Focused processed hACE2?+?XBB1.5 RBD (PDB ID: 8VKP, EMDB ID: EMD-43325). complexes derived from donor serum are deposited together under the following EMDB access: Unfavorable Stain EM Reconstructions of SARS-CoV-2 spike proteins mixed with polyclonal antibodies from donor 4 (EMDB ID: EMD-43326)?Source data are provided with this paper. Abstract The XBB.1.5 KGF variant of SARS-CoV-2 has rapidly achieved global dominance and exhibits a high growth advantage over previous variants. Preliminary reports CO-1686 (Rociletinib, AVL-301) suggest that the success of XBB.1.5 stems from mutations within its spike glycoprotein, causing immune evasion and enhanced receptor binding. We present CO-1686 (Rociletinib, AVL-301) receptor binding studies that demonstrate retention of binding contacts with the human ACE2 receptor and a striking decrease in binding to mouse ACE2 due to the revertant R493Q mutation. Despite considerable evasion of antibody binding, we spotlight a region around the XBB.1.5 spike protein receptor binding domain (RBD) that is recognized by serum antibodies from a donor with hybrid immunity, collected prior to the emergence of the XBB.1.5 variant. T cell assays reveal high frequencies of XBB.1.5 spike-specific CD4+ and CD8+ T cells amongst donors with cross immunity, with the CD4+ T cells skewed towards a Th1 cell phenotype and having attenuated effector cytokine secretion as compared to ancestral spike protein-specific cells. Thus, while the XBB.1.5 variant has retained efficient human receptor binding and gained antigenic alterations, it remains susceptible to recognition by T cells induced via vaccination and previous infection. Subject terms: Cryoelectron microscopy, Immune evasion, SARS-CoV-2, Antibodies, T cells New variants of SARS-CoV-2 computer virus can evolve such that antibodies that recognised previous versions are not able to recognise newer versions. Here the authors characterise antibody binding to the XBB.1.5 variant and how antibodies and T cells from persons infected with earlier versions of SARS-CoV-2 are able to recognise and/or bind to the XBB.1.5 spike protein. Introduction In late 2021, the emergence of the original Omicron SARS-CoV-2 variant (BA.1) ushered in a new chapter of the COVID-19 pandemic. While previously emerged variants (Alpha, Beta, Gamma, Delta) contained up to 10 mutations in their spike glycoproteins, Omicron variants contained an unprecedented >30 spike mutations1. Given the role of the spike protein as the major immunogen within SARS-CoV-2 vaccines, the primary consequence of the high number of mutations within the Omicron variants was reduced vaccine efficacy2C4. Secondary effects of Omicron mutations include altered viral tropism and the acquired ability to participate several mammalian ACE2 receptors (mouse, rat, bat, etc.)5C7. Specifically, early Omicron lineage spike proteins (BA.1 and BA.2) acquired the ability to bind mouse ACE2 (mACE2) with high affinity, generating the hypothesis of spillover transmission into mice followed by spillback transmission into humans as underlying the emergence CO-1686 (Rociletinib, AVL-301) of these highly mutated lineages. Throughout 2022, several sub-lineagesBA.1.1, BA.2, and BA.5successively supplanted the original BA.1 variant, with a high degree of mutational plasticity observed within the amino terminal domain (NTD) and receptor binding domain (RBD) (Fig.?1A). In late CO-1686 (Rociletinib, AVL-301) 2022, a recombination occurred between Omicron sub-lineages BA.2.75 and BA.2.10.1 resulting in a new variant XBB.1 and its further sub-lineage XBB.1.5, the latter of which rapidly became the most dominantly sequenced lineage in the United States (Fig.?1B). Given that XBB.1.5 is the first recombinant SARS-CoV-2 lineage to achieve global dominance, and the stark growth advantage it has over earlier Omicron sub-lineages, we investigated the XBB.1.5 spike glycoprotein from the perspectives of receptor binding, antibody evasion, and T cell specificity, and report our findings here. We find that while the XBB.1.5 maintains a high binding affinity for human ACE2 as similar to previous variants, it has a diminished affinity for mouse ACE2, representing a departure from earlier SARS-CoV-2 variants. We show that despite significant antibody escape, the XBB.1.5 spike protein is still recognized by pre-existing antibodies and T cells from donors with hybrid immunity. Open in a separate window Fig. 1 Mutational profile and prevalence of the XBB.1.5 SARS-CoV-2.