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The characterization of the antibody response to SARS-CoV-2 and its determinants are key for the understanding of COVID-19. The identification of vulnerable populations to the infection and to its socioeconomic impact is indispensable for inclusive policies. We conducted an age-stratified cross-sectional community-based seroprevalence survey between June 12th and 19th 2020—during the easing of lockdown—in Cizur, Spain. We quantified IgG, IgM and IgA levels against SARS-CoV-2 spike and its receptor-binding domain in a sample of 728 randomly selected, voluntarily registered inhabitants. We estimated a 7.9% seroprevalence in the general population, with the lowest seroprevalence among children under ten (n = 3/142, 2.1%) and the highest among adolescents (11–20 years old, n = 18/159, 11.3%). We found a heterogeneous immune-response profile across participants regarding isotype/antigen-specific seropositivity, although levels generally correlated. Those with technical education level were the most financially affected. Fifty-five percent had visited a supermarket and 43% a sanitary centre since mid-February 2020. When comparing by gender, men had left the household more frequently. In conclusion, few days after strict lockdown, the burden of SARS-CoV-2 infection was the lowest in children under 10. The findings also suggest that a wider isotype-antigen panel confers higher sensitivity. Finally, the economic impact biases should be considered when designing public health measures.

We evaluated the kinetics of antibody responses to Two years into the COVID-19 pandemic and 1 year after the start of vaccination rollout, the world faced a peak of cases associated with the highly contagious Omicron variant of concern (VoC) of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) spike (S) and nucleocapsid (N) antigens over five cross-sectional visits (January-November 2021), and the determinants of pre-booster immunoglobulin levels, in a prospective cohort of vaccinated primary health care workers in Catalonia, Spain. Antibodies against S antigens after a full primary vaccination course, mostly with BNT162b2, decreased steadily over time and were higher in pre-exposed (n = 247) than naïve (n = 200) individuals, but seropositivity was maintained at 100% (100% IgG, 95.5% IgA, 30.6% IgM) up to 319 days after the first dose. Antibody binding to variants of concern was highly maintained for IgG compared to wild type but significantly reduced for IgA and IgM, particularly for Beta and Gamma. Factors significantly associated with longer-term antibodies included age, sex, occupation, smoking, adverse reaction to vaccination, levels of pre-vaccination SARS-CoV-2 antibodies, interval between disease onset and vaccination, hospitalization, oxygen supply, post COVID and symptomatology. Earlier morning vaccination hours were associated with higher IgG responses in pre-exposed participants. Symptomatic breakthroughs occurred in 9/447 (2.01%) individuals, all among naïve (9/200, 4.5%) and generally boosted antibody responses. Additionally, an increase in IgA and/or IgM seropositivity to variants, and N seroconversion at later time points (6.54%), indicated asymptomatic breakthrough infections, even among pre-exposed. Seropositivity remained highly stable over almost a year after vaccination. However, gradually waning of anti-S IgGs that correlate with neutralizing activity, coupled to evidence of an increase in breakthrough infections during the Delta and Omicron predominance, provides a rationale for booster immunization.

COVID-19 affects children to a lesser extent than adults but they can still get infected and transmit SARS-CoV-2 to their contacts. Field deployable non-invasive sensitive diagnostic techniques are needed to evaluate the infectivity dynamics of SARS-CoV-2 in pediatric populations and guide public health interventions, particularly if this population is not fully vaccinated. We evaluated the utility of high-throughput Luminex assays to quantify saliva IgM, IgA and IgG antibodies against five SARS-CoV-2 spike (S) and nucleocapsid (N) antigens in a contacts and infectivity longitudinal study in 122 individuals (52 children and 70 adults). We compared saliva versus serum/plasma samples in infected children and adults diagnosed by weekly RT-PCR over 35 days (n=62), and those who consistently tested negative over the same follow up period (n=60), in the Summer of 2020 in Barcelona, Spain. Saliva antibody levels in SARS-CoV-2 RT-PCR positive individuals were significantly higher than in negative individuals and correlated with those measured in sera/plasmas. Asymptomatic infected individuals had higher levels of anti-S IgG than symptomatic individuals, suggesting a protective anti-disease role for antibodies. Higher anti-S IgG and IgM levels in serum/plasma and saliva, respectively, in infected children compared to infected adults could also be related to stronger clinical immunity in them. Among infected children, males had higher levels of saliva IgG to N and RBD than females. Despite overall correlation, individual clustering analysis suggested that responses that may not be detected in blood could be patent in saliva, and vice versa. In conclusion, measurement of SARS-CoV-2-specific saliva antibodies should be considered as a complementary non-invasive assay to serum/plasma to determine COVID-19 prevalence and transmission in pediatric populations before and after vaccination campaigns. The project has been funded by Stavros Niarchos Foundation (SNF), Banco Santander and other private donors of KidsCorona, and Fundació Privada Daniel Bravo Andreu. RR had the support of the Health Department, Catalan Government (PERIS SLT017/20/000224). Development of SARS-CoV-2 reagents was partially supported by the NIAID Centers of Excellence for Influenza Research and Surveillance (CEIRS) contract HHSN272201400008C. ISGlobal receives support from the Spanish Ministry of Science and Innovation through the “Centro de Excelencia Severo Ochoa 2019-2023” Program (CEX2018-000806-S), and support from the Generalitat de Catalunya through the CERCA Program. CISM is supported by the Government of Mozambique and the Spanish Agency for International Development (AECID). Peer Reviewed Article signat per 20 autors/es: Carlota Dobaño (1,2), Selena Alonso (1), Marta Vidal (1), Alfons Jiménez (1,3), Rocío Rubio (1), Rebeca Santano (1), Diana Barrios (1), Gemma Pons Tomas (4), María Melé Casas (4), María Hernández García (4), Mònica Girona-Alarcón (5,6), Laura Puyol (1), Barbara Baro (1), Pere Millat-Martínez (1), Sara Ajanovic (1), Núria Balanza (1), Sara Arias (1), Natalia Rodrigo Melero (7), Carlo Carolis (7), Aleix García-Miquel (8), Elisenda Bonet-Carné (8,9,10), Joana Claverol (5,11), Marta Cubells (5,11), Claudia Fortuny (5,12), Victoria Fumadó (5,12), Anna Codina (13), Quique Bassat (1,3,4,14,15), Carmen Muñoz-Almagro (3,5,16,17), Mariona Fernández de Sevilla (3,4,5), Eduard Gratacós (8,9,18), Luis Izquierdo (1,2), Juan José García-García (3,4,5), Ruth Aguilar (1), Iolanda Jordan (3,5,6) and Gemma Moncunill (1,2) // (1) ISGlobal, Hospital Clínic - Universitat de Barcelona, Barcelona, Spain, (2) Consorcio de Investigación Biomédica en Red (CIBER) de Enfermedades Infecciosas, Madrid, Spain, (3) Consorcio de Investigación Biomédica en Red de Epidemiología y Salud Pública (CIBERESP), Madrid, Spain, (4) Pediatrics Department, Hospital Sant Joan de Déu, Universitat de Barcelona, Esplugues, Spain, (5) Institut de Recerca Sant Joan de Déu, Esplugues, Spain, (6) Paediatric Intensive Care Unit, Hospital Sant Joan de Déu, Universitat de Barcelona, Barcelona, Spain, (7) Biomolecular Screening and Protein Technologies Unit, Centre for Genomic Regulation (CRG), The Barcelona Institute of Science and Technology, Barcelona, Spain, (8) Fetal Medicine Research Center (Hospital Clínic and Hospital Sant Joan de Déu), Universitat de Barcelona, Barcelona, Spain, (9) Institut d’Investigacions Biomèdiques August Pi i Sunyer (IDIBAPS), Barcelona, Spain, (10) Universitat Politècnica de Catalunya, BarcelonaTech, Barcelona, Spain, (11) Fundació Sant Joan de Déu, Barcelona, Spain, (12) Infectious Diseases Department, Hospital Sant Joan de Déu, Barcelona, Spain, (13) Biobank Hospital Sant Joan de Déu, Barcelona, Spain, (14) Centro de Investigação em Saúde de Manhiça (CISM), Maputo, Mozambique, (15) Catalan Institution for Research and Advanced Studies (ICREA), Barcelona, Spain, (16) Department of Medicine, Universitat Internacional de Catalunya, Barcelona, Spain, (17) Molecular Microbiology Department, Hospital Sant Joan de Déu, Esplugues, Spain, (18) Center for Biomedical Research on Rare Diseases (CIBER-ER), Madrid, Spain

SARS-CoV-2 infected pregnant women are at increased risk of severe COVID-19 than non-pregnant women and have a higher risk of adverse pregnancy outcomes like intrauterine/fetal distress and preterm birth. However, little is known about the impact of SARS-CoV-2 infection on maternal and neonatal immunological profiles. In this study, we investigated the inflammatory and humoral responses to SARS-CoV-2 in maternal and cord blood paired samples. Thirty-six pregnant women were recruited at delivery at Hospital Sant Joan de Déu, Barcelona, Spain, between April-August 2020, before having COVID-19 available vaccines. Maternal and pregnancy variables, as well as perinatal outcomes, were recorded in questionnaires. Nasopharyngeal swabs and maternal and cord blood samples were collected for SARS-CoV-2 detection by rRT-PCR and serology, respectively. We measured IgM, IgG and IgA levels to 6 SARS-CoV-2 antigens (spike [S], S1, S2, receptor-binding domain [RBD], nucleocapsid [N] full-length and C-terminus), IgG to N from 4 human coronaviruses (OC43, HKU1, 229E and NL63), and the concentrations of 30 cytokines, chemokines and growth factors by Luminex. Mothers were classified as infected or non-infected based on the rRT-PCR and serology results. Sixty-four % of pregnant women were infected with SARS-CoV-2 (positive by rRT-PCR during the third trimester and/or serology just after delivery). None of the newborns tested positive for rRT-PCR. SARS-CoV-2 infected mothers had increased levels of virus-specific antibodies and several cytokines. Those with symptoms had higher cytokine levels. IFN-α was increased in cord blood from infected mothers, and in cord blood of symptomatic mothers, EGF, FGF, IL-17 and IL-15 were increased, whereas RANTES was decreased. Maternal IgG and cytokine levels showed positive correlations with their counterparts in cord blood. rRT-PCR positive mothers showed lower transfer of SARS-CoV-2-specific IgGs, with a stronger effect when infection was closer to delivery. SARS-CoV-2 infected mothers carrying a male fetus had higher antibody levels and higher EGF, IL-15 and IL-7 concentrations. Our results show that SARS-CoV-2 infection during the third trimester of pregnancy induces a robust antibody and cytokine response at delivery and causes a significant reduction of the SARS-CoV-2-specific IgGs transplacental transfer, with a stronger negative effect when the infection is closer to delivery. This work was supported by the Fundació Privada Daniel Bravo Andreu. RR had the support of the Health Department, Catalan Government (PERIS SLT017/20/000224). LI work was supported by the PID2019-110810RB-I00 grant from the Spanish Ministry of Science & Innovation. We acknowledge support from the Spanish Ministry of Science and Innovation and State Research Agency through the “Centro de Excelencia Severo Ochoa 2019-2023” Program (CEX2018-000806-S), and support from the Generalitat de Catalunya through the CERCA Program.

Background: Surveillance tools to estimate viral transmission dynamics in young populations are essential to guide recommendations for school opening and management during viral epidemics. Ideally, sensitive techniques are required to detect low viral load exposures among asymptomatic children. We aimed to estimate SARS-CoV-2 infection rates in children and adult populations in a school-like environment during the initial COVID-19 pandemic waves using an antibody-based field-deployable and non-invasive approach. Methods: Saliva antibody conversion defined as ≥ 4-fold increase in IgM, IgA, and/or IgG levels to five SARS-CoV-2 antigens including spike and nucleocapsid constructs was evaluated in 1509 children and 396 adults by high-throughput Luminex assays in samples collected weekly in 22 summer schools and 2 pre-schools in 27 venues in Barcelona, Spain, from June 29th to July 31st, 2020. Results: Saliva antibody conversion between two visits over a 5-week period was 3.22% (49/1518) or 2.36% if accounting for potentially cross-reactive antibodies, six times higher than the cumulative infection rate (0.53%) assessed by weekly saliva RT-PCR screening. IgG conversion was higher in adults (2.94%, 11/374) than children (1.31%, 15/1144) (p=0.035), IgG and IgA levels moderately increased with age, and antibodies were higher in females. Most antibody converters increased both IgG and IgA antibodies but some augmented either IgG or IgA, with a faster decay over time for IgA than IgG. Nucleocapsid rather than spike was the main antigen target. Anti-spike antibodies were significantly higher in individuals not reporting symptoms than symptomatic individuals, suggesting a protective role against COVID-19. Conclusion: Saliva antibody profiling including three isotypes and multiplexing antigens is a useful and user-friendlier tool for screening pediatric populations to detect low viral load exposures among children, particularly while they are not vaccinated and vulnerable to highly contagious variants, and to recommend public health policies during pandemics. This work was supported by the Departament de Salut, Generalitat de Catalunya (grant number SLT006/17/00109). L.I. work was supported by PID2019-110810RB-I00 grant from the Spanish Ministry of Science & Innovation. Development of SARS-CoV-2 reagents was partially supported by the National Institute of Allergy and Infectious Diseases Centers of Excellence for Influenza Research and Surveillance (contract number HHSN272201400008C). ISGlobal receives support from the Spanish Ministry of Science and Innovation through the “Centro de Excelencia Severo Ochoa 2019-2023” Program (CEX2018-000806-S), and support from the Generalitat de Catalunya through the CERCA Program

Objectives SARS‐CoV‐2 infection induces virus‐reactive memory B cells expressing unmutated antibodies, which hints at their emergence from naïve B cells. Yet, the dynamics of virus‐specific naïve B cells and their impact on immunity and immunopathology remain unclear. Methods We longitudinally profiled SARS‐CoV‐2‐specific B‐cell responses in 25 moderate‐to‐severe COVID‐19 patients by high‐dimensional flow cytometry and isotyping and subtyping ELISA. We also explored the relationship of B‐cell responses to SARS‐CoV‐2 with the activation of effector and regulatory cells from the innate or adaptive immune system. Results We found a virus‐specific antibody response with a broad spectrum of classes and subclasses during acute infection, which evolved into an IgG1‐dominated response during convalescence. Acute infection was associated with increased mature B‐cell progenitors in the circulation and the unexpected expansion of virus‐targeting naïve‐like B cells. The latter further augmented during convalescence together with virus‐specific memory B cells. In addition to a transitory increase in tissue‐homing CXCR3+ plasmablasts and extrafollicular memory B cells, most COVID‐19 patients showed persistent activation of CD4+ and CD8+ T cells along with transient or long‐lasting changes of key innate immune cells. Remarkably, virus‐specific antibodies and the frequency of naïve B cells were among the major variables defining distinct immune signatures associated with disease severity and inflammation. Conclusion Aside from providing new insights into the complexity of the immune response to SARS‐CoV‐2, our findings indicate that the de novo recruitment of mature B‐cell precursors into the periphery may be central to the induction of antiviral immunity., We longitudinally studied moderate‐to‐severe COVID‐19 patients to dissect SARS‐CoV‐2‐specific B cell responses overtime. We found a broad virus‐specific antibody response during acute infection, which evolved into an IgG1‐dominated response. Moreover, we identified a persistent expansion of virus‐targeting naïve‐like and memory B cells in COVID‐19 patients, as well as unique immune signatures that associated with disease severity and inflammation.

mRNA-based vaccines effectively induce protective neutralizing antibodies against SARS-CoV-2, the etiological agent of COVID-19. Yet, the kinetics and compositional patterns of vaccine-induced antibody responses to the original strain and emerging variants of concern remain largely unknown. Here we characterized serum antibody classes and subclasses targeting the spike receptor-binding domain of SARS-CoV-2 wild type and α, β, γ and δ variants in a longitudinal cohort of SARS-CoV-2 naïve and COVID-19 recovered individuals receiving the mRNA-1273 vaccine. We found that mRNA-1273 vaccine recipients developed a SARS-CoV-2-specific antibody response with a subclass profile comparable to that induced by natural infection. Importantly, these antibody responses targeted both wild type SARS-CoV-2 as well as its α, β, γ and δ variants. Following primary vaccination, individuals with pre-existing immunity showed higher induction of all antibodies but IgG3 compared to SARS-CoV-2-naïve subjects. Unlike naïve individuals, COVID-19 recovered subjects did not mount a recall antibody response upon the second vaccine dose. In these individuals, secondary immunization resulted in a slight reduction of IgG1 against the receptor-binding domain of β and γ variants. Despite the lack of recall humoral response, vaccinees with pre-existing immunity still showed higher titers of IgG1 and IgA to all variants analyzed compared to fully vaccinated naïve individuals. Our findings indicate that mRNA-1273 vaccine triggered cross-variant antibody responses with distinct profiles in vaccinees with or without pre-existing immunity and suggest that individuals with prior history of SARS-CoV-2 infection may not benefit from the second mRNA vaccine dose with the current standard regimen. This study was supported by the COVID-19 call grant from Generalitat de Catalunya, Department of Health (to GM), grant Miguel Servet research program (to GM) and by National Health Institute Carlos III (ISCIII) through the project COV20_00508 grant (Co-funded by European Regional Development Fund/European Social Fund “a way to make Europe) (to RG)

Unraveling the long-term kinetics of antibodies to SARS-CoV-2 and the individual characteristics influencing it, including the impact of pre-existing antibodies to human coronaviruses causing common cold (HCoVs), is essential to understand protective immunity to COVID-19 and devise effective surveillance strategies. IgM, IgA and IgG levels against six SARS-CoV-2 antigens and the nucleocapsid antigen of the four HCoV (229E, NL63, OC43 and HKU1) were quantified by Luminex, and antibody neutralization capacity was assessed by flow cytometry, in a cohort of health care workers followed up to 7 months (N = 578). Seroprevalence increases over time from 13.5% (month 0) and 15.6% (month 1) to 16.4% (month 6). Levels of antibodies, including those with neutralizing capacity, are stable over time, except IgG to nucleocapsid antigen and IgM levels that wane. After the peak response, anti-spike antibody levels increase from ~150 days post-symptom onset in all individuals (73% for IgG), in the absence of any evidence of re-exposure. IgG and IgA to HCoV are significantly higher in asymptomatic than symptomatic seropositive individuals. Thus, pre-existing cross-reactive HCoVs antibodies could have a protective effect against SARS-CoV-2 infection and COVID-19 disease., Long-term characterisation of SARS-CoV-2 antibody kinetics is needed to understand the protective role of the immune response. Here the authors describe antibody levels and neutralisation activity in healthcare workers over seven months and investigate the role of immunity to endemic human coronaviruses.

mRNA-based vaccines effectively induce protective neutralizing antibody responses against SARS-CoV-2, the etiological agent of COVID-19. The specific compositional patterns of these responses remain largely unknown. We found that SARS-CoV-2-naïve individuals receiving the first dose of an mRNA vaccine developed a SARS-CoV-2-specific antibody response with a subclass profile comparable to that induced by the natural infection, except IgA2, which did not increase. SARS-CoV-2-naïve subjects also mounted a robust virus-specific recall response after receiving the second dose. This response increased all IgG subclasses, but boosted neither IgM nor IgA1 and IgA2 subclasses. In contrast, individuals recovered from COVID-19 mounted peak virus-specific antibody responses upon primary immunization and did not further augment such responses following secondary immunization. Remarkably, compared to SARS-CoV-2-naïve subjects, individuals with pre-existing immunity showed increased levels of all virus-specific antibodies but IgG3 following primary vaccination. By dissecting the heterogeneity of mRNA vaccine-induced humoral responses to SARS-CoV-2, our findings indicate that the induction of optimal immune protection may require the development of personalized vaccination programs.

Surveillance tools to estimate infection rates in young populations are essential to guide recommendations for school reopening and management during viral epidemics. Ideally, field-deployable non-invasive, sensitive techniques are required to detect low viral load exposures among asymptomatic children. We determined SARS-CoV-2 antibody conversion by high-throughput Luminex assays in saliva samples collected weekly in 1,509 children and 396 adults in 22 Summer schools and 2 pre-schools in 27 venues in Barcelona, Spain, from June 29thto July 31st2020, between the first and second COVID-19 pandemic waves. Saliva antibody conversion defined as ≥4-fold increase in IgM, IgA and/or IgG levels to SARS-CoV-2 antigens between two visits over a 5-week period was 3.22% (49/1518), or 2.36% if accounting for potentially cross-reactive antibodies, six times higher than the cumulative infection rate (0.53%) by weekly saliva RT-PCR screening. IgG conversion was higher in adults (2.94%, 11/374) than children (1.31%, 15/1144) (p=0.035), IgG and IgA levels moderately increased with age, and antibodies were higher in females. Most antibody converters increased both IgG and IgA antibodies but some augmented either IgG or IgA, with a faster decay over time for IgA than IgG. Nucleocapsid rather than spike was the main antigen target. Anti-spike antibodies were significantly higher in individuals not reporting symptoms than symptomatic individuals, suggesting a protective role against COVID-19. To conclude, saliva antibody profiling including three isotypes and multiplexing antigens is a useful and more user-friendly tool for screening pediatric populations to determine SARS-CoV-2 exposure and guide public health policies during pandemics.

COVID-19 affects children to a lesser extent than adults but they can still get infected and transmit SARS-CoV-2 to their contacts. Field deployable non-invasive sensitive diagnostic techniques are needed to evaluate the infectivity dynamics of the coronavirus in pediatric populations and guide public health interventions.We evaluated the utility of high-throughput Luminex-based assays applied to saliva samples to quantify IgM, IgA and IgG antibodies against five SARS-CoV-2 spike (S) and nucleocapsid (N) antigens in the context of a contacts and infectivity longitudinal study. We compared the antibody levels obtained in saliva versus serum/plasma samples from a group of children and adults tested weekly by RT-PCR over 35 days and diagnosed as positive (n=58), and a group of children and adults who consistently tested negative over the follow up period (n=61), in the Summer of 2020 in Barcelona, Spain.Antibody levels in saliva samples from individuals with confirmed RT-PCR diagnosis of SARS-CoV-2 infection were significantly higher than in negative individuals and correlated with those measured in sera/plasmas. Higher levels of anti-S IgG were found in asymptomatic individuals that could indicate protection against disease in infected individuals. Higher anti-S IgG and IgM levels in serum/plasma and saliva, respectively, in infected children compared to infected adults could also be related to stronger clinical immunity in them. Among infected children, males had higher levels of saliva IgG to N and RBD than females. Despite overall correlation, individual clustering analysis suggested that responses that may not be detected in blood could be patent in saliva, and vice versa, and therefore that both measurements are complementary.In addition to serum/plasma, measurement of SARS-CoV-2-specific saliva antibodies should be considered as a complementary non-invasive assay to better estimate the percentage of individuals who have experienced coronavirus infection. Saliva antibody detection could allow determining COVID-19 prevalence in pediatric populations, alternative to bleeding or nasal swab, and serological diagnosis following vaccination.

Unraveling the long-term kinetics of antibodies to SARS-CoV-2 and its determinants, including the impact of pre-existing antibodies to human coronaviruses causing common cold (HCoVs), is essential to understand protective immunity to COVID-19 and devise effective surveillance strategies. IgM, IgA and IgG levels against six SARS-CoV-2 antigens and the nucleocapsid antigen of the four HCoV (229E,NL63, OC43 and HKU1) were quantified by Luminex, and antibody neutralization capacity was assessed by flow cytometry, in a cohort of health care workers followed-up for 6 months (N = 578). Seroprevalence increased over time from 13.5% (month 0) and 15.6% (month 1) to 16.4% (month 6). Levels of antibodies, including those with neutralizing capacity, were stable over time, except IgG to nucleocapsid antigen and IgM levels that waned. After the peak response, anti-spike antibody levels increased from ∼150 days post-symptom onset in all individuals (73% for IgG), in the absence of any evidence of re-exposure. Pre-existing antibodies to alpha-HCoV were lower in individuals who subsequently seroconverted for SARS-CoV-2. IgG and IgA to HCoV were significantly higher in asymptomatic than symptomatic seropositive individuals. Thus, pre-existing cross-reactive HCoVs antibodies could have a protective effect against SARS-CoV-2 infection and COVID-19 disease.

SummaryThe production of SARS-CoV-2-specific neutralizing antibodies is widely considered as a key mechanism for COVID-19 resolution and protection. However, beyond their protective function, antibodies to SARS-CoV-2 may also participate in disease pathogenesis. To explore the potential relationship between virus-specific humoral responses and COVID-19 immunopathology, we measured serum antibody classes and subclasses to the receptor-binding domain of the SARS-CoV-2 spike protein and the nucleoprotein in a cohort of hospitalized COVID-19 patients with moderate to severe disease. We found that RBD-specific IgG1 and IgG3 dominated the humoral response to SARS-CoV-2, were more abundant in severe patients, and positively correlated with several clinical parameters of inflammation. In contrast, a virus-specific IgA2 response skewed toward RBD rather than NP associated with a more favorable clinical course. Interestingly, RBD-dominant IgA2 responses were mostly detected in patients with gastrointestinal symptoms, suggesting the possible involvement of intrinsically tolerogenic gut immune pathways in the attenuation of virus-induced inflammation and disease resolution.

The blood-brain barrier (BBB) is a formidable obstacle for delivery of biologic therapeutics to central nervous system (CNS) targets. Whilst the BBB prevents passage of the vast majority of molecules, it also selectively transports a wide variety of molecules required to maintain brain homeostasis. Receptor-mediated transcytosis is one example of a macromolecule transport system that is employed by cells of the BBB to supply essential proteins to the brain and which can be utilized to deliver biologic payloads, such as antibodies, across the BBB. In this study, we performed phage display selections on the mouse brain endothelial cell line, bEND.3, to enrich for antibody single-chain variable fragments (scFvs) that could compete for binding with a known BBB-crossing antibody fragment, FC5. A number of these scFvs were converted to IgGs and characterized for their ability to bind to mouse, rat and human brain endothelial cells, and subsequent ability to transport across the BBB. We demonstrated that these newly identified BBB-targeting IgGs had increased brain exposure when delivered peripherally in mice and were also able to transport a biologically active molecule, interleukin-1 receptor antagonist (IL-1RA), into the CNS. The antagonism of the interleukin-1 system within the CNS can result in the relief of neuropathic pain. We demonstrated that the BBB-targeting IgGs were able to elicit an analgesic response in a mouse model of nerve ligation-induced hypersensitivity when fused to IL-1RA.