Poorly Neutralizing Cross-Reactive Antibodies against the Fusion Loop of West Nile Virus Envelope Protein ProtectIn Vivovia Fcγ Receptor and Complement-Dependent Effector Mechanisms

Abstract

The human antibody response to flavivirus infection is dominantly directed against a cross-reactive epitope on the fusion loop of domain II (DII-FL) of the envelope (E) protein. Although antibodies against this epitope fail to recognize fully mature West Nile virus (WNV) virions and accordingly neutralize infection poorly in vitro,their functional properties in vivo remain less well understood. Here, we show that while passive transfer of poorly neutralizing monoclonal antibodies (MAb) and polyclonal antibodies against the DII-FL epitope protect against lethal WNV infection in wild-type mice, they fail to protect mice lacking activating Fc␥ receptors (Fc␥R) and the complement opsonin C1q. Consistent with this, an aglycosyl chimeric mouse-human DII-FL MAb (E28) variant that lacks the ability to engage Fc␥R and C1q also did not protect against WNV infection in wild-type mice. Using a series of immunodeficient mice and antibody depletions of individual immune cell populations, we demonstrate that the nonneutralizing DII-FL MAb E28 does not require T, B, or NK cells, inflammatory monocytes, or neutrophils for protection. Rather, E28 treatment decreased viral load in the serum early in the course of infection, which resulted in blunted dissemination to the brain, an effect that required phagocytic cells, C1q, and Fc␥RIII (CD16). Overall, these studies enhance our understanding of the functional significance of immunodominant, poorly neutralizing antibodies in the polyclonal human antiflavivirus response and highlight the limitations of current in vitro surrogate markers of protection, such as cell-based neutralization assays, which cannot account for the beneficial effects conferred by these antibodies. West Nile virus (WNV) is a zoonotic mosquito-transmittedFlavivirus that can infect and cause disease in humans and many other vertebrate animals. The Flavivirus genus also contains other human pathogens of global relevance, including dengue virus (DENV), yellow fever virus, Japanese encephalitis virus, and tick-borne encephalitis virus. Most human WNV infections are asymptomatic, but about 20% of infected individuals experience a mild fever, and less than 1% develop severe neuroinvasive disease (67). Risk factors for symptomatic disease include an age of greater than 55 years, a compromised immune status, genetic variation in the OAS1 gene, and a CC5⌬32 genotype (17,26,40,41). Although WNV first appeared in the Western Hemisphere in 1999 in New York and spread rapidly through North America, surprisingly few human clinical infections have been reported in Central and South America, despite the migration of avian hosts and appropriate vectors for transmission (35,56).WNV infection requires attachment to cell surface receptors, which remain poorly defined, endocytosis, and acid-catalyzed fusion of the virus within the late endosome. After translation of input-strand RNA and viral replication, progeny virion assembly occurs within the endoplasmic reticulum (ER), with the capsid protein and genomic RNA associating with pre...

Anti-inflammatory activity of intravenous immunoglobulins protects against West Nile virus encephalitis

“…1c) despite being incapable of neutralizing virus infectivity (Table 1). This result showed that WNV neutralizing activity of IVIG was not critical for protection from WNV encephalitis and that alternative mechanisms could mediate protection (Vogt et al, 2011), which we anticipated based on our demonstration that IVIG protection against HSV1 encephalitis was independent of neutralizing antibodies (Ramakrishna et al, 2011).…”

“…IVIG has a broad repertoire of neutralizing antibodies for various pathogens and neutralization is commonly assumed to be the mechanism of protection against viral infection. However, we have shown in the HSV1 model that IVIG devoid of neutralizing antibodies is as effective as IVIG in protection against encephalitis (Ramakrishna et al, 2011) and the reports that non-neutralizing antibodies can protect against WNV infection are consistent with this (Chung et al, 2006;Mehlhop et al, 2005;Vogt et al, 2011). Studies investigating IVIG protection from WNV infection have based dosing on its neutralizing activity and have thus used IVIG at suboptimal doses (0.1-0.6 g kg 21 ) to elicit antiinflammatory responses.…”

Structural basis of potent Zika–dengue virus antibody cross-neutralization

“…One surface exposed by this 'breathing' is the fusionloop epitope (FLE), which is a dominant cross-reactive antigenic site 9 . Although antibodies to this site can protect by complement-mediated mechanisms, as shown in a mouse model for West Nile virus 10 , they are poorly neutralizing and lead to antibody-dependent enhancement [11][12][13][14][15] , thereby aggravating Flavivirus pathogenesis and complicating the development of safe and effective vaccines.…”

Human antibody responses after dengue virus infection are highly cross-reactive to Zika virus

“…S1B). The fusion loop has been described as a target for broadly cross-reactive antibodies against DENV (21,22) as well as other flaviviruses (23)(24)(25) and could be one of the epitopes targeted by the cross-reactive antibodies described in our study. In addition, despite the amino acid differences between the DENV2 and ZIKV E proteins compared, the two proteins share nearly identical structures (Fig.…”