| Abstract: | Human respiratory syncytial virus (HRSV) fusion (F) protein is an essential component of the virus envelope that mediates fusion of the viral and cell membranes, and, therefore, it is an attractive target for drug and vaccine development. Our aim was to analyze the neutralizing mechanism of anti-F antibodies in comparison with other low-molecular-weight compounds targeted against the F molecule. It was found that neutralization by anti-F antibodies is related to epitope specificity. Thus, neutralizing and nonneutralizing antibodies could bind equally well to virions and remained bound after ultracentrifugation of the virus, but only the former inhibited virus infectivity. Neutralization by antibodies correlated with inhibition of cell-cell fusion in a syncytium formation assay, but not with inhibition of virus binding to cells. In contrast, a peptide (residues 478 to 516 of F protein [F478-516]) derived from the F protein heptad repeat B (HRB) or the organic compound BMS-433771 did not interfere with virus infectivity if incubated with virus before ultracentrifugation or during adsorption of virus to cells at 4°C. These inhibitors must be present during virus entry to effect HRSV neutralization. These results are best interpreted by asserting that neutralizing antibodies bind to the F protein in virions interfering with its activation for fusion. Binding of nonneutralizing antibodies is not enough to block this step. In contrast, the peptide F478-516 or BMS-433771 must bind to F protein intermediates generated during virus-cell membrane fusion, blocking further development of this process.Human respiratory syncytial virus (HRSV), a member of the Pneumovirus genus of the Paramyxoviridae family, is the main cause of severe lower respiratory tract infections in very young children (36), and it is a pathogen of considerable importance in the elderly (24, 26) and in immunocompromised adults (22). Currently, there is no effective vaccine against the virus although it is known that passive administration of neutralizing antibodies to individuals at high risk is an effective immunoprophylaxis (37, 38).The HRSV genome is a single-stranded negative-sense RNA molecule of approximately 15 kb that encodes 11 proteins (16, 53). Two of these proteins are the main surface glycoproteins of the virion. These are (i) the attachment (G) protein, which mediates virus binding to cells (44), and (ii) the fusion (F) protein, which promotes both fusion of the viral and cell membranes at the initial stages of the infectious cycle and fusion of the membrane of infected cells with those of adjacent cells to form characteristic syncytia (72). These two glycoproteins are the only targets of neutralizing antibodies either induced in animal models (19, 63, 65, 70) or present in human sera (62).The G protein is a highly variable type II glycoprotein that shares neither sequence identity nor structural features with the attachment protein of other paramyxoviruses (75). It is synthesized as a precursor of about 300 amino acids (depending on the strain) that is modified posttranslationally by the addition of a large number of N- and O-linked oligosaccharides and is also palmitoylated (17). The G protein is oligomeric (probably a homotetramer) (23) and promotes binding of HRSV to cell surface proteoglycans (35, 40, 49, 67). Whether this is the only interaction of G with cell surface components is presently unknown.The F protein is a type I glycoprotein that is synthesized as an inactive precursor of 574 amino acids (F0) which is cleaved by furin during transport to the cell surface to yield two disulfide-linked polypeptides, F2 from the N terminus and F1 from the C terminus (18). Like other viral type I fusion proteins, the mature F protein is a homotrimer which is in a prefusion, metastable, conformation in the virus particle. After fusion, the F protein adopts a highly stable postfusion conformation. Stability of the postfusion conformation is determined to great extent by two heptad repeat (HR) sequences, HRA and HRB, present in the F1 chain. Mixtures of HRA and HRB peptides form spontaneously heterotrimeric complexes (43, 51) that assemble in six-helix bundles (6HB), consisting of an internal core of three HRA helices surrounded by three antiparallel HRB helices, as determined by X-ray crystallography (79).The three-dimensional (3D) structure of the HRSV F protein has not been solved yet. Nevertheless, the structures of the pre- and postfusion forms of two paramyxovirus F proteins have revealed substantial conformational differences between the pre- and postfusion conformations (77, 78). The present hypothesis about the mechanism of membrane fusion mediated by paramyxovirus F proteins proposes that, following binding of the virus to the cell surface, the prefusion form of the F glycoprotein is activated, and membrane fusion is triggered. The F protein experiences then a series of conformational changes which include the exposure of a hydrophobic region, called the fusion peptide, and its insertion into the target membrane. Subsequent refolding of this intermediate leads to formation of the HRA and HRB six-helix bundle, concomitant with approximation of the viral and cell membranes that finally fuse, placing the fusion peptide and the transmembrane domain in the same membrane (4, 20). The formation of the 6HB and the associated free energy change are tightly linked to the merger of the viral and cellular membranes (60).Antibodies play a major role in protection against HRSV. Animal studies have demonstrated that immunization with either F or G glycoproteins induces neutralizing antibodies and protects against a viral challenge (19, 63, 70). Furthermore, transfer of these antibodies (31, 56) or of anti-F or anti-G monoclonal antibodies (MAbs) protects mice, cotton rats, or calves against either a human or bovine RSV challenge, respectively (65, 68, 73). Likewise, infants at high risk of severe HRSV disease are protected by the prophylactic administration of immunoglobulins with high anti-HRSV neutralizing titers (33). Finally, a positive correlation was found between high titers of serum neutralizing antibodies and protection in adult volunteers challenged with HRSV (34, 74), while an inverse correlation was found between high titers of neutralizing antibodies and risk of infection in children (29) and in the elderly (25).Whereas all the anti-G monoclonal antibodies reported to date are poorly neutralizing (1, 28, 48, 71), some anti-F monoclonal antibodies have strong neutralization activity (1, 3, 5, 28, 46). It is believed that HRSV neutralization by anti-G antibodies requires simultaneous binding of several antibodies to different epitopes, leading to steric hindrance for interaction of the G glycoprotein with the cell surface. Indeed, it has been shown that neutralization is enhanced by mixtures of anti-G monoclonal antibodies (1, 50), mimicking the effect of polyclonal anti-G antibodies. In contrast, highly neutralizing anti-F monoclonal antibodies do not require cooperation by other antibodies to block HRSV infectivity efficiently (1).In addition to neutralizing antibodies, other low-molecular-weight compounds directed against the F protein are potent inhibitors of HRSV infectivity. Synthetic peptides that reproduce sequences of heptad repeat B inhibit both membrane fusion promoted by the F protein and HRSV infectivity (42). Also, other small molecules obtained by chemical synthesis have been shown to interact with F protein and inhibit HRSV infectivity. These HRSV entry inhibitors have been the topic of intense research in recent years (55).This study explores the mechanisms of HRSV neutralization by different inhibitors of membrane fusion, including anti-F monoclonal antibodies, an HRB peptide, and the synthetic compound BMS-433771 (13-15). The results obtained indicate that antibodies and low-molecular-weight compounds block membrane fusion at different stages during virus entry. |
