The Marburgvirus-Neutralizing Human Monoclonal Antibody MR191 Targets a Conserved Site to Block Virus Receptor Binding

Abstract

Summary Since its first identification 50 years ago, Marburgviruses have emerged several times, with 83–90% lethality in the largest outbreaks. Although no vaccines or therapeutics are available for human use, the human antibody MR191 provides complete protection in nonhuman primates when delivered several days after inoculation of a lethal marburgvirus dose. The detailed neutralization mechanism of MR191 remains outstanding. Here we present a 3.2 Å crystal structure of MR191 complexed with a trimeric marburgvirus surface glycoprotein (GP). MR191 neutralizes by occupying the conserved receptor-binding site and competing with the host receptor Niemann-Pick C1. The structure illuminates previously disordered regions of GP including the stalk, fusion loop, CX6CC switch, and an N-terminal region of GP2 that wraps about the outside of GP1 to anchor a marburgvirus-specific “wing” antibody epitope. Virus escape mutations mapped far outside the MR191 receptor-binding site footprint suggest a role for these other regions in the GP quaternary structure.

Structural Basis of Pan-Ebolavirus Neutralization by an Antibody Targeting the Glycoprotein Fusion Loop

“…This is likely due to a combination of a clash with the recently described MARV wing anchor at the base of the GP (King et al, 2018) (Figure S4A), as well as sequence divergence at the critical contact residue Q560, which is an arginine (R561) in MARV, a bulkier residue that likely clashes with the CDRH3(Figure S4B). It has been demonstrated that the MLDs hang over the sides of the MARV GP, in contrast to their conformation in ebolaviruses, which possibly further shields the ADI-15878 epitope in marburgviruses (Hashiguchi et al, 2015).…”

“…We also modeled a large portion of HR2 down to residue 625 (Figure 2A), which adopts a three-helix bundle within the GP2 and has been previously modeled for EBOV/May (Zhao et al, 2016) and MARV (King et al, 2018). Notably, HR2in MARV is slightly different from the ebolaviruses, with a slightly wider three-helix bundle.…”

“…A side- by-side comparison of known viral GP structures demonstrates the high level of similarities within the ebolavirus genus and in the core (Figure 2E). However, MARV lacks a structured glycan cap, contains an additional immunodominant epitope at the base of the GP, and displays its MLDs in a different spatial arrangement (Fusco et al, 2015; Hashiguchi et al, 2015; King et al, 2018). …”

“…The RBS interacts with the host receptor NPC1 during entry and is structurally conserved across all filoviruses (Wang et al, 2016). While the RBS has been shown to elicit pan-filoviral antibodies, potency and efficacy is variable because the ebolaviruses require the proteolytic removal of the MLDs and glycan cap to expose the RBS (Bale et al, 2011; Bornholdt et al, 2016a; Miller et al, 2012; Wang et al, 2016), while the marburgviruses do not (Flyak et al, 2015; Gnirss et al, 2012; King et al, 2018). The HR2 domain has also been proposed as a hotspot of filoviral vulnerability (Flyak et al, 2016, 2018; Wec et al, 2017; Ya- mayoshi and Kawaoka, 2017), but antibodies that target this region have limited cross-reactivity.…”

“…The HR1 and IFL on filoviral GPs are highly conserved in sequence and structure across genera, as demonstrated by comparing the structures of EBOV, SUDV, and MARV GPs, making this epitope an attractive target for therapeutic antibody development (Bale et al, 2012; Dias et al, 2011; Hashiguchi et al, 2015; King et al, 2018; Lee et al, 2008; Zhao et al, 2016). HR1 is composed of an alpha helix, which cradles GP1 and contains a highly conserved glycan at N563.…”