The MRC OX-44 antigen marks a functionally relevant subset among rat thymocytes

Tetraspanin-enriched microdomains: The building blocks of migrasomes

The migrasome is a newly discovered organelle of migrating cells. Migrasomes play diverse physiological roles including mitochondrial quality control, lateral transfer of material between cells, and delivery of signaling molecules to spatially defined locations. The formation of migrasomes is dependent on tetraspanins, a group of membrane proteins containing four transmembrane domains, which form membrane microdomains named tetraspanin-enriched microdomains (TEMs). In this review, we will discuss the mechanisms for migrasome biogenesis, with a focus on the role of TEMs and the organizing principles underlying the formation of TEMs.

Leukocyte Tetraspanin CD53 Restrains α3 Integrin Mobilization and Facilitates Cytoskeletal Remodeling and Transmigration in Mice

The importance of tetraspanin proteins in regulating migration has been demonstrated in many diverse cellular systems. However, the function of the leukocyte-restricted tetraspanin CD53 remains obscure. We therefore hypothesized that CD53 plays a role in regulating leukocyte recruitment and tested this hypothesis by examining responses of CD53-deficient mice to a range of inflammatory stimuli. Deletion of CD53 significantly reduced neutrophil recruitment to the acutely inflamed peritoneal cavity. Intravital microscopy revealed that in response to several inflammatory and chemotactic stimuli, absence of CD53 had only minor effects on leukocyte rolling and adhesion in postcapillary venules. In contrast, Cd53^-/- mice showed a defect in leukocyte transmigration induced by TNF, CXCL1 and CCL2, and a reduced capacity for leukocyte retention on the endothelial surface under shear flow. Comparison of adhesion molecule expression in wild-type and Cd53^-/- neutrophils revealed no alteration in expression of β₂ integrins, whereas L-selectin was almost completely absent from Cd53^-/- neutrophils. In addition, Cd53^-/- neutrophils showed defects in activation-induced cytoskeletal remodeling and translocation to the cell periphery, responses necessary for efficient transendothelial migration, as well as increased α₃ integrin expression. These alterations were associated with effects on inflammation, so that in Cd53^-/- mice, the onset of neutrophil-dependent serum-induced arthritis was delayed. Together, these findings demonstrate a role for tetraspanin CD53 in promotion of neutrophil transendothelial migration and inflammation, associated with CD53-mediated regulation of L-selectin expression, attachment to the endothelial surface, integrin expression and trafficking, and cytoskeletal function.

Monoclonal antibodies to rat leukocyte surface antigens, MHC antigens, and immunoglobulins

The CD nomenclature used for human-leukocyte surface antigens is now being widely applied to naming their homologs in other species. This appendix catalogs those CD antigens that have been clearly defined in the rat. There are also many other antigens defined in the rat, but only those for which good biochemical data are available, such as amino acid sequences, are given here. The most commonly used antibodies are summarized.

CD53, a thymocyte selection marker whose induction requires a lower affinity TCR-MHC interaction than CD69, but is up-regulated with slower kinetics

The molecular mechanisms that govern the survival, maturation and export of thymocytes are the subject of intense study, and candidates for involvement in these processes might be identified by their differential expression during thymocyte selection. One such molecule is the tetraspanin CD53, which is not expressed on most CD4(+)CD8(+) double-positive (DP) cells in the normal mouse. We have examined CD53 expression on DP from several class I- and class II-restricted TCR transgenic (Tg) mice, and have found a strong correlation between CD53 expression and positive selection. CD53 expression in DP was formally demonstrated to be dependent upon MHC recognition as evidenced by studying DP from MHC-deficient mice which totally lack expression of this molecule. This link between selection and CD53 expression was reminiscent of CD69, and indeed the majority of selected DP from normal mice that express CD53 also express CD69. We compared CD53 and CD69 induction in vitro using pre-selected thymocytes from TCR-Tg mice that were stimulated either with mAb against TCR or with antigen-presenting cells (APC) pulsed with peptides. The data shows that with either stimulus, CD69 is induced rapidly on the thymocyte surface with expression detected in as little as 2 h. CD53 induction is slower with maximal expression taking up to 20 h. We also stimulated pre-selected thymocytes from the OT-1 TCR-Tg strain with APC pulsed with peptides of varying affinities for the TCR. Here low-affinity peptides which induce CD69 expression poorly were able to induce significant levels of CD53 expression. These data demonstrate that the induction of CD53 and CD69 upon selection is not identical. Thus a combination of the CD69 and CD53 selection markers may be a powerful tool to isolate thymocytes that have either been very recently selected or have arisen from differing MHC--TCR affinity interactions during selection.

Divergent cell-specific effects of activin-A on thymocyte proliferation stimulated by phytohemagglutinin, and interleukin 1beta or interleukin 6 in vitro

Activin-A is a member of the transforming growth factor-beta (TGF-beta) cytokine family. Based on studies in several cell systems, activin-A has been postulated to be a specific inhibitor of the actions of the inflammatory cytokine, interleukin 6. In cultures of adult rat thymocytes, activin-A inhibited sub-optimal phytohemagglutinin-induced and interleukin 1beta-stimulated proliferation, as measured by [(3)H]-thymidine incorporation in vitro. In contrast with TGF-beta1, which exerted similar inhibitory effects on thymocyte proliferation, activin-A activity was reduced by increasing the concentration of phytohemagglutinin or addition of the reducing agent, beta-mercaptoethanol. Both activin-A and TGF-beta1 inhibited the in vitro production of interleukin 6 by thymocytes in the presence of phytohemagglutinin and interleukin 1beta. In the presence of exogenous interleukin 6, however, both activin-A and TGF-beta1 stimulated thymocyte proliferation. These data suggest that activin-A inhibits thymocyte growth and differentiation, at least in part, by inhibiting endogenous production of interleukin 6, but stimulates thymocyte growth when exogenous interleukin 6 is present in vitro. These data indicate that activin interacts with other cytokines to exert complex regulation of T cell development, and is not an inhibitor of interleukin 6 action in all cell systems.

Selective enrichment of tetraspan proteins on the internal vesicles of multivesicular endosomes and on exosomes secreted by human B-lymphocytes

Association of major histocompatibility complex (MHC) class II molecules with peptides occurs in a series of endocytic vacuoles, termed MHC class II-enriched compartments (MIICs). Morphological criteria have defined several types of MIICs, including multivesicular MIICs, which are composed of 50-60-nm vesicles surrounded by a limiting membrane. Multivesicular MIICs can fuse with the plasma membrane, thereby releasing their internal vesicles into the extracellular space. The externalized vesicles, termed exosomes, carry MHC class II and can stimulate T-cells in vitro. In this study, we show that exosomes are enriched in the co-stimulatory molecule CD86 and in several tetraspan proteins, including CD37, CD53, CD63, CD81, and CD82. Interestingly, subcellular localization of these molecules revealed that they were concentrated on the internal membranes of multivesicular MIICs. In contrast to the tetraspans, other membrane proteins of MIICs, such as HLA-DM, Lamp-1, and Lamp-2, were mainly localized to the limiting membrane and were hardly detectable on the internal membranes of MIICs nor on exosomes. Because internal vesicles of multivesicular MIICs are thought to originate from inward budding of the limiting membrane, the differential distribution of membrane proteins on the internal and limiting membranes of MIICs has to be driven by active protein sorting.

Ligation of CD53/OX44, a tetraspan antigen, induces homotypic adhesion mediated by specific cell-cell interactions

The CD53 antigen is a member of the tetraspan family of proteins with unknown function. Stimulation of rat IR938F B-cell lymphoma cells with monoclonal antibody MRC OX44 (anti-rat CD53) triggered a homotypic adhesion reaction which reached a maximum effect at 24 hr. This effect occurred at 37 degrees C but not at 4 degrees C. Adhesion was prevented by removal of divalent cations, Ca2+ and Mg2+, with EGTA and EDTA as chelating agents. The adhesion induced by MRC OX44 was inhibited by cycloheximide and actinomycin D, suggesting that de novo protein synthesis was required for this effect. The addition of mAb WT1 against rat LFA-1 (CD11a) antigen had no effect on adhesion, suggesting that the cell-cell interaction is not mediated by the expression of LFA-1 antigen. The intracellular signals required to induce adhesion were inhibited by two tyrosine kinase inhibitors, genistein and piceatannol. Wortmannin, a selective inhibitor of phosphoinositide 3-kinase activity, completely blocked adhesion. Two protein kinase C inhibitors, H7 and bisindolylmaleimide, inhibited the adhesion, suggesting that part of the signal is mediated by PKC. Electron microscopy of aggregated cells showed that the interaction is localized to short membrane regions, where contact areas of higher density in opposing zones from both cells were detected. We postulate that there is a common adhesion mechanism that is modulated by several tetraspan family members and associated proteins. This adhesion structure might represent a novel form of cell communication among lymphoid cells.

Susceptibility of different subsets of immature thymocytes to apoptosis

In the present study the susceptibility of different subsets of immature rat thymocytes to undergo apoptosis was examined. Unfractionated rat thymocytes were negatively enriched into immature double positive (CD4+ CD8+), immature single positive (CD4- CD8+ CD3-) and triple negative (CD4- CD8- CD3-) thymocytes. These enriched subsets of immature thymocytes were then exposed to various apoptotic stimuli such as dexamethasone, etoposide and thapsigargin which readily induced apoptosis in unfractionated rat thymocytes. We found that the double positive thymocytes and their precursor cells, i.e. the single positive immature thymocytes, were equally sensitive to apoptosis after treatment with the apoptotic stimuli. In sharp contrast, the early migrants or precursor-containing thymocytes which are triple negative have a lower spontaneous apoptosis rate and were relatively resistant to all the apoptotic stimuli. These findings showed a breakpoint in thymocyte sensitivity to apoptosis which occurs after the onset of CD8 expression, suggesting that susceptibility of thymocytes to apoptosis is developmentally regulated.

Putative liver progenitor cells: conditions for long-term survival in culture

Oval cells proliferate extensively in the livers of animals exposed to oncogenic insults, are bipotent and are believed to be related to the so far unidentified liver stem cell. In normal liver, cells antigenically related to oval cells and expressing liver and epithelial markers are considered to be liver progenitor cells. We isolated, by fluorescence-activated cell sorting or magnetic bead sorting, cells expressing the oval cell antigens OC.2 or OC.3 from the liver of normal newborn or day 12 embryonal age rats. Magnetic bead sorting of positive cells was as efficient as fluorescence-activated cell sorting. A two-chamber culture system was devised in which cells were plated onto transwell filters coated with type IV collagen and cultured in a serum-free Ham's F12 medium supplemented with free fatty acids and bovine serum albumin. Under these conditions, cells remained viable for up to 6 weeks and their antigenic phenotype was unchanged throughout. Approximately 30% of sorted cells expressed epithelial and/or liver-specific markers. Growth factors mitogenic for epithelial cells and hepatocytes did not elicit cell proliferation. These results provide an important background for further studies designed to determine the biological significance of OC.2+ and OC.3+ cells in normal liver, to test the liver stem cell hypothesis and to develop protocols for the expansion in vitro of normal liver progenitors.

The use of intracellular single-chain antibody fragments to inhibit specifically the expression of cell surface molecules

One possible method to inhibit specifically the function of a protein inside a cell is to express an intracellular antibody combining site that can block function or prevent expression of the targeted molecule. In this report the parameters involved in the production and expression of functional, endoplasmic reticulum-retained, single chain Fv antibody fragments (scFv) were investigated. These intracellular scFv constructs were tested for their ability to inhibit specifically the expression of a CHO cell line pretransfected with the relevant cell surface antigen CD2. No scFv was detected in the cell supernatant although functional scFv, as assayed by ELISA, was detected in an NP-40 soluble fraction if an N-linked glycosylation site had been introduced into the antibody construct. This demonstrates that functional antibody combining sites can be produced even though they are not secreted. Inhibition of CD2 was obtained but was not complete and differed between clones. Levels of scFv could be increased by gene amplification but the level of functional binding activity remained constant and no further inhibition of CD2 expression was obtained. Immunofluorescence analysis at the single-cell level of the permeabilised transfected cell lines showed that less than 8% of cells expressed detectable levels of intracellular scFv, indicating selection against cells producing high levels of single-chain antibody. This selection was not seen when comparable single-chain TCR constructs, known to be retained intracellularly, were used. Thus, production of scFv with binding activity is not sufficient for good inhibition of gene expression although introduction of an N-linked glycosylation site is beneficial. The best strategy is probably to screen a panel of scFv constructs and use those that are secreted rather than those that are retained intracellularly.

Ectopic expression of human and feline CD9 in a human B cell line confers beta 1 integrin-dependent motility on fibronectin and laminin substrates and enhanced tyrosine phosphorylation

Few molecules have been shown to confer cell motility. Although the motility-arresting properties of anti-CD9 monoclonal antibody (mAb) suggest the transmembrane 4 superfamily (TM4SF) member CD9 can induce a motorgenic signal, gene transfection studies have failed to confirm this hypothesis. We report here that ectopic expression of human CD9 (CD9h) and feline CD9 (CD9f) in the CD9-negative, poorly motile, human B cell line Raji dramatically enhances migration across fibronectin- and laminin-coated polycarbonate filters. Migration of Raji/CD9h and Raji/CD9f on either substrate was inhibited by the anti-CD9 mAb 50H.19 and by the anti-beta 1 integrin mAb AP-138. Migration of Raji/CD9h on laminin was potently inhibited by the anti-VLA-6 integrin mAb GoH3 and by the anti-VLA-4 integrin mAb 44H6, whereas migration of Raji/CD9h on fibronectin was inhibited only by mAb 44H6. Since CD9h-transfected Raji cells adhered to fibronectin as effectively as mock transfectants, expression of CD9 enhanced motility, but not adhesion. CD9-enhanced migration was inhibited by the protein tyrosine kinase inhibitor herbimycin A suggesting that tyrosine phosphorylation played a role in the generation of a motorgenic signal. Raji/CD9h transfectants adherent to fibronectin expressed 6-fold higher levels of phosphotyrosine than Raji. Raji/CD9f transfectants also phosphorylated proteins on tyrosine more effectively than Raji including a protein of 110 kDa which was phosphorylated on the motility-inducing substrates laminin and fibronectin, but not on bovine serum albumin. Our results support a role for CD9 in the amplification of a motorgenic signal in B cells involving beta 1 integrins and the activation of protein tyrosine kinases.

A tetraspan membrane glycoprotein produced in the human intestinal epithelium and liver that can regulate cell density-dependent proliferation

The human cell line HT-29 provides a model system for studying regulation of proliferation and differentiation in intestinal epithelial cell lineages: (i) HT-29 cells cultured in glucose resemble undifferentiated multipotent transit cells located in the lower half of intestinal crypts; (ii) proliferating HT-29 cells cultured in inosine resemble committed cells located in the upper half of the crypt; (iii) nonproliferating, confluent HT-29-inosine cells have features of differentiated enterocytes and goblet cells that overlie small intestinal villi. A cDNA library prepared from HT-29-inosine cells was screened with a series of subtracted cDNA probes to identify proteins that regulate proliferation/differentiation along the crypt-villus axis. A cDNA was recovered that encodes a 202-amino acid protein with four predicted membrane spanning domains and two potential sites for N-linked glycosylation. Levels of this new member of the superfamily of tetraspan membrane proteins (TMPs) increase dramatically as nondividing epithelial cells exit the proliferative compartment of the crypt-villus unit and migrate onto the villus. The protein is also produced in nondividing hepatocytes that have the greatest proliferative potential within liver acini. Three sets of observations indicate that in the appropriate cellular context, intestinal and liver (il)-TMP can mediate density-associated inhibition of proliferation. (i) Accumulation of il-TMP glycoforms precedes terminal differentiation of HT-29-inosine cells and occurs as they undergo density-dependent cessation of growth. il-TMP levels are lower and glycosylation less extensive in HT-29-glucose cells, which do not undergo growth arrest at confluence. (ii) HeLa cells normally do not produce il-TMP. Forced expression of il-TMP inhibits proliferation as cells approach confluence. The extent of il-TMP glycosylation in the transfected cells is similar to that observed in HT-29-inosine cells and greater than in HT-29-glucose cells. (iii) SW480 cells are derived from a human colon adenocarcinoma and do not express il-TMP. Like nontransfected HeLa cells, they do not stop dividing at confluence, whether grown in medium containing glucose or inosine. Expression of il-TMP has no effect on the growth properties of SW480 cells. The extent of il-TMP glycosylation in SW480-glucose cells is similar to that noted in HT-29-glucose cells, lending further support to the notion that il-TMP's activity is related to its state of N-glycosylation.

Characterization of mouse CD53: epitope mapping, cellular distribution and induction by T cell receptor engagement during repertoire selection

The pan-leukocyte antigen CD53 is a member of the poorly understood transmembrane 4 superfamily (TM4SF) of cell membrane glycoproteins. CD53 is proposed to play a role in thymopoiesis, since rat CD53 is expressed on immature CD4-8-thymocytes and the functionally mature single-positive subset, but is largely absent from the intermediate CD4+8+ cells. We have characterized CD53 in the mouse through the production of two new monoclonal antibodies, MRC OX-79 and OX-80, which were raised against the RAW 264 cell line and screened on recombinant CD53 fusion proteins. The epitopes recognized by both antibodies are dependent on disulfide bonding and map to the major extracellular region of CD53, requiring the presence of a single threonine residue at position 154. Mouse CD53 has a molecular mass of 35-45 kDa and is expressed on virtually all peripheral leukocytes, but not on cells outside the lymphoid or myeloid lineages. CD53 expression distinguishes subpopulations of thymocytes in the mouse and resembles the expression pattern of rat CD53. Amongst the immature CD4-8-thymocytes, mouse CD53 is clearly detectable on the earliest CD44high25- subset, but down-regulated on the later CD44high25+, CD44low25+ and CD44low25- stages. Also, the subsequent transient TcR-/low CD4-8+ cells and most CD4+8+ thymocytes express little or no CD53. This is consistent with the idea that cells which are committed to enter the selectable CD4+8+ compartment switch off CD53. The effect of T cell receptor (TcR) engagement on the re-expression of CD53 on CD4+8+ thymocytes was studied both ex vivo and in vitro using F5 mice, transgenic for the H-2b/influenza nucleoprotein-peptide-specific TcR, back-crossed onto an H-2q or H-2b background of RAG-2-deficient mice. CD4+8+ thymocytes from non-selecting H-2q F5 mice are CD53 negative, but in vitro stimulation through the TcR dramatically induces CD53 expression. In contrast, a fraction of CD4+8+ thymocytes from positively selecting H-2b F5 transgenic mice express CD53. Therefore TcR engagement by selecting major histocompatibility complex peptide complexes, or surrogate ligands, induces CD53 expression on otherwise CD53-negative, non-selected CD4+8+ thymocytes. Whether CD53 itself participates as a signaling molecule in further stages of thymic selection is still a matter of speculation.

Association of the transmembrane 4 superfamily molecule CD53 with a tyrosine phosphatase activity

Cell surface proteins of the transmembrane 4 superfamily (TM4SF) are a newly characterized family of proteins which are presumed to span the plasma membrane four times. The function of this family of molecules is poorly understood, but based on monoclonal antibody studies there is some evidence that they may be involved in transmembrane signal transduction and regulation of cell proliferation, differentiation, or both, in a number of different cell types. CD53 is a member of this family that is expressed on leukocytes, and transduces activation signals through unknown mechanisms that may involve phosphorylation events. However, CD53 has never been shown to associate directly with kinases. Here, we show by immunoprecipitation from cell lysates of lymph nodes and a thymoma cell line, that immune complexes of rat CD53 contain tyrosine phosphatase activity. The CD53-associated phosphatase was able to dephosphorylate in vitro the phosphorylated tyrosine kinase Lck, as well as a synthetic substrate, and its activity was abrogated by a tyrosine phosphatase inhibitor. Although its identity has not been established, it is clear from depletion experiments that it is not CD45. CD63, a second member of the TM4SF, also co-precipitates a phosphatase activity from rat basophilic leukemia cells. These results demonstrate that the TM4SF members associate with tyrosine phosphatases. It seems possible that such associated phosphatases may contribute to the signal transduction capacity of TM4SF molecules.

Expression of cell interaction molecules by immature rat thymocytes during passage through the CD4+8+ compartment: developmental regulation and induction by T cell receptor engagement of CD2, CD5, CD28, CD11a, CD44 and CD53

Rat thymocytes of the T cell receptorlow (TcRlow) CD4+8+ subset which is the target of repertoire selection are heterogeneous with respect to expression of the cell interaction (CI) molecules CD2, CD5, CD11a/CD18 (LFA-1), CD28 and CD44. We show that this heterogeneity is due to the developmental regulation of these CI molecules during passage through the CD4+8+ compartment, and to up-regulation by TcR engagement. Thus, cohorts of CD4+8+ cells differentiating synchronously in vitro from their direct precursors, the immature CD4-8+ cells, were homogeneous with regard to CI molecule expression. Upon entry into the CD4+8+ compartment, they expressed relatively high levels of CD2 and CD44, and moderate levels of CD5, CD28 and CD11a. CD2, CD28 and CD44 were slightly down-regulated during the following 2 days, whereas CD5 slightly increased and CD11a remained constant. TcR stimulation using immobilized monoclonal antibodies resulted in rapid and dramatic up-regulation of CD2, CD5 and CD28 and, to a lesser extent, of CD11a and CD44. Finally CD53, a triggering structure absent from unstimulated CD4+8+ thymocytes was also rapidly induced by TcR stimulation. Inclusion of interleukin (IL)-2, IL-4, or IL-7 in this in vitro differentiation system did not affect the levels of CI molecules studied. Since the high levels of CI molecules induced by TcR-stimulation correspond to those found in vivo on TcRintermediate thymocytes known to be undergoing repertoire selection, these results suggest that upregulation of CI molecules by TcR engagement provides a mechanism by which thymocytes that have entered the selection process gain preferential access to further interactions with stromal and lymphoid cells in the thymus.

T cells from BB-DP rats show a unique cytokine mRNA profile associated with the IDDM1 susceptibility gene, Lyp

Diabetes prone biobreeding rats display several abnormalities in T cell numbers, T cell function and T cell surface phenotype which are associated with the onset of spontaneous disease. One of the most pronounced abnormalities in these animals is a marked T cell lymphopenia which is evident in both CD4+ and CD8+ peripheral T cell subsets. To gain a better understanding as to the nature of T cell responses in these animals, we have utilized RT-PCR to analyze the cytokine mRNA profiles of mitogen activated peripheral T cells derived from lymphopenic and non-lymphopenic animals. Our results suggest that inheritance of the lymphopenia gene, Lyp, is associated with a unique cytokine profile most similar to that previously described for mouse medullary thymocytes. In addition, cell surface staining of peripheral T cells from diabetes prone animals revealed a high frequency of Thyl+ cells, which is characteristic of both thymocytes and recent thymic emigrants. Following thymectomy, T cell responsiveness to a number of different stimuli is greatly reduced on a cell for cell basis as is the absolute number of surviving T cells. Taken collectively, our results suggest that the majority of the peripheral T cell pool in these diabetic prone rats consists of short lived, recent thymic emigrants which most likely also contain the effector cells required for initiation of diabetes.

The ins and outs of the transmembrane 4 superfamily

The recently discovered transmembrane 4 superfamily comprises a group of cell-surface proteins that are characterized by the presence of four hydrophobic domains, which are presumed to be membrane spanning. At least seven of these molecules are expressed on leukocytes, and it seems likely that they mediate signal transduction events that play a role in the regulation of cell development, activation, growth and motility.

Ontogeny of rat thymic dendritic cells

In the present study we have combined various in vivo and in vitro approaches to analyse the appearance and development throughout ontogeny and postnatal life of the dendritic cell (DC) populations of rat thymus. The in situ ultrastructural study demonstrated immature interdigitating cells (IDC)/DC in the thymus of 17-day-old embryonic rats, but thymic stromal cell cultures from 16-day-old fetal rats seemed to contain DC precursors which, after several days in culture, produced strongly class II major histocompatibility complex (MHC)-positive, mature DC. According to morphology and class II MHC expression we also defined three different DC populations in the late embryonic rat thymus; two of them, which remained in the adult rat thymus, could represent distinct developmental stages within the IDC/DC lineage. The third cell subset might be involved in a massive process of negative selection, presumably occurring at the end of fetal life in the rat thymus. In supporting the existence of thymic DC subpopulations, we also demonstrated a differential expression of various cell markers, including CD4, CD8, CD25, adhesion molecules and the antigen recognized by OX44 monoclonal antibody (mAb), on thymic DC during both embryonic and adult life. Their possible significance for the attributed functions to thymic DC are discussed extensively.

Induction of nitric oxide release by MRC OX-44 (anti-CD53) through a protein kinase C-dependent pathway in rat macrophages

Many membrane proteins are implicated in the control of cell function by triggering specific signaling pathways. There is a new family of membrane proteins, defined by its structural motifs, which includes several lymphoid antigens, but lacks a function. To study its biological role, we determined which signaling pathways are affected by the CD53 antigen, a prototypic member of this family, in rat macrophages. Activation of CD53 by cross-linking results in an increase in inositol phosphates and diacylglycerol and in Ca2+ mobilization, which are insensitive to pertussis or cholera toxins. There is a translocation of protein kinase C to the membrane accompanied by nitric oxide (NO) release in macrophages. This effect is the result of the expression of the inducible nitric oxide synthase (iNOS), which is dependent on protein kinase C and protein synthesis. These results have linked a new receptor with a specific pathway of NO induction and thus have opened up a novel aspect of NO regulation in cell biology.

Analysis of cell surface antigens on glucocorticoid-treated rat thymocytes with monoclonal antibodies

The effects of glucocorticoid (GC) on thymocytes have been utilized to investigate the maturation and differentiation of thymocytes, but these experiments have mainly been performed on mouse thymocytes. We investigated the cell surface antigens expressed by LEW rat thymocytes during thymic reconstitution after GC treatment. Three-color flow cytofluorometric analysis of CD4, CD8 and the T cell antigen receptor (TCR alpha beta) clearly demonstrated that normal rat thymocytes contain CD4-8+ TCR alpha beta- and CD4+8- TCR alpha beta- cells. After GC treatment, we observed significant increases in the percentages of CD4-8+ TCR alpha beta- and CD4+8- TCR alpha beta- cells. The extent of the increase in the percentage of CD4-8+ TCR alpha beta- cells was greater than that of CD4+8- TCR alpha beta- thymocytes. Two-color analysis of TCR alpha beta and major histocompatibility complex (MHC) class I antigen showed that GC treatment significantly increased the percentage of TCR alpha beta- MHC class Ihi cells. Three-color analysis of CD4, CD8 and MHC class I demonstrated that normal rat thymocytes contain CD4-8- MHC class Ihi cells, which increased in number after GC treatment. These results indicate that rat thymocytes contain no fewer CD4-8+ TCR alpha beta- and CD4+8- TCR alpha beta+ cells than do mouse thymocytes, and that CD4-8+ TCR alpha beta- cells predominate over CD4+8- TCR alpha beta- cells in LEW rat thymus. Rat CD4-8- cells seemed to be divided into two subsets of TCR alpha beta- MHC class Ihi and TCR alpha beta- MHC class I- cells.

Selective inhibition of immature CD4-CD8+ thymocyte proliferation, but not differentiation, by the thymus atrophy-inducing compound di-n-butyltin dichloride

Effects of the thymus atrophy-inducing organotin compound di-n-butyltin dichloride (DBTC) on the differentiation and proliferation of immature rat thymocyte subsets were studied in vivo and in vitro. Incubation of freshly isolated CD4-CD8- or immature CD4-CD8+ (characterized as CD4-CD53-) thymocytes with 10(-7) M DBTC for 18 hr did not affect cell recovery or their ability to differentiate to CD4-CD8+ cells and CD4+CD8+ or to CD4+CD8+ cells, respectively. The same treatment decreased the spontaneous as well as the phytohaemagglutinin (PHA)-induced proliferation in both subsets. However, the inhibition of proliferation by DBTC of immature CD4-CD8+, but not of CD4-CD8- thymocytes, appeared to increase with their growth rate. Data show that differentiation of immature thymocytes can proceed independently of proliferation and that DBTC causes thymus atrophy by selectively inhibiting the proliferation of immature CD4-CD8+ thymocytes. Administration to rats of DBTC via the diet for 14 days resulted in an initial decrease of thymoblast number by day 2, followed by a decrease in the total number of thymocytes by day 4. Total thymocyte numbers were lowest on day 7 and did not significantly change thereafter. CD4/CD8 thymocyte subset distributions were similar to controls on day 4, but on day 7 of feeding a marked reduction of the percentage of CD4+CD8+ thymocytes and consequently an increase of the percentages of the three other CD4/CD8 subsets were found. Thereafter, the CD4/CD8 subset distribution recovered, reaching near control values on day 14, despite the very low numbers of thymoblasts and of total thymocytes at that time. Data together indicate that DBTC reduces the production of CD4+CD8+ and mature single-positive thymocytes by selectively inhibiting immature CD4-CD8+ thymocyte proliferation but without affecting the differentiation capacity of these cells. This suggests that thymocyte proliferation and differentiation are separately regulated processes.

Analysis of early reconstitution events in the SCID mouse thymus following rat bone marrow cell transplantation

In this work, we provide comprehensive evidence that sublethally irradiated Thy-1.2 SCID mice can be used as a model system for thymus homing and reconstitution after intravenous transfer of rat bone marrow cells. Full short-term SCID mouse thymus reconstitutions were obtained using a plastic nonadherent low-density rat bone marrow cell subset. Cell counts and flow cytometric analysis showed that at 3 weeks post-transfer the SCID mouse thymus contained up to 41 x 10(6) Thy-1.1high rat lymphoid cells comprising the expected percentages and distribution of CD2+, CD5+, CD3+, alpha beta TCR+ and CD4+ CD8+ cells. As seen on cryostat sections, bone marrow-derived MHC class II+ accessory cells had already developed by 2 weeks post-transfer, preceding the thymocyte expansion apparent at 3 weeks. Thus, analysis of the early events of SCID thymus reconstitution by rat bone marrow cells shows that they closely resemble those described in congenic animals and points out the temporally distinct development of dendritic cells and thymocytes. The SCID mouse-rat chimera model system represents a new in vivo tool for manipulating rat T-cell differentiation from bone marrow-resident precursor cells and in addition supports our previous xenogeneic reconstitution studies performed in organ culture.

Recovery from chemically induced thymus atrophy starts with CD4- CD8- CD2high TcR alpha beta-/low thymocytes and results in an increased formation of CD4- CD8- TcR alpha beta high thymocytes

Regeneration of the thymus was studied in rats that were treated with a single oral dose of the organotin compound di-n-butyltin dichloride (DBTC). After an initial maximum depletion of cortical BrdU+ thymocytes on day 2 after treatment, repopulation appeared to start on day 3 as indicated by an increased number of BrdU+ cells in the subcapsular region. On day 5, when thymocyte depletion was most pronounced, a relative increase of BrdU+ cells was observed all over the cortex. In comparison with controls, the thymoblast population on day 5 appeared to harbour increased numbers of CD4- CD8- and immature CD4- CD8+ CD53- thymoblasts, while the number of CD4+ CD8+ blasts had decreased. In comparison with day 3, however, the number of CD4+ CD8+ blasts had increased again. Results together have been interpreted as indicative for thymus regeneration starting from CD4- CD8- blasts which differentiate to immature CD4- CD8+ and then to CD4+ CD8+ blasts. Further characterization revealed that the majority of the CD4- CD8- and CD4- CD8+ CD53- blasts expressed high levels of CD2 and no or low levels of T-cell receptor (TcR) alpha beta. The high expression of CD2 on repopulating thymoblasts may be an additional indication of their activated state and for a role of interaction with the ligand LFA-3 on thymic epithelial cells during this phase of thymocyte differentiation. The number of CD4- CD8- TcR alpha beta high cells was increased on day 5 after dosing. The origin of this population and the possible implication of its development during thymus regeneration after chemically induced thymus atrophy are discussed.

T cell receptor ligation induces interleukin (IL) 2R beta chain expression in rat CD4,8 double positive thymocytes, initiating an IL-2-dependent differentiation pathway of CD8 alpha+/beta- T cells

The role of interleukin (IL)2 in intrathymic T cell development is highly controversial, and nothing is known about IL-2R expression on thymocytes of the T cell receptor (TCR) alpha/beta lineage undergoing TCR-driven differentiation events. We analyze here IL-2R alpha and beta mRNA expression in an in vitro system where newly generated rat CD4,8 double positive (DP) thymocytes respond to TCR ligation plus IL-2 (but not to either stimulus alone) with rapid differentiation to functional CD8 single positive T cells (Hünig, T., and R. Mitnacht. 1991. J. Exp. Med. 173:561). TCR ligation induced expression of IL-2R beta (but not alpha) chain mRNA in DP thymocytes. Addition of IL-2 then lead to functional maturation and expression of the IL-2R alpha chain. To investigate if the CD8 T cells generated via this IL-2R beta-driven pathway in vitro correspond to the bulk of CD8 T cells seeding peripheral lymphoid organs in vivo, we compared their phenotype to that of lymph node CD8 T cells. Surprisingly, analysis of CD8 cell surface expression using a novel anti-CD8 monoclonal antibody specific for the alpha/beta heterodimeric isoform, and of CD8 alpha and beta chain mRNA revealed that T cells generated by TCR ligation plus IL-2 resemble thymus-independent rather than thymus-derived CD8 cells in that they express CD8 alpha without beta chains. These findings demonstrate that TCR crosslinking induces functional IL-2R on immature DP rat thymocytes. In addition, they show that at least in vitro, CD8 alpha/alpha T cells are generated from TCR-stimulated DP thymocytes (which express the CD8 alpha/beta in the heterodimeric isoform) along an IL-2-driven pathway of T cell differentiation.

Epitope mapping of anti-rat CD53 monoclonal antibodies. Implications for the membrane orientation of the Transmembrane 4 Superfamily

CD53 is a pan-leukocyte glycoprotein which is a member of the recently described Transmembrane 4 Superfamily (TM4SF) of membrane proteins that are predicted to span the lipid bilayer four times. The major hydrophilic region of murine CD53 was expressed as a glutathione-S-transferase fusion protein, and the epitopes of four mouse anti-rat CD53 monoclonal antibodies (mAb) (OX-44, 2D1, 6E2 and 7D2) were mapped to this region using mouse/rat chimeric fusion proteins. The epitopes of OX-44, 6E2 and 7D2 are restored by the substitution of a single isoleucine residue for threonine at position 154 in the mouse protein. The 2D1 epitope is non-linear and appears to require the juxtaposition of isoleucine at position 154 with one or more of the amino acids arginine (132), methionine (133) and serine (140). All of these epitopes are shown to be sensitive to reduction, thus indicating the importance of disulfide bonding in the correct folding of the CD53 hydrophilic domain. Moreover, as these four mAb recognize CD53 at the cell surface, the data provide direct molecular evidence for the proposed membrane orientation of the TM4SF.

Association of Src-like protein tyrosine kinases with the CD2 cell surface molecule in rat T lymphocytes and natural killer cells

The cell surface molecule CD2 has a signaling role in the activation of T lymphocytes and natural killer cells. Because perturbation of CD2 leads to the appearance of tyrosine-phosphorylated proteins, we investigated the possibility that CD2 associates with cytoplasmic protein tyrosine kinases. As determined by in vitro kinase assays and phosphoamino acid analysis, protein tyrosine kinase activity coprecipitated with CD2 from rat T lymphocytes, T lymphoblasts, thymocytes, interleukin-2-activated natural killer cells, and RNK-16 cells (a rat natural killer cell line). In each case, both p56lck and p59fyn were identified in the CD2 immunoprecipitate. In the thymus, the association between CD2 and these kinases occurred predominately in a small subset of thymocytes that had the cell surface phenotype of mature T cells, indicating that the association is a regulated event and occurs late in T-cell ontogeny. The finding that CD2 is associated with p56lck and p59fyn in detergent lysates suggests that interactions with these Src-like protein kinases play a critical role in CD2-mediated signal transduction.

Association of Src-like protein tyrosine kinases with the CD2 cell surface molecule in rat T lymphocytes and natural killer cells

The cell surface molecule CD2 has a signaling role in the activation of T lymphocytes and natural killer cells. Because perturbation of CD2 leads to the appearance of tyrosine-phosphorylated proteins, we investigated the possibility that CD2 associates with cytoplasmic protein tyrosine kinases. As determined by in vitro kinase assays and phosphoamino acid analysis, protein tyrosine kinase activity coprecipitated with CD2 from rat T lymphocytes, T lymphoblasts, thymocytes, interleukin-2-activated natural killer cells, and RNK-16 cells (a rat natural killer cell line). In each case, both p56lck and p59fyn were identified in the CD2 immunoprecipitate. In the thymus, the association between CD2 and these kinases occurred predominately in a small subset of thymocytes that had the cell surface phenotype of mature T cells, indicating that the association is a regulated event and occurs late in T-cell ontogeny. The finding that CD2 is associated with p56lck and p59fyn in detergent lysates suggests that interactions with these Src-like protein kinases play a critical role in CD2-mediated signal transduction.

Molecular associations between the T-lymphocyte antigen receptor complex and the surface antigens CD2, CD4, or CD8 and CD5

The T-cell antigen receptor (TCR) complex is the key structure involved in signal transduction in T cells. To analyze associations between the TCR complex and other molecules, immunoprecipitations were carried out, followed by phosphorylation of molecules in vitro by tyrosine kinases associated with the precipitated molecules. This provided a sensitive assay for molecular complexes, and associations were demonstrated between the TCR complex and the surface antigens CD2, CD4, or CD8 and CD5 in normal rat T cells. The complexes were readily seen in immunoprecipitates from Brij 96 but not Nonidet P-40 detergent extracts. The multimolecular complexes are associated with the internal tyrosine kinases p56lck and p59fyn. The presence of p56lck associated with CD4 or CD8 was also examined in early thymocytes, natural killer cells, and macrophages. The kinase was present in all cases except that of normal macrophages.

The extent of clonal structure in different lymphoid organs

To gain insight into the clonal organization of lymphoid organs, we studied the distribution in situ of donor-derived cells in near-physiological chimeras. We introduced RT7b fetal liver cells into nonirradiated congenic RT7a neonatal rats. The chimerism 6-20 wk after injection ranged from 0.3 to 20%. The numbers of cell clones simultaneously contributing to cell generation in a particular histological feature were deduced from the variance in donor cell distribution. In bone marrow and thymus, donor-derived lymphoid cells were found scattered among host cells, indicating a high mobility of cells. In bone marrow, donor cells were evenly distributed over the entire marrow, even at low chimerism. This indicates that leukopoiesis is maintained by the proliferation of many clones. In the thymus, the various lobules showed different quantities of donor-derived lymphoid cells. Mathematical analysis of these differences indicated that 17-18 cell division cycles occur in the cortex. In spleen, the distribution of donor-derived cells over the germinal centers indicated that 5 d after antigenic stimulation, germinal centers develop oligoclonally. The main conclusions of this work are that (a) bone marrow and thymus are highly polyclonal; (b) 17-18 divisions occur between prothymocyte and mature T cell; and (c) lymphoid cells disperse rapidly while proliferating and differentiating.

Differential expression of granzyme A and granzyme B proteases and their secretion by fresh rat natural killer cells (NK) and lymphokine-activated killer cells with NK phenotype (LAK-NK)

Granzymes A-G are a family of serine proteases localized in the cytoplasmic granules of cytotoxic T lymphocytes (Masson, D. et al. Cell 1987. 49: 679) and granzyme A is secreted by T lymphocytes in response to antigenic stimulation. Granzyme A is also expressed by natural killer (NK) and lymphokine-activated killer (LAK) cells. Here we show that fresh rat NK cells constitutively express granzyme B and that granzyme A and granzyme B are differentially regulated in unstimulated NK cells vs. LAK cells with NK phenotype (LAK-NK cells). We also show that both granzymes A and B are secreted in a calcium-dependent manner, by NK and LAK-NK cells in response to stimuli which trigger NK cell functions.

Effects of the color additive caramel color III and 2-acetyl-4(5)-tetrahydroxybutylimidazole (THI) on the immune system of rats

Administration of ammonia caramel color (AC) to rats may decrease blood lymphocyte counts, specifically in rats fed a diet low in vitamin B6. This effect is associated with 2-acetyl-4(5)-(1,2,3,4-tetrahydroxybutyl)imidazole (THI). To characterize and compare the effects of AC and THI and to study the influence of dietary pyridoxine, two studies in rats were conducted. Weanling rats fed a diet containing 2-3 ppm pyridoxine and exposed to 4% AC or 5.72 ppm THI in drinking water for 4 weeks showed reduced cell numbers in spleen and popliteal lymph nodes, as well as in the blood. Flow cytometric analyses demonstrated a comparable reduction in B and T lymphocytes. In blood, spleen, and popliteal lymph nodes, CD4+ lymphocytes were more reduced than CD8+ cells. The number of bone marrow cells was not affected. Although thymus weight and cell number were not affected either, a decreased cortex over medulla area ratio and an increase in medullary cell density largely due to an increase in CD4+ thymocytes was observed. Decreased numbers of ED2+ macrophages were observed in the thymic cortex and in the spleen red pulp. All effects observed were either less pronounced or absent in a study with rats fed a diet containing 11-12 ppm pyridoxine. The effects of AC and THI on the immune system were similar. Whereas AC exposure was associated with changes in vitamin B6 status, THI did not induce obvious effects on vitamin B6 parameters. It is proposed that the effects of AC and THI on the immune system are not caused by a disturbance of vitamin B6 metabolism, but may in fact result from a disturbance of a specific PLP-dependent process.

The OX-44 molecule couples to signaling pathways and is associated with CD2 on rat T lymphocytes and a natural killer cell line

The MRC OX-44 molecule, which is expressed on all peripheral leukocytes, identifies the subset of thymocytes capable of proliferating in response to alloantigens and lectins (Paterson, D.J., J.R. Green, W.A. Jefferies, M. Puklavec, and A.F. Williams. 1987. J. Exp. Med. 165:1). When we isolated monoclonal antibodies (mAbs) on the basis of their ability to activate the phosphatidylinositol signaling pathway in RNK-16 cells (a rat leukemia line with natural killer activity), three of the resulting mAbs recognized the OX-44 molecule. Addition of these mAbs to RNK-16 elicits protein tyrosine phosphorylation, generates inositol phosphates, and increases the concentration of cytoplasmic free calcium. These responses require the addition of intact mAb and are not observed with F(ab')2 fragments. One of these mAbs (7D2) is mitogenic for freshly isolated rat splenic T cells and synergizes with a mAb to the T cell antigen receptor in this activation. A 50-60-kD glycoprotein coprecipitates with the OX-44 molecule from RNK-16 cells and rat splenic T cells. Peptide mapping and reprecipitation studies indicate that the coprecipitating molecule is CD2. Thus, the OX-44 molecule can couple to multiple signaling pathways and associates with CD2 on both RNK-16 and rat T cells.

CD8 gamma delta cells: presence in the adult rat thymus and generation in vitro from CD4-/CD8- thymocytes in the presence of interleukin 2

Three to fifteen percent of peripheral T cells in adults express the recently described gamma delta T-cell antigen receptor (TcR) heterodimer. A small subpopulation of gamma delta cells express the CD8 accessory molecule. In this study, we analyzed the potential of highly purified CD4-/CD8-, double negative (DN) rat precursor thymocytes to give rise to gamma delta cells. We observed that in the presence of interleukin 2 (IL-2) and concanavalin A (ConA), both DN and CD8 cells expressing the gamma delta TcR were generated in vitro. We then examined the rat thymus for these cells and confirmed the presence of a previously undescribed CD8 TcR-alpha beta- subset in the rat thymus, expressing high levels of TcR-gamma and delta messages with no detectable TcR-alpha transcripts, similar to the cells generated in vitro in the presence of IL-2 and ConA.

Rat "first-wave" mature thymocytes: cycling lymphoblasts that are sensitive to activation-induced cell death but rescued by interleukin 2

The sequential appearance of thymocyte subsets in rat ontogeny was studied using the surface markers CD4, CD8, and the alpha/beta T cell receptor (here referred to as TcR). It was noted that the first TcRhigh thymocytes, appearing around birth, are not resting lymphocytes but cycling blast cells. These "first wave" TcRhigh cells are medullary in location and predominantly of the CD4/CD8 "single-positive" phenotype. Only about 5% express the light chain of the interleukin (IL)2 receptor, indicating that binding of IL 2 to high-affinity receptors is not driving proliferation of these blasts. Newborn TcR high blast cells were purified and analyzed in vitro. When cultured without further additions, they rapidly stopped cycling. Stimulation with cross-linked anti-TcR monoclonal antibody plus IL 2 resulted in vigorous and rapid proliferation that exhibited accelerated kinetics as compared to peripheral resting T cells. In contrast, TcR cross-linking without exogenous IL 2 induced cell death. This TcR-induced cell death involved fragmentation of DNA characteristic of apoptosis that was readily detectable within 18 h of culture. Addition of IL 2 to these short-term cultures prevented TcR-induced DNA fragmentation. Together, these results suggest that in newly formed TcRhigh thymocytes, TcR engagement results in clonal deletion if the IL 2 receptor remains unoccupied but allows clonal expansion if IL 2 is provided. This mechanism may be operative in the establishment of self-tolerance during T cell development.

The rat leukocyte antigen MRC OX-44 is a member of a new family of cell surface proteins which appear to be involved in growth regulation

Moloney murine leukemia virus (MoMuLV)-induced rat T-cell lymphomas express discrete 1.8-, 2.2-, and 4-kb mRNA transcripts hybridizing under conditions of reduced stringency to a probe derived from a region upstream of the first exon of the Tpl-1/Ets-1 gene. Screening a cDNA library from one rat T-cell lymphoma with this genomic probe yielded 15 cDNA clones which were derived from 10 different genes. One of these genes, defined by the cDNA clone pRcT7a, was expressed as a 1.8-kb mRNA transcript in spleen and thymus but not in other normal rat tissues. Expression of the gene defined by the pRcT7a cDNA clone in a series of MoMuLV-induced rat T-cell lymphomas showed a perfect correlation with the expression of the rat leukocyte antigen MRC OX-44. Because of this observation, the pRcT7a clone was sequenced and it was shown to identify a gene coding for a 219-amino-acid protein. The homology between pRcT7a and the Tpl-1 probe used for its detection mapped within the 3' untranslated region of the pRcT7a cDNA clone. The pRcT7a protein, which exhibits four putative transmembrane regions and three putative glycosylation sites, contains a region which is nearly identical in sequence to a peptide derived from the rat leukocyte antigen MRC OX-44. This finding suggested that the pRcT7a cDNA clone defines the gene coding for OX-44. To confirm this finding, a pRcT7a construct in the retrovirus vector pZipNeo was introduced into the OX-44- T-cell lymphoma line 2788. Immunostaining with the MRC OX-44 monoclonal antibody followed by flow cytometry revealed that following gene transfer, the 2788 cells became OX-44+. Sequence comparisons revealed that pRcT7a/MRC OX-44 is a member of a family of genes which includes the melanoma-specific antigen ME491; the human leukocyte antigen CD37; the protein TAPA-1, which is expressed on the surface of human T cells and appears to be involved in growth regulation; the human gastrointestinal tumor antigen CO-029; and the Schistosoma mansoni-associated antigen Sm23.

The rat leukocyte antigen MRC OX-44 is a member of a new family of cell surface proteins which appear to be involved in growth regulation

Moloney murine leukemia virus (MoMuLV)-induced rat T-cell lymphomas express discrete 1.8-, 2.2-, and 4-kb mRNA transcripts hybridizing under conditions of reduced stringency to a probe derived from a region upstream of the first exon of the Tpl-1/Ets-1 gene. Screening a cDNA library from one rat T-cell lymphoma with this genomic probe yielded 15 cDNA clones which were derived from 10 different genes. One of these genes, defined by the cDNA clone pRcT7a, was expressed as a 1.8-kb mRNA transcript in spleen and thymus but not in other normal rat tissues. Expression of the gene defined by the pRcT7a cDNA clone in a series of MoMuLV-induced rat T-cell lymphomas showed a perfect correlation with the expression of the rat leukocyte antigen MRC OX-44. Because of this observation, the pRcT7a clone was sequenced and it was shown to identify a gene coding for a 219-amino-acid protein. The homology between pRcT7a and the Tpl-1 probe used for its detection mapped within the 3' untranslated region of the pRcT7a cDNA clone. The pRcT7a protein, which exhibits four putative transmembrane regions and three putative glycosylation sites, contains a region which is nearly identical in sequence to a peptide derived from the rat leukocyte antigen MRC OX-44. This finding suggested that the pRcT7a cDNA clone defines the gene coding for OX-44. To confirm this finding, a pRcT7a construct in the retrovirus vector pZipNeo was introduced into the OX-44- T-cell lymphoma line 2788. Immunostaining with the MRC OX-44 monoclonal antibody followed by flow cytometry revealed that following gene transfer, the 2788 cells became OX-44+. Sequence comparisons revealed that pRcT7a/MRC OX-44 is a member of a family of genes which includes the melanoma-specific antigen ME491; the human leukocyte antigen CD37; the protein TAPA-1, which is expressed on the surface of human T cells and appears to be involved in growth regulation; the human gastrointestinal tumor antigen CO-029; and the Schistosoma mansoni-associated antigen Sm23.

T cell receptor-mediated selection of functional rat CD8 T cells from defined immature thymocyte precursors in short-term suspension culture

Recent results have indicated that positive and negative repertoire selection act on the major population of CD4,8 double-positive (DP) thymocytes that express 5-10-fold less T cell receptor (TCR) than mature T cells (i.e., they are TCRlow). Since DP cells obtained ex vivo are heterogeneous with regard to their stage within thymic selection, a homogeneous population of virgin DP cells suitable for selection studies was generated in vitro from their immediate precursors, the CD8 single-positive (SP) immature blast cells. To mimic TCR-mediated selection signals, these virgin DP cells were then cultured for another 2 d in the presence of immobilized anti-TCR monoclonal antibodies with or without interleukin 2 (IL-2). Daily monitoring of recovery and phenotype showed that without TCR stimulation, the cells remained DP and became small, TCRlow cells that were lost with a half-life of 1 d, regardless of the presence of IL-2. TCR stimulation resulted in rapid downregulation of CD4 and CD8, maintenance of a larger cell size, and induction of the CD53 antigen that marks mature and CD4,8 double-negative rat thymocytes. In the absence of IL-2, viability decreased as rapidly as without TCR stimulation. Addition of IL-2 rescued TCR-stimulated virgin DP cells and prevented CD8 downregulation, so that 50-80% of input DP cells were recovered after 2 d as CD4-8+53+ cells. After release from modulation, these in vitro generated CD8 SP cells quantitatively upregulated the TCR to the TCRhigh phenotype and were readily induced to proliferate and exhibit cytotoxic T lymphocyte (CTL) activity in a polyclonal readout. Evidence is presented implicating an IL-2 receptor (IL-2R) not containing the p55 chain (i.e., most likely the p70 intermediate affinity IL-2R) in the TCR plus IL-2-driven in vitro differentiation of virgin DP cells towards the mature CD8 SP phenotype.

The human leucocyte surface antigen CD53 is a protein structurally similar to the CD37 and MRC OX-44 antigens

CD53 is an N-glycosylated pan-leucocyte antigen of 35-42,000 Mr. The sequence of the CD53 polypeptide deduced from a cDNA clone is 219 amino acids in length. It appears to lack a conventional leader sequence because the deduced NH2-terminal amino acid sequence is very similar to the rat MRC OX-44 and human CD37 antigens. The CD53 molecule is likely to consist of four transmembrane regions and a major extracellular hydrophilic loop containing two potential N-glycosylation sites. It is suggested that the CD53 glycoprotein is the true human homologue of the rat OX-44 antigen, rather than the CD37 antigen of more restricted expression and lower NH2-terminal sequence similarity to OX-44.

T-cell lymphoma lines derived from rat thymomas induced by Moloney murine leukemia virus: phenotypic diversity and its implications

The phenotype of 27 Moloney murine leukemia virus-induced rat thymic lymphomas and 36 cell lines derived from these tumors was determined by using 18 monoclonal antibodies directed against hematopoietic cell surface determinants. The cell lines and the primary tumors from which they were derived were clonally related as determined by the pattern of provirus integration and the pattern of rearrangement of the T-cell receptor beta and delta and Igh loci. The differentiation phenotype of the primary tumors and the cell lines derived from them were related. The differences observed between the primary tumors and the cell lines could be explained either by the selection of subpopulations of tumor cells during establishment in culture or by the phenotypic instability of the tumor cells. One cell line (LE3Sp) underwent the transition from a CD4+ CD8+ to a CD4+ CD8- phenotype following exposure to interleukin-2 in culture. Both the primary tumors and the cell lines derived from them express a wide range of phenotypes which correspond to multiple stages in T-cell development. This observation suggests that the pleiomorphism of retrovirus-induced lymphomas, which had been suggested previously from the analysis of mouse tumors, is an intrinsic property of the process of oncogenesis and is not due to the transformation of different types of cells by spontaneously arising leukemogenic variants of the inoculated virus. The wide spectrum of phenotypes expressed by these tumors suggests that Moloney murine leukemia virus may infect and transform T cells at various stages of development. Alternatively, the target cells may be immature T-cell precursors which, following transformation, continue to differentiate. A host of early findings, suggesting that the repertoire of target cells is restricted to poorly differentiated hematopoietic progenitors, and the ability of the LE3Sp cell line to differentiate in culture indicate that the latter possibility may be more likely. The data in this report address the extent and mechanism of the phenotypic variability of retrovirus-induced rodent T-cell lymphomas. In addition, they demonstrate the potential usefulness of the T-cell lymphoma lines we have established in studies of oncogenesis and T-cell differentiation.

Repopulation of the atrophied thymus in diabetic rats by insulin-like growth factor I

Atrophy of the thymus is one of the consequences of severe insulin deficiency. We describe here that the weight and the architecture of the thymus of diabetic rats is restored towards normal not only by insulin but also by insulin-like growth factor I (IGF-I) treatment. In contrast to insulin, this effect of IGF-I occurs despite persisting hyperglycemia and adrenal hyperplasia. We also investigated the in vivo effect of IGF-I on replication and differentiation of thymocytes from streptozotocin-induced diabetic rats. Thymocytes from diabetic rats incorporated less [3H]thymidine than did thymocytes from healthy rats. Insulin, as well as IGF-I treatment of diabetic rats increased [3H]thymidine incorporation by thymocytes. Flow cytometry of thymocytes labeled with monoclonal antibodies revealed a decreased expression of the Thy-1 antigen in diabetic rats compared with control rats. In addition, a major deficiency of thymocytes expressing simultaneously the W3/25 and the Ox8 antigens (corresponding to immature human CD4+/CD8+ thymocytes) was observed. These changes were restored towards normal by insulin as well as by IGF-I treatment. The antibody response to a T cell-dependent antigen (bovine serum albumin) was comparable in normal and diabetic rats. We conclude that IGF-I has important effects on the thymocyte number and the presence of CD4+/CD8+ immature cells in the thymus of diabetic rats despite persisting hyperglycemia. However, helper T-cell function for antibody production appears to be preserved even in the severely diabetic state.

STB1, a mouse lymphocyte marker found on T cell and B cell subpopulations

A set of monoclonal antibodies has been raised to cell surface markers which are either hyperexpressed, or exclusive to activated murine lymphocytes. One antigen is present on all B cells of the Ly1 lineage and defines a novel subpopulation of classical B cells, most of which are activated B cells. In Western blots, this antigen appears as a single glycoprotein of 95 kDa molecular weight but of greater interest, is its expression at high levels on all mature peripheral T cells but on only 30% of thymocytes. This marker is expressed at high levels on all CD4 and CD8 thymocytes, most double-negative (CD4-CD8-) thymocytes, and at low levels on double-positive (CD4+CD8+) thymocytes. It may prove useful as a developmental marker for subdividing the double-positive (CD4+CD8+) and double negative (CD4-CD8-) thymocyte populations. Taken together these results indicate that mab 3.17 recognises a novel lymphocyte antigen expressed on subsets of T and B cells, which is hyperexpressed on lymphocyte activation.

Astrocytes as antigen presenting cells for primary and secondary T cell responses: effect of astrocyte infection by murine hepatitis virus

CD4+ T cell lines specific for murine hepatitis virus (MHV) - JHM or myelin basic protein (MBP) proliferated when cultured together with MHC class I and II positive syngeneic rat astrocytes and either inactivated virus or MBP as antigen. The magnitude of the T cell proliferative response was comparable to that seen when thymocytes were used as a source of antigen presenting cells (APC). In contrast, MHC class I and II positive astrocytes were unable to significantly stimulate the proliferation of highly purified populations of naive CD4+ and CD8+ T cells in an allogeneic mixed lymphocyte reaction (MLR). Both T cell populations proliferated when mixed with allogeneic lymph node cells. Infection of the astrocytes with a variant of MHV-JHM (PI-AS22D) did not alter this cells incapacity to stimulate the naive CD4+ and CD8+ T cells to proliferate.

Subsets of thymopoietic rat thymocytes defined by expression of the CD2 antigen and the MRC OX-22 determinant of the leukocyte-common antigen CD45

The MRC OX-22 monoclonal antibody recognizes a restricted determinant of the rat leukocyte-common antigen (L-CA, CD45), which is expressed on most peripheral T cells and all B cells. In contrast only 2%-3% of thymocytes are OX-22+ and these are now shown to be mostly of the immature CD4-CD8- (double-negative, DN), phenotype with very few of the mature phenotype cells being OX-22+. Analysis of immunoperoxidase sections suggests that the DN OX-22+ cells are located in the cortex. Among the DN cells about 60% are OX-22+ and a similar percentage are positive for CD2 antigen. Double staining showed that OX-22+CD2-, OX-22+CD2+, and OX-22-CD2+ populations can be defined and that these three sets account for approximately 95% of the DN cells. Measurement of the thymopoietic activity of DN subsets showed that OX-22+CD2- and OX-22+CD2+ cells have regenerative capacity whilst OX-22-CD2+ cells do not.

Surface phenotype of rat bone marrow-derived mononuclear phagocytes

Utilizing a panel of currently available monoclonal antibodies, the surface phenotype of a pure population of resting rat bone marrow-derived mononuclear phagocytes (BMM phi) was analyzed by means of flow cytometry. The present work provides an extensive list of surface markers expressed by BMM phi and also outlines advantages and limitations of flow cytometric analysis of this cell type. The results show that the majority of surface markers considered to be expressed selectively by T lymphocytes, such as Thy-1, CD2 and CD5 antigens, leukosialin (W3/13), or an alloantigen of peripheral T cells, are not expressed by BMM phi. On the other hand, the CD8 antigen and the leukocyte common antigen recognized by MRC OX-33, considered to represent specific markers of cytotoxic T cells and/or peripheral B cells, are expressed on a variable, often considerable proportion of BMM phi. Monoclonal antibodies W3/25, MRC OX-35, and MRC OX-38, directed against epitopes on the CD4 molecule, labeled a variable proportion of BMM phi. Among the 39 monoclonal antibodies examined, none appeared to recognize an epitope which is expressed selectively by mononuclear phagocytes.

The primary structure of the human leukocyte antigen CD37, a species homologue of the rat MRC OX-44 antigen

Comparison of NH2-terminal protein sequence from the rat OX-44 antigen with the sequence of the human CD37 antigen deduced from a cDNA clone shows that these antigens are species homologues. The CD37 sequence is 244 amino acids in length and lacks a conventional leader sequence. The molecule is likely to have an NH2-terminal cytoplasmic domain followed by three transmembrane sequences that lie within the first 110 amino acids. The rest of the molecule is hydrophillic and contains three sites for N-linked glycosylation.

The phenotypic and molecular characterization of Nb2 lymphoma cells activated with IL-2 and human growth hormone

The Nb2 rat lymphoma cell line has the unique property that its growth is dependent on lactogenic pituitary hormones. Cell surface staining with monoclonal antibodies showed expression of class I MHC alloantigens of the RT1u haplotype, but no expression of class II MHC antigens. Staining for differentiation markers was strongly positive with antibodies OX52, W3/13 and OX44. Partial and weaker staining was obtained with CD2, P4/16 and the transferrin receptor. Nb2 cells were negative with CD5, OX40 and CD4, whilst CD8 stained only a minor fraction (1%) and certain variant clones of the cell line. This general pattern of staining is consistent with the phenotype of a small subpopulation of immature T cells. Nb2 cells proliferated in response to recombinant human IL-2, although they did not stain with antibodies against the IL-2 receptor. Enhancement of the stimulation by IL-2 in the presence of a submitogenic concentration of hGH indicated a synergism between these two hormones, and responses were suppressed by a similar dose of cyclosporin A (ID50=2 microg/ml). Although IL-2 could not be identified in culture supernatants, the presence of mRNA for IL-2, IL-2R and IL-4 was demonstrated by dot blot analysis. Finally, evidence that the Nb2 lymphoma is of T-cell lineage was given by Northern blot detection of mRNA for the alpha and beta chains of the T-cell receptor.