Intercellular communication and cell-cell adhesion

The seven-transmembrane spanning topography of the Alzheimer disease-related presenilin proteins in the plasma membranes of cultured cells

To ascertain the membrane topography of the multi-transmembrane spanning presenilin proteins PS-1 and PS-2, anti-peptide antibodies were raised to several specific amino acid sequences in the two proteins, and, after their specificity was ascertained, the anti-peptide antibodies were used in immunofluorescent labeling of live PS-transfected, cultured DAMI cells, which are impermeable to the antibodies, as well as of their fixed and permeabilized counterparts. In such experiments, antibodies that specifically stain the intact live cells must label epitopes of the PS proteins that are on the exterior face of the plasma membrane whereas those antibodies that do not stain the live cells but do stain the fixed and permeabilized cells must label epitopes that face the cytoplasmic side of the membrane. The results obtained were entirely in accord with the predictions of the seven-transmembrane spanning topography (like that of rhodopsin and the beta-adrenergic receptor) and were totally inconsistent with the expectations for either the six- or eight-transmembrane topographies that have been proposed.

On the spurious endoproteolytic processing of the presenilin proteins in cultured cells and tissues

It has been widely reported that the presenilin proteins PS-1 and PS-2 in extracts derived from a variety of cultured cells and from tissues are fragmented extensively by endoproteolytic processing events. It generally has been presumed that this endoproteolysis is a physiologically normal intracellular event following presenilin expression, which might play an important role in the still unknown functions of these molecules in connection with Alzheimer disease. We demonstrate herein, however, that, if a variety of cultured cells and several mouse tissues are examined under conditions minimizing cell trauma, the presenilin molecules in the extracts are found to be intact but that, if the cells and tissues are prepared under somewhat more stressful conditions, the endoproteolytic fragments are then observed. We conclude that these particular endoproteolytic events are not the result of physiologically normal processing of the presenilins but are rather artifacts occurring during the common procedures of specimen preparation.

Immunology of sarcoidosis

Because of its association with cutaneous anergy, sarcoidosis was originally viewed as a defect of cellular immunity. Supporting that misperception were early studies of peripheral blood lymphocytes that found lymphopenia and impaired lymphocyte responses to mitogens and recall antigens. The clue to a vast underlying network of complex hyperactive cellular immune functions was discovered in the paradoxical finding of in vitro spontaneous lymphoblastic transformation and lymphokine production. Subsequently, investigative focus shifted to the activated, proliferating T-helper lymphocytes, the lymphokines of which were found to function in the recruitment and retention of monocytes for granuloma development. T-helper lymphocytes also contributed to the mechanism of hypergammaglobulinemia through their influence on B cells. The most intriguing question about sarcoid immunology is the initiating factor that triggers the T-lymphocyte activation and proliferation in the first place. There is much to suggest that antigen processing and presentation launches the process. Because lymphocyte activation and proliferation antedate granuloma formation at K-S skin test sites and in the lung, we combined the harvesting technique of BAL with the K-S bioassay to show that granulomagenic antigen is being processed by monocyte-macrophages. The finding of autologous monocyte-macrophage granulomagenicity raises the distinct possibility that sarcoidosis is a unique cell-mediated type of autoimmune process. The isolation and identification of the granulomagenic factor is the exciting research frontier ahead.

Cell surface expression of the Alzheimer disease-related presenilin proteins

The presenilin proteins PS-1 and PS-2 are crucially involved in Alzheimer disease (AD), but their molecular functions are not known. They are integral membrane proteins, but whether they can be expressed at the surface of cells has been in dispute. Here we show by immunofluorescence experiments, using anti-peptide antibodies specific for either PS-1 or PS-2, that live cultured DAMI cells and differentiated human NT2N neuronal cells are specifically immunolabeled for their endogenous as well as transfected presenilins, although the cells cannot be immunolabeled for their intracellular tubulin, unless they are first fixed and permeabilized. These and other results establish that portions of the presenilins are indeed expressed at the surfaces of these cells. These findings support our previous proposal that the presenilins on the surface of a cell engage in intercellular interactions with the beta-amyloid precursor protein on the surface of a neighboring cell, as a critical step in the molecular and cellular mechanisms that lead to AD.

Internalization of Escherichia coli by human renal epithelial cells is associated with tyrosine phosphorylation of specific host cell proteins

Human renal epithelial cells are capable of internalizing Escherichia coli regardless of whether the bacteria are isolated from individuals with pyelonephritis or from healthy volunteers. In this study, we investigated the role of host cell tyrosine kinase activity in internalization. We found that internalization of both fecal and pyelonephritis isolates is blocked by tyrosine kinase inhibitors. We found increased intensity of two tyrosine-phosphorylated proteins, with relative mobilities of approximately 123,000 and 110,000, in Western blots of extracts from human renal epithelial cells infected with E. coli. The increased intensity of these tyrosine-phosphorylated proteins was observed only in the Triton X-100-insoluble fraction, suggesting that these proteins could be associated with the cytoskeleton. Increased tyrosine phosphorylation of these proteins upon E. coli infection was observed in both transformed and primary human renal epithelial cells and in cells infected with several different strains of E. coli isolated from the feces of healthy individuals or from the blood or urine of patients with pyelonephritis. The increased tyrosine phosphorylation of these proteins required live bacteria and was blocked by tyrosine kinase inhibition but not by protein synthesis inhibitors or cytochalasin D. These experiments establish a strong link between E. coli internalization and host cell signaling through tyrosine kinases in human kidney cells and provide evidence that specific proteins are involved in these processes.

Decreased IgG-Fc gamma RII dissociation kinetics in the presence of a protein antigen

Total internal reflection fluorescence microscopy has been used to examine the interaction of a mouse monoclonal IgG2b, in the absence and presence of its protein antigen, with mouse Fc gamma RII in substrate-supported planar membranes. Equilibrium association and kinetic dissociation constants were measured for the antibody S6-34.11, which is specific for bovine prothrombin fragment 1 (BF1). These measurements showed that BF1 induces a statistically significant decrease (30-40%) in the IgG-Fc gamma RII dissociation kinetics. A corresponding increase in the equilibrium association constant was not observed, perhaps because the statistical accuracy of the equilibrium measurements is lower than that for the kinetic measurements. The consequences of these results for understanding the mechanism by which macrophages recognize and ingest opsonized targets are discussed.

Immunosuppression by inhibition of cellular adhesion mediated by leukocyte function-associated antigen-1/intercellular adhesion molecule-1 in murine cardiac transplantation

BACKGROUND

Donor alloantigen-specific tolerance to vascularized allografts can be induced by several treatments, but the immunological mechanism(s) of these effects remain unclear. One hypothesis is that allograft unresponsiveness is correlated with a shift in the pattern of expression of the T helper 1 versus T helper 2 T-cell cytokines. We report here an extensive analysis of murine cardiac allografts, during normal first set rejection and in mice treated with anti-adhesion molecule monoclonal antibodies (mAbs), a regimen that results in prolonged unresponsiveness.

METHODS

A combination of immunohistochemical staining with a panel of mAbs, and in situ hybridization with a panel of digoxigenin-labeled riboprobes, was performed on frozen-tissue sections of cardiac allografts.

RESULTS

In several strain combinations, injection of anti-leukocyte function-associated antigen-1 and anti-intercellular adhesion molecule-1, from day 0 to day 6 after transplantation, results in significant long-term survival. Examination of tolerated cardiac allografts by in situ hybridization and immunohistochemical staining shows an altered cytokine expression pattern, although the frequency of CD3 and CD4 cells is not dramatically reduced. These allografts show a decreased frequency of interferon-gamma and interleukin (IL)-2-expressing cells and a slightly increased frequency of cells expressing IL-4 and IL-10, compared with unmodified acute rejection. A direct role of these changes in T-cell cytokine expression is demonstrated by reversal of tolerance induction and rejection of the allograft by in vivo injection of either anti-IL-10 or anti-IL-4 mAb.

CONCLUSIONS

Although there are significant differences in the frequency of different cellular subsets and patterns of cytokine gene expression, these differences are quantitatively subtle, suggesting a delicately balanced immune response that can develop a pattern of specific unresponsiveness, with relatively minor alterations in the specific T-cell response.

Human monocytes activate porcine endothelial cells, resulting in increased E-selectin, interleukin-8, monocyte chemotactic protein-1, and plasminogen activator inhibitor-type-1 expression

Monocytes (Mo) are thought to be important effector cells in early xenograft rejection. Effects of Mo-endothelial cell (EC) interactions on EC activation in vitro were studied by coculturing human Mo or human monocytoid cell lines, U937 and THP-1, with porcine EC. Without preactivation, U937 cells and Mo induced mRNA for the EC-specific adhesion receptor, E-selectin, expressed only on activated cells, after 2 hr. Surface protein was maximal when equal numbers of EC and Mo were cocultured. Increased mRNA expression of the chemokines, interleukin-8 and monocyte chemotactic protein-1, and the antifibrinolytic protein plasminogen activator inhibitor type-1, confirmed EC activation. Like E-selectin, plasminogen activator inhibitor type-1 mRNA was rapidly induced and returned to baseline after 24 hr, whereas chemokine gene expression was slower and more prolonged. Interleukin-1 receptor antagonist failed to modulate induction of E-selectin. Soluble tumor necrosis factor (TNF) alpha receptor inhibited E-selectin induced by TNF alpha, but not by U937 cells, and mRNA and protein on EC in Mo-EC mixtures cocultured at 1:1 ratios were not significantly reduced. The TNF alpha inhibitor did reduce E-selectin expression (30-40%), as well as induced chemokine gene expression (80%), at higher Mo-EC ratios. Despite this, minimal TNF alpha was detectable in supernatants. These results, along with the transwell experiments that confirmed a requirement for Mo-EC contact, suggest that membrane-bound TNF alpha may be involved. Thus, Mo-EC interactions in the porcine to human combination activated several EC functions, suggesting that initial Mo contact with the vessel wall of a xenogeneic graft may promote leukocyte recruitment, inflammation, and maintenance of thrombus, resulting in eventual organ destruction.

Specific transcellular binding between membrane proteins crucial to Alzheimer disease

Molecular genetic studies of families suffering from genetic forms of early onset Alzheimer disease (AD) have identified three genes and their protein products as being crucially involved in the etiology of AD. The three proteins are all integral membrane proteins. One of them is beta-APP, the precursor of the beta-amyloid found in the characteristic neuritic plaques present in the brains of AD patients. The other two, S182 and STM2, are homologous in amino acid sequence to one another but are unrelated to beta-APP. It is shown here, using cultured cells transfected for each of these proteins, that beta-APP binds specifically and transcellularly to either S182 or STM2. We propose that this transcellular binding may not only be important in normal neuronal physiology and development but may be directly involved in the process of formation of beta-amyloid from beta-APP.

Regulation of leukocyte integrin function: affinity vs. avidity

Leukocytes circulate freely in the bloodstream until receiving signals which activate adhesive mechanisms essential for immune responsiveness. Key mediators of these adhesion events are heterodimeric cell surface receptors called integrins. It is now apparent that several components may contribute to successful integrin-mediated adhesion: alterations in individual receptors lead to enhanced affinity for ligand; integrin clustering causes an increase in avidity; by spreading, the adhering cell is less susceptible to shear force. Model systems have allowed us to examine the contribution of each of these factors in generating adhesion. In more physiologically relevant situations, it can now be questioned whether integrin-mediated adhesion is regulated via alterations in receptor affinity or avidity, or whether both these mechanisms are involved.

Intracellular trafficking of CTLA-4 and focal localization towards sites of TCR engagement

T lymphocyte receptor CTLA-4 binds costimulatory molecules CD80 (B7-1) and CD86 (B7-2) with high avidity and negatively regulates T cell activation. CTLA-4 functions at the cell surface, yet is primarily localized in intracellular vesicles. Here, we demonstrate cycling of CTLA-4 between intracellular stores and the cell surface. Intracellular vesicles containing CTLA-4 overlapped with endocytic compartment(s) and with perforin-containing secretory granules. Cell surface expression of CTLA-4 was rapidly increased by raising intracellular calcium levels. During T cell activation, intracellular and cell surface CTLA-4 became focused towards sites of TCR activation. Cycling and directional control of CTLA-4 expression may regulate its functional interaction with APCs bearing peptide-MHC complexes of appropriate specificity and avidity.

Orchestrated information transfer underlying leukocyte endothelial interactions

The specificity and efficiency of leukocyte binding to endothelial cells (ECs) depends on coordinated information transfer from the underlying tissue to endothelium and from there to the leukocyte. We address three distinct information-transfer points in this system: 1, How does the leukocyte read information from the EC? This process is best accounted for by the paradigm of a multi-step adhesion cascade optimized for rapid information readout; it consists of primary adhesion (rolling/tethering), triggering, and strong adhesion. Recent studies with T cells, monocytes, and eosinophils confirm the generality of the paradigm. The concept of primary adhesion has been expanded to involve not only the selectins, but also certain integrins; furthermore, it depends on receptor concentration on leukocyte microvilli. 2. What information from the underlying tissue does the EC transform into signals for the leukocytes? And what rules govern that process? We illustrate the principles with chemokines, believed to participate in the triggering step. The endothelium displays chemokines either (a) directly by "posting" them from other cells or (b) by integrating a variety of tissue and environmental signals and "relaying" that information by producing its own chemokines and surface adhesion molecules. The rules for this endothelial transduction include specificity coupled with redundancy, amplification, synergy, and coordinated induction of ensembles of molecules. Finally, 3. How does the relevant information reach the endothelium? Simple diffusion is sufficient to deliver signals from cells close to the vessel. However, longer range soluble mediator transport appears to be facilitated by fiber bundles, particularly those ensheathed by fibroblastic reticular cells in the lymph node.

Significance of the inflammatory response in brain ischemia

Leukocytes appear to have a central role in the inflammatory response that develops during acute brain ischemia. This brief review adduces evidence that leukocytes accumulate in focal zones of acute brain ischemia at a sufficiently early stage to participate in the process of progressive ischemic brain damage and that partial inhibition of that accumulation, by various measures, can attenuate ischemic brain injury. Mechanisms of leukocyte adhesion are discussed in detail and an inference is put forward that leukocytes are an important factor in progressive ischemic injury, but almost certainly act in concert with a number of other similarly important factors. On this basis, leukocyte inhibition may have demonstrable benefit in acute stroke, but ultimately be found to only partially spare potentially salvageable tissue in the ischemic zone.

Expressions of very late antigen-6 and vitronectin receptor, and their interactions to laminin and vitronectin during tonsillar B-cell activation

This study examined the expressions of a-subunits of very late antigen-6 (VLA-6; alpha 6) and vitronectin receptor (VNR; alpha V) on tonsillar B cells and interactions between those integrins and their respective ligands, laminin (LM) and vitronectin (VN). alpha 6 and alpha V were expressed on about 30 to 40% of tonsillar B cells. When purified tonsillar B cells were separated by a discontinuous Percoll gradient, the number of alpha 6- and alpha V-positive cells decreased as the cell density went down, while the number of activated cells went up. After in vitro activation of tonsillar B cells by Staphylococcus aureus Cowan I strain (SAC), the expressions of alpha 6 and alpha V and their adhesiveness to LM or VN decreased significantly. Increased proliferation of B cells was observed when tonsillar B cells were cultured with immobilized LM or VN. The results of immunohistological staining showed VLA-6, VNR, LM and VN in the follicular area. These results suggest that the expressions of VLA-6 and VNR on tonsillar B cells may be decreased during B cell activation, and the interaction between VLA-6, VNR, and LM, VN may give a costimulatory effect on B cell activation in the follicular area of the tonsil.

Catenin-dependent and -independent functions of vascular endothelial cadherin

Vascular endothelial cadherin (VE-cadherin, cadherin-5, or 7B4) is an endothelial specific cadherin that regulates cell to cell junction organization in this cell type. Cadherin linkage to intracellular catenins was found to be required for their adhesive properties and for localization at cell to cell junctions. We constructed a mutant form of VE-cadherin lacking the last 82 amino acids of the cytoplasmic domain. Surprisingly, despite any detectable association of this truncated VE-cadherin to catenin-cytoskeletal complex, the molecule was able to cluster at cell-cell contacts in a manner similar to wild type VE-cadherin. Truncated VE-cadherin was also able to promote calcium-dependent cell to cell aggregation and to partially inhibit cell detachment and migration from a confluent monolayer. In contrast, intercellular junction permeability to high molecular weight molecules was severely impaired by truncation of VE-cadherin cytoplasmic domain. These results suggest that the VE-cadherin extracellular domain is enough for early steps of cell adhesion and recognition. However, interaction of VE-cadherin with the cytoskeleton is necessary to provide strength and cohesion to the junction. The data also suggest that cadherin functional regulation might not be identical among the members of the family.

Identification of the border between fibronectin type III homologous repeats 2 and 3 of the neural cell adhesion molecule L1 as a neurite outgrowth promoting and signal transducing domain

To determine the domains of the neural cell adhesion molecule L1 involved in neurite outgrowth, we have generated monoclonal antibodies against L1 and investigated their effects on neurite outgrowth of small cerebellar neurons in culture. When the 10 antibodies were coated as substrate, only antibody 557.B6, which recognizes an epitope represented by a synthetic peptide comprising amino acids 818 to 832 at the border between the fibronectin type III homologous repeats 2 and 3, was as efficacious as L1 in promoting neurite outgrowth, increasing intracellular levels of Ca2+, and stimulating the turnover of inositol phosphates. These findings suggest that neurite outgrowth and changes in these second messengers are correlated. Such a correlation was confirmed by the ability of Ca2+ channel antagonists and pertussis toxin to inhibit neurite outgrowth on L1 and antibody 557.B6. These observations indicate for the first time a distinct site on cell surface-bound L1 as a prominent signal-transducing domain through which the recognition events appear to be funneled to trigger neurite outgrowth, increase turnover of inositol phosphates, and elevate intracellular levels of Ca2+.

Cytotoxic T lymphocyte-associated molecule-4, a high-avidity receptor for CD80 and CD86, contains an intracellular localization motif in its cytoplasmic tail

CD28 and CTLA-4, T cell receptors for B7-1 (CD80) and B7-2 (CD86) molecules on antigen-presenting cells, transmit costimulatory signals important for optimal T cell activation. Despite sharing sequence homology and common ligands, these receptors have distinct binding properties and patterns of expression. The function of CTLA-4 during T cell activation is not well understood, although an important role is suggested by complete amino acid sequence conservation of its cytoplasmic tail in all species studied to date. We report here a role of the cytoplasmic tail of CTLA-4 in regulating its subcellular localization and cell surface expression. In activated human peripheral blood T cells, or in several transfected or transduced cell types, CTLA-4 is not primarily a cell surface protein, but rather is localized intracellularly in a region which overlaps the Golgi apparatus. Transfer of 11 cytoplasmic residues, 161TTGVYVKMPPT, from the CTLA-4 cytoplasmic tail to the homologous position in CD28 was sufficient to confer intracellular localization. Mutation of the tyrosine residue (Tyr165) in this motif to phenylalanine resulted in increased surface expression of CTLA-4. Thus, the subcellular localization of CTLA-4 is controlled by a tyrosine-containing motif within its cytoplasmic domain. Contained within this motif is a binding site for SH2 domains of the p85 subunit of phosphatidylinositol 3-kinase.

Structure and function of C-CAM1: effects of the cytoplasmic domain on cell aggregation

C-CAMs are epithelial cell-adhesion molecules of the immunoglobulin supergene family with sequences highly homologous to carcinoembryonic antigen (CEA). C-CAMs and their human homologues, biliary glycoproteins, are unique among the CEA-family proteins in that they have cytoplasmic domains. Furthermore, alternative splicing generates C-CAM isoforms with different cytoplasmic domains, suggesting that the cytoplasmic domains of C-CAM may play important roles in regulating the function or functions of C-CAM. By using both sense and antisense approaches, we have shown that C-CAM1 is a tumour suppressor in prostate carcinogenesis. This observation raises the possibility that the cytoplasmic domain of C-CAM1 may be involved in signal transduction or interaction with cytoskeletal elements to elicit the tumour suppressor function. The cytoplasmic domain of C-CAM1 contains several potential phosphorylation sites, including putative consensus sequences for cyclic AMP-dependent kinase and tyrosine kinase. One of the potential tyrosine phosphorylation sites is located within the antigen-receptor homology (ARH) domain. The ARH domain of the membrane-bound IgM molecule is necessary for signal transduction in B-cells. These structural features suggest that the cytoplasmic domain of C-CAM1 may be important for signal transduction. To test this possibility, we generated several site-directed C-CAM1 mutants and tested their ability to support adhesion and their abilities to be phosphorylated in vivo. Results from these studies revealed that Tyr-488 is phosphorylated in vivo. However, replacing this tyrosine with phenylalanine did not significantly compromise its adhesion function. Similarly, Ser and Thr residues are phosphorylated in vivo, but deletion of the potential cyclic AMP-dependent kinase site did not significantly reduce the adhesion function. These results suggest that the kinase phosphorylation sites in the cytoplasmic domain of C-CAM1 are not required for the adhesion function. However, these phosphorylation sites are probably involved in the regulation of C-CAM-mediated signal transduction. Thus, there are probably distinct structural requirements for the adhesion and the signal transduction functions of C-CAM. Incidentally, a C-CAM1 deletion mutant containing a 10-amino-acid cytoplasmic domain was able to support adhesion activity. This is in contrast to our previous finding that a C-CAM isoform, C-CAM3, with a 6-amino-acid cytoplasmic domain could not support cell adhesion. This result indicates that the extra four amino acids, which are absent in C-CAM3 and contain a potential Ser/Thr phosphorylation site, are important for the adhesion function.

T-cell recognition of antigen. A process controlled by transient intermolecular interactions

As recently as ten years ago, the nature of the T-cell receptor for antigen was a mystery, as was the precise role of histocompatibility molecules in antigen-presentation to T cells. Although T-cell receptors have now been cloned and crystal structures of MHC/peptide molecules exist, our understanding of the parameters that characterize this interaction and other interactions relevant to T-cell immunity are still unclear. The engineering of soluble forms of proteins that mediate T-cell recognition of antigen has allowed the first measurements of these parameters. Interestingly, many of these interactions are of a transient nature, with very rapid off-rates. These data suggest a model whereby highly reversible intermolecular interactions mediate the cell-cell association. The association of adhesion molecules is probably the first step in the stabilization of a conjugate, because they are more numerous than any antigen-specific interaction, followed later by TCR-MHC engagements. Diffusion within each lipid bilayer should allow the congregation of MHC/TCR interactions at the cell-cell interface, with peptide-specific TCR interactions outcompeting irrelevant interactions. Rapid off-rates for both the antigen-specific and nonspecific interactions may be necessary to maintain reversibility, yet allow a rapid approach to equilibrium and consequent signaling when a specific antigen is present or disengagement when it is not.

Calcium response of helper T lymphocytes to antigen-presenting cells in a single-cell assay

We developed a dynamic, single-cell assay involving alternating differential interference contrast and fluorescence microscopy, together with digital imaging, for both viewing the physical interaction of live helper T lymphocytes (Th cells) with antigen-presenting cells (APCs) and monitoring the increases in the intracellular free calcium concentration of the Th cell, an early event in Th cell activation. We obtained Th-APC conjugates by allowing the Th cells to migrate toward and interact with APCs that either settled nearby or had been micromanipulated in close proximity to the Th cells. Th cell motility played an important role in initiating Th-APC contacts but not in determining the Th cell calcium response. We found that the intracellular calcium responses of individual Th cells are heterogeneous and an all-or-none phenomenon, independent of antigen concentration. However, the fraction of Th-APC conjugates involving responding Th cells is an increasing function of the antigen concentration. Finally, we measured some characteristics of the developing Th-APC contact area. We used all of these data together with previously developed mathematical models to estimate that only 1 to 20 major histocompatibility class II-antigen complexes are required in the initial Th-APC contact area to elicit a Th cell calcium response.

Notch signaling

The Notch/Lin-12/Glp-1 receptor family mediates the specification of numerous cell fates during development in Drosophila and Caenorhabditis elegans. Studies on the expression, mutant phenotypes, and developmental consequences of unregulated receptor activation have implicated these proteins in a general mechanism of local cell signaling, which includes interactions between equivalent cells and between different cell types. Genetic approaches in flies and worms have identified putative components of the signaling cascade, including a conserved family of extracellular ligands and two cellular factors that may associate with the Notch Intracellular domain. One factor, the Drosophila Suppressor of Hairless protein, is a DNA-binding protein, which suggests that Notch signaling may involve relatively direct signal transmission from the cell surface to the nucleus. Several vertebrate Notch receptors have also been discovered recently and play important roles in normal development and tumorigenesis.

Lymphocyte memory and affinity selection

The persistence of antigen-specific immune memory appears to require the presence of antigen--suggesting that memory may be due to restimulation of "memory" lymphocytes by persisting antigen. Persistence of antigen, in a form capable of stimulating B cell proliferation, on long-lived, follicular dendritic cells of lymphoid tissue is well documented. Existence of an analogous mechanism for T cell memory maintenance is controversial but can not be ruled out. Here we examine the consequences of immune memory maintenance by antigen-specific lymphocyte restimulation, and estimate the duration of memory as a function of model parameters. We show that the competition for restimulation among memory cell populations results in the selection of the clone having the highest overall affinity for the retained antigen. Thus affinity selection, an important attribute of immunity, is a constitutive property of memory maintenance by antigen-specific restimulation. In the case of B cells, affinity selection is predicted to continue to increase antibody affinity even after somatic mutation stops, and thus may be an important component of affinity maturation. Finally, we discuss several other hypotheses proposed to explain immune memory, including T cell stimulation by cross-reactive antigens.

Sustained signaling leading to T cell activation results from prolonged T cell receptor occupancy. Role of T cell actin cytoskeleton

Using antigen-specific T cell clones and peptide-pulsed antigen-presenting cells (APCs) we investigated the mechanisms that lead to sustained signaling, known to be required for activation of effector function. Four lines of evidence indicate that the T cell actin cytoskeleton plays a crucial role in T cell activation by antigen-pulsed APCs, but is not required when T cell receptor (TCR) is cross-linked by soluble antibodies. First, addition of antibodies to the major histocompatibility complex molecules recognized by the TCR aborts the ongoing intracellular calcium concentration ([Ca2+]i) increase in performed T-APC conjugates, indicating that the sustained signaling requires the continuous occupancy of TCR. Second, time-lapse image recording shows that T lymphocytes conjugated to peptide-pulsed APCs undergo a sustained [Ca2+]i increase, which is accompanied by the formation of a large and changing area of contact between the two opposing membranes. Third, drugs that disrupt the actin cytoskeleton, Cytochalasin D and and C2 Clostridium botulinum toxin induce a rapid block of [Ca2+]i rise, coincident with a block of the cyclic changes in T cell shape. Finally, the addition of Cytochalasin D or of anti-MHC antibodies to preformed conjugates inhibits interferon gamma production in an 1-antigen dose- and time-dependent fashion. These results identify T cell actin cytoskeleton as a major motor for sustaining signal transduction and possibly for driving TCR cross-linking and offer an explanation for how T cells equipped with low affinity TCR can be triggered by a small number of complexes on APCs.

Structure--function relationships in gap junctions

Gap junctions are metabolic and electrotonic pathways between cells and provide direct cooperation within and between cellular nets. They are among the cellular structures most frequently investigated. This chapter primarily addresses aspects of the assembly of the gap junction channel, considering the insertion of the protein into the membrane, the importance of phosphorylation of the gap junction proteins for coupling modulation, and the formation of whole channels from two hemichannels. Interactions of gap junctions with the subplasmalemmal cytoplasm on the one side and with tight junctions on the other side are closely considered. Furthermore, reviewing the significance and alterations of gap junctions during development and oncogenesis, respectively, including the role of adhesion molecules, takes up a major part of the chapter. Finally, the literature on gap junctions in the central nervous system, especially between astrocytes in the brain cortex and horizontal cells in the retina, is summarized and new aspects on their structure-function relationship included.

Isolated plasma membranes induce the loss of oriented detyrosinated microtubules and other contact inhibition-like responses in migrating NRK cells

We have previously shown that detyrosinated microtubules (Glu MTs), which are oriented toward the direction of locomotion in motile fibroblasts, disappear from the area adjacent to cell-cell contact soon after a cell-cell collision. To identify cell surface molecules that trigger this phenomenon, we have established a system in which this and other cellular reactions to cell-cell contact can be reproduced by the addition of isolated plasma membranes. Experimental wounds were made in confluent monolayers of NRK cells, and cells at the wound margin were allowed to develop oriented Glu MTs. Test samples were added to these cells and after a 1 hour incubation the distributions of Glu MTs, tyrosinated MTs (Tyr MTs) and microfilaments were determined by immunofluorescence. When plasma membranes isolated from NRK cells were added, oriented Glu MTs disappeared from the leading lamella of target cells and instead a small number of Glu MTs were found clustered around the nucleus. As observed for cell-cell contact, plasma membranes did not significantly affect the distribution of Tyr MTs. We also found that both cell-cell contact and membrane treatment caused the collapse of lamellipodia and loss of associated staining with antiactin antibody. Time-lapse recordings of directed locomotion of NRK cells showed that membranes suppressed the forward movement of cells. The loss of Glu MTs from the leading lamella was the most amenable response for quantification and we used it to examine the biochemical properties of the membrane activity. The ability of membranes to induce the loss of oriented Glu MTs was observed at as low as 4 micrograms/ml of membrane protein and was detectable 10 minutes after membrane addition. The loss of oriented Glu MTs was reversible upon removal of membranes, demonstrating that the membranes were not toxic to the cells. The oriented Glu MT reducing activity could be solubilized from the membranes by detergent, was enriched in a plasma membrane fraction, and was labile to heat and acid treatment. In summary, we have successfully reconstituted a number of responses of contact inhibition using solubilized preparations of membranes. Our preliminary results suggest that there is a specific factor in plasma membranes that is capable of triggering contact inhibition. With the assay we have developed, it should now be possible to dissect contact inhibition of motility at the molecular level.

Autocrine stimulation of a human lung mesothelioma cell line is mediated through the transforming growth factor alpha/epidermal growth factor receptor mitogenic pathway

Malignant cells frequently acquire a certain independency of exogenous growth factors via the coexpression of epidermal growth factor receptor (EGFR) and epidermal growth factor (EGF)-related molecules. In the present study we investigate a possible involvement of EGF-related molecules in the growth of human lung mesothelioma. Four well-characterised cell lines are analysed for their responsiveness to exogenous EGF and transforming growth factor alpha (TGF-alpha) as well as for coexpression of EGFR and EGF/TGF-alpha. Both growth factors are able to stimulate DNA synthesis in three cell lines, although the degree of responsiveness is very variable, but neither EGF nor TGF-alpha has an effect on the cell line ZL34. In contrast, no heterogeneity is observed in the expression of EGFR, which is similarly high in all cell lines. Analysis of cell supernatants reveals that, whereas no EGF is detected, TGF-alpha is released by two cell lines. Furthermore, these two cell lines, ZL5 and ZL34, are shown to express the membrane anchored precursor pro-TGF-alpha. Thus, coexpression of EGFR and TGF-alpha is observed on two mesothelioma cell lines. The potential autocrine mitogenic role of TGF-alpha in these two cell lines was tested using neutralising antibodies against TGF-alpha and EGFR. In ZL5 cells DNA synthesis was not affected by the presence of neutralising antibodies, indicating that an external autocrine mitogenic pathway is not active in these cells. In ZL34 cells, however, the potential autocrine loop could be disrupted, as DNA synthesis was significantly reduced in the presence of neutralising antibodies. This result gives strong evidence for an autocrine role of TGF-alpha in the growth of the mesothelioma cell line ZL34.

The protein tyrosine kinase p56lck regulates cell adhesion mediated by CD4 and major histocompatibility complex class II proteins

The CD4 protein is expressed on a subset of human T lymphocytes that recognize antigen in the context of major histocompatibility complex (MHC) class II molecules. Using Chinese hamster ovary (CHO) cells expressing human CD4, we have previously demonstrated that the CD4 protein can mediate cell adhesion by direct interaction with MHC class II molecules. In T lymphocytes, CD4 can also function as a signaling molecule, presumably through its intracellular association with p56lck, a member of the src family of protein tyrosine kinases. In the present report, we show that p56lck can affect cell adhesion mediated by CD4 and MHC class II molecules. The expression of wild-type p56lck in CHO-CD4 cells augments the binding of MHC class II+ B cells, whereas the expression of a mutant p56lck protein with elevated tyrosine kinase activity results in decreased binding of MHC class II+ B cells. Using site-specific mutants of p56lck, we demonstrate that the both the enzymatic activity of p56lck and its association with CD4 are required for this effect on CD4/MHC class II adhesion. Further, the binding of MHC class II+ B cells induces CD4 at the cell surface to become organized into structures resembling adhesions-type junctions. Both wild-type and mutant forms of p56lck influence CD4-mediated adhesion by regulating the formation of these structures. The wild-type lck protein enhances CD4/MHC class II adhesion by augmenting the formation of CD4-associated adherens junctions whereas the elevated tyrosine kinase activity of the mutant p56lck decreases CD4-mediated cell adhesion by preventing the formation of these structures.

Soluble intercellular adhesion molecule-1 in primary biliary cirrhosis: relationship with disease stage, immune activity and cholestasis

Intercellular adhesion molecule-1, strongly expressed on the interlobular and proliferating bile ducts in primary biliary cirrhosis, is important in the migration and adhesion of inflammatory cells from the circulation to these structures. A soluble form has been found to be elevated in serum in primary biliary cirrhosis. Our aim was to check on the role of soluble intercellular adhesion molecule-1 in primary biliary cirrhosis with particular reference to its specificity by comparison with other disease control groups and to assess its relationship with stage of disease activity, circulating lymphocyte activation and cholestasis. Soluble intercellular adhesion molecule-1 (enzyme-linked immunosorbent assay) and liver biochemistry were measured in 41 patients with primary biliary cirrhosis, 9 with primary sclerosing cholangitis, 12 with alcoholic liver disease and 17 healthy controls. In subgroups of patients with primary biliary cirrhosis, lymphocyte activation and hepatic bile acid uptake and excretory rates were determined. Soluble intercellular adhesion molecule-1 was significantly higher in all three disease groups. Levels in primary biliary cirrhosis and primary sclerosing cholangitis were similar and significantly higher than alcoholic liver disease. Soluble intercellular adhesion molecule-1 expression was greater in late primary biliary cirrhosis than early disease and correlated with histological progression. Correlations were also found with alkaline phosphatase, gamma-glutamyl transpeptidase and conjugated bilirubin. A trend toward an inverse correlation with hepatic excretory rate was found, but no correlation was detected with circulating lymphocyte interleukin-2 receptor expression.(ABSTRACT TRUNCATED AT 250 WORDS)

Transient intercellular adhesion: the importance of weak protein-protein interactions

Intercellular adhesion is a complex phenomenon central to the development, structure and functioning of all multicellular organisms. Adhesion is mediated by distinct families of cell-adhesion molecules (CAMs), and recent studies have identified key characteristics of CAMs that influence their function. Affinity and kinetic analyses using a novel technique based on surface plasmon resonance have shown that CAM interactions that mediate transient cell adhesion may have surprisingly low affinities and extremely fast dissociation rate constants.

N-cadherin and Ng-CAM/8D9 are involved serially in the migration of newly generated neurons into the adult songbird brain

In the adult avian forebrain, neurons continue to be produced in the subependymal zone (SZ), from which they migrate upon radial fibers. To identify ligands regulating this process, we studied N-cadherin and Ng-CAM/8D9 expression in HVC, a neurogenic region of the canary neostriatum. N-cadherin was relatively restricted to the SZ and was expressed by dividing, [3H]thymidine-labeled precursor cells. However, cellular N-cadherin was down-regulated prior to neuronal migration from the SZ. Addition of anti-N-cadherin Fab hastened neuronal migration from adult SZ explants, without influencing neuronal number. Unlike N-cadherin, Ng-CAM/8D9 was expressed by migrating neurons. Anti-8D9 Fab inhibited neuronal migration upon cultured ependymoglia, which did not express Ng-CAM/8D9. Thus, the departure of new neurons from the adult SZ may require their suppression of N-cadherin, whereas their subsequent migration and survival may depend upon neuronal expression of Ng-CAM/8D9 and its interaction with a heterophilic radial cell receptor.

Antigen recognition by helper T cells elicits a sequence of distinct changes of their shape and intracellular calcium

BACKGROUND

Helper T-cell activation is initiated in vivo when the T-cell receptor complex recognizes an antigen fragment associated with MHC class II molecules on the surface of an antigen-presenting cell. In most previous studies of this phenomenon, T cells were stimulated not with antigen-presenting cells, but with CD3-specific antibodies. This approach provided considerable understanding of the cascade of molecular events triggered by T-cell receptor stimulation. However, the specific consequences of cell-cell interactions are still poorly understood. We therefore used a dual imaging system that provides simultaneous transmission and fluorescence images to study the morphological changes and variations of intracellular calcium concentration ([Ca2+]i) triggered in a human CD4+ antigen-specific T-cell clone in response to antigen presented by a class II-transfected murine fibroblast.

RESULTS

T cells loaded with the Ca(2+)-sensitive fluorescent dye Fura-2 were individually monitored for half an hour following their contact with a monolayer of antigen-pulsed antigen-presenting cells. The response was found to have three distinct phases. During the first few minutes after contact, the T cell moves over the antigen-presenting cells, as if 'scanning' them. After several minutes, an oscillating [Ca2+]i response begins, accompanied by the immobilization of the cell and the retraction of pseudopodia. This rounding-up was probably Ca(2+)-dependent, as it could also be triggered by ionomycin or thapsigargin. Later during the [Ca2+]i response, the T cell becomes flattened and further elongated, suggesting increased adhesion to antigen-presenting cells.

CONCLUSIONS

The physiological signal for T-cell activation, antigen recognition, is a three-step process reminiscent of the three steps previously observed in the interaction between neutrophils and endothelial cells. During these successive steps, a mobile, weakly interacting T cell is transformed into an immobile cell fully engaged in the activation pathway. Thus, antigenic recognition is not instantaneous, but evolves slowly by progressive amplification of the signal given by a few antigen molecules, eventually resulting in T-cell activation.

Development of the locomotory muscle of the chaetognath Sagitta. 2. Stereological study of fibre growth and differentiation processes

The cellular growth and differentiation of A and B fibers was studied using classical stereological methods and the results have then been analysed using linear regression analysis and hypothesis testing procedures. At a given level, the development of the muscular tissue slows down and stabilises, at this same level the growth of the two types of fibres slows down sharply. This is not the case for the cellular organelles. The growth of the contractile apparatus is continuous in each fibre, but the density of myofibrils is higher in the A fibres. The surface of the SR increases in a different fashion in the two types of fibre, but its volumic density evolves in a similar fashion. The mitochondria develop differently in the two types of fibre, neither their shape nor their distribution are comparable from one type of fibre to the other. Despite this overall difference between the mitochondrial populations the individual mitochondria growth mechanisms seem to be comparable. These parameters reflect the organisation and the development of the fiber groups and cellular architecture. They indicate the existence of unknown morphogenetic signals and fields, in this epithelial tissue having paracellular paths communicating with sea water. Muscle development seems to be largely a myogenic property regulated by various extrinsic factors, which are examined.

Development of the locomotory muscle of the chaetognath Sagitta. 1. Quantitative and qualitative aspects of the body and muscle tissue development within the phylum

The Chaetognath primary muscle constitutes the main tissue of the body-wall. It is made up of four main longitudinal bands. Muscle fibres are separated from the nervous system. During muscle growth, four myogenic zones produce two types of fibres A and B, forming units. These units ordered in an epithelial-like tissue, show various kind of intercellular junctions. For a given animal, the overall number of A and B fibres units is practically the same in each band. The four myogenic areas are the centre for a two-phase production of A and B fibres which appear at a regular rate in each quadrant, by the simultaneous production of three sets of A fibres and two sets of B fibres. The former differentiate at once, the latter some time later, and always at the same moment in their lives. In this way the number of sets of A and B fibres increases during the animal's life. This hyperplasia is accompanied by the hypertrophy of the cells which make up each group. For any given animal, the ventral quadrants are thicker than the dorsal quadrants. Chaetognath locomotory muscle is a good model on which to study the development of a skeletal muscle. Its development is closely related to the hydroskeleton growth, to the animal's shape and various other constructional factors which play a central role in this very isolated phylum.

Association of the transmembrane TGF-alpha precursor with a protein kinase complex

A variety of growth factors including transforming growth factor-alpha (TGF-alpha) are synthesized as transmembrane precursors. The short cytoplasmic domain of the transmembrane TGF-alpha precursor lacks any apparent motif associated with signal transduction. However, the sequence conservation of this cytoplasmic domain and its abundance of cysteine residues, reminiscent of the cytoplasmic domains of CD4 and CD8, suggest a biological function. In this study, we showed that transmembrane TGF-alpha was rapidly internalized after interaction with a specific antibody and that this internalization was greatly decreased when the COOH-terminal 31 amino acids were removed. Chemical cross-linking experiments revealed two associated proteins of 86 and 106 kD which coimmunoprecipitated with the TGF-alpha precursor. The association of p86 was dependent on the presence of the COOH-terminal cytoplasmic 31 amino acids of the TGF-alpha precursor, whereas p106 still remained associated when this segment was deleted. In addition, p106 was tyrosine-phosphorylated and exposed on the cell surface. The protein complex associated with transmembrane TGF-alpha displayed kinase activities towards tyrosine, serine, and threonine residues. These activities were not associated with transmembrane TGF-alpha when the COOH-terminal segment was truncated. The association of a protein kinase complex with transmembrane TGF-alpha may provide the basic elements for a "reverse" mode of signaling through the cytoplasmic domain of this growth factor, which may lead to two-directional communication during ligand-receptor interaction.

Dynamic adhesion of CD8-positive cells to antibody-coated surfaces: the initial step is independent of microfilaments and intracellular domains of cell-binding molecules

Cell adhesion is a multistep, metabolically active process usually requiring several minutes or even hours to complete. This results in the formation of strong bonds that cannot be ruptured by mechanical forces encountered by living cells in their natural environment. However, the first seconds after contact formation are much more sensitive to external conditions and may be the critical step of adhesion. This step is very difficult to monitor without disturbing the observed system. We addressed this problem by studying the interaction between anti-CD8-coated or control surfaces and murine lymphoid cell lines bearing wild-type CD8 molecules, or genetically engineered molecules bearing extracellular CD8 domains and transmembranar and intracytoplasmic domains of class I histocompatibility molecules, or with extensive deletion of intracytoplasmic domains. We used a new method that consisted of monitoring the motion of cells driven along adhesive surfaces by a hydrodynamic force weaker than the reported strength of single ligand-receptor bonds, but sufficient to make free cells move with an easily detectable velocity of several micrometers per second. Cells exhibited short-term (< or = 0.5 s) adhesions to the surface with a frequency of about one event per 30-s period of contact. These events did not require specific antigen-antibody bonds. However, when anti-CD8 were present, strong adhesion was achieved within < 1 s, since most arrests were longer than a standard observation period of 1 min. This bond strengthening was not affected by cytochalasin, and it did not require intact intracellular domains on binding molecules. It is concluded that the initial step in strong adhesion may be viewed as a passive, diffusion-driven formation of a new specific bonds.

Immunohistochemical detection of neural cell adhesion molecule and laminin in X-linked dystrophic dogs and mdx mice

Although dystrophin deficiency is known to be the genetic and biochemical defect causing Duchenne muscular dystrophy (DMD), much remains unknown about the underlying factors affecting clinical and pathological expression of the disease. Two animal forms of muscular dystrophy resembling DMD have been described. Neural cell adhesion molecule (NCAM) and laminin expression were examined in the proliferation-competent mdx mouse and non-regenerative "golden retriever muscular dystrophy dog" (GRMD). The results showed that (1) NCAM expression was greater in dystrophic dogs and mice than in age-matched normal animals, (2) myoblast-specific NCAM was greater in mdx mice than in dystrophic dogs, and (3) laminin strongly labelled mdx and GRMD myofibre membranes but was also sometimes found in individual interstitial cells of mdx muscle. Expression of these proteins may partly determine the clinicopathological expression of dystrophin deficiency.

Cytosolic interaction between deltex and Notch ankyrin repeats implicates deltex in the Notch signaling pathway

Genetic data from Drosophila have suggested a functional relationship between the novel cytoplasmic protein encoded by the deltex locus and the transmembrane receptor encoded by Notch. We have demonstrated a direct interaction between these proteins from expression studies conducted in cultured cells, in yeast, and in the imaginal wing disc. deltex binds specifically to the Notch ankyrin repeats, a region that is crucial for Notch signaling and that constitutes the most conserved domain among Notch family members. In addition, we present a new Notch allele, Nsu42c, that is associated with a missense mutation within the fifth ankyrin repeat. In addition to representing a new class of viable Notch allele, this mutation behaves similarly to mutations of deltex and further implicates the ankyrin repeats in Notch function.