Functional maturation of thymic lymphocyte populations in vitro

Thymus-deriving natural regulatory T cell generation in vitro: role of the source of activation signals

In this research we have examined different sources of activation signals in order to optimize culture conditions for in vitro generation of thymus-deriving natural regulatory T cells (nTregs). We have established a novel model using JAWS II dendritic cell line of immature phenotype and compared it to commonly used methods for the generation of Tregs from peripheral lymphoid organs or blood T cells. In our model the first activation signal is provided by anti-CD3 monoclonal antibodies while the second is delivered by costimulatory molecules expressed on JAWS II cells. The presence of JAWS II cells co-cultured in vitro with unsorted thymocytes directly isolated from the thymus gland creates environment favoring SP CD4+ differentiation, provides the apoptotic cells clearance, maintains the survival of thymocytes and facilitate nTreg generation. Moreover the usage of immature dendritic cells stimuli enables to conduct research on agents affecting nTreg survival, proliferation and development in conditions of cell-to-cell contact of undifferentiated thymocytes with dendritic cells.

Biological expressions of lymphocyte activation : I. Effects of phytomitogens on antibody synthesis in vitro

The effects of nonspecific phytomitogens on primary plaque-forming cell (PFC) responses of mouse spleen cells to heterologous erythrocytes in vitro were studied. Spleen cell cultures treated with concanavalin A or phytohemagglutinin in vitro or established with spleen cells derived from mice injected with concanavalin A 24 h previously were similarly affected. In both cases, submitogenic doses resulted in substantial enhancement of PFC responses, whereas 10-fold larger doses were profoundly inhibitory. In contrast to the suppressive effects of mitogenic doses of phytomitogens added at culture initiation, addition of these same doses to cultures 48 h later resulted in increased PFC responses. This enhancement could be observed within 1 h after treatment and consequently could not be ascribed only to mitotic expansion of the antibody-synthesizing clone. Activation of spleen cells with specific antigen before mitogen treatment was not required for expression of the enhancing or suppressing effects on PFC responses. IgM and IgG PFC responses were similarly affected. Studies of cell interactions revealed that as few as 10⁵ spleen cells obtained from mice treated with concanavalin A in vivo synergistically enhanced the PFC responses of 10⁷ normal spleen cells. This enhancement was mediated by mitogen-activated T lymphocytes which were resistant to 2000 R irradiation 24 h after activation. The relevance of these observations to emerging concepts of helper and suppressor T cell activity is discussed.

Asynchronism of Thymocyte Development In Vivo and In Vitro

Although fetal thymus organ culture (FTOC) has become widely used to investigate T-cell development, the differences between thymocyte development in vivo and in vitro (in FTOC) remain largely unknown. In this study, the viability and numbers of thymocytes recovered from embryonic thymus lobes in different gestation days (gd) mice or from 15 day embryonic thymus lobes cultured for different days in FTOC system were evaluated. The expression of CD3, CD4, CD8, CD95 ligand (CD95L), and CD69 on thymocytes were analyzed by FACS. The results showed that thymocytes, either in vivo or in vitro, could differentiate from double negative (DN) cells to double positive (DP) cells and to single positive (SP) cells. But the number of total thymocytes and the percentage of DP cells in vitro were less than that in vivo, and the expression of CD95L and CD69 on thymocytes in vitro was higher than that in vivo. Our results suggested that although thymocyte development in vitro could recapitulate thymic development in vivo, the proliferation of thymocytes in vitro was less intensive than that in vivo; the differentiation of thymocytes in vitro was delayed compared with that in vivo; and the apoptosis and activation of thymocytes in vitro were higher than that in vivo. In conclusion, FTOC is a useful system for the study of T cell differentiation, but it is necessary to interpret the results from in vitro studies carefully since the thymocyte development in vitro is asynchronous from that in vivo.

Contact-mediated maturational effects of thymic stromal cells on murine thymocytes in culture

The effect of direct contact between thymic stromal cells and thymocytes on differentiation markers and functions of the latter was studied. The thymic stromal cells included two epithelial and one fibroblast cell lines, previously described. Murine thymocytes were incubated with confluent monolayers of these cells or their supernatants for 24 hr, using monolayers of non-thymic cells and their supernatants as controls. Then, the thymocytes were tested for changes in expression of several surface antigens [Thy-1, Lyt-1, Lyt-2, L3T4, IL-2-receptor (IL-2R)], spontaneous [3H]thymidine incorporation (STI), lectin-induced proliferative response (PR) and lymphokine (IL-2 and IL-3) production. All three thymic stromal cell lines reduced the expression of Thy-1, Lyt-1 and Lyt-2 significantly. The expression of L3T4 was also reduced in some of the experiments, while IL-2R was not expressed by the thymocytes, neither before nor after the co-culture. The thymic stromal cell lines also increased the spontaneous [3H]thymidine incorporation and lymphokine production by the thymocytes and inhibited their proliferative response to lectins. Under the same experimental conditions, the culture supernatants of the thymic stromal cells and the non-thymic cells did not have any effect on the thymocytes, either when collected and used separately or when used in a co-culture system which allowed thymocyte contact with the medium but not with the stromal cells (Transwell system). These results suggest a specific effect of thymic stromal cells, epithelial as well as fibroblasts, on thymocyte maturation. The effect is mediated by direct cell contact and not by secreted factors.

In vitro effect of a thymic epithelial culture supernate or thymosin fraction 5 on rabbit platelet aggregation and intracellular cyclic AMP levels

Supernates of thymic epithelial cell culture (STEC) strongly inhibit aggregation induced by addition of adenosine diphosphate (ADP: 1 microM) or thrombin (0.5 unit per ml) to washed platelet suspensions and accelerated the restoration from ADP-triggered aggregation. At the same time, STEC increased the level of platelet adenosine 3',5'-cyclic monophosphate (cyclic AMP) in a dose-dependent manner. Depending on the concentration used, thymosin fraction 5 increased the level of intracellular cyclic AMP ranging between 5 and 100 micrograms per ml, as well as inhibiting ADP-induced platelet aggregation. The activities of both STEC and thymosin fraction 5 were found to act exclusively on cyclic AMP phosphodiesterase activity in platelets. In contrast the supernates from Chang, HeLa, or HCC-M cells did not affect platelet aggregation induced by ADP, but slightly increased the cyclic AMP level (Chang, HeLa). Within 2 min after the treatment with STEC, more than 50% of the maximum inhibitory activity on platelet aggregation and increases in intracellular cyclic AMP were observed. These activities disappeared following STEC treatment with pronase E. STEC activity was found predominantly in the 1,000-50,000-dalton fractions. These activities were not altered when STEC was treated by adenosine deaminase. The level of prostaglandin E (PGE) derivatives in STEC was about two times that found in the control culture medium. These data suggest that the biological activity of STEC in the platelets might be attributed to thymosinlike polypeptides and PGE1.

Histochemical localization of adenosine triphosphatase activity in thymus: a light microscopical and ultrastructural study

Ultrastructural localization of ATPase at high pH in the presence of Ca2+ showed that activity in thymocyte precursors was stronger than in mature thymocytes. The activity was localized in the nuclear envelope, rough endoplasmic reticulum, Golgi apparatus and mitochondria. The difference in activity was attributed to a marked decrease in ATPase-containing organelles, mainly the endoplasmic reticulum in the mature thymocytes. This appears to be related to the proliferative activity of the cells rather than to the immunological maturity of the thymocytes. A very strong activity, also localized in the same organelles, was present in the macrophages and interdigitating cells which might have a secretory function and possibly contribute to thymocyte maturation. The Ca2+-ATPase activity in the nuclear envelope-endoplasmic reticulum system suggests that these may be the sites for storage and regulation of cytoplasmic calcium.

Indomethacin-induced sialic acid-mediated changes in surface markers from "cortical type" to "medullary type" in murine thymoma line EL-4

The effect of indomethacin (INDO), a specific inhibitor of prostaglandin (PG) synthesis, on the expression of markers during thymocyte differentiation such as Thy-1, H-2K, and peanut agglutinin (PNA) receptor was examined by immunofluorescence, using poorly differentiated thymoma, EL-4, as the indicator cells. EL-4 cells grown in the culture medium in the presence of INDO exhibited lower levels of Thy-1 and PNA, and a higher level of H-2K, compared with the EL-4 cells grown in medium without INDO. The decrease in PNA level by INDO was attributed to an increased density of sialic acid bound to PNA receptors on the cell surface, because treatment with neuraminidase (Nase) released more sialic acid from such cells, as compared with control cells, and markedly increased the detectable amounts of PNA receptors. On the other hand, a decrease of Thy-1 or an increase of H-2K may be ascribed to the decrease of Nase-resistant sialic acid on the cell surface, determined by analyses with FITC-LPA, sialic acid-specific lectin, and by metabolic labeling of surface sialic acid. These results suggest that the PG-system modulates the metabolism of sialic acid located on the thymocyte surface and alters the expression of surface markers of thymocytes.

A scanning electron-microscopic study of the rat thymus with special reference to cell types and migration of lymphocytes into the general circulation

The three-dimensional structure of the rat thymus was studied by combined scanning- and transmission electron microscopy. The thymus consists mainly of four types of cells: epithelial cells, lymphocytes, macrophages, and interdigitating cells (IDCs). The epithelial cells form a meshwork in the thymus parenchyma. Cortical epithelial cells are stellate in shape, while the medullary cells comprise two types: stellate and large vacuolated elements. A continuous single layer of epithelial cells separates the parenchyma from connective tissue formations of the capsule, septa and vessels. Surrounding the blood vessels, this epithelial sheath is continuous in the cortex, while it is partly interrupted in the medulla, suggesting that the blood-thymus barrier might function more completely in the cortex. Cortical lymphocytes are round and vary in size, whereas medullary lymphocytes are mainly small, although they vary considerably in surface morphology. Two types of large wandering cells, macrophages and IDCs, could be distinguished, as well as intermediate forms. IDCs sometimes embraced or contacted lymphocytes, suggesting their role in the differentiation of the latter cells. Perivascular channels were present around venules and some arterioles in the cortico-medullary region and in the medulla. A few lymphatic vessels were present in extended perivascular spaces. The present study suggests the possible existence of two routes of passage of lymphocytes into the general circulation. One is via the lymphatics, while the other is through the postcapillary venules into the blood circulation. Our SEM images give evidence that lymphocytes use an intracellular route, i.e., the endothelium of venules.

In vitro growth and phenotypic characterization of mesodermal-derived and epithelial components of normal and abnormal human thymus

Long-term in vitro cultures of human thymic tissue were established and phenotypically characterized using monoclonal reagents that define distinct components of the human thymic microenvironment. The epithelial component of the thymus, defined by monoclonal antibodies TE-3, TE-4, BBTECS, and AE1 (anti-keratin) was isolated from the mesodermal component, defined by antibody TE-7, and maintained separately in long-term culture. The epithelial cells were subcultured repeatedly and recovered from storage in liquid nitrogen. The in vitro phenotype of the cultured cells was compared to that of cultured human epidermal cells. A subpopulation of cultured thymic epithelial cells along with a subpopulation of cultured epidermal cells expressed antigens (TE-8, TE-15) characteristic of late stages of keratinized epithelial cell differentiation. Thus, we have established a system whereby components of the human thymic microenvironment can be cultivated in vitro while maintaining the capacity to differentiate. This approach can be used to evaluate the role of components of the thymic microenvironment at various stages of differentiation on developing T lymphocytes. In addition, keratin-containing thymic epithelial cells were successfully cultured from thymuses obtained from patients with myasthenia gravis and thymoma. Cultivation of abnormal thymic epithelium will provide insight into aberrant T lymphocyte-thymic epithelial interaction.

Thymus-derived macrophages in long-term culture: release of IL-1, stimulation of MLR and expression of tumoricidal activity

The present report examines the behaviour of a slowly proliferating pure population of thymus-derived macrophages in long-term culture, regarding their ability to secrete interleukin-1 (IL-1) and prostaglandin E2 (PGE2), to stimulate a mixed leucocyte reaction (MLR) and to lyse tumour cells in vitro. Following stimulation with LPS, high levels of IL-1 were released to the medium. IL-1 release was significantly augmented by the addition of indomethacin during stimulation. Thymus-derived macrophages constitutively secreted significant levels of PGE2. These cells served as excellent stimulators in a one-way MLR, substantiating the claim that pure populations of macrophages effectively stimulate an allogeneic response in vitro. Thymus-derived macrophages showed tumoricidal activity following activation with either high concentrations of LPS or suboptimal concentrations of LPS and T-cell lymphokine. These findings portray a close interrelationship and reciprocal regulation between thymus-derived macrophages and T lymphocytes.

Nonimmune lymphocyte-macrophage interaction. I. Quantification by an automated colorimetric assay

Previous studies have demonstrated a spontaneous, nonimmune interaction between lymphocytes and macrophages. This paper describes an automated colorimetric assay based on the dye, rose bengal, to quantify this interaction. The procedure entails allowing lymphocytes to adhere to preformed macrophage monolayers in the wells of microplates and then staining bound lymphocytes with rose bengal. Dye uptake and the consequent number of lymphocytes bound were quantified using an automated spectrophotometer developed for reading microplates. This procedure was used to confirm and extend the basic parameters of the system. The interaction was found to be temperature dependent but the kinetics and percentage of cells binding varied with the source of lymphocytes. However, all lymphocyte populations tested, namely, mature and immature thymocytes, T and B lymphocytes, and a range of thymoma cell lines, bound to macrophages. Furthermore, all macrophage populations examined had the ability to bind lymphocytes. The interaction also showed no strain specificity and generally lacked species specificity. It is proposed that the interaction is a highly dynamic process that enables lymphocytes to scan the surface of macrophages for self and/or foreign antigens.

Characterization of human thymic epithelial cells grown in serum-free medium

Thymic epithelial cells have a critical influence on T-cell differentiation. In order to characterize these cells in humans, a serum-free growth medium was developed for their long-term culture. Important components of this medium included transferrin, epidermal growth factor, prostaglandin E1, and selenious acid. The presence of a keratin cytoskeleton, tonofilaments, and desmosomes confirmed the epithelial nature of these cells. Indirect immunofluorescence study of these epithelial cells demonstrated the presence of Ia and B-2 microglobulin antigens. The availability of highly enriched thymic epithelial cultures should simplify the functional characterization of this cell.

Induction of T-cell maturation by a cloned line of thymic epithelium (TEPI)

A cloned cell line of thymic origin has been characterized as epithelial in nature. A description of the procedures for derivation and cloning of the cell line includes use of epidermal growth factor. The thymic epithelial (TEPI) cell line is Ia antigen positive, forms desmosomes, and produces an extracellular fibronectin matrix. The supernatant from confluent monolayers of TEPI was tested for its ability to promote thymocyte functional activity. TEPI supernatant (TEPI SN) was demonstrated to greatly enhance the response of peanut agglutinin-positive thymocytes to alloantigen, as measured by cell-mediated lympholysis. Furthermore, preincubation of peanut agglutinin-positive thymocytes with TEPI SN prior to allostimulation resulted in marked enhancement, thus distinguishing it from interleukin 2. Finally, TEPI SN was demonstrated to induce interleukin 2 production by peanut agglutinin-positive thymocytes in the presence of concanavalin A. This activity was demonstrated not to be due to interleukin 1, which is absent in TEPI SN. Preliminary biochemical analysis indicates that the biological activity is associated with a Mr 50,000 entity. The data suggest that TEPI produces a soluble factor capable of inducing function of an immature thymocyte subpopulation into an IL 2 producer.

Characterization of the population of phagocytic cells in thymic cell suspensions. A morphological and cytochemical study

Rat thymic phagocytic cells were characterized in vitro using various light- and electron-microscopical techniques. Thymic cell suspensions were mechanically prepared and enriched for non-lymphoid cells, which were predominantly phagocytic and of three types. Type I showed acid phosphatase (APh) activity in small granules dispersed throughout the cytoplasm and were mostly Ia antigen-positive, although the Ia membrane label varied in intensity and distribution among individual cells. Only a few cells had endogenous peroxidase activity. The type-I cells could not be clearly distinguished morphologically from type-II or -III cells, and most likely comprise precursors of both these cell types. Type-II were large pale cells with many slender cell processes. These cells had APh activity centrally positioned, were strongly positive for Ia on the cell membrane and were negative for endogenous peroxidase. The cytoplasm frequently contained Birbeck granules, which unequivocally classifies these cells as the in vitro equivalent of the interdigitating cells present in the medullary area of the thymus in situ. Type-III cells were rounded with a smooth or ruffled cell membrane and contained vacuoles and many phagolysosomes. They were strongly positive for APh which was present throughout the cytoplasm. About 50% of these cells were positive for endogenous peroxidase in a pattern resembling resident macrophages. The cells were negative for Ia antigens. Type-III cells mostly likely represent the macrophages found in the cortical area of the thymus.

Functional activity of a transformed thymic epithelial cell line

In an attempt to obtain pure populations of nonlymphoid thymic cells, monolayers of thymic cells enriched for low-density cells were transformed with SV40. This paper describes the characteristics of the initial cell lines which resulted from this approach. These cell lines appeared to be of epithelial origin by morphologic criteria (extensive surface microvilli and pronounced intercellular tight functions) and by biochemical analysis, since they possessed mouse keratin proteins. They did not possess I-region-associated antigens on their surface, markers that have been detected on normal thymic epithelium. Functionally, these transformed cell lines secreted a factor capable of inducing thymocytes to mature to functional cytotoxic effector cells directed against allogeneic stimulators. This factor did not appear to be interleukin 1, interleukin 2, or colony-stimulating factor; did not result in thymocyte proliferation; and did not affect peripheral T cells. The cell lines described in this report should prove useful in delineating the steps by which immature T cells acquire their immunocompetence in the thymus.

Immunological properties of thymus cell subpopulations: rat thymic dendritic cells are potent accessory cells and stimulators in a mixed leukocyte culture

Rat thymus cells were fractionated by centrifugation on a discontinuous bovine serum albumin gradient into two subpopulations: one of high density that accounted for greater than 90% of the recovered cells, and a minor low-density subpopulation containing 4 to 10% of the total cells. The high-density subpopulation consisted mainly of uniform small-sized thymocytes, whereas the low-density subpopulation contained mostly larger-sized cells. High-density thymus cells did not function either as stimulators in a mixed leukocyte reaction or as accessory cells required for T-cell response to mitogens, Con A and sodium periodate, as determined by 3H-thymidine incorporation. Dense thymus cells also responded poorly to allogeneic and to mitogenic stimulation, even when accessory cells were added. In contrast, the low-density thymus cells responded well to allogeneic stimulation and to both mitogens. In addition, low-density thymus cells possessed stimulatory activity in mixed leukocyte cultures, as well as accessory activity for mitogenic responses. Both activities were found to reside in dendritic cells that were purified extensively (70-90% of the preparation) with good yield. When tested as accessory cells for T-cell responses to periodate, thymic dendritic cells were as potent as lymph node dendritic cells on a per cell basis. A small number of thymic dendritic cells was able to cause marked enhancement in T-lymphocyte proliferation in response to stimulation. By immunofluorescence thymic dendritic cells were shown to be Ia-positive, but Thy 1.1 -negative.

Ultrastructure of the rat thymus: the micro-environment of T-lymphocyte maturation

The ultrastructure of the micro-environment of the fully functional rat thymus was studied. The thymus consists of two discrete compartments, viz., an epithelial and a mesenchymal compartment. Thymus fibroblasts/fibrocytes, mast cells and granulocytes, are restricted to the mesenchymal compartment. The thymocyte maturation process seems to occur in the epithelial compartment in a network of reticular epithelial cells. The cortex is finely meshed and filled with proliferating thymocytes and some scattered macrophages. Moreover, in the medulla vacuolated epithelial cells from part of a loosely meshed reticulum which is filled with thymocytes and interdigitating cells (IDCs). IDCs frequently contain Birbeck granules and appear to be phagocytic. Together with macrophages, they probably enter the thymus, predominantly in the cortico-medullary region, and cross the separating wall between the two compartments. Some functional aspects of the non-lymphoid cells and in particular the IDCs, which form the micro-environment of the thymus, are discussed with respect to T-cell development.

Evaluation of the morphological and functional characteristics of murine thymic non-lymphoid cells grown in vitro

During the past few years, several investigators have attempted to grow the epithelial component of the murine thymus as a monolayer in vitro in order to study its proposed physiological role in T-lymphocyte differentiation. However, using for the most part identical culture techniques, we have provided morphological, histochemical and functional evidence that the majority of murine thymic nonlymphoid cells grown in culture are epithelioid macrophages and that these macrophages and/or their soluble factors may be responsible for many functions previously attributed to thymic epithelium or "purified thymic hormones". This study emphasizes the absolute necessity and importance of extensive morphological and histochemical evaluation of cultured cells prior to their use in physiological studies. Furthermore, any conclusion regarding the function of thymic non-lymphoid cells in vitro must first take into account numerous species differences in the morphological and histochemical characteristics of these cells as noted both in our laboratory as well as by other investigators.

Analysis of cellular heterogeneity in mouse thymus cultures

Analysis of 5 to 6 d primary cultures of cells derived from murine thymus glands revealed a heterogeneous population of cells rather than "pure" reticuloepithelial cell cultures as was assumed previously by other investigators. The monolayer cultures consisted of at least three cell types: thymus epithelial cells, macrophagelike epithelioid cells, and fibroblasts. Surprisingly, about 50% of the cells had positive cytochemical staining reactions for acid phosphatase and nonspecific esterase. The sme cells phagocytized carbon particles, latex beads, and yeast. Furthermore, these cells could be removed from the initial cell suspension by phagocytosis of carbonyl iron, followed by magnetic separation, but once they had adhered to the substratum they were resistant to trypsin removal. All of these findings supported the conclusion that about 50% of the cells in the monolayers were macrophages. The other cells present were thymus epithelial cells and a small number of fibroblasts. Both of the latter types of cell were cytochemically negative, did not phagocytize particulate material, and were not removed by carbonyl iron treatment, but were removed by treating the monolayer with trypsin. The findings in this report indicated that epithelioid morphology alone was inadequate to identify correctly the cell types found in thymus cultures and that the use of such cultures as a model to study in vitro the maturation of certain immunological functions has been based on assumptions here shown to be incorrect.

Thymic microenvironment and cultures derived from mouse thymic explants. A morphological study

Cultures derived from thymus fragments of embryonic (18-19 day old), newborn or one month old C57BL mice have been characterized functionally l(phagocytic and nonspecific esterase activities) and morphologically by means of light, scanning (SEM) and transmission (TEM) electron microscopy. The observations show the heterogeneity of the cell populations composing the monolayers. After a few days incubation macrophages appear as the predominating cell type, while epithelial cells usually constitute no more than 30% of the cells. Experiments designed to determine the fate of lymphocytes adhering to the monolayers lead us to believe (on the basis of SEM morphometric analysis) that the survival of lymphocytes attached either to thymic macrophages or to epithelial cells is improved during the first days of coculture. This survival enhancement does not, however, appear to be a specific inductive effect since a similar survival increase is found when lymphocytes adhere to non-thymic cells. In contrast with the monolayer, the explant provides a three-dimensional culture system able to preserve intact thymic microenvironmental conditions since numerous lymphocytes are found even in five week old cultures which were not overlaid with thymocytes or spleen cells.

Round cells of the epidermis: clues from studies on neoplastic lymphocytes of cutaneous T cell lymphoma

Neoplastic cells of cutaneous T cell lymphoma (CTCL) appear to be of monoclonal origin and frequently are nonspecific helpers of normal B cell differentiation. A natural progression from epidermotropic (mycosis fungoides and Sézary syndrome) to nonepidermotropic, more widely disseminated T cell neoplasms generally occurs. Affinity of CTLC cells for the epidermis may result from their having membrane receptors for histocompatibility (Ia) antigens present in skin. Cultured human epidermal cells produce a thymopoietin-like molecule, an indication of a role for skin in T cell differentiation.

Thymic nurse cells. Lymphoepithelial cell complexes in murine thymuses: morphological and serological characterization

We describe a new cellular component of normal mouse thymuses, which is isolated by fractionated trypsin dissociation of minced thymus tissue followed by repeated unit gravity sedimentation. These cells are of unusually large size, with diameters of 30 mum and more. They represent cellular complexes of single large cells filled with high numbers of lymphoid cells. The majority of the engulfed lymphoid cells is not only fully intact, as judged by morphological criteria, but, moreover, includes a high proportion of mitotic figures. Electron microscopic investigations reveal the epithelial character of the large thymic nurse cells (TNC). The peripherally situated cytoplasmic tonofilament streams, and characteristic vacuoles filled with coarse, unidentified material, closely resemble cytoplasmic organelles found in the cortical reticuloepithelial cells described in situ. The internalized lymphocytes are located within caveolae lined by plasma membranes. These TNC caveolae are completely sequestered, and have lost any communication with the extracellular space, as demonstrated by the inability of an electrondense marker, cationized ferritin, to diffuse into the perilymphocytic clefts. The structural interactions between the membranes of the engulfed thymocytes with the surrounding TNC caveolar membranes were investigated both in ultrathin sections and in freeze-etch preparates. Two distinct contact types between both membranes were discerned: (a) complete, close contact along the entire lymphocyte circumference, and (b) more frequently, contact restricted to discrete, localized areas. Judging from their size and distribution, the localized contacts could correspond particle aggregates of freeze-etch preparates, which morphologically resemble certain stages of gap junction. Furthermore, we regularly found square arrays of particles of uniform size, which so far have been thought to be typical for cell membranes actively engaged in ion exchange. Tight junction-like particle arrays, which were present on TNC outer membranes, and probably represented disrupted contacts between adjacent TNC in the intact tissue, could not be found on caveolar or lymphocyte membranes. Finally, one of the most conspicuous specializations of the TNC caveolar membrane were membrane invaginations, which were arranged mainly in groups, and which probably reflect endo- or exocytotoxic events. We investigated the surface antigen phenotype of TNC by indirect immunofluorescence, with monoclonal antibodies against determinants of H-2- complex subregions as well as against lymphocyte differentiation markers. Semiquantification was reached with flow cytofluorimetry, followed by morphological control by fluorescence microscopy. The surface antigen formula of TNC is: Ig(-), Thy-l(-), H-2K(++), I-A (++), I-E/C(+), H-D(++), Ly-1(-), Ly-2(-), Qat-4(-), Qat-5(-), and peanut agglutinin (PNA)(-). Thymic macrophages, which were identified by double fluorescence, with rhodamine- coupled zymosan as a phagocytosis marker, were serologically identical with TNC. Free thymocytes, in contrast, had the following antigen formula: Ig(-), Thy-1(++), H-2K(+/-), I-A(-), I-E/C(-), H-2D(+/-), Ly-1(+/-), Ly-2(+), Qat- 4(-), Qat-5(-), and PNA(+). The unprecedented finding of high numbers of dividing thymocytes sojourning within thymic epithelial cells, and the particular specializations of the TNC caveolar membranes surrounding these engulfed thymocytes is the basis of a hypothesis that postulates that an intraepithelial differentiation cycle is one essential step in, intrathymic T lymphocyte generation.

Monocyte-induced potentiation of bovine fetal thymocyte mitogenic responses to concanavalin A

Peripheral blood monocytes significantly potentiated the mitogenic response of bovine fetal thymocytes to Concanavalin A as measured by incorporation of [3H] thymidine into cellular DNA. Mononuclear cells obtained from either normal or Mycobacterium bovis sensitized cattle were cultured with or without purified protein derivative (PPD) for 24 hours at which time bovine fetal thymocytes and concanavalin A were added. After 3 days of culture, both activated or non-activated monocytes significantly potentiated Con A-induced blastogenic responses. of monocytes from thymocyte cultures completely abrogated thymocyte responses to Concanavalin A.

Macrophage-lymphocyte interaction and genetic control of immune responsiveness

We have reviewed briefly some of the diverse functions of macrophages in the immune response. Clearly, this population of cells interact physically with lymphoid cells, are required for activation of T cells, and process various protein antigens. Finally, we have studied the immune response to insulin in order to unify these previous data in such a way to demonstrate the active role of macrophages in the regulation of the immune response. The function of the Ir gene in the guinea pigs appears to be an intramolecular selection of discrete regions within the antigen for recognition by the T cell. The data presented suggest that this function operates at the level of the macrophage.

Mitogen-induced interferon production by normal and steroid-resistant mouse thymocytes

We have investigated the proliferative response and interferon production in cultures of mouse thymocytes stimulated with two different mitogens, PHA (phytohemagglutinin), or Con A (concanavalin A). Normal thymocytes proliferated weakly and did not produce detectable interferon levels in response to both mitogens. Supplementing this cell population with macrophages or adding 2-mercaptoethanol to the culture medium strongly enhanced the proliferative response to both mitogens, but only in response to PHA marginal levels of interferon could be detected. When the steroid-resistant population was tested, both PHA and Con A induced strong proliferative responses; in this case significant interferon levels could be obtained after stimulation with PHA, but only borderline levels with Con A. Peripheral lymphocytes from the spleen responded identically to both mitogens with respect to interferon production as well as proliferation. The data suggest that distinct differentiation pathways may exist for T lymphocytes producing interferon in response to different mitogens.

Binding of bursal, thymic and splenic lymphocytes to macrophages

This study was carried out to examine the potential for chicken lymphocytes from the bursa of Fabricius, the thymus and the spleen to interact physically with glass-adherent peripheral blood monocytes (macrophages). Lymphocytes from all three sources were bound by macrophages. Under optimal conditions, over 90% of the macrophages had lymphocytes bound. Splenic lymphocytes from either bursectomized or normal chickens were capable of being bound by macrophages. The degree of macrophage—lymphocyte interaction varied with the duration of incubation and with the concentration of lymphocytes in culture. The number of lymphocytes bound to macrophages was reduced when binding was allowed to take place at 4°, or if macrophage monolayers were pre-treated with iodoacetate. Incubation of the macrophage cultures with cytochalasin B or pre-treatment of the macrophage monolayers with trypsin also reduced lymphocyte binding. Chicken immunoglobulin, fresh chicken serum, or antigen—antibody complexes had no apparent affect on macrophage—lymphocyte interaction. These data indicated the following: (1) the majority of macrophages derived from peripheral blood monocytes have a receptor for lymphocytes; (2) functionally immature thymic and bursal lymphocytes as well as splenic lymphocytes can be bound by macrophages; and (3) macrophage—lymphocyte interaction in the chicken has similar properties to that seen in mammals.

Establishment of an epithelial cell line from rat thymus

A cell line (IT-26R21), composed only of epithelial cells, was established from normal rat thymus. Thymuses were treated with both collagenase and trypsin. Four months after the initiation of cultures, epithelial cells in packed colonies formed a monolayer and no other cells were found in cultures. Thereafter, epithelial cells have been subcultured with trypsin and EDTA, and are currently at the 30th subculture. Based upon the fine structure of the thymus in vivo, IT-26R21 cells were identified as epithelial cells from the thymus, because of their mosaic-like arrangement, desmosomes and tonofilaments. Other features also supported their origin and identity.

Mouse thymus reticulo-epithelial (RE) cells in vitro: isolation cultivation and preliminary characterization

The reticulo-epithelial (RE) cells of the thymus are presumably playing a crucial role in the differentiation of the T lineage lymphoid cells, but how this happens is still a matter for speculation. This paper describes a method for rapid preparation of thymic RE cells with as little damage as possible, their culture, and the analysis of their membrane antigens and of other cytological properties. The cultured cells are pleiomorphic, but at least two types can be distinguished, one being round and very villous, the other one being flat and very cystic. Thymus RE cells have species specific surface antigens and large amounts of H-2 antigen. The possible presence of theta antigen is unclear. Most cells have no detectable Tla antigen. In vitro, they show some uptake of normal mouse serum immunoglobulins. RE cells show a surface migration of ligand-bound membrane antigen; such a capping is much slower than for lymphocytes, and is inhibited by 10 mM NaN3; The drug also causes the apparition of long microprojections (or retraction fibres) on the villous RE cell type, as is also caused by a slight fixation with formalin. Type C virus particles are found in RE cells from AKR mice as young as 1 day.

The differentiation of murine thymocytes in vivo and in vitro

The differentiation of lymphoid cells in the early foetal mouse thymus was studied in vivo and in organ culture. The lymphoid precursors found in the 13 and 14 day foetal thymus constituted about 50% of the total thymus cell population. These T precursors were large blast-like cells which already expressed the Thy-1 antigen but were mostly TL negative. Both in vivo and in vitro the blasts were rapidly replaced by a population of typical small lymphocytes which were strongly Thy-1 and TL positive but Ig negative. In organ cultures grown under optimal conditions, large bimodal increases in cell numbers occurred. An initial population of about 2 x 10(4) T precursors per thymus lobe gave rise to nearly 10(6) predominantly Thy-1 and TL positive small lymphocytes by the 6--8th day in vitro. After this time lymphocyte numbers decreased until about the 10th day when they again increased to form a second peak of small lymphocytes on the 12--13th day. These cells were also predominantly Thy-1 positive but the majority were now TL negative. No Ig positive B lymphocytes were detected either by immunofluorescence or by autoradiography using polyvalent anti-MIg sera and no plasma cells were detected by electronmicroscopy. At all times however, minor subpopulations of Thy-1 negative small cells were present. The production of small lymphocytes during the 1st week of culture was critically dependent on culture conditions and particularly on the batch of FCS used. The population developing during the 2nd week required less stringent conditions and was less dependent on FCS. The culture systems described should prove useful in the study of T-lymphocyte differentiation.

Suppression of antibody response by group A streptococcal pyrogenic exotoxin and characterization of the cells involved

The effect of purified streptococcal pyrogenic exotoxins (SPE) on the antibody response to sheep erythrocytes was studied in cultures of mouse spleen cells. Purified SPE types A, B, and C shared the ability to suppress the day 4 direct plaque-forming cell response when added to cultures. SPE A and C were most suppressive at concentrations of 0.1 to 1 ng per culture, while SPE B was active at 1 microgram per culture. Pretreatment of mice with SPE A, 3 h before removal of their spleens for culture, also produced suppression. Cell populations were separated from spleens of normal and toxin-treated mice and recombined in culture to test the cellular site of action of SPE immunosuppression. When nonadherent cells (lymphocytes) and adherent cells (macrophages) from control and SPE-treated mice were separated and recombined, the plaque-forming cell response depended on the source of lymphocytes. Macrophages from toxin-treated mice functioned normally in the presence of control lymphocytes. In a further experiment, toxin pretreatment failed to suppress the plaque-forming cell response of spleen cells that were T-cell depleted and reconstituted with control thymocytes. When the T lymphocytes were removed from toxin-treated spleen cell suspensions, the remaining cells were able to respond normally to antigen if normal helper T cells were provided. The results suggest that the suppressive activity of SPE on antibody production is mediated by altered activity of T lymphocytes.

Macrophage-induced thymic lymphocyte maturation

Guinea-pig peritoneal macrophages were found to influence the functional maturation of thymic lymphocytes. Autologous thymic lymphocytes obtained from macrophage co-cultures responded to three different mitogens and were reduced in their ability to reassociate spontaneously with macrophages. Neither of these properties were found in thymic lymphocytes that had not been cultured with macrophages. These functional changes appeared to be specific for macrophages since thymic lymphocytes incubated with skin fibroblasts failed to respond to the test mitogens. Furthermore, they were not the result of either the inactivation, by macrophages, of a putative suppressor thymocyte or a soluble macrophage product. In addition to influencing the functional maturation of thymic lymphocytes, macrophages also appeared to play a direct role in inducing the mitogen response of functionally mature cells.

Increase in T cell mitogen responsiveness in rat thymocytes by thymic epithelial culture supernatant

Supernatants from rat thymic epithelial cultures (TES) increased [14C]thymidine incorporation into rat thymocytes stimulated with phytohemagglutinin (PHA) or concanavalin A, whereas supernatants from nonthymic control tissue cultures had no effect. It was demonstrated that the increased mitogen responsiveness was at least partially due to an increase in the number of responsive cells. Furthermore, our data suggest that the target cell for TES resides among the cortisone-sensitive population of thymocytes. The continuous presence of TES provided the best conditions for enhancement of mitogen stimulation. The addition of TES had very little effect on mitogen-stimulated spleen and lymph node lymphocyte cultures. The data suggest that, in the presence of TES, a cortisone-sensitive, PHA-nonresponsive subpopulation of rat thymocytes is induced to respond to PHA.

Genetic selection of mice for quantitative responsiveness of lymphocytes to phytohemagglutinin

A two-way selection was performed in mice according to the quantitative in vitro response of lymph node lymphocytes to the mitogenic activity of phytohemagglutinin (PHA). The foundation population was composed of outbred mice produced by reciprocal mating of equal numbers of mice from four different colonies. The selective breeding was carried out by mating of mice at each generation giving the best or the lowest response, respectively. The progressive interline separation produced by 6 generations of selective breeding demonstrates that responsiveness to PHA is submitted to polygenic regulation. The heritability of the character investigated is 0.28 +/- 0.08. The interline separation is also found with another T mitogen, concanavalin A (Con A). In spleen cells PHA and Con A produce a similar interline difference. In contrast, the purified protein derivative of tuberculin (PPD) stimulated both lines equally, and E. coli lipopolysaccharide gave only a slightly higher response in high line. This finding implies that our selection based upon response to PHA did not influence B cell function.

Generation of T-helper cells in vitro. I. Cellular and antigen requirements

A sequential mouse cell culture system is described for the induction and assay of T-helper cells. Unprimed, cortisone-resistant, nylon wool-purified thymocytes cultured with adherent peritoneal exudate cells can be primed in vitro with soluble carrier protein to generate carrier-reactive helper cells. These cultured cells enhance the anti-hapten plaque-forming response of hapten-primed spleen cell cultures to hapten carrier conjugates. The culture conditions, cellular manipulations, and antigen requirements for the optimal induction of helper cells with these purified cell populations is presented. The active helper cell generated in this culture system is a thymus-derived cell which requires macrophages for its induction and must be proliferate in vitro before the manifestation of helper-cell function. Helper cells generated in vitro stimulate both carrier-specific and nonspecific enhancement of splenic anti-hapten responses. The carrier-specific and nonspecific enhancement can be distinguished by the requirement for antigen in the helper cell and spleen cell cultures, the dose of helper cells added to the spleen cell cultures, and by the requirement for additional splenic adherent accessory cell interactions.

Hyperactive T-cell function in young NZB mice; increased proliferative responses to allogenic cells

The one-way mixed lymphocyte reaction was employed to study proliferative responses to antigens by mature, immunocompetent T cells from NZB mice 3 weeks to 4 months old. Compared to cells from control mice of the same H-2 type, thymus, spleen and lymph node cells from NZB mice were hyperactive in this response. The results are discussed in relation to possible effects of chronic stimulation by endogenous type C leukaemia virus upon differentiation of functional T cells or upon regulation by T cells of other T-cell functions, including augmentation of antibody responses.

Human T-cell heterogeneity as delineated with a specific human thymus lymphocyte antiserum. In vitro effects on mitogen response mixed leukocyte culture, cell-mediated lymphocytotoxicity, and lymphokine production

Human peripheral blood lymphocytes (PBL) were evaluated by their responses to phytohemmagglutinin (PHA-P), concanavallin A (con-A), and pokeweed mitogen (PWM), both before and after treatment with an antiserum against human thymic lymphocyte antigens (HTLA) that had been made T-cell-specific by multiple absorptions with immunoglobulin EAC-positive lymphoblast cell lines (B cells). Cells treated with HTLA were examined for their ability to react in a mixed lymphocyte culture (MLC) and to form killer cells in a cell-mediated lymphocytotoxicity (CML) system. Sensitized cells were also examined for their ability to respond to purified protein derivative (PPD) by blastogenesis, migration inhibitory factor release (MIP), and lymphotoxin (LT) production, both before and after treatment with HTLA and complement. The HTLA was in itself highly stimulatory to PBL. However, with the addition of complement and subsequent cell destruction, a marked decrease in its stimulatory response was noted. PBL treated with HTLA and complement exhibited marked inhibition of responsiveness to con-A with little decrease in PHA-P -OR PWM stimulation except at very high concentration of HTLA. MLC reaction was inhibited only when responder cells were treated with HTLA + C'. Treatment of stimulator cells with HTLA + C' did not significantly alter the MLC response. The HTLA + C'-treated cells failed to form killer cells in the CML reaction and inhibited PPD-induced blasto-genesis from PPD-sensitized individuals; however, treatment of sensitized cells with HTLA + C' had little effects on the release of MIF and LT. It is suggested that subpopulations of T-cells carry surface antigens that bind with this specific antisera, and that the con-A-responsive cells, the responder cells in the MLC, and killer T-cells comprise a separate subset from cells responding to PHA-P or PWM, OR THE MIF-and LT-producing cells.

Thymosin-induced differentiation of murine thymocytes in allogeneic mixed lymphocyte cultures

Calf thymosin is shown to enhance the one-way MLR of CBA thymocytes cultured with allogeneic mitomycin-C- treated C57BL/J6 spleen cells. Thymosin does not enhance the one-way MLR of CBA thymocytes cultured with syngeneic mitomucin-C-treated spleen cells. Based on this finding we present a relatively simple, rapid and quantitative in vitro microculture hioassay for inducers of T-cell differentiation and propose that thymosin treatment, when accompanied by antigen presentation, induces the two-step maturational sequence of pre-T yields T1 yields T2.

Immunological mechanisms in the pathogenesis of virus-induced murine leukemia. II. Characterization of autoreactive thymocytes

The nature of the reaction and the type of effector cells involved in the reactivity of thymocytes from Moloney murine leukemia virus (MuLV-M)-carrier mice against normal syngeneic target cells has been further characterized. The reaction is mediated by viable MuLV-infected thymocytes. Lysates of carrier thymocytes are not more effective in causing target cell reduction than are lysates of normal thymus cells. Soluble mediators do not appear to be involved in the reaction. The thymocytes mediating the reaction are non-adherent cells positive for the theta-C3H and H-2-k alloantigens, but are negative for detectable murine IgG determinants. Their resistance to corticosteroid treatment further identifies them as functionally mature immunocytes.

In vitro culture of functional human thymic epithelium

Cultures of pure epithelial cells have been obtained from human thymus. Incubation of spleen cells from adult thymectomized mice on these cultures rendered RFC sensitive to inhibition by anti-theta serum, whereas no effect was obtained with fibroblasts or spleen cell cultures. The fact that theta conversion was obtained when spleen cells were incubated in the thymic culture placed within a Millipore chamber indicates that theta conversion was due to the effect of a humoral factor.