*1.1. Filoviridae Phylogeny*

The first filovirus genus to be identified was *Marburgvirus* in 1967 composed of one species *Marburg marburgvirus* with two viruses: Marburg virus (MARV) and the very closely related Ravn virus (RAVV). Both viruses cause Marburg virus disease (MVD), a highly lethal form of viral haemorrhagic fever, with the largest outbreak occurring between 2004 and 2005 in Angola, with 252 infected individuals and a case fatality rate (CFR) of 90% [1].

The second and most notorious genus of filovirus, *Ebolavirus*, contains six species each with one virus; *Zaire ebolavirus*, Ebola virus (EBOV); *Sudan ebolavirus*, Sudan virus (SUDV); *Bundibugyo ebolavirus*, Bundibugyo virus (BDBV); *Tai Forest ebolavirus*, Tai Forest virus (TAFV); *Reston ebolavirus*, Reston virus (RESTV) and *Bombali ebolavirus*, Bombali virus (BOMV). The first four of these six viruses are known to cause Ebola virus disease (EVD) in humans, with a CFR frequently reported between 40% and 90%. However, this is likely an overestimate as many EBOV infections may go unreported [2]. EBOV is predominantly responsible for the EVD outbreaks of the greatest magnitude, with the largest being the 2013–2016 West African outbreak [3,4].

Most recently, new filoviruses have been discovered, which have not yet been associated with outbreaks in humans. In 2011, a third genus *Cuevavirus* was discovered

**Citation:** Hargreaves, A.; Brady, C.; Mellors, J.; Tipton, T.; Carroll, M.W.; Longet, S. Filovirus Neutralising Antibodies: Mechanisms of Action and Therapeutic Application. *Pathogens* **2021**, *10*, 1201. https:// doi.org/10.3390/pathogens10091201

Academic Editors: Philipp A. Ilinykh and Kai Huang

Received: 20 July 2021 Accepted: 12 September 2021 Published: 16 September 2021

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with a sole species *Lloviu cuevavirus* including one virus, Lloviu virus (LLOV) [5]. Whilst infectious LLOV still remains to be isolated, anti-LLOV antibodies have been detected in bats [6,7]. In 2019, a new filovirus species *Mˇenglà dianlovirus*, including Mengla virus (MLAV) was discovered in China as the sole species of a new genus *Dianlovirus* [8]. Still to date, neither BOMV, LLOV nor MLAV are known to cause viral haemorrhagic fever with its pathogenicity in humans still to be determined [8,9]. However, both BOMV and MLAV Glycoprotein (GP) pseudoviruses as well as LLOV virus-like particles (VLPs) demonstrate a broad tissue tropism in cell lines from different animals, replicate similarly to ebolaviruses and use the Niemann pick type C1 (NPC1) as an entry receptor indicating a potential for spillover events [3,8,10]. Two much more divergent species of filovirus *Huángjiao tham- ¯ novirus* including Huángjiao virus (HUJV) and ¯ *X¯ılang striavirus ˇ* including X¯ılang virus ˇ (XILV), which belong to the genera *Thamnovirus* and *Striavirus* respectively, have been also described. These filoviruses infect fish [11].

While non-EBOV filoviruses will be mentioned, this review will be more focused on EBOV due to the ongoing impact of this pathogen on world health and the recent developments in antibody-based therapeutics and EBOV vaccines.

#### *1.2. Genome Organisation of Ebolavirus*

With the exception of the more divergent *Thamnovirus* and *Striavirus* genera, all filoviruses encode seven structural proteins: Nucleoprotein (NP), Viral Protein (VP) VP35, VP40, GP, VP30, VP24 and L polymerase (L) as shown in Figure 1A.

**Figure 1.** (**A**) A comparison of the EBOV and MARV genome structures highlighting key differences such as how EBOV

has multiple GP gene products and where there are differences in overlapping genes. (**B**) A diagram of the EBOV GP with domains highlighted based on the work of Lee et al. 2009 [12]. Domains of the EBOV GP as they appear on the gene: GP1, SP (signal peptide), GP1 base residues: (33–69, 95–104, 158–167 and 175–189), GP1 Head: (70–94, 105–157, 168–175 and 214–226), Glycan Cap: (227–310), MLD Mucin-like-Domain, and in GP2, N-terminus of GP2, Internal Fusion Loop: (511–553), HR1 (554–598), HR2 (599–630), MPER Membrane proximal external region, TM transmembrane domain, CT Cytoplasmic tail. Regions highlighted in brown are uncharacterised as they could not be assigned to a domain due to differences with the protein structure used and the domains as listed by Lee et al. 2009 (32, 191–195, 210–211, 470–478) and the protein structure used from Zhao et al. [12,13]. Regions of the EBOV GP in white boxes with blue outlines are regions that could not be shown via X-ray diffraction and so do not appear on the GP structure in the diagram, in addition to residues (28–30, 196–209, 284–285, 294–300, 431–469, 632–669) [12,13]. Diagram was created with BioRender.com (©BioRender 2021, accessed in June 2021) and the protein structure was generated in The PyMOL Molecular Graphics System, Version 2.0 Schrödinger, Delano Scientific LLC, Berkeley, CA, USA using the PDB accession: 5JQ3.

> The NP is the main component of the ribonucleoprotein or nucleocapsid, though VP30 and VP24 are also required for the stability of the nucleocapsid and together make the changes in NP required to incorporate the viral RNA. L polymerase is an RNA-dependent RNA polymerase which complexes with the polymerase co-factor VP35 and is responsible for transcribing the viral RNA, while the initiation of transcription is activated by VP30. L polymerase also functions in regulating and editing the viral RNA e.g., in the case of GP where three different gene products are produced [14]. VP40 is considered the matrix protein and is crucial for viral assembly and budding. It is also worth noting that many of the viral proteins, particularly VP24, VP30, VP35 and VP40 have functions linked to host pathology or immune evasion. For instance, VP24 and VP35 inhibit interferon (IFN) pathways (reviewed in Cantoni and Rossman, 2018) [15].
