Nucleotide sequence of bovine herpesvirus type 1 glycoprotein gIII, a structural model for gIII as a new member of the immunoglobulin superfamily, and implications for the homologous glycoproteins of other herpesviruses

The gene encoding bovine herpesvirus type 1 (BHV-1) glycoprotein gIII was mapped, cloned, and sequenced. The gene is situated between map units 0.122 and 0.135 and encodes a predicted protein of 521 amino acids. The identity of the sequenced gene has been verified previously by expression of immunologically authentic recombinant BHV-1 gIII (Fitzpatrick et al. (1988) J. Virol. 62, 4239-4248). Comparison of the BHV-1 gIII amino acid sequence with the homologous glycoproteins of other alphaherpesviruses revealed significant homology in the carboxy-terminal half of the molecules, including six invariant cysteine residues. A 96 amino acid domain with significant homology to class II major histocompatibility complex (MHC) antigen constant domains was identified in the conserved carboxy-terminal half of BHV-1 gIII. This domain is flanked by two other similarly sized domains which may be related to other immunoglobulin (Ig) superfamily domains. These homologies support a model for the structure of BHV-1 gIII as a new member of the Ig superfamily. Elements of the model may be applicable to the homologous glycoproteins of other herpesviruses and relevant to the immunobiology of herpesvirus infections.     

Cloning and sequencing of the cDNA encoding the human homologue of the murine immunoglobulin-associated protein B29.

Membrane-bound immunoglobulins (Ig) on the surface of murine B cells are noncovalently associated with a heterodimeric protein complex of MB-1 and B29 (also called Ig-alpha and Ig-beta). The Ig-associated proteins are predicted to regulate the assembly and transport of the Ig complex to the cell surface and to couple membrane-bound Ig to intracellular signal transduction pathways. We have isolated and sequenced a full-length cDNA clone encoding the human homologue of the B29 protein. The predicted amino acid sequence was compared to its murine counterpart, to MB-1 and to the human T cell receptor (TcR)-associated CD3 proteins. The alignment of the human B29 protein with its murine counterpart revealed 90% homology in the C-terminal portion comprising the cytoplasmic tails, the transmembrane regions and the adjacent 26 amino acids of the extracellular regions. Only 59% homology was found in the rest of the Ig-like extracellular domains. The high degree of conservation observed for the C-terminal amino acids suggested that these domains of the proteins play important functional roles for the Ig complex. Indicative of this was the conservation of the antigen receptor tail motif D-(X)7-E/D-(X)2-Y-(X)2-L-(X)7-Y-(X)2-L/I which is thought to be a component of signal transduction pathways. This motif is also found in the human and murine MB-1 proteins as well as in the TcR-associated CD3 molecules. Further regions of homology between B29, MB-1 and the CD3 proteins included extracellular residues which were predicted to maintain the Ig-like structure, and hydrophilic residues within the transmembrane regions which may be utilized during the intracellular assembly and transport of the oligomeric Ig/MB-1/B29 or TcR/CD3 complexes. Thus the similarities found between B29, MB-1 and the CD3 proteins suggest conserved functions for both the Ig- and TcR-associated proteins.     

CD80 and CD86 IgC domains are important for quaternary structure,receptor binding and co-signaling function

CD86 and CD80, the ligands for the co-stimulatory molecules CD28 and CTLA-4, are members of the Ig superfamily. Their structure includes Ig variable-like (IgV) domains, Ig constant-like (IgC) domains and intracellular domains. Although crystallographic studies have clearly identified the IgV domain to be responsible for receptor interactions, earlier studies suggested that both Ig domains are required for full co-signaling function. Herein, we have used deletion and chimeric human CD80 and CD86 molecules in co-stimulation assays to study the impact of the multimeric state of IgV and IgC domains on receptor binding properties and on co-stimulatory function in a peptide-specific T cell activation model. We report for the first time the presence of CD80 dimers and CD86 monomers in living cells. Moreover, we show that the IgC domain of both molecules inhibits multimer formation and greatly affects binding to the co-receptors CD28 and CTLA-4. Finally, both IgC and intracellular domains are required for full co-signaling function. These findings reveal the distinct but complementary roles of CD80 and CD86 IgV and IgC domains in T cell activation.     

CD80 and CD86 C domains play an important role in receptor binding and co-stimulatory properties

CD80 and CD86 expressed on the surface of antigen-presenting cells interact with cytotoxic T lymphocyte antigen-4 [CTLA-4 (CD152)] expressed on activated T cells and mediate critical T cell inhibitory signals. CD80 and CD86 are type I glycoproteins, and are made up of two extracellular (EC) Ig-like domains-a transmembrane region and a cytoplasmic tail. The N-terminal (V domain) and membrane-proximal (C) domains share homology with the variable region (V) and the constant region (C) of Ig respectively. Co-crystallographic structures of both CD80 and CD86 bound to CTLA-4 indicate that there is no direct interaction of the C domain of either CD80 or CD86 with the CTLA-4. In contrast, previous mutagenesis studies have identified specific amino acids within the C domain of CD80 that are critical for CTLA-4 binding. To further understand the importance of C domains in the functioning of CD80 and CD86, we constructed chimeric human CD80 and CD86 molecules by swapping their respective C domains, and tested their ability to stimulate T cells. A Chinese hamster ovary (CHO) cell line expressing CD86 activated murine T cells. In contrast, CHO cells expressing either CD80 or a chimeric construct of the CD86 V domain and the CD80 C domain showed a significantly reduced activation. Our studies further demonstrated that the decreased activation by cells expressing the CD80 or a chimera containing CD80 C domain is most likely due to enhanced CTLA-4 binding. From these results we conclude that C domains play a critical, albeit indirect, role in determining CTLA-4 binding affinities and co-stimulatory properties.     

NK cell receptor gene of the KIR family with two IG domains but highest homology to KIR receptors with three IG domains

The killer cell inhibitory receptors (KIRs) are surface glycoproteins expressed by natural killer (NK) cells and some T cells. They recognize polymorphic human HLA class I molecules. Two families of KIRs have been identified and named p58 and p70. The p58 family of genes encode type I membrane proteins with two extracellular immunoglobulin (Ig) domains, while the p70 genes have three Ig domains. We here report the cloning and characterization of a novel KIR cDNA obtained from tumor cell lines with NK reactivity (YT and NK-92). This gene is also expressed in the normal cell line NK 3.3 and in NK cells obtained from some but not all normal donors. The clone, KIR103AS, has an open reading frame consistent with a KIR with two extracellular Ig domains, a transmembrane region and a 114 amino acid long cytoplasmic domain containing a single src homology 2 (SH2) binding motif. The membrane distal Ig domain of KIR 103AS has highest homology with the first Ig domain of p70 KIRs and differs significantly from the first Ig domain of p58 KIRs. The second, membrane proximal Ig domain of KIR103AS has similar and high homology with the membrane proximal Ig domains of both p70 and p58 KIRs. The extracellular domains of KIR 103AS therefore share characteristic features with both p70 and p58 genes: the domain structure is identical to p58 KIRs but the sequence homology matches closely with p70 KIRs. The putative transmembrane and cytoplasmic domains are distinctly different from all previously reported KIR cDNAs.     

Human Ig superfamily CTLA-4 gene: chromosomal localization and identity of protein sequence between murine and human CTLA-4 cytoplasmic domains

The mouse CTLA-4 gene has been shown to code for an activated lymphocyte-associated sequence belonging to the Ig gene superfamily. We now report on the molecular cloning and study of the human corresponding gene isolated from a genomic library and designated Hu-CTLA-4. The Hu-CTLA-4 gene exists as a single copy per human haploid genome and maps to band q33 of chromosome 2. It comprises 3 exons notwithstanding the leader sequence. The first exon encodes a V-like domain of 116 amino acids, the second one a hydrophobic putative transmembrane region of 37 amino acids and the third one a 34 amino acid putative cytoplasmic domain. Whereas the overall homology between the human and murine CTLA-4 proteins is 76%, there is, remarkably, a complete identity of their cytoplasmic domains. This complete interspecies conservation comes in support of an important role for this domain in CTLA-4 function.     

The putative invertebrate adaptive immune protein Litopenaeus vannamei Dscam (LvDscam) is the first reported Dscam to lack a transmembrane domain and cytoplasmic tail

It has recently been suggested that Dscam (Down syndrome cell adhesion molecule), a member of the immunoglobulin superfamily (IgSF), plays an essential role in the alternative adaptive immune system of invertebrates. Here, we isolated and characterized the first shrimp Dscam from Litopenaeus vannamei. The LvDscam protein had an extracellular domain but lacked the expected transmembrane domain and cytoplasmic tail, both of which are found in all other members of the Dscam family (and may also be found in other L. vannamei Dscams that have not yet been isolated). In nervous tissue, expression levels of LvDscam were unexpectedly low. Phylogenetic analysis suggests that LvDscam is far from the Dscams found in other invertebrates. Nevertheless, the domain architecture of the extracellular region of LvDscam is similar to other invertebrate Dscams, and it exhibits the typical configuration of 10 immunoglobulin (Ig) domains, 6 fibronectin type 3 domains (FNIII) and one cell attachment sequence (RGD). Cloning and characterization of a total of 62 cDNAs from hemocytes collected from WSSV-free, WSSV-persistent and WSSV-acute-infected shrimp revealed 23 alternative amino acid sequences in the N-terminal of Ig2, 30 in the N-terminal of Ig3 and 13 in the Ig7 domain. This implies that LvDscam can potentially encode at least 8970 unique isoforms. Further analysis suggested that the LvDscam Ig2 and Ig3 regions are more functionally important than Ig7 in the shrimp's specific immune response against WSSV. We discuss how this tail-less, soluble Dscam can still play an active role in alternative adaptive immune response even while its axonal guidance functionality may be impaired.     

The hinge region of the CD8 alpha chain: structure, antigenicity, and utility in expression of immunoglobulin superfamily domains.

The lymphocyte surface CD8 antigen is a heterodimer with each chain containing a single Ig-related domain, a hinge-like sequence, a transmembrane segment, and a short cytoplasmic sequence. A soluble form of the rat CD8 alpha chain was produced by introducing a stop codon into the cDNA at the end of the region encoding the extracellular sequence and expressed in Chinese hamster ovary cells. sCD8 alpha was produced at 20 mg/l, and consisted of monomers, dimers, and higher aggregates. The latter could be minimized, but not eliminated, by removal of one of the two cysteine residues in the hinge region by mutation and by growth in serum-free medium. The positions of the N- and O-linked glycosylation sites and the disulphide bond in the Ig-like domain were determined. The MRC OX-8 antibody was shown to react with a region from the CD8 alpha hinge containing 24 amino acids and the antigenic determinant was sensitive to neuraminidase digestion. A construct encoding the Ig-like domain of rat CD8 alpha without the hinge was not expressed in CHO cells, indicating the importance of the hinge region for expression. It seemed possible that the CD8 alpha hinge might facilitate expression of other Ig-related domains and such expression could be detected using the MRC OX-8 antibody. To test the system cDNA constructs were made with the rat CD8 alpha hinge spliced to the V-like domain of mouse CD8 alpha, to the V alpha and V beta domains of a T lymphocyte antigen receptor, and to one or both of the Ig-like domains of the MRC OX-47 membrane antigen. All these forms were expressed as soluble proteins that were detected with the MRC OX-8 antibody. This method may prove useful for the expression of Ig superfamily domains for raising antibodies and other studies.     

Leukocyte Ig-like receptor complex (LRC) in mice and men

Here, we compare the architecture of membrane receptors with extracellular Ig-like domains located within the leukocyte Ig-like receptor complex (LRC) of humans and mice. The receptors can be classified broadly into four groups, based on the homology of their Ig-like domains and gene architecture. Receptors in the first group are characterized by the presence of the Ig constant type 2-1 (IgC2-1) and variant Ig (vlg) domains, and include the leukocyte Ig-like receptors (LILRs) and murine paired Ig-activating receptors (PIRs). The second group of receptors possess an IgC2-2 domain and comprise the killer-cell Ig-like receptors (KIRs) and platelet collagen receptor glycoprotein VI (GPVI). The third group consists of receptors with IgC2-1, and IgC2-3 or IgC2-4 domains, and includes the receptor for IgA Fc (FCAR), NKp46 and murine Ly94. The fourth group, with a single extracellular IgC2-1 domain, consists of the leukocyte-associated Ig-like receptors (LAIRs). The genomic organization of and evolutionary associations between these receptors and their domains are examined.     

Cloning of two members of the SIRPα family of protein tyrosine phosphatase binding proteins in cattle that are expressed on monocytes and a subpopulation of dendritic cells and which mediate binding to CD4 T cells

Recent experimental studies have greatly clarified the function of cell surface molecules in the induction and modulation of T cell responses by antigen-presenting cells (APC). However, the differences in ability to stimulate T cells evident for different types and subpopulations of the same APC, such as dendritic cell subsets, is less well understood. This report details an investigation of an antigen expressed on monocytes that is also expressed on a subset of cattle afferent lymph veiled cells (ALVC). A cDNA library derived from cattle monocytes was screened with monoclonal antibodies (mAb) for expression in COS-7 cells. Using separate mAb for screening, two cDNA were cloned, the sequences of which showed a single long open reading frame encoding a predicted type I glycoprotein of 506 amino acids that contained three immunoglobulin superfamily domains and a long 112-amino acid cytoplasmic tail. We have termed this antigen MyD-1, reflecting its myeloid and dendritic cell distribution. Analysis of the EMBL database revealed that the molecule is a member of the recently described family of signal regulatory proteins (SIRP). The outeremost Ig domain was of the adhesion/receptor I-type, suggesting that MyD-1 might bind to a ligand on another cell. Evidence for this was subsequently obtained by demonstrating that COS-7 cells transfected with MyD-1 cDNA bound CD4 T cells and this binding was blocked by specific mAb. The potential importance of this interaction was supported by the finding that the proliferation of resting memory CD4 T cells to ovalbumin-pulsed monocytes was significantly reduced in the presence of mAb to MyD-1. A role for the molecule in the modulation of the monocyte/dendritic APC response is also predicted from the existence of multiple potential tyrosine phosphorylation sites in the cytoplasmic domain, including the presence of an immunoreceptor tyrosine-based inhibitory motif (ITIM) and the observation that the SIRPα family members have been shown to bind to SHP-1 and SHP-2. Together these data indicate a possible functional importance for MyD-1 in the regulation of monocyte and dendritic cell function.     

Identification of SAF-2, a novel siglec expressed on eosinophils, mast cells, and basophils

Background: Eosinophils, basophils, and mast cells are believed to be the central tenet cells in allergic conditions including allergic rhinitis, asthma, and eczema. The molecular mechanisms underlying the recruitment of these cells to sites of allergic inflammation are poorly understood. Objectives: Our aim was to identify a common adhesion molecule that could potentially be responsible for mediating the recruitment of the allergic cell types to the lungs and other sites of allergy. Methods: We have cloned a sialoadhesin molecule from a human eosinophil library with the use of expressed sequence tag technology and characterized its expression on allergic cells by the use of flow cytometry and specific mAbs. Results: With the use of expressed sequence tag sequencing, we have identified a novel siglec molecule, SAF-2. SAF-2 has homology with other sialoadhesin family members (CD33 and siglec-5) and belongs to a subgroup of the Ig superfamily. SAF-2 is a 431–amino acid protein composed of 3 Ig domains with a 358–amino acid extracellular domain and a 47–amino acid tail. SAF-2 is highly restricted to eosinophils, basophils, and mast cells. Antibodies to SAF-2 do not modulate Ca⁺⁺ mobilization or chemotaxis of human eosinophils induced by eotaxin. Conclusion: SAF-2 is a highly restricted sialoadhesin molecule, which may be useful in the detection and/or modulation of allergic cells. (J Allergy Clin Immunol 2000;105:1093-1100.)     

Molecular cloning characterization and expression of porcine immunoreceptor SIRPalpha

SWC3 is a porcine CD that has been the reference marker of myeloid lineage. It is expressed in every myelomonocytic cell from early bone marrow precursors. We have identified the molecule recognized by anti-SWC3 antibodies as a member of the signal-regulatory proteins (SIRPs)alpha family. Here, we describe the cloning of a cDNA coding for a porcine SIRPalpha protein. The sequence is 2470 nucleotides long and contains an open reading frame encoding a 507 amino acid sequence. The predicted polypeptide was composed of a 30 amino acids putative signal peptide, a 342 amino acid extracellular region, a 23 amino acid transmembrane segment and a 112 amino acid cytoplasmic domain. Analysis of the sequence reveals a high degree of homology with known SIRPs in other species, being easily identified the three extracellular Ig type domains and two cytoplasmic ITIM motifs characteristic of this molecule. The gene coding for porcine SIRPalpha has been mapped to porcine chromosome 17, in a region syntenic to the human chromosome 20 where SIRP genes have been mapped. During the analysis of SIRP gene expression in tissues by RT-PCR, we noticed the existence of a shorter mRNA, and cloned the corresponding cDNA. This coded for a splicing variant of SIRPalpha that lacked the two membrane proximal Ig domains. In transfection experiments, we have been able to show that anti-SWC3 antibodies recognize both forms of the molecule, mapping the SWC3 epitopes to the N-terminal IgV type domain.     

The role of the extracellular domain in the biology of the coxsackievirus and adenovirus receptor

The Coxsackievirus B and Adenovirus Receptor (CAR) plays a dual role as a homotypic junctional adhesion protein and as a viral receptor. CAR is a transmembrane protein and a member of the Immunoglobulin (Ig) superfamily with two extracellular Ig-like domains. The most distal Ig-like domain (D1) mediates the homophilic interaction and is also responsible for the high-affinity binding of the adenovirus (Ad) fiber protein. Currently, no activity has been ascribed to the proximal Ig-like domain (D2). To further understand the function of the extracellular domain in the biological activities of CAR, we created extracellular deletion mutants and evaluated cellular localization, adhesion, and viral infection. Deletion of any segment of the extracellular domain results in loss of adhesion and mislocalization as explained by a model, termed "diffusion trapping," that suggests adhesion is the driving force in junctional localization. Loss of junctional localization and adhesion was particularly apparent in polarized human airway epithelia, where mutant CAR expression was basolateral but not limited to the lateral junctions between cells. Surprisingly, the D2 domain was required for adenovirus fiber-knob binding and infection. In summary, the entire extracellular domain of CAR is of vital importance to the biology of this highly conserved and important protein.     

FCRLA is a resident endoplasmic reticulum protein that associates with intracellular Igs, IgM, IgG and IgA

Fc receptor-like A (FCRLA) is an unusual member of the extended Fc receptor family. FCRLA has homology to receptors for the Fc portion of Ig (FCR) and to other FCRL proteins. However, unlike these other family representatives, which are typically transmembrane receptors with extracellular ligand-binding domains, FCRLA has no predicted transmembrane domain or N-linked glycosylation sites and is an intracellular protein. We show by confocal microscopy and biochemical assays that FCRLA is a soluble resident endoplasmic reticulum (ER) protein, but it does not possess the amino acid sequence KDEL as an ER retention motif in its C-terminus. Using a series of deletion mutants, we found that its ER retention is most likely mediated by the amino terminal partial Ig-like domain. We have identified ER-localized Ig as the FCRLA ligand. FCRLA is unique among the large family of Fc receptors, in that it is capable of associating with multiple Ig isotypes, IgM, IgG and IgA. Among hemopoietic cells, FCRLA expression is restricted to the B lineage and is most abundant in germinal center B lymphocytes. The studies reported here demonstrate that FCRLA is more broadly expressed among human B lineage cells than originally reported; it is found at significant levels in resting blood B cells and at varying levels in all B-cell subsets in tonsil.     

The MRC OX-47 antigen is a member of the immunoglobulin superfamily with an unusual transmembrane sequence

The MRC OX-47 monoclonal antibody recognizes a membrane antigen present at low levels on many lymphocytes but whose expression is markedly increased on activation with mitogens. cDNA clones for the OX-47 antigen were isolated from an expression library and the protein sequence deduced. It contains a leader sequence giving a mature protein of 251 amino acids with a single putative transmembrane region, a cytoplasmic domain of 40 amino acids and an extracellular domain of 187 amino acids that contained two immunoglobulin-like domains. The putative transmembrane sequence includes a glutamic acid residue within the hydrophobic sequence. The presence of acidic residues within the hydrophobic sequence of transmembrane sequences usually indicates association with other polypeptides and this is predicted for the OX-47 antigen. A sequence of 37 amino acids that included all the transmembrane region was identical to that of the chicken HT7 antigen present on endothelium in brain and erythroblasts. The level of protein sequence identity in the Ig-like domains was lower but HT7 is almost certainly the chicken homologue of the rat OX-47 antigen. The ligand and function of the molecule are unknown. In addition to lymphoblasts the OX-47 antigen was localized on a variety of other cell types including various immature cells, endothelia and cells with excitable membranes.     

ICAM-1 isoforms: specific activity and sensitivity to cleavage by leukocyte elastase and cathepsin G

The extracellular moiety of ICAM-1 consists of five Ig-like domains, the first and third domains mediating adhesion to integrin ligands. The ICAM-1 gene, however, gives rise to the expression of five alternative splice variants containing two, three, or four Ig-like domains. In this work, we have investigated whether the rearrangement of the architecture of ICAM-1 affects its structural properties and function. We showed that, in contrast to the common form, all alternative isoforms of ICAM-1 were susceptible to cleavage by leukocyte elastase and cathepsin G. We found that the length of an isoform did not influence the susceptibility to proteolysis. The molecular diversity provided by the skipping of entire Ig domains and the level of expression on the APC, however, significantly influenced their ability to potentiate the proliferation of T cells. Finally, we found that the expression of minor ICAM-1 isoforms encoding the third Ig-like domains was sufficient to sustain neutrophil infiltration in the liver and confer exon-5-targeted ICAM-1-deficient mice susceptibility to LPS-induced septic shock. These findings not only demonstrate that ICAM-1 isoforms are fully functional, but support the concept that alternative RNA splicing in the Ig superfamily may fulfill distinct roles during the development of the immune response.     

Killer cell immunoglobulin-like receptors (KIR) of the African-origin sabaeus monkey: evidence for recombination events in the evolution of KIR

Killer cell immunoglobulin (Ig)-like receptors (KIR) were characterized in the West African sabaeus monkey (Chlorocebus sabaeus) to elucidate the mechanism by which diversity evolves in this family of molecules. Complementary DNA encoding four forms of KIR molecules, including KIR3DL, KIR2DL4, KIR2DL5, and KIR3DH forms, were identified in two unrelated sabaeus monkeys. A novel hybrid form showing features found in both KIR2DL5 and KIR3DH was also identified. Both the KIR3DL and KIR3DH forms from the sabaeus monkey were considerably more polymorphic than any KIR form identified in great apes or humans. The polymorphic residues of the three Ig-like domains were frequently located in structural loops, indicating that point mutations have occurred in these regions. The three Ig-like domains of the KIR3D forms of six primate species were found to have different patterns of clustering in phylogenetic trees, suggesting that each Ig-like domain has a distinct phylogenetic history. This variation in relationships suggests that repeated recombination events have occurred between the Ig-like domains during the evolution of the KIR family in primates. Recombination between individual Ig-like domains, in addition to point mutations, provides a mechanism for generating the diversity of the KIR genes.