XBB.1.5 RBD-Based Bivalent Vaccines Induced Antibody Responses Against SARS-CoV-2 Variants in Mice.

(1) Background: The currently circulating variant of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) exhibits resistance to antibodies induced by vaccines. The World Health Organization recommended the use of monovalent XBB.1 sublineages (e.g., XBB.1.5) as an antigenic component in 2023. (2) Objective: In this study, we aimed to develop vaccines based on the XBB.1.5 receptor-binding domain (RBD) to combat the recently emerged SARS-CoV-2 XBB and JN.1 variants, as well as previously circulating variants. (3) Methods: Glycoengineered Pichia pastoris was utilized to produce a recombinant XBB.1.5 RBD protein with mammalian-like and fucose-free N-glycosylation. The XBB.1.5 RBD was mixed with Al(OH)(3):CpG adjuvants to prepare monovalent vaccines. Thereafter, the XBB.1.5 RBD was mixed with the Beta (B.1.351), Delta (B.1.617.2), or Omicron (BA.2) RBDs (1:1 ratio), along with Al(OH)(3):CpG, to prepare bivalent vaccines. BALB/c mice were immunized with the monovalent and bivalent vaccines. Neutralizing antibody titers were assessed via pseudovirus and authentic virus assays; humoral immune responses were analyzed by RBD-binding IgG subtypes. (4) Results: The monovalent vaccine induced higher neutralizing antibody titers against Delta, BA.2, XBB.1.5, and JN.1 compared to those in mice immunized solely with Al(OH)(3):CpG, as demonstrated by pseudovirus virus assays. The XBB.1.5/Delta RBD and XBB.1.5/Beta RBD-based bivalent vaccines provided potent protection against the BA.2, XBB.1.5, JN.1, and KP.2 variants, as well as the previously circulating Delta and Beta variants. All monovalent and bivalent vaccines induced high levels of RBD-binding IgG (IgG1, IgG2a, IgG2b, and IgG3) antibodies in mice, suggesting that they elicited robust humoral immune responses. The serum samples from mice immunized with the XBB.1.5 RBD-based and XBB.1.5/Delta RBD-based vaccines could neutralize the authentic XBB.1.16 virus. (5) Conclusions: The XBB.1.5/Beta and XBB.1.5/Delta RBD-based bivalent vaccines are considered as potential candidates for broad-spectrum vaccines against SARS-CoV-2 variants.