Monotherapy with antibody 1C3 partially protects Ebola virus-exposed macaques

Abstract

A cocktail of human monoclonal antibodies 1C3 and 1C11 previously protected macaques from a lethal exposure to either Ebola virus (EBOV) or Sudan virus (SUDV). 1C3 is of particular interest because its paratope strongly binds with unique stoichiometry to the glycoprotein head of several orthoebolaviruses, resulting in neutralization of EBOV and SUDV. Therefore, we evaluated the protective activity of 1C3 as a standalone therapeutic in macaques exposed to either EBOV or SUDV. Two doses of 1C3 monotherapy, administered 4 and 7 days post-exposure, did not protect SUDV-exposed macaques and partially protected EBOV-exposed macaques. Notably, in a macaque that succumbed to EBOV infection, we identified two mutually exclusive escape mutations that emerged immediately after the first dose and resulted in two amino acid changes at the 1C3 binding site. We also detected a subconsensus treatment-emergent mutation likely affecting the 1C3 binding site in all three deceased SUDV-exposed macaques. Our findings highlight combination treatment with 1C11 as critical for protection, particularly against SUDV, and in vivo activity of unpartnered 1C3 as susceptible to rapid EBOV and SUDV escape under therapeutic pressure.

Importance: A cocktail of human monoclonal antibodies 1C3 and 1C11 previously protected macaques exposed to a lethal dose of either Ebola virus (EBOV) or Sudan virus (SUDV). Since the unique binding characteristics of 1C3 are of particular interest, we evaluated its protective activity as monotherapy in macaques exposed to either EBOV or SUDV. Two doses of 1C3 alone did not protect SUDV-exposed macaques and only partially protected EBOV-exposed macaques. Importantly, failure to protect was associated with the rapid emergence of previously in vitro-identified escape mutations at the 1C3 binding site, highlighting the importance of its use in combination with 1C11 for protection against fatal disease outcome and avoiding rapid EBOV and SUDV escape. Findings have broader implications for the wise use of combination-based monoclonal antibody therapeutics to improve outcomes and prevent resistance in filovirid diseases.

Keywords: 1C11; 1C3; EBOV; Ebola virus; SUDV; Sudan virus; escape mutation; glycoprotein; monoclonal antibody; orthoebolavirus; partial protection.

Fig 1

Percent survival of rhesus monkeys and crab-eating macaques after intramuscular exposure on day 0 to EBOV or SUDV, respectively, followed by intravenous injection of 25 mg/kg of 1C3 on day 4 and day 7.

Fig 2

Clinical euthanasia scores recorded for macaques exposed to EBOV or SUDV. Scores are by day (D) and per observation recorded in the morning (AM) or afternoon (PM) and are additionally numbered when more than one observation occurred in the morning or afternoon during the peak disease period. The EBOV scoring scale was 0–4. Scores of 4 or 3, in combination with a rectal body temperature of equal to or less than 34°C, necessitated euthanasia. The SUDV scoring scale was 0–29. Any cumulative scores equal to or above 9 triggered euthanasia. All macaques that met euthanasia criteria or were found dead are shown in magenta; survivors are indicated in purple and controls in green.

Fig 3

Detectable plasma viremia in macaques over the course of EBOV or SUDV infection. The infectious titers of EBOV and SUDV were determined by plaque assay titration and are shown as PFU per milliliter of plasma. Real-time reverse transcription PCR (RT-qPCR) targeting the glycoprotein gene (GP) of EBOV and the nucleoprotein gene (NP) of SUDV was used to determine the gene equivalents (GEq) per milliliter of plasma. All macaques that met euthanasia criteria or were found dead are indicated in magenta, survivors are indicated in purple, and controls are indicated in green.

Fig 4

Structure of the EBOV glycoprotein (GP_1,2; green) with 1C3 light chain (yellow) and heavy chain (blue). Residues 119 and 172 (red) undergo substitutions S119N and R172Q. Binding of antibody 1C11 (gray) is shown for reference.

Fig 5

Left: frequency of G-to-A mutations at nucleotide positions 6,393 and 6,552 leading to amino acid substitutions S119N and R172Q in sequences of EBOV from plasma collected on day 7 and spleen and tracheobronchial lymph node of RM 3 harvested on day 8. Right: relative abundance (as a ratio of 1.0) suggesting exclusivity or coexistence of S119N and R172Q in the same sample type. Based on Fisher’s exact test, a significant difference (P < 0.001) was found in the abundance of either S119N or R172Q in spleen and lymph node.

Fig 6

Left: frequency of amino acid change for P124L in SUDV GP_1,2 was significantly (P < 0.001) higher in plasma and tissue homogenates from 1C3-treated macaques (magenta) than from untreated controls (green). Right: change in P124L was detectable at a frequency of 0.1–0.4 in plasma, spleen, liver, and axillary lymph node from CEM 1 and CEM 3. In CEM 2, GP₁ P124L was also detectable in all tissues but at >0.1 frequency (black dashed line) in plasma and lymph node only.