SARS-CoV-2 Omicron lineage XBB spike structures, conformations, antigenicity, and receptor recognition (preprint)

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.

Neutralization Escape by SARS-CoV-2 Omicron Subvariant BA.2.86 (preprint)

The continued evolution of SARS-CoV-2 may lead to evasion of vaccine immunity and natural immunity. A highly mutated Omicron variant BA.2.86 has recently been identified with over 30 amino acid changes in Spike compared with BA.2 and XBB.1.5. As of September 4, 2023, BA.2.86 has been identified in 37 sequences from 10 countries, which is likely an underestimate due to limited surveillance. The ability of BA.2.86 to evade NAbs compared with other currently circulating Omicron variants remains unknown. Our data show that NAb responses to BA.2.86 were lower than to BA.2 but were comparable or slightly higher than to the current circulating recombinant variants XBB.1.5, XBB.1.16, EG.5, EG.5.1, and FL.1.5.1.

Mutational basis of serum cross-neutralization profiles elicited by infection or vaccination with SARS-CoV-2 variants (preprint)

A series of SARS-CoV-2 variants emerged during the pandemic under selection for neutralization resistance. Convalescent and vaccinated sera show consistently different cross-neutralization profiles depending on infecting or vaccine variants. To understand the basis of this heterogeneity, we modeled serum cross-neutralization titers for 165 sera after infection or vaccination with historically prominent lineages tested against 18 variant pseudoviruses. Cross-neutralization profiles were well captured by models incorporating autologous neutralizing titers and combinations of specific shared and differing mutations between the infecting/vaccine variants and pseudoviruses. Infecting/vaccine variant-specific models identified mutations that significantly impacted cross-neutralization and quantified their relative contributions. Unified models that explained cross-neutralization profiles across all infecting and vaccine variants provided accurate predictions of holdout neutralization data comprising untested variants as infecting or vaccine variants, and as test pseudoviruses. Finally, comparative modeling of 2-dose versus 3-dose mRNA-1273 vaccine data revealed that the third dose overcame key resistance mutations to improve neutralization breadth.

Waning Immunity Against XBB.1.5 Following Bivalent mRNA Boosters (preprint)

The SARS-CoV-2 Omicron variant has continued to evolve. XBB is a recombinant between two BA.2 sublineages, XBB.1 includes the G252V mutation, and XBB.1.5 includes the G252V and F486P mutations. XBB.1.5 has rapidly increased in frequency and has become the dominant virus in New England. The bivalent mRNA vaccine boosters have been shown to increase neutralizing antibody (NAb) titers to multiple variants, but the durability of these responses remains to be determined. We assessed humoral and cellular immune responses in 30 participants who received the bivalent mRNA boosters and performed assays at baseline prior to boosting, at week 3 after boosting, and at month 3 after boosting. Our data demonstrate that XBB.1.5 substantially escapes NAb responses but not T cell responses after bivalent mRNA boosting. NAb titers to XBB.1 and XBB.1.5 were similar, suggesting that the F486P mutation confers greater transmissibility but not increased immune escape. By month 3, NAb titers to XBB.1 and XBB.1.5 declined essentially to baseline levels prior to boosting, while NAb titers to other variants declined less strikingly.

SARS-CoV-2 variant transition dynamics are associated with vaccination rates, number of co-circulating variants, and natural immunity (preprint)

Background: Throughout the COVID-19 pandemic, the SARS-CoV-2 virus has continued to evolve, with new variants outcompeting existing variants and often leading to different dynamics of disease spread. Methods: In this paper, we performed a retrospective analysis using longitudinal sequencing data to characterize differences in the speed, calendar timing, and magnitude of 13 SARS-CoV-2 variant waves/transitions for 215 countries and sub-country regions, between October 2020 and October 2022. We then clustered geographic locations in terms of their variant behavior across all Omicron variants, allowing us to identify groups of locations exhibiting similar variant transitions. Finally, we explored relationships between heterogeneity in these variant waves and time-varying factors, including vaccination status of the population, governmental policy, and the number of variants in simultaneous competition. Findings: This work demonstrates associations between the behavior of an emerging variant and the number of co-circulating variants as well as the demographic context of the population. We also observed an association between high vaccination rates and variant transition dynamics prior to the Mu and Delta variant transitions. Interpretation: These results suggest the behavior of an emergent variant may be sensitive to the immunologic and demographic context of its location. Additionally, this work represents the most comprehensive characterization of variant transitions globally to date. Funding: Laboratory Directed Research and Development (LDRD), Los Alamos National Laboratory

Substantial Neutralization Escape by the SARS-CoV-2 Omicron Variant BQ.1.1 (preprint)

Omicron BA.5 has been the globally dominant SARS-CoV-2 variant and has demonstrated substantial neutralization escape compared with prior variants. Additional Omicron variants have recently emerged, including BA.4.6, BF.7, BA.2.75.2, and BQ.1.1, all of which have the Spike R346T mutation. In particular, BQ.1.1 has rapidly increased in frequency, and BA.5 has recently declined to less than half of viruses in the United States. Our data demonstrate that BA.2.75.2 and BQ.1.1 escape NAbs induced by infection and vaccination more effectively than BA.5. BQ.1.1 NAb titers were lower than BA.5 NAb titers by a factor of 7 in two cohorts of individuals who received the monovalent or bivalent mRNA vaccine boosters. These findings provide the immunologic context for the rapid increase in BQ.1.1 prevalence in regions where BA.5 is dominant and have implications for both vaccine immunity and natural immunity.

Structural basis for breadth development in a HIV-1 neutralizing antibody (preprint)

Antibody affinity maturation enables adaptive immune responses to a wide range of pathogens. In some individuals broadly neutralizing antibodies develop to recognize rapidly mutating pathogens with extensive sequence diversity. Vaccine design for pathogens such as HIV-1 and influenza have therefore focused on recapitulating the natural affinity maturation process. Here, we determined structures of antibodies in complex with HIV-1 Envelope for all observed members and ancestral states of a broadly neutralizing HIV-1 antibody clonal B cell lineage. These structures track the development of neutralization breadth from the unmutated common ancestor and define affinity maturation at high spatial resolution. By elucidating contacts mediated by key mutations at different stages of antibody development we have identified sites on the epitope-paratope interface that are the focus of affinity optimization. Thus, our results identify bottlenecks on the path to natural affinity maturation and reveal solutions for these that will inform immunogen design aimed at eliciting a broadly neutralizing immune response by vaccination. Summary Somatic hypermutation drives affinity maturation of germline-encoded antibodies leading to the development of their pathogen neutralization function 1 . Rational vaccine design efforts that aim to recapitulate affinity maturation rely on information from antibodies elicited and matured during natural infection. High-throughput next generation sequencing and methods for tracing antibody development have allowed close monitoring of the antibody maturation process. Since maturation involves both affinity-enhancing and affinity-independent diversification, the precise effect of each observed mutation, their role in enhancing affinity to antigens, and the order in which the mutations accumulated are often unclear. These gaps in knowledge most acutely hinder efforts directed at difficult targets such as pan-HIV, pan-Influenza, and pan-Coronavirus vaccines. In HIV-1 infection, antibody maturation over several years is required to achieve neutralization breadth. Here, we determined structures of antibodies in complex with HIV-1 Envelope trimers for all observed members and ancestral states of a broadly neutralizing HIV-1 antibody clone to examine affinity maturation as neutralization breadth developed from the unmutated common ancestor. Structural determination of epitope-paratope interfaces revealed details of the contacts evolving over a timescale of several years. Structures along different branches of the clonal lineage elucidated differences in the branch that led to broad neutralization versus off-track paths that culminated in sub-optimal neutralization breadth. We further determined structures of the evolving Envelope revealing details of the virus-antibody co-evolution through visualization of how the virus constructs barriers to evade antibody-mediated neutralization and the mechanisms by which the developing antibody clone circumvents these barriers. Together, our structures provide a detailed time-resolved imagery of the affinity maturation process through atomic level descriptions of virus-antibody co-evolution leading to broad HIV neutralization. While the findings from our studies have direct relevance to HIV-1, the principles of affinity optimization and breadth development elucidated in our study should have broad relevance to other pathogens.

Characterization of SARS-CoV-2 Omicron BA.2.75 clinical isolates (preprint)

The prevalence of the Omicron subvariant BA.2.75 is rapidly increasing in India and Nepal. In addition, BA.2.75 has been detected in at least 34 other countries and is spreading globally. However, the virological features of BA.2.75 are largely unknown. Here, we evaluated the replicative ability and pathogenicity of BA.2.75 clinical isolates in Syrian hamsters. Although we found no substantial differences in weight change among hamsters infected with BA.2, BA.5, or BA.2.75, the replicative ability of BA.2.75 in the lungs was higher than that of BA.2 and BA.5. Of note, BA.2.75 caused focal viral pneumonia in hamsters, characterized by patchy inflammation interspersed in alveolar regions, which was not observed in BA.5-infected hamsters. Moreover, in competition assays, BA.2.75 replicated better than BA.5 in the lungs of hamsters. These results suggest that BA.2.75 can cause more severe respiratory disease than BA.5 and BA.2 and should be closely monitored.

Structural diversity of the SARS-CoV-2 Omicron spike (preprint)

Aided by extensive spike protein mutation, the SARS-CoV-2 Omicron variant overtook the previously dominant Delta variant. Spike conformation plays an essential role in SARS-CoV-2 evolution via changes in receptor binding domain (RBD) and neutralizing antibody epitope presentation affecting virus transmissibility and immune evasion. Here, we determine cryo-EM structures of the Omicron and Delta spikes to understand the conformational impacts of mutations in each. The Omicron spike structure revealed an unusually tightly packed RBD organization with long range impacts that were not observed in the Delta spike. Binding and crystallography revealed increased flexibility at the functionally critical fusion peptide site in the Omicron spike. These results reveal a highly evolved Omicron spike architecture with possible impacts on its high levels of immune evasion and transmissibility.

Booster of mRNA-1273 Vaccine Reduces SARS-CoV-2 Omicron Escape from Neutralizing Antibodies (preprint)

Data obtained on SARS-CoV-2 variant Omicron suggest that Omicron poses an increased risk of symptomatic breakthrough infections in people who receive only 2 doses of mRNA-1273. Administration of a booster mRNA vaccine may substantially reduce this risk.

Network analysis outlines strengths and weaknesses of emerging SARS-CoV-2 Spike variants (preprint)

The COVID-19 pandemic, caused by the SARS-CoV-2 virus, has triggered myriad efforts to dissect and understand the structure and dynamics of this complex pathogen. The Spike glycoprotein of SARS-CoV-2 has received special attention as it is the means by which the virus enters the human host cells. The N-terminal domain (NTD) is one of the targeted regions of the Spike protein for therapeutics and neutralizing antibodies against COVID-19. Though its function is not well-understood, the NTD is reported to acquire mutations and deletions that can accelerate the evolutionary adaptation of the virus driving antibody escape. Cellular processes are known to be regulated by complex interactions at the molecular level, which can be characterized by means of a graph representation facilitating the identification of key residues and critical communication pathways within the molecular complex. From extensive all-atom molecular dynamics simulations of the entire Spike for the wild-type and the dominant variant, we derive a weighted graph representation of the protein in two dominant conformations of the receptor-binding-domain; all-down and one-up. We implement graph theory techniques to characterize the relevance of specific residues at facilitating roles of communication and control, while uncovering key implications for fitness and adaptation. We find that many of the reported high-frequency mutations tend to occur away from the critical residues highlighted by our graph theory analysis, implying that these mutations tend to avoid targeting residues that are most critical for protein allosteric communication. We propose that these critical residues could be candidate targets for novel antibody therapeutics. In addition, our analysis provides quantitative insights of the critical role of the NTD and furin cleavage site and their wide-reaching influence over the protein at large. Many of our conclusions are supported by empirical evidence while others point the way towards crucial simulation-guided experiments.

Exploring the role of glycans in the interaction of SARS-CoV-2 RBD and human receptor ACE2 (preprint)

COVID-19 is a highly infectious respiratory disease caused by the novel coronavirus SARS-CoV-2. It has become a global pandemic and its frequent mutations may pose new challenges for vaccine design. During viral infection, the Spike RBD of SARS-CoV-2 binds the human host cell receptor ACE2, enabling the virus to enter the host cell. Both the Spike and ACE2 are densely glycosylated, and it is unclear how distinctive glycan types may modulate the interaction of RBD and ACE2. Detailed understanding of these determinants is key for the development of novel therapeutic strategies. To this end, we perform extensive all-atom simulations of the (i) RBD-ACE2 complex without glycans, (ii) RBD-ACE2 with oligomannose MAN9 glycans in ACE2, and (iii) RBD-ACE2 with complex FA2 glycans in ACE2. These simulations identify the key residues at the RBD-ACE2 interface that form contacts with higher probabilities, thus providing a quantitative evaluation that complements recent structural studies. Notably, we find that this RBD-ACE2 contact signature is not altered by the presence of different glycoforms, suggesting that RBD-ACE2 interaction is robust. Applying our simulated results, we illustrate how the recently prevalent N501Y mutation may alter specific interactions with host ACE2 that facilitate the virus-host binding. Furthermore, our simulations reveal how the glycan on Asn90 of ACE2 can play a distinct role in the binding and unbinding of RBD. Finally, an energetics analysis shows that MAN9 glycans on ACE2 decrease RBD-ACE2 affinity, while FA2 glycans lead to enhanced binding of the complex. Together, our results provide a more comprehensive picture of the detailed interplay between virus and human receptor, which is much needed for the discovery of effective treatments that aim at modulating the physical-chemical properties of this virus.

Recombination and low-diversity confound homoplasy-based methods to detect the effect of SARS-CoV-2 mutations on viral transmissibility (preprint)

The SARS-CoV-2 variant carrying the Spike protein mutation G614 was first detected in late January 2020 and within a few months became the dominant form globally. In the months that followed, many studies, both in vitro and in animal models, showed that variants carrying this mutation were more infectious and more readily transmitted than the ancestral Wuhan form. Here we investigate why a recently published study by van Dorp et al. failed to detect such higher transmissibility of the G614 variant using homoplasy-based methods. We show that both low diversity and recombination confound the methods utilized by van Dorp et al. and significantly decrease their sensitivity. Furthermore, though they claim no evidence of recombination in their dataset, we and several other studies identify a subset of the sequences as recombinants, possibly enough to affect their statistic adversely.

Information retrieval in an infodemic: the case of COVID-19 publications (preprint)

In the context of searching for COVID-19 related scientific literature, we present an information retrieval methodology for effectively finding relevant publications for different information needs. We discuss different components of our architecture consisting of traditional information retrieval models, as well as modern neural natural language processing algorithms. We present recipes to better adapt these components to the case of an infodemic, where, from one hand, the number of publications has an exponential growth and, from the other hand, the topics of interest evolve as the pandemic progresses. The methodology was evaluated in the TREC-COVID challenge, achieving competitive results with top ranking teams participating in the competition. In retrospect to this challenge, we provide additional insights with further useful impacts.

The SARS-CoV-2 Spike Variant D614G Favors an Open Conformational State (preprint)

The COVID-19 pandemic underwent a rapid transition with the emergence of a SARS-CoV-2 variant that carried the amino acid substitution D614G in the Spike protein that became globally prevalent. The G-form is both more infectious in vitro and associated with increased viral loads in infected people. To gain insight into the mechanism underlying these distinctive characteristics, we employed multiple replicas of microsecond all-atom simulations to probe the molecular-level impact of this substitution on Spikes closed and open states. The open state enables Spike interactions with its human cellular receptor, ACE2. Here we show that changes in the inter-protomer energetics due to the D614G substitution favor a higher population of infection-capable (open) states. The inter-protomer interactions between S1 and S2 subunits in the open state of the D-form are asymmetric. This asymmetry is resolved in the G-form due to the release of tensile hydrogen bonds resulting in an increased population of open conformations. Thus, the increased infectivity of the G-form is likely due to a higher rate of profitable binding encounters with the host receptor. It is also predicted to be more neutralization sensitive due to enhanced exposure of the receptor binding domain, a key target region for neutralizing antibodies.

D614G Spike Mutation Increases SARS CoV-2 Susceptibility to Neutralization. (preprint)

The SARS-CoV-2 Spike protein acquired a D614G mutation early in the COVID-19 pandemic that appears to confer on the virus greater infectivity and is now the globally dominant form of the virus. Certain of the current vaccines entering phase 3 trials are based on the original D614 form of Spike with the goal of eliciting protective neutralizing antibodies. To determine whether D614G mediates neutralization-escape that could compromise vaccine efficacy, sera from Spike-immunized mice, nonhuman primates and humans were evaluated for neutralization of pseudoviruses bearing either D614 or G614 Spike on their surface. In all cases, the G614 pseudovirus was moderately more susceptible to neutralization. The G614 pseudovirus also was more susceptible to neutralization by monoclonal antibodies against the receptor binding domain and by convalescent sera from people known to be infected with either the D614 or G614 form of the virus. These results indicate that a gain in infectivity provided by D614G came at the cost of making the virus more vulnerable to neutralizing antibodies, and that the mutation is not expected to be an obstacle for current vaccine development.

Spike mutation pipeline reveals the emergence of a more transmissible form of SARS-CoV-2 (preprint)

We have developed an analysis pipeline to facilitate real-time mutation tracking in SARS-CoV-2, focusing initially on the Spike (S) protein because it mediates infection of human cells and is the target of most vaccine strategies and antibody-based therapeutics. To date we have identified thirteen mutations in Spike that are accumulating. Mutations are considered in a broader phylogenetic context, geographically, and over time, to provide an early warning system to reveal mutations that may confer selective advantages in transmission or resistance to interventions. Each one is evaluated for evidence of positive selection, and the implications of the mutation are explored through structural modeling. The mutation Spike D614G is of urgent concern; it began spreading in Europe in early February, and when introduced to new regions it rapidly becomes the dominant form. Also, we present evidence of recombination between locally circulating strains, indicative of multiple strain infections. These finding have important implications for SARS-CoV-2 transmission, pathogenesis and immune interventions.

Emergence of SARS-CoV-2 through Recombination and Strong Purifying Selection (preprint)

COVID-19 has become a global pandemic caused by a novel coronavirus SARS-CoV-2. Understanding the origins of SARS-CoV-2 is critical for deterring future zoonosis and for drug discovery and vaccine development. We show evidence of strong purifying selection around the receptor binding motif (RBM) in the spike gene and in other genes among bat, pangolin and human coronaviruses, indicating similar strong evolutionary constraints in different host species. We also demonstrate that SARS-CoV-2s entire RBM was introduced through recombination with coronaviruses from pangolins, possibly a critical step in the evolution of SARS-CoV-2s ability to infect humans. Similar purifying selection in different host species and frequent recombination among coronaviruses suggest a common evolutionary mechanism that could lead to new emerging human coronaviruses. One Sentence SummaryExtensive Recombination and Strong Purifying Selection among coronaviruses from different hosts facilitate the emergence of SARS-CoV-2