymic microenvironment plays a key role in the intrathymic T-cell differentiation. It is composed of a tridimensional network of epithelial cells whose physiology is controlled by extrinsic circuits such as neuroendocrine axes. Herein we show that the expression of extracellular matrix ligands and receptor by cultured thymic epithelial cells is upregulated by prolactin (PRL) and growth hormone (GH), the latter apparently occurring via insulin-like growth factor I (IGF-I). Thymocyte release from the lymphoepithelial complexes, thymic nurse cells, as well as the reconstitution of these complexes are enhanced by PRL, GH or IGF-I. Treatment of a mouse thymic epithelial cell line with these hormones induced an increase in thymocyte adhesion, an effect significantly prevented in the presence of antibodies to fibronectin, laminin or respective receptors VLA-5 and VLA-6. Our data suggest that the in vitro changes in thymocyte/thymic epithelial cell interactions induced by pituitary hormones are partially mediated by the enhancement of extracellular matrix ligands and receptors.

Formation of spheroid structures in a human colon carcinoma cell line involves a complex series of intercellular rearrangements

The structural remodelling of tissues that occurs in vivo during animal morphogenesis can often prove difficult to study. Here we investigate the organizational processes of the LIM 1863 colon carcinoma cell line as it transforms from a single-cell stage into multicellular spherical structures called 'organoids'. The organoids can be dissociated into a viable single-cell suspension when cultured in calcium-depleted medium, and then induced to reform the organoid structure by the readdition of calcium. Previous studies have shown that initial cell attachment under these conditions is characterized by a novel mechanism of cell engulfment termed 'clutching'. This investigation reveals the subsequent appearance of junctional complexes between groups of 'clutched' cells prior to lumen formation, and the ultimate 'declutching' of entrapped cells as a means of cell rearrangement. Intact actin filaments but not microtubules were required for the initial clutching events, while inhibition of microtubule polymerization resulted in aberrant apical protein polarization, but did not affect the development of a luminal space within the spheroids. Single cells exhibited pools of intracellular microvilli contained in vacuolar apical compartments, which were resistant to the effects of cytoskeleton-disrupting drugs. However, these structures did not seem to be responsible for the swift development of the luminal surface observed in these cells. Two other cell lines, MDCK and DU 4475, were found to exhibit similar clutching conformations when induced to form three-dimensional structures, suggesting that this may be a widespread mechanism of cell rearrangement that reflects the process of organ morphogenesis in vivo.

Chicken thymic nurse cells: an overview

Thymic nurse cells are multicellular complexes located in the subcortical area of the thymus of all avian, mammalian and amphibian species investigated so far. Since their first description in 1980 many studies have been carried out to characterize their morphological and functional properties. The purpose of this review is to summarize recent morphological as well a functional analyses of chicken thymic nurse cells which suggest a role of these cell complexes in T cell selection.

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.

Extracellular matrix components of the mouse thymus microenvironment. IV. Modulation of thymic nurse cells by extracellular matrix ligands and receptors

Extracellular matrix (ECM) proteins can influence cell migration and differentiation in a variety of cell systems. Within the thymus, these molecules are heterogeneously distributed, and their physiological role is poorly understood. This prompted us to carry out in vitro studies using the thymic nurse cell (TNC) model. We observed that fibronectin and laminin accelerate spontaneous in vitro release of thymocytes from TNC, whereas anti-ECM antibodies exhibited a blocking effect. Similar results were obtained with anti-ECM receptor reagents. Moreover, these antibodies abrogated in vitro reconstitution of TNC complexes and thymocyte adhesion to TNC-derived epithelial cultures. Our results indicate that lymphocyte traffic in TNC (comprising both entrance into and exit from the epithelial structure) is affected by interactions involving extracellular matrix ligands and receptors. In this respect, the dynamic analysis of thymic nurse cell complexes should be regarded as a relevant in vitro tool for functional studies of distinct adhesion molecules in intrathymic lymphocyte traffic.

Thymic nurse cells in culture: morphological and antigenic characterization

Epithelial monolayers were derived from thymic nurse cells (TNC), and were seeded onto collagen-coated dishes immediately after their isolation from young adult C3H-murine thymuses. Different media and supplements were tested in order to obtain cultures that were as pure as possible. Primary cultures were enriched in epithelial cells but always contained non-epithelial components among which fibroblasts predominated. Immunodetection of keratins, and repeated light- and electron-microscopic observations established the epithelial nature of the elongated cells derived from TNC; these elongated cells were cortical reticular cells, and were different from medullary globular cells that immediately adopted a mosaic pattern in vitro. At the beginning of the culture, the necrosis of cortical lymphocytes appeared to be toxic for epithelial cells; when epithelial cells survived, they showed a temporary lipid accumulation. After a 5-day culture, they still synthesized DNA but lost this capacity thereafter and dedifferentiated. The lympho-epithelial symbiosis appeared to be necessary to maintain some epithelial characteristics of the cultured cells, such as the clear vesicles and the expression of Ia antigens. In sub-cultures, the monolayers were almost purely epithelial in nature but growth was no longer observed. The cells remained reticular in shape, as they were in vivo, but their cytoplasm and their nucleus became larger and numerous cells were multinucleated. Confluence was not obtained with classical media even after mitogenic stimulation. The frequent observation of strongly keratinized areas suggested a process of terminal differentiation; this could not be avoided by using low serum concentration.

Extracellular matrix proteins in intrathymic T-cell migration and differentiation?

Intrathymic T-cell migration and differentiation is not completely understood. Here, Wilson Savino and colleagues argue that certain interactions between differentiating thymocytes and thymic epithelial cells are mediated by extracellular matrix proteins and that these interactions influence intrathymic migration events and thymocyte differentiation.

Thymic nurse cells exclusively bind and internalize CD4+CD8+ thymocytes

Thymic nurse cells (TNC) contain 20-200 thymocytes within specialized vacuoles in their cytoplasm. The purpose of the uptake of thymocytes by TNCs is unknown. TNCs also have the capacity to present self-antigens, which implies that they may serve a function in the process of thymic education. We have recently reported the development of thymic nurse cell lines that have the ability to bind and internalize T cells. Here, we use one of these TNC lines to identify the thymocyte subpopulation(s) involved in this internalization process. TNCs exposed to freshly isolated thymocytes bind and internalize CD4 and CD8 expressing thymocytes (CD4+CD8+ or double positives) exclusively. More specifically, a subset of the double-positive thymocyte population displayed binding capacity. These double-positive cells express cell surface alpha beta type T cell antigen receptor (TCR), as well as CD3 epsilon. Binding was not inhibited in the presence of antibodies against CD3, CD4, CD8, Class I antigens, or Class II antigens. These results describe two significant events in T cell development. First, TNCs exclusively bind and internalize a subset of alpha beta TCR expressing double-positive T cells. Also, binding is facilitated through a mechanism other than TCR recognition of major histocompatibility complex antigens. This suggests that thymocyte internalization may be independent of the process used by TNCs to present self-antigen.

Thymocyte maturation following interaction with thymus-derived macrophages

Murine C57BL/6 thymocytes were cultivated together with syngeneic thymus-derived macrophages (TDM phi) for up to 96 hr to determine whether TDM phi participate in thymocyte maturation. The expression level of H-2b and Thy-1.2 antigens served as thymocyte differentiation surface markers as analyzed by flow cytometry. Indirect immunofluorescent staining profiles of the thymocytes demonstrate a dramatic increase in H-2b expression and a profound decrease in Thy-1.2 expression during cultivation with TDM phi. A similar phenomenon was observed when enriched populations of immature thymocytes were cocultivated with TDM phi. These changes were not observed when thymocytes were cultivated alone or with trypsin-treated TDM phi; neither were they observed when cortisone-resistant thymocytes manifesting mature characteristics were cultivated together with TDM phi. These findings suggest that interaction of thymocytes with TDM phi, involving binding and engulfment, results in the appearance of mature thymocyte subsets.

The immortalization of thymic nurse cells by SV40 virus

Thymic nurse cells (TNCs) are stromal elements that contain between 20 and 200 T cells within their cytoplasm. Because of this unique feature they are believed to play a role in thymocyte development. Unfortunately, it has been difficult to obtain pure TNCs in quantities sufficient for extensive evaluation of their thymic function. As a result, only a limited amount of information is available that characterizes TNCs or the T cell population(s) found within their cytoplasm. We have now used SV40 to infect and immortalize TNCs from C57BL/6 mice. SV40-transformed TNCs were found to specifically bind and internalize cells from an immature thymocyte line isolated in our laboratory. These results describe a method of obtaining pure populations of TNCs for future studies of their thymic function, and suggest that binding to specific subpopulations of lymphoblasts may be necessary for internalization.

Thymic nurse cells: differentiation of thymocytes within complexes

Thymic nurse cell complexes (TNC-c) were isolated from thymuses of BDF1 mice at pre-determined intervals during the 12-week latency period that precedes the development of leukemias. T-cell leukemias were induced by a single i.v. injection of 50 mg/kg of methylnitrosourea (MNU). In order to clarify processes taking place in TNC-c, the complexes of mice after MNU injection were compared with TNC-c of age-matched control mice, with respect to their number per thymus, the distribution of TNC-c according to their size (the number of intra-TNC thymocytes reflects the type of TNC-c), the number of intra-TNC thymocytes that undergo DNA synthesis, and the phenotype of thymocytes inside TNC-c. During the latency period of leukemogenesis, the effects of MNU were shown to involve, in addition to changes in number of TNC-c, a decrease in the number of thymocytes incorporating labeled thymidine, viz., the number of dividing cells, thus affecting the size distribution of TNC-c types. Intra-TNC thymocytes of control mice were heterogeneous in their phenotype and represented cells at varying stages of their maturation cycle. MNU administration was followed by selective differentiation of thymocytes within TNC-c to Lyt 1-thymocytes in some and to Lyt 2-thymocytes in others. Lyt 1 and Lyt 2 being specific antigens expressed by thymocytes.

Selective abnormalities in the thymic microenvironment associated with avian scleroderma, an inherited fibrotic disease of L200 chickens

As a prelude to deciphering the mechanisms of intrathymic T-cell maturation we produced a panel of 18 monoclonal antibodies (mAbs) against chicken thymic stromal elements. Eleven of these detected epithelial cells. They were: pan-epithelial; subcapsule and peri-vascular (pan type 1 epithelium); subcapsular, perivascular and medulla; medulla; or cortex. Of particular interest were the sub-specificities within these regions, especially the subcapsular region. Four mAbs stained both epithelial and non-epithelial cells in discrete regions. In addition, three mAbs recognized only non-epithelial cells. One identified macrophages scattered throughout the thymus, another the connective tissue and another the medullary vascular endothelium. These reagents have provided an extensive profile of the thymic stromal architecture and revealed that these cells are equally as complex as the T cells whose differentiation they induce and regulate. While the mAbs provide a valuable means for studying the mechanisms of normal thymopoiesis, their clinical significance is unknown. UCD line 200 chickens develop an autoimmune disease manifest by dermal and internal organ fibrosis, T cell infiltrates of skin and other affected organs and production of multiple autoantibodies. We have used our panel of mAbs to evaluate the thymic microenvironment in these autoimmunity-prone chickens. A comparative analysis with control chickens revealed striking deficiencies in the L200 subcapsular regions coupled with excessive expression of MHC class II antigens, particularly in the cortex. We hypothesize that these abnormalities induce altered T-cell differentiation, thereby predisposing the L200 chickens to autoimmune disease.

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.

Selection of the T-cell repertoire in transgenic mice expressing a transplantation antigen in distinct thymus subsets

Transgenic mice that expressed a transplantation antigen, H-2Kb, in an unusual tissue distribution have been developed. Gene-regulatory elements from the immunoglobulin heavy-chain locus (Emu enhancer and heavy chain promoter) were linked to the class I Kb gene and the construct microinjected into fertilized mouse eggs of a different haplotype. It was expected that such gene-regulatory elements would direct expression of the foreign class I molecules only to B and T lymphocytes. However, expression was also detected in a subset of thymus medullary epithelium. The Kb molecules expressed on this thymic subset were unable to positively select T cells for passage to the periphery. The mice were, however, tolerant of the cell types expressing the foreign Kb molecules and were also tolerant of Kb presented as skin grafts. These results suggest that not all components of thymic epithelium are involved in positive selection of T cells and that transplantation antigens expressed on non-dendritic cells can induce tolerance.

Thymic nurse cells: morphological study during their isolation from murine thymus

Thymic nurse cells (TNC), which are multicellular complexes composed of epithelial cells and thymocytes, were obtained from C3H-mice thymuses. They were described by means of light and electron microscopy. The morphology of epithelial cells forming isolated TNC compared to that of small tissue fragments obtained by enzymatic digestion revealed that TNC could be derived from all parts of the thymus: cortex, corticomedullary junction and medulla, the cortex being their principal source. This variety of origin, the presence of several epithelial cells inside a single TNC, the presence of non-lymphoid cells, and the various locations of cleaved desmosomes confirmed that their aspect "in vitro" as round and sealed structures can be considered to be an artifact due to the isolation technique used. Indeed, during this procedure, they are formed by a process of wrapping of the epithelial cytoplasm around the tightly associated thymocytes. All three epithelial cell types: cortical reticular cells, medullary reticular cells, and medullary globular cells can form TNC.

Nurse cells of the bursa of Fabricius: do they exist?

Cell-cell interactions in B lymphocyte development have so far been incompletely characterized, mostly due to lack of a special organ for B cell maturation in the mammalian species. Certain well-known lymphostromal interactions in the thymus have raised the question whether similar interactions with nurse cells would also operate in the development of B cells. We have tested this hypothesis in the chicken bursa of Fabricius, an organ specific for the B cell maturation. To identify possible nurse cells, with viable lymphocytes enclosed, the cells in the bursa of Fabricius were dispersed with collagenase and trypsin. Light and electron microscopic examination of bursa cell suspensions showed four types of aggregates, identified by low magnification light microscopy as potential nurse cell-like complexes. Electron microscopy revealed that all aggregates consisted of epithelial cells, and complexes of epithelial cells with lymphocytes enclosed were not observed. These findings indicate that interactions similar to those seen in the avian and mammalian thymus between epithelial nurse cells and T lymphocytes are not a part of the avian B cell differentiation process.

Thymic nurse cells: division of thymocytes within complexes

Thymic nurse cell complexes (TNC-c), isolated from mouse thymuses at 1 and 2 h after i.v. injection of 6-(3H)thymidine, were analyzed in autoradiographs of semithin serial sections with regard to their size and the distribution of labeled thymocytes in individual types of complexes. The total number of thymocytes per complex reflects the type of complex. In a parallel study, localization of labeled thymocytes within individual zones of thymic cortex was examined. Thymocyte division within complexes may yield sequential complex generations differing in number per complex. However, thymocytes within complexes differ from each other in division kinetics. Half of the thymocytes that had been labeled 1 h after injection divided within 2 h. The rapidly dividing fraction of thymocytes were distributed within small complexes containing 2-8 cells and corresponded to the distribution of labeled cells in the outer thymic cortex. The proportion of labeled cells within large complexes resembled the distribution of labeled cells in the deep cortex. The data support the view that microenvironmental factors within TNC-c are responsible for both inducing thymocytes to enter the cell cycle and the negative selection (cell death) of some thymocytes.

Stromal cells of hemopoietic origin

Hemopoiesis is a multistep process involving stem cell renewal, commitment, differentiation, maturation and consequent positioning of the cells within the tissue. Stromal cells are a major component of the hemopoietic microenvironment. The in vitro culture of cloned stromal cells has enabled detailed analysis of their functions and has provided answers relating to the contribution of stromal cells to the control of hemopoiesis. Cultured stromal cells were found to support the renewal of stem cells through a mechanism that did not seem to involve already known cytokines. Cloned stromal cells from both marrow and thymus supported the in vitro accumulation of myeloid as well as T and B lymphoid cells. Thus, cloned stromal cells had the ability to induce multilineage hemopoiesis, irrespective of the organ from which they were derived. Invariably, stromal cells tended to select in culture for hemopoietic cells at early differentiation stages and restricted the accumulation of mature cells. These functions may be part of the mechanism that protects the stem cell pool from excess differentiation.

Kinetic analysis of thymocyte attachment to thymus stromal cells in culture by using phase-contrast and scanning electron microscopy

Direct cellular contact between thymocytes and thymus stromal cells within the thymus appears to contribute to the maturation of thymocytes. Thymocyte-stromal cell complexes, formed in vivo, have been isolated by others and postulated to play a role in T-cell differentiation. These previous studies have been hampered, however, by a time-consuming isolation procedure from which only small numbers of these complexes are recovered. We have examined a model to study thymocyte-stromal cell complexes in vitro in which thymocytes are added to primary cultures of thymus stromal cells. In the present study, we found that thymocytes were histotypically selective in their attachment to thymus stromal cells. We also investigated the kinetics of thymocyte attachment to these thymus stromal cells. Cultures were examined at selected time intervals from 5 min through 3 days of incubation. Thymocyte attachment to stromal cells was a biphasic interaction, with maximum surface attachment at 15 min of cocultivation, followed by migration of thymocytes into the cultures. Morphological studies were confirmed by using 3H-leucine-labeled thymocytes and liquid scintigraphy. With increased time in culture, thymocytes became amoeboid and migrated between the layers of stromal cells where thymocyte mitotic figures were seen at 4 and 8 hr. In some cases it appeared that stromal cells, which often grew two to three cell layers deep, played an active role in enclosing thymocytes within the cultures. Large numbers of viable thymocytes were observed in the cultures at 24 hr. The number of thymocytes then decreased progressively on days 2 and 3, when relatively few were found within the layers of the culture.

Nature of the thymocytes associated with dendritic cells and macrophages in thymic rosettes

Thymic rosettes, structures consisting of 3-30 thymic lymphoid cells attached to a central macrophage or dendritic cell, were released from mouse thymus tissue by collagenase digestion. They were shown to be preexistent structures within the thymus, but to be subject to extensive exchange with free thymocytes under certain conditions. An isolation procedure was developed, using a new technique of zonal unit-gravity elutriation, which minimized exchange and produced a completely pure sample of the larger rosettes. The rosette-associated thymocytes were analyzed by two- and three-color immunofluorescent staining and flow cytometry. The dominant cell type was a small, CD4+CD8+, cortical-type thymocyte. However, all of the established thymus subpopulations defined by CD4 and CD8, including CD4-CD8+ and CD4+CD8- mature thymocytes and CD4-CD8- early thymocytes, were also present in rosettes. Very few of the cells present were of an intermediate or transitional phenotype. Rosette-associated thymocytes were somewhat enriched in large dividing thymocytes, in CD4-CD8- thymocytes, and in mature thymocytes expressing the T-cell antigen receptor-CD3 complex. Their most striking characteristic was a marked depletion in small thymocytes lacking surface H-2K expression, a major population among free thymocytes. The physiological role of the rosette structure is discussed, and it is suggested that the heterogeneity of the associated thymocytes in part reflects the existence of different types of rosettes in different areas of the thymus.

Normal thymic cortical epithelial cells developmentally regulate the expression of a B-lineage transformation-associated antigen

Nonlymphoid, stromal cells in the mouse thymus are believed to be important in T cell maturation and have been proposed to play a central role in the acquisition of major histocompatibility complex (MHC) restriction and self-tolerance by maturing thymocytes. Both cortical and medullary epithelial cells in the thymus express high levels of class II (A) major histocompatibility antigens (MHC Ags). We show here that a specific subset of these A+ epithelial cells express a transformation-associated antigen (6C3Ag) found previously on the surfaces of Abelson murine leukemia virus-transformed pre-B cells and on those bone marrow-derived stromal cell clones which support normal and preneoplastic pre-B cell proliferation. Among solid lymphoid organs, only the thymus contains 6C3Ag+ cells and within the thymus, this antigen is found exclusively on A+ epithelial cells in cortical regions. It is striking that the expression of the 6C3Ag on thymic epithelium is developmentally regulated, suggesting a role for this lymphostromal antigen in the maturation of the thymic microenvironment.

In vitro behavior of thymic nurse cell-like complexes from mechanically and enzymatically dissociated frog tadpole thymuses

Cellular complexes, analogous by virtue of their external appearance, size, and number of seemingly internalized thymocytes to thymic nurse cells (TNCs) of endothermic vertebrates, were seen in short-term cultures (6-8 days) of mechanically and enzymatically dissociated thymuses of leopard frog tadpoles. Most TNC-like complexes from mechanically disrupted thymuses were covered with many thymocytes that morphologically resembled the "internalized" thymocytes. With time in culture, most complexes remained spherical and lost their externally adherent and "internalized" thymocytes. Some complexes, however, adhered to the glass substratum by means of macrophage-like cells. After one typically appearing TNC from a mechanically dissociated thymus had released its "internalized" thymocytes and spread completely over the glass substratum, it could be seen to consist actually of 9-10 stromal cells with the appearance of epithelial cells, macrophages, and dendritic cells. TNC-like structures from enzymatically dissociated thymuses had few, if any, attached thymocytes. Although these structures closely resembled murine TNCs initially, they displayed abnormal transformations within a few days of culture. Our observations led us to question the assumption that all TNCs from mechanically as well as enzymatically isolated TNCs from vertebrate thymuses are single cells. Rather, some if not all of the so-called TNC may actually be entities composed of several stromal cell types that enclose thymocytes. We suggest that this configuration seen in vitro may reflect the architecture of the compartmentalized reticular stromal cell meshwork that characterizes the intact thymus.

A chemotactic factor for rat thymocytes may regulate T-lymphocyte migration toward the thymic microenvironment

Using a modified Boyden chamber assay, extracts or culture supernatants of rat thymic stromal cells, or thymocytes were examined by chemotactic activity to rat leukocytes. Rat thymocytes responded chemotactically to the aqueous extract as well as to culture supernatants of thymic stromal cells. However, neither the extract and culture medium from concanavalin A-stimulated thymocytes nor any component of rat serum has shown such an activity. The thymic extract was fractionated into three molecular species with chemotactic activity for thymocytes. The thymocyte chemotactic factor(s) (TCFs) in the extract was distinct from known lymphocytic chemotactic factors, such as interleukin-1 (IL-1), IL-2, C5a, and the culture supernatant of stimulated thymocytes. In vitro, TCFs could attract, in addition to thymocytes, bone marrow cells, fetal liver cells, and nylon-wool nonadherent lymphocytes from peripheral blood and spleen. Lymph node cells, neutrophils, macrophages, and B cells from peripheral blood could not respond to TCFs. Thymocytes also responded to the extract of splenic stromal cells. Unlike the thymic extract, however, the splenic extract was chemotactically active for lymphocytes from lymph nodes but not for bone marrow cells. These results indicate that thymic stromal cells secrete a chemotactic factor(s) for a relatively immature type of T-lineage cells, which may by a thymus-homing progenitor T cell, while spleen may contain an attractant for a relatively mature type of T-lineage cells.

A light microscopical study of isolated follicular dendritic cell-clusters in human tonsils

In order to re-examine the cellular structure of isolated FDC-culsters in the germinal centers of human tonsils, a light microscopical analysis was made. Approximately 14 FDC-clusters were recovered from one enucleated germinal center using the enzyme digestion technique. The minimum unit of the FDC-clusters was composed of one FDC and 8 to 9 lymphocytes. Most of the FDC-clusters were representative of the microenvironment of the light zone at the germinal center in situ. Half of the engulfed centrocytes were supposed to be at the Go phase, and the others at the G1 to G2 phase. It is suspected that the helper-T cell has some relationship to the FDC microenvironment, and that the suppressor-T cell does not. Most of the CIgG-containing cells in the germinal centers were considered to have infiltrated into the interspace of the FDC microenvironment.

In situ characterization in freeze-fractured mouse thymuses of lymphoepithelial complexes ultrastructurally similar to isolated thymic nurse cells

Scanning and transmission electron microscopy of the cracked surfaces of cryofractured pre-fixed C57BL/Ka mouse thymus reveals the existence of cell complexes, distinct from the surrounding cell organization, in which groups of lymphocytes are delimited by large cytoplasmic sheets or envelopes. These complexes, located in the subcapsular and cortical regions, display morphological features similar to that of the thymic nurse cells (TNCs), which can be isolated from the mouse or human thymus enzymatically dissociated. They can be considered as dynamic systems able to modify their three-dimensional organization, namely with regard to intrathymic cellular traffic involved in T-lymphocyte maturation.

Zonal unit-gravity elutriation. A new technique for separating large cells and multicellular complexes from cell suspensions

A new and simple technique, zonal unit-gravity elutriation, has been devised for separating very large cells, multicellular complexes, or small organisms from suspensions consisting mainly of small cells. The separation vessel is a conical chamber with an entrance at the lower, narrower part of the cone and an exit at the upper, wider part of the cone via a dome-shaped lid. A baffle at the entrance prevents turbulence from incoming fluid. Chambers of differing widths and wall slopes are chosen depending on the sedimentation rate of the particles to be separated. A small volume of the cell suspension is placed in the chamber on the bench in a cold-room. Medium stabilized by a shallow density gradient is pumped into the base of the chamber and ascends, creating a decreasing velocity gradient. Cells sediment at unit-gravity against this ascending counterstream, and are separated into bands according to sedimentation velocity. By adjusting the flow rate of the medium, different sizes of cells can be separated. Tumor cells can be enriched, and larger blast cells can be separated from small cells in lymphoid cell suspensions. The procedure produces complete separation of thymic nurse cells (epithelial-lymphoid complexes) from free thymocytes in digested thymus suspensions and produces substantial enrichment of thymic rosettes (macrophage-lymphoid complexes). A very favorable situation for applying this technique is the isolation of Taenia taeniaformis larvae, which can be completely purified from infected liver suspensions, representing a 4 X 10(5)-fold enrichment of the parasites, with high recovery, in a single 30 min operation.

The limited immunocompetence of thymocytes within murine thymic nurse cells

Thymic nurse cells, cortical epithelial cells enclosing 20-200 lymphocytes, were prepared from mouse thymus by enzyme digestion and repetitive sedimentation. Individual nurse cells were then isolated free of any exogenous thymocytes by micromanipulation, and the endogeneous thymocytes released from inside the nurse cells by a brief period of culture. The thymocytes from within individual nurse cells were tested, at the one cell/well level, for their capacity to proliferate in high cloning efficiency mitogen-stimulated limiting dilution cultures. The resultant clones were tested for their cytolytic capacity in a lectin-mediated isotype-release assay. Most intra-nurse cell thymocytes were unresponsive, like typical cortical thymocytes, but an average of 1/30, or around 2-6 lymphocytes/nurse cell, were able to proliferate in response to concanavalin A. The clones produced were of a relatively small size, similar to those characteristic of helper-lineage T cells. No cytolytic clones at all were obtained, despite stringent positive controls showing an efficient cytolytic response from known sources of cytolytic precursor cells. This finding disagrees with earlier studies on nurse cell lymphocytes, where there may have been a possibility of contamination with exogenous thymocytes. These results suggest either that the nurse cell represents a selective environment for helper-lineage T cell differentiation, or that further steps after the nurse cell stage are needed to produce mature cytolytic-lineage T cells.

Thymic non-lymphoid cells

In formulating this summary of our simon-pure knowledge of the structure/function relationships in the thymus, we decided that the time may have come to introduce a suitable dose of cynicism to balance the sometimes hopeless optimism of the past. Are the non-lymphoid cells of the thymus necessary for thymic function? Probably, but not to the extent or uniqueness that some authors including ourselves have previously claimed; T cells can probably differentiate in other tissues but may acquire their preference for MHC class II in the thymus. Mouse thymic lymphoid cell traffic and surface phenotype has recently been summarized pictorally by Scollay and Shortman [95]. Briefly stated, within the thymus, cells are hatched, matched and then dispatched. Minimally, the non-lymphoid cells act either as scenically varied obstacles along the way, nurseries for newborn T cells, or as tombstones for life's disenfranchized, effete and autoaggressive thymocytes. Hassall's corpuscles are morphological structures unique to the thymus, which are most useful to medical students for identification of this tissue. Their function remains one of life's great mysteries. Morphologically, they are suitable companions to the more recently described strange multicellular complexes of lymphocytes and epithelial cells which might be functionally important. The thymus of the much studied inbred, environmentally mollycoddled, laboratory mouse has been often and majestically described. It is probably typical for that of man and most mammals. It may, however, be unrepresentative of the thymus of stressed and parasitized wild animals. Diseases of the thymus generally can be categorized as not having enough thymus, having a neoplastic thymus or having a thymus which does not work properly. The bottom line in our knowledge of thymic nonlymphoid cells is that if you are born without them, you get sick and die; unless, of course, you are a nude mouse in Omaha, in which case you just freeze to death.