Antibody binding in proximity to the receptor/glycoprotein complex leads to a basal level of virus neutralization

Binding of transmembrane mucins to galectin-3 limits herpesvirus 1 infection of human corneal keratinocytes

Epithelial cells lining mucosal surfaces impose multiple barriers to viral infection. At the ocular surface, the carbohydrate-binding protein galectin-3 maintains barrier function by cross-linking transmembrane mucins on the apical glycocalyx. Despite these defense mechanisms, many viruses have evolved to exploit fundamental cellular processes on host cells. Here, we use affinity assays to show that herpes simplex virus type 1 (HSV-1), but not HSV-2, binds human galectin-3. Knockdown of galectin-3 in human corneal keratinocytes by small interfering RNA significantly impaired HSV-1 infection, but not expression of nectin-1, indicating that galectin-3 is a herpesvirus entry mediator. Interestingly, exposure of epithelial cell cultures to transmembrane mucin isolates decreased viral infectivity. Moreover, HSV-1 failed to elute the biological counterreceptor MUC16 from galectin-3 affinity columns, suggesting that association of transmembrane mucins to galectin-3 provides protection against viral infection. Together, these results indicate that HSV-1 exploits galectin-3 to enhance virus attachment to host cells and support a protective role for transmembrane mucins under physiological conditions by masking viral entry mediators on the epithelial glycocalyx.

Neutralization efficiency is greatly enhanced by bivalent binding of an antibody to epitopes in the V4 region and the membrane-proximal external region within one trimer of human immunodeficiency virus type 1 glycoproteins

Most antibodies are multivalent, with the potential to bind with high avidity. However, neutralizing antibodies commonly bind to virions monovalently. Bivalent binding of a monoclonal antibody (MAb) to a virion has been documented only in a single case. Thus, the role of high avidity in antibody-mediated neutralization of viruses has not been defined clearly. In this study, we demonstrated that when an artificial 2F5 epitope was inserted in the gp120 V4 region so that an HIV-1 envelope glycoprotein (Env) trimer contains a natural 2F5 epitope in the gp41 membrane-proximal envelope region (MPER) and an artificially engineered 2F5 epitope in the gp120 V4 region, bivalent 2F5 IgG achieved greatly enhanced neutralization efficiency, with a 50% inhibitory concentration (IC(50)) decrease over a 2-log scale. In contrast, the monovalent 2F5 Fab fragment did not exhibit any appreciable change in neutralization efficiency in the same context. These results demonstrate that bivalent binding of 2F5 IgG to a single HIV-1 Env trimer results in dramatic enhancement of neutralization, probably through an increase in binding avidity. Furthermore, we demonstrated that bivalent binding of MAb 2F5 to the V4 region and MPER of an HIV-1 Env trimer can be achieved only in a specific configuration, providing an important insight into the structure of a native/infectious HIV-1 Env trimer. This specific binding configuration also establishes a useful standard that can be applied to evaluate the biological relevance of structural information on the HIV-1 Env trimer.

The human immunodeficiency virus type 1 envelope spike of primary viruses can suppress antibody access to variable regions

The human immunodeficiency virus type 1 (HIV-1) envelope spike is a heavily glycosylated trimeric structure in which protein surfaces conserved between different HIV-1 isolates are particularly well hidden from antibody recognition. However, even variable regions on the spike tend to be less antigenic and immunogenic than one might have anticipated for external structures. Here we show that the envelope spike of primary viruses has an ability to restrict antibody recognition of variable regions. We show that access to an artificial epitope, introduced at multiple positions across the spike, is frequently limited, even though the epitope has been inserted at surface-exposed regions on the spike. Based on the data, we posit that restricted antibody access may be the result, at least in part, of a rigidification of the epitope sequence in the context of the spike and/or a highly effective flexible arrangement of the glycan shield on primary viruses. Evolution of the HIV envelope structure to incorporate extra polypeptide sequences into nominally accessible regions with limited antibody recognition may contribute to reducing the magnitude of antibody responses during infection and allow the virus to replicate unhindered by antibody pressure for longer periods.

Structural basis for potent cross-neutralizing human monoclonal antibody protection against lethal human and zoonotic severe acute respiratory syndrome coronavirus challenge

Severe acute respiratory syndrome coronavirus (SARS-CoV) emerged in 2002, and detailed phylogenetic and epidemiological analyses have suggested that it originated from animals. The spike (S) glycoprotein has been identified as a major component of protective immunity, and 23 different amino acid changes were noted during the expanding epidemic. Using a panel of SARS-CoV recombinants bearing the S glycoproteins from isolates representing the zoonotic and human early, middle, and late phases of the epidemic, we identified 23 monoclonal antibodies (MAbs) with neutralizing activity against one or multiple SARS-CoV spike variants and determined the presence of at least six distinct neutralizing profiles in the SARS-CoV S glycoprotein. Four of these MAbs showed cross-neutralizing activity against all human and zoonotic S variants in vitro, and at least three of these were mapped in distinct epitopes using escape mutants, structure analyses, and competition assays. These three MAbs (S109.8, S227.14, and S230.15) were tested for use in passive vaccination studies using lethal SARS-CoV challenge models for young and senescent mice with four different homologous and heterologous SARS-CoV S variants. Both S227.14 and S230.15 completely protected young and old mice from weight loss and virus replication in the lungs for all viruses tested, while S109.8 completely protected mice from weight loss and clinical signs in the presence of viral titers. We conclude that a single human MAb can confer broad protection against lethal challenge with multiple zoonotic and human SARS-CoV isolates, and we identify a robust cocktail formulation that targets distinct epitopes and minimizes the likely generation of escape mutants.