Physiological enzymatic cleavage of leukocyte membrane molecules

Serum levels of soluble vascular cell adhesion molecule-1 (sVCAM-1) are elevated in advanced stages of non-Hodgkin's lymphomas

The serum levels of soluble vascular cell adhesion molecule-1 (sVCAM-1) were measured in 116 patients with non-Hodgkin's lymphomas (NHL) tested previously for soluble intercellular adhesion molecule-1 (sICAM-1). In contrast to Hodgkin's disease and chronic lymphocytic leukaemia, the sVCAM-1 levels in NHL patients were not significantly different from the levels of healthy controls (n=31). However, sVCAM-1 was elevated in advanced stage disease, i.e. stages III+IV. Elevated serum levels of sVCAM-1 were associated with significantly poorer disease-free (p = 0.024) and overall (p = 0.02) survival. sVCAM-1 correlated poorly with other known prognostic variables (LDH, sTK and beta2m) and with sICAM-1. None of the tested markers added prognostic information for disease-free survival independently of Ann Arbor stage and B-symptoms. The expression of VCAM-1 and ICAM-1 in tumour biopsies from 15 patients representing 7 different histologies were examined and compared with the serum levels of the soluble adhesion molecules. No correlation was found between the adhesion molecule expression by vascular endothelium and the corresponding serum levels.

Targeting of gliadin peptides, CD8, alpha/beta-TCR, and gamma/delta-TCR to Golgi complexes and vacuoles within celiac disease enterocytes

Celiac disease (CD) is characterized by autodestruction of enterocytes after exposure of genetically susceptible individuals to dietary gluten. To define the transport pathways of proteins involved in the celiac immune response, we wished to determine the subcellular compartments of the intestinal mucosa where wheat gliadin peptides colocalize with receptors of T lymphocytes, including alpha/beta-TCR, gamma/delta-TCR, and CD8. Semithin and ultrathin frozen section of jejunal biopsies from CD patients and controls were used to perform immunofluorescence and immunogold labeling as well as in situ hybridization experiments. In patients with active CD, we detected gliadin peptides in vacuoles and Golgi complexes of enterocytes. CD8, alpha/beta-TCR, and gamma/delta-TCR were found in vacuoles and Golgi complexes within these gliadin-containing enterocytes in addition to the surface of intraepithelial and mucosal T lymphocytes. In contrast, we observed that the localization of CD4, CD3, T cell-restricted intracellular antigen (TIA), and leukocyte common antigen (LCA) was restricted to lymphocytes in CD patients. We further detected labeling signals for gliadin peptides, CD8, alpha/beta-TCR, and gamma/delta-TCR at the basal membrane of enterocytes that were interdigitated by extensions of lymphocytes. In situ hybridization experiments revealed that CD8 and gamma/delta-TCR were not expressed by CD enterocytes. We conclude that CD8, alpha/beta-TCR, and gamma/delta-TCR are targeted to Golgi complexes and vacuoles of small intestinal enterocytes in active CD. The observed process may be involved in the pathogenesis of CD enterocytes. We propose a mechanism for the uptake of CD8, alpha/beta-TCR, and gamma/delta-TCR by the basolateral membrane of small intestinal enterocytes.

A functional soluble form of the murine mannose receptor is produced by macrophages in vitro and is present in mouse serum

A soluble form of the mannose receptor (sMR) has been found in conditioned medium of primary macrophages in vitro and in mouse serum. sMR was released as a single species, had a smaller size than the cell-associated form, and accumulated in macrophage-conditioned medium, in a cytokine-regulated manner, to levels comparable with those found for cell-associated mannose receptor. Pulse-chase experiments showed that sMR production in culture occurred by constitutive cleavage of pre-existing full-length protein. A binding assay was developed to determine the sugar specificity of sMR and its ability to interact with pathogens and particulate antigens (i.e. Candida albicans and zymosan). Protease inhibitor studies suggested that sMR was produced by cleavage of an intact mannose receptor by a matrix metalloprotease or ADAM metalloprotease. A role for sMR in the immune response is proposed based on its binding properties, regulation by cytokines, and the previous discovery of putative ligands for the cysteine-rich domain of the mannose receptor in lymph nodes and spleen.

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.

Adenosine Triphosphate–Induced Shedding of CD23 and L-Selectin (CD62L) From Lymphocytes Is Mediated by the Same Receptor but Different Metalloproteases

CD23 is a transmembrane protein expressed on the surface of B-lymphocytes that binds IgE, CD21, CD11b, and CD11c. High concentrations of soluble CD23 and L-selectin are found in the serum of patients with B-chronic lymphocytic leukemia (B-CLL). Because extracellular adenosine triphosphate (ATP) causes shedding of L-selectin via activation of P2Z/P2X7 receptors expressed on B-CLL lymphocytes, we studied the effect of ATP on shedding of CD23. ATP-induced shedding of CD23 at an initial rate of 12% of that for L-selectin, whereas the EC50 for ATP was identical (35 μmol/L) for shedding of both molecules. Furthermore, benzoylbenzoyl ATP also produced shedding of CD23 and L-selectin with the same agonist EC50 values for both (10 μmol/L). Inactivation of the P2Z/P2X7 receptor by preincubation with oxidized ATP abolished ATP-induced shedding of both molecules. Moreover, KN-62, the most potent inhibitor for the P2Z/P2X7 receptor, inhibited ATP-induced shedding of both CD23 and L-selectin with the same IC50 (12 nmol/L). Ro 31-9790, a membrane permeant zinc chelator that inhibits the phorbol-ester-stimulated shedding of L-selectin, also inhibited shedding of CD23 from B-CLL lymphocytes. However, the IC50 for this inhibition by Ro31-9790 was different for L-selectin and CD23 (83 v 6 μmol/L, respectively). Although L-selectin was completely shed by incubation of cells with phorbol-ester, CD23 was not lost under these conditions. The data show that extracellular ATP induces shedding of L-selectin and CD23 from B-CLL lymphocytes by an action mediated by the P2Z/P2X7 receptor. However, different membrane metalloproteases seem to mediate the shedding of L-selectin and CD23.

© 1998 by The American Society of Hematology.

The glycosylphosphatidylinositol-anchored form and the transmembrane form of CD58 are released from the cell surface upon antibody binding

The adhesion molecule CD58 is expressed on the cell surface in both a transmembrane form and a glycosylphosphatidylinositol (GPI)-anchored form. Here we report that CD58 is released from JY cells following cross-linking by immobilized anti-CD58 monoclonal antibodies. Antibodies to other cell surface proteins, as well as PMA and LPS, did not trigger CD58 release. The release resulted from membrane cleavage, since biotin-labeled CD58 was released from biotinylated cells, and down-modulation of CD58 surface expression accompanied accumulation of soluble CD58 IN culture media. We have previously reported the isolation of JY variant cells, which lack expression of GPI anchored proteins and thus express only the transmembrane form of CD58. Here we show that these variant cells release CD58 upon crosslinking, indicating that the transmembrane isoform is released, probably by proteolysis. Antibodies directed to the cytoplasmic domain of CD58, in contrast to antibodies against an extracellular epitope of CD58, did not react with released CD58, supporting a membrane cleavage mechanism. It is also shown that CD58, released from [3H]ethanolamine-labeled JY cells, contained ethanolamine. This result demonstrated that the GPI-anchored CD58 can be released in parallel to the transmembrane isoform and that this release does not result from proteolytic cleavage, since cleavage by a protease would have removed the ethanolamine. The present data suggest that the two isoforms of CD58 are released upon antibody binding and that their release is mediated by distinct mechanisms.

Delta9-tetrahydrocannabinol suppresses macrophage costimulation by decreasing heat-stable antigen expression

Delta9-tetrahydrocannabinol (THC) suppresses several immunologic functions of macrophages. The costimulatory activity of a THC-exposed macrophage hybridoma was investigated by its ability to elicit interleukin-2 secretion by a helper T cell hybridoma activated with immobilized monoclonal anti-CD3 antibody. THC added at culture initiation inhibited the T cell response in a dose-dependent manner. When the macrophages were fixed with paraformaldehyde before culture, THC had no effect on T cell stimulation. However, macrophages, which were preincubated with THC and then fixed, were impaired in delivering costimulatory signals to T cells cultured without THC. The drug's inhibitory effect on macrophage costimulatory activity was reversible. THC exposure also decreased macrophage expression of heat-stable antigen (HSA). Antibody blocking experiments showed that HSA expressed on the macrophages provided an important costimulatory signal, whereas B7-1 and B7-2 molecules had a minor role. Treatment of the macrophages with phosphatidylinositol-specific phospholipase C cleaved HSA, but not the transmembrane B7 molecules, from the cell surface. Similar to THC, enzyme treatment significantly diminished macrophage costimulatory activity, which was also reversible. After drug or enzyme removal, HSA expression returned to the control level by 4 h. Therefore, THC suppresses macrophage costimulatory activity by diminishing cell surface expression of HSA.

Adenosine Triphosphate–Induced Shedding of CD23 and L-Selectin (CD62L) From Lymphocytes Is Mediated by the Same Receptor but Different Metalloproteases

CD23 is a transmembrane protein expressed on the surface of B-lymphocytes that binds IgE, CD21, CD11b, and CD11c. High concentrations of soluble CD23 and L-selectin are found in the serum of patients with B-chronic lymphocytic leukemia (B-CLL). Because extracellular adenosine triphosphate (ATP) causes shedding of L-selectin via activation of P2Z/P2X7 receptors expressed on B-CLL lymphocytes, we studied the effect of ATP on shedding of CD23. ATP-induced shedding of CD23 at an initial rate of 12% of that for L-selectin, whereas the EC50 for ATP was identical (35 μmol/L) for shedding of both molecules. Furthermore, benzoylbenzoyl ATP also produced shedding of CD23 and L-selectin with the same agonist EC50 values for both (10 μmol/L). Inactivation of the P2Z/P2X7 receptor by preincubation with oxidized ATP abolished ATP-induced shedding of both molecules. Moreover, KN-62, the most potent inhibitor for the P2Z/P2X7 receptor, inhibited ATP-induced shedding of both CD23 and L-selectin with the same IC50 (12 nmol/L). Ro 31-9790, a membrane permeant zinc chelator that inhibits the phorbol-ester-stimulated shedding of L-selectin, also inhibited shedding of CD23 from B-CLL lymphocytes. However, the IC50 for this inhibition by Ro31-9790 was different for L-selectin and CD23 (83 v 6 μmol/L, respectively). Although L-selectin was completely shed by incubation of cells with phorbol-ester, CD23 was not lost under these conditions. The data show that extracellular ATP induces shedding of L-selectin and CD23 from B-CLL lymphocytes by an action mediated by the P2Z/P2X7 receptor. However, different membrane metalloproteases seem to mediate the shedding of L-selectin and CD23.

© 1998 by The American Society of Hematology.

Glycoprotein-inspired materials promote the proteolytic release of cell surface L-selectin

The proteolytic release, or shedding, of a cell surface protein can serve a regulatory role; the process liberates a soluble form of the protein into circulation while downregulating its cell surface concentration. The characteristics that render a protein susceptible to proteolytic cleavage are not known. We hypothesized that the clustering of a protein at the cell surface might target it for proteolysis. To test this hypothesis, we synthesized molecules that display multiple copies of sulfated galactose residues, termed neoglycopolymers, that are designed to mimic natural ligands for the cell adhesion protein L-selectin. We found that treatment of human neutrophils with the neoglycopolymers resulted in a dose-dependent loss of L-selectin from the cell surface, while monovalent compounds and unsulfated neoglycopolymers had no effect. Because L-selectin is an important mediator in the inflammatory response, such compounds could lead to novel antiinflammatory drugs. Moreover, molecules that control receptor shedding can be used to alter cellular responsiveness to specific ligands or to promote responses at distal sites; consequently, these results have broad implications for regulating the location and presentation of important biomolecules.

Soluble vascular cell adhesion molecule-1 (sVCAM-1) is an independent prognostic marker in Hodgkin's disease

Serum levels of soluble vascular cell adhesion molecule-1 (sVCAM-1. sCD106) were significantly elevated in patients with Hodgkin's disease (HD) (n = 101) compared to controls (n= 31) (P<0.0001). sVCAM-1 correlated with histology, stage, B-symptoms, and prognostic markers (sICAM-1, sCD30, sIL-2R, LDH). sVCAM-1, sICAM-1 and sCD30 added independent prognostic information for both disease-free and overall survival. 14 biopsies from 13 patients with HD were immunostained for VCAM-1 and ICAM-1. The vascular endothelium stained positive for VCAM-1 in 10/12 evaluable biopsies and for ICAM-1 in all evaluable biopsies. A stromal expression of both adhesion molecules precluded a precise evaluation of HRS-cells. This led us to investigate VCAM-1 (and ICAM-1) expression in six Hodgkin cell lines (HDLM-2, L428, L540, L591, DEV, KM-H2). Two cell lines stained positive for VCAM-1 (HDLM-2, L591). All cell lines stained positive for ICAM-1. sVCAM-1 is a new prognostic marker in HD; its predictive power equals or surpasses that of sCD30 and sICAM-1. Furthermore, two Hodgkin cell lines stained positive for VCAM-1. This indicates that VCAM-1 may be expressed by some HD tumour cells in vivo.

Circulating Form of Human Vascular Adhesion Protein-1 (VAP-1): Increased Serum Levels in Inflammatory Liver Diseases

Vascular adhesion protein-1 (VAP-1) is a dimeric 170-kDa endothelial transmembrane molecule that under normal conditions is most strongly expressed on the high endothelial venules of peripheral lymph nodes and on hepatic endothelia. It is a glycoprotein that mediates tissue-selective lymphocyte adhesion in a sialic acid-dependent manner. In this study, we report the detection of a soluble form of VAP-1 in circulation. We developed a quantitative sandwich ELISA using novel anti-VAP-1 mAbs and used it to determine the levels of soluble VAP-1 (sVAP-1) in the serum of healthy individuals and in patients with inflammatory diseases. In healthy persons, circulating sVAP-1 concentrations were 49 to 138 ng/ml. Immunoblotting studies revealed that the apparent molecular mass of dimeric sVAP-1 is slightly (∼10 kDa) higher than that of transmembrane VAP-1 under nonreducing conditions. In contrast, the electrophoretic mobilities of monomeric sVAP-1 and transmembrane VAP-1 were similar after reduction and boiling. Adhesion assays showed that the circulating sVAP-1 modulates lymphocyte binding to endothelial cells. Inflammation can cause an elevation of serum sVAP-1 levels, because sVAP-1 concentrations in patients with certain liver diseases were two- to fourfold higher than those in normal individuals. In contrast, rheumatoid arthritis and inflammatory bowel diseases were not associated with elevated levels of sVAP-1. These findings indicate that there is a functionally active, soluble form of VAP-1 in circulation and suggest that the serum level of sVAP-1 might be a useful marker of disease activity in inflammatory liver diseases.

Tonic neurogenic inhibition of interleukin-6 secretion from murine spleen caused by opioidergic transmission

The peripheral nervous system and the immune system were shown to have neurohumoral interactions. This study extends observations that demonstrated neuronal modulation of spontaneous interleukin-6 (IL-6) secretion in the spleen by norepinephrine (NE) and beta-endorphin. Spontaneous IL-6 secretion in vivo was markedly reduced by removal of macrophages with the clodronate technique. Furthermore, spontaneous IL-6 secretion was significantly inhibited at physiological concentrations of cortisol (10(-7) M). In the presence of 10(-7) M cortisol, addition of norepinephrine (NE; 10(-5) M) and isoproterenol (10(-6) and 10(-5) M) significantly increased spontaneous IL-6 secretion (+20%; P = 0.0280, P = 0.0005, and P = 0.0050, respectively). In contrast, addition of beta-endorphin significantly inhibited spontaneous IL-6 secretion in the presence of 10(-7) M cortisol (-40%; 10(-11) M, P = 0.0410; 10(-10) M, P = 0.0005). To study the effect of endogenously released transmitters on spontaneous IL-6 secretion, spleen slices were electrically stimulated with 1, 5, 10, 50, and 100 Hz. Spontaneous IL-6 secretion was markedly reduced at a frequency of 10 Hz with 10(-7) M cortisol present (P < 0.0001). This indicates that the combination of nerve firing at 5-10 Hz and physiological cortisol conditions inhibits spontaneous IL-6 secretion. Inhibition of spontaneous IL-6 secretion from spleen macrophages is most probably due to a net inhibitory effect of opioidergic transmission under these conditions.

Transforming the cell surface through proteolysis

Protein shedding, or the proteolytic cleavage of a protein from the surface of a cell, is emerging as an important mechanism in the regulation of cellular activity but it is poorly understood. Growing evidence suggesting that protein shedding and protein function are closely linked may lead to new strategies for the treatment of a wide range of diseases.

Mutation of Proline 211 Reduces Shedding of the Human p75 TNF Receptor

We have identified a residue necessary for the cleavage of human p75 TNF-R from the cell surface by deletion and mutagenesis analysis of the membrane-proximal domain between amino acids 147 and 221. Deletion analysis of this area showed that residues between amino acids 207 and 216 are required for shedding. Site-directed mutagenesis of proline 211 to glycine reduced PMA-induced shedding of human p75 TNF-R from COS-7 or Raw 264.7 cells. Mutation of glycine 210 to aspartic acid did not affect receptor shedding. Mutation of serine 212 to leucine did not affect the PMA-induced shedding from the surface of COS-7 cells, but reduced the efficiency of shedding from the surface of Raw 264.7 macrophages by PMA or LPS. Deletion of putative elastase cleavage sites at amino acids 151 to 153, 162 to 163, and 177 to 178 (also a putative metalloprotease site similar to the cleavage site of TNF-α) or mutation of a serine-threonine-serine motif resembling a similar motif at the site of L-selectin cleavage at amino acids 202 to 204 did not reduce shedding of p75 TNF-R after treatment of cells with PMA. This work shows that a single amino acid mutation at proline 211 of human p75 TNF-R can prevent shedding from the cell surface, and that deletion of other previously proposed putative cleavage sites of the human p75 TNF-R does not prevent its shedding.

Immortalized rat proximal tubule cells produce membrane bound and soluble megalin

Megalin (gp330), a glycoprotein receptor found on renal proximal tubule cells and several other epithelial cells, is deduced to be a type I integral membrane protein, but may also exist as a cell surface form lacking a cytoplasmic domain. Furthermore, soluble megalin products have been detected in urine, and in culture medium of a rat yolk sac carcinoma cell line, combined with receptor associated protein (RAP). Permanent renal cell lines expressing megalin were unavailable until the recent description of two immortalized rat proximal tubule cell lines (IRPTC). The present study demonstrated megalin on IRPTC surface by immunofluorescence, without surface staining for RAP, which was, however, readily detected within cells. Antibodies to ectodomain megalin epitopes immunoprecipitated megalin products both from cell lysates and culture medium, whereas antibodies to cytoplasmic domain epitopes precipitated megalin only from lysates. Western blots showed two major megalin products in medium, a prominent band at approximately 200 kDa, and a fainter band above 400 kDa, slightly below intact megalin in cell lysates. Anti-receptor associated protein antibodies immunoprecipitated megalin from IRPTC lysates, but not from media. We propose that portions of megalin are spontaneously produced by IRPTC, probably either by cleavage in the ectodomain or release of forms lacking a cytoplasmic domain.

Role of metalloproteases in the release of the IL-1 type II decoy receptor

The IL-1 type II receptor (decoy RII) is a nonsignaling molecule the only established function of which is to capture IL-1 and prevent it from interacting with signaling receptor. The decoy RII is released in a regulated way from the cell surface. Here, we reported that hydroxamic acid inhibitors of matrix metalloproteases inhibit different pathways of decoy RII release, including the following: (a) the slow (18 h) gene expression-dependent release from monocytes and polymorphonuclear cells exposed to dexamethasone; (b) rapid release (minutes) from myelomonocytic cells exposed to tumor necrosis factor, chemoattractants, or phorbol myristate acetate; (c) phorbol myristate acetate-induced release from decoy RII-transfected fibroblasts and B cells. Inhibition of release was associated with increased surface expression of decoy RII. Inhibitors of other protease classes did not substantially affect release. However, serine protease inhibitors increased the molecular mass of the decoy RII released from polymorphonuclear cells from 45 to 60 kDa. Thus, irrespective of the pathway responsible for release and of the cellular context, matrix metalloproteases, rather than differential splicing, play a key role in production of soluble decoy RII.

Expression characteristics of the human CD97 antigen

Molecules whose expression is increased upon stimulation on leukocyte populations are of considerable interest because insights into their structure and function extend knowledge of the intracellular and intercellular events that accompany cellular activation. One such molecule is CD97, a cell surface antigen that is rapidly upregulated upon activation on lymphocytes. Increasing CD97 levels on peripheral blood lymphocytes (PBL) induced by different stimuli were found to be largely independent on de novo RNA and protein synthesis during the early stage of activation. Thus, inhibition of CD97 surface expression by cycloheximide was not noticeable or insignificant for 15 min to 4 h after stimulation. Furthermore, a fraction of intracellular CD97 decreased within 2 h suggesting redistribution of CD97 protein. Later, de novo protein synthesis apparently contributes to the induction of high CD97 surface density and inhibition by cycloheximide was more pronounced. Upregulation of CD97 on PBL involves protein kinase C-, tyrosine protein kinase- and Ca2+-dependent intracellular pathways. The effect on CD97 surface expression of the phorbol ester, phorbol 12-myristate 13-acetate (PMA), is different in PBL and Jurkat T cells. Whereas it stimulated after 22 h strong CD97 increase on PBL it suppressed CD97 expression on Jurkat T cells. CD97 expression, which is strong and constitutive on myelo-monocytic cells, is shown to be not leukocyte-restricted. Thus, CD97 transcripts were found in most of the investigated nonhematopoietic cell types and CD97 protein was detected on the cell surface at low amount. In a previous report, sequence data of a CD97 cDNA suggested a protein homologous to the secretin receptor superfamily and consisting of 722 amino acids with 8 potential glycosylation sites. According to this finding, glycoproteins displaying apparent molecular weights in the region of 70-85 kDa were detected in all investigated cell types. These molecules may represent differentially glycosylated and sialylated molecular forms of the same polypeptide. The findings support the notion that CD97 is broadly distributed and possesses a differentially regulated expression behavior on leukocytes and non-hematopoietic cells.

Detection of a soluble form of the leukocyte surface antigen CD48 in plasma and its elevation in patients with lymphoid leukemias and arthritis

Proteins with glycosylphosphatidylinositol (GPI) anchors exhibit a range of activities and some of these proteins exist in both a membrane-associated and a soluble form. CD48 is a 47-kd GPI-linked glycoprotein which is expressed on T and B lymphocytes, monocytes, and many lymphoid malignancies. The biological function of CD48 is unknown. We describe the detection of a soluble form of CD48 in plasma and serum. Its level was quantified by an immunoenzymometric assay (IEMA) specific for soluble CD48. While soluble CD48 was detected in the plasma of healthy individuals (median = 29 ng/ml; range, 15-48 ng/ml), elevated levels were detected in some patients with lymphoproliferative disease (median = 41 ng/ml; range, 9-213 ng/ml, arthritis (median = 42 ng/ml; range, 13-67 ng/ml), and acute EBV infection (174 ng/ml). Soluble CD48 was also detectable in tissue culture supernatants from the Raji lymphoid cell line. The mechanism of CD48 release from cells is unclear. The finding of significant levels of soluble CD48 in plasma and the development of a sensitive IEMA for its measurement will facilitate further studies on its normal function and its role in disease.

Purification and properties of a membrane aminopeptidase from Ascaris suum muscle that degrades neuropeptides AF1 and AF2

We have identified on the membranes of the locomotory muscle of Ascaris suum an amastatin-sensitive aminopeptidase that hydrolyses the bioactive neuropeptides AF1 (KNEFIRF-NH2) and AF2 (KHEYLRF-NH2), by cleavage of the Lys1-Asn2 and Lys1-His2 peptide bonds, respectively. AF2 (1.2 nmol of HEYLRF-NH2 formed min[-1] (mg protein[-1])) was hydrolysed at a faster rate compared to AF1 (0.2 nmol of NEFIRF-NH2 formed min[-1] (mg protein[-1])). AF1 hydrolysis by the aminopeptidase was inhibited by the amastatin (IC50, 9.0 microM), leuhistin (IC50, 1.25 microM) but was insensitive to puromycin, indicating a similarity to mammalian aminopeptidase N. The enzyme was also inhibited by arphamenine B (IC50, 9.0 microM), (2S, 3R)-3-amino-2-hydroxy-4-(4-nitrophenyl)butanoyl-L-leucine (IC50, 8.0 microM), bestatin (IC50, 15.0 microM) and 1 mM 1-10 bis-phenanthroline. The detergent Triton X-100 solubilised enzyme had a pI of 5.0 and after 1000-fold purification by ion-exchange chromatography, appeared to have a Mr of around 240,000 by SDS-PAGE. The purified aminopeptidase had a Km of 534 microM for the hydrolysis of AF1 and cleaved Phe1 from FMRF-NH2, but was unable to hydrolyse DFMRF-NH2 or FDMRF-NH2. The aminopeptidase that we have described in this report might have a role in the extracellular metabolism and inactivation of neuropeptides acting on the locomotory muscle of A. suum.

Differential regulation and function of CD73, a glycosyl-phosphatidylinositol-linked 70-kD adhesion molecule, on lymphocytes and endothelial cells

CD73, otherwise known as ecto-5'-nucleotidase, is a glycosyl-phosphatidylinositol-linked 70-kD molecule expressed on different cell types, including vascular endothelial cells (EC) and certain subtypes of lymphocytes. There is strong evidence for lymphocyte CD73 having a role in several immunological phenomena such as lymphocyte activation, proliferation, and adhesion to endothelium, but the physiological role of CD73 in other cell types is less clear. To compare the biological characteristics of CD73 in different cell types, we have studied the structure, function, and surface modulation of CD73 on lymphocytes and EC. CD73 molecules on lymphocytes are shed from the cell surface as a consequence of triggering with an anti-CD73 mAb, mimicking ligand binding. In contrast, triggering of endothelial CD73 does not have any effect on its expression. Lymphocyte CD73 is susceptible to phosphatidylinositol phospholipase, whereas only a small portion of CD73 on EC could be removed by this enzyme. Furthermore, CD73 on EC was unable to deliver a tyrosine phosphorylation inducing signal upon mAb triggering, whereas triggering of lymphocyte CD73 can induce tyrosine phosphorylation. Despite the functional differences, CD73 molecules on lymphocytes and EC were practically identical structurally, when studied at the protein, mRNA, and cDNA level. Thus, CD73 is an interesting example of a molecule which lacks structural variants but yet has a wide diversity of biological functions. We suggest that the ligand-induced shedding of lymphocyte CD73 represents an important and novel means of controlling lymphocyte-EC interactions.

Novel pathogenic mechanism of microbial metalloproteinases: liberation of membrane-anchored molecules in biologically active form exemplified by studies with the human interleukin-6 receptor

Certain membrane-anchored proteins, including several cytokines and cytokine receptors, can be released into cell supernatants through the action of endogenous membrane-bound metalloproteinases. The shed molecules are then able to fulfill various biological functions; for example, soluble interleukin-6 receptor (sIL-6R) can bind to bystander cells, rendering these cells sensitive to the action of IL-6. Using IL-6R as a model substrate, we report that the metalloproteinase from Serratia marcescens mimics the action of the endogenous shedding proteinase. Treatment of human monocytes with the bacterial protease led to a rapid release of sIL-6R into the supernatant. This effect was inhibitable with TAPI [N-(D,L-[2-(hydroxyaminocarbonyl)methyl]-4-methylpentanoyl) L-3-(2' naphthyl)-alanyl-L-alanine, 2-aminoethyl amide], a specific inhibitor of the membrane-bound intrinsic metalloproteinase, but not with other conventional proteinase inhibitors. sIL-6R-liberating activity was also detected in culture supernatants of Staphylococcus aureus, Pseudomonas aeruginosa, and Listeria monocytogenes, organisms that are known to produce metalloproteinases. sIL-6R released through the action of S. marcescens metalloproteinase retained biological activity and rendered IL-6-unresponsive human hepatoma cells sensitive to stimulation with IL-6. This was shown by Northern (RNA) blot detection of haptoglobin mRNA and by quantitative measurements of de novo-synthesized haptoglobin in cell supernatants. Analysis of immunoprecipitated, radiolabeled sIL-6R revealed that the bacterial protease cleaved IL-6R at a site distinct from that utilized by the endogenous protease. These studies show that membrane-anchored proteins can be released in active form through cleavage at multiple sites, and they uncover a novel mechanism via which microbial proteases possibly provoke long-range biological effects in the host organism.

Selective killing of human monocytes and cytokine release provoked by sphingomyelinase (beta-toxin) of Staphylococcus aureus

The best-known activity of Staphylococcus aureus sphingomyelinase C, alias beta-toxin, is as a hemolysin that provokes hot-cold lysis of erythrocytes which contain substantial amounts of sphingomyelin in the plasma membrane. Sheep erythrocytes are most susceptible, and we found that one hemolytic unit, representing the toxin concentration that elicits 50% hemolysis of 2.5 X 10(8) erythrocytes per ml, corresponds to 0.05 enzyme units or to approximately 0.25 microg of sphingomyelinase per ml. The cytotoxic action of beta-toxin on nucleated cells has not been described in any detail before, and the present investigation was undertaken to fill this information gap. We now identify beta-toxin as a remarkably potent monocytocidal agent. At a concentration of 0.001 U/ml, corresponding to approximately 5 ng/ml, beta-toxin killed over 50% of human monocytes (10(6) cells per ml) within 60 min. By contrast, 1 to 5 microg of beta-toxin per ml had no cytocidal effects on human granulocytes, fibroblasts, lymphocytes, or erythrocytes. A selective monocytocidal action was also observed with sphingomyelinase C from Bacillus cereus and a Streptomyces sp., whereas phospholipase A2 and phospholipase D at 100 U/ml were without effect. Monocytes succumbing to the action of beta-toxin processed and released interleukin-1beta, soluble interleukin-6 receptor, and soluble CD14 into the supernatant. Thus, monocyte killing by beta-toxin is associated with cytokine-related events that are important for the initiation and progression of infectious disease. These findings uncover a potentially important role for sphingomyelinase as a determinant of microbial pathogenicity.

Pore-forming toxins trigger shedding of receptors for interleukin 6 and lipopolysaccharide

Cleavage of membrane-associated proteins with the release of biologically active macromolecules is an emerging theme in biology. However, little is known about the nature and regulation of the involved proteases or about the physiological inducers of the shedding process. We here report that rapid and massive shedding of the interleukin 6 receptor (IL-6R) and the lipopolysaccharide receptor (CD14) occurs from primary and transfected cells attacked by two prototypes of pore-forming bacterial toxins, streptolysin O and Escherichia coli hemolysin. Shedding is not induced by an streptolysin O toxin mutant which retains cell binding capacity but lacks pore-forming activity. The toxin-dependent cleavage site of the IL-6R was mapped to a position close to, but distinct from, that observed after stimulation with phorbol myristate acetate. Soluble IL-6R that was shed from toxin-treated cells bound its ligand and induced an IL-6-specific signal in cells that primarily lacked the IL-6R. Transsignaling by soluble IL-6R and soluble CD14 is known to dramatically broaden the spectrum of host cells for IL-6 and lipopolysaccharide, and is thus an important mechanism underlying their systemic inflammatory effects. Our findings uncover a novel mechanism that can help to explain the long-range detrimental action of pore-forming toxins in the host organism.

Human lymphocytes shed a soluble form of CD21 (the C3dg/Epstein-Barr virus receptor, CR2) that binds iC3b and CD23

We report on a soluble (s) form of CD21 (the C3dg/Epstein-Barr virus receptor, CR2) that is spontaneously released by B and T lymphocytes. Immunoprecipitation with anti-CD21 mAb of culture supernatants of surface and biosynthetically labeled B and T cell lines revealed a single band with an apparent molecular mass of 135 kDa. The molecule exhibited a molecular mass 10 kDa lower than that of membrane CD21. The release of soluble CD21 (sCD21) was time dependent and correlated with a parallel decrease in the expression of the membrane-associated molecule. The protein was also found in culture supernatants of tonsillar B cells and normal human thymocytes. Epitopic analysis using combinations of anti-CD21 monoclonal antibodies (mAb) indicated that sCD21 and membrane CD21 were similarly recognized by mAb directed against short consensus repeats (SCR) 1-2, SCR 4-5 and SCR 9-11. Affinity-purified sCD21 was capable of binding to purified human iC3b and to human recombinant CD23, as assessed by enzyme-linked immunosorbent assay and by using the BIAcore technology. In addition, normal human serum was found to contain a soluble form of CD21 that exhibited a similar molecular mass to that of the molecule shed by B and T cells in culture. The serum form of CD21 was recognized by all anti-CD21 mAb that we tested and showed a high reactivity with mAb directed against SCR 1-2. Our observations suggest that B and T cells shed the extracellular portion of CD21 and release a soluble molecule that retains the ligand-binding properties of CD21, thus having a potential role in immunoregulation.

Bacterial superantigens induce V beta-specific T cell receptor internalization

Staphylococcal enterotoxins can cause toxic shock syndrome and autoimmune diseases. Circulating T cells from these diseases have a very wide range of expression in particular T cell receptor (TCR) beta chain variable regions (V beta). One possibility for this wide range of TCR V beta expression is that during acute infection with organisms secreting superantigens (SAg) these potent molecules might modulate TCR expression. To test this hypothesis, we investigated the potential effects of SAg on TCR V beta cell surface expression. Peripheral blood mononuclear cells (PBMC) from healthy donors were incubated with staphylococcal SAg. Toxic shock syndrome toxin-1 (TSST-1) induced downregulation of V beta 2 expression, whereas staphylococcal enterotoxin (SE) B induced V beta 3-and V beta 12-specific downregulation. TSST-1 did not interfere with anti-V beta 2 mAb binding. Therefore, this downregulation was not due to steric hindrance of Ab binding by TSST-1. TSST-1 induced V beta 2 downregulation was time-, dose- and temperature-dependent. CD3 expression decreased in parallel with reduction of V beta expression. CD4 and CD8 expression were only slightly decreased. CD2, CD25 and HLA-DR expression were upregulated following TSST-1 stimulation of T cell lines. To investigate the fate of TCR after toxin stimulation, V beta 8+ Jurkat T cells were incubated with SEE which is known to stimulate V beta 8+ T cells, and analysed with fluoresence microscopy, and immunoprecipitation and Western blotting. After SEE stimulation, there was an increase in V beta 8 molecules found in the cytoplasm which correlated with loss of cell surface V beta 8 molecules, suggesting internalization of cell surface V beta 8 molecules was induced by SEE stimulation. Shedding of V beta 8 molecules into the culture supernatant was not detected. These data demonstrate that SAg mediated downregulation of TCR expression occurs primarily as the result of TCR internalization. This downregulation phenomenon may have physiological and pathological consequences in patients infected with Staphylococcus aureus.

Elevated levels of the soluble form of bone marrow stromal cell antigen 1 in the sera of patients with severe rheumatoid arthritis

OBJECTIVE

Bone marrow stromal cell antigen 1 (BST-1) is a novel glycosyl phosphatidylinositol-anchored ectoenzyme, which is overexpressed on bone marrow stromal and synovial cell lines derived from patients with rheumatoid arthritis (RA). To investigate the pathophysiologic roles of BST-1 in RA, we established an enzyme-linked immunosorbent assay (ELISA) system to detect the soluble form of BST-1 (sBST-1) and examined levels of sBST-1 in the sera of RA patients.

METHODS

Concentrations of sBST-1 in sera from healthy donors and from patients with RA, osteoarthritis, Sjögren's syndrome, and systemic lupus erythematosus were measured with the ELISA.

RESULTS

In 7% of the RA patient samples (10 of 143), concentrations of serum sBST-1 were higher (approximately 30-50-fold) than in non-RA samples. Serum sBST-1 concentrations showed no correlation with age, C-reactive protein level, or rheumatoid factor level. All RA patients with high concentrations of serum sBST-1 had severe disease with involvement of several large joints.

CONCLUSION

We believe the measurement of serum sBST-1 may have prognostic value, but further analysis is necessary to clarify the clinical significance of elevated sBST-1 in RA.

Cellular mechanisms of adaptation of grafts to antibody

New, more effective, strategies of immunosuppression, including those recently designed to induce durable T cell tolerance (by grafting allogeneic or xenogeneic haematopoietic cells into T lymphocyte-depleted recipients), leave humoral rejection as the main barrier to transplantation of vascularized organs between different species. Recent experimental work indicates that hyperacute rejection can be prevented by manipulations of antibodies and complement. In this paper, we review the mechanisms governing the interaction of antibodies with cell surface antigens in vitro and in vivo, and their cellular consequences. Evidence is presented that, in appropriate conditions, antibodies can protect by effecting modification of graft antigenicity (adaptation or accommodation).

Soluble T cell receptors: detection and quantitative assay in fluid phase via ELISA or immuno-PCR

To establish the concentration range in which soluble murine T cell receptors (sTCR), derived from the Th2 clone D10, exhibited biological activity, and to follow production and purification of D10 sTCR, we devised four quantitative immunoassays: three ELISA systems, and an immuno-PCR assay. The direct ELISA, employed hamster anti-TCR beta monoclonal antibody (H57), which detects all types of alpha beta TCR, regardless of their variable regions, and had a detection limit of about 6 ng/ml sTCR. The indirect sandwich ELISA employed anti-V beta 8 as capture antibody, and had a detection limit of 600 pg/ml. With the direct sandwich ELISA, that also employed anti-V beta 8, TCR concentrations as low as 100 pg/ml could be detected. The ELISA assays were specific for soluble alpha beta TCR, and showed no cross-reactivity when employing two control hamster anti-gamma delta TCR mAbs (GL3 and UC7), or with anti-TCR beta and monoclonal hamster IgG as a control antigen. Further, we demonstrated that in some assays where use of passive binding ELISA plates resulted in a high background, replacement with covalent binding ELISA plates resulted in an acceptable low background value. With the immuno-PCR assay, concentrations of sTCR as little as 0.8 pg/ml could be detected. In summary, the assays described here may prove valuable in investigating the occurrence and amount of sTCR in vitro and in vivo.

Chemokines regulate cellular polarization and adhesion receptor redistribution during lymphocyte interaction with endothelium and extracellular matrix. Involvement of cAMP signaling pathway

Leukocyte recruitment is a key step in the inflammatory reaction. Several changes in the cell morphology take place during lymphocyte activation and migration: spheric-shaped resting T cells become polarized during activation, developing a well defined cytoplasmic projection designated as cellular uropod. We found that the chemotactic and proinflammatory chemokines RANTES, MCP-1, and, to a lower extent, MIP-1 alpha, MIP-1 beta, and IL-8, were able to induce uropod formation and ICAM-3 redistribution in T lymphoblasts adhered to ICAM-1 or VCAM-1. A similar chemokine-mediated effect was observed during T cells binding to the fibronectin fragments of 38- and 80-kD, that contain the binding sites for the integrins VLA-4 and VLA-5, respectively. The uropod structure concentrated the ICAM-3 adhesion molecule (a ligand for LFA-1), and emerged to the outer milieu from the area of contact between lymphocyte and protein ligands. In addition, we found that other adhesion molecules such as ICAM-1, CD43, and CD44, also redistributed to the lymphocyte uropod upon RANTES stimulation, whereas a wide number of other cell surface receptors did not redistribute. Chemokines displayed a selective effect among different T cell subsets; MIP-1 beta had more potent action on CD8+ T cells and tumor infiltrating lymphocytes (TIL), whereas RANTES and MIP-1 alpha targeted selectively CD4+ T cells. We have also examined the involvement of cAMP signaling pathway in uropod formation. Interestingly, several cAMP agonists were able to induce uropod formation and ICAM-3 redistribution, whereas H-89, a specific inhibitor of the cAMP-dependent protein kinase, abrogated the chemokine-mediated uropod formation, thus pointing out a role for cAMP-dependent signaling in the development of this cytoplasmic projection. Since the lymphocyte uropod induced by chemokines was completely abrogated by Bordetella pertussis toxin, the formation of this membrane projection appears to be dependent on G proteins signaling pathways. In addition, the involvement of myosin-based cytoskeleton in uropod formation and ICAM-3 redistribution in response to chemokines was suggested by the prevention of this phenomenon with the myosin-disrupting agent butanedione monoxime. Interestingly, this agent also inhibited the ICAM-3-mediated cell aggregation, but not the cell adhesion to substrata. Altogether, these results demonstrate that uropod formation and adhesion receptor redistribution is a novel function mediated by chemokines; this phenomenon may represent a mechanism that significantly contributes to the recruitment of circulating leukocytes to inflammatory foci.