Long-term humoral signatures following acute pediatric COVID-19 and Multisystem Inflammatory Syndrome in Children.

BACKGROUND: Although most children experience mild symptoms during acute SARS-CoV-2 infection, some develop the severe post-COVID-19 complication, Multisystem Inflammatory Syndrome in Children (MIS-C). While acute presentations of COVID-19 and MIS-C have been well immunophenotyped, little is known about the lasting immune profile in children after acute illness. METHODS: Children 2 months-20 years of age presenting with either acute COVID-19 (n = 9) or MIS-C (n = 12) were enrolled in a Pediatric COVID-19 Biorepository at a single medical center. We deeply profiled humoral immune responses and circulating cytokines following pediatric COVID-19 and MIS-C. RESULTS: Twenty-one children and young adults provided blood samples at both acute presentation and 6-month follow-up (mean: 6.5 months; standard deviation: 1.77 months). Pro-inflammatory cytokine elevations resolved after both acute COVID-19 and MIS-C. Humoral profiles continue to mature after acute COVID-19, displaying decreasing IgM and increasing IgG over time, as well as stronger effector functions, including antibody-dependent monocyte activation. In contrast, MIS-C immune signatures, especially anti-Spike IgG1, diminished over time. CONCLUSIONS: Here, we show the mature immune signature after pediatric COVID-19 and MIS-C, displaying resolving inflammation with recalibration of the humoral responses. These humoral profiles highlight immune activation and vulnerabilities over time in these pediatric post-infectious cohorts. IMPACT: The pediatric immune profile matures after both COVID-19 and MIS-C, suggesting a diversified anti-SARS-CoV-2 antibody response after resolution of acute illness. While pro-inflammatory cytokine responses resolve in the months following acute infection in both conditions, antibody-activated responses remain relatively heightened in convalescent COVID-19. These data may inform long-term immunoprotection from reinfection in children with past SARS-CoV-2 infections or MIS-C.

Selective SARS-CoV2 BA.2 escape of antibody Fc/Fc-receptor interactions.

The number of mutations in the omicron (B.1.1.529) BA.1 variant of concern led to an unprecedented evasion of vaccine induced immunity. However, despite rise in global infections, severe disease did not increase proportionally and is likely linked to persistent recognition of BA.1 by T cells and non-neutralizing opsonophagocytic antibodies. Yet, the emergence of new sublineage BA.2, which is more transmissible than BA.1 despite relatively preserved neutralizing antibody responses, has raised the possibility that BA.2 may evade other vaccine-induced responses. Here, we comprehensively profiled the BNT162b2 vaccine-induced response to several VOCs, including omicron BA.1 and BA.2. While vaccine-induced immune responses were compromised against both omicron sublineages, vaccine-induced antibody isotype titers, and non-neutralizing Fc effector functions were attenuated to the omicron BA.2 spike compared to BA.1. Conversely, FcγR2a and FcγR2b binding was elevated to BA.2, albeit lower than BA.1 responses, potentially contributing to persistent protection against severity of disease.

Circulating Spike Protein Detected in Post-COVID-19 mRNA Vaccine Myocarditis.

BACKGROUND: Cases of adolescents and young adults developing myocarditis after vaccination with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2)-targeted mRNA vaccines have been reported globally, but the underlying immunoprofiles of these individuals have not been described in detail. METHODS: From January 2021 through February 2022, we prospectively collected blood from 16 patients who were hospitalized at Massachusetts General for Children or Boston Children's Hospital for myocarditis, presenting with chest pain with elevated cardiac troponin T after SARS-CoV-2 vaccination. We performed extensive antibody profiling, including tests for SARS-CoV-2-specific humoral responses and assessment for autoantibodies or antibodies against the human-relevant virome, SARS-CoV-2-specific T-cell analysis, and cytokine and SARS-CoV-2 antigen profiling. Results were compared with those from 45 healthy, asymptomatic, age-matched vaccinated control subjects. RESULTS: Extensive antibody profiling and T-cell responses in the individuals who developed postvaccine myocarditis were essentially indistinguishable from those of vaccinated control subjects, despite a modest increase in cytokine production. A notable finding was that markedly elevated levels of full-length spike protein (33.9±22.4 pg/mL), unbound by antibodies, were detected in the plasma of individuals with postvaccine myocarditis, whereas no free spike was detected in asymptomatic vaccinated control subjects (unpaired t test; P<0.0001). CONCLUSIONS: Immunoprofiling of vaccinated adolescents and young adults revealed that the mRNA vaccine-induced immune responses did not differ between individuals who developed myocarditis and individuals who did not. However, free spike antigen was detected in the blood of adolescents and young adults who developed post-mRNA vaccine myocarditis, advancing insight into its potential underlying cause.

BNT162b2 induces robust cross-variant SARS-CoV-2 immunity in children.

Currently available mRNA vaccines are extremely safe and effective to prevent severe SARS-CoV-2 infections. However, the emergence of variants of concerns (VOCs) has highlighted the importance of high population-based vaccine rates to effectively suppress viral transmission and breakthrough infections. While initially left out from vaccine efforts, children have become one of the most affected age groups and are key targets to stop community and household spread. Antibodies are central for vaccine-induced protection and emerging data points to the importance of additional Fc effector functions like opsononophagocytosis or cytotoxicity, particularly in the context of VOCs that escape neutralizing antibodies. Here, we observed delayed induction and reduced magnitude of vaccine-induced antibody titers in children 5-11 years receiving two doses of the age-recommended 10 µg dose of the Pfizer SARS-CoV-2 BNT162b2 vaccine compared to adolescents (12-15 years) or adults receiving the 30 µg dose. Conversely, children mounted equivalent or more robust neutralization and opsonophagocytic functions at peak immunogenicity, pointing to a qualitatively more robust humoral functional response in children. Moreover, broad cross-VOC responses were observed across children, with enhanced IgM and parallel IgG cross-reactivity to VOCs in children compared to adults. Collectively, these data argue that despite the lower magnitude of the BNT162b2-induced antibody response in children, vaccine-induced immunity in children target VOCs broadly and exhibit enhanced functionality that may contribute to the attenuation of disease.

Neutrophil profiles of pediatric COVID-19 and multisystem inflammatory syndrome in children.

Multisystem inflammatory syndrome in children (MIS-C) is a delayed-onset, COVID-19-related hyperinflammatory illness characterized by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) antigenemia, cytokine storm, and immune dysregulation. In severe COVID-19, neutrophil activation is central to hyperinflammatory complications, yet the role of neutrophils in MIS-C is undefined. Here, we collect blood from 152 children: 31 cases of MIS-C, 43 cases of acute pediatric COVID-19, and 78 pediatric controls. We find that MIS-C neutrophils display a granulocytic myeloid-derived suppressor cell (G-MDSC) signature with highly altered metabolism that is distinct from the neutrophil interferon-stimulated gene (ISG) response we observe in pediatric COVID-19. Moreover, we observe extensive spontaneous neutrophil extracellular trap (NET) formation in MIS-C, and we identify neutrophil activation and degranulation signatures. Mechanistically, we determine that SARS-CoV-2 immune complexes are sufficient to trigger NETosis. Our findings suggest that hyperinflammatory presentation during MIS-C could be mechanistically linked to persistent SARS-CoV-2 antigenemia, driven by uncontrolled neutrophil activation and NET release in the vasculature.

Differential Severe Acute Respiratory Syndrome Coronavirus 2 Antibody Profiles After Allergic Reactions to Messenger RNA Coronavirus Disease 2019 Vaccine.

Allergic symptoms after messenger RNA (mRNA) coronavirus disease 2019 (COVID-19) vaccines occur in up to 2% of recipients. Compared to nonallergic controls (n = 18), individuals with immediate allergic reactions to mRNA COVID-19 vaccines (n = 8) mounted lower immunoglobulin G1 (IgG1) to multiple antigenic targets in severe acute respiratory syndrome coronavirus 2 spike following vaccination, with significantly lower IgG1 to full-length spike (P = .04). Individuals with immediate allergic reactions to mRNA COVID-19 vaccines bound Fcγ receptors similarly to nonallergic controls. Although there was a trend toward an overall reduction in opsonophagocytic function in individuals with immediate allergic reactions compared to nonallergic controls, allergic patients produced functional antibodies exhibiting a high ratio of opsonophagocytic function to IgG1 titer.

SARS-CoV-2 mRNA vaccination elicits robust antibody responses in children.

Although children have been largely spared from coronavirus disease 2019 (COVID-19), the emergence of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) variants of concern (VOCs) with increased transmissibility, combined with fluctuating mask mandates and school reopenings, has led to increased infections and disease among children. Thus, there is an urgent need to roll out COVID-19 vaccines to children of all ages. However, whether children respond equivalently to adults to mRNA vaccines and whether dosing will elicit optimal immunity remain unclear. Here, we aimed to deeply profile the vaccine-induced humoral immune response in 6- to 11-year-old children receiving either a pediatric (50 µg) or adult (100 µg) dose of the mRNA-1273 vaccine and to compare these responses to vaccinated adults, infected children, and children who experienced multisystem inflammatory syndrome in children (MIS-C). Children elicited an IgG-dominant vaccine-induced immune response, surpassing adults at a matched 100-µg dose but more variable immunity at a 50-µg dose. Irrespective of titer, children generated antibodies with enhanced Fc receptor binding capacity. Moreover, like adults, children generated cross-VOC humoral immunity, marked by a decline of omicron-specific receptor binding domain, but robustly preserved omicron spike protein binding. Fc receptor binding capabilities were also preserved in a dose-dependent manner. These data indicate that both the 50- and 100-µg doses of mRNA vaccination in children elicit robust cross-VOC antibody responses and that 100-µg doses in children result in highly preserved omicron-specific functional humoral immunity.

Preserved recognition of Omicron spike following COVID-19 messenger RNA vaccination in pregnancy.

BACKGROUND: SARS-CoV-2 infection is associated with enhanced disease severity in pregnant women. Despite the potential of COVID-19 vaccines to reduce severe disease, vaccine uptake remained relatively low among pregnant women. Just as coordinated messaging from the Centers for Disease Control and Prevention and leading obstetrics organizations began to increase vaccine confidence in this vulnerable group, the evolution of SARS-CoV-2 variants of concerns, including the Omicron variant, raised new concerns about vaccine efficacy because of their ability to escape vaccine-induced neutralizing antibodies. Early data point to a milder disease course following infection with the Omicron variant in vaccinated individuals. Thus, these data suggest that alternate vaccine-induced immunity beyond neutralization may continue to attenuate Omicron variant-induced disease, such as Fc-mediated antibody activity. OBJECTIVE: This study aimed to test whether vaccine-induced antibodies raised during pregnancy continue to bind to and leverage Fc receptors to protect against variants of concern including the Omicron variant. STUDY DESIGN: The receptor binding domain or whole spike-specific antibody isotype binding titers and Fc gamma receptor binding directed toward variants of concern, including the Omicron variant, were analyzed in pregnant women after receiving the full dose regimen of either the Pfizer/BioNTech BNT62b2 (n=10) or Moderna mRNA-1273 (n=10) vaccination using a multiplexing Luminex assay. RESULTS: Reduced isotype recognition of the Omicron receptor binding domain was observed following administration of either vaccine with relatively preserved, albeit reduced, recognition of the whole Omicron spike by immunoglobulin M and G antibodies. Despite the near complete loss of Fc receptor binding to the Omicron receptor binding domain, Fc receptor binding to the Omicron spike was more variable but largely preserved. CONCLUSION: Reduced binding titers to the Omicron receptor binding domain aligns with the observed loss of neutralizing activity. Despite the loss of neutralization, preserved, albeit reduced, Omicron spike recognition and Fc receptor binding potentially continue to attenuate disease severity in pregnant women.

Durability and Cross-Reactivity of SARS-CoV-2 mRNA Vaccine in Adolescent Children.

Emergent SARS-CoV-2 variants and waning humoral immunity in vaccinated individuals have resulted in increased infections and hospitalizations. Children are not spared from infection nor complications of COVID-19, and the recent recommendation for boosters in individuals ages 12 years or older calls for broader understanding of the adolescent immune profile after mRNA vaccination. We tested the durability and cross-reactivity of anti-SARS-CoV-2 serologic responses over a six-month time course in vaccinated adolescents against the SARS-CoV-2 D614G ("wild type") and Omicron antigens. Serum from 77 adolescents showed that anti-Spike antibodies wane significantly over six months. After completion of a two-vaccine series, cross-reactivity against Omicron-specific receptor-binding domain (RBD) was seen. Functional humoral activation against wild type and Omicron SARS-CoV-2 also declines over time in vaccinated adolescent children. Evidence of waning mRNA-induced vaccine immunity underscores vulnerabilities in long-term pediatric protection against SARS-CoV-2 infection, while cross-reactivity highlights the additional benefits of vaccination. Characterization of adolescent immune signatures post-vaccination will inform guidance on vaccine platforms and timelines, and ultimately optimize immunoprotection of children.

A modified vaccinia Ankara vaccine expressing spike and nucleocapsid protects rhesus macaques against SARS-CoV-2 Delta infection.

SARS-CoV-2 vaccines should induce broadly cross-reactive humoral and T cell responses to protect against emerging variants of concern (VOCs). Here, we inactivated the furin cleavage site (FCS) of spike expressed by a modified vaccinia Ankara (MVA) virus vaccine (MVA/SdFCS) and found that FCS inactivation markedly increased spike binding to human ACE2. After vaccination of mice, the MVA/SdFCS vaccine induced eightfold higher neutralizing antibodies compared with MVA/S, which expressed spike without FCS inactivation, and protected against the Beta variant. We next added nucleocapsid to the MVA/SdFCS vaccine (MVA/SdFCS-N) and tested its immunogenicity and efficacy via intramuscular (IM), buccal (BU), or sublingual (SL) routes in rhesus macaques. IM vaccination induced spike-specific IgG in serum and mucosae (nose, throat, lung, and rectum) that neutralized the homologous (WA-1/2020) and heterologous VOCs, including Delta, with minimal loss (<2-fold) of activity. IM vaccination also induced both spike- and nucleocapsid-specific CD4 and CD8 T cell responses in the blood. In contrast, the SL and BU vaccinations induced less spike-specific IgG in secretions and lower levels of polyfunctional IgG in serum compared with IM vaccination. After challenge with the SARS-CoV-2 Delta variant, the IM route induced robust protection, the BU route induced moderate protection, and the SL route induced no protection. Vaccine-induced neutralizing and non-neutralizing antibody effector functions positively correlated with protection, but only the effector functions correlated with early protection. Thus, IM vaccination with MVA/SdFCS-N vaccine elicited cross-reactive antibody and T cell responses, protecting against heterologous SARS-CoV-2 VOC more effectively than other routes of vaccination.

Omicron variant Spike-specific antibody binding and Fc activity are preserved in recipients of mRNA or inactivated COVID-19 vaccines.

The Omicron variant of SARS-CoV-2 has been shown to evade neutralizing antibodies elicited by vaccination or infection. Despite the global spread of the Omicron variant, even among highly vaccinated populations, death rates have not increased concomitantly. These data suggest that immune mechanisms beyond antibody-mediated virus neutralization may protect against severe disease. In addition to neutralizing pathogens, antibodies contribute to control and clearance of infections through Fc effector mechanisms. Here, we probed the ability of vaccine-induced antibodies to drive Fc effector activity against the Omicron variant using samples from individuals receiving one of three SARS-CoV-2 vaccines. Despite a substantial loss of IgM, IgA, and IgG binding to the Omicron variant receptor binding domain (RBD) in samples from individuals receiving BNT162b2, mRNA-1273, and CoronaVac vaccines, stable binding was maintained against the full-length Omicron Spike protein. Compromised RBD binding IgG was accompanied by a loss of RBD-specific antibody Fcγ receptor (FcγR) binding in samples from individuals who received the CoronaVac vaccine, but RBD-specific FcγR2a and FcγR3a binding was preserved in recipients of mRNA vaccines. Conversely, Spike protein-specific antibodies exhibited persistent but reduced binding to FcγRs across all three vaccines, although higher binding was observed in samples from recipients of mRNA vaccines. This was associated with preservation of FcγR2a and FcγR3a binding antibodies and maintenance of Spike protein-specific antibody-dependent natural killer cell activation. Thus, despite the loss of Omicron neutralization, vaccine-induced Spike protein-specific antibodies continue to drive Fc effector functions, suggesting a capacity for extraneutralizing antibodies to contribute to disease control.

Serological Markers of SARS-CoV-2 Reinfection.

As public health guidelines throughout the world have relaxed in response to vaccination campaigns against SARS-CoV-2, it is likely that SARS-CoV-2 will remain endemic, fueled by the rise of more infectious SARS-CoV-2 variants. Moreover, in the setting of waning natural and vaccine immunity, reinfections have emerged across the globe, even among previously infected and vaccinated individuals. As such, the ability to detect reexposure to and reinfection by SARS-CoV-2 is a key component for global protection against this virus and, more importantly, against the potential emergence of vaccine escape mutations. Accordingly, there is a strong and continued need for the development and deployment of simple methods to detect emerging hot spots of reinfection to inform targeted pandemic response and containment, including targeted and specific deployment of vaccine booster campaigns. In this study, we identify simple, rapid immune biomarkers of reinfection in rhesus macaques, including IgG3 antibody levels against nucleocapsid and FcγR2A receptor binding activity of anti-RBD antibodies, that are recapitulated in human reinfection cases. As such, this cross-species analysis underscores the potential utility of simple antibody titers and function as price-effective and scalable markers of reinfection to provide increased resolution and resilience against new outbreaks. IMPORTANCE As public health and social distancing guidelines loosen in the setting of waning global natural and vaccine immunity, a deeper understanding of the immunological response to reexposure and reinfection to this highly contagious pathogen is necessary to maintain public health. Viral sequencing analysis provides a robust but unrealistic means to monitor reinfection globally. The identification of scalable pathogen-specific biomarkers of reexposure and reinfection, however, could significantly accelerate our capacity to monitor the spread of the virus through naive and experienced hosts, providing key insights into mechanisms of disease attenuation. Using a nonhuman primate model of controlled SARS-CoV-2 reexposure, we deeply probed the humoral immune response following rechallenge with various doses of viral inocula. We identified virus-specific humoral biomarkers of reinfection, with significant increases in antibody titer and function upon rechallenge across a range of humoral features, including IgG1 to the receptor binding domain of the spike protein of SARS-CoV-2 (RBD), IgG3 to the nucleocapsid protein (N), and FcγR2A receptor binding to anti-RBD antibodies. These features not only differentiated primary infection from reexposure and reinfection in monkeys but also were recapitulated in a sequencing-confirmed reinfection patient and in a cohort of putatively reinfected humans that evolved a PCR-positive test in spite of preexisting seropositivity. As such, this cross-species analysis using a controlled primate model and human cohorts reveals increases in antibody titers as promising cross-validated serological markers of reinfection and reexposure.

mRNA-1273 vaccine-induced antibodies maintain Fc effector functions across SARS-CoV-2 variants of concern.

SARS-CoV-2 mRNA vaccines confer robust protection against COVID-19, but the emergence of variants has generated concerns regarding the protective efficacy of the currently approved vaccines, which lose neutralizing potency against some variants. Emerging data suggest that antibody functions beyond neutralization may contribute to protection from the disease, but little is known about SARS-CoV-2 antibody effector functions. Here, we profiled the binding and functional capacity of convalescent antibodies and Moderna mRNA-1273 COVID-19 vaccine-induced antibodies across SARS-CoV-2 variants of concern (VOCs). Although the neutralizing responses to VOCs decreased in both groups, the Fc-mediated responses were distinct. In convalescent individuals, although antibodies exhibited robust binding to VOCs, they showed compromised interactions with Fc-receptors. Conversely, vaccine-induced antibodies also bound robustly to VOCs but continued to interact with Fc-receptors and mediate antibody effector functions. These data point to a resilience in the mRNA-vaccine-induced humoral immune response that may continue to offer protection from SARS-CoV-2 VOCs independent of neutralization.

Epidemiological and Immunological Features of Obesity and SARS-CoV-2.

Obesity is a key correlate of severe SARS-CoV-2 outcomes while the role of obesity on risk of SARS-CoV-2 infection, symptom phenotype, and immune response remain poorly defined. We examined data from a prospective SARS-CoV-2 cohort study to address these questions. Serostatus, body mass index, demographics, comorbidities, and prior COVID-19 compatible symptoms were assessed at baseline and serostatus and symptoms monthly thereafter. SARS-CoV-2 immunoassays included an IgG ELISA targeting the spike RBD, multiarray Luminex targeting 20 viral antigens, pseudovirus neutralization, and T cell ELISPOT assays. Our results from a large prospective SARS-CoV-2 cohort study indicate symptom phenotype is strongly influenced by obesity among younger but not older age groups; we did not identify evidence to suggest obese individuals are at higher risk of SARS-CoV-2 infection; and remarkably homogenous immune activity across BMI categories suggests immune protection across these groups may be similar.

Humoral signatures of protective and pathological SARS-CoV-2 infection in children.

The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) pandemic continues to spread relentlessly, associated with a high frequency of respiratory failure and mortality. Children experience largely asymptomatic disease, with rare reports of multisystem inflammatory syndrome in children (MIS-C). Identifying immune mechanisms that result in these disparate clinical phenotypes in children could provide critical insights into coronavirus disease 2019 (COVID-19) pathogenesis. Using systems serology, in this study we observed in 25 children with acute mild COVID-19 a functional phagocyte and complement-activating IgG response to SARS-CoV-2, similar to the acute responses generated in adults with mild disease. Conversely, IgA and neutrophil responses were significantly expanded in adults with severe disease. Moreover, weeks after the resolution of SARS-CoV-2 infection, children who develop MIS-C maintained highly inflammatory monocyte-activating SARS-CoV-2 IgG antibodies, distinguishable from acute disease in children but with antibody levels similar to those in convalescent adults. Collectively, these data provide unique insights into the potential mechanisms of IgG and IgA that might underlie differential disease severity as well as unexpected complications in children infected with SARS-CoV-2.

Multisystem inflammatory syndrome in children is driven by zonulin-dependent loss of gut mucosal barrier.

BACKGROUNDWeeks after SARS-CoV-2 infection or exposure, some children develop a severe, life-threatening illness called multisystem inflammatory syndrome in children (MIS-C). Gastrointestinal (GI) symptoms are common in patients with MIS-C, and a severe hyperinflammatory response ensues with potential for cardiac complications. The cause of MIS-C has not been identified to date.METHODSHere, we analyzed biospecimens from 100 children: 19 with MIS-C, 26 with acute COVID-19, and 55 controls. Stools were assessed for SARS-CoV-2 by reverse transcription PCR (RT-PCR), and plasma was examined for markers of breakdown of mucosal barrier integrity, including zonulin. Ultrasensitive antigen detection was used to probe for SARS-CoV-2 antigenemia in plasma, and immune responses were characterized. As a proof of concept, we treated a patient with MIS-C with larazotide, a zonulin antagonist, and monitored the effect on antigenemia and the patient's clinical response.RESULTSWe showed that in children with MIS-C, a prolonged presence of SARS-CoV-2 in the GI tract led to the release of zonulin, a biomarker of intestinal permeability, with subsequent trafficking of SARS-CoV-2 antigens into the bloodstream, leading to hyperinflammation. The patient with MIS-C treated with larazotide had a coinciding decrease in plasma SARS-CoV-2 spike antigen levels and inflammatory markers and a resultant clinical improvement above that achieved with currently available treatments.CONCLUSIONThese mechanistic data on MIS-C pathogenesis provide insight into targets for diagnosing, treating, and preventing MIS-C, which are urgently needed for this increasingly common severe COVID-19-related disease in children.

Discrete SARS-CoV-2 antibody titers track with functional humoral stability.

Antibodies serve as biomarkers of infection, but if sustained can confer long-term immunity. Yet, for most clinically approved vaccines, binding antibody titers only serve as a surrogate of protection. Instead, the ability of vaccine induced antibodies to neutralize or mediate Fc-effector functions is mechanistically linked to protection. While evidence has begun to point to persisting antibody responses among SARS-CoV-2 infected individuals, cases of re-infection have begun to emerge, calling the protective nature of humoral immunity against this highly infectious pathogen into question. Using a community-based surveillance study, we aimed to define the relationship between titers and functional antibody activity to SARS-CoV-2 over time. Here we report significant heterogeneity, but limited decay, across antibody titers amongst 120 identified seroconverters, most of whom had asymptomatic infection. Notably, neutralization, Fc-function, and SARS-CoV-2 specific T cell responses were only observed in subjects that elicited RBD-specific antibody titers above a threshold. The findings point to a switch-like relationship between observed antibody titer and function, where a distinct threshold of activity-defined by the level of antibodies-is required to elicit vigorous humoral and cellular response. This response activity level may be essential for durable protection, potentially explaining why re-infections occur with SARS-CoV-2 and other common coronaviruses.

Loss of Bcl-6-Expressing T Follicular Helper Cells and Germinal Centers in COVID-19.

Humoral responses in coronavirus disease 2019 (COVID-19) are often of limited durability, as seen with other human coronavirus epidemics. To address the underlying etiology, we examined post mortem thoracic lymph nodes and spleens in acute SARS-CoV-2 infection and observed the absence of germinal centers and a striking reduction in Bcl-6+ germinal center B cells but preservation of AID+ B cells. Absence of germinal centers correlated with an early specific block in Bcl-6+ TFH cell differentiation together with an increase in T-bet+ TH1 cells and aberrant extra-follicular TNF-α accumulation. Parallel peripheral blood studies revealed loss of transitional and follicular B cells in severe disease and accumulation of SARS-CoV-2-specific "disease-related" B cell populations. These data identify defective Bcl-6+ TFH cell generation and dysregulated humoral immune induction early in COVID-19 disease, providing a mechanistic explanation for the limited durability of antibody responses in coronavirus infections, and suggest that achieving herd immunity through natural infection may be difficult.

The Loss of Bcl-6 Expressing T Follicular Helper Cells and the Absence of Germinal Centers in COVID-19.

Humoral responses in COVID-19 disease are often of limited durability, as seen with other human coronavirus epidemics. To address the underlying etiology, we examined postmortem thoracic lymph nodes and spleens in acute SARS-CoV-2 infection and observed the absence of germinal centers, a striking reduction in Bcl-6+ germinal center B cells but preservation of AID+ B cells. Absence of germinal centers correlated with an early specific block in Bcl-6+TFH cell differentiation together with an increase in T-bet+TH1 cells and aberrant extra-follicular TNF-a accumulation.  Parallel peripheral blood studies revealed loss of transitional and follicular B cells in severe disease and accumulation of SARS-CoV-2-specific "disease-related" B cell populations. These data identify defective Bcl-6+TFH cell generation and dysregulated humoral immune induction early in COVID-19 disease, providing a mechanistic explanation for the limited durability of antibody responses in coronavirus infections and suggest that achieving herd immunity through natural infection may be difficult. Funding: This work was supported by NIH U19 AI110495 to SP, NIH R01 AI146779 to AGS, NIH R01AI137057 and DP2DA042422 to DL, BMH was supported by NIGMS T32 GM007753, TMC was supported by T32 AI007245. Funding for these studies from the Massachusetts Consortium of Pathogen Readiness, the Mark and Lisa Schwartz Foundation and Enid Schwartz is also acknowledged. Conflict of Interest: None. Ethical Approval: This study was performed with the approval of the Institutional Review Boards at the Massachusetts General Hospital and the Brigham and Women's Hospital.