Maturation of West Nile Virus Modulates Sensitivity to Antibody-Mediated Neutralization

Abstract

West Nile virions incorporate 180 envelope (E) proteins that orchestrate the process of virus entry and are the primary target of neutralizing antibodies. The E proteins of newly synthesized West Nile virus (WNV) are organized into trimeric spikes composed of pre-membrane (prM) and E protein heterodimers. During egress, immature virions undergo a protease-mediated cleavage of prM that results in a reorganization of E protein into the pseudo-icosahedral arrangement characteristic of mature virions. While cleavage of prM is a required step in the virus life cycle, complete maturation is not required for infectivity and infectious virions may be heterogeneous with respect to the extent of prM cleavage. In this study, we demonstrate that virion maturation impacts the sensitivity of WNV to antibody-mediated neutralization. Complete maturation results in a significant reduction in sensitivity to neutralization by antibodies specific for poorly accessible epitopes that comprise a major component of the human antibody response following WNV infection or vaccination. This reduction in neutralization sensitivity reflects a decrease in the accessibility of epitopes on virions to levels that fall below a threshold required for neutralization. Thus, in addition to a role in facilitating viral entry, changes in E protein arrangement associated with maturation modulate neutralization sensitivity and introduce an additional layer of complexity into humoral immunity against WNV. West Nile virus (WNV) virions incorporate 180 envelope (E) proteins that are the primary target of neutralizing antibodies. As newly formed WNV virions are released from infected cells, the E proteins undergo a significant organizational change associated with maturation into an infectious virus. However, this process is not always efficient, as populations of infectious WNV include virions that did not complete the maturation process and may be heterogeneous with respect to the arrangement of E proteins on the virion. In this study, we found that neutralization by antibodies specific for epitopes commonly recognized in vivo is strongly impacted by the maturation state of WNV. Our studies suggest that maturation of WNV reduces the accessibility of some, but not all, epitopes on the virion for antibody binding. Virions that retain some immature character can be neutralized by monoclonal antibodies that fail to block infection of populations of WNV composed solely of mature virions. Similar results were found using polyclonal human serum obtained from volunteers of two clinical trials of candidate WNV vaccines. These studies identify unappreciated aspects of the antigenic complexity of WNV and highlight the importance of understanding the heterogenous forms of WNV that may be introduced into or replicating within the host.