Antiviral neutralizing antibodies: from in vitro to in vivo activity

Abstract

Neutralizing antibodies (nAbs) are being increasingly used as passive antiviral reagents in prophylactic and therapeutic modalities and to guide viral vaccine design. In vivo, nAbs can mediate antiviral functions through several mechanisms, including neutralization, which is defined by in vitro assays in which nAbs block viral entry to target cells, and antibody effector functions, which are defined by in vitro assays that evaluate nAbs against viruses and infected cells in the presence of effector systems. Interpreting in vivo results in terms of these in vitro assays is challenging but important in choosing optimal passive antibody and vaccine strategies. Here, I review findings from many different viruses and conclude that, although some generalizations are possible, deciphering the relative contributions of different antiviral mechanisms to the in vivo efficacy of antibodies currently requires consideration of individual antibody-virus interactions.

Fig. 1. Mechanisms of neutralization of an enveloped virus by neutralizing antibodies in vitro.

Antibody-mediated neutralization of enveloped viruses in vitro can operate by several mechanisms. a, Preventing the attachment of virions to host cell receptors by the disassembly or conformational modification of viral spike proteins. b, Aggregation of virions, impeding attachment to host cell receptors. c, Directly blocking binding of viral spike protein to host cell receptors through steric obstruction. d, Blocking the fusion of viral and host cell membranes by steric obstruction. e, Blocking conformational changes in spike protein required for virus entry into host cells. f, For viruses that enter endosomes, blocking entry into the cytoplasm by blocking endosomal cleavage and/or endosomal receptor binding. g, Blocking viral egress from the cell; although this is not strictly a mechanism of neutralization, it would be observed in multiple-round, but not single-round, neutralization assays and can occur through the aggregation of progeny virions at the surface of infected cells. The figure illustrates neutralization of an enveloped virus. For non-enveloped viruses, there is additionally intracytoplasmic neutralization involving the ubiquitin ligase TRIM21, which targets antibody-bound virus to the proteasome for degradation. Finally, the mechanisms that prevent viruses gaining entry to host cells can also prevent viruses spreading from one cell to another directly by inhibiting attachment of the infected cell to the uninfected cell and/or by inhibiting fusion of the membranes of the two infected cells.

Fig. 2. Model to scale of antibody-mediated neutralization of HIV.

The large size of a human IgG neutralizing antibody (Protein Data Bank (PDB) ID: 1HZH) relative to the molecules that are involved in the entry of HIV into host cells — the viral envelope (Env) spike protein (PDB ID: 3J5M), and CD4 (PDB ID: 1WIO) and CCR5 (PDB ID: 1OPT) receptors on host cells — suggests that antibody can readily interfere with viral attachment to and/or fusion with the host cell through steric obstruction.

Fig. 3. Antiviral activities of neutralizing antibodies mediated by Fc-dependent effector functions.

a, In the presence of complement, neutralizing antibody (nAb)-coated virions are susceptible to enhanced neutralization (by steric hindrance) and to complement-mediated lysis. b, In the presence of crystallizable fragment receptor (FcR)-bearing effector cells, nAb-coated virions can be sequestered and prevented from binding to host cells or can be taken up by effector cells such as macrophages by antibody-dependent cellular phagocytosis (ADCP). c, Virus-infected cells coated with nAbs are susceptible to killing by effector cells such as natural killer (NK) cells through FcR-dependent, antibody-dependent cellular cytotoxicity (ADCC).

Fig. 4. Common themes associated with neutralizing antibody functions in vivo.

The activities of neutralizing antibodies (nAbs) against select virus types and routes of exposure are illustrated schematically. ADE, antibody-dependent enhancement.