The human non-lineage antigen CD46 (HuLy-m5) and primate retroviral gp70 molecules share protein-defined antigenic determinants

Phylogeny-directed search for murine leukemia virus-like retroviruses in vertebrate genomes and in patients suffering from myalgic encephalomyelitis/chronic fatigue syndrome and prostate cancer

Gammaretrovirus-like sequences occur in most vertebrate genomes. Murine Leukemia Virus (MLV) like retroviruses (MLLVs) are a subset, which may be pathogenic and spread cross-species. Retroviruses highly similar to MLLVs (xenotropic murine retrovirus related virus (XMRV) and Human Mouse retrovirus-like RetroViruses (HMRVs)) reported from patients suffering from prostate cancer (PC) and myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS) raise the possibility that also humans have been infected. Structurally intact, potentially infectious MLLVs occur in the genomes of some mammals, especially mouse. Mouse MLLVs contain three major groups. One, MERV G3, contained MLVs and XMRV/HMRV. Its presence in mouse DNA, and the abundance of xenotropic MLVs in biologicals, is a source of false positivity. Theoretically, XMRV/HMRV could be one of several MLLV transspecies infections. MLLV pathobiology and diversity indicate optimal strategies for investigating XMRV/HMRV in humans and raise ethical concerns. The alternatives that XMRV/HMRV may give a hard-to-detect “stealth” infection, or that XMRV/HMRV never reached humans, have to be considered.

Interaction of glycoprotein H of human herpesvirus 6 with the cellular receptor CD46

Human herpesvirus 6 (HHV-6) employs the complement regulator CD46 (membrane cofactor protein) as a receptor for fusion and entry into target cells. Like other known herpesviruses, HHV-6 encodes multiple glycoproteins, several of which have been implicated in the entry process. In this report, we present evidence that glycoprotein H (gH) is the viral component responsible for binding to CD46. Antibodies to CD46 co-immunoprecipitated an approximately 110-kDa protein band specifically associated with HHV-6-infected cells. This protein was identified as gH by selective depletion with an anti-gH monoclonal antibody, as well as by immunoblot analysis with a rabbit hyperimmune serum directed against a gH synthetic peptide. In reciprocal experiments, a monoclonal antibody against HHV-6 gH was found to co-immunoprecipitate CD46. Studies using monoclonal antibodies directed against specific CD46 domains, as well as engineered constructs lacking defined CD46 regions, demonstrated a close correspondence between the CD46 domains involved in the interaction with gH and those previously shown to be critical for HHV-6 fusion (i.e. short consensus repeats 2 and 3).

Tissue-specific and allelic expression of the complement regulator CD46 is controlled by alternative splicing

CD46 (membrane cofactor protein) is a human cell surface glycoprotein with cofactor activity for factor I-mediated cleavage of complement components C3b and C4b. The CD46 protein from normal lymphocytes resolves on sodium dodecyl sulfate-polyacrylamide gel electrophoresis as two major bands of 66 and 56 kDa. CD46 cDNA encodes four extracellular short consensus repeat domains, a Ser/Thr/Pro (STP)-rich region, a transmembrane region and a cytoplasmic tail. We now show that exquisite control of mRNA splicing is responsible for the heterogeneous expression of CD46 isoforms. Differential splicing of 5 exons generates at least 14 CD46 mRNA variants whose expression is stringently regulated by allelic, tissue-specific and malignancy-related factors, as: (a) leukemic cells and Epstein-Barr virus-transformed B cells preferentially incorporate the first of three STP exons (exon 7) into mRNA, and produce a larger CD46 isoform of 74 kDa, (b) an allelic difference in the proportion of 66- and 56-kDa CD46 isoforms on lymphocytes corresponds to the preferential inclusion or exclusion of the second STP exon (exon 8), (c) the third STP exon (exon 9) is specifically deleted in some placentae, (d) spermatozoa delete both exons 12 and 13, encoding a shorter transmembrane region and a unique cytoplasmic tail and (e) all tissues tested differentially splice exon 13, resulting in two alternative cytoplasmic tails. The distribution of the 14 alternatively spliced RNA transcripts correlated with the presence of protein isoforms of the predicted size, indicating that alternative splicing leads to heterogeneity of CD46 glycoproteins.

Human astrocytes express membrane cofactor protein (CD46), a regulator of complement activation

Expression of membrane cofactor protein (CD46) on cultured human astrocytes was demonstrated by indirect immunofluorescence microscopy and flow cytometry following staining with a monoclonal antibody specific for CD46. Western transfer and immunoblotting detected a doublet of Mr 66,000 and 56,000. Analysis of astrocyte mRNA revealed the presence of multiple alternatively spliced transcripts encoding different extracellular regions or cytoplasmic tails of CD46. Astrocytes were also shown to express decay accelerating factor, but not the type 1 complement receptor. Upregulation of astrocyte CD46 occurred following cytomegalovirus infection. These results indicate that astrocytes express proteins involved in regulation of complement activation and protection against autologous complement.

Identification of four different CD46 (MCP) molecules with anti-peptide antibodies

Four protein isoforms of the human CD46 molecule (MCP) have been identified using rabbit antisera against synthetic peptides corresponding to predicted cytoplasmic carboxyl-terminal sequences. Two different cytoplasmic tails, CD46 CYT 1 and CD46 CYT 2, were detected in CD46 molecules isolated from almost every cell type examined. Bands of 56 and 66 kDa were obtained from SDS-PAGE analyses of various cell types with both CYT 1 and CYT 2 antisera, which indicated that the size polymorphism of the CD46 molecule is not due to variations in the cytoplasmic tail and that each species contained two different sized molecules.

Alternatively spliced RNAs encode several isoforms of CD46 (MCP), a regulator of complement activation

Five alternative cDNA clones were isolated for CD46, also known as the membrane cofactor protein (MCP) for the factor I-mediated cleavage of the complement convertases. One of these cDNA clones (a) was identical to an earlier MCP clone. The other four CD46 clones contained the four NH2-terminal short consensus repeat (SCR) units of MCP, but differed at the region encoding the carboxyl-terminal of the protein which includes an extracellular segment rich in Ser, Thr, and Pro residues, a hydrophobic membrane-spanning domain, and a 33 amino acid cytoplasmic tail. The different CD46 cDNAs have variously: (b) inserted a 93 base pair (bp) exon resulting in a new cytoplasmic tail of 26 amino acids; (c) deleted a 42 bp exon from the extracellular Ser/Thr rich region: (d) used a cryptic splice acceptor sequence to delete 37 bp from an exon encoding transmembrane sequence; or (e) failed to splice the intron after the four SCR units. These were shown by northern blot and polymerase chain reaction to arise by alternative splicing of CD46 RNA. Forms (a), (b), and (c) of CD46 RNA are common in placental RNA, but (d) was rare, and (e) was incompletely processed and therefore aberrant. The polymerase chain reaction (PCR) was used to map the sites of the intron/exon junctions and demonstrate further possible splice variants of CD46. The alternative RNAs for CD46 may correlate to the different isoforms of CD46 found in different tissues, tumors, and in serum.

Human non-lineage antigen, CD46 (HuLy-m5): purification and partial sequencing demonstrates structural homology with complement-regulating glycoproteins

CD46, until recently known as HuLy-m5, is a non-lineage restricted surface antigen ubiquitously expressed by almost all human cells except erythrocytes. The CD46 antigen is identified by the E4.3 monoclonal antibody (mAb) and exists at the surface of human peripheral blood lymphocytes (PBLs) as two acidic, non-disulfide bonded chains, alpha and beta, of Mr 66,000 and 56,000. Receptor density analysis showed that CD46 was of moderately low abundance on PBLs with 7.5 x 10(3) molecules present on each cell. The two chains of CD46 were purified (144,000-fold) by immunoaffinity-chromatography with E4.3 mAb from the plasma membranes of a human spleen infiltrated with chronic myelogenous leukemia cells. Amino acid sequence analysis of the NH2-terminal of both alpha and beta chains yielded the same sequence; XEEPPQ/TFEAMELIGKPKPYYEIGE. Peptide mapping studies confirmed that both CD46 chains were closely related, except for one peptide fragment. This amino acid sequence is identical to that of the NH2-terminal of the recently cloned membrane co-factor protein (MCP), a membrane protein that binds the C3b and C4b fragments of complement and acts as a co-factor for I protein-mediated decay of the complement convertases. CD46 shares a cross-reactive epitope with some primate retroviruses, and this may indicate that some retroviruses mimic the mechanisms used by autologous human cells to evade complement-mediated immune clearance.