The development and kinetics of functional antibody-dependent cell-mediated cytotoxicity (ADCC) to SARS-CoV-2 spike protein

A Short Update on the Use of Monoclonal Antibodies in COVID-19

Monoclonal antibodies in the prophylaxis and treatment of COVID-19 have been crucial in reducing severe infections when vaccines were unavailable. However, as the virus and its variants have changed over time, the effectiveness of monoclonal antibodies has been questioned. This technical note highlights the need to assess the antiviral activity of these antibodies against new variants and adapt treatment strategies accordingly. On the one hand, in vitro studies have suggested reduced susceptibility of the latest variants to monoclonal antibodies, whereas clinical data still show benefits in reducing severe illness and mortality, indicating that laboratory results do not always mirror real-world outcomes. As a result, although resistance to monoclonal antibodies can develop over time, they could still have an important role in COVID-19 treatment, especially when used in combination, and ongoing research aims to identify effective antibodies against new variants.

In Silico Design and Evaluation of a Novel Therapeutic Agent Against the Spike Protein as a Novel Treatment Strategy for COVID-19 Treatment

BACKGROUND

Coronavirus disease 2019 (COVID-19) is a viral respiratory disease that is associated with severe damage to other human organs. It causes by a novel coronavirus, and it is spreading all over the world. To date, there is some approved vaccine or therapeutic agent which could be effective against this disease. But their effectiveness against mutated strains is not studied completely. The spike glycoprotein on the surface of the coronaviruses gives the virus the ability to bind to host cell receptors and enter cells. Inhibition of attachment of these spikes can lead to virus neutralization by inhibiting viral entrance.

AIMS

In this study, we tried to use the virus entrance strategy against itself by utilizing virus receptor (ACE-2) in order to design an engineered protein consisting of a human Fc antibody fragment and a part of ACE-2, which reacts with virus RBD, and we also evaluated this interaction by computational methods and in silico methods. Subsequently, we have designed a new protein structure to bind with this site and inhibit the virus from attaching to its cell receptor, mechanically or chemically.

METHODS

Various in silico software, bioinformatics, and patent databases were used to retrieve the requested gene and protein sequences. The physicochemical properties and possibility of allergenicity were also examined. Three-dimensional structure prediction and molecular docking were also performed to develop the most suitable therapeutic protein.

RESULTS

The designed protein consisted of a total of 256 amino acids with a molecular weight of 28984.62 and 5.92 as a theoretical isoelectric point. Instability and aliphatic index and grand average of hydropathicity are 49.99, 69.57 and -0.594, respectively.

CONCLUSIONS

In silico studies can provide a good opportunity to study viral proteins and new drugs or compounds since they do not need direct exposure to infectious agents or equipped laboratories. The suggested therapeutic agent should be further characterized in vitro and in vivo.

SARS-CoV-2 Antibody Profiles in Maternal Serum and Breast Milk Following mRNA COVID-19 Vaccination: A Longitudinal Prospective Observational Cohort Study

COVID-19 vaccination during pregnancy protects infants against symptomatic COVID-19. Vaccination of lactating mothers may offer additional protection, but our understanding of immune responses in breast milk is limited. We, therefore, performed a single-center prospective cohort study of lactating mothers who received a COVID-19 mRNA primary vaccine series to evaluate the durability, breadth, and neutralizing capacity of the antibody responses in breast milk. Spike IgG- and IgA-binding antibodies of ancestral SARS-CoV-2 in serum and breast milk were quantified over 9 months using Meso Scale Discovery (MSD) V-PLEX assays, and ancestral titers were compared to four variants of concern (Alpha, Beta, Delta, Gamma) at a single time point. Neutralizing antibodies against ancestral SARS-CoV-2 and Omicron BA.4/5 were compared before and after vaccination using a pseudovirus-neutralization assay. Eleven lactating mothers received either Pfizer BNT162b2 (7/11) or Moderna mRNA-1273 (4/11) vaccine primary series. IgG and IgA titers increased in serum and breast milk following each dose, peaking 1-4 weeks after series completion. Titers remained significantly elevated for 7-9 months, except for in breast milk IgA which returned to baseline within 1 month. Furthermore, binding antibodies against all included variants were detected in breast milk collected 1-3 weeks after series completion. However, while vaccination induced a strong neutralizing response against ancestral SARS-CoV-2 in serum and more modest response in breast milk, it did not induce neutralizing antibodies against Omicron BA.4/5 in either specimen type. This study demonstrates that maternal COVID-19 mRNA vaccination may enhance immune protection for infants through breast milk via increased IgG- and IgA-binding-and-neutralizing antibodies; although, variant-specific boosters may be required to optimize immune protection.

The Potential of Probiotics as Ingestible Adjuvants and Immune Modulators for Antiviral Immunity and Management of SARS-CoV-2 Infection and COVID-19

Probiotic bacteria are able to modulate general antiviral responsiveness, including barrier functionality and innate and adaptive immune responses. The COVID-19 pandemic, resulting from SARS-CoV-2 infection, has created a need to control and treat this viral infection and its ensuing immunopathology with a variety of approaches; one such approach may involve the administration of probiotic bacteria. As with most viral infections, its pathological responses are not fully driven by the virus, but are significantly contributed to by the host's immune response to viral infection. The potential adoption of probiotics in the treatment of COVID-19 will have to appreciate the fine line between inducing antiviral immunity without over-provoking immune inflammatory responses resulting in host-derived immunopathological tissue damage. Additionally, the effect exerted on the immune system by SARS-CoV-2 evasion strategies will also have to be considered when developing a robust response to this virus. This review will introduce the immunopathology of COVID-19 and the immunomodulatory effects of probiotic strains, and through their effects on a range of respiratory pathogens (IAV, SARS-CoV, RSV), as well as SARS-CoV-2, will culminate in a focus on how these bacteria can potentially manipulate both infectivity and immune responsiveness via barrier functionality and both innate and adaptive immunity. In conclusion, the harnessing of induction and augmentation of antiviral immunity via probiotics may not only act as an ingestible adjuvant, boosting immune responsiveness to SARS-CoV-2 infection at the level of barrier integrity and innate and adaptive immunity, but also act prophylactically to prevent infection and enhance protection afforded by current vaccine regimens.

Serologic responses to COVID-19 vaccination in children with history of multisystem inflammatory syndrome (MIS-C)

Understanding the serological responses to COVID-19 vaccination in children with history of MIS-C could inform vaccination recommendations. We prospectively enrolled seven children hospitalized with MIS-C and measured SARS-CoV-2 binding IgG antibodies to spike protein variants longitudinally pre- and post-Pfizer-BioNTech BNT162b2 primary series COVID-19 vaccination. We found that SARS-CoV-2 variant cross-reactive IgG antibodies variably waned following acute MIS-C, but were significantly boosted with vaccination and maintained for up to 3 months. We then compared post-vaccination binding, pseudovirus neutralizing, and functional antibody-dependent cell-mediated cytotoxicity (ADCC) titers to the reference strain (Wuhan-hu-1) and Omicron variant (B.1.1.529) among previously healthy children (n = 16) and children with history of MIS-C (n = 7) or COVID-19 (n = 8). Despite the breadth of binding antibodies elicited by vaccination in all three groups, pseudovirus neutralizing and ADCC titers were significantly reduced to the Omicron variant.

Pre-pandemic SARS-CoV-2-specific IFN-γ and antibody responses were low in Ugandan samples and significantly reduced in HIV-positive specimens

Introduction

We investigated whether prior SARS-CoV-2-specific IFN-γ and antibody responses in Ugandan COVID-19 pre-pandemic specimens aligned to this population's low disease severity.

Methods

We used nucleoprotein (N), spike (S), NTD, RBD, envelope, membrane, SD1/2-directed IFN-γ ELISpots, and an S- and N-IgG antibody ELISA to screen for SARS-CoV-2-specific cross-reactivity.

Results

HCoV-OC43-, HCoV-229E-, and SARS-CoV-2-specific IFN-γ occurred in 23, 15, and 17 of 104 specimens, respectively. Cross-reactive IgG was more common against the nucleoprotein (7/110, 15.5%; p = 0.0016, Fishers' Exact) than the spike (3/110, 2.72%). Specimens lacking anti-HuCoV antibodies had higher rates of pre-epidemic SARS-CoV-2-specific IFN-γ cross-reactivity (p-value = 0.00001, Fishers' exact test), suggesting that exposure to additional factors not examined here might play a role. SARS-CoV-2-specific cross-reactive antibodies were significantly less common in HIV-positive specimens (p=0.017; Fishers' Exact test). Correlations between SARS-CoV-2- and HuCoV-specific IFN-γ responses were consistently weak in both HIV negative and positive specimens.

Discussion

These findings support the existence of pre-epidemic SARS-CoV-2-specific cellular and humoral cross-reactivity in this population. The data do not establish that these virus-specific IFN-γ and antibody responses are entirely specific to SARS-CoV-2. Inability of the antibodies to neutralise SARS-CoV-2 implies that prior exposure did not result in immunity. Correlations between SARS-CoV-2 and HuCoV-specific responses were consistently weak, suggesting that additional variables likely contributed to the pre-epidemic cross-reactivity patterns. The data suggests that surveillance efforts based on the nucleoprotein might overestimate the exposure to SARS-CoV-2 compared to inclusion of additional targets, like the spike protein. This study, while limited in scope, suggests that HIV-positive people are less likely than HIV-negative people to produce protective antibodies against SARS-CoV-2.

Bulk IgG glycosylation predicts COVID-19 severity and vaccine antibody response

Although vaccination efforts have expanded, there are still gaps in our understanding surrounding the immune response to SARS-CoV-2. Measuring IgG Fc glycosylation provides insight into an infected individual's inflammatory state, among other functions. We set out to interrogate bulk IgG glycosylation changes from SARS-CoV-2 infection and vaccination, using plasma from mild or hospitalized COVID-19 patients, and from vaccinated individuals. Inflammatory glycans are elevated in hospitalized COVID-19 patients and increase over time, while mild patients have anti-inflammatory glycans that increase over time, including increased sialic acid correlating with RBD antibody levels. Vaccinated individuals with low RBD antibody levels and low neutralization have the same IgG glycan traits as hospitalized COVID-19 patients. In addition, a small vaccinated cohort reveals a decrease in inflammatory glycans associated with peak IgG concentrations and neutralization. This report characterizes the bulk IgG glycome associated with COVID-19 severity and vaccine responsiveness and can help guide future studies into SARS-CoV-2 protective immunity.

COVID-19 and plasma cells: Is there long-lived protection?

Infection with SARS-CoV-2, the etiology of the ongoing COVID-19 pandemic, has resulted in over 450 million cases with more than 6 million deaths worldwide, causing global disruptions since early 2020. Memory B cells and durable antibody protection from long-lived plasma cells (LLPC) are the mainstay of most effective vaccines. However, ending the pandemic has been hampered by the lack of long-lived immunity after infection or vaccination. Although immunizations offer protection from severe disease and hospitalization, breakthrough infections still occur, most likely due to new mutant viruses and the overall decline of neutralizing antibodies after 6 months. Here, we review the current knowledge of B cells, from extrafollicular to memory populations, with a focus on distinct plasma cell subsets, such as early-minted blood antibody-secreting cells and the bone marrow LLPC, and how these humoral compartments contribute to protection after SARS-CoV-2 infection and immunization.

The humoral response and antibodies against SARS-CoV-2 infection

Two and a half years into the COVID-19 pandemic, we have gained many insights into the human antibody response to the causative SARS-CoV-2 virus. In this Review, we summarize key observations of humoral immune responses in people with COVID-19, discuss key features of infection- and vaccine-induced neutralizing antibodies, and consider vaccine designs for inducing antibodies that are broadly protective against different variants of the SARS-CoV-2 virus.

A Qualitative Comparison of the Abbott SARS-CoV-2 IgG II Quant Assay against Commonly Used Canadian SARS-CoV-2 Enzyme Immunoassays in Blood Donor Retention Specimens, April 2020 to March 2021

Our group has previously used laboratory and commercially developed assays to understand the IgG responses to SARS-CoV-2 antigens, including nucleocapsid (N), spike (S), and receptor binding domain (RBD), in Canadian blood donors. In this current study, we analyzed 17,428 available and previously characterized retention samples collected from April 2020 to March 2021. The analysis compared the characteristics of the Abbott SARS-CoV-2 IgG II Quant assay (Abbott anti-spike [S], Abbott, Chicago, IL) against four other IgG assays. The Abbott anti-S assay has a qualitative threshold of 50 AU/mL. The four comparator assays were the Abbott anti-nucleocapsid (N) assay and three commonly used Canadian in-house IgG enzyme-linked immunosorbent assays (ELISAs) recognizing distinct recombinant viral antigens, full-length spike glycoprotein, glycoprotein RBD, and nucleocapsid. The strongest qualitative relationship was between Sinai RBD and the Abbott anti-S assay (kappa, 0.707; standard error [SE] of kappa, 0.018; 95% confidence interval, 0.671 to 0.743). We then scored each previously characterized specimen as positive when two anti-SARS-COV-2 assays identified anti-SARS-CoV-2 IgG in the specimen. Using this composite reference standard approach, the sensitivity of the Abbott anti-S assay was 95.96% (95% confidence interval [CI], 93.27 to 97.63%). The specificity of the Abbott anti-S assay was 99.35% (95% CI, 99.21 to 99.46%). Our study provides context on the use of commonly used SARS-CoV-2 serologies in Canada and identifies how these assays qualitatively compare to newer commercial assays. Our next steps are to assess how well the Abbott anti-S assays quantitatively detect wild-type and SARS-CoV-2 variants of concern. IMPORTANCE We describe the qualitative test characteristics of the Abbott SARS-CoV-2 IgG II Quant assay against four other anti-SARS-CoV-2 IgG assays commonly used in Canada. Although there is no gold standard for identifying anti-SARS-CoV-2 seropositivity, aggregate standards can be used to assess seropositivity. In this study, we used a specimen bank of previously well-characterized specimens collected between April 2020 and March 2021. The Abbott anti-S assay showed the strongest qualitative relationship with a widely used laboratory-developed IgG assay for the SARS-CoV-2 receptor binding domain. Using the composite reference standard approach, we also showed that the Abbott anti-S assay was highly sensitive and specific. As new anti-SARS-CoV-2 assays are developed, it is important to compare their test characteristics against other assays that have been extensively used in prior research.

Molecular characteristics, immune evasion, and impact of SARS-CoV-2 variants

The persistent COVID-19 pandemic since 2020 has brought an enormous public health burden to the global society and is accompanied by various evolution of the virus genome. The consistently emerging SARS-CoV-2 variants harboring critical mutations impact the molecular characteristics of viral proteins and display heterogeneous behaviors in immune evasion, transmissibility, and the clinical manifestation during infection, which differ each strain and endow them with distinguished features during populational spread. Several SARS-CoV-2 variants, identified as Variants of Concern (VOC) by the World Health Organization, challenged global efforts on COVID-19 control due to the rapid worldwide spread and enhanced immune evasion from current antibodies and vaccines. Moreover, the recent Omicron variant even exacerbated the global anxiety in the continuous pandemic. Its significant evasion from current medical treatment and disease control even highlights the necessity of combinatory investigation of the mutational pattern and influence of the mutations on viral dynamics against populational immunity, which would greatly facilitate drug and vaccine development and benefit the global public health policymaking. Hence in this review, we summarized the molecular characteristics, immune evasion, and impacts of the SARS-CoV-2 variants and focused on the parallel comparison of different variants in mutational profile, transmissibility and tropism alteration, treatment effectiveness, and clinical manifestations, in order to provide a comprehensive landscape for SARS-CoV-2 variant research.

Cell-based reporter assays for measurements of antibody-mediated cellular cytotoxicity and phagocytosis against SARS-CoV-2 spike protein

The COVID-19 pandemic caused by SARS-CoV-2 infections has led to excess deaths worldwide. Neutralizing antibodies (nAbs) against viral spike protein acquired from natural infections or vaccinations contribute to protection against new- and re-infections. Besides neutralization, antibody-mediated cellular cytotoxicity (ADCC) and phagocytosis (ADCP) are also important for viral clearance. However, due to the lack of convenient methods, the ADCC and ADCP responses elicited by viral infections or vaccinations remain to be explored. Here, we developed cell-based assays using target cells stably expressing SARS-CoV-2 spikes and Jurkat-NFAT-CD16a/CD32a effector cells for ADCC/ADCP measurements of monoclonal antibodies and human convalescent COVID-19 plasmas (HCPs). In control samples (n = 190), the specificity was 99.5% (95%CI: 98.4–100%) and 97.4% (95%CI: 95.1–99.6%) for the ADCC and ADCP assays, respectively. Among 87 COVID-19 HCPs, 83 (sensitivity: 95.4%, 95%CI: 91.0–99.8%) and 81 (sensitivity: 93.1%, 95%CI: 87.8–98.4%) showed detectable ADCC (titer range: 7.4–1721.6) and ADCP activities (titer range: 4–523.2). Notably, both ADCC and ADCP antibody titers positively correlated with the nAb titers in HCPs. In summary, we developed new tools for quantitative ADCC and ADCP analysis against SARS-CoV-2, which may facilitate further evaluations of Fc-mediated effector functions in preventing and treating against SARS-CoV-2.

Functional antibody responses to SARS-CoV-2 variants in children with COVID-19, MIS-C, and after two doses of BNT162b2 vaccination

Background

Although neutralizing antibodies to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) correlate with protection against coronavirus disease 2019 (COVID-19), little is known about the neutralizing and antibody-dependent cell-mediated cytotoxicity (ADCC) responses to COVID-19, multisystem inflammatory syndrome in children (MIS-C), and COVID-19 vaccination in children.

Methods

We enrolled children 0–21 years of age with a history of COVID-19 (n = 13), MIS-C (n = 13), or 2 doses of BNT162b2 vaccination (n = 14) into a phlebotomy protocol. We measured pseudovirus neutralizing and functional ADCC antibodies to SARS-CoV-2 variants, including Omicron (B.1.1.529).

Results

The primary BNT162b2 vaccination series elicited higher neutralizing and ADCC responses with greater breadth to SARS-CoV-2 variants than COVID-19 or MIS-C, although these were diminished against Omicron.

Conclusions

Serologic responses were significantly reduced against variants, particularly Omicron.

Impact of an Immune Modulator Mycobacterium-w on Adaptive Natural Killer Cells and Protection Against COVID-19

The kinetics of NKG2C+ adaptive natural killer (ANK) cells and NKG2A+inhibitory NK (iNK) cells with respect to the incidence of SARS-CoV-2 infection were studied for 6 months in a cohort of healthcare workers following the administration of the heat-killed Mycobacterium w (Mw group) in comparison to a control group. In both groups, corona virus disease 2019 (COVID-19) correlated with lower NKG2C+ANK cells at baseline. There was a significant upregulation of NKG2C expression and IFN-γ release in the Mw group (p=0.0009), particularly in those with a lower baseline NKG2C expression, along with the downregulation of iNK cells (p<0.0001). This translated to a significant reduction in the incidence and severity of COVID-19 in the Mw group (incidence risk ratio-0.15, p=0.0004). RNA-seq analysis at 6 months showed an upregulation of the ANK pathway genes and an enhanced ANK-mediated antibody-dependent cellular cytotoxicity (ADCC) signature. Thus, Mw was observed to have a salutary impact on the ANK cell profile and a long-term upregulation of ANK-ADCC pathways, which could have provided protection against COVID-19 in a non-immune high-risk population.

Natural killer cell-mediated ADCC in SARS-CoV-2-infected individuals and vaccine recipients

COVID‐19, caused by SARS‐CoV‐2, has emerged as a global pandemic. While immune responses of the adaptive immune system have been in the focus of research, the role of NK cells in COVID‐19 remains less well understood. Here, we characterized NK cell‐mediated SARS‐CoV‐2 antibody‐dependent cellular cytotoxicity (ADCC) against SARS‐CoV‐2 spike‐1 (S1) and nucleocapsid (NC) protein. Serum samples from SARS‐CoV‐2 resolvers induced significant CD107a‐expression by NK cells in response to S1 and NC, while serum samples from SARS‐CoV‐2‐negative individuals did not. Furthermore, serum samples from individuals that received the BNT162b2 vaccine induced strong CD107a expression by NK cells that increased with the second vaccination and was significantly higher than observed in infected individuals. As expected, vaccine‐induced responses were only directed against S1 and not against NC protein. S1‐specific CD107a responses by NK cells were significantly correlated to NK cell‐mediated killing of S1‐expressing cells. Interestingly, screening of serum samples collected prior to the COVID‐19 pandemic identified two individuals with cross‐reactive antibodies against SARS‐CoV‐2 S1, which also induced degranulation of NK cells. Taken together, these data demonstrate that antibodies induced by SARS‐CoV‐2 infection and anti‐SARS‐CoV‐2 vaccines can trigger significant NK cell‐mediated ADCC activity, and identify some cross‐reactive ADCC‐activity against SARS‐CoV‐2 by endemic coronavirus‐specific antibodies.

Durability of ChAdOx1 nCoV-19 vaccination in people living with HIV

Duration of protection from SARS-CoV-2 infection in people living with HIV (PWH) following vaccination is unclear. In a substudy of the phase II/III the COV002 trial (NCT04400838), 54 HIV+ male participants on antiretroviral therapy (undetectable viral loads, CD4+ T cells > 350 cells/μL) received 2 doses of ChAdOx1 nCoV-19 (AZD1222) 4-6 weeks apart and were followed for 6 months. Responses to vaccination were determined by serology (IgG ELISA and Meso Scale Discovery [MSD]), neutralization, ACE-2 inhibition, IFN-γ ELISpot, activation-induced marker (AIM) assay and T cell proliferation. We show that, 6 months after vaccination, the majority of measurable immune responses were greater than prevaccination baseline but with evidence of a decline in both humoral and cell-mediated immunity. There was, however, no significant difference compared with a cohort of HIV-uninfected individuals vaccinated with the same regimen. Responses to the variants of concern were detectable, although they were lower than WT. Preexisting cross-reactive T cell responses to SARS-CoV-2 spike were associated with greater postvaccine immunity and correlated with prior exposure to beta coronaviruses. These data support the ongoing policy to vaccinate PWH against SARS-CoV-2, and they underpin the need for long-term monitoring of responses after vaccination.

Development of flow cytometry-based assays to assess the ability of antibodies to bind to SARS-CoV-2-infected and Spike-transfected cells and mediate NK cell degranulation

Since the beginning of the SARS-CoV-2 pandemic, antibody responses and antibody effector functions targeting SARS-CoV-2-infected cells have been understudied. Consequently, the role of these types of antibodies in SARS-CoV-2 disease (COVID-19) and immunity is still undetermined. To provide tools to study these responses, we used plasma from SARS-CoV-2-infected individuals (n=50) and SARS-CoV-2 naive healthy controls (n=20) to develop four specific and reproducible flow cytometry-based assays: (i) two assessing antibody binding to, and antibody-mediated NK cell degranulation against, SARS-CoV-2-infected cells and (ii) two assessing antibody binding to, and antibody-mediated NK cell degranulation against, SARS-CoV-2 Spike-transfected cells. All four assays demonstrated the ability to detect the presence of these functional antibody responses in a specific and reproducible manner. Interestingly, we found weak to moderate correlations between the four assays (Spearman rho ranged from 0.50-0.74), suggesting limited overlap in the responses captured by the individual assays. Lastly, while we initially developed each assay with multiple dilutions in an effort to capture the full relationship between antibody titers and assay outcome, we explored the relationship between fewer antibody dilutions and the full dilution series for each assay to reduce assay costs and improve assay efficiency. We found high correlations between the full dilution series and fewer or single dilutions of plasma. Use of single or fewer sample dilutions to accurately determine the response rates and magnitudes of the responses allows for high-throughput use of these assays platforms to facilitate assessment of antibody responses elicited by SARS-CoV-2 infection and vaccination in large clinical studies. This article is protected by copyright. All rights reserved.

Functional antibody-dependent cell mediated cytotoxicity (ADCC) responses to vaccine and circulating influenza strains following vaccination

Novel cell-based assays were developed to assess antibody-dependence cellular cytotoxicity (ADCC) antibodies against both vaccine and a representative circulation strain HA and NA proteins for the 2014-15 influenza season. The four assays using target cells stably expressing one of the four proteins worked well. In pre- and post-vaccine sera from 70 participants in a pre-season vaccine trial, we found ADCC antibodies and a rise in ADCC antibody titer against target cells expressing the 4 proteins but a much higher titer for the vaccine than the circulating HA in both pre-and post-vaccine sera. These differences in HA ADCC antibodies were not reflected in differences in HA binding antibodies. Our observations suggested that relatively minor changes on the subtype HA can result in large differences in ADCC activity.

Overview of Memory NK Cells in Viral Infections: Possible Role in SARS-CoV-2 Infection

NK cells have usually been defined as cells of the innate immune system, although they are also involved in adaptative responses. These cells belong to the innate lymphocyte cells (ILC) family. They remove unwanted cells, tumoral cells and pathogens. NK cells are essential for viral infection clearance and are involved in tolerogenic responses depending on the dynamic balance of the repertoire of activating and inhibitory receptors. NK plasticity is crucial for tissue function and vigilant immune responses. They directly eliminate virus-infected cells by recognising viral protein antigens using a non-MHC dependent mechanism, recognising viral glycan structures and antigens by NCR family receptors, inducing apoptosis by Fas-Fas ligand interaction, and killing cells by antibody-dependent cell cytotoxicity via the FcγIII receptor. Activating receptors are responsible for the clearance of virally infected cells, while inhibitory KIR receptor activation impairs NK responses and facilitates virus escape. Effective NK memory cells have been described and characterised by a low NKG2A and high NKG2C or NKG2D expression. NK cells have also been used in cell therapy. In SARS-CoV-2 infection, several contradicting reports about the role of NK cells have been published. A careful analysis of the current data and possible implications will be discussed.

Antibody-dependent cellular cytotoxicity response to SARS-CoV-2 in COVID-19 patients

Antibody-dependent cellular cytotoxicity (ADCC) responses to viral infection are a form of antibody regulated immune responses mediated through the Fc fragment. Whether severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) triggered ADCC responses contributes to COVID-19 disease development is currently not well understood. To understand the potential correlation between ADCC responses and COVID-19 disease development, we analyzed the ADCC activity and neutralizing antibody response in 255 individuals ranging from asymptomatic to fatal infections over 1 year post disease. ADCC was elicited by 10 days post-infection, peaked by 11-20 days, and remained detectable until 400 days post-infection. In general, patients with severe disease had higher ADCC activities. Notably, patients who had severe disease and recovered had higher ADCC activities than patients who had severe disease and deceased. Importantly, ADCC activities were mediated by a diversity of epitopes in SARS-COV-2-infected mice and induced to comparable levels against SARS-CoV-2 variants of concern (VOCs) (B.1.1.7, B.1.351, and P.1) as that against the D614G mutant in human patients and vaccinated mice. Our study indicates anti-SARS-CoV-2 ADCC as a major trait of COVID-19 patients with various conditions, which can be applied to estimate the extra-neutralization level against COVID-19, especially lethal COVID-19.

Impaired Antibody-Dependent Cellular Cytotoxicity in a Spanish Cohort of Patients With COVID-19 Admitted to the ICU

SARS-CoV-2 infection causes COVID-19, ranging from mild to critical disease in symptomatic subjects. It is essential to better understand the immunologic responses occurring in patients with the most severe outcomes. In this study, parameters related to the humoral immune response elicited against SARS-CoV-2 were analysed in 61 patients with different presentations of COVID-19 who were recruited in Hospitals and Primary Healthcare Centres in Madrid, Spain, during the first pandemic peak between April and June 2020. Subjects were allocated as mild patients without hospitalization, severe patients hospitalized or critical patients requiring ICU assistance. Critical patients showed significantly enhanced levels of B cells with memory and plasmablast phenotypes, as well as higher levels of antibodies against SARS-CoV-2 with neutralization ability, which were particularly increased in male gender. Despite all this, antibody-dependent cell-mediated cytotoxicity was defective in these individuals. Besides, patients with critical COVID-19 also showed increased IgG levels against herpesvirus such as CMV, EBV, HSV-1 and VZV, as well as detectable CMV and EBV viremia in plasma. Altogether, these results suggest an enhanced but ineffectual immune response in patients with critical COVID-19 that allowed latent herpesvirus reactivation. These findings should be considered during the clinical management of these patients due to the potential contribution to the most severe disease during SARS-CoV-2 infection.

Evidence for antibody as a protective correlate for COVID-19 vaccines

A correlate of protection (CoP) is urgently needed to expedite development of additional COVID-19 vaccines to meet unprecedented global demand. To assess whether antibody titers may reasonably predict efficacy and serve as the basis of a CoP, we evaluated the relationship between efficacy and in vitro neutralizing and binding antibodies of 7 vaccines for which sufficient data have been generated. Once calibrated to titers of human convalescent sera reported in each study, a robust correlation was seen between neutralizing titer and efficacy (ρ = 0.79) and binding antibody titer and efficacy (ρ = 0.93), despite geographically diverse study populations subject to different forces of infection and circulating variants, and use of different endpoints, assays, convalescent sera panels and manufacturing platforms. Together with evidence from natural history studies and animal models, these results support the use of post-immunization antibody titers as the basis for establishing a correlate of protection for COVID-19 vaccines.