In a recent study posted to the bioRxiv* pre-print server, a team of researchers assessed the neutralizing activity of XAV-19, a glyco-humanized polyclonal antibody derived from hyperimmunized animal hosts, against the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) Omicron variant of concern (VOC).
The unprecedented health impact of the coronavirus disease 2019 (COVID-19) pandemic has led to more research and development of various therapy options like polyclonal antibodies (pAbs) against SARS-CoV-2.
One of them, XAV-19, is a glyco-humanized polyclonal antibody derived from pigs and can bind to multiple target epitopes in SARS-CoV-2 spike proteins and cause neutralization of SARS-CoV-2 Alpha, Beta, Gamma, and Delta VOCs. However, its effect on the newly emerged SARS-CoV-2 Omicron VOC still needs to be investigated.
About the study
The present study evaluates the impact of neutralization by XAV-19 on the interaction between SARS-CoV-2 Omicron angiotensin-converting enzyme-2 (ACE-2) and receptor-binding domain (RBD) and the subsequent neutralization of the Omicron VOC.
The study used the Omicron-type recombinant RBD protein and SARS-CoV-2 Wuhan strains, Alpha, Beta, Gamma, Delta, and Omicron VOCs isolated from COVID-19-infected patients. The binding ability of XAV-19 to the Omicron RBD proteins and to the Wuhan strains was evaluated using enzyme-linked immunosorbent assay (ELISA) with the addition of 3,3′,5,5′-tetramethylbenzidine (TMB) reagent. The XAV-19-induced neutralization of the interaction between the Omicron RBD and its ACE-2 receptor was measured by ELISA. The assay included triplicate measurements to evaluate the neutralization activity of a mixture of cilgavimab and tixagevimab against RBD-ACE-2 receptor interaction. The impact of the mixture and XAV-19 on the interaction was compared.
SARS-CoV-2 Wuhan strains were isolated from reverse transcription-polymerase chain reaction (RT-PCR)-confirmed SARS-CoV-2-infected patients using nasopharyngeal swabs or sputum sample inoculated with Vero cells. Viral stocks were produced by passing the isolates once through the Vero cells and then titrated by a limiting dilution assay. This assay allowed median tissue culture infective dose (TCID50) calculation. A whole virus replication assay was also used to analyze the neutralizing ability of XAV-19 against the five SARS-CoV-2 strains isolated.
The results showed that the XAV-19 antibody responded to all the SARS-CoV-2 Wuhan RBD peptides. Dominant target epitopes were present in only four regions of the XAV-19 antibody, as confirmed by proteolytic epitope mapping. These regions contained the six amino acids responsible for interaction between the RBD and the ACE-2 counter receptor, which could explain the neutralizing ability of XAV-19.
Out of the 15 Omicron mutations present in the RBD, seven of the amino acids were crucial for ACE-2 interaction; two of these seven amino acids were included in the XAV-19 target epitopes. In the case of the tixagevimab and cilgavimab combination, five amino acids of the target epitope corresponded to the amino acids in the Omicron mutation.
The intensity of XAV-19 binding attained an efficacy plateau at comparable concentrations of 1 µg/ml against Wuhan and Omicron RBD. An RBD/ACE-2 binding competition assay showed that the full neutralization potency of XAV-19 against ACE-2-interaction was equal for both Wuhan and Omicron RBD. Comparison of this assay with the combination of cilgavimab and tixagevimab revealed only 60% neutralization potency.
The inhibitory effect of XAV-19 on the live virus isolates was tested by using Vero E6 cells and recording the infection of the inoculated Vero cells after four days by calculating the viral load and cytopathic effect (CPE) by quantitative reverse transcription PCR (RT-qPCR). XAV-19 completely neutralized the CPE of the live virus isolates or reduced 100% of the viral load with different NC50 concentrations.
The current study findings show that XAV-19 neutralized the RBD of the Omicron VOC and the RBD of the Wuhan strain at similar doses. The neutralizing ability of the combination of cilgavimab and tixagevimab was also measured by ELISA and it was observed that it exhibited limited neutralization compared to XAV-19. Owing to the weak binding of the Omicron VOC to ACE2 and the lack of Omicron mutations in regions of the XAV-19 target epitope domain, XAV-19 could exhibit more potent neutralization against Omicron compared to any other SARS-CoV-2 variant.
The researchers suggested that COVID-19-infected patients should be administered with pAbs during the early days of infection to allow full neutralization of SARS-CoV-2. The wide range of potency of XAV-19 and other pAbs in neutralizing different SARS-CoV-2 variants have made them a notable drug candidate for the treatment of SARS-CoV-2 and its potential VOCs.
bioRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.