Lasting first impression: Pre-existing immunity restricts mucosal antibody responses during Omicron breakthrough (preprint)

Understanding mucosal antibody responses from SARS-CoV-2 infection and/or vaccination is crucial to develop strategies for longer term immunity, especially against emerging viral variants. We profiled serial paired mucosal and plasma antibodies from: COVID-19 vaccinated only vaccinees (vaccinated, uninfected), COVID-19 recovered vaccinees (convalescent, vaccinated) and individuals with breakthrough Delta or Omicron BA.2 infections (vaccinated, infected). Saliva from COVID-19 recovered vaccinees displayed improved antibody neutralizing activity, Fc{gamma}R engagement and IgA compared to COVID-19 uninfected vaccinees. Furthermore, repeated mRNA vaccination boosted SARS-CoV-2-specific IgG2 and IgG4 responses in both mucosa biofluids (saliva and tears) and plasma. IgG, but not IgA, responses to breakthrough COVID-19 variants were dampened and narrowed by increased pre-existing vaccine-induced immunity to the ancestral strain. Salivary antibodies delayed initiation of boosting following breakthrough COVID-19 infection, especially Omicron BA.2, however, rose rapidly thereafter. Our data highlight how pre-existing immunity shapes mucosal SARS-CoV-2-specific antibody responses and has implications for long-term protection from COVID-19.

Monoclonal antibodies against human coronavirus NL63 spike (preprint)

The COVID-19 pandemic has illustrated the potential for monoclonal antibody therapeutics as prophylactic and therapeutic agents against pandemic viruses. No such therapeutics currently exist for other human coronaviruses. NL63 is a human alphacoronavirus that typically causes the common cold and uses the same receptor, ACE2, as the highly pathogenic SARS-CoV and SARS-CoV-2 pandemic viruses. In a cohort of healthy adults, we characterised humoral responses against the NL63 spike protein. While NL63 spike and receptor binding domain-specific binding antibodies and neutralisation activity could be detected in plasma of all subjects, memory B cells against NL63 spike were variable and relatively low in frequency compared to that against SARS-CoV-2 spike. From these donors, we isolated a panel of antibodies against NL63 spike and characterised their neutralising potential. We identified potent neutralising antibodies that recognised the receptor binding domain (RBD) and other non-RBD epitopes within spike.

Durable reprogramming of neutralising antibody responses following breakthrough Omicron infection (preprint)

SARS-CoV-2 breakthrough infection of vaccinated individuals is increasingly common with the circulation of highly immune evasive and transmissible Omicron variants. Here, we report the dynamics and durability of recalled spike-specific humoral immunity following BA.1 or BA.2 breakthrough infection, with longitudinal sampling up to 8 months post-infection. Both BA.1 and BA.2 infection robustly boosted neutralisation activity against the infecting strain while expanding breadth against other Omicron strains. Cross-reactive memory B cells against both ancestral and Omicron spike were predominantly expanded by infection, with limited recruitment of de novo Omicron-specific B cells or antibodies. Modelling of neutralisation titres predicts that protection from symptomatic reinfection against antigenically similar strains will be remarkably durable, but is undermined by novel emerging strains with further neutralisation escape.

Rapid recall and de novo T cell responses during SARS-CoV-2 breakthrough infection (preprint)

While the protective role of neutralising antibodies against COVID-19 is well-established, questions remain about the relative importance of cellular immunity. Using 6 pMHC-multimers in a cohort with early and frequent sampling we define the phenotype and kinetics of recalled and primary T cell responses following Delta or Omicron breakthrough infection. Recall of spike-specific CD4+ T cells was rapid, with cellular proliferation and extensive activation evident as early as 1 day post-symptom onset. Similarly, spike-specific CD8+ T cells were rapidly activated but showed variable levels of expansion. Strikingly, high levels of SARS-CoV-2-specific CD8+ T cell activation at baseline and peak were strongly correlated with reduced peak SARS-CoV-2 RNA levels in nasal swabs and accelerated clearance of virus. Our study demonstrates rapid and extensive recall of memory T cell populations occurs early after breakthrough infection and suggests that CD8+ T cells contribute to the control of viral replication in breakthrough SARS-CoV-2 infections.

Broad immunity to SARS-CoV-2 variants of concern mediated by a SARS-CoV-2 receptor-binding domain protein vaccine (preprint)

The SARS-CoV-2 global pandemic has fuelled the generation of vaccines at an unprecedented pace and scale. However, many challenges remain, including: the emergence of vaccine-resistant mutant viruses, vaccine stability during storage and transport, waning vaccine-induced immunity, and concerns about infrequent adverse events associated with existing vaccines. Here, we report on a protein subunit vaccine comprising the receptor-binding domain (RBD) of the ancestral SARS-CoV-2 spike protein, dimerised with an immunoglobulin IgG1 Fc domain. These were tested in conjunction with three different adjuvants: a TLR2 agonist R4-Pam2Cys, an NKT cell agonist glycolipid alpha-Galactosylceramide, or MF59 squalene oil-in-water adjuvant. Each formulation drove strong neutralising antibody (nAb) responses and provided durable and highly protective immunity against lower and upper airway infection in mouse models of COVID-19. We have also developed an RBD-human IgG1 Fc vaccine with an RBD sequence of the highly immunoevasive beta variant (N501Y, E484K, K417N). This beta variant RBD vaccine, combined with MF59 adjuvant, induced strong protection in mice against the beta strain as well as the ancestral strain. Furthermore, when used as a third dose booster vaccine following priming with whole spike vaccine, anti-sera from beta-RBD-Fc immunised mice increased titres of nAb against other variants including alpha, delta, delta+, gamma, lambda, mu, and omicron BA.1 and BA.2. These results demonstrated that an RBD-Fc protein subunit/MF59 adjuvanted vaccine can induce high levels of broad nAbs, including when used as a booster following prior immunisation of mice with whole ancestral-strain Spike vaccines. This vaccine platform offers a potential approach to augment some of the currently approved vaccines in the face of emerging variants of concern, and it has now entered a phase I clinical trial.

Altered affinity to ACE2 and reduced Fc functional antibodies to SARS-CoV-2 RBD variants (preprint)

The emergence of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) variants remains a formidable challenge to worldwide public health. The receptor binding domain (RBD) of the SARS-CoV-2 spike protein is a hotspot for mutations, reflecting its critical role at the ACE2 interface during viral entry. We comprehensively investigated the impact of RBD mutations, including 6 variants of concern (VOC) or interest (Alpha, Beta, Gamma, Delta, Kappa and Omicron) and 33 common point mutations, on IgG recognition, Fc{gamma}R-engagement, and ACE2-binding inhibition in plasma from BNT162b2-vaccine recipients (two-weeks following second dose) and mild-to-moderate COVID-19 convalescent subjects using our custom bead-based 39-plex array. We observed that IgG-recognition and Fc{gamma}R-binding antibodies were most profoundly decreased against Beta and Omicron RBDs, as well as point mutations G446S, found in Omicron, and N501T, a key mutation found in animal adapted SARS-CoV-2 viruses. Measurement of RBD-ACE2 binding affinity via Biolayer Interferometry showed all VOC RBDs have enhanced affinity to human ACE2. Furthermore we demonstrate that human ACE2 polymorphisms, E35K (rs1348114695), K26R (rs4646116) and S19P (rs73635825), have altered binding kinetics to the RBD of VOCs potentially affecting virus-host interaction and thereby host susceptibility.

At least three doses of leading vaccines essential for neutralization of SARS-CoV-2 Omicron variant (preprint)

Plasma samples taken at different time points from donors who received either AstraZeneca (Vaxzevria) or Pfizer (Comirnaty) or Moderna (Spikevax) coronavirus disease-19 (COVID-19) vaccine were assessed in virus neutralization assays against Delta and Omicron variants of concern and a reference isolate (VIC31). With the Pfizer vaccine there was 6-8 fold reduction in 50% neutralizing antibody titres (NT50) against Delta and VIC31 at 6 months compared to 2 weeks after the second dose; followed by 25-fold increase at 2 weeks after the third dose. Neutralisation of Omicron was only consistently observed 2 weeks after the third dose, with most samples having titres below the limit of detection at earlier timepoints. Moderna results were similar to Pfizer at 2 weeks after the second dose, while the titres for AstraZeneca samples derived from older donors were 7-fold lower against VIC31 and below the limit of detection against Delta and Omicron. Age and gender were not found to significantly impact our results. These observations indicate that vaccine matching may be needed, and that at least a third dose of these vaccines is necessary to generate sufficient neutralising antibodies against emerging variants of concern, especially Omicron, amidst the challenges of ensuring vaccine equity worldwide.

Heterologous SARS-CoV-2 IgA neutralising antibody responses in convalescent plasma (preprint)

Following infection with SARS-CoV-2, virus-specific antibodies are generated which can both neutralise virions and clear infection via Fc effector functions. The importance of IgG antibodies for protection and control of SARS-CoV-2 has been extensively reported. In comparison, other antibody isotypes including IgA have been poorly characterized. Here we characterized plasma IgA from 41 early convalescent COVID-19 subjects for neutralisation and Fc effector functions. We find that convalescent plasma IgA from >60% of the cohort have the capacity to inhibit the interaction between wild-type RBD and ACE2. Furthermore, a third of the cohort induced stronger IgA-mediated inhibition of RBD binding to ACE2 than IgG, when tested at equivalent concentrations. Plasma IgA and IgG from the cohort, broadly recognize similar RBD epitopes and showed similar ability to inhibit ACE2 from binding 22 of 23 different prevalent RBD proteins with single amino acid mutations. Plasma IgA was largely incapable of mediating antibody-dependent phagocytosis in comparison to plasma IgG. Overall, convalescent plasma IgA contributes to neutralisation towards wild-type RBD and various RBD single mutants in most subjects, although this response is heterogeneous and less potent than IgG.

Dynamics of immune recall following SARS-CoV-2 vaccination or breakthrough infection (preprint)

Vaccination against SARS-CoV-2 results in protection from acquisition of infection as well as improved clinical outcomes even if infection occurs, likely reflecting a combination of residual vaccine-elicited immunity and the recall of immunological memory. Here, we define the early kinetics of spike-specific humoral and T cell immunity after vaccination of seropositive individuals, and after breakthrough infection in vaccinated individuals. Intensive and early longitudinal sampling reveals the timing and magnitude of recall, with the phenotypic activation of B cells preceding an increase in neutralizing antibody titres. In breakthrough infections, the delayed kinetics of humoral immune recall provides a mechanism for the lack of early control of viral replication but likely underpins accelerated viral clearance and the protective effects of vaccination against severe COVID-19.

Integrated immune networks in SARS-CoV-2 infected pregnant women reveal differential NK cell and unconventional T cell activation (preprint)

Although pregnancy poses a greater risk for severe COVID-19, the underlying immunological changes associated with SARS-CoV-2 infection during pregnancy are poorly understood. We defined immune responses to SARS-CoV-2 in pregnant and non-pregnant women during acute and convalescent COVID-19 up to 258 days post symptom onset, quantifying 217 immunological parameters. Additionally, matched maternal and cord blood were collected from COVID-19 convalescent pregnancies. Although serological responses to SARS-CoV-2 were similar in pregnant and non-pregnant women, cellular immune analyses revealed marked differences in key NK cell and unconventional T cell responses during COVID-19 in pregnant women. While NK cells, {gamma}{delta} T cells and MAIT cells displayed pre-activated phenotypes in healthy pregnant women when compared to non-pregnant age-matched women, activation profiles of these pre-activated NK and unconventional T cells remained unchanged at acute and convalescent COVID-19 in pregnancy. Conversely, activation dynamics of NK and unconventional T cells were prototypical in non-pregnant women in COVID-19. In contrast, activation of {beta} CD4+ and CD8+ T cells, T follicular helper cells and antibody-secreting cells was similar in pregnant and non-pregnant women with COVID-19. Elevated levels of IL-1{beta}, IFN-{gamma}, IL-8, IL-18 and IL-33 were also found in pregnant women in their healthy state, and these cytokine levels remained elevated during acute and convalescent COVID-19. Collectively, our study provides the first comprehensive map of longitudinal immunological responses to SARS-CoV-2 infection in pregnant women, providing insights into patient management and education during COVID-19 pregnancy.

What level of neutralising antibody protects from COVID-19? . (preprint)

Both previous infection and vaccination have been shown to provide potent protection from COVID-19. However, there are concerns that waning immunity and viral variation may lead to a loss of protection over time. Predictive models of immune protection are urgently needed to identify immune correlates of protection to assist in the future deployment of vaccines. To address this, we modelled the relationship between in vitro neutralisation levels and observed protection from SARS-CoV-2 infection using data from seven current vaccines as well as convalescent cohorts. Here we show that neutralisation level is highly predictive of immune protection. The 50% protective neutralisation level was estimated to be approximately 20% of the average convalescent level (95% CI = 14-28%). The estimated neutralisation level required for 50% protection from severe infection was significantly lower (3% of the mean convalescent level (CI = 0.7-13%, p = 0.0004). Given the relationship between in vitro neutralization titer and protection, we then used this to investigate how waning immunity and antigenic variation might affect vaccine efficacy. We found that the decay of neutralising titre in vaccinated subjects over the first 3-4 months after vaccination was at least as rapid as the decay observed in convalescent subjects. Modelling the decay of neutralisation titre over the first 250 days after immunisation predicts a significant loss in protection from SARS-CoV-2 infection will occur, although protection from severe disease should be largely retained. Neutralisation titres against some SARS-CoV-2 variants of concern are reduced compared to the vaccine strain and our model predicts the relationship between neutralisation and efficacy against viral variants. Our analyses provide an evidence-based prediction of SARS-CoV-2 immune protection that will assist in developing vaccine strategies to control the future trajectory of the pandemic.

Adaptive immunity to human coronaviruses is widespread but low in magnitude (preprint)

Endemic human coronaviruses (hCoV) circulate worldwide but cause minimal mortality. Although seroconversion to hCoV is near ubiquitous during childhood, little is known about hCoV-specific T cell memory in adults. We quantified CD4 T cell and antibody responses to hCoV spike antigens in 42 SARS-CoV-2 uninfected individuals. T cell responses were widespread within conventional memory and cTFH compartments but did not correlate with IgG titres. SARS-CoV-2 cross-reactive T cells were observed in 48% of participants and correlated with HKU1 memory. hCoV-specific T cells exhibited a CCR6+ central memory phenotype in the blood, but were enriched for frequency and CXCR3 expression in human lung draining lymph nodes. Overall, hCoV-specific humoral and cellular memory are independently maintained, with a shared phenotype existing among coronavirus-specific CD4 T cells. This understanding of endemic coronavirus immunity provides insight into the homeostatic maintenance of immune responses that are likely to be critical components of protection against SARS-CoV-2.

Evolution of immunity to SARS-CoV-2 (preprint)

The durability of infection-induced SARS-CoV-2 immunity has major implications for public health mitigation and vaccine development. Animal studies and the scarcity of confirmed re-infection suggests immune protection is likely, although the durability of this protection is debated. Lasting immunity following acute viral infection requires maintenance of both serum antibody and antigen-specific memory B and T lymphocytes and is notoriously pathogen specific, ranging from life-long for smallpox or measles4, to highly transient for common cold coronaviruses (CCC). Neutralising antibody responses are a likely correlate of protective immunity and exclusively recognise the viral spike (S) protein, predominantly targeting the receptor binding domain (RBD) within the S1 sub-domain. Multiple reports describe waning of S-specific antibodies in the first 2-3 months following infection. However, extrapolation of early linear trends in decay might be overly pessimistic, with several groups reporting that serum neutralisation is stable over time in a proportion of convalescent subjects. While SARS-CoV-2 specific B and T cell responses are readily induced by infection, the longitudinal dynamics of these key memory populations remains poorly resolved. Here we comprehensively profiled antibody, B and T cell dynamics over time in a cohort recovered from mild-moderate COVID-19. We find that binding and neutralising antibody responses, together with individual serum clonotypes, decay over the first 4 months post-infection, as expected, with a similar decline in S-specific CD4+ and circulating T follicular helper (cTFH) frequencies. In contrast, S-specific IgG+ memory B cells (MBC) consistently accumulate over time, eventually comprising a significant fraction of circulating MBC. Modelling of the concomitant immune kinetics predicts maintenance of serological neutralising activity above a titre of 1:40 in 50% of convalescent subjects to 74 days, with probable additive protection from B and T cells. Overall, our study suggests SARS-CoV-2 immunity after infection is likely t 66 o be transiently protective at a population level. SARS-CoV-2 vaccines may require greater immunogenicity and durability than natural infection to drive long-term protection.

Prime-boost protein subunit vaccines against SARS-CoV-2 are highly immunogenic in mice and macaques (preprint)

SARS-CoV-2 vaccines are advancing into human clinical trials, with emphasis on eliciting high titres of neutralising antibodies against the viral spike (S). However, the merits of broadly targeting S versus focusing antibody onto the smaller receptor binding domain (RBD) are unclear. Here we assessed prototypic S and RBD subunit vaccines in homologous or heterologous prime-boost regimens in mice and non-human primates. We find S is highly immunogenic in mice, while the comparatively poor immunogenicity of RBD was associated with limiting germinal centre and T follicular helper cell activity. Boosting S-primed mice with either S or RBD significantly augmented neutralising titres, with RBD-focussing driving moderate improvement in serum neutralisation. In contrast, both S and RBD vaccines were comparably immunogenic in macaques, eliciting serological neutralising activity that generally exceed levels in convalescent humans. These studies confirm recombinant S proteins as promising vaccine candidates and highlight multiple pathways to achieving potent serological neutralisation.

SARS-CoV-2 ORF9c Is a Membrane-Associated Protein that Suppresses Antiviral Responses in Cells (preprint)

Disrupted antiviral immune responses are associated with severe COVID-19, the disease caused by SAR-CoV-2. Here, we show that the 73-amino-acid protein encoded by ORF9c of the viral genome contains a putative transmembrane domain, interacts with membrane proteins in multiple cellular compartments, and impairs antiviral processes in a lung epithelial cell line. Proteomic, interactome, and transcriptomic analyses, combined with bioinformatic analysis, revealed that expression of only this highly unstable small viral protein impaired interferon signaling, antigen presentation, and complement signaling, while inducing IL-6 signaling. Furthermore, we showed that interfering with ORF9c degradation by either proteasome inhibition or inhibition of the ATPase VCP blunted the effects of ORF9c. Our study indicated that ORF9c enables immune evasion and coordinates cellular changes essential for the SARS-CoV-2 life cycle. One-sentence summarySARS-CoV-2 ORF9c is the first human coronavirus protein localized to membrane, suppressing antiviral response, resembling full viral infection.

Identification of potential key genes for SARS-CoV-2 infected human bronchial organoids based on bioinformatics analysis (preprint)

There is an urgent need to understand the pathogenesis of the severe acute respiratory syndrome coronavirus clade 2 (SARS-CoV-2) that leads to COVID-19 and respiratory failure. Our study is to discover differentially expressed genes (DEGs) and biological signaling pathways by using a bioinformatics approach to elucidate their potential pathogenesis. The gene expression profiles of the GSE150819 datasets were originally produced using an Illumina NextSeq 500 (Homo sapiens). KEGG (Kyoto Encyclopedia of Genes and Genomes) and GO (Gene Ontology) were utilized to identify functional categories and significant pathways. KEGG and GO results suggested that the Cytokine-cytokine receptor interaction, P53 signaling pathway, and Apoptosis are the main signaling pathways in SARS-CoV-2 infected human bronchial organoids (hBOs). Furthermore, NFKBIA, C3, and CCL20 may be key genes in SARS-CoV-2 infected hBOs. Therefore, our study provides further insights into the therapy of COVID-19.

Suboptimal SARS-CoV-2-specific CD8+ T-cell response associated with the prominent HLA-A*02:01 phenotype (preprint)

An improved understanding of human T-cell-mediated immunity in COVID-19 is important if we are to optimize therapeutic and vaccine strategies. Experience with influenza shows that infection primes CD8+ T-cell memory to shared peptides presented by common HLA types like HLA-A2. Following re-infection, cross-reactive CD8+ T-cells enhance recovery and diminish clinical severity. Stimulating peripheral blood mononuclear cells from COVID-19 convalescent patients with overlapping peptides from SARS-CoV-2 Spike, Nucleocapsid and Membrane proteins led to the clonal expansion of SARS-CoV-2-specific CD8+ and CD4+ T-cells in vitro, with CD4+ sets being typically robust. For CD8+ T-cells taken directly ex vivo, we identified two HLA-A*02:01-restricted SARS-CoV-2 epitopes, A2/S269-277 and A2/Orf1ab3183-3191. Using peptide-HLA tetramer enrichment, direct ex vivo assessment of the A2/S269+CD8+ and A2/Orf1ab3183+CD8+ populations indicated that the more prominent A2/S269+CD8+ set was detected at comparable frequency ([~]1.3x10-5) in acute and convalescent HLA-A*02:01+ patients. But, while the numbers were higher than those found in uninfected HLA-A*02:01+ donors ([~]2.5x10-6), they were low when compared with frequencies for influenza-specific (A2/M158) and EBV-specific (A2/BMLF1280) ([~]1.38x10-4) populations. Phenotypic analysis ex vivo of A2/S269+CD8+ T-cells from COVID-19 convalescents showed that A2/S269+CD8+ T-cells were predominantly negative for the CD38, HLA-DR, PD-1 and CD71 activation markers, although the majority of total CD8+ T-cells were granzyme and/or perforin-positive. Furthermore, the bias towards naive, stem cell memory and central memory A2/S269+CD8+ T-cells rather than effector memory populations suggests that SARS-CoV2 infection may be compromising CD8+ T-cell activation. Priming with an appropriate vaccine may thus have great value for optimizing protective CD8+ T-cell immunity in COVID-19.