Structural diversity of the hagfish variable lymphocyte receptors

Variable lymphocyte receptors (VLRs) are recently discovered leucine-rich repeat (LRR) family proteins that mediate adaptive immune responses in jawless fish. Phylogenetically it is the oldest adaptive immune receptor and the first one with a non-immunoglobulin fold. We present the crystal structures of one VLR-A and two VLR-B clones from the inshore hagfish. The hagfish VLRs have the characteristic horseshoe-shaped structure of LRR family proteins. The backbone structures of their LRR modules are highly homologous, and the sequence variation is concentrated on the concave surface of the protein. The conservation of key residues suggests that our structures are likely to represent the LRR structures of the entire repertoire of jawless fish VLRs. The analysis of sequence variability, prediction of protein interaction surfaces, amino acid composition analysis, and structural comparison with other LRR proteins suggest that the hypervariable concave surface is the most probable antigen binding site of the VLR.

Rapid isolation of IgNAR variable single-domain antibody fragments from a shark synthetic library

The immunoglobulin isotype IgNAR (Novel Antigen Receptor) was discovered in the serum of the nurse shark (Ginglymostoma cirratum) and wobbegong shark (Orectolobus maculates) as a homodimer of two protein chains, each composed of a single variable domain (V) domain and five constant domains. The IgNAR variable domain contains an intact antigen-binding site and functions as an independent domain able to react to antigen with both high specificity and affinity. Here we describe the successful construction of a synthetic phage-displayed library based upon a single anti-lysozyme clone HEL-5A7 scaffold, which was previously selected from an immune IgNAR variable domain library. The complementarity-determining region 3 (CDR3) loop of this clone was varied in both length and composition and the derived library was used to pan against two model proteins, lysozyme and leptin. A single anti-lysozyme clone (Ly-X20) and anti-leptin clone (Lep-12E1) were selected for further study. Both clones were shown to be functionally expressed in Escherichia coli, extremely thermostable and bind to corresponding antigens specifically. The results here demonstrate that a synthetic IgNAR variable domain library based on a single framework scaffold can be used as a route to generate antigen binders quickly, easily and without the need of immunization.

Angiogenic oligosaccharides of hyaluronan induce protein tyrosine kinase activity in endothelial cells and activate a cytoplasmic signal transduction pathway resulting in proliferation

We have recently shown that the degradation products of hyaluronan of 3 to 10 disaccharides (o-HA), but not native high molecular weight hyaluronan, can induce angiogenesis in vivo and, as such, o-HA is an important regulator of the neovascularization process. As a continuation of this work, we have studied the cytoplasmic signal transduction pathways responsible for o-HA-activated endothelial cell proliferation. We show that the addition of o-HA (1 microg/ml) to bovine aortic endothelial cells induces tyrosine phosphorylation of multiple proteins within 1 minute and that the activity remains above basal levels for at least 24 hours. Increased phosphorylation of the CD44 receptor was also observed. Pretreatment of cells with an anti-CD44-receptor antibody (5 microg/ml) or the tyrosine kinase inhibitor genistein (10 microM) inhibited both o-HA-induced proliferation (p < 0.05) and protein tyrosine phosphorylation. In comparison, native hyaluronan had little effect on tyrosine phosphorylation across the same time period. Protein kinase C (PKC) activity was increased 2- to 3-fold in the membranes of cells treated with o-HA, and a pretreatment with phorbol 12,13-dibutyrate (PDBu) to down-regulate PKC significantly inhibited o-HA-induced cell proliferation (p < 0.05). Examination by Western blotting showed that only the betaI and epsilon isoforms remained translocated to the membrane for at least 24 hours. These isoforms seem to be involved in modulating the proliferative effects of o-HA, because the transient translocation of PKC isoforms by PDBu was not sufficient to induce mitogenesis. Furthermore, we show that PKC activation of the cytoplasmic kinase cascade (Raf-1 kinase, MAP kinase kinase [MEK-1], and extracellular signal-regulated kinase [ERK-1]) by o-HA culminated in the nuclear translocation of ERK-1. This pathway is essentially linear, as shown by the ability of specific enzyme inhibitors (PDBu and PD98059) to prevent both activation of ERK-1- and o-HA-induced proliferation. We conclude that phosphorylation of the CD44 receptor results in an increase in tyrosine phosphorylation, leading to the activation of a cytoplasmic cascade and cell proliferation; this concurs with previous work, which showed that o-HA-induced proliferation of endothelial cells is CD44-receptor-mediated and accompanied by early response gene activation.

Activation of protein-tyrosine phosphatase SH-PTP2 by a tyrosine-based activation motif of a novel brain molecule

BIT (a brain immunoglobulin-like molecule with tyrosine-based activation motifs) is a brain-specific membrane protein which has two cytoplasmic TAMs (tyrosine-based activation motifs). Using the Far Western blotting technique, we detected association of a 70-kDa protein with the tyrosine-phosphorylated TAMs of BIT. A mouse brain cDNA library in lambdagt11 was screened for this association, and two positive clones encoding tyrosine phosphatase SH-PTP2 were isolated. SH-PTP2 has two SH2 domains and is believed to function as a positive mediator in receptor tyrosine kinase signaling. SH-PTP2 and BIT were coimmunoprecipitated from phosphorylated rat brain lysate, and BIT was a major tyrosine-phosphorylated protein associated with SH-PTP2 in this lysate. This interaction was also observed in Jurkat T cells transfected with BIT cDNA depending on tyrosine phosphorylation of BIT. Bisphosphotyrosyl peptides corresponding to BIT-TAMs stimulated SH-PTP2 activity 33-35-fold in vitro, indicating that two SH2 domains of SH-PTP2 simultaneously interact with two phosphotyrosines of BIT-TAM. Our findings suggest that the tyrosine phosphorylation of BIT results in stimulation of the signal transduction pathway promoted by SH-PTP2 and that BIT is probably a major receptor molecule in the brain located just upstream of SH-PTP2.

A novel "chimeric" antibody class in cartilaginous fish: IgM may not be the primordial immunoglobulin

Using a degenerate oligonucleotide primer specific for immunoglobulin (Ig) constant type 1 (C-1 set) domain genes, products were amplified by the reverse transcriptase-polymerase chain reaction from nurse shark spleen cDNA. The deduced protein sequence of one of these clones reveals a novel Ig class in cartilaginous fish. A complete mRNA could encode a mature protein bearing an amino-terminal variable (V) domain, followed by six C-1 set domains, and ending in a carboxy-terminal tail typical of secreted IgM, IgA, and the new antigen receptor (NAR). The two amino-terminal C domains are orthologous to IgX (or IgR), an Ig heavy (H) chain class in the skate, and the last four domains are homologous to the carboxy-terminal four domains of NAR. We designate this "chimeric" Ig class IgNARC for Ig new antigen receptor from cartilaginous fish. Like NAR, but unlike shark IgM, IgNARC is encoded by very few V and C genes which apparently are not closely linked. The number of bands that hybridize with exon-specific probes varies with genomic DNA from individual sharks, suggestive of different numbers of IgNARC genes in different animals. A protein of approximately 95 kDa, which is likely to be the IgNARC H chain, is immunoprecipitated with both light chain-specific monoclonal antibodies and with antisera generated to a peptide comprising the IgNARC carboxy-terminal tail. We conclude that the arsenal of secreted antigen receptors in cartilaginous fish is greater than previously believed. In addition, our data cast doubt on the dogma that IgM is the primordial Ig isotype.

Avid AL, a synthetic ligand affinity gel mimicking immobilized bacterial antibody receptor for purification of immunoglobulin G

Avid AL is an affinity gel designed for the purification of immunoglobulin G (IgG). The gel was prepared by first reacting Sepharose with 3,5-dichloro-2,4,6-trifluoropyridine and 4-dimethylaminopyridine and then with 2-mercaptoethanol. The IgG purified by Avid AL is about 95% pure. The binding parameters of Avid AL for the whole IgG, Fab and Fc fragment and the stability of gel were investigated. The IgG bound to Avid AL can be eluted with an acidic buffer or with a novel neutral buffer containing electron donors. The development of such a mild neutral elution buffer is described. Application of Avid AL in a rapid gram-scale IgG purification was demonstrated. The possible mechanism of IgG binding is discussed.

Affinity chromatographic isolation of the melanoma adhesion receptor for the IIICS region of fibronectin and its identification as the integrin alpha 4 beta 1

Eukaryotic cells adhere to at least two different regions of the fibronectin molecule: a central domain present in all fibronectin isoforms, and the type III connecting segment domain (IIICS), the expression of which is controlled by complex alternative splicing of precursor mRNA. Using affinity chromatography on a matrix containing a synthetic peptide ligand (CS1) representing the strongest active site within the IIICS, we have isolated the human melanoma cell receptor recognizing this region of fibronectin. The receptor is a complex of two polypeptides with subunit molecular masses of 145 and 125 kDa. This heterodimeric structure resembles that of receptors for other extracellular matrix proteins. Immunological analysis with specific antibodies identified these polypeptides as the integrin subunits alpha 4 and beta 1. In addition, antifunctional monoclonal antibodies directed against either alpha 4 or beta 1, but not against other integrin subunits, were potent inhibitors of CS1-mediated melanoma cell spreading. Furthermore, when the function of the central cell-binding domain was blocked, anti-alpha 4 and anti-beta 1 antibodies abolished spreading of A375-M cells on fibronectin, indicating that alpha 4 beta 1 is an authentic fibronectin receptor. Taken together, these results identify the human fibronectin IIICS receptor as the integrin heterodimer alpha 4 beta 1.

Identification of a putative antigen receptor on fish nonspecific cytotoxic cells with monoclonal antibodies

In the present study mAb were derived against flow cytometry (FCM) purified fish (Ictalurus punctatus) nonspecific cytotoxic cells (NCC). mAb 5C6.10.4 and 6D3.2.10 produced 60 to 65% inhibition of lysis of NC-37 target cells (a human B-lymphoblastoid cell line) by unfractionated NCC. mAb 2B2.4.9 and 6D3.4.4 were noninhibitors of cytotoxicity. All mAb were the same isotype (IgM) and were cloned by limiting dilution (2x). Inhibitory activity was specific for the effector cells because the mAb had no effect on NCC cytotoxicity when only the target cells were treated. Inhibition could be produced by preincubation of the mAb with NCC or by no preincubation, and inhibition was not reversible. Killing by FCM-sorted NCC of NC-37 target cells was inhibited almost 100% by mAb 5C6.10.4. Inhibitor mAb also significantly reduced NCC killing of MOLT-4, K562, P815, U937, Daudi, YAC-1, and HL-60 cells. Experiments also were conducted to determine at which stage of the lytic cycle the mAb acted. Both inhibitor mAb significantly inhibited conjugate formation between effector and NC-37 target cells. The technique of FCM was combined with competitive binding experiments to determine that the Ag recognized by both inhibitor and noninhibitor mAb was found on the membranes of the same cells. These results were confirmed by demonstrating (by using FCM) that FITC-labeled inhibitor and biotinylated noninhibitor mAb bound to the same cells. FCM also was next used to determine mAb binding to various effector cell populations. Inhibitor and noninhibitor mAb bound to approximately 25% (5C6.10.4) and 39% (6D3.4.4) of fish anterior kidney cells; to 42% (5C6.10.4) and 54% (6D3.4.4) of fish spleen cells; and to 2.5% (5C6.10.4 and 6D3.4.4) of fish peripheral blood. mAb were used to purify the target cell binding structure found on NCC. Con A-Sepharose purified mAb were used as the fixed ligand for Affi-Gel-10 affinity chromatography experiments. FCM-purified NCC were solubilized and the receptor was purified by using this technique. Analysis of the NCC-purified receptor by 12% SDS-PAGE indicated that the mAb purified structure may be composed of a dimeric molecule consisting of 41 kDa and 38 kDa proteins. The same dimer was purified by using either inhibitory (6D3.2.10) or noninhibitory (6D3.4.4) mAb. Similar results were obtained with immunoprecipitation experiments by using mAb 5C6.10.4. These studies demonstrate that the Ag-binding receptor structure on fish NCC may be comprised of a dimeric complex.

Isolation of specific lymphocyte receptors by high-performance immunoaffinity chromatography

Isolation of antigen-specific lymphocyte receptors can be achieved by high-performance immunoaffinity chromatography, using immobilized monoclonal antibodies directed against the antigen. Primed lymphocytes are allowed to react with their target antigen prior to disruption and membrane solubilization. The bound antigen acts both as a preservative for the receptor and as a ligand on which the immobilized antibody captures the antigen-receptor complex. Dissociation of the antigen-antibody complex also releases the antigen from the receptor, and both antigen and receptor can be separated and recovered for further study.

The Vero cell receptor for the hepatitis B virus small S protein is a sialoglycoprotein

The Vero (African green monkey kidney-derived) cell line is capable of binding recombinant hepatitis B surface antigen (rHBsAg) particles containing only the small surface (S) protein of hepatitis B virus (HBV). This binding activity appears to be due to a single major population of receptors (M. E. Peeples et al., Virology 160, 135-142 (1987]. Since infectious HBV particles also contain the small S protein, it is possible that the Vero cell receptor might also function as an HBV receptor. The initial physical characterization of this receptor is reported here. Treatment of Vero cells with each of four proteases reduced their binding activity by 70% or greater, indicating that the receptor is partially protein in nature. Binding activity was also reduced by pretreating cells with neuraminidase or low levels of sodium periodate, indicating that sialic acid also plays a major role in the receptor activity. Consistent with this interpretation, N-acetylneuraminic acid and N-acetylneuraminyl-lactose were able to competitively inhibit rHBsAg particle attachment to Vero cells. The protein nature of the Vero cell receptor was confirmed by the demonstration that chymotrypsin treatment which resulted in 70% loss of binding had little effect on the cell sialic acid content. Therefore, the Vero cell receptor for rHBsAg particles is a sialoglycoprotein.

Isolation and characterization of the receptor on human neutrophils that mediates cellular adherence

The receptor on human neutrophils (polymorphonuclear leukocytes or PMN) that mediates cellular adherence has been purified from the peripheral blood PMN obtained from an individual with chronic myelogenous leukemia (CML). This receptor consists of two noncovalently associated subunits, designated alpha M (Mac-1 alpha, CD11b) (Mr = 170,000) and beta (Mac-1 beta, CDw18) (Mr = 100,000), respectively, which are identical on normal and CML PMN. The subunits were purified by monoclonal antibody 60.1-Sepharose (anti-alpha M) affinity chromatography and separated in 5-nmol quantities by high pressure liquid chromatography on a TSK-4000 gel filtration column. Subunits were characterized by amino acid composition, NH2-terminal amino acid sequence, and carbohydrate content. The NH2-terminal sequence of the human PMN alpha M subunit contains regions of homology with the human platelet glycoprotein IIb alpha. We conclude that nanomole amounts of individual alpha M and beta subunits of the receptor on human PMN that mediates cellular adherence can be isolated and separated using CML PMN.

Identification and chemical synthesis of a host cell receptor binding site on hepatitis B virus

Hepatitis B virus (HBV) has not yet been propagated in vitro, and knowledge concerning its reaction with receptors on target cells remained scant. We have located within the HBV envelope proteins a sequence mediating the attachment of HBV to human hepatoma HepG2 cells. A synthetic peptide analog (PLGFFPDHQLDPAFGANSNNPDWDFNP) is recognized by both cell receptors and anti-HBV antibodies and elicits antibodies reacting with native HBV. The synthetic peptide is a promising immunogen expected to elicit protective antibodies based on the concept of the attachment blockade pathway of virus neutralization. The approach described here, based on anti-peptide antisera and the binding of peptide analogs to cell receptors is generally applicable for the delineation of cell receptor binding sites on viruses with known gene sequences.

Identification and characterization of mouse small intestine mucosal receptors for Escherichia coli K-12(K88ab)

Adhesion of 3H-labeled Escherichia coli K-12(K88ab) to CD-1 mouse small intestine mucus and brush border preparations, immobilized on polystyrene, was studied. E. coli K12(K88ab) was shown to adhere readily to either crude mucus or brush border preparations, but not to bovine serum albumin. In contrast, the nearly isogenic E. coli K-12 strain, i.e., lacking the K88ab plasmid, did not bind well to either mucus, brush borders, or bovine serum albumin. The adhesion of E. coli K-12(K88ab) to both mucus and brush borders required pilus expression (i.e., growth at temperatures greater than 18 degrees C) and was inhibited by pretreatment of either mucus or brush borders with trypsin, pronase, or sodium metaperiodate and by the presence of D-galactosamine. Crude mucus was fractionated by gel filtration, and the proteins in receptor-containing fractions were separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Separated proteins were Western blotted to nitrocellulose. Adhesion of 35SO4-labeled E. coli K-12(K88ab) and 35SO4-labeled E. coli K-12 to Western blots followed by autoradiography revealed two E. coli K-12(K88ab)-specific mucus receptor proteins (57 and 64 kilodaltons). Brush borders contained the same two receptor proteins present in mucus and an additional 91-kilodalton receptor protein.

Lymphocyte recognition of lymph node high endothelium. VI. Evidence of distinct structures mediating binding to high endothelial cells of lymph nodes and Peyer's patches

Lymphocytes migrate from blood into lymph nodes (LN) and Peyer's patches (PP) of rats specifically at segments of venules lined by high endothelium (HEV). We previously identified and isolated a lymphocyte surface component termed high endothelial binding factor (HEBF) that appears to be involved in lymphocyte adhesion to high endothelial cells of LN. HEBF has also been isolated from thoracic duct lymph and is antigenically related to the cell surface component. Soluble HEBF derived from detergent lysates of thoracic duct lymphocytes (TDL) or directly from lymph has affinity for HEVLN in vitro, and is able to block sites where lymphocytes would normally attach. In the present study, lymphocyte binding sites of HEVLN and HEVPP were investigated through the use of lymph-derived HEBF and rabbit antibody to this factor. The results show that treatment of rat TDL with anti-HEBF Fab did not block binding to HEVPP, even though adhesion to HEVLN was reduced by 80% or more. Similarly, HEBF isolated by anti-HEBF F(ab')2 affinity chromatography blocked lymphocyte binding sites of HEVLN but not HEVPP. This material is therefore designated HEBFLN, and antibody to it is designated anti-HEBFLN Ig. Fractionation of thoracic duct lymph revealed that it contained an antigenically distinct component, HEBFPP, which blocked lymphocyte binding to HEVPP but not to HEVLN. Lymph components precipitating between 40 and 60% (NH4)2SO4 saturation contained both factors, which were separated from the bulk of lymph proteins by DEAE-Sepharose chromatography and then from each other by fractionation on the anti-HEBFLN F(ab')2-Sepharose column. The unbound fraction from this column contained HEBFPP, which was then partially purified by CM-Sepharose filtration. HEBFPP appeared to be a glycoprotein because it was destroyed by trypsin, bound to lentil lectin, and was eluted with alpha-methyl-mannoside. Together, the results demonstrate the existence of two antigenically distinct species of HEBF, and imply that lymphocyte binding sites of HEVLN and HEVPP are structurally different. We interpret the results to mean that distinct high endothelial adhesion molecules on lymphocytes mediate their entry into LN and PP.

An alternative pathway of T-cell activation: a functional role for the 50 kd T11 sheep erythrocyte receptor protein

A series of seven monoclonal antibodies was produced against the T-lineage-specific 50 kd T11 sheep erythrocyte rosette (SRBC) receptor protein in order to define the function of the molecule. Three distinct epitopes were detected: T11(1), the SRBC binding site expressed on all T lymphocytes and thymocytes; T11(2), an epitope unrelated to the SRBC binding site but with a similar distribution; and T11(3), a neo-epitope expressed only upon T-cell activation. Simultaneous triggering of T11(2) and T11(3) epitopes by monoclonal antibodies induces T lymphocytes to proliferate and mediate their functional programs in the absence of antigen and/or antigen-presenting cells. This antigen-independent mode of triggering is distinct from that involving the T3-Ti antigen receptor complex and represents an alternate pathway of T-cell activation. Given that T11 is the earliest T-lineage surface glycoprotein to appear in thymic ontogeny and is thus expressed before T3-Ti, the former may be involved in clonal expansion and/or differentiation during early development.

Monoclonal antibodies against the antigen receptor on a cloned T-cell hybrid

A pigeon cytochrome c-specific and Ia molecule-restricted T-cell hybrid was used as an immunogen in order to obtain monoclonal anti-antigen receptor antibodies. Two antibodies were isolated that specifically bound to and inhibited interleukin (IL) 2 release from only the immunizing clone. Lectin-induced IL 2 release was not affected by these antibodies. Binding assays with purified and iodinated monoclonal antibody indicated that there were approximately equal to 25,000 binding sites on the T-cell hybrid. Immunoprecipitation and NaDodSO4/polyacrylamide gel electrophoresis of detergent lysates from surface-labeled hybrid cells revealed a heterodimeric structure composed of chains of apparent Mrs 45,000-50,000 and 40,000-44,000. The chains were linked by intermolecular disulfide bonds, and the difference in migration of the isolated chains under reducing and nonreducing conditions was consistent with the presence of intramolecular disulfide bonds. The molecule that has been identified is a candidate for the antigen-specific receptor on the immunizing T-cell clone.

Influence of proteolysis inhibitors and immunoadsorption technique on the composition of rabbit antigen-specific receptor preparations. Presence of Fc receptors

Addition of proteolysis inhibitors during the isolation procedure of rabbit arsanylated bovine IgG specific receptors decreased significantly the amount of low-molar-mass proteins in all receptor preparations. Rabbit ARS-BGG-specific receptor preparations isolated by immunoadsorption technique contain molecules which do not react with antigen and antibodies against immunoglobulins and have identical molar mass and chymotryptic peptide composition as those of isolated Fc receptors. It is suggested that during isolation of antigen-specific receptors from the surface of lymphoid cells, Fc receptors react with complexes composed of antigen and Ig+ receptors on the surface of immunoadsorbent and are isolated together with antigen-specific receptors.

The functional role of Langerhans cells

Langerhans cells represent a subpopulation of mammalian epidermal cells. The recent findings that these cells are the only epidermal cells which express Fc-IgG receptors, C3 receptors and Ia antigens support the early suggestion that they are related to cells from the monocyte-macrophage-histiocyte series. In this report, evidence is presented that epidermal Langerhans cell can replace Ia-bearing macrophages in their capacity to induce antigen-specific and allogeneic T cell activation. The possible clinical implications of these findings are discussed with regard to the role of Langerhans cells as sensitizing factors in contact hypersensitivity and skin graft rejection.

H-Y antigen in 46,XY gonadal dysgenesis

Presence of H-Y antigen has been correlated with testicular differentiation, and absence of H-Y with failure of testicular differentiation, in a variety of mammalian species. To determine more precisely the relationship between expression of H-Y antigen and development of the testis, we studied the cells of phenotypic females with the 46,XY male karyotype. Blood leukocytes were typed H-Y+ in five XY females with gonadal dysgenesis, although in other studies blood leukocytes from XY females with gonadal dysgenesis were typed H-Y-. Thus mere presence of H-Y antigen is not sufficient to guarantee normal differentiation of the testis. In the present paper we review evidence for an additional factor in gonadal organogenesis, the H-Y antigen receptor. We infer that testicular development requires engagement of H-Y and its receptor. It follows that XY gonadal dysgenesis is the consequence of functional absence of the H-Y testis inducer as in the following conditions: failure of synthesis of H-Y or failure of specific binding of H-Y.