Phosphatidylinositol-anchored membrane proteins and lymphocyte activation

Decay-accelerating factor (CD55): a versatile acting molecule in human malignancies

The decay-accelerating factor (DAF, CD55) physiologically serves as an inhibitor of the complement system. Moreover, DAF is broadly expressed in malignant tumors. Here, DAF seems to dispose of several different functions reaching far beyond its immunological role, e.g., promotion of tumorigenesis, decrease of complement mediated tumor cell lysis, autocrine loops for cell rescue and evasion of apoptosis, neoangiogenesis, invasiveness, cell motility, and metastasis via oncogenic tyrosine kinase pathway activation, and specific seven-span transmembrane receptors (CD97) binding. Furthermore, DAF has already been included in diagnostic or therapeutic studies. Thereby, studies applying monoclonal anti-DAF antibodies and anti-DAF vaccination for a targeted therapy have been enrolled recently.

T-cell-activating monoclonal antibodies, reacting with both leukocytes and erythrocytes, recognize the guinea pig Thy-1 differentiation antigen: characterization and cloning of guinea pig CD90

A glycophosphatidylinositol (GPI)-linked differentiation antigen expressed on guinea pig T and B lymphocytes was identified by several monoclonal antibodies; it has been shown previously that this membrane protein induced strong polyclonal T cell proliferation upon antibody binding and costimulation by PMA. Purification by immunoadsorption and microsequencing revealed that this T-cell-activating protein is the homologue of Thy-1 or CD90. In contrast to the Thy-1 antigen of most other species, guinea pig Thy-1 has a much higher molecular weight, which is due to a more extensive N-linked glycosylation, bringing the molecular weight of the total antigen up to 36 kDa. Molecular cloning of guinea pig Thy-1 indicated that the deduced molecular weight of the protein backbone is 12,777 after removal of an N-terminal 19-amino-acid leader peptide and cleavage of the 31 amino acids for GPI anchoring the C-terminal end. Sequence comparison showed that guinea pig Thy-1 has an 82% homology to human and a 72% homology to mouse Thy-1 on the amino acid level. Immunohistological staining of cryostat sections revealed intensive staining with the monoclonal antibody H154 on fibroblasts, fibrocytes, Kupffer cells, alveolar macrophages, and mesangial cells. As observed in the human, mouse, and rat, Thy-1 is abundant in the guinea pig brain. Unlike Thy-1 expression in other species, guinea pig Thy-1 is strongly expressed on most resting, nonactivated B cells and, to a lesser extent, on erythrocytes. While treatment of erythrocytes and lymphocytes with GPI-specific phospholipase C largely decreased reactivity with mAb H154, T cells retained the proliferative response to antibody and phorbol esters.

Cloning, expression, and mutagenesis of phosphatidylinositol-specific phospholipase C from Staphylococcus aureus: a potential staphylococcal virulence factor

Staphylococcus aureus secretes a phosphatidylinositol (PI)-specific phospholipase C (PI-PLC) which is able to hydrolyze the membrane lipid PI and membrane protein anchors containing glycosyl-PI. The gene for PI-PLC (plc) was cloned from S. aureus into Escherichia coli. Oligonucleotide probes based on partial protein sequence and polyclonal antibodies raised against the purified protein were used to identify positive clones. E. coli transformed with a plasmid containing the plc gene expressed PI-PLC enzyme activity which was abolished by mutagenesis with a tetracycline resistance gene. The plc gene was present in all 15 S. aureus strains examined but not in any of 6 coagulase-negative staphylococcal species. The plc gene contained 984 bp and coded for a mature protein with a calculated molecular mass of 34,107 Da. Amino acid sequence comparisons indicated that the staphylococcal plc gene was similar (51 to 56%) to the PI-PLCs from Bacillus cereus, Bacillus thuringiensis, and Listeria monocytogenes. The recombinant PI-PLC expressed in E. coli was purified and exhibited biochemical properties identical to those of the native PI-PLC from S. aureus. PI-PLC production was decreased in agr mutant strains of S. aureus. However, PI-PLC production by both agr+ and agr mutant strains exhibited a similar dependence on the type of medium used. These data suggested that PI-PLC production was regulated by both agr-dependent and agr-independent mechanisms.

Cross-linking of CD59 and of other glycosyl phosphatidylinositol-anchored molecules on neutrophils triggers cell activation via tyrosine kinase

Many membrane proteins are attached via a glycosyl phosphatidylinositol (GPI) anchor. Proteins anchored in this way make no direct contact with the interior of the cell, therefore a role in signaling or activation would seem unlikely. Nevertheless, cross-linking of GPI-anchored proteins on human and murine T lymphocytes has been shown to cause calcium transients and cell activation. Our studies address the non-lethal events caused by the membrane attack complex of complement, which include release of Ca2+ from intracellular stores, and have suggested that the GPI-anchored complement inhibitor CD59 may be involved in signaling these events. We here report that cross-linking of CD59 on human neutrophils using specific monoclonal antibody and second antibody caused a rapid increase in intracellular free Ca2+ concentration (Ca2+ transient) due to release of Ca2+ from stores and also caused neutrophil oxidase activation. All antibodies against CD59 tested were effective and cross-linking of any other GPI-anchored protein expressed on neutrophils also initiated an increase in intracellular free Ca2+ concentration, whereas cross-linking of transmembrane proteins caused little or no response. A tyrosine kinase-dependent activation pathway was indicated by the demonstration of tyrosine phosphorylation on cross-linking and by blocking of the Ca2+ transient with the tyrosine kinase inhibitor herbimycin.

T cell proliferation induced by monoclonal antibodies to a phosphatidylinositol-linked differentiation antigen of guinea pig lymphocytes

Monoclonal antibodies (mAb) to differentiation antigens frequently influence the in vitro function of antigen-bearing cells. We characterized a 32-36-kDa membrane protein expressed on guinea pig lymphocytes and Langerhans cells. A series of independently derived mAb to this protein, now called guinea pig T cell activation antigen (gpTAA), induced strong proliferation of T cells in vitro. Cross-linking of the mAb by a secondary antibody (rabbit anti-mouse Ig) and costimulation with phorbol 12-myristate 13-acetate were required for activation. Treatment of the cells with phosphatidylinositol-specific phospholipase C greatly reduced the amount of antigen expressed on the cell surface as measured by flow cytometry analysis. This finding indicates that the antigen is anchored to the cell membrane via phosphatidylinositol linkage as shown similarly for other membrane proteins with T cell activating properties, e.g. Thy-1 and Ly-6. The guinea pig protein differs, however, in its molecular weight and tissue distribution from similar proteins identified in the mouse or in the rat system. Unlike Thy-1, gpTAA is also expressed on B Lymphocytes and Langerhans cells. Considering the previously described involvement in cellular adhesion, and the functional characteristics reported here, gpTAA might represent a new species of differentiation antigen with T cell-activating capacity.

Phosphatidylinositol membrane anchors and T-cell activation

A number of lymphocyte proteins are attached to the cell membrane via glycophosphatidylinositol-anchoring domains. Antibodies specific for several of these proteins are potent mitogens for T cells. In this article, Peter Robinson reviews recent evidence for the involvement of these membrane anchors in cell signalling events and discusses their possible significance in providing antigen-independent costimulatory signals to lymphoid cells.

Isolation and characterization of a phosphatidylinositol-glycan-anchor-specific phospholipase D from bovine brain

In recent years an increasing number of proteins has been shown to be membrane-anchored by a covalently attached PtdIns-glycan residue. In mammalian cells little is known about PtdIns-glycan-specific phospholipases which might play a role in the metabolism of PtdIns-glycan-anchored proteins. In order to identify PtdIns-glycan-specific phospholipases, a rapid and sensitive assay for such enzymes was developed using the PtdIns-glycan-anchored amphiphilic membrane form of acetylcholinesterase as substrate. The rate of product formation was monitored by the increase in soluble hydrophilic acetylcholinesterase in the aqueous phase after separation in Triton X-114. With this assay we established the presence of a PtdIns-glycan-specific phospholipase in bovine brain. This enzyme was soluble and could be partially purified by a heat step followed by chromatography on DEAE-cellulose and by gel filtration on Sepharose CL-6B. PtdIns-glycan-specific phospholipase had a high affinity for the PtdIns-glycan anchor of the substrate (Km = 52 nM) and did not degrade either PtdCho or PtdIns. Hydrophobic labeling of the anchor of the substrate with 3-trifluoromethyl-3-(m-[125I]iodophenyl)diazirine [( 125I]TID) caused a marked decrease in the cleavage rate and methylation of the amino group of the glucosamine residue of the anchor decreased the cleavage rate to zero. Using [125I]TID-labeled substrate, diradylglycerol phosphate was identified as the second product showing that the cleavage specificity of PtdIns-glycan-specific phospholipase was that of a phospholipase D. PtdIns-glycan-specific phospholipase D was inhibited by mercurials, omicron-phenanthroline and EGTA. It was stimulated by Ca2+ in micromolar concentrations indicating that PtdIns-glycan-phospholipase D is a Ca2(+)-regulated enzyme.

Inositol is a constituent of detergent-solubilized immunoaffinity-purified rat liver 5'-nucleotidase

myo-Inositol analysis of detergent-solubilized immunoaffinity-purified rat liver 5'-nucleotidase showed the presence of 1 mol of myo-inositol/mol of enzyme monomer. This provides unequivocal evidence that the ectoenzyme 5'-nucleotidase is attached to liver membranes by a glycosyl-phosphatidylinositol lipid anchor.

A glycophospholipid anchor is required for Qa-2-mediated T cell activation

A number of lymphocyte surface proteins are anchored in the cell membrane by glycophosphatidyl inositol (known as GPI) linkages instead of hydrophobic protein domains. Treatment of mouse T lymphocytes with antibodies specific for two such proteins, Thy-1 and Ly-6, are known to induce proliferation. We have found that antibodies specific for Qa-2, a GPI-anchored class I histocompatibility antigen, can also activate mouse T cells. To determine whether the GPI-anchor is important for this pathway of cell activation, we produced transgenic mice expressing either normal GPI-anchored Qa-2, or Qa-2 molecules with a membrane-spanning protein domain derived from H-2. Our studies show that only lymphocytes from transgenic mice carrying GPI-anchored forms of Qa-2 can be activated in vitro by Qa-2-specific antibodies. We also show that transgenic mouse T cells expressing a GPI-anchored form of H-2Db can be activated by anti-H-2Db antibodies. These results strongly indicate that the GPI-anchor is critical for this pathway of T cell activation.