RESUMO
A recombinant lineage of the SARS-CoV-2 Omicron variant, named XBB, appeared in late 2022 and evolved descendants that successively swept local and global populations. XBB lineage members were noted for their improved immune evasion and transmissibility. Here, we determine cryo-EM structures of XBB.1.5, XBB.1.16 and EG.5 spike (S) ectodomains to reveal enhanced occupancy of the receptor inaccessible closed state. Interprotomer receptor binding domain (RBD) interactions previously observed in BA.1 and BA.2 were retained to reinforce the 3-RBD-down state. Improved stability of XBB.1.5 and XBB.1.16 RBD compensated for loss of stability caused by early Omicron mutations, while the F456L substitution reduced EG.5 RBD stability. Long-range impacts of S1 subunit mutations affected conformation and epitope presentation in the S2 subunit. Taken together, our results feature a theme of iterative optimization of S protein stability as Omicron continues to evolve, while maintaining high affinity receptor binding and bolstering immune evasion.