Antibody Neutralization of Emerging SARS-CoV-2: EG.5.1 and XBC.1.6

Abstract

SARS-CoV-2 variants EG.5.1 and XBC.1.6 have recently emerged, attracting increased attention due to their rapid expansion globally and in Australia, respectively. EG.5.1 evolved from Omicron subvariant XBB.1.9, harboring additional Q52H and F456L spike substitutions. The F456L mutation is located within the epitopes of many class-1 monoclonal antibodies (mAbs) directed to the receptor-binding domain (RBD), raising concerns about further antibody evasion. XBC.1.6, a descendant of a Delta-BA.2 recombinant, carries 15 additional spike mutations. The extent to which antibody evasion contributes to the growth advantage of XBC.1.6 in Australia remains to be determined. To assess the antibody evasion properties of the emergent variants, we conducted pseudovirus neutralization assays using sera from individuals who received three doses of COVID-19 mRNA monovalent vaccines plus one dose of a BA.5 bivalent vaccine, as well as from patients with BQ or XBB breakthrough infection. The assays were also performed using a panel of 14 mAbs that retained neutralizing activity against prior XBB subvariants. Our data suggested that EG.5.1 was slightly but significantly more resistant (< 2-fold) to neutralization by BQ and XBB breakthrough sera than XBB.1.16, which is known to be antigenically similar to XBB.1.5. Moreover, the F456L mutation in EG.5.1 conferred heightened resistance to certain RBD class-1 mAbs. In contrast, XBC.1.6 was more sensitive to neutralization by sera and mAbs than the XBB subvariants. Notably, XBB breakthrough sera retained only weak neutralization activity against XBB subvariants. In summary, EG.5.1 and XBC.1.6 exhibited distinct antibody evasion properties. The recent global expansion of EG.5.1 might be attributable, in part, to its enhanced neutralization resistance. That XBB breakthrough infections did not elicit a robust antibody neutralization response against XBB subvariants is indicative of immunological imprinting. The high prevalence of XBC.1.6 in Australia is not due to enhanced antibody evasion.