Original antigenic sin responses to Betacoronavirus spike proteins are observed in a mouse model, but are not apparent in children following SARS-CoV-2 infection

Describing Elephants: An Update on the Immunopathology of Multisystem Inflammatory Syndrome in Children

First described in 2020, multi-system inflammatory syndrome in children (MIS-C) is an, initially life-threatening, disease characterised by severe inflammation and following exposure to SARS-CoV-2. The immunopathology of MIS-C involves a hyperinflammation characterised by a cytokine storm and activation of both the innate and adaptive immune system, eventually leading to multi-organ failure. Several etiological theories are described in literature. Firstly, it is suggested that the gut plays an important role in the translocation of microbial products to the systemic circulation. Additionally, the production of autoantibodies that develop after the initial infection with SARS-CoV-2 might lead to many of its broad clinical symptoms. Finally, the superantigen theory where non-specific binding of the SARS-CoV-2 spike glycoprotein to the T-cell receptor leads to a subsequent activation of T cells, generating a powerful immune response. Despite the sudden outbreak of MIS-C and alarming messages, as of 2024, cases have declined drastically and subsequently show a less severe clinical spectrum. However, subacute cases not meeting current diagnostic criteria might be overlooked even though they represent a valuable research population. In the future, research should focus on adjusting these criteria to better understand the broad pathophysiology of MIS-C, aiding early detection, therapy, and prediction.

Challenges and Prospects in Developing Future SARS-CoV-2 Vaccines: Overcoming Original Antigenic Sin and Inducing Broadly Neutralizing Antibodies

The impacts of the COVID-19 pandemic led to the development of several effective SARS-CoV-2 vaccines. However, waning vaccine efficacy as well as the antigenic drift of SARS-CoV-2 variants has diminished vaccine efficacy against SARS-CoV-2 infection and may threaten public health. Increasing interest has been given to the development of a next generation of SARS-CoV-2 vaccines with increased breadth and effectiveness against SARS-CoV-2 infection. In this Brief Review, we discuss recent work on the development of these next-generation vaccines and on the nature of the immune response to SARS-CoV-2. We examine recent work to develop pan-coronavirus vaccines as well as to develop mucosal vaccines. We further discuss challenges associated with the development of novel vaccines including the need to overcome "original antigenic sin" and highlight areas requiring further investigation. We place this work in the context of SARS-CoV-2 evolution to inform how the implementation of future vaccine platforms may impact human health.

Pre-existing Immunity to Endemic Human Coronaviruses Does Not Affect the Immune Response to SARS-CoV-2 Spike in a Murine Vaccination Model

Endemic human coronaviruses (HCoVs) have been evidenced to be cross-reactive to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Although a correlation exists between the immunological memory to HCoVs and coronavirus disease 2019 (COVID-19) severity, there is little experimental evidence for the effects of HCoV memory on the efficacy of COVID-19 vaccines. Here, we investigated the Ag-specific immune response to COVID-19 vaccines in the presence or absence of immunological memory against HCoV spike Ags in a mouse model. Pre-existing immunity against HCoV did not affect the COVID-19 vaccine-mediated humoral response with regard to Ag-specific total IgG and neutralizing Ab levels. The specific T cell response to the COVID-19 vaccine Ag was also unaltered, regardless of pre-exposure to HCoV spike Ags. Taken together, our data suggest that COVID-19 vaccines elicit comparable immunity regardless of immunological memory to spike of endemic HCoVs in a mouse model.

Serum Fc-Mediated Monocyte Phagocytosis Activity Is Stable for Several Months after SARS-CoV-2 Asymptomatic and Mildly Symptomatic Infection

We investigated the temporal profile of multiple components of the serological response after asymptomatic or mildly symptomatic SARS-CoV-2 infection, in a cohort of 67 previously SARS-CoV-2 naive young adults, up to 8.5 months after infection. We found a significant decrease of spike IgG and neutralization antibody titers from early (11 to 56 days) to late (4 to 8.5 months) time points postinfection. Over the study period, S1-specific IgG levels declined significantly faster than that of the S2-specific IgG. Further, serum antibodies from PCR-confirmed participants cross-recognized S2, but not S1, of the betacoronaviruses HKU1 and OC43, suggesting a greater degree of cross-reactivity of S2 among betacoronaviruses. Antibody-Dependent Natural Killer cell Activation (ADNKA) was detected at the early time point but significantly decreased at the late time point. Induction of serum Antibody-Dependent Monocyte Phagocytosis (ADMP) was detected in all the infected participants, and its levels remained stable over time. Additionally, a reduced percentage of participants had detectable neutralizing activity against the Beta (50%), Gamma (61 to 67%), and Delta (90 to 94%) variants, both early and late postinfection, compared to the ancestral strain (100%). Antibody binding to S1 and RBD of Beta, Gamma, Delta (1.7 to 2.3-fold decrease), and Omicron (10 to 16-fold decrease) variants was also significantly reduced compared to the ancestral SARS-CoV-2 strain. Overall, we found variable temporal profiles of specific components and functionality of the serological response to SARS-CoV-2 in young adults, which is characterized by lasting, but decreased, neutralizing activity and antibody binding to S1, stable ADMP activity, and relatively stable S2-specific IgG levels. IMPORTANCE Adaptive immunity mediated by antibodies is important for controlling SARS-CoV-2 infection. While vaccines against COVID-19 are currently widely distributed, a high proportion of the global population is still unvaccinated. Therefore, understanding the dynamics and maintenance of the naive humoral immune response to SARS-CoV-2 is of great importance. In addition, long-term responses after asymptomatic infection are not well-characterized, given the challenges in identifying such cases. Here, we investigated the longitudinal humoral profile in a well-characterized cohort of young adults with documented asymptomatic or mildly symptomatic SARS-CoV-2 infection. By analyzing samples collected preinfection, early after infection and during late convalescence, we found that, while neutralizing activity decreased over time, high levels of serum S2 IgG and Antibody-Dependent Monocyte Phagocytosis (ADMP) activity were maintained up to 8.5 months after infection. This suggests that a subset of antibodies with specific functions could contribute to long-term protection against SARS-CoV-2 in convalescent unvaccinated individuals.

Updated vaccine protects against SARS-CoV-2 variants including Omicron (B.1.1.529) and prevents transmission in hamsters

Current COVID-19 vaccines are based on prototypic spike sequences from ancestral 2019 SARS-CoV-2 strains. However, the ongoing pandemic is fueled by variants of concern (VOC) escaping vaccine-mediated protection. Here we demonstrate how immunization in hamsters using prototypic spike expressed from yellow fever 17D (YF17D) as vector blocks ancestral virus (B lineage) and VOC Alpha (B.1.1.7) yet fails to fully protect from Beta (B.1.351). However, the same YF17D vectored vaccine candidate with an evolved antigen induced considerably improved neutralizing antibody responses against VOCs Beta, Gamma (P.1) and the recently predominant Omicron (B.1.1.529), while maintaining immunogenicity against ancestral virus and VOC Delta (B.1.617.2). Thus vaccinated animals resisted challenge by all VOCs, including vigorous high titre exposure to the most difficult to cover Beta, Delta and Omicron variants, eliminating detectable virus and markedly improving lung pathology. Finally, vaccinated hamsters did not transmit Delta variant to non-vaccinated cage mates. Overall, our data illustrate how current first-generation COVID-19 vaccines may need to be updated to maintain efficacy against emerging VOCs and their spread at community level.

Original antigen sin and COVID-19: implications for seasonal vaccination

INTRODUCTION

Original antigenic sin describes the phenomenon in which immunity against pathogens or antigens is shaped by the host's first exposure to a related pathogen or antigen.

AREAS COVERED

When primary immunity is boosted not by the homologous but by a cross-reacting vaccine, the newly formed antibodies may react better with the primary antigen than with the antigen actually eliciting the response. This form of immune imprinting, which has been observed with influenza, dengue, human immunodeficiency virus, and other pathogens, has profound implications for the approach to seasonal vaccination against a variety of diseases, including COVID-19.

EXPERT OPINION

Public health agencies and regulatory bodies have consistently recommended repeated vaccination every few months as a way to protect against COVID-19. However, the risks and benefits of this approach requires scrutiny given the concern for original antigenic sin in response to SARS-CoV-2. This manuscript examines what is known about immune imprinting and looks ahead to explore how this phenomenon may impact seasonal vaccination against emerging SARS-CoV-2 subvariants such as BA.4, BA.5, and BA.5.1, which have been associated increased transmissibility due to enhanced immune escape.

Antibody affinity and cross-variant neutralization of SARS-CoV-2 Omicron BA.1, BA.2 and BA.3 following third mRNA vaccination

There is limited knowledge on durability of neutralization capacity and antibody affinity maturation generated following two versus three doses of SARS-CoV-2 mRNA vaccines in naïve versus convalescent individuals (hybrid immunity) against the highly transmissible Omicron BA.1, BA.2 and BA.3 subvariants. Virus neutralization titers against the vaccine-homologous strain (WA1) and Omicron sublineages are measured in a pseudovirus neutralization assay (PsVNA). In addition, antibody binding and antibody affinity against spike proteins from WA1, BA.1, and BA.2 is determined using surface plasmon resonance (SPR). The convalescent individuals who after SARS-CoV-2 infection got vaccinated develop hybrid immunity that shows broader neutralization activity and cross-reactive antibody affinity maturation against the Omicron BA.1 and BA.2 after either second or third vaccination compared with naïve individuals. Neutralization activity correlates with antibody affinity against Omicron subvariants BA.1 and BA.2 spikes. Importantly, at four months post-third vaccination the neutralization activity and antibody affinity against the Omicron subvariants is maintained and trended higher for the individuals with hybrid immunity compared with naïve adults. These findings about hybrid immunity resulting in superior immune kinetics, breadth, and durable high affinity antibodies support the need for booster vaccinations to provide effective protection from emerging SARS-CoV-2 variants like the rapidly spreading Omicron subvariants.

Here the authors show that a third SARS-CoV-2 vaccination significantly boosts neutralizing antibodies against Omicron subvariants and that hybrid immunity (infection and vaccination) results in broader neutralization activity and cross-reactive antibody affinity maturation.

SARS-CoV-2 S2-targeted vaccination elicits broadly neutralizing antibodies

Several variants of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) have emerged during the current coronavirus disease 2019 (COVID-19) pandemic. Although antibody cross-reactivity with the spike glycoproteins (S) of diverse coronaviruses, including endemic common cold coronaviruses (HCoVs), has been documented, it remains unclear whether such antibody responses, typically targeting the conserved S2 subunit, contribute to protection when induced by infection or through vaccination. Using a mouse model, we found that prior HCoV-OC43 S-targeted immunity primes neutralizing antibody responses to otherwise subimmunogenic SARS-CoV-2 S exposure and promotes S2-targeting antibody responses. Moreover, vaccination with SARS-CoV-2 S2 elicited antibodies in mice that neutralized diverse animal and human alphacoronaviruses and betacoronaviruses in vitro and provided a degree of protection against SARS-CoV-2 challenge in vivo. Last, in mice with a history of SARS-CoV-2 Wuhan-based S vaccination, further S2 vaccination induced broader neutralizing antibody response than booster Wuhan S vaccination, suggesting that it may prevent repertoire focusing caused by repeated homologous vaccination. These data establish the protective value of an S2-targeting vaccine and support the notion that S2 vaccination may better prepare the immune system to respond to the changing nature of the S1 subunit in SARS-CoV-2 variants of concern, as well as to future coronavirus zoonoses.

Fatal COVID-19 outcomes are associated with an antibody response targeting epitopes shared with endemic coronaviruses

The role of immune responses to previously seen endemic coronavirus epitopes in severe acute respiratory coronavirus 2 (SARS-CoV-2) infection and disease progression has not yet been determined. Here, we show that a key characteristic of fatal outcomes with coronavirus disease 2019 (COVID-19) is that the immune response to the SARS-CoV-2 spike protein is enriched for antibodies directed against epitopes shared with endemic beta-coronaviruses and has a lower proportion of antibodies targeting the more protective variable regions of the spike. The magnitude of antibody responses to the SARS-CoV-2 full-length spike protein, its domains and subunits, and the SARS-CoV-2 nucleocapsid also correlated strongly with responses to the endemic beta-coronavirus spike proteins in individuals admitted to an intensive care unit (ICU) with fatal COVID-19 outcomes, but not in individuals with nonfatal outcomes. This correlation was found to be due to the antibody response directed at the S2 subunit of the SARS-CoV-2 spike protein, which has the highest degree of conservation between the beta-coronavirus spike proteins. Intriguingly, antibody responses to the less cross-reactive SARS-CoV-2 nucleocapsid were not significantly different in individuals who were admitted to an ICU with fatal and nonfatal outcomes, suggesting an antibody profile in individuals with fatal outcomes consistent with an "original antigenic sin" type response.

Seasonal coronaviruses and SARS-CoV-2: effects of preexisting immunity during the COVID-19 pandemic

Although the coronavirus disease 2019 (COVID-19) epidemic is still ongoing, vaccination rates are rising slowly and related treatments and drugs are being developed. At the same time, there is increasing evidence of preexisting immunity against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in humans, mainly consisting of preexisting antibodies and immune cells (including T cells and B cells). The presence of these antibodies is mainly due to the seasonal prevalence of four common coronavirus types, especially OC43 and HKU1. The accumulated relevant evidence has suggested that the target of antibodies is mainly the S2 subunit of S protein, followed by evolutionary conservative regions such as the nucleocapsid (N) protein. Additionally, preexisting memory T and B cells are also present in the population. Preexisting antibodies can help the body protect against SARS-CoV-2 infection, reduce the severity of COVID-19, and rapidly increase the immune response post-infection. These multiple effects can directly affect disease progression and even the likelihood of death in certain individuals. Besides the positive effects, preexisting immunity may also have negative consequences, such as antibody-dependent enhancement (ADE) and original antigenic sin (OAS), the prevalence of which needs to be further established. In the future, more research should be focused on evaluating the role of preexisting immunity in COVID-19 outcomes, adopting appropriate policies and strategies for fighting the pandemic, and vaccine development that considers preexisting immunity.

Cross-reactive immunity against the SARS-CoV-2 Omicron variant is low in pediatric patients with prior COVID-19 or MIS-C

Neutralization capacity of antibodies against Omicron after a prior SARS-CoV-2 infection in children and adolescents is not well studied. Therefore, we evaluated virus-neutralizing capacity against SARS-CoV-2 Alpha, Beta, Gamma, Delta and Omicron variants by age-stratified analyses (<5, 5-11, 12-21 years) in 177 pediatric patients hospitalized with severe acute COVID-19, acute MIS-C, and in convalescent samples of outpatients with mild COVID-19 during 2020 and early 2021. Across all patients, less than 10% show neutralizing antibody titers against Omicron. Children <5 years of age hospitalized with severe acute COVID-19 have lower neutralizing antibodies to SARS-CoV-2 variants compared with patients >5 years of age. As expected, convalescent pediatric COVID-19 and MIS-C cohorts demonstrate higher neutralization titers than hospitalized acute COVID-19 patients. Overall, children and adolescents show some loss of cross-neutralization against all variants, with the most pronounced loss against Omicron. In contrast to SARS-CoV-2 infection, children vaccinated twice demonstrated higher titers against Alpha, Beta, Gamma, Delta and Omicron. These findings can influence transmission, re-infection and the clinical disease outcome from emerging SARS-CoV-2 variants and supports the need for vaccination in children.

Rapid Generation of In-House Serological Assays Is Comparable to Commercial Kits Critical for Early Response to Pandemics: A Case With SARS-CoV-2

Introduction

Accurate and sensitive measurement of antibodies is critical to assess the prevalence of infection, especially asymptomatic infection, and to analyze the immune response to vaccination during outbreaks and pandemics. A broad variety of commercial and in-house serological assays are available to cater to different laboratory requirements; however direct comparison is necessary to understand utility.

Materials and Methods

We investigate the performance of six serological methods against SARS-CoV-2 to determine the antibody profile of 250 serum samples, including 234 RT-PCR-confirmed SARS-CoV-2 cases, the majority with asymptomatic presentation (87.2%) at 1-51 days post laboratory diagnosis. First, we compare to the performance of two in-house antibody assays: (i) an in-house IgG ELISA, utilizing UV-inactivated virus, and (ii) a live-virus neutralization assay (PRNT) using the same Cambodian isolate as the ELISA. In-house assays are then compared to standardized commercial anti-SARS-CoV-2 electrochemiluminescence immunoassays (Elecsys ECLIAs, Roche Diagnostics; targeting anti-N and anti-S antibodies) along with a flow cytometry based assay (FACS) that measures IgM and IgG against spike (S) protein and a multiplex microsphere-based immunoassay (MIA) determining the antibodies against various spike and nucleoprotein (N) antigens of SARS-CoV-2 and other coronaviruses (SARS-CoV-1, MERS-CoV, hCoVs 229E, NL63, HKU1).

Results

Overall, specificity of assays was 100%, except for the anti-S IgM flow cytometry based assay (96.2%), and the in-house IgG ELISA (94.2%). Sensitivity ranged from 97.3% for the anti-S ECLIA down to 76.3% for the anti-S IgG flow cytometry based assay. PRNT and in-house IgG ELISA performed similarly well when compared to the commercial ECLIA: sensitivity of ELISA and PRNT was 94.7 and 91.1%, respectively, compared to S- and N-targeting ECLIA with 97.3 and 96.8%, respectively. The MIA revealed cross-reactivity of antibodies from SARS-CoV-2-infected patients to the nucleocapsid of SARS-CoV-1, and the spike S1 domain of HKU1.

Conclusion

In-house serological assays, especially ELISA and PRNT, perform similarly to commercial assays, a critical factor in pandemic response. Selection of suitable immunoassays should be made based on available resources and diagnostic needs.

Longitudinal Analysis of Coronavirus-Neutralizing Activity in COVID-19 Patients

The severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) pandemic has now been continuing for more than two years. The infection causes COVID-19, a disease of the respiratory and cardiovascular system of variable severity. Here, the humoral immune response of 80 COVID-19 patients from the University Hospital Frankfurt/Main, Germany, was characterized longitudinally. The SARS-CoV-2 neutralization activity of serum waned over time. The neutralizing potential of serum directed towards the human alpha-coronavirus NL-63 (NL63) also waned, indicating that no cross-priming against alpha-coronaviruses occurred. A subset of the recovered patients (n = 13) was additionally vaccinated with the mRNA vaccine Comirnaty. Vaccination increased neutralization activity against SARS-CoV-2 wild-type (WT), Delta, and Omicron, although Omicron-specific neutralization was not detectable prior to vaccination. In addition, the vaccination induced neutralizing antibodies against the more distantly related SARS-CoV-1 but not against NL63. The results indicate that although SARS-CoV-2 humoral immune responses induced by infection wane, vaccination induces a broad neutralizing activity against multiple SARS-CoVs, but not to the common cold alpha-coronavirus NL63.

Boosting of cross-reactive antibodies to endemic coronaviruses by SARS-CoV-2 infection but not vaccination with stabilized spike

Preexisting antibodies to endemic coronaviruses (CoV) that cross-react with SARS-CoV-2 have the potential to influence the antibody response to COVID-19 vaccination and infection for better or worse. In this observational study of mucosal and systemic humoral immunity in acutely infected, convalescent, and vaccinated subjects, we tested for cross-reactivity against endemic CoV spike (S) protein at subdomain resolution. Elevated responses, particularly to the β-CoV OC43, were observed in all natural infection cohorts tested and were correlated with the response to SARS-CoV-2. The kinetics of this response and isotypes involved suggest that infection boosts preexisting antibody lineages raised against prior endemic CoV exposure that cross-react. While further research is needed to discern whether this recalled response is desirable or detrimental, the boosted antibodies principally targeted the better-conserved S2 subdomain of the viral spike and were not associated with neutralization activity. In contrast, vaccination with a stabilized spike mRNA vaccine did not robustly boost cross-reactive antibodies, suggesting differing antigenicity and immunogenicity. In sum, this study provides evidence that antibodies targeting endemic CoV are robustly boosted in response to SARS-CoV-2 infection but not to vaccination with stabilized S, and that depending on conformation or other factors, the S2 subdomain of the spike protein triggers a rapidly recalled, IgG-dominated response that lacks neutralization activity.

Immunology of SARS-CoV-2 infection in children

Children and adolescents exhibit a broad range of clinical outcomes from SARS-CoV-2 infection, with the majority having minimal to mild symptoms. Additionally, some succumb to a severe hyperinflammatory post-infectious complication called multisystem inflammatory syndrome in children (MIS-C), predominantly affecting previously healthy individuals. Studies characterizing the immunological differences associated with these clinical outcomes have identified pathways important for host immunity to SARS-CoV-2 and innate modulators of disease severity. In this Review, we delineate the immunological mechanisms underlying the spectrum of pediatric immune response to SARS-CoV-2 infection in comparison with that of adults.

Pre-existing humoral immunity to human common cold coronaviruses negatively impacts the protective SARS-CoV-2 antibody response

SARS-CoV-2 infection causes diverse outcomes ranging from asymptomatic infection to respiratory distress and death. A major unresolved question is whether prior immunity to endemic, human common cold coronaviruses (hCCCoVs) impacts susceptibility to SARS-CoV-2 infection or immunity following infection and vaccination. Therefore, we analyzed samples from the same individuals before and after SARS-CoV-2 infection or vaccination. We found hCCCoV antibody levels increase after SARS-CoV-2 exposure, demonstrating cross-reactivity. However, a case-control study indicates that baseline hCCCoV antibody levels are not associated with protection against SARS-CoV-2 infection. Rather, higher magnitudes of pre-existing betacoronavirus antibodies correlate with more SARS-CoV-2 antibodies following infection, an indicator of greater disease severity. Additionally, immunization with hCCCoV spike proteins before SARS-CoV-2 immunization impedes the generation of SARS-CoV-2-neutralizing antibodies in mice. Together, these data suggest that pre-existing hCCCoV antibodies hinder SARS-CoV-2 antibody-based immunity following infection and provide insight on how pre-existing coronavirus immunity impacts SARS-CoV-2 infection, which is critical considering emerging variants.

Heterologous humoral immunity to human and zoonotic coronaviruses: Aiming for the achilles heel

Coronaviruses are evolutionarily successful RNA viruses, common to multiple avian, amphibian and mammalian hosts. Despite their ubiquity and potential impact, knowledge of host immunity to coronaviruses remains incomplete, partly owing to the lack of overt pathogenicity of endemic human coronaviruses (HCoVs), which typically cause common colds. However, the need for deeper understanding became pressing with the zoonotic introduction of three novel coronaviruses in the past two decades, causing severe acute respiratory syndromes in humans, and the unfolding pandemic of coronavirus disease 2019 (COVID-19), caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). This renewed interest not only triggered the discovery of two of the four HCoVs, but also uncovered substantial cellular and humoral cross-reactivity with shared or related coronaviral antigens. Here, we review the evidence for cross-reactive B cell memory elicited by HCoVs and its potential impact on the puzzlingly variable outcome of SARS-CoV-2 infection. The available data indicate targeting of highly conserved regions primarily in the S2 subunits of the spike glycoproteins of HCoVs and SARS-CoV-2 by cross-reactive B cells and antibodies. Rare monoclonal antibodies reactive with conserved S2 epitopes and with potent virus neutralising activity have been cloned, underscoring the potential functional relevance of cross-reactivity. We discuss B cell and antibody cross-reactivity in the broader context of heterologous humoral immunity to coronaviruses, as well as the limits of protective immune memory against homologous re-infection. Given the bidirectional nature of cross-reactivity, the unprecedented current vaccination campaign against SARS-CoV-2 is expected to impact HCoVs, as well as future zoonotic coronaviruses attempting to cross the species barrier. However, emerging SARS-CoV-2 variants with resistance to neutralisation by vaccine-induced antibodies highlight a need for targeting more constrained, less mutable parts of the spike. The delineation of such cross-reactive areas, which humoral immunity can be trained to attack, may offer the key to permanently shifting the balance of our interaction with current and future coronaviruses in our favour.