Abstract
This study aims to characterize the mannose receptor (MR) gene in sheep and its role in ovine visna/maedi virus (VMV) infection. The deduced amino acid sequence of ovine MR was compatible with a transmembrane protein having a cysteine-rich ricin-type amino-terminal region, a fibronectin type II repeat, eight tandem C-type lectin carbohydrate-recognition domains (CRD), a transmembrane region, and a cytoplasmic carboxy-terminal tail. The ovine and bovine MR sequences were closer to each other compared to human or swine MR. Concanavalin A (ConA) inhibited VMV productive infection, which was restored by mannan totally in ovine skin fibroblasts (OSF) and partially in blood monocyte-derived macrophages (BMDM), suggesting the involvement of mannosylated residues of the VMV ENV protein in the process. ConA impaired also syncytium formation in OSF transfected with an ENV-encoding pN3-plasmid. MR transcripts were found in two common SRLV targets, BMDM and synovial membrane (GSM) cells, but not in OSF. Viral infection of BMDM and especially GSM cells was inhibited by mannan, strongly suggesting that in these cells the MR is an important route of infection involving VMV Env mannosylated residues. Thus, at least three patterns of viral entry into SRLV-target cells can be proposed, involving mainly MR in GSM cells (target in SRLV-induced arthritis), MR in addition to an alternative route in BMDM (target in SRLV infections), and an alternative route excluding MR in OSF (target in cell culture). Different routes of SRLV infection may thus coexist related to the involvement of MR differential expression.
Highlights
Visna/maedi virus (VMV) and caprine arthritis encephalitis virus (CAEV) belong to the small ruminant lentivirus (SRLV) group, within the non-oncogenic lentivirus genus of the Retroviridae family, which includes the human immunodeficiency virus (HIV), simian immunodeficiency virus (SIV), feline immunodeficiency virus (FIV), bovine immunodeficiency virus (BIV) and equine infectious anaemia virus (EIAV)
Studies aimed to identify the SRLV cellular receptor have proposed different candidate molecules, including a 30 kDa membrane-associated proteoglycan substituted with a chondroitin sulphate glycosaminoglycan chain(s) [7]; MHC class II molecules, which incubated with VMV inhibit viral infection, even though infection is not inhibited by class II-specific antibodies [8]; CD4 and CXCR4 molecules, which have been proposed as optional auxiliary components of a VMV receptor that facilitate VMV-mediated membrane fusion events [9]; and a complex formed by three membrane proteins of 15, 30 and 50 kDa identified as a Visna binding protein [10]
Characterization of the ovine mannose receptor (MR) encoding sequence and comparison with other MRs Taking into account the relevance of interaction of glycosylated ENV with the soluble C-type lectin Concanavalin A (ConA) on VMV infection and cell fusion, we attempted the identification of the gene encoding a cell surface-anchored C-type lectin, the ovine MR, to determine subsequently its possible role in VMV infection
Summary
Visna/maedi virus (VMV) and caprine arthritis encephalitis virus (CAEV) belong to the small ruminant lentivirus (SRLV) group, within the non-oncogenic lentivirus genus of the Retroviridae family, which includes the human immunodeficiency virus (HIV), simian immunodeficiency virus (SIV), feline immunodeficiency virus (FIV), bovine immunodeficiency virus (BIV) and equine infectious anaemia virus (EIAV). Studies aimed to identify the SRLV cellular receptor have proposed different candidate molecules, including a 30 kDa membrane-associated proteoglycan substituted with a chondroitin sulphate glycosaminoglycan chain(s) [7]; MHC class II molecules, which incubated with VMV inhibit viral infection, even though infection is not inhibited by class II-specific antibodies [8]; CD4 and CXCR4 molecules, which have been proposed as optional auxiliary components of a VMV receptor (or receptor complex) that facilitate VMV-mediated membrane fusion events [9]; and a complex formed by three membrane proteins of 15, 30 and 50 kDa identified as a Visna binding protein [10] None of these molecules has been established as the main essential receptor, an issue which has stimulated research on alternative candidates
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