The uropod-bearing lymphocyte of the guinea pig. Evidence for thymic origin

An ezrin-rich, rigid uropod-like structure directs movement of amoeboid blebbing cells

Melanoma cells can switch between an elongated mesenchymal-type and a rounded amoeboid-type migration mode. The rounded 'amoeboid' form of cell movement is driven by actomyosin contractility resulting in membrane blebbing. Unlike elongated A375 melanoma cells, rounded A375 cells do not display any obvious morphological front-back polarisation, although polarisation is thought to be a prerequisite for cell movement. We show that blebbing A375 cells are polarised, with ezrin (a linker between the plasma membrane and actin cytoskeleton), F-actin, myosin light chain, plasma membrane, phosphatidylinositol (4,5)-bisphosphate and β1-integrin accumulating at the cell rear in a uropod-like structure. This structure does not have the typical protruding shape of classical leukocyte uropods, but, as for those structures, it is regulated by protein kinase C. We show that the ezrin-rich uropod-like structure (ERULS) is an inherent feature of polarised A375 cells and not a consequence of cell migration, and is necessary for cell invasion. Furthermore, we demonstrate that membrane blebbing is reduced at this site, leading to a model in which the rigid ezrin-containing structure determines the direction of a moving cell through localised inhibition of membrane blebbing.

Hypersensitivity pneumonitis in man. Light- and electron-microscopic studies of 18 lung biopsies

Light- and electron-microscopic changes produced by hypersensitivity pneumonitis were analyzed in open lung biopsies taken from 18 patients with chronic forms of the disease. The main changes observed were: alveolitis (both luminal and mural), granulomas, intraalveolar buds, and interstitial fibrosis. The cells infiltrating the alveolar walls were mainly lymphocytes. Occasionally these lymphocytes presented irregularities in the contours of the nuclear membranes and resembled Sézary cells. In one patient, a few lymphocytes were found that resembled "hand-mirror" cells. Intraalveolar macrophages often had a foamy appearance. Granulomas, present in two-thirds of the patients, differed in several respects from those in sarcoidosis: they were smaller, more loosely arranged, and poorly limited; they had a higher content of lymphocytes; and they were located more frequently in alveolar tissue than in the vicinity of bronchioles and vessels. Intraalveolar buds, also present in about two thirds of the patients, were composed mainly of fibroblasts, myofibroblasts, and macrophages in a loose connective tissue that was rich in proteoglycan material. Capillaries and epithelial cells were rarely seen in buds. Alveolar buds appear to develop by a process of disruption of the epithelial lining layer, due to alveolitis, followed by intraalveolar exudation and by subsequent intraalveolar migration of connective tissue cells interacting with macrophages. Severe fibrotic and alveolar epithelial changes were observed in four patients; milder changes were frequent in most other patients. It is concluded that hypersensitivity pneumonitis usually has distinctive morphologic features; these may help to distinguish the resultant pulmonary fibrosis from that due to other causes.

Evidence for cytotoxic T-lymphocyte-target cell interaction in brains of mice infected intracerebrally with lymphocytic choriomeningitis virus

Murine lymphocytic choriomeningitis is a T-cell-mediated pathologic immune phenomenon. The name of this experimental illness was derived from the principal histopathologic alterations of the central nervous system (CNS) of adult mice infected intracerebrally with lymphocytic choriomeningitis virus, i.e., lymphocytic infiltrations of plexus choroidei and meninges. The general assumption that the main event in the pathogenesis is damage to virus-infected target cells by cytotoxic T-lymphocytes is plausible but direct evidence is scarce. We have studied the ultrastructural alterations of both types of cells that are thought to participate in this immunopathologic interaction. Lymphocytes with signs of T-cell transformation were first evident on day 4 after infection. One day later, lymphoblasts, often extending uropods and containing cytoplasmic dense and compound multivesicular bodies, predominated. They were sometimes seen in intimate contact with connective tissue cells of the leptomeninx and epithelial cells of the choroid plexuses which were shown to be infected by immunofluorescence procedure. Lymphoblasts occasionally invaginated the cytoplasm of the putative target cells with cytoplasmic processes, and were even found inside the latter, exhibiting the phenomenon of emperipolesis. Lymphocytic transformation was at its maximum 6 days after infection. At this time, individual leptomeningeal cells and groups of plexus epithelial cells showed signs of cytolysis, and in a few instances these damaged cells were in close spatial association with lymphoblasts. Similar observations have been reported by others who studied the interaction between cytotoxic T-lymphocytes and their appropriate targeted cells in vitro. We interpret our findings as providing direct evidence for the assumption that one link in the chain of events leading to the cerebral form of lymphocytic choriomeningitis of the mouse is damage to virus-infected leptomeningeal and plexus cells by cytotoxic T-lymphocytes.

Morphologic observations of contact-induced lysis of EBV-infected B lymphocytes by autologous hand mirror T cells

To study the possible immunologic role of hand mirror lymphocytes (HML) in the control of Epstein-Barr virus (EBV)-infected B lymphocytes in patients with infectious mononucleosis (IM), we studied the in vitro interactions of these cells by time-lapse video light microscopy. Peripheral blood T lymphocytes isolated from three patients in the early recovery phase of IM were mixed with their autologous EBV-infected B cells. Motile lymphocytes with their characteristic hand mirror shape were observed to attach by their uropods to the B cells. The HML remained attached for variable periods ranging from 45-75 min. Following detachment, B cells that were in contact with HML underwent lysis. Mixtures of T cells from healthy donors and EBV-infected cell lines exhibited only rare uropod formation with no attachment or lysis of B cells. The present experiment indicates that contact-induced lysis of EBV-infected B lymphocytes is operative in IM and that this process is mediated by the HML.

A defect in the formation of uropod-bearing lymphocytes (hand-mirror cells) in patients with the Wiskott-Aldrich syndrome

The formation of a uropod by lymphocytes (hand-mirror cells) represents a morphologic stage of immune activation and motility in lymphocytes. Immune complexes have been previously shown to induce hand-mirror-cell (HMC) formation in human lymphocytes and have been associated with increased number of HMC in acute leukemia. We studied the ability of immune complexes to induce HMC in patients with Wiskott-Aldrich syndrome (WAS), a disease of cellular immune deficiency. Our findings indicate that lymphocytes from patients with WAS adhere to immune complexes but have a defect in their ability to form HMC when compared with normal human lymphocytes. These results support the concept that immune complexes induce HMC formation and that the HMC is related to the normal cellular immune response.

Scanning and transmission electron microscopy study of antibody-dependent lymphocyte-mediated cytotoxicity on measles virus-infected cells

The structural events related to antibody-dependent lymphocyte-mediated cytotoxicity (ADLC) have been studied on measles virus-infected cells using human peripheral blood lymphocytes (PBL) and anti-measles virus serum. The first event in ADLC was a recognition process occurring with 15 min after contact between the infected cells and lymphocytes. Plasma membrane and microvilli of adsorbed PBL were specifically attached to virus-induced ridges over nucleocapsids and to viral buds. After 30 min, a fraction of adsorbed PBL (K cells) changed shape and extended long filipodia toward the target cells which, in turn, showed long villi contacting the PBL. At 4 h, when cytotoxicity as measured by chromium release was maximum, K cells had flattened and numerous blebs and ruffles formed on their surface. The K-cell alterations varied in intensity with the type of measles-infected target cell, but frequently the K cells appeared irreversibly damaged. T- and non-T-cell fractions were separated, and in situ erythrocyte rosettes were used as markers for subpopulations which were easily recognized by scanning electron microscopy. Most of the cytotoxic K cells were identified as non-T cells carrying Fc receptors for immunoglobulin G. However, a small subpopulation of cells bearing both sheep erythrocyte and Fc receptors was also found to be involved in ADLC by chromium release assay as well as by electron microscopy. Some of these interacting T cells extended a long uropod on the target cell, but their intracellular structure remained unaltered through ADLC, in contrast with the other T cells and the non-t killer cells. This suggests that perhaps some T killer cells might remain functional after the cytotoxic interaction with a target cell.

Infectious mononucleosis and acute lymphoblastic leukemia--hand mirror cells: a qualitative and quantitative ultrastructural study

The hand mirror cell (HMC) in infectious mononucleosis (IM) is a T cell which appears at the time of improvement and apparently has an important role in immune surveillance. Fifty electron micrographs of HMC from an unusual untreated patient with ALL-HMC variant were compared to 50 electron micrographs of HMC from five patients beginning to recover from IM. Quantitative differences between the IM-HMC and ALL-HMC were observed in multivariate analysis of variance (p < 0.001), nuclear perimeter (p < 0.0001), nuclear length (p < 0.0001), cytoplasmic length (p < 0.0001), total cell area (p < 0.0001), cytoplasmic area (p < 0.0001) and nuclear cytoplasmic ratio (p < 0.004), and numbers of mitochondria (p < 0.001). The qualitative differences included greater nuclear irregularity in the IM-HMC, and prominent nucleolonema in some of the ALL-HMC. In addition, the IM-HMC showed more polyribosomes, parallel tubular arrays and microtubules, but much less glycogen. Since the IM-HMC apparently enter effectively into immune surveillance, and some cases of ALL have HMC, the ultrastructural similarities and differences above contribute to a better understanding of the importance of this unique cell configuration in benign and malignant disorders.

Specific binding of T lymphocytes to macrophages IV. Dependence on cations, temperature and cytochalasin B-sensitive mechanisms

Peritoneal exudate lymphocytes (PEL) from immune guinea pigs adhere to macrophages carrying the relevant antigen and are thereby stimulated to proliferate in culture. The resultant PEL represent a population highly enriched with regard to their capacity to specifically rebind to antigen-pulsed macrophages. We have studied the mechanisms underlying specific binding of lymphocytes to macrophages by examining the effects of physical and chemical modifications of the two cell types. Specific binding was inhibited by fixation of cells, metabolic inhibitors, low temperatures, cytochalasin B and divalent cation depletion. After specific binding has taken place, cation depletion, but not cytochalasin B or low temperatures, disrupts binding. These observations indicate that specific binding occurs by a series of discrete events that can be operationally distinguished.

Ultrastructural cytochemical demonstration of peroxidase-positive monocyte granules: an additional method for studying the origin of mononuclear cells in encephalitic lesions

Unlike lymphocytes, blood monocytes possess in their cytoplasm peroxidase-positive (azurophil) granules (ppg) which largely correspond to the homonymous organelles of neutrophil granulocytes. We tested whether ppg, demonstrated cytochemically at the submicroscopic level, could serve as markers of monocyte-derived reactive mononuclear cells in encephalitic lesions. Samples of cerebrocortical tissue from adult albino mice with experimental yellow fever virus encephalitis were incubated in a medium containing diaminobenzidine and H2O2 for localization of peroxidatic activity. Mononuclear cells exhibiting ppg were found (1) in the lumen of brain venules, (2) in different stages of migration through the walls of such vessels, (3) in perivascular areas, (4) in the glioneuropil, either loosely scattered or forming small clusters, (5) in a satellite position to neurons, and (6) in leptomeningitic inflitrates. Several mononuclear elements harboring ppg had assumed an elongated, rod cell-like outline. Amongst the peroxidase-negative mononuclears were fully developed brain macrophages and elements showing morphologic features characteristic of activated lymphocytes. Most mononuclear cells without ppg resembled the peroxidase-reactive ones. The results of this study provide direct evidence in favor of a monocytic origin of, at least, numerous reactive mononuclear elements in encephalitic lesions. The approach followed in the present study is not suitable for quantitative investigations of the histogenesis of mononuclear cells responding to brain injuries, since emigrated blood monocytes rapidly lose their ppg, particularly, when they display enhanced phagocytic activity.

Reactive and non-reactive lymphocytes in experimental allergic encephalomyelitis

Myelin basic protein (BP),induces an increased in vitro proliferative response of peritoneal exudate cells (PEC), compared with that given by pheripheral blood lymphocytes (PBL), in guinea-pigs with experimental allergic encephaloMYELITIS (EAE). This response is determined by the nature of the lymphocytes in such exudates. We have also found that: (a) the majority of peritoneal lymphocytes from non-sensitized animals are E-rosetting (T cells) (59 percent) or null cells (greater than 40 percent) with EAC-rosetting (B cells) comprising 1 percent or less of cells; (b) both T cells and null cells respond equally to BP as determined by a technique combining rosette-formation and autoradiography; (c) the increased in vitro peritoneal lymphocyte response in animals with EAE cannot be explained solely by the number of null cells since peripheral blood and lymph nodes also contain appreciable numbers of null cells.

[T-cell leukemias of adulthood]

9 adult patients suffering from different forms of T-cell-malignancies were investigated: 4 patients with T-ALL; 1-T-ALL-CLL mixed form (prolymphocytic); 2 T-CLL; 2 Sézary-syndrome. The clinical peculiarities of the different forms of leukemias were compared: involvement of lymph nodes and spleen, of the central nervous system and the skin was frequent; in contrast to the findings in Sézary-syndrome, bone marrow infiltration was prominent. Light and electron microscopic morphology of the malignant cells are described. In all cases a strong activity of acid phosphatase was demonstrated, in one patient prominent deposits of glycogen. The T-cell-quality of the respective malignant cell population as well as the B-T-cell distribution of the remaining "normal" lymphocytes were shown by the following cell markers: demonstration of T-cell-antigen, resp. membrane immunoglobulins with the aid of specific heterologous antisera conjugated with peroxidase, 125iodine or fluoresceine; complement consumtion or cytotoxicity with such antisera; spontaneous rosette formation with sheep red cells or with acrylic acid beads. Usually, there was a good coincidence in results obtained with the different markers. In two patients, however, T-cells demonstrated by anti-T-globulin were not able to form T-rosettes. Responsiveness of the malignant T-cells and also of the remaining "normal" blood lymphocytes to different mitogens usually was depressed, immunoglobulin levels in the blood mostly were normal. Taking all findings into consideration, T-cell-leukemias of the adult represent a special group of hematological malignancies; the different subgroups show similarities; transitional forms occur.

[Electronmicroscopic and immunohistochemical studies on human lymphocytes]

Lymphocytes from the blood of healthy individuals and of patients suffering from CLL were investigated by electron microscopy and peroxidase-immunohistochemistry. B-lymphocytes were labelled by heterologous, peroxidase-conjugated antisera directed against the Id-determinants of their membranes. T-lymphocytes were labelled by an indirect method: specific incubation with a specific anti-T-cell-globulin from the rabbit; labelling-incubation with a peroxidase-conjugated anti-rabbit-IgG-globulin from the sheep. In addition, T-lymphocytes were identified by their ability to form rosettes with sheep erythrocytes spontaneously. The quantitative results were: about 80% T-lymphocytes and about 24% B-lymphocytes in normal persons, the opposite results in CLL. T- and B-lymphocytes were photographed electron microscopically; the number of organelles in the single cells was evaluated: lysosomes in the average are more numerous in T-lymphocytes, ergastoplasm in B-lymphocytes, mitochondria are equally distributed in both groups of cells. There is so much overlapping, however, that the single cell only with the aid of immunochemistry or rosette formation can be identified as a B- or T-cell. In both, the T- and the B-cell-series, different forms of lymphocytes can be distinguished according to the degree of cell differentiation. Some further problems, as specificity of the antisera and labelling of the cells by means of their Fc-receptor are discussed.

The adherence of human Fc receptor-bearing lymphocytes to antigen-antibody complexes. II. Morphologic alterations induced by the substrate

The adhesion of human Fc receptor-bearing lymphocytes to immobilized antigen-antibody complexes is accompanied by marked alterations in cell shape, resulting in flattening of greater than 90% of the adherent cells. In addition, about 65% of the adherent cells become elongated, with distinct uropods being present in about 1/3 of these cells. Scanning electron microscopy demonstrates that most of the surface microvilli are lost, while ruffled membranes and long microextensions are formed during the shape change. Time-lapse cinematography shows that the major shape changes occur within a few minutes after contact with the substrate, and that the adherent cells undergo translational motility. Both flattening and elongation of the adherent cells are inhibited by low temperature, chelating agents, cytochalasin B, and vinblastine, while sodium azide selectively inhibits elongation and uropod formation. It is argued that these morphological changes result from an active response of the cell to the immobilized complexes, and that such alterations may be related mechanistically to the ability of the cells to kill antibody-coated target cells.

Ultrastructural studies of rabbit synovitis induced by autologous IgG fragments. II. Infiltrating cells in the sublining layer

The synovial sublining layer of rabbits with synovitis induced by intra-articular injection of cathepsin D digested autologous IgG fragments (Fab2) has been examined under the electron microscope. Twelve or more injections of autologous Fab2 led to chronic synovitis with dense mononuclear cell infiltrates containing lymphocytes, blastic cells, plasma cells and macrophages. In the infiltrates there was evidence that the lymphocytes had been activated prior to transformation into mature plasma cells. Indirect evidence suggests that T lymphocyte activation also occurred in these infiltrates. Cellular contacts between macrophages and lymphocytes or plasma cells as well as between M cells and lymphocytes were demonstrated. These contacts are tentatively interpreted as a feature of ongoing immune processes in the synovium.

Local immune response in experimental pyelonephritis in the rabbit. I. Morphological and functional features of the lymphocytic infiltrate

The cellular activity of circulating lymphocytes and lymphocytes isolated from the infected kidney of animals with experimental haematogenous pyelonephritis was evaluated. The incorporation of [3H-methyl]thymidine into DNA by lymphocytes was studied with mitogens such as phytohaemagglutinin (PHA), pokeweek mitogen (PWM) and goat anti-rabbit IgG (GARIG). Lymphocytes from infected kidney had a high baseline DNA synthesis compared to circulating lymphocytes from days 5 to 27 of infection. Infected kidney lymphocytes failed to respond to PHA, PWM, or GARIG, whereas circulating lymphocytes did respond to these mitogens. Uropod-bearing lymphocytes, which were shown to be T lymphocytes, were present from days 5 to 77 of infection. B lymphocytes, as determined by surface immunofluorescent technique, were present by day 12, coincident with the onset of local synthesis of antibody. These studies reveal that in pyelonephritis, the cellular response goes through sequential changes and indicate a dynamic interrelationship between T and B lymphocytes at an infected site.

Separation of T cell subpopulations capable of DNA synthesis, lymphotoxin release, and regulation of antigen and phytohemagglutinin responses on the basis of density and adherence properties

T memory cells specifically responsive to ovalbumin and performing the diverse functions of DNA synthesis, lymphotoxin release, and regulation can be isolated in enriched numbers in the most buoyant fractions (A+B) of bovine serum albumin gradients on day 9 after sensitization. At least 20-30% of these cells are capable of mounting a blastogenic response to ovalbumin. A+B cells responding to ovalbumin with DNA synthesis have adherent properties and are further enriched on passage through glass wool. The subpopulations capable of entering into blastogenesis and DNA synthesis and of lymphotoxin release are unresponsive to T mitogens. A+B cells are capable of either potentiating or suppressing DNA synthetic responses to both phytohemagglutinin and antigen when added to 5 X 10(5) D cells in different proportions. Potentiation or suppression of phytohemagglutinin responses were observed with 1 X 10(5) A+B cells, and total suppression was observed with A+B in the range of 4 X 10(3) to 2 X 10(4). The response to antigen was sometimes inhibited in the same cell combinations that gave a potentiated response to phytohemagglutinin and vice versa. Regulatory cells in this system were not macrophages since their effect was not mimicked by addition of peritoneal macrophages, and ablation of macrophages by carrageenan affected neither the potentiation nor suppression.

In vitro selection and extended culture of antigen-specific T lymphocytes. I. Description of selection culture procedure and initial characterization of selected cells

Specific selection of antigen-responsive guinea pig peritoneal exudate lymphocytes (PELs) was achieved by a selection culture procedure. This procedure involved the addition of PELs from immune donors to monolayers of antigen-pulsed adherent peritoneal exudate cells from nonprimed syngeneic donors. PELs which failed to adhere were discarded at 24 and 48 h; after 1 wk of culture, lymphocytes were obtained which were highly responsive to the antigen for which they were selected but which demonstrated little or no response to other antigens to which the original donor of the lymphocyte was immune. These selected cells were largely T lymphocytes and could be maintained in culture for 2-5 wk in an antigen-responsive state and, in 20-30% of cases, for 8-10 wk in an antigen-independent state.

Cell adherence as a serious source of error in the haemacytometer count of leucocytes in artificial suspensions

This report demonstrates a previously disregarded source of error in the count of lymphocytes that have been fixed in Turk's dilution fluid (acetic acid and gentian violet). If the dilution fluid does not contain any serum protein or other proteins, a large proporation of the lymphocytes will stick to the walls of the mixing vessel. The use of plastic or siliconized vessels does not prevent this. The number of cells which in that way will be eliminated from the suspension is dependent on several variables, such as the type of mixing procedure and mixing vessel, the compensation of the dilution fluid and the cell density. As a consequence it is difficult to give any correction factor for such a count in a haemacytometer counting chamber. More than 30% of the cells may disappear under generally obtained conditions. This source of error is to some extent also involved in the white blood cell count, despite the presence of about 1% protein in the dilution fluid from the serum and lysed erythrocytes. To prevent this loss of cells either bone serum albumin, to a concentration of 5%, or the detergent Certrimide, to a concentration of 2-5 mM, may be added to the dilution fluid.

Morphological definition of thymocyte subpopulations

An attempt was made to distinguish immature from mature, immunocompetent thymocytes in the adult mouse using morphological criteria. All procedures enriching for competent thymocytes also enrich small cells whose morphology is very similar to the small thymocytes that have been described in the medulla of the thymus (Abe and Ito, 1970). A heterogeneity of competent thymocytes is likely since the enriched populations always contain a number of medium sized cells. These latter cells show spontaneous DNA-synthetic activity and are probably the source of thymus proliferative activity. Owing to their characteristic morphology these two cell types might represent the precursors for each of the two differentiated T cell types that have been found to occur in delayed type hypersensitivity (Matter, 1974).

Macrophage-lymphocyte clusters in the immune response to soluble protein antigen in vitro. II. Ultrastructure of clusters formed during the early response

Macrophage-lymphocyte clusters are formed when lymph node cells and autologous peritoneal exudate cells from guinea pigs immunized with tubercle bacilli are cultured in the presence of purified protein derivative of tuberculin (PPD) for 20 h. We have studied the ultrastructure of these clusters employing transmission and scanning electron microscopy. The most simple macrophage-lymphocyte cluster consisted of one macrophage, one large central lymphocyte with a blastoid appearance attached to the macrophage with a broad area of contact, and from a few to more than 20 small peripheral lymphocytes attached to the central lymphocyte by their uropods. Some clusters were of more complex type, containing two or three macrophages or one macrophage with more than one central lymphocyte attached to the surface, but even in these clusters each peripheral lymphocyte was attached only to one central lymphocyte. By morphological criteria the peripheral lymphocytes were T lymphocytes.

Binding of aggregated gamma-globulin to activated T lymphocytes in the guinea pig

Heat-aggregated guinea pig gamma-globulin was shown to bind to the surface membrane of a subclass of guinea pig T lymphocytes. Cells of this subpopulation were identified as T lymphocytes because these cells did not stain for surface Ig (a B-cell marker) but did form spontaneous E-rosettes with rabbit erythrocytes (a T-cell marker). A strikingly high proportion of such aggregate-binding (Agg(+)), E-rosette-forming (E-rosette(+)), but surface Ig-negative (Ig(-)) cells were found in an inflammatory exudate. Thus purified peritoneal exudate lymphocytes (PELs) are known to consist of over 90% T cells, and 59% of these cells bound aggregates. 10% of these Agg+ Ig- E-rosette+ cells were found in draining lymph node cell populations and none in thymus cell populations. The high frequency amongst PELs suggested that these Aggregate+ Ig- E-rosette+ cells might be activated T cells as these are known to occur in high proportion in PEL populations. Confirmatory evidence for this postulate was provided by the striking increase (from 10% to 46%) of Ig- E-rosette+ cells that bound aggregates when lymph node cells were activated by antigen stimulation in vitro.

Ligand-induced movement of lymphocyte surface macromolecules. IV. Stimulation of cell motility by anti-Ig and lack of relationship to capping

Microscopic examination of spleen lymphocytes discloses a small number moving at random at a given time. The majority of lymphocytes with this spontaneous movement are thymic derived. Addition of anti-Ig antibodies stimulates random movement of B lymphocytes. This movement depends upon a bivalent antibody and a metabolically active cell. The movement is inhibited by DFP, suggesting the involvement of a serine esterase. Also the anti-Ig stimulated movement of the lymphocyte is inhibited by cytochalasin B or by not allowing the cells to settle onto a surface. Lymphocytes treated with DFP or cytochalasin B, or untreated lymphocytes in suspension, capped the anti-Ig-Ig complexes. Hence, one can dissociate the surface capping of anti-Ig-Ig complexes from cell movement. We postulate that capping may result from superficial movements of the surface and/or from membrane flow, both of which are not related to actual translation of the cell on a surface. Four effects have now been observed following combination of a ligand with the antigen receptor on the B lymphocytes: redistribution on the surface of the complexes; pinocytosis and catabolism; shedding into the extracellular environment; and stimulation of translational movement.

Function of macrophages in antigen recognition by guinea pig T lymphocytes. I. Requirement for histocompatible macrophages and lymphocytes

Antigen activation of DNA synthesis in immune thymus-derived lymphocytes of guinea pigs requires the cooperation of macrophages and lymphocytes. We have investigated the role of histocompatibility determinants in this macrophage-lymphocyte interaction using cells from inbred strain 2 and 13 guinea pigs. The data demonstrate that efficient presentation of macrophage-associated antigen to the lymphocyte requires identity between macrophage and lymphocyte at some portion of the major histocompatibility complex. The failure of allogeneic macrophages to effectively initiate immune lymphocyte proliferation was not the result of the presence of an inhibitor of blastogenesis released in mixtures of allogeneic cells, peculiarities of the antigen or lymphoid cells employed, nor differing kinetics of activation by allogeneic macrophages. In addition, data were presented that demonstrated that alloantisera inhibit lymphocyte DNA synthesis by functional interference with macrophage-lymphocyte interaction.

Separation of thymus-derived and marrow-derived rat lymphocytes on glass bead columns

Rat thoracic duct lymphocytes were passed through columns of fine siliconed glass beads according to the method of Shortman (1966). On the basis of the degree of labelling with uridine in vitro and of surface antigenic markers it was shown that marrow-derived small lymphocytes were preferentially retained on the columns, causing the concentration of thymus-derived small lymphocytes in the filtered cells. Filtered cells retained normal graft-versus-host (GVH) activity. The proportion of thymus-derived small lymphocytes in normal rat lymph was estimated at a maximum of 63–87 per cent. Previous reports on the immunocompetence of filtered cells may readily be explained by these findings.

Regulation of lymphocyte responses in vitro: potentiation and inhibition of rat lymphocyte responses to antigen and mitogens by cytochalasin B

Cytochalasin B, at concentration between 0.02 and 0.2 mug/ml, was slightly stimulatory to lymph-node cells from normal rats and greatly potentiated their response to phytohemagglutinin and low concentrations of concanavalin A (mitogens for thymus-derived lymphocytes); it also potentiated the response of thymocytes to phytohemagglutinin. The response of lymphnode cells to lipopolysaccharide endotoxin (a mitogen for thymus-independent lymphocytes) was also enhanced, but only at concentrations in the usual inhibitory range, possibly by inhibition of a "suppressor T-cell" response. Sensitized lymphocytes responding to antigen were not stimulated at all, except at a very high cell density, where inhibition of a "suppressor cell" response was also considered likely. At concentrations of 5-10 mug/ml or higher, cytochalasin B inhibited all responses tested.