Macrophages phagocytose thymic lymphocytes with productively rearranged T cell receptor alpha and beta genes

An assay for the quantitative measurement of in vitro phagocytosis of early apoptotic thymocytes by murine resident peritoneal macrophages

Research into the mechanisms by which apoptotic cells are phagocytosed has grown considerably over recent years, together with a growing appreciation of the importance of clearance of redundant cells for tissue homeostasis. However, studies addressing the efficacy of phagocytosis have been rare. The few studies reported to date were either attempts to determine apoptotic cell clearance from the circulation or were focused on clearance in inflammation. We now describe an in vitro assay which permits the quantitative measurement of phagocytosis of apoptotic cells by murine resident peritoneal macrophages. The apoptotic cells used in the assay were murine thymocytes incubated with dexamethasone for only 3 h. Most apoptotic thymocytes were annexin V positive and propidium iodide negative and therefore still in the earlier stages of apoptosis. The assay was completed 7 h after the isolation of both macrophages and thymocytes, while macrophage culture time was only 4 h. Because of this short-term culture it is likely that the resident peritoneal macrophages largely maintained their in vivo phenotype. Using BALB/c macrophages and thymocytes, the maximal in vitro phagocytosis exceeded five thymocytes per macrophage in 1 h and two of these thymocytes were taken up within 10 min. Therefore, in vitro phagocytosis by resident peritoneal macrophages was rapid and of high capacity, as it is postulated to be in vivo. Under selected conditions, the mean uptake was 4.45+/-0.70 (mean +/- SD, n = 31) thymocytes per macrophage in 1 h. The inter-assay coefficient of variation, also representing the biological variability, was found to be 15.7%. The average intra-assay coefficient of variation was 13.6%. This assay permits comparisons of phagocytic efficacy between different strains of mice in vitro. In addition, a method of preparation is described which allows long-term storage of experimental results. Finally, our data suggests that internalization, but not binding of apoptotic cells to short-term cultured resident peritoneal macrophages, is critically dependent on the presence of serum. This allows separate analysis of binding and internalization of apoptotic cells with the assay, without the necessity to use agents blocking internalization.

Human monocyte-derived macrophage phagocytosis of senescent eosinophils undergoing apoptosis. Mediation by alpha v beta 3/CD36/thrombospondin recognition mechanism and lack of phlogistic response

Eosinophils may mediate tissue injury in a number of allergic diseases. Previously, we reported that eosinophils constitutively undergo apoptosis (programmed cell death) in culture. As this led to phagocytosis of the intact senescent cell by macrophages, we proposed that apoptosis represented an injury-limiting eosinophil disposal mechanism. Ingestion of apoptotic neutrophils by human monocyte-derived macrophages (M phi s) was found to be mediated by adhesive interactions between thrombospondin and the M phi alpha v beta 3 vitronectin receptor integrin and M phi CD36. As this failed to elicit a pro-inflammatory response from M phi s, we sought evidence that this specific, nonphlogistic clearance mechanism may operate in eosinophil disposal. In this study, we found that M phi ingestion of apoptotic eosinophils was specifically inhibited by monoclonal antibodies to M phi alpha v beta 3, CD36, and thrombospondin and by other inhibitors of this recognition mechanism including RGD peptide and amino sugars. Furthermore, not only did M phi ingestion of intact apoptotic eosinophils fail to stimulate release of the phlogistic eicosanoid thromboxane, but there was also a lack of increased release of the pro-inflammatory cytokine granulocyte/macrophage colony-stimulating factor. However, increased release of these mediators was observed when M phi s took up senescent post-apoptotic eosinophils that had been cultured long enough to lose plasma membrane integrity. The data indicate that the nonphlogistic alpha v beta 3/CD36/thrombospondin macrophage recognition mechanism is available for clearance of intact senescent eosinophils undergoing apoptosis. Furthermore, our findings suggest that, by contrast, phagocytosis of post-apoptotic eosinophils may elicit undesirable pro-inflammatory responses.

Morphological study of thymus stromal cells (TEL-2 cell) which play a role in the elimination of double positive immature thymocytes by phagocytosis

BACKGROUND

The process of the selective elimination of immature double positive thymocytes, during culture with TEL-2 cells (an epithelial cell line from the thymus stroma), is initiated by the contact between thymocytes and TEL-2 cells (Nakashima et al. 1990 Eur. J. Immunol., 20: 47-53; Hirata et al. 1991 Anat. Rec., 230: 524-530).

METHODS

Our approach was to follow the process of thymocyte internalization as a sequelae of this interaction with scanning electron microscopy (SEM), transmission electron microscopy (TEM), and electron microscopic cytochemistry for the hydrolytic enzyme acid phosphatase (Ac Pase).

RESULTS

With SEM, numerous thymocytes lay underneath the TEL-2 cells. An enhanced activity of the TEL-2 cell membrane, consisting of several thin thread-like projections either with or without an expanded termination, was involved with contacting the thymocytes. With TEM, close appositions were noted either at adjacent segments of the cellular membrane or between thick plump local protrusions of the plasma membrane. Blunt pseudopodia and deep invaginations on indented portions of the TEL-2 surface engulfed one or more thymocytes. With Ac Pase histochemistry: 1) the incorporation of thymocytes into a phagosome was indicated by the Ac Pase negative thymocytes enclosed in a membrane beneath the protuberance of a TEL-2 cell surface without fusion to the TEL-2 lysosomes; 2) most of these thymocytes were morphologically intact, whereas the rest were already damaged, having changes similar to apoptosis; and 3) a few Ac Pase positive dense bodies of the TEL-2 cell, mostly with the morphology of secondary lysosomes, fused with the thymocyte-enclosing membrane after which the digestion of several thymocytes proceeded.

CONCLUSIONS

With the prominent activity of the plasma membrane involving the initial attachment and subsequent phagocytosis, a murine thymic epithelial cell line TEL-2 plays a demonstrating role in the mechanism of the elimination of double positive thymocytes.

Programmed cell death of the normal human neutrophil: an in vitro model of senescence

The present study provides experimental data which indicate that the neutrophil is ideal for studying programmed cell death or apoptosis in vitro. Neutrophils can be obtained from human peripheral blood in large numbers with minimal experimental manipulation and are easily separated from other leukocytes, providing nearly pure cell suspensions. The neutrophil life span in vitro is sufficiently short to allow observations to be made within eight hours after experimental manipulation. Neutrophils can also be easily maintained in serum-free, chemically defined media which can be systematically altered, thereby defining specific variables that influence the apoptotic process. Since the neutrophils do not need an exogenous trigger to undergo programmed cell death, it is also an excellent model to study senescence. It was determined from this study that neutrophils undergo apoptosis most efficiently at 37 degrees C, a temperature requirement for physiologic cell death. Neutrophils undergo apoptosis at a slightly faster rate and maintain membrane integrity better when incubated in a tissue culture medium (e.g., RPMI 1640) compared with a balanced salt solution (e.g., HBBB). Cycloheximide, an inhibitor of protein synthesis, was shown to accelerate apoptosis in a dose-dependent manner. The presence of Zn++ significantly decreased the rate of apoptosis, whereas the presence of Ca++ and Mg++ had no apparent effect. These studies indicate that the process of senescence, culminating in cell death, is subject to modulation by a variety of agents and experimental conditions. In addition, the ultrastructural features of neutrophils undergoing programmed cell death in vitro were compared in detail to those occurring in vivo and were found to be comparable.

Intrathymic presentation by dendritic cells and macrophages: their role in selecting T cells with specificity for internal and external nominal antigen

The present study focused on the question of whether intrathymic dendritic cells and macrophages (DC/M phi) are involved in the processes of T-cell repertoire selection and establishment of tolerance towards nominal antigen. Proliferation of thymocytes (TC) was determined under limiting dilution (LD) conditions after depletion of DC/M phi and after reconstitution of TC, which were depleted of cells expressing major histocompatibility complex (MHC) class II antigens, with thymic DC/M phi. Trinitrophenyl (TNP) [coupled to ovalbumin (OVA)] was used as an internal antigen in prenatally trinitrobenzenesulphonic acid (TNBS)-treated mice and as an external antigen in prenatally untreated mice. Intrathymic DC/M phi were clearly involved in selecting the repertoire of T cells specific for external antigen: they presented the antigen and initiated proliferation of thymic T cells, which were depleted of MHC class II antigen-expressing cells. But they were not the only cells to present nominal antigen in the thymic environment. Intrathymic DC/M phi could also deliver negative signals. This became apparent when evaluating presentation of TNP in prenatally TNBS-treated mice. Thymus-derived DC/M phi from prenatally TNBS-treated mice could not initiate proliferation of TC in response to TNP-OVA. Instead, when prenatally TNBS-treated mice received an antigenic challenge [TNP-sheep red blood cells (SRBC)], thymic DC/M phi inhibited proliferation of cortisone-resistant TC from untreated and prenatally TNBS-treated mice. This can be explained by assuming that in the process of establishing tolerance, intrathymic DC/M phi may exert cytotoxic/cytostatic activity.

Morphological analysis of the cellular interaction between thymocytes and a thymic stromal cell line (TEL-2)

In a previous study (Nakashima et al., Eur. J. Immunol., 20: 47-53, 1990), a cloned stromal cell line TEL-2 was established from Balb/c mouse thymus. Incubation of thymocytes with TEL-2 cells resulted in the selective elimination of CD4 and CD8 double-positive thymocytes from the culture. In the present report, both phase-contrast and scanning electron microscopes were used to examine, at various time intervals, TEL-2 cells cocultivated with thymocytes in order to elucidate the kinetic sequence of their cellular interaction. The thymocytes attached to the TEL-2 cell surface were more numerous at early times (30 min to 1 h), and their number decreased gradually with time. In contrast, the thymocytes that migrated into the TEL-2 cell layers were less abundant at early times, their number increasing with time thereafter. Destruction of the regular arrangement of TEL-2 cells was found at later than 1 h, suggesting active cellular interaction. The thymocytes adherent to the TEL-2 cell surface were found to be of various shapes and often showed variable profiles, e.g., extending small cytoplasmic processes along the surface of TEL-2 cells or appearing ameboidal. A remarkable feature of the TEL-2 cells was that they formed numerous "round spaces" at the surface of the TEL-2 cell layers. The thymocytes were often located around "round spaces," and some were seen migrating into TEL-2 cell layers through these round spaces. In addition, complementary examinations by transmission electron microscopy revealed that the internalization of thymocytes into TEL-2 cells occurs inside the TEL-2 cell layers after migration.(ABSTRACT TRUNCATED AT 250 WORDS)

Detection of apoptosis of immature CD4+8+ thymocytes by flow cytometry

Apoptosis, i.e., programmed cell death, may be the mechanism by which both autoreactive and unselected immature CD4+8+ thymocytes are eliminated in the thymus. In the present paper we describe a simple and rapid flow cytometric method which permits one to study the induction and kinetics of apoptosis of CD4+8+ thymocytes using in vivo and in vitro suspension cultures. Analysing the level of surface expression of CD4 and CD8 molecules, forward light scatter and side (90 degrees) scatter as well as staining with ethidium bromide, three distinct stages of apoptosis of CD4+8+ thymocytes were defined. By counting cells passing through different stages of apoptosis one can attempt to quantitate this process. This method should be useful for in vitro studies on the mechanisms of negative and positive selection of CD4+8+ thymocytes, i.e., induction and inhibition of apoptosis respectively.

Thymic accessory cell complexes in vitro and in vivo: morphological study

Murine thymic macrophages and interdigitating cells, also called thymic accessory cells, were characterized by means of light- and electron microscopy. The cells were studied in suspension, during isolation by enzymatic digestion and in vivo. They were observed as isolated cells or as components of multicellular complexes, some of which were rosettes and were composed of lymphoid cells centered on each type of accessory cell. We also noted other cell complexes including macrophages that resembled classical epithelial nurse cells. We consider that multicellular complexes represent lymphostromal associations already existing in vivo, because we observed them at the periphery of thymic pieces undergoing enzymatic treatment. The heterogeneity of macrophages that we observed in vitro was also noted in vivo. In vivo macrophages were of three types: classical phagocytic cells distributed throughout the gland, cortical elongated cells in close contact with lymphoid blast cells, and atypical nurse cells containing mitotic cells and located in the inner cortex. The morphological aspects of the latter two cell types suggest that cortical macrophages in vivo have other roles: they can be interpreted as images of positive or negative cell selection. We also believe that rosettes are formed by elongated cortical macrophages when they are enzymatically isolated from the thymus.

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.

Cell components required for deletion of an autoreactive T cell repertoire

T cells become tolerant to self antigens during their development in the thymus. Clonal deletion of thymocytes bearing T cell receptor (TcR) which recognize self antigens is a major mechanism for generating tolerance. In the present study we have used allogeneic bone marrow (BM) chimeras, prepared with various combinations of mouse strains and focusing especially on expressions of I-E molecules and Mls-1a antigens on the cell surface, to investigate both immunohistochemically and by flow cytometry the cell components that contribute to the clonal deletion of T cells positive for V beta 6 TcR. The V beta 6 TcR expression is strongly associated with T cell recognition of both I-E and Mls-1a antigens. We found that I-E+ cells derived from donor BM (and thus not of recipient lineage) represented a primary requirement for deletion of Mls-1a-reactive thymocytes which bear V beta 6 TcR. Immunohistochemical analysis revealed that the donor-derived I-E+ cells were distributed mainly to the thymic medulla and that the V beta 6+ cells were eliminated from the thymic medulla between 2 and 3 weeks following BM transplantation. In contrast, Mls-1a+ cells of either donor or recipient origin might be responsible for the deletion, even though cortical epithelial cells appeared not to express Mls-1a antigens.

Selective elimination of double-positive immature thymocytes by a thymic epithelial cell line

A cloned epithelial cell line, TEL-2, has been established from the stroma tissues of normal mouse thymus. Incubation of mouse thymocytes on TEL-2 cells resulted in the selective elimination of double-positive (CD4+CD8+) cells from the culture, whereas single-positive (CD4+CD8- or CD4-CD8+) thymocytes remaining in the culture were concentrated in non-integrated cell population. The CD3- or CD3 low-positive thymocytes were also eliminated by the TEL-2 cells from the culture, followed by the concentration of CD3 high-positive cells in the culture. Only intact viable thymocytes were integrated into TEL-2 cells. Electron microscopic examination showed that the integrated cells into TEL-2 cytoplasm were gradually degenerated. Mature single-positive T cells, mature B cells or double-negative thymocytes were not integrated into TEL-2 cells. The TEL-2 cell may provide information on the mechanism of selective disappearance of double-positive immature cells from the thymus.

Thymic dendritic cells and B cells: isolation and function

The thymus is the primary organ in which T cells undergo rearrangement of T cell receptor alpha and beta genes, positive selection for affinity to self MHC products, and elimination (negative selection) of reactivity to self antigens. These events require an interaction of the developing T cell with other cell types in the thymus. The latter include epithelial cells, macrophages, dendritic cells, and the recently described thymic B cells the majority of which are CD5+. Here we review the identification and isolation of thymic dendritic cells and CD5+ B cells. We consider phenotype, ontogeny, and function, including possible contributions to the induction of self tolerance. Thymic dendritic cells are similar to spleen dendritic cells, but are larger and exhibit a few differences in phenotype. Dendritic cells from both organs are equally potent accessory cells for the MLR and lectin-induced, T cell proliferation. Thymic dendritic cells have higher levels of Fc receptors and support anti-CD3 dependent mitogenesis. Thymic CD5+ B cells share phenotypic features with peritoneal CD5+ B cells. However thymic B cells neither proliferate nor form antibody producing cells in response to the stimulation with LPS or anti-IgM plus IL-4, but do respond to stimulation with MHC class II-restricted helper T cells. Thymic dendritic cells and CD5+ B cells both appear at a similar time in ontogeny, about 14 d of gestation, which is the time T cell differentiation begins to take place. Dendritic cells from spleen, which are potent activators for peripheral T cells, are also potent inactivators for thymic-derived cytotoxic T cells. A correlation between reactivity to MIs products and the expression of TCR-V beta genes is well documented, and B cells are the primary APC for this antigen. Therefore, thymic CD5+ B cells may be a good tool for the investigation of tolerance to M1s products.

The myelopoietic inducing potential of mouse thymic stromal cells

The thymus has generally been considered as being solely involved in T cell maturation. In this study we have demonstrated that mouse thymic stroma can also support myelopoiesis. Bone marrow from mice treated with 5-fluorouracil was depleted of cells expressing Mac-1, CD4, and CD8 and incubated on lymphocyte-free monolayer cultures of adherent thymic stromal cells. After 7 days there was a marked increase in nonadherent cells, the majority of which were Mac-1+, FcR+, and HSA+. These proliferating bone marrow cells also expressed markers (MTS 17 and MTS 37) found on thymic stromal cells. Such cells were not found in thymic cultures alone, in bone marrow cultured alone, or on control adherent cell monolayers. Supernatants from the cultured thymic stroma, however, were able to induce these cell types in the bone marrow precursor population. Incubation of normal thymocytes with a monolayer of these in vitro cultivated Mac-1+, MTS 17+, MTS 37+ myeloid cells leads to selective phagocytosis of CD4+ CD8+ cells. Hence, this study demonstrates that the thymic adherent cells can induce myelopoiesis in bone marrow-derived precursor cells and provide a form of self-renewal for at least one population of thymic stromal cells. Furthermore, these induced cells are capable of selective phagocytosis of CD4+ CD8+ thymocytes and may provide one mechanism for the selective removal of such cells from the thymus.

Lymph node macrophage heterogeneity: the phenotypic and functional characterization of two distinct populations of macrophages from rat lymph node

The isolation and characterization of lymph node macrophages (M phi) has shown a hitherto unknown heterogeneity. Two types of M phi were distinguished by morphology, monoclonal antibody staining and functional assays. The type I M phi failed to express surface Ia even when activated, a characteristic which has only previously been reported for splenic marginal zone M phi; despite studies suggesting an antigen presentation role for the M phi, the failure to express surface Ia would seem to eliminate an interaction with T helper cells for the type I M phi in the lymph node. In contrast, the type I M phi, other characteristics of clustering with activated B cells in vitro, the colocalization of the type I M phi and activated B cells in situ, the specific uptake of thymus-independent type 2 antigens and the failure to undergo respiratory burst activity all suggest a M phi-B cell interaction, possibly of a trophic nature. The defective microbicidal activity of the type I M phi may have been compensated for by the type II M phi, which expresses both strong respiratory burst activity and surface Ia expression when freshly isolated. However, unlike the inflammatory M phi the activated phenotype of the type II M phi did not appear to be interferon-gamma dependent because type II M phi could also be isolated from nude rat lymph nodes.