Single spike mutation differentiating XBB.1 and XBB.1.5 enhances SARS-CoV-2 cell-to-cell transmission and facilitates serum-mediated enhancement.

Introduction: The ongoing emergence of SARS-CoV-2 variants poses significant challenges to existing therapeutics. The spike (S) glycoprotein is central to both viral entry and cell-to-cell transmission via syncytia formation, a process that confers resistance to neutralizing antibodies. The mechanisms underlying this resistance, particularly in relation to spike-mediated fusion, remain poorly understood. Methods: We analyzed two clinical SARS-CoV-2 isolates differing by a single amino acid substitution in the S protein. Using biochemical and cell-based assays, we evaluated entry kinetics, syncytia formation, and the neutralizing efficacy of convalescent sera. These parameters were further correlated with S-mediated cell-cell fusion activity. Results: The single amino acid substitution significantly altered entry kinetics and enhanced syncytia formation. This modification did not diminished the neutralizing capacity of convalescent sera, but it increased the efficiency of S-induced cell-cell fusion. These findings highlight the mutation's impact on viral transmissibility and immune evasion. Discussion: Our study demonstrates that even minor changes in the S protein can profoundly influence SARS-CoV-2 transmissibility and resistance to antibody-mediated neutralization. Understanding the molecular basis of S-mediated cell-cell fusion is crucial for anticipating the impact of emerging variants and developing next-generation therapeutic strategies. These insights provide a framework for predicting variant fitness and optimizing treatment approaches against future SARS-CoV-2 variants.