The association of the protein tyrosine kinases p56lck and p60fyn with the glycosyl phosphatidylinositol-anchored proteins Thy-1 and CD48 in rat thymocytes is dependent on the state of cellular activation

The CD48 receptor mediates Staphylococcus aureus human and murine eosinophil activation

BACKGROUND

Allergy is characterized by eosinophilia and an increased susceptibility to microbial infection. Atopic dermatitis (AD) is typically associated with Staphylococcus aureus (SA) colonization. Some of the mechanisms by which SA and its exotoxins interact with eosinophils remain elusive. CD48, a glycosylphosphatidylinositol-anchored receptor belonging to the CD2 family, participates in mast cells-SA stimulating cross-talk, facilitates the formation of the mast cell/eosinophils effector unit and as expressed by eosinophils, mediates experimental asthma.

OBJECTIVE

To investigate the role of CD48 expressed on human peripheral blood and mouse bone marrow-derived eosinophils (BMEos) in their interaction with heat-killed SA and its three exotoxins, Staphylococcal enterotoxin B (SEB), protein A (PtA) and peptidoglycan (PGN).

METHODS

Eosinophils were obtained from human peripheral blood and BM of WT and CD48-/- mice. SA was heat killed and eosinophils-SA/exotoxins interactions were analyzed by confocal microscopy, adhesion and degranulation, cell viability, cytokine release and cell signalling. In addition, peritonitis was induced by SEB injection into CD48-/- and WT mice. CD48 expression was studied in AD patients' skin and as expressed on their leucocytes in the peripheral blood.

RESULTS

We provide evidence for the recognition and direct physical interaction between eosinophils and SA/exotoxins. Skin of AD patients showed a striking increase of eosinophil-associated CD48 expression while on peripheral blood leucocytes it was down-regulated. SA/exotoxins enhanced CD48 eosinophil expression, bound to CD48 and caused eosinophil activation and signal transduction. These effects were significantly decreased by blocking CD48 on human eosinophils or in BMEos from CD48-/- mice. We have also explored the role of CD48 in a SEB-induced peritonitis model in CD48-/- mice by evaluating inflammatory peritoneal cells, eosinophil numbers and activation.

CONCLUSIONS

These data demonstrate the important role of CD48 in SA/exotoxins-eosinophil activating interactions that can take place during allergic responses and indicate CD48 as a novel therapeutic target for allergy and especially of AD.

Isolation and characterization of hepatic stem cells, or "oval cells," from rat livers

The pace of research on the potential therapeutic uses of liver stem cells or "oval cells" has accelerated significantly in recent years. Concurrent advancements in techniques for the isolation and characterization of these cells have helped fuel this research. Several models now exist for the induction of oval cell proliferation in rodents. Protocols for the isolation and culture of these cells have evolved to the point that they may be set up in any laboratory equipped for cell culture. The advent of magnetic cell sorting has eliminated reliance on expensive flow cytometric sorting equipment to generate highly enriched populations of oval cells. Our laboratory has had much success in using the oval cell surface marker Thy-1 in combination with magnetic sorting to produce material suitable for testing the influence of a myriad of chemical signaling molecules on the oval cell phenotype. This chapter will describe our basic strategy for oval cell induction and isolation. Additionally, two in vitro procedures are described which the reader may find useful in the early stages of developing an oval cell research project.

Thy1 up-regulates FasL expression in lung myofibroblasts via Src family kinases

We have previously demonstrated that myofibroblasts from lungs with bleomycin-induced fibrosis overexpress FasL molecules. Two subpopulations of fibroblasts, distinguished by their expression of Thy1 molecules, have been shown in the lungs of both mice and humans. Thy1-mediated FasL induction has been reported in T cells through the use of anti-Thy1 (G7). We therefore sought to determine whether FasL expression in lung myofibroblasts is associated with and/or dependent on Thy1 expression, and to examine the underlying mechanism of regulation. We show that myofibroblast Thy1 expression is associated with increased FasL expression. Moreover, we directly show that Thy1 activation induces FasL up-regulation. Initially, Thy1 activation causes translocation of FasL to the membrane surface, and later induces de novo synthesis of FasL at the mRNA and protein levels. In contrast to Thy1 activation, TNF-alpha and IFN-gamma do not induce FasL myofibroblast up-regulation. Using Src family kinase (SFKs) inhibitor (PP2), we show the general involvement of SFKs in Thy1 signal transduction leading to FasL up-regulation; and, using specific siRNA, we show the particular involvement of Fyn, one protein in the SFK family. These results demonstrate that Thy1 in myofibroblasts is not just a marker, but is a functional protein that transmits signals into the cell, up-regulating its FasL expression.

CD48 is an allergen and IL-3-induced activation molecule on eosinophils

Eosinophils are involved in a variety of allergic, parasitic, malignant, and idiopathic disorders by releasing a variety of factors including specific granule proteins, lipid mediators, and proinflammatory and immunoregulatory cytokines and chemokines. In addition, they interact with various cell types in the inflamed tissue. Yet, the mechanism of eosinophil activation is still poorly understood. Recently, we described the expression and function of the CD2-subfamily of receptors and especially 2B4 on human eosinophils. In this study we focus on CD48, the high-affinity ligand of 2B4. CD48 is a GPI-anchored protein involved in cellular activation, costimulation, and adhesion, but has not been studied on eosinophils. We demonstrate that human eosinophils from atopic asthmatics display enhanced levels of CD48 expression and that IL-3 up-regulates CD48 expression. Furthermore, cross-linking CD48 on human eosinophils triggers release of eosinophil granule proteins. Assessment of CD48 expression in a murine model of experimental asthma revealed that CD48 is induced by allergen challenge and partially regulated by IL-3. Additionally, anti-IL-3 reduces CD48 expression and the degree of airway inflammation. Thus, CD48 is an IL-3-induced activating receptor on eosinophils, likely involved in promoting allergic inflammation.

The CD2 family of natural killer cell receptors

The CD2 family of receptors is evolutionarily conserved and widely expressed on cells within the hematopoietic compartment. In recent years several new members have been identified with important roles in the immune system. CD2 family members regulate natural killer (NK) cell lytic activity and inflammatory cytokine production when engaged by ligands on tumor cells. Furthermore, a subfamily of CD2 receptors, the CD 150-like molecules, has been implicated in the pathogenesis of X-linked lymphoproliferative disease (XLP). Many of these receptors have now been shown to bind homophilically or heterophilically to other molecules within the family. With these discoveries a novel mechanism for lymphocyte regulation has emerged: CD2 family members on NK cells engage ligands on neighboring NK cells, leading to NK cell stimulation. Moreover, heterotypic stimulatory interactions between NK cells and other leukocytes also occur. In this manner, CD2 family members may provide interlymphocyte communication that maintains organization within the hematopoietic compartment and amplifies immune responses. This review discusses these multiple roles for CD2 family members, focusing specifically on the regulation of NK cells.

2B4/CD48-mediated regulation of lymphocyte activation and function

2B4 (CD244) is a member of the CD2 subset of the Ig superfamily. This molecule is expressed on innate immune cells, including NK cells, and on subsets of T cells. The 2B4 molecule interacts with CD48, which is widely expressed on hemopoietic cells. Although earlier reports demonstrated a role for 2B4 as an activating receptor in both mice and humans, recent studies of 2B4-deficient mice have suggested that 2B4 functions predominantly as an inhibitory receptor in mice. In addition, 2B4 may also act as a costimulatory ligand for cells expressing CD48. Thus, the 2B4 molecule is more multifunctional than previously understood. In this study, we delineate the current view of 2B4-CD48 interactions among lymphocytes and other cells.

Requirement of homotypic NK-cell interactions through 2B4(CD244)/CD48 in the generation of NK effector functions

2B4 belongs to the CD2 subset of the IgG family of receptors. Members in this family have been shown to function as coreceptors via homophilic or heterophilic interactions. Both 2B4 and CD2 bind to CD48, another member of this family. Because all 3 molecules are expressed on natural killer (NK) cells, it raises a possibility that the binding of 2B4 and CD2 to CD48 among NK cells may have functional consequences. Using specific monoclonal antibodies and gene-deficient NK cells, we found that 2B4/CD48, but not CD2/CD48, interaction is essential for IL-2-driven expansion and activation of murine NK cells. In the absence of 2B4/CD48 interaction, NK cytotoxicity and IFN-gamma secretion on tumor target exposure is severely impaired. Impaired activation of NK cells in 2B4-deficient mice was also demonstrated by poor NK-mediated clearance of syngeneic tumor cells in these mice. Functional impairment of NK cells in the absence of 2B4/CD48 interactions was accompanied by defective calcium signaling, suggesting that the early signaling pathway of NK receptors is inhibited. Finally, homotypic interactions among NK cells through 2B4/CD48 was visualized by specific localization of GFP-tagged 2B4 onto NK-NK conjugation sites. Thus, these data identify a novel mechanism whereby NK effector function is regulated via homotypic 2B4/CD48 interactions.

2B4 (CD244) is a non-MHC binding receptor with multiple functions on natural killer cells and CD8+ T cells

2B4 (CD244) is expressed by memory-phenotype CD8(+) T cells and all natural killer (NK) cells. The ligand for 2B4, CD48, is expressed on hematopoietic cells. 2B4 is conserved in humans and mice, and a number of reports have linked 2B4 with activation of lymphocytes. We have employed 2B4-deficient mice and antibody blocking to analyze 2B4 function both in vitro and in vivo and found that 2B4 is a receptor with multiple functions. 2B4 is required for optimal activation of CD8(+) T cells and NK cells--in this context 2B4 requires interaction with CD48 on neighboring lymphocytes, demonstrating that homotypic interaction within NK cell or T cell populations augments immunity. When 2B4 is engaged by CD48 on a target cell, 2B4 conversely inhibits NK effector function. As an inhibitory receptor, 2B4 is unconventional as it is not regulated by MHC class I molecules. In this review we will discuss the significance of these multiple functions and the events that may regulate differential 2B4 signaling outcome.

Functional role played by the glycosylphosphatidylinositol anchor glycan of CD48 in interleukin-18-induced interferon-gamma production

Interleukin (IL)-18 induces T cells and natural killer cells to produce not only interferon-gamma but also other cytokines by binding to the IL-18 receptor (IL-18R) alpha and beta subunits. However, little is known about how IL-18, IL-18Ralpha, and IL-18Rbeta form a high-affinity complex on the cell surface and transduce the signal. We found that IL-18 and IL-18Ralpha bind to glycosylphosphatidylinositol (GPI) glycan via the third mannose 6-phosphate diester and the second beta-GlcNAc-deleted mannose 6-phosphate of GPI glycan, respectively. To determine which GPI-anchored glycoprotein is involved in the complex of IL-18 and IL-18Ralpha, IL-18Ralpha of IL-18-stimulated KG-1 cells was immunoprecipitated together with CD48 by anti-IL-18Ralpha antibody. More than 90% of CD48 was detected as beta-GlcNAc-deleted GPI-anchored glycoprotein, and soluble recombinant human CD48 without GPI glycan bound to IL-18Ralpha, indicating that CD48 is associated with IL-18Ralpha via both the peptide portion and the GPI glycan. To investigate whether the carbohydrate recognition of IL-18 is involved in physiological activities, KG-1 cells were digested with phosphatidylinositol-specific phospholipase C before IL-18 stimulation. Phosphatidylinositol-specific phospholipase C treatment inhibited the phosphorylation of tyrosine kinases and the following IL-18-dependent interferon-gamma production. These observations suggest that the complex formation of IL-18.IL-18Ralpha. CD48 via both the peptide portion and GPI glycan triggers the binding to IL-18Rbeta, and the IL-18.IL-18Ralpha.CD48.IL-18Rbeta complex induces cellular signaling.

2B4 co-stimulation: NK cells and their control of adaptive immune responses

NK cells have primarily been defined by their ability to kill infected cells, tumor cells and some normal cells expressing low levels of MHC class I molecules. NK cells have also been shown to affect adaptive immune responses by their production of both pro- and anti-inflammatory cytokines. Recently it has been shown that adaptive immune responses can be enhanced or maintained also through direct lymphocyte-lymphocyte interactions. One of these interactions was identified to occur between 2B4 and CD48, where 2B4 acted as a co-stimulatory ligand for both NK cells and T cells. In the current article, we discuss the role of 2B4 in the development of adaptive immune responses and the role of NK-T cell interactions in these responses.

Role of Thy-1 in in vivo and in vitro neural development and regeneration of dorsal root ganglionic neurons

We have examined the expression of Thy-1, an abundant glycosylphosphatidylinositol (GPI)-anchored glycoprotein, in dorsal root ganglia (DRG) and associated nerve fascicles, during postnatal development and following a nerve crush. The expression levels of Thy-1 in DRG neurons, dorsal roots, and central processes in spinal cord were rather low at postnatal day 2, and gradually increased as DRG neurons matured. During early development, the expression of Thy-1 within DRG neurons was low and equally distributed between plasma membrane and cytosol. With maturation, the staining intensities of Thy-1 in both the plasma membrane and the cytosol of DRG neurons became increased. We also studied Thy-1 expression in the regeneration of mature DRG neurons following the crush injury of sciatic nerve. Two days after the crush injury, Thy-1 expression dramatically decreased in the DRG neurons on the lesion side. Between 4 and 7 days after the injury, the expression of Thy-1 gradually increased and returned to a normal level 1 week after the sciatic nerve crush. The time course of the up-regulation of Thy-1 expression during regeneration matched that of the recovery of sensory functions, such as pain withdraw reflex, placing reflex, and the score of Basso-Beattie-Bresnahan Locomotor Rating Scale. Taken together, our results suggest that Thy-1 expression is developmentally regulated and is closely associated with the functional maturation of DRG neurons during both postnatal development and nerve regeneration. Furthermore, perturbation of Thy-1 function with anti-Thy-1 antibodies promoted neurite outgrowth from primary cultured DRG neurons, again confirming the inhibitory role of Thy-1 on neurite outgrowth.

NK Cells Stimulate Proliferation of T and NK Cells through 2B4/CD48 Interactions

Few studies have addressed the consequences of physical interactions between NK and T cells, as well as physical interactions among NK cells themselves. We show in this study that NK cells can enhance T cell activation and proliferation in response to CD3 cross-linking and specific Ag through interactions between 2B4 (CD244) on NK cells and CD48 on T cells. Furthermore, 2B4/CD48 interactions between NK cells also enhanced proliferation of NK cells in response to IL-2. Overall, these results suggest that NK cells augment the proliferation of neighboring T and NK cells through direct cell-cell contact. These results provide new insights into NK cell-mediated control of innate and adaptive immunity and demonstrate that receptor/ligand-specific cross talk between lymphocytes may occur in settings other than T-B cell or T-T cell interactions.

c-Src-dependent cross-talk between CEACAM6 and alphavbeta3 integrin enhances pancreatic adenocarcinoma cell adhesion to extracellular matrix components

Carcinoembryonic antigen-related cell adhesion molecule 6 (CEACAM6) is an immunoglobulin superfamily member with a diversity of extracellular ligands that is implicated in the initiation and progression of a variety of malignancies. We sought to characterize the effects of CEACAM6 crosslinking on pancreatic adenocarcinoma cellular interaction with the extracellular matrix (ECM) components fibronectin and vitronectin. Antibody-mediated CEACAM6 crosslinking was performed and the ability of BxPC3 cells, which inherently overexpress CEACAM6, to adhere to fibronectin and vitronectin was quantified. The roles of the archetypal fibronectin (alpha5beta1 integrin) and vitronectin (alphavbeta3 integrin) receptors were determined. The effects of c-Src inhibition were investigated using the Src family kinase inhibitor 4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine (PP2) and c-Src specific RNA interference. CEACAM6 crosslinking initiates c-Src-dependent cross-talk between CEACAM6 and alphavbeta3 integrin, leading to increased ECM component adhesion. CEACAM6-mediated signaling events may contribute to the invasive and metastatic potential of pancreatic adenocarcinoma cells by promoting their interaction with ECM components.

Rafts, little caves and large potholes: how lipid structure interacts with membrane proteins to create functionally diverse membrane environments

This chapter reviews how diverse lipid microdomains form in the membrane and partition proteins into different functional units that regulate cell trafficking, signalling and movement. We will concentrate upon five major issues: 1. the diversity of lipid structure that produces diverse microenvironments into which different subsets of proteins partition; 2. why ordered lipid domains exclude proteins, and the conditions required for select subsets of proteins to enter these domains; 3. the coupling of the inner and outer leaflets within ordered microdomains; 4. the effect of ordered lipid domains upon membrane properties including curvature and hydrophobicity that affect membrane fission, fusion and extension of filopodia; 5. the biological effects of these structural constraints; in particular how the properties of these domains combine to provide a very different signalling, trafficking and membrane fusion environment to that found in disordered (fluid mosaic) membrane. In addressing these problems, the review draws upon studies ranging from molecular dynamic modelling of lipid interactions, through physical studies of model membrane systems to structural and biological studies of whole cells, examining in the process problems inherent in visualising and purifying these microdomains. While the diversity of structure and function of ordered lipid microdomains is emphasised, some general roles emerge. In particular, the basis for having quite different, non-interacting ordered lipid domains on the same membrane is evident in the diversity of lipid structure and plays a key role in sorting signalling systems. The exclusion of ordered membrane from coated pits, and hence rapid endocytosis, is suggested to underlie the ability of highly ordered domains to establish stable secondary signalling systems required, for instance, in T cell receptor, insulin and neurotrophin signalling.

The SAP and SLAM families in immune responses and X-linked lymphoproliferative disease

SAP (signalling lymphocytic activation molecule (SLAM)-associated protein) is a T- and natural killer (NK)-cell-specific protein containing a single SH2 domain encoded by a gene that is defective or absent in patients with X-linked lymphoproliferative syndrome (XLP). The SH2 domain of SAP binds with high affinity to the cytoplasmic tail of the haematopoietic cell-surface glycoprotein SLAM and five related receptors. SAP regulates signal transduction of the SLAM-family receptors by recruiting SRC kinases. Similarly, the SAP-related proteins EAT2A and EAT2B are thought to control signal transduction that is initiated by SLAM-related receptors in professional antigen-presenting cells. In this review, we discuss recent findings on the structure and function of proteins of the SAP and SLAM families.

Positive and negative regulation of T-cell activation through kinases and phosphatases

The sequence of events in T-cell antigen receptor (TCR) signalling leading to T-cell activation involves regulation of a number of protein tyrosine kinases (PTKs) and the phosphorylation status of many of their substrates. Proximal signalling pathways involve PTKs of the Src, Syk, Csk and Tec families, adapter proteins and effector enzymes in a highly organized tyrosine-phosphorylation cascade. In intact cells, tyrosine phosphorylation is rapidly reversible and generally of a very low stoichiometry even under induced conditions due to the fact that the enzymes removing phosphate from tyrosine-phosphorylated substrates, the protein tyrosine phosphatases (PTPases), have a capacity that is several orders of magnitude higher than that of the PTKs. It follows that a relatively minor change in the PTK/PTPase balance can have a major impact on net tyrosine phosphorylation and thereby on activation and proliferation of T-cells. This review focuses on the involvement of PTKs and PTPases in positive and negative regulation of T-cell activation, the emerging theme of reciprocal regulation of each type of enzyme by the other, as well as regulation of phosphotyrosine turnover by Ser/Thr phosphorylation and regulation of localization of signal components.

The roles of membrane microdomains (rafts) in T cell activation

Detergent-resistant membrane microdomains enriched in sphingolipids, cholesterol and glycosylphosphatidylinositol-anchored proteins play essential roles in T cell receptor (TCR) signaling. These 'membrane rafts' accumulate several cytoplasmic lipid-modified molecules, including Src-family kinases, coreceptors CD4 and CD8 and transmembrane adapters LAT and PAG/Cbp, essential for either initiation or amplification of the signaling process, while most other abundant transmembrane proteins are excluded from these structures. TCRs in various T cell subpopulations may differ in their use of membrane rafts. Membrane rafts also seem to be involved in many other aspects of T cell biology, such as functioning of cytokine and chemokine receptors, adhesion molecules, antigen presentation, establishing cell polarity or interaction with important pathogens. Although the concept of membrane rafts explains several diverse biological phenomena, many basic issues, such as composition, size and heterogeneity, under native conditions, as well as the dynamics of their interactions with TCRs and other immunoreceptors, remain unclear, partially because of technical problems.

Lipid rafts and little caves. Compartmentalized signalling in membrane microdomains

Lipid rafts are liquid-ordered membrane microdomains with a unique protein and lipid composition found on the plasma membrane of most, if not all, mammalian cells. A large number of signalling molecules are concentrated within rafts, which have been proposed to function as signalling centres capable of facilitating efficient and specific signal transduction. This review summarizes current knowledge regarding the composition, structure, and dynamic nature of lipid rafts, as well as a number of different signalling pathways that are compartmentalized within these microdomains. Potential mechanisms through which lipid rafts carry out their specialized role in signalling are discussed in light of recent experimental evidence.

Molecular mechanisms for protein kinase A-mediated modulation of immune function

Protein kinase A (PKA) is a serine/threonine kinase that regulates a number of cellular processes important for immune activation and control. Modulation of signal transduction by PKA is a complex and diverse process, and differential isozyme expression, holoenzyme composition and subcellular localization contribute specificity to the PKA signalling pathway. In lymphocytes, phosphorylation by PKA has been demonstrated to regulate antigen receptor-induced signalling both by altering protein-protein interactions and by changing the enzymatic activity of target proteins. PKA substrates involved in immune activation include transcription factors, members of the MAP kinase pathway and phospholipases. The ability of PKA type I to regulate activation of signalling components important for formation of the immunological synapse, demonstrates that the cAMP signalling pathway can directly modulate proximal events in lymphocyte activation. Furthermore, the recent discovery that PKA regulates Src kinases through modulation of Csk, supports the notion that PKA is involved in the fine-tuning of immune receptor signalling in lipid rafts.

Recombinant human interleukins IL-1alpha, IL-1beta, IL-4, IL-6, and IL-7 show different and specific calcium-independent carbohydrate-binding properties

A method was developed for the determination of putative lectin activities of cytokines. It involved the immunoblotting measurement of the quantity of these cytokines unbound to a series of different immobilized glycoconjugates and displacement of the bound cytokines with oligosaccharides of known structures. This method allows demonstrating that the following interleukins specifically recognize different oligosaccharide structures in a calcium-independent mechanism: interleukin-1alpha binds to the biantennary disialylated N-glycan completed with two Neu5Acalpha2-3 residues; interleukin-1beta to a GM4 sialylated glycolipid Neu5Acalpha2-3Galbeta1-Cer having very long and unusual long-chain bases; interleukin-4 to the 1,7 intramolecular lactone of N-acetyl-neuraminic acid; interleukin-6 to compounds having N-linked and O-linked HNK-1-like epitopes; and interleukin-7 to the sialyl-Tn antigen. Because the glycan ligands are rare structures in human circulating cells, it is suggested that such activities could be essential for providing specific signaling systems to cells having both the receptors and the oligosaccharide ligands of the interleukin at their cell surface.

Phosphoprotein associated with glycosphingolipid-enriched microdomains (PAG), a novel ubiquitously expressed transmembrane adaptor protein, binds the protein tyrosine kinase csk and is involved in regulation of T cell activation

According to a recently proposed hypothesis, initiation of signal transduction via immunoreceptors depends on interactions of the engaged immunoreceptor with glycosphingolipid-enriched membrane microdomains (GEMs). In this study, we describe a novel GEM-associated transmembrane adaptor protein, termed phosphoprotein associated with GEMs (PAG). PAG comprises a short extracellular domain of 16 amino acids and a 397-amino acid cytoplasmic tail containing ten tyrosine residues that are likely phosphorylated by Src family kinases. In lymphoid cell lines and in resting peripheral blood alpha/beta T cells, PAG is expressed as a constitutively tyrosine-phosphorylated protein and binds the major negative regulator of Src kinases, the tyrosine kinase Csk. After activation of peripheral blood alpha/beta T cells, PAG becomes rapidly dephosphorylated and dissociates from Csk. Expression of PAG in COS cells results in recruitment of endogenous Csk, altered Src kinase activity, and impaired phosphorylation of Src-specific substrates. Moreover, overexpression of PAG in Jurkat cells downregulates T cell receptor-mediated activation of the transcription factor nuclear factor of activated T cells. These findings collectively suggest that in the absence of external stimuli, the PAG-Csk complex transmits negative regulatory signals and thus may help to keep resting T cells in a quiescent state.

Specificity in signal transduction among glycosylphosphatidylinositols of Plasmodium falciparum, Trypanosoma brucei, Trypanosoma cruzi and Leishmania spp

Glycosylphosphatidylinositols (GPIs) and related glycoconjugates of parasite origin have been shown to regulate both the innate and acquired immune systems of the host. This is achieved through the activation of novel GPI-dependent signalling pathways in macrophages, lymphocytes and other cell types. Parasite GPIs impart at least two distinct signals to host cells through the structurally distinct inositolphosphoglycan (IPG) and fatty acid domains. Binding of IPG to as yet uncharacterized cell surface receptor(s) leads to activation of src-family protein tyrosine kinases: depending upon structure, GPI-derived fatty acids can either activate or antagonize protein kinase C, and may enter the sphingomyelinase pathway. The degree of fatty acid saturation may also contribute to signalling activity. Thus, variation in structure of parasite GPIs imparts different properties of signal transduction upon this class of glycolipid. The divergent activities of GPIs from various protozoal taxa reflect global aspects of the host/parasite relationship, suggesting that GPI signalling is a central determinant of disease in malaria, leishmaniasis and both American and African trypanosomiases.

Compartmentation of Fyn kinase with glycosylphosphatidylinositol-anchored molecules in oligodendrocytes facilitates kinase activation during myelination

In many cell types, glycosylphosphatidylinositol (GPI)-anchored proteins are sequestered in detergent-resistant membrane rafts. These are plasma membrane microdomains enriched in glycosphingolipids and cholesterol and are suggested to be platforms for cell signaling. Concomitant with the synthesis of myelin glycosphingolipids, maturing oligodendrocytes progressively associate GPI-anchored proteins, including the adhesion molecules NCAM 120 and F3, in rafts. Here we show that these microdomains include Fyn and Lyn kinases. Both kinases are maximally active in myelin prepared from young animals, correlating with early stages of myelination. In the rafts, Fyn kinase is tightly associated with NCAM 120 and F3. In contrast, in oligodendrocyte progenitor cells lacking rafts or in raft-free membrane domains of more mature cells, F3 does not associate with Fyn. The addition of anti-F3 antibodies to oligodendrocytes results in stimulation of Fyn kinase specifically in rafts. Compartmentation of oligodendrocyte GPI-anchored proteins in rafts is thus a prerequisite for association with Fyn, permitting kinase activation. Interaction of oligodendrocyte F3 with axonal ligands such as L1 and ensuing kinase activation may play a crucial role in initiating myelination.

The engagement of beta1 integrins on promonocytic cells promotes phosphorylation of Syk and formation of a protein complex containing Lyn and beta1 integrin

The protein-tyrosine kinase Syk participates in signal transduction pathways downstream from multiple immune recognition receptors. Recent evidence indicates that Syk is also functionally coupled to cell surface integrins, which mediate interactions between the actin cytoskeleton and extracellular matrix proteins. The interactions of undifferentiated, promonocytic HL60 or U937 cells with fibronectin or anti-beta1 integrin antibodies leads to an apparent activation and tyrosine phosphorylation of Syk that is independent of tight cellular adhesion and spreading. In response to fibronectin or anti-beta1 integrin antibodies, beta1 integrins become associated with a complex of proteins that include the Lyn protein tyrosine kinase and endogenous kinase substrates of 29 and 75-80 kDa. Lyn becomes transiently activated following integrin engagement and co-localizes with the actin cytoskeleton. These studies suggest a major role for Lyn in coupling beta1 integrins to the activation of Syk.

Microdomain-dependent regulation of Lck and Fyn protein-tyrosine kinases in T lymphocyte plasma membranes

Src family protein-tyrosine kinases are implicated in signaling via glycosylphosphatidylinositol (GPI)-anchored receptors. Both kinds of molecules reside in opposite leaflets of the same sphingolipid-enriched microdomains in the lymphocyte plasma membrane without making direct contact. Under detergent-free conditions, we isolated a GPI-enriched plasma membrane fraction, also containing transmembrane proteins, selectively associated with sphingolipid microdomains. Nonionic detergents released the transmembrane proteins, yielding core sphingolipid microdomains, limited amounts of which could also be obtained by detergent-free subcellular fractionation. Protein-tyrosine kinase activity in membranes containing both GPI-anchored and transmembrane proteins was much lower than in core sphingolipid microdomains but was strongly reactivated by nonionic detergents. The inhibitory mechanism acting on Lck and Fyn kinases in these membranes was independent of the protein-tyrosine phosphatase CD45 and was characterized as a mixed, noncompetitive one. We propose that in lymphocyte plasma membranes, Lck and Fyn kinases exhibit optimal activity when juxtaposed to the GPI- and sphingolipid-enriched core microdomains but encounter inhibitory conditions in surrounding membrane areas that are rich in glycerophospholipids and contain additional transmembrane proteins.

Spontaneous clustering of Thy-1 antigens on CD4+ CD8+ thymocytes lacking TCR engagement by MHC/peptide complexes

While much is known concerning CD4+ CD8+ thymocytes positively or negatively selected through interaction of their TCR with self peptides bound to self-MHC molecules, little is known of the majority of CD4+ CD8+ thymocytes lacking this interaction. We developed a monoclonal antibody (mAb) 1D11, the ligand of which (1D11-L) is expressed on 60-70% of CD4+ CD8+ thymocytes but not on other subsets of thymocytes and peripheral T cells. 1D11-L expression on CD4+ CD8+ thymocytes reversely correlates with their TCR engagement, in vitro and in vivo. In addition, 1D11-L+ CD4+ CD8+ thymocytes were more susceptible than 1D11-L- CD4+ CD8- thymocytes to apoptosis. We also found that T lineage cells other than CD4+ CD8+ thymocytes and a Thy-1-expressing fibroblast cell line became positive for 1D11-L by cross-linking their Thy-1 antigens with anti-Thy-1 mAb but not with their Fab fragment, suggesting that 1D11 recognizes multimerized Thy-1 antigens. Confocal laser scanning microscopy revealed that Thy-1 antigens as well as 1D11-L are clustered on some CD4+ CD8+ thymocytes but not on the other subsets of thymocytes. Taken together, we suggest that clustering of Thy-1 antigens spontaneously and specifically occurs on CD4+ CD8+ thymocytes lacking TCR engagement by MHC/peptide complexes.

Antibody cross-linking of the glycosylphosphatidylinositol-linked protein CD59 on hematopoietic cells induces signaling pathways resembling activation by complement

CD59 is a glycosylphosphatidylinositol-anchored cell surface glycoprotein involved in protecting cells from host-mediated complement attack. Studies have shown that antibody cross-linking of CD59 induces a series of intracellular signaling events including the activation of protein-tyrosine kinases (PTK). To further characterize these events, antibodies and complement 8, one of the natural ligands of CD59, were used to activate CD59. Antibody-induced cross-linking of CD59 on the surface of THP-1 and U937 hematopoietic cell lines as well as exposure to complement 8 induces a rapid increase in the tyrosine phosphorylation of several proteins within the cell. Consistent with an early role for the Src family PTKs in these signaling events, we found that transient activation of Hck- and CD59-mediated signaling was abrogated in the presence of the Src family PTK-selective inhibitor PP1. Although the molecular mechanism by which CD59 communicates to Hck is unknown, cellular fractionation studies indicated that both CD59 and Hck are compartmentalized in plasma membrane microdomains. We also detected tyrosine phosphorylation of the adaptor proteins p120 and Shc, and the cytoplasmic nonreceptor tyrosine kinase Syk. The identification of CD59-mediated signaling events may help explain why paroxysmal nocturnal hemoglobinuria patients, who are deficient in glycosylphosphatidylinositol-linked proteins including CD59, are susceptible to proliferative disorders.

Determination of the non-ionic detergent insolubility and phosphoprotein associations of glycosylphosphatidylinositol-anchored proteins expressed on T cells

Glycosylphosphatidylinositol (GPI)-anchored proteins are poorly solublized in non-ionic detergents such as Triton X-100 and Nonidet P40, but are easily solublized by detergents with high critical micelle concentrations such as octylglucoside. This solubility profile has been suggested to be due to the localization of GPI-anchored proteins to lipid microdomains rich in cholesterol and sphingolipids. Additionally, GPI-anchored proteins expressed on haemopoietic cells have been shown to associate with src-family tyrosine kinases and heterotrimeric G proteins. Despite these observations, the non-ionic detergent insolubility of GPI-anchored proteins on haemopoietic cells has not been quantified nor has a relationship between the non-ionic detergent insolubility of these proteins and their association with signal-transduction molecules been identified. Here we show that GPI-anchored proteins found on T-cell tumours and activated T cells, although significantly more insoluble then transmembrane proteins, are not uniform in their detergent insolubility. Whereas CD59 was between 4% and 13% soluble, CD48 was between 13% and 25% soluble, CD55 was between 20% and 30% soluble, and CD109 was between 34% and 75% soluble. The ability of these GPI-anchored proteins to associate with phosphoproteins was correlated with their detergent insolubility: the more detergent-insoluble that a GPI-anchored protein was, the greater the level of phosphoprotein associations. These experiments reveal a relationship between non-ionic detergent insolubility and association with signal-transduction molecules and suggest a cause-and-effect relationship between these two properties. In total, these experiments support the hypothesis that the association of GPI-anchored proteins with signalling molecules is due to their sorting to lipid microdomains.

Signal transduction in leucocytes via GPI-anchored proteins: an experimental artefact or an aspect of immunoreceptor function?

Membrane proteins anchored in the membrane via a glycolipid glycosylphosphatidylinositol (GPI) as well as some glycolipids are able to transduce signals and induce diverse functional responses in cells upon their cross-linking via antibodies or natural ligands. In some cases this signaling capacity seems to be due to associations of these molecules with specific transmembrane proteins. GPI-anchored proteins are components of membrane microdomains enriched in glycosphingolipids and cholesterol and devoid of most transmembrane proteins. These membrane specializations are relatively resistant to solubilization in solutions of some mild detergents at low temperatures. These 'GPI-microdomains' contain also cytoplasmic signaling molecules such as Src-family protein tyrosine kinases and trimeric G-proteins. Thus, at least some signaling elicited upon cross-linking of GPI-anchored proteins and glycolipids may be due to perturbation of the signaling molecules associated with these microdomains. It is suggested that these specialized areas of the membrane rich in signaling molecules interact with immunoreceptors (TCR, BCR, Fc receptors) cross-linked upon their interactions with ligands and importantly contribute to initiation of proximal phases of their signaling pathways.

Thy-1 is a component common to multiple populations of synaptic vesicles

Thy-1, a glycosylphosphatidylinositol-linked integral membrane protein of the immunoglobulin superfamily, is a component of both large dense-core and small clear vesicles in PC12 cells. A majority of this protein, formerly recognized only on the plasma membrane of neurons, is localized to regulated secretory vesicles. Thy-1 is also present in synaptic vesicles in rat central nervous system. Experiments on permeabilized PC12 cells demonstrate that antibodies against Thy-1 inhibit the regulated release of neurotransmitter; this inhibition appears to be independent of any effect on the Ca2+ channel. These findings suggest Thy-1 is an integral component of many types of regulated secretory vesicles, and plays an important role in the regulated vesicular release of neurotransmitter at the synapse.

The nature of membrane anchorage determines kinase association and detergent solubility of CD4

The presence of a glycosylphosphatidylinositol (GPI) anchor on a membrane protein is thought to influence aspects of the protein's biochemistry. While it has been demonstrated that a GPI-anchor is sufficient for altering the detergent solubility of integral membrane proteins, it has not been shown that the anchor is sufficient for changing the phosphoprotein associations of membrane proteins. In order to define the influence of GPI-anchors on the biochemistry of membrane proteins we compared the phosphoprotein associations and detergent solubility of wild-type and GPI-anchored CD4 expressed on HSB cell transfectants. While wild-type CD4 was mostly associated with lck kinase, GPI-anchored CD4 was associated with the 'GPI-anchored pattern of phosphoproteins'. The Triton X-100 solubilities of the two forms of CD4 were also distinct: wild-type CD4 was > 95% soluble, whereas GPI-anchored CD4 was only 65% soluble. These results underscore the deterministic role of the GPI-anchor in the properties associated with GPI-anchored proteins.

Signal transduction via glycosyl phosphatidylinositol-anchored proteins in T cells is inhibited by lowering cellular cholesterol

Glycosylphosphatidylinositol (GPI)-anchored proteins can deliver costimulatory signals to lymphocytes, but the exact pathway of signal transduction involved is not yet characterized. GPI-anchored proteins are fixed to the cell surface solely by a phospholipid moiety and are clustered in distinct membrane domains that are formed by an unique lipid composition requiring cholesterol. To elucidate the role of membrane lipids for signal transduction via GPI-anchored proteins, we studied the influence of reduced cellular cholesterol content on calcium signaling via GPI-anchored CD59 and CD48 in Jurkat T cells. Lowering cholesterol by different inhibitors of cellular cholesterol synthesis suppressed calcium response via GPI-anchored proteins by about 50%, whereas stimulation via CD3 was only minimally affected (<10%). The decrease in overall calcium response via GPI-anchored proteins was reflected by inhibition of calcium release from intracellular stores. Cell surface expression of GPI-anchored proteins was not changed quantitatively by lowering cellular cholesterol, and neither was the pattern of immunofluorescence in microscopic examination. In addition, the distribution of GPI-anchored proteins in detergent-insoluble complexes remained unaltered. These results suggest that cellular cholesterol is an important prerequisite for signal transduction via GPI-anchored proteins beyond formation of membrane domains.

Signal transduction in macrophages by glycosylphosphatidylinositols of Plasmodium, Trypanosoma, and Leishmania: activation of protein tyrosine kinases and protein kinase C by inositolglycan and diacylglycerol moieties

The perturbation of various glycosylphosphatidylinositol (GPI)-anchored surface proteins imparts profound regulatory signals to macrophages, lymphocytes and other cell types. The specific contribution of the GPI moieties to these events however is unclear. This study demonstrates that purified GPIs of Plasmodium falciparum, Trypanosoma brucei, and Leishmania mexicana origin are sufficient to initiate signal transduction when added alone to host cells as chemically defined agonists. GPIs (10 nM-1 microM) induce rapid activation of the protein tyrosine kinase (PTK) p59(hck) in macrophages. The minimal structural requirement for PTK activation is the evolutionarily conserved core glycan sequence Man alpha1-2Man alpha1-6Man alpha1-4GlcN1-6myo-inositol. GPI-associated diacylglycerols independently activate the calcium-independent epsilon isoform of protein kinase C. Both signals collaborate in regulating the downstream NF-kappa B/rel-dependent gene expression of interleukin 1alpha, tumor necrosis factor (TNF) alpha, and inducible NO synthase. The alkylacyl-glycerol-containing iM4 GIPL of L. mexicana, however, is unable to activate protein kinase C and inhibits TNF expression in response to other agonists, establishing signaling specificity among structurally distinct GPIs. GPI alone appears sufficient to mimic the activities of malaria parasite extracts in the signaling pathway leading to TNF expression. A mAb to GPI blocks TNF induction by parasite extracts indicating that GPI is a necessary agent in this response. As protozoal GPIs are closely related to their mammalian counterparts, the data indicate that GPIs do indeed constitute a novel outside-in signaling system, acting as both agonists and second messenger substrates, and imparting at least two separate signals through the structurally distinct glycan and fatty acid domains. These activities may underlie aspects of pathology and immune regulation in protozoal infections.

Regulation of host cell function by glycosylphosphatidylinositols of the parasitic protozoa

Antigenic variation, antigenic drift, molecular mimicry, intracellular localization and sequestration in privileged sites are important mechanisms of immune evasion by infectious organisms. Added to this however is the phenomenon by which pathogens deliberately regulate host cell function by the production of glycolipids with agonistic or antagonistic signal transduction capacity. Such pro-active glycolipids are often pathogenicity factors, but they also serve as immunomodulators and immunosuppressants, and these activities may serve as mechanisms of immune evasion. Here we review glycosylphosphatidylinositols and related structures, a novel class of glycolipid common to eukaryotic parasites and their hosts, which recent studies suggest may play a role in immune evasion and immunosuppression by regulating host cell function via the activation or suppression of endogenous host signalling pathways.

The association between glycosylphosphatidylinositol-anchored proteins and heterotrimeric G protein alpha subunits in lymphocytes

Glycosylphosphatidylinositol (GPI)-anchored proteins are nonmembrane spanning cell surface proteins that have been demonstrated to be signal transduction molecules. Because these proteins do not extend into the cytoplasm, the mechanism by which cross-linking of these molecules leads to intracellular signal transduction events is obscure. Previous analysis has indicated that these proteins are associated with src family member tyrosine kinases; however, the role this interaction plays in the generation of intracellular signals is not clear. Here we show that GPI-anchored proteins are associated with alpha subunits of heterotrimeric GTP binding proteins (G proteins) in both human and murine lymphocytes. When the GPI-anchored proteins CD59, CD48, and Thy-1 were immunoprecipitated from various cell lines or freshly isolated lymphocytes, all were found to be associated with a 41-kDa phosphoprotein that we have identified, by using specific antisera, as a mixture of tyrosine phosphorylated G protein alpha subunits: a small amount of Gialpha1, and substantial amounts of Gialpha2 and Gialpha3. GTP binding assays performed with immunoprecipitations of CD59 indicated that there was GTP-binding activity associated with this molecule. Thus, we have shown by both immunochemical and functional criteria that GPI-anchored proteins are physically associated with G proteins. These experiments suggest a potential role of G proteins in the transduction of signals generated by GPI-anchored molecules expressed on lymphocytes of both mouse and human.

Association of 75/80-kDa phosphoproteins and the tyrosine kinases Lyn, Fyn, and Lck with the B cell molecule CD20. Evidence against involvement of the cytoplasmic regions of CD20

CD20, a non-glycosylated cell-surface protein expressed exclusively on B lymphocytes, is one of a family of 4-pass transmembrane molecules that also includes the beta chain of the high affinity receptor for IgE. The precise function of CD20 is unknown, although in vitro effects of CD20-specific antibodies on resting B cells indicate that it is able to transduce an extracellular signal affecting the G0/G1 cell cycle transition. Previous studies have demonstrated that CD20-initiated intracellular signals involve tyrosine kinase activation and that CD20 is tightly associated with both serine and tyrosine kinases. Here, analysis of CD20-associated molecules has revealed that CD20 is associated with the Src family tyrosine kinases p56/53lyn, p56lck, and p59fyn and with 75/80-kDa proteins phosphorylated in vivo on tyrosine residues. Mutagenesis of CD20 was performed to define regions of CD20 involved in intermolecular interactions. Mutants were analyzed in the human T lymphoblastoid cell line Molt-4, in which ectopically expressed wild-type CD20 associated with p59fyn, p56lck, and 75/80-kDa phosphoproteins. Deletion of major portions of the cytoplasmic regions of CD20 did not abolish its association with either p75/80 or tyrosine kinases. The interaction between CD20 and the Src-related kinases is therefore likely to be independent of CD20 cytoplasmic domains and may occur indirectly. The interaction may be mediated by the p75/80 phosphoproteins, which were found to be tightly associated with the Src family kinases isolated from the CD20 complex.

Urokinase plasminogen activator receptor, beta 2-integrins, and Src-kinases within a single receptor complex of human monocytes

The glycosylphosphatidylinositol (GPI)-anchored membrane protein urokinase plasminogen activator-receptor (uPA-R; CD87) is one of the key molecules involved in migration of leukocytes and tumor cells. uPA bound to uPA-R provides the cell proteolytic potential used for degradation of extracellular matrix. uPA-R is also involved in induction of cell adhesion and chemotaxis. Here, we provide a molecular explanation for these uPA-R-related cellular events. By size fractionation of monocyte lysate and affinity isolation on its natural ligand uPA, we demonstrate uPA-R as a component of a receptor complex of relatively large size. Reprecipitation and immunoblotting techniques allowed us to detect the protein tyrosine kinases (PTKs) p60fyn, p53/56lyn, p58/64hck, and p59fgr as components of this "uPA-R complex". Activation of monocytes even with enzymatically inactivated uPA resulted in induction of tyrosine phosphorylation, suggesting modulation of uPA-R-associated PTKs upon ligand binding. In spite of their presence in large complexes, we did not find the GPI-linked proteins CD14, CD58, and CD59 in the uPA-R complex, which indicates the presence of different receptor domains containing GPI-linked proteins in monocytes. However, we identified the leukocyte integrins LFA-1 and CR3 as components of the uPA-R complex as indicated by coisolation of these molecules, as well as by cocapping and comodulation of uPA-R and leukocyte integrins on the monocyte surface. The assemblage of uPA-R, PTKs and membrane spanning beta 2-integrins in one receptor complex indicates functional cooperation. In regard to the involvement of these molecules in pericellular proteolysis, signal transduction, as well as adhesion and chemotactic movement, we suggest uPA-R complex as a potential cellular device for cell migration.

An alternative way of CD4 and CD8 association with protein kinases of the Src family

The T-lymphocyte co-receptors of MHC glycoproteins CD4 and CD8 are known to be associated with the protein tyrosine kinase Lck via cysteine-containing sequences in the cytoplasmic domains of CD4 and CD8 and in the N-terminal domain of Lck. Here we demonstrate that a fraction of CD4 and CD8 molecules are associated with very large, detergent-resistant complexes containing several glycosylphosphatidylinositol-anchored proteins, (glyco)lipids, and protein tyrosine kinases Lck and Fyn but apparently no other major transmembrane proteins. Association of Lck and Fyn with these large complexes is, in contrast to simple CD4/CD8-Lck complexes, not sensitive to alkylation with iodoacetamide. These large complexes therefore represent an alternative way of association of CD4 and CD8 with the protein tyrosine kinases, which may play a role in signaling through these receptors.

Recombinant glycosyl-phosphatidylinositol-anchored proteins are not associated with protein kinases in transfected thymoma cells

The cross-linking by antibody of some glycosyl-phosphatidyl-inositol (GPI)-anchored proteins on the plasma membrane of T cells leads to cell activation. Phosphorylation of proteins on tyrosine residues has a central role in the control of T cell activation, and non-receptor protein tyrosine kinases can be coprecipitated with immune complexes of GPI-anchored proteins in T cell lysates. In order to investigate the nature of this interaction, two recombinant GPI-anchored proteins were constructed (using the GPI signal sequence from Thy-1), and their associations with protein tyrosine kinases in stable transfectants of a mouse thymoma have been investigated. One recombinant GPI protein is the extracellular domain of the human complement receptor-1, normally an integral membrane protein, and the other is the secreted protein, human tissue inhibitor of metalloproteinases. The latter protein should be foreign to the cell surface and yet has been expressed as a GPI-anchored protein at levels equivalent to the highly expressed antigens Thy-1 and Ly6.A2 on mouse thymoma cells. Neither of the two recombinant proteins, when immunoprecipitated from NP40 lysates of transfected cells, was associated with protein tyrosine kinases in contrast with the natural endogenous GPI-anchored proteins Thy-1 and Ly6.A2 in non-transfected parental cells. Moreover, high expression of foreign recombinant GPI protein appears to interfere with the association of the natural GPI proteins with protein tyrosine kinases.

Heat-stable antigen/CD24 on mouse T lymphocytes: evidence for a costimulatory function

Heat-stable antigen (HSA)/mouse CD24 (formerly termed Nectadrin) is a membrane glycoprotein with an unusual structure consisting of a small protein core and extensive glycosylation. It is expressed by hematopoietic cells but not by mature T lymphocytes. HSA on accessory cells is an important costimulatory molecule required for the clonal expansion of T lymphocytes. HSA is also involved in cell-cell adhesion events and the isolated antigen has been shown to possess self-binding properties. In the present study we have re-investigated the role of HSA in T cell proliferation. We find that following stimulation of T lymphocytes with concanavalin A or of CD4+ T lymphocytes with a combination of anti-CD3/CD28 monoclonal antibodies (mAb) the HSA antigen is transiently expressed. The expression correlated with the appearance of CD25 and a CD2 activation epitope at the cell surface. Induction of HSA was also seen in vivo on V beta 8+ T lymphocytes in BALB/c mice that were injected with Staphylococcal enterotoxin B. Biosynthetic labeling and analysis of mRNA by reverse transcriptase-polymerase chain reaction showed that HSA was synthesized by activated T lymphocytes. A combination of anti-CD3 and mAb 79 to HSA was incapable of inducing proliferation of purified CD4+ T lymphocytes. However, the antibody strongly enhanced the response obtained with a combination of anti-CD3/CD28 mAb. The augmenting effect of the HSA-specific mAb was dose dependent. Since HSA is bound to the membrane via a glycosyl-phosphatidylinositol (GPI) anchor and GPI-anchored molecules have been implicated in lymphocyte activation, it is conceivable that HSA is not only a costimulatory molecule on accessory cells but is also a signaling molecule in T lymphocytes. The possibility of a homotypic HSA/HSA interaction between T lymphocytes and accessory cells is discussed.