Production and characterisation of a SARSCoV-2 S-protein RBD homodimer with increased avidity for specific antibodies.

Monitoring of the proportion of immune individuals and the effectiveness of vaccination in a population involves evaluation of several important parameters, including the level of virus-neutralising antibodies. In order to combat the COVID-19 pandemic, it is essential to develop approaches to detecting SARS-CoV-2 neutralising antibodies by safe, simple and rapid methods that do not require live viruses. To develop a test system for enzyme-linked immunosorbent assay (ELISA) that detects potential neutralising antibodies, it is necessary to obtain a highly purified recombinant receptor-binding domain (RBD) of the spike (S) protein with high avidity for specific antibodies. The aim of the study was to obtain and characterise a SARSCoV-2 S-protein RBD homodimer and a recombinant RBD-expressing cell line, as well as to create an ELISA system for detecting potential neutralising antibodies. Material(s) and Method(s): the genetic construct was designed in silico. To generate a stable producer cell line, the authors transfected CHO-S cells, subjected them to antibiotic pressure, and selected the optimal clone. To isolate monomeric and homodimeric RBD forms, the authors purified the recombinant RBD by chromatographic methods. Further, they analysed the activity of the RBD forms by Western blotting, bio-layer interferometry, and indirect ELISA. The analysis involved monoclonal antibodies GamXRH19, GamP2C5, and h6g3, as well as serum samples from volunteers vaccinated with Gam-COVID-Vac (Sputnik V) and unvaccinated ones. Result(s): the authors produced the CHO-S cell line for stable expression of the recombinant SARS-CoV-2 S-protein RBD. The study demonstrated the recombinant RBD's ability to homodimerise after fed-batch cultivation of the cell line for more than 7 days due to the presence of unpaired cysteines. The purified recombinant RBD yield from culture broth was 30-50 mg/L. Monomeric and homodimeric RBD forms were separated using gel-filtration chromatography and characterised by their ability to interact with specific monoclonal antibodies, as well as with serum samples from vaccinated volunteers. The homodimeric recombinant RBD showed increased avidity for both monoclonal and immune sera antibodies. Conclusion(s): the homodimeric recombinant RBD may be more preferable for the analysis of levels of antibodies to the receptor-binding domain of the SARS-CoV-2 S protein.Copyright © 2023 Authors. All rights reserved.

Noncovalent antibody catenation on a target surface greatly increases the antigen-binding avidity.

Immunoglobulin G (IgG) antibodies are widely used for diagnosis and therapy. Given the unique dimeric structure of IgG, we hypothesized that, by genetically fusing a homodimeric protein (catenator) to the C-terminus of IgG, reversible catenation of antibody molecules could be induced on a surface where target antigen molecules are abundant, and that it could be an effective way to greatly enhance the antigen-binding avidity. A thermodynamic simulation showed that quite low homodimerization affinity of a catenator, e.g. dissociation constant of 100 µM, can enhance nanomolar antigen-binding avidity to a picomolar level, and that the fold enhancement sharply depends on the density of the antigen. In a proof-of-concept experiment where antigen molecules are immobilized on a biosensor tip, the C-terminal fusion of a pair of weakly homodimerizing proteins to three different antibodies enhanced the antigen-binding avidity by at least 110 or 304 folds from the intrinsic binding avidity. Compared with the mother antibody, Obinutuzumab(Y101L) which targets CD20, the same antibody with fused catenators exhibited significantly enhanced binding to SU-DHL5 cells. Together, the homodimerization-induced antibody catenation would be a new powerful approach to improve antibody applications, including the detection of scarce biomarkers and targeted anticancer therapies.

SARS-CoV-2 IgG Spike antibody levels and avidity in natural infection or following vaccination with mRNA-1273 or BNT162b2 vaccines.

Certain aspects of the immunogenicity and effectiveness of the messenger ribonucleic acid (mRNA) vaccines (mRNA-1273 and BNT162b2) developed in response to the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) pandemic are still uncharacterized. Serum or plasma samples from healthy donor recipients of either vaccine (BNT162b2 n = 53, mRNA-1273 n = 49; age 23-67), and individuals naturally infected with SARS-CoV-2 (n = 106; age 18-82) were collected 0-2 months post-infection or 1- and 4 months after second dose of vaccination. Anti-Spike antibody levels and avidity were measured via an enzyme-linked immunosorbent assay (ELISA). Overall, vaccination induced higher circulating anti-Spike protein immunoglobulin G (IgG) antibody levels and avidity compared to infection at similar time intervals. Both vaccines produced similar anti-Spike IgG concentrations at 1 month, while mRNA-1273 demonstrated significantly higher circulating antibody concentrations after 4 months. mRNA-1273 induced significantly higher avidity at month 1 compared to BNT162b2 across all age groups. However, the 23-34 age group was the only group to maintain statistical significance by 4 months. Male BNT162b2 recipients were approaching statistically significant lower anti-Spike IgG avidity compared to females by month 4. These findings demonstrate enhanced anti-Spike IgG levels and avidity following vaccination compared to natural infection. In addition, the mRNA-1273 vaccine induced higher antibody levels by 4 months compared to BNT162b2.

Binding and avidity signatures of polyclonal sera from individuals with different exposure histories to SARS-CoV-2 infection, vaccination, and Omicron breakthrough infections.

BACKGROUND: The number of exposures to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and to vaccination antigens impact the magnitude and avidity of the polyclonal response. METHODS: We studied binding and avidity of different antibody isotypes to the spike, the receptor binding domain (RBD), and the nucleoprotein (NP) of wild type (WT) and BA.1 SARS-CoV-2 in convalescent, mRNA vaccinated, mRNA boosted, hybrid immune individuals, and in individuals with breakthrough cases during the peak of the BA.1 wave. RESULTS: We found an increase in spike binding antibodies and antibody avidity with increasing number of exposures to infection and/or vaccination. Nucleoprotein antibodies were detectible in convalescent individuals and a proportion of breakthrough cases, but displayed low avidity. Omicron breakthrough infections elicited high levels of cross-reactive antibodies between WT and BA.1 antigens in vaccinated individuals without prior infection directed against the spike and receptor binding domain (RBDs). The magnitude of the antibody response and avidity correlated with neutralizing activity against WT virus. CONCLUSIONS: The magnitude and quality of the antibody response increased with the number of antigen exposures, including breakthrough infections. However, cross-reactivity of the antibody response after BA.1 breakthroughs, was impacted by the number of prior antigenic exposures.

Production and characterisation of a SARSCoV-2 S-protein RBD homodimer with increased avidity for specific antibodies

Monitoring of the proportion of immune individuals and the effectiveness of vaccination in a population involves evaluation of several important parameters, including the level of virus-neutralising antibodies. In order to combat the COVID-19 pandemic, it is essential to develop approaches to detecting SARS-CoV-2 neutralising antibodies by safe, simple and rapid methods that do not require live viruses. To develop a test system for enzyme-linked immunosorbent assay (ELISA) that detects potential neutralising antibodies, it is necessary to obtain a highly purified recombinant receptor-binding domain (RBD) of the spike (S) protein with high avidity for specific antibodies. The aim of the study was to obtain and characterise a SARSCoV-2 S-protein RBD homodimer and a recombinant RBD-expressing cell line, as well as to create an ELISA system for detecting potential neutralising antibodies. Material(s) and Method(s): the genetic construct was designed in silico. To generate a stable producer cell line, the authors transfected CHO-S cells, subjected them to antibiotic pressure, and selected the optimal clone. To isolate monomeric and homodimeric RBD forms, the authors purified the recombinant RBD by chromatographic methods. Further, they analysed the activity of the RBD forms by Western blotting, bio-layer interferometry, and indirect ELISA. The analysis involved monoclonal antibodies GamXRH19, GamP2C5, and h6g3, as well as serum samples from volunteers vaccinated with Gam-COVID-Vac (Sputnik V) and unvaccinated ones. Result(s): the authors produced the CHO-S cell line for stable expression of the recombinant SARS-CoV-2 S-protein RBD. The study demonstrated the recombinant RBD's ability to homodimerise after fed-batch cultivation of the cell line for more than 7 days due to the presence of unpaired cysteines. The purified recombinant RBD yield from culture broth was 30-50 mg/L. Monomeric and homodimeric RBD forms were separated using gel-filtration chromatography and characterised by their ability to interact with specific monoclonal antibodies, as well as with serum samples from vaccinated volunteers. The homodimeric recombinant RBD showed increased avidity for both monoclonal and immune sera antibodies. Conclusion(s): the homodimeric recombinant RBD may be more preferable for the analysis of levels of antibodies to the receptor-binding domain of the SARS-CoV-2 S protein.Copyright © 2023 Authors. All rights reserved.

DYNAMICS IN MATURATION OF SARS-CoV-2 RBD-SPECIFIC IgG ANTIBODY AVIDITY DEPENDING ON IMMUNIZATION TIMEFRAME AND TYPE.

The aim is to examine dynamics of avidity maturation of IgG antibodies against SARS-CoV-2 RBD depending on the type of immunization (vaccination or infection), as well as on the duration and frequency of immunization. Materials and methods. The study was performed on two sample cohorts collected at two time points during COVID-19 pandemic. The first cohort (group No. 1) consisted of 87 samples of blood sera obtained from COVID-19 convalescents in the period from March to September 2020. The second cohort included 204 samples obtained in September 2021 from two patient groups. Group No. 2 (n = 64) - patients immunized with a full course of Gam-Covid-Vac, group No. 3 (n = 140) - COVID-19 convalescent patients and subjects vaccinated with Gam-Covid-Vac ("hybrid immunity"). Results and conclusion. The dynamics of avidity maturation for SARS-CoV-2 RBD IgG antibodies depending on the method and frequency of immunization, showed that the most effective immunity was formed in COVID-19 convalescent patients and subjects vaccinated with a full course of Gam-Covid-Vac. The "hybrid" immunity showed not only a significantly higher (compared with groups No. 1 and No. 2) level of IgG antibodies (median 228 BAU/ml vs 75 or 119 BAU/ml, p < 0.001), but also a higher level of avidity (IA 90.5% vs 54.5 and 76.6, respectively, p < 0.001, 4M urea). In the test for assessing the avidity index with the denaturing agent 8M urea in patients with "hybrid immunity", the median level of IA was 25% versus 14.8% and 16% in COVID-19 convalescents and vaccinated subjects (p < 0.001), only in 8 patients IA was higher than 50%. While comparing a single infection of COVID-19 with a full course of Gam-Covid-Vac, it was shown that vaccination leads to higher IgG levels (median values in groups 119 and 75 BAU/ml, p < 0.001) and to a higher avidity index (median 76.6% vs 54.5%). Thus, the more rapid induction of high-avidity antibodies was in vaccinated individuals at early stages of immunization (up to 4 months), during the period when IgG avidity maturation has not yet been completed. Our results showed that during this period vaccination leads to production of antibodies with avidity index at median level of 82% versus 36% in COVID-19 convalescents at similar time point.Copyright © 2023 Saint Petersburg Pasteur Institute. All rights reserved.

SARS-CoV-2 antibody avidity responses in natural infection and vaccination

Background: In ongoing SARS CoV-2 pandemic, understanding antibody responses have played a key role in measuring extent of exposure, protection from reinfection, vaccine efficacy and serodiagnosis. Antibody avidity is total binding strength of immunoglobulin G (IgG) toward its target epitope. High antibody avidity has been correlated with effective neutralization of the SARSCoV-2 virus. However, the data on avidity responses against COVID-19 infection and vaccination are limited. Objective(s): To understand the avidity responses among sera of naturally infected, recovered COVID-19 patients;naive Covaxin, Covishield vaccinees and breakthrough infections. Material(s) and Method(s): In this study, we utilized an in-house developed SARS-CoV-2 anti-spike receptor binding domain (SRBD) IgG ELISA to optimize the avidity assay. A panel of anti-SARS-CoV- 2 SRBD IgG positive serum samples were treated with known concentration of a chaotropic agent (urea) for disruption of the noncovalent interactions of the antigen-antibody complex. This disruption causes low avidity antibodies to dissociate which gives the percentage of high avidity antibodies present in a serum sample. Additionally, the optimized assay was used to understand the avidity responses among sera belonging to individuals naturally infected and recovered after COVID-19, naive Covaxin and Covishield vaccinees;followed by breakthrough infections. Result(s) and Conclusion(s): The anti-SRBD avidity progressively elevated over a period of twelve months. Moreover, overall antibody avidity responses were similar in the case of natural infection and naive two doses of Covaxin and Covishield vaccinated individuals. However, avidity responses were high among individuals with a breakthrough infection as compared to naive vaccinees.

Production and characterisation of a SARSCoV-2 S-protein RBD homodimer with increased avidity for specific antibodies.

Monitoring of the proportion of immune individuals and the effectiveness of vaccination in a population involves evaluation of several important parameters, including the level of virus-neutralising antibodies. In order to combat the COVID-19 pandemic, it is essential to develop approaches to detecting SARS-CoV-2 neutralising antibodies by safe, simple and rapid methods that do not require live viruses. To develop a test system for enzyme-linked immunosorbent assay (ELISA) that detects potential neutralising antibodies, it is necessary to obtain a highly purified recombinant receptor-binding domain (RBD) of the spike (S) protein with high avidity for specific antibodies. The aim of the study was to obtain and characterise a SARSCoV-2 S-protein RBD homodimer and a recombinant RBD-expressing cell line, as well as to create an ELISA system for detecting potential neutralising antibodies. Material(s) and Method(s): the genetic construct was designed in silico. To generate a stable producer cell line, the authors transfected CHO-S cells, subjected them to antibiotic pressure, and selected the optimal clone. To isolate monomeric and homodimeric RBD forms, the authors purified the recombinant RBD by chromatographic methods. Further, they analysed the activity of the RBD forms by Western blotting, bio-layer interferometry, and indirect ELISA. The analysis involved monoclonal antibodies GamXRH19, GamP2C5, and h6g3, as well as serum samples from volunteers vaccinated with Gam-COVID-Vac (Sputnik V) and unvaccinated ones. Result(s): the authors produced the CHO-S cell line for stable expression of the recombinant SARS-CoV-2 S-protein RBD. The study demonstrated the recombinant RBD's ability to homodimerise after fed-batch cultivation of the cell line for more than 7 days due to the presence of unpaired cysteines. The purified recombinant RBD yield from culture broth was 30-50 mg/L. Monomeric and homodimeric RBD forms were separated using gel-filtration chromatography and characterised by their ability to interact with specific monoclonal antibodies, as well as with serum samples from vaccinated volunteers. The homodimeric recombinant RBD showed increased avidity for both monoclonal and immune sera antibodies. Conclusion(s): the homodimeric recombinant RBD may be more preferable for the analysis of levels of antibodies to the receptor-binding domain of the SARS-CoV-2 S protein.Copyright © 2023 Authors. All rights reserved.

DYNAMICS IN MATURATION OF SARS-CoV-2 RBD-SPECIFIC IgG ANTIBODY AVIDITY DEPENDING ON IMMUNIZATION TIMEFRAME AND TYPE

The aim is to examine dynamics of avidity maturation of IgG antibodies against SARS-CoV-2 RBD depending on the type of immunization (vaccination or infection), as well as on the duration and frequency of immunization. Materials and methods. The study was performed on two sample cohorts collected at two time points during COVID-19 pandemic. The first cohort (group No. 1) consisted of 87 samples of blood sera obtained from COVID-19 convalescents in the period from March to September 2020. The second cohort included 204 samples obtained in September 2021 from two patient groups. Group No. 2 (n = 64) — patients immunized with a full course of Gam-Covid-Vac, group No. 3 (n = 140) — COVID-19 convalescent patients and subjects vaccinated with Gam-Covid-Vac ("hybrid immunity”). Results and conclusion. The dynamics of avidity maturation for SARS-CoV-2 RBD IgG antibodies depending on the method and frequency of immunization, showed that the most effective immunity was formed in COVID-19 convalescent patients and subjects vaccinated with a full course of Gam-Covid-Vac. The "hybrid” immunity showed not only a significantly higher (compared with groups No. 1 and No. 2) level of IgG antibodies (median 228 BAU/ml vs 75 or 119 BAU/ml, p < 0.001), but also a higher level of avidity (IA 90.5% vs 54.5 and 76.6, respectively, p < 0.001, 4M urea). In the test for assessing the avidity index with the denaturing agent 8M urea in patients with "hybrid immunity”, the median level of IA was 25% versus 14.8% and 16% in COVID-19 convalescents and vaccinated subjects (p < 0.001), only in 8 patients IA was higher than 50%. While comparing a single infection of COVID-19 with a full course of Gam-Covid-Vac, it was shown that vaccination leads to higher IgG levels (median values in groups 119 and 75 BAU/ml, p < 0.001) and to a higher avidity index (median 76.6% vs 54.5%). Thus, the more rapid induction of high-avidity antibodies was in vaccinated individuals at early stages of immunization (up to 4 months), during the period when IgG avidity maturation has not yet been completed. Our results showed that during this period vaccination leads to production of antibodies with avidity index at median level of 82% versus 36% in COVID-19 convalescents at similar time point. © 2023 Saint Petersburg Pasteur Institute. All rights reserved.

The quantity and quality of anti-SARS-CoV-2 antibodies show contrariwise association with COVID-19 severity: lessons learned from IgG avidity.

Gaining more appreciation on the protective/damaging aspects of anti-SARS-CoV-2 immunity associated with disease severity is of great importance. This study aimed to evaluate the avidity of serum IgG antibodies against SARS-CoV-2 spike (S) and nucleocapsid (N) in hospitalized symptomatic COVID-19 patients and asymptomatic RT-PCR-confirmed SARS-CoV-2 carriers as well as to compare antibody avidities with respect to vaccination status, vaccination dose and reinfection status. Serum levels of anti-S and anti-N IgG were determined using specific ELISA kits. Antibody avidity was determined by urea dissociation assay and expressed as avidity index (AI) value. Despite higher IgG levels in the symptomatic group, AI values of both anti-S and anti-N IgG were significantly lower in this group compared to asymptomatic individuals. In both groups, anti-S AI values were elevated in one-dose and two-dose vaccinees versus unvaccinated subjects, although significant differences were only detected in the symptomatic group. However, anti-N avidity showed no significant difference between the vaccinated and unvaccinated subgroups. Almost all vaccinated patients of different subgroups (based on vaccine type) had higher anti-S IgG avidity, while the statistical significance was detected only between those receiving Sinopharm compared to the unvaccinated subgroup. Also, statistically significant differences in antibody AIs were only found between primarily infected individuals of the two groups. Our findings indicate a key role for anti-SARS-CoV-2 IgG avidity in protection from symptomatic COVID-19 and calls for the incorporation of antibody avidity measurement into the current diagnostic tests to predict effective immunity toward SARS-CoV-2 infection or even for prognostic purposes.

FUNCTIONALLY IMPAIRED ANTIBODY RESPONSE TO BNT162B2 BOOSTER VACCINATION IN CVID IgG RESPONDERS.

BACKGROUND: While previous studies described the production of IgG-antibodies in a subgroup of CVID-patients following mRNA-vaccinations with bnt162b2 SARS-CoV2 (CVID responders), the functionality of these antibodies in terms of avidity as measured by the dissociation rate constant (kdis) and the antibody response to booster immunization has not been studied. OBJECTIVE: In CVID responders and healthy individuals the avidity of anti-SARS-CoV-2 serum-antibodies and their neutralization capacity as measured by surrogate virus neutralizing antibodies were analyzed in addition to IgG-, IgM- and IgA-antibody levels and the response of circulating follicular T-helper cells after a third vaccination with BNT162b2 SARS-CoV2 mRNA-vaccine. METHODS: Binding IgG, IgA and IgM serum levels were analyzed by ELISA in CVID-patients responding to the primary vaccination (CVID responders, n=10) and healthy controls (n=41). The binding-avidity of anti-spike antibodies was investigated using biolayer interferometry in combination with biotin-labelled receptor-binding-domain (RBD) of SARS-CoV2 spike-protein and streptavidin-labelled sensors. Antigen-specific recall T-cell responses were assessed by measuring activation-induced markers by flow cytometry. RESULTS: After the third vaccination with BNT162b2 IgG-, IgM and IgA-antibody levels, sVNT levels and antibody avidity were lower in CVID responders as compared to healthy. In contrast αSpike-avidity was comparable in CVID responders and healthy individuals following primary vaccination. Follicular T-helper cell response to booster vaccination in CVID-responders was significantly reduced when compared to healthy individuals. CONCLUSION: Impaired affinity-maturation during booster-response provides new insight into CVID pathophysiology.

Avidity engineering of human heavy-chain-only antibodies mitigates neutralization resistance of SARS-CoV-2 variants.

Emerging SARS-CoV-2 variants have accrued mutations within the spike protein rendering most therapeutic monoclonal antibodies against COVID-19 ineffective. Hence there is an unmet need for broad-spectrum mAb treatments for COVID-19 that are more resistant to antigenically drifted SARS-CoV-2 variants. Here we describe the design of a biparatopic heavy-chain-only antibody consisting of six antigen binding sites recognizing two distinct epitopes in the spike protein NTD and RBD. The hexavalent antibody showed potent neutralizing activity against SARS-CoV-2 and variants of concern, including the Omicron sub-lineages BA.1, BA.2, BA.4 and BA.5, whereas the parental components had lost Omicron neutralization potency. We demonstrate that the tethered design mitigates the substantial decrease in spike trimer affinity seen for escape mutations for the hexamer components. The hexavalent antibody protected against SARS-CoV-2 infection in a hamster model. This work provides a framework for designing therapeutic antibodies to overcome antibody neutralization escape of emerging SARS-CoV-2 variants.

Antibody Avidity Maturation Following Recovery From Infection or the Booster Vaccination Grants Breadth of SARS-CoV-2 Neutralizing Capacity.

BACKGROUND: Cross-neutralizing capacity of antibodies against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) variants is important in mitigating (re-)exposures. Role of antibody maturation, the process whereby selection of higher affinity antibodies augments host immunity, to determine SARS-CoV-2 neutralizing capacity was investigated. METHODS: Sera from SARS-CoV-2 convalescents at 2, 6, or 10 months postrecovery, and BNT162b2 vaccine recipients at 3 or 25 weeks postvaccination, were analyzed. Anti-spike IgG avidity was measured in urea-treated ELISAs. Neutralizing capacity was assessed by surrogate neutralization assays. Fold change between variant and wild-type neutralization inferred the breadth of neutralizing capacity. RESULTS: Compared with early-convalescent, avidity indices of late-convalescent sera were significantly higher (median, 37.7 [interquartile range 28.4-45.1] vs 64.9 [57.5-71.5], P < .0001). Urea-resistant, high-avidity IgG best predicted neutralizing capacity (Spearman r = 0.49 vs 0.67 [wild-type]; 0.18-0.52 vs 0.48-0.83 [variants]). Higher-avidity convalescent sera better cross-neutralized SARS-CoV-2 variants (P < .001 [Alpha]; P < .01 [Delta and Omicron]). Vaccinees only experienced meaningful avidity maturation following the booster dose, exhibiting rather limited cross-neutralizing capacity at week 25. CONCLUSIONS: Avidity maturation was progressive beyond acute recovery from infection, or became apparent after the booster vaccine dose, granting broader anti-SARS-CoV-2 neutralizing capacity. Understanding the maturation kinetics of the 2 building blocks of anti-SARS-CoV-2 humoral immunity is crucial.

The relationship between SARS-CoV-2 neutralising antibody titres and avidity in plasma collected from convalescent non-vaccinated and vaccinated blood donors.

Convalescent plasma (CP) treatment of COVID-19 has shown significant therapeutic effect when administered early (e.g. Argentinian trial showing reduced hospitalisation) but has in general been ineffective (e.g. REMAP-CAP trial without improvement during hospitalisation). To investigate whether the differences in CP used could explain the different outcomes, we compared neutralising antibodies, anti-spike IgG and avidity of CP used in the REMAP-CAP and Argentinian trials and in convalescent vaccinees. We found no difference between the trial plasmas emphasising initial patient serostatus as treatment efficacy predictor. By contrast, vaccinee convalescent plasma showed significantly higher titres and avidity, being preferable for future CP-treatment.

RBD and Spike DNA-Based Immunization in Rabbits Elicited IgG Avidity Maturation and High Neutralizing Antibody Responses against SARS-CoV-2.

Neutralizing antibodies (nAbs) are a critical part of coronavirus disease 2019 (COVID-19) research as they are used to gain insight into the immune response to severe acute respiratory syndrome-related coronavirus 2 (SARS-CoV-2) infections. Among the technologies available for generating nAbs, DNA-based immunization methods are an alternative to conventional protocols. In this pilot study, we investigated whether DNA-based immunization by needle injection in rabbits was a viable approach to produce a functional antibody response. We demonstrated that three doses of DNA plasmid carrying the gene encoding the full-length spike protein (S) or the receptor binding domain (RBD) of SARS-CoV-2 induced a time-dependent increase in IgG antibody avidity maturation. Moreover, the IgG antibodies displayed high cross neutralization by live SARS-CoV-2 and pseudoviruses neutralization assays. Thus, we established a simple, low cost and feasible DNA-based immunization protocol in rabbits that elicited high IgG avidity maturation and nAbs production against SARS-CoV-2, highlighting the importance of DNA-based platforms for developing new immunization strategies against SARS-CoV-2 and future emerging epidemics.

Cross neutralization of SARS-CoV-2 omicron subvariants after repeated doses of COVID-19 mRNA vaccines.

We have measured the humoral response to messenger RNA (mRNA) vaccines in COVID-19 naïve and convalescent individuals. Third doses of mRNA COVID-19 vaccines induced a significant increase in potency and breadth of neutralization against SARS-CoV-2 variants of concern (VoC) including Omicron subvariants BA.1, BA.2, and BA.2.12.1, that were cross-neutralized at comparable levels and less for BA.4/5. This booster effect was especially important in naïve individuals that only after the third dose achieved a level that was comparable with that of vaccinated COVID-19 convalescents except for BA.4/5. Avidity of RBD-binding antibodies was also significantly increased in naïve individuals after the third dose, indicating an association between affinity maturation and cross neutralization of VoC. These results suggest that at least three antigenic stimuli by infection or vaccination with ancestral SARS-CoV-2 sequences are required to induce high avidity cross-neutralizing antibodies. Nevertheless, the circulation of new subvariants such as BA.4/5 with partial resistance to neutralization will have to be closely monitored and eventually consider for future vaccine developments.

Repeated SARS-CoV-2 vaccination in cancer patients treated with immune checkpoint inhibitors: induction of high-avidity anti-RBD neutralizing antibodies.

BACKGROUND: Cancer patients are more vulnerable to COVID-19 and are thus given high priority in vaccination campaigns. In solid cancer patients treated with checkpoint inhibitors, we evaluated the amount of anti-RBD and neutralizing antibodies and antibody avidity after two or three doses of the vaccine. METHODS: Thirty-eight solid cancer patients, 15 untreated hematological patients and 21 healthy subjects were enrolled in the study. Blood was collected before the first dose (T0), 21 days after the second (T2) and in 18 solid cancer patients also 15 days after the third dose of vaccine (T3). IgG, IgM and IgA anti-RBD antibodies were detected by ELISA. Neutralizing antibodies were measured testing the inhibition of RBD binding to ACE2. Antibody avidity was evaluated in 18 patients by a urea avidity ELISA. RESULTS: IgG anti-RBD antibodies were produced in 65.8% of the cancer patients at T2, and in 60% of hematological patients at levels lower than healthy controls. IgM and IgA anti-RBD antibodies were also produced in 5.3% and 21% cancer patients, respectively. At T3, a significant increase in anti-RBD IgG levels was observed. Neutralizing antibodies were produced in 68.4% of cancer patients as compared with 93% of untreated hematological patients and 100% of controls, at titers lower than in healthy subjects. At T3, neutralizing antibodies and avidity of IgG anti-RBD increased; 6/18 patients negative at T2 developed neutralizing antibodies at T3. CONCLUSION: The data indicate that in cancer patients mRNA vaccine induces high avidity anti-RBD antibodies and neutralizing antibodies that increase after the third dose. The process of induction and selection of high-affinity antibodies is apparently unaffected by the treatment with anti-PD-1 or anti-PD-L1 antibodies.

VERIFICATION OF A CASE OF MIXED INFECTION WITH LYME DISEASE, TICK-BORNE ENCEPHALITIS AND COVID-19

Background. The relationship between pathogens of new diseases and tick-borne infections is an underinvestigated direction in the problem of infectious diseases. The aim. To show the features ofidentifying the markers ofBorrelia burgdorferi, tick-borne encephalitis and SARS-CoV-2 pathogens on the example of a case of a triple mixed infection (Lyme disease, tick-borne encephalitis and COVID-19) and using comprehensive studies. Methods. In 2019-2021, a comprehensive study of 7 blood samples from a patient with mixed infection was carried out. We used real-time polymerase chain reaction (PCR), enzyme immunoassay (ELISA) and determined antigen, IgM, IgG antibodies, and avidity index (AI) of IgG antibodies. Results. Ixodid tick-borne borreliosis was diagnosed in a patient 5 months after contagion. Only high-avid Lyme-IgG antibodies were detected. Low-avid Lyme-IgG antibodies appeared against the background of a reduced general condition. Atthe same time, high-avidIgG(cut-offindex (COI)- 7.8)andIgM(COI= 1.2)antibod-ies to the TBE virus were detected. In July 2020, the patient was infected with SARS-CoV-2. TBE virus which passed into the body simultaneously with Borrelia in the fall of 2019 was activated. Although the patient did not have specific symptoms of TBE, in subsequentbloodsamples (No. 4, 5, 6)we foundTBEVantigen (opticaldensity (OD) - 4.3;1.9and2.0respectively)andIgM(COI= 1.3;0,9and0respectively). These results were recognized as TBEV activation, which contributed to an increase in the avidity of IgG antibodies (AI = 65 %;100 % and 63 % respectively). IgM antibodies to SARS-CoV-2 virus were not detected, as opposed to the high levels of IgG (COI = 8.2;8.1;8.4 and 14.7 respectively). Conclusions. Therefore, using not only the common methods of diagnosing (PCR andELISA), butalso the determination ofthe antibody avidity degree, we have shown that when analyzing a case of a triple mixed infection, B. burgdorferi dominates in the human body and causes a long-term chronic course of the disease. © 2022 Acta Biomedica Scientifica. All rights reserved.

Labeled TEMPO-Oxidized Mannan Differentiates Binding Profiles within the Collectin Families.

Establishing the rapid and accurate diagnosis of sepsis is a key component to the improvement of clinical outcomes. The ability of analytical platforms to rapidly detect pathogen-associated molecular patterns (PAMP) in blood could provide a powerful host-independent biomarker of sepsis. A novel concept was investigated based on the idea that a pre-bound and fluorescent ligand could be released from lectins in contact with high-affinity ligands (such as PAMPs). To create fluorescent ligands with precise avidity, the kinetically followed TEMPO oxidation of yeast mannan and carbodiimide coupling were used. The chemical modifications led to decreases in avidity between mannan and human collectins, such as the mannan-binding lectin (MBL) and human surfactant protein D (SP-D), but not in porcine SP-D. Despite this effect, these fluorescent derivatives were captured by human lectins using highly concentrated solutions. The resulting fluorescent beads were exposed to different solutions, and the results showed that displacements occur in contact with higher affinity ligands, proving that two-stage competition processes can occur in collectin carbohydrate recognition mechanisms. Moreover, the fluorescence loss depends on the discrepancy between the respective avidities of the recognized ligand and the fluorescent mannan. Chemically modulated fluorescent ligands associated with a diversity of collectins may lead to the creation of diagnostic tools suitable for multiplex array assays and the identification of high-avidity ligands.

Regimen of Coronavirus Disease 2019 Vaccination Influences Extent and Kinetics of Antibody Avidity.

We investigated antibody titers and avidity after heterologous versus homologous coronavirus disease 2019 vaccination over 6 months after the second dose. We found a significantly higher avidity in regimens including at least 1 dose of the adenoviral vector vaccine ChAdOx1-S compared with 2 doses of the mRNA vaccine BNT162b2.