Neutralization of respiratory syncytial virus after cell attachment

Host and Viral Factors in Respiratory Syncytial Virus Infection

Respiratory syncytial virus (RSV) is a major worldwide pathogen for which there is still no effective vaccine or antiviral treatment available, and immunoprophylaxis with RSV-specific antibodies (e.g., palivizumab) is used in limited clinical settings. In this review, we discuss virus-host interactions relevant to RSV pathobiology and how advances in cell and systems biology have accelerated knowledge in this area. We also highlight recent advances in understanding the relationship between RSV bronchiolitis and sequelae of recurrent wheezing and asthma, new findings into an intriguing interaction between RSV and air pollution, and exciting developments toward the goal of realizing a safe and effective RSV vaccine.

The RSV fusion receptor: not what everyone expected it to be

This article reviews current knowledge about respiratory syncytial virus (RSV) binding and entry into cells. The recent discovery of Nucleolin as a fusion receptor for RSV opens new avenues for developing interventions, while raising questions concerning RSV pathobiology and tropism. We also discuss characteristics of a good RSV drug target.

Identification of nucleolin as a cellular receptor for human respiratory syncytial virus

Human respiratory syncytial virus (RSV) causes a large burden of disease worldwide. There is no effective vaccine or therapy, and the use of passive immunoprophylaxis with RSV-specific antibodies is limited to high-risk patients. The cellular receptor (or receptors) required for viral entry and replication has yet to be described; its identification will improve understanding of the pathogenesis of infection and provide a target for the development of novel antiviral interventions. Here we show that RSV interacts with host-cell nucleolin via the viral fusion envelope glycoprotein and binds specifically to nucleolin at the apical cell surface in vitro. We observed decreased RSV infection in vitro in neutralization experiments using nucleolin-specific antibodies before viral inoculation, in competition experiments in which virus was incubated with soluble nucleolin before inoculation of cells, and upon RNA interference (RNAi) to silence cellular nucleolin expression. Transfection of nonpermissive Spodoptera frugiperda Sf9 insect cells with human nucleolin conferred susceptibility to RSV infection. RNAi-mediated knockdown of lung nucleolin was associated with a significant reduction in RSV infection in mice (P = 0.0004), confirming that nucleolin is a functional RSV receptor in vivo.

Respiratory syncytial virus-neutralizing monoclonal antibodies motavizumab and palivizumab inhibit fusion

Respiratory syncytial virus (RSV) is a major cause of virus-induced respiratory disease and hospitalization in infants. Palivizumab, an RSV-neutralizing monoclonal antibody, is used clinically to prevent serious RSV-related respiratory disease in high-risk infants. Motavizumab, an affinity-optimized version of palivizumab, was developed to improve protection against RSV. These antibodies bind RSV F protein, which plays a role in virus attachment and mediates fusion. Determining how these antibodies neutralize RSV is important to help guide development of new antibody drugs against RSV and, potentially, other viruses. This study aims to uncover the mechanism(s) by which palivizumab and motavizumab neutralize RSV. Assays were developed to test the effects of these antibodies at distinct steps during RSV replication. Pretreatment of virus with palivizumab or motavizumab did not inhibit virus attachment or the ability of F protein to interact with the target cell membrane. However, pretreatment of virus with either of these antibodies resulted in the absence of detectable viral transcription. These results show that palivizumab and motavizumab act at a point after F protein initiates interaction with the cell membrane and before virus transcription. Palivizumab and motavizumab also inhibited F protein-mediated cell-to-cell fusion. Therefore, these results strongly suggest that these antibodies block both cell-to-cell and virus-to-cell fusion, since these processes are likely similar. Finally, palivizumab and motavizumab did not reduce viral budding. Based on models developed from numerous studies of viral fusion proteins, our results indicate that these antibodies may prevent conformational changes in F protein required for the fusion process.

Different mechanisms of antibody-mediated neutralization of parvoviruses revealed using the Fab fragments of monoclonal antibodies

Antibody binding and neutralization are major host defenses against viruses, yet the mechanisms are often not well understood. Eight monoclonal antibodies and their Fab fragments were tested for neutralization of canine parvovirus and feline panleukopenia virus. All IgGs neutralized >85% of virus infectivity. Two Fabs neutralized when present at 5 nM, while the others gave little or no neutralization even at 20-100 nM. The antibodies bind two antigenic sites on the capsids which overlap the binding site of the host transferrin receptor (TfR). There was no specific correlation between Fab binding affinity and neutralization. All Fabs reduced capsid binding of virus to purified feline TfR in vitro, but the highly neutralizing Fabs were more efficient competitors. All partially prevented binding and uptake of capsids by feline TfR on cells. The virus appears adapted to allow some infectivity in the presence of at least low levels of antibodies.

Orally active fusion inhibitor of respiratory syncytial virus

BMS-433771 was found to be a potent inhibitor of respiratory syncytial virus (RSV) replication in vitro. It exhibited excellent potency against multiple laboratory and clinical isolates of both group A and B viruses, with an average 50% effective concentration of 20 nM. Mechanism-of-action studies demonstrated that BMS-433771 inhibits the fusion of lipid membranes during both the early virus entry stage and late-stage syncytium formation. After isolation of resistant viruses, resistance was mapped to a series of single amino acid mutations in the F1 subunit of the fusion protein. Upon oral administration, BMS-433771 was able to reduce viral titers in the lungs of mice infected with RSV. This new class of orally active RSV fusion inhibitors offers potential for clinical development.

Genetic and structural determinants of virus neutralizing antibodies

Neutralizing antibodies (Abs) are the principal protective mechanism against disease caused by reinfection with viruses. Ab-mediated neutralization of viruses is a complex process comprising multiple mechanisms. Every structural aspect of Abs is potentially capable of modulating the level of neutralizing activity or the mechanisms of neutralization. The focus of our laboratory is to understand the genetic and structural basis of Ab-mediated neutralization of human viral pathogens. We demonstrated the unexpected finding that virus antigen-binding fragments of Abs (Fabs) mediate potent virus neutralizing effects in vivo. This work has led to a broad investigation of the importance of the genetics, chemistry, and structure of the combining site to the neutralizing activity of antiviral Abs. Ongoing work in our laboratory reveals that effect or functions specified by the Ab isotype such as polymer formation, interactions with complement, interactions with Fc receptors, and the ability to transcytose mucosal epithelia, also modulate the mechanism and level of neutralizing effects mediated by antiviral Abs.

Postattachment neutralization of a primary strain of HIV type 1 in peripheral blood mononuclear cells is mediated by CD4-specific antibodies but not by a glycoprotein 120-specific antibody that gives potent standard neutralization

De novo infecting HIV-1 or virus released from an infected cell in vivo attaches relatively quickly to a target cell, but the rate of fusion-entry of such virus is slow, with 50% entry taking > or =2 hr. It is thus desirable that antibodies stimulated by any vaccine or given in immunotherapy are able to neutralize not only free virus, but also virus attached to the target cell. Here we investigated postattachment neutralization (PAN) of a primary HIV-1 strain (JRCSF) in peripheral blood mononuclear cells and of a T cell line-adapted strain (IIIB) in C8166 T lymphoblastoid cells, using the highly potent gp120-specific human monoclonal b12 monoclonal IgG, and monoclonal antibodies specific for the CD4 primary cell receptor. In addition, we improved the experimental protocols of related studies by using a pulse of antibody, thus avoiding the complication of neutralizing progeny virus. We found that b12 IgG PAN was inefficient, with PAN of IIIB needing a 1000-fold greater concentration of antibody than was required for standard neutralization, and PAN of JRCSF being detected erratically only at 4 degrees C and unphysiologically high concentrations (300 microg/ml). Nonetheless, under identical conditions a 10-microg/ml pulse of the CD4-specific MAb Q4120 gave up to 99% PAN of JRCSF, and more than 95% even when added 3 hr after infection at 37 degrees C. Possible mechanisms by which PAN by CD4- specific antibodies is mediated are discussed. We suggest that such anti-CD4 antibodies should be considered as a component of HIV-1 immunotherapy.

Hemagglutinin 1-specific immunoglobulin G and Fab molecules mediate postattachment neutralization of influenza A virus by inhibition of an early fusion event

In standard neutralization (STAN), virus and antibody are reacted together before inoculation of target cells, and inhibition of almost any of the processes concerned in the early interaction of virus and cell, including inhibition of virus attachment to cell receptors, can be the cause of neutralization by a particular monoclonal antibody (MAb). To simplify the interpretation of antibody action, we carried out a study of postattachment neutralization (PAN), where virus is allowed to attach to target cells before neutralizing antibody is introduced. We used influenza virus A/PR/8/34 (H1N1) and monoclonal immunoglobulin G (IgG) molecules and their Fabs specific to antigenic sites Sb (tip), Ca2 (loop), and Cb (hinge) of the hemagglutinin 1 (HA1) protein. All IgGs and Fabs gave PAN, although with reduced efficiency compared with STAN. Thus, bivalent binding of antibody was not essential for PAN. By definition, none of these MAbs gave PAN by inhibiting virus attachment, and they did not elute attached virus from the target cell or inhibit endocytosis of virus. However, virus-cell fusion, as demonstrated by R18 fluorescence dequenching or hemolysis of red blood cells, was inhibited in direct proportion to neutralization and in a dose-dependent manner and was thus likely to be responsible for the observed neutralization. However, to get PAN, it was necessary to inhibit the activation of the prefusion intermediate, the earliest known form on the fusion pathway that is created when virus is incubated at pH 5 and 4 degrees C. PAN antibodies may act by binding HA trimers in contact with the cell and/or trimers in the immediate vicinity of the virus-cell contact point and so inhibit the recruitment of additional receptor-HA complexes.