Immunogenicity and Neutralizing Activity Comparison of SARS-CoV-2 Spike Full-Length and Subunit Domain Proteins in Young Adult and Old-Aged Mice

Characterization of monoclonal antibodies targeting SARS-CoV-2 spike glycoprotein: Reactivity against Delta and Omicron BA.1 variants

The cross-species transmissibility of SARS-CoV-2 infection has necessitated development of specific reagents for detecting infection in various animal species. The spike glycoprotein of SARS-CoV-2, which is involved in viral entry, is a highly immunogenic protein. To develop assays targeting this protein, we generated eight monoclonal antibodies (mAbs) against the S1 and seven against the S1/S2 protein (ectodomain) of SARS CoV-2. Based on neutralization capability and reactivity profile observed in ELISA, the mAbs generated against the S1/S2 antigen exhibited a broader spectrum of epitope specificity than those produced against the S1 domain alone. The full-length ectodomain induced antibodies that could neutralize the two most important variants of the virus encountered during the pandemic, namely Delta and Omicron. The availability of these reagents could greatly enhance the development of precise diagnostics for detecting COVID-19 infections in various host species and contribute to the advancement of mAb-based therapeutics.

Heterologous booster vaccination enhances antibody responses to SARS-CoV-2 by improving Tfh function and increasing B-cell clonotype SHM frequency

Heterologous prime-boost has broken the protective immune response bottleneck of the COVID-19 vaccines. however, the underlying mechanisms have not been fully elucidated. Here, we investigated antibody responses and explored the response of germinal center (GC) to priming with inactivated vaccines and boosting with heterologous adenoviral-vectored vaccines or homologous inactivated vaccines in mice. Antibody responses were dramatically enhanced by both boosting regimens. Heterologous immunization induced more robust GC activation, characterized by increased Tfh cell populations and enhanced helper function. Additionally, increased B-cell activation and antibody production were observed in a heterologous regimen. Libra-seq was used to compare the differences of S1-, S2- and NTD-specific B cells between homologous and heterologous vaccination, respectively. S2-specific CD19+ B cells presented increased somatic hypermutations (SHMs), which were mainly enriched in plasma cells. Moreover, a heterologous booster dose promoted the clonal expansion of B cells specific to S2 and NTD regions. In conclusion, the functional role of Tfh and B cells following SARS-CoV-2 heterologous vaccination may be important for modulating antibody responses. These findings provide new insights for the development of SARS-CoV-2 vaccines that induce more robust antibody response.

Multivalent and Sequential Heterologous Spike Protein Vaccinations Effectively Induce Protective Humoral Immunity against SARS-CoV-2 Variants

The emergence of new SARS-CoV-2 variants continues to cause challenging problems for the effective control of COVID-19. In this study, we tested the hypothesis of whether a strategy of multivalent and sequential heterologous spike protein vaccinations would induce a broader range and higher levels of neutralizing antibodies against SARS-CoV-2 variants and more effective protection than homologous spike protein vaccination in a mouse model. We determined spike-specific IgG, receptor-binding inhibition titers, and protective efficacy in the groups of mice that were vaccinated with multivalent recombinant spike proteins (Wuhan, Delta, Omicron), sequentially with heterologous spike protein variants, or with homologous spike proteins. Trivalent (Wuhan + Delta + Omicron) and sequential heterologous spike protein vaccinations were more effective in inducing serum inhibition activities of receptor binding to spike variants and virus neutralizing antibody titers than homologous spike protein vaccination. The higher efficacy of protection was observed in mice with trivalent and sequential heterologous spike protein vaccination after a challenge with a mouse-adapted SARS-CoV-2 MA10 strain compared to homologous spike protein vaccination. This study provides evidence that a strategy of multivalent and sequential heterologous variant spike vaccination might provide more effective protection against emerging SARS-CoV-2 variants than homologous spike vaccination and significantly alleviate severe inflammation due to COVID-19.

Second Boost of Omicron SARS-CoV-2 S1 Subunit Vaccine Induced Broad Humoral Immune Responses in Elderly Mice

Currently approved COVID-19 vaccines prevent symptomatic infection, hospitalization, and death from the disease. However, repeated homologous boosters, while considered a solution for severe forms of the disease caused by new SARS-CoV-2 variants in elderly individuals and immunocompromised patients, cannot provide complete protection against breakthrough infections. This highlights the need for alternative platforms for booster vaccines. In our previous study, we assessed the boost effect of the SARS-CoV-2 Beta S1 recombinant protein subunit vaccine (rS1Beta) in aged mice primed with an adenovirus-based vaccine expressing SARS-CoV-2-S1 (Ad5.S1) via subcutaneous injection or intranasal delivery, which induced robust humoral immune responses (1). In this follow-up study, we demonstrated that a second booster dose of a non-adjuvanted recombinant Omicron (BA.1) S1 subunit vaccine with Toll-like receptor 4 (TLR4) agonist RS09 (rS1RS09OM) was effective in stimulating strong S1-specific immune responses and inducing significantly high neutralizing antibodies against the Wuhan, Delta, and Omicron variants in 100-week-old mice. Importantly, the second booster dose elicits cross-reactive antibody responses, resulting in ACE2 binding inhibition against the spike protein of SARS-CoV-2 variants, including Omicron (BA.1) and its subvariants. Interestingly, the levels of IgG and neutralizing antibodies correlated with the level of ACE2 inhibition in the booster serum samples, although Omicron S1-specific IgG level showed a weaker correlation compared to Wuhan S1-specific IgG level. Furthermore, we compared the immunogenic properties of the rS1 subunit vaccine in young, middle-aged, and elderly mice, resulting in reduced immunogenicity with age, especially an impaired Th1-biased immune response in aged mice. Our findings demonstrate that the new variant of concern (VOC) rS1 subunit vaccine as a second booster has the potential to offer cross-neutralization against a broad range of variants and to improve vaccine effectiveness against newly emerging breakthrough SARS-CoV-2 variants in elderly individuals who were previously primed with the authorized vaccines.

Heterologous DNA-prime/protein-boost immunization with a monomeric SARS-CoV-2 spike antigen redundantizes the trimeric receptor-binding domain structure to induce neutralizing antibodies in old mice

A multitude of alterations in the old immune system impair its functional integrity. Closely related, older individuals show, for example, a reduced responsiveness to severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) vaccines. However, systematic strategies to specifically improve the efficacy of vaccines in the old are missing or limited to simple approaches like increasing the antigen concentration or injection frequencies. We here asked whether the intrinsic, trimeric structure of the SARS-CoV-2 spike (S) antigen and/or a DNA- or protein-based antigen delivery platform affects priming of functional antibody responses particularly in old mice. The used S-antigens were primarily defined by the presence/absence of the membrane-anchoring TM domain and the closely interlinked formation/non-formation of a trimeric structure of the receptor binding domain (S-RBD). Among others, we generated vectors expressing prefusion-stabilized, cell-associated (TM+) trimeric "S2-P" or secreted (TM-) monomeric "S6-PΔTM" antigens. These proteins were produced from vector-transfected HEK-293T cells under mild conditions by Strep-tag purification, revealing that cell-associated but not secreted S proteins tightly bound Hsp73 and Grp78 chaperones. We showed that both, TM-deficient S6-PΔTM and full-length S2-P antigens elicited very similar S-RBD-specific antibody titers and pseudovirus neutralization activities in young (2-3 months) mice through homologous DNA-prime/DNA-boost or protein-prime/protein-boost vaccination. The trimeric S2-P antigen induced high S-RBD-specific antibody responses in old (23-24 months) mice through DNA-prime/DNA-boost vaccination. Unexpectedly, the monomeric S6-PΔTM antigen induced very low S-RBD-specific antibody titers in old mice through homologous DNA-prime/DNA-boost or protein-prime/protein-boost vaccination. However, old mice efficiently elicited an S-RBD-specific antibody response after heterologous DNA-prime/protein-boost immunization with the S6-PΔTM antigen, and antibody titers even reached similar levels and neutralizing activities as in young mice and also cross-reacted with different S-variants of concern. The old immune system thus distinguished between trimeric and monomeric S protein conformations: it remained antigen responsive to the trimeric S2-P antigen, and a simple change in the vaccine delivery regimen was sufficient to unleash its reactivity to the monomeric S6-PΔTM antigen. This clearly shows that both the antigen structure and the delivery platform are crucial to efficiently prime humoral immune responses in old mice and might be relevant for designing "age-adapted" vaccine strategies.

SARS-CoV-2 S1 Subunit Booster Vaccination Elicits Robust Humoral Immune Responses in Aged Mice

The emergence of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) variants has raised concerns about reduced vaccine effectiveness and the increased risk of infection, and while repeated homologous booster shots are recommended for elderly and immunocompromised individuals, they cannot completely protect against breakthrough infections. In our previous study, we assessed the immunogenicity of an adenovirus-based vaccine expressing SARS-CoV-2 S1 (Ad5.S1) in mice, which induced robust humoral and cellular immune responses (E. Kim, F. J. Weisel, S. C. Balmert, M. S. Khan, et al., Eur J Immunol 51:1774-1784, 2021, https://doi.org/10.1002/eji.202149167). In this follow-up study, we found that the mice had high titers of anti-S1 antibodies 1 year after vaccination, and one booster dose of the nonadjuvanted rS1Beta (recombinant S1 protein of SARS-CoV-2 Beta [B.1.351]) subunit vaccine was effective at stimulating strong long-lived S1-specific immune responses and inducing significantly high neutralizing antibodies against Wuhan, Beta, and Delta strains, with 3.6- to 19.5-fold increases. Importantly, the booster dose also elicited cross-reactive antibodies, resulting in angiotensin-converting enzyme 2 (ACE2) binding inhibition against spikes of SARS-CoV-2, including Omicron variants, persisting for >28 weeks after booster vaccination. Interestingly, the levels of neutralizing antibodies were correlated not only with the level of S1 binding IgG but also with ACE2 inhibition. Our findings suggest that the rS1Beta subunit vaccine candidate as a booster has the potential to offer cross-neutralization against broad variants and has important implications for the vaccine control of newly emerging breakthrough SARS-CoV-2 variants in elderly individuals primed with adenovirus-based vaccines like AZD1222 and Ad26.COV2.S. IMPORTANCE Vaccines have significantly reduced the incidences of severe coronavirus disease 2019 (COVID-19) cases and deaths. However, the emergence of SARS-CoV-2 variants has raised concerns about their increased transmissibility and ability to evade neutralizing antibodies, especially among elderly individuals who are at higher risks of mortality and reductions of vaccine effectiveness. To address this, a heterologous booster vaccination strategy has been considered as a solution to protect the elderly population against breakthrough infections caused by emerging variants. This study evaluated the booster effect of an S1 subunit vaccine in aged mice that had been previously primed with adenoviral vaccines, providing valuable preclinical evidence for elderly people vaccinated with the currently approved COVID-19 vaccines. This study confirms the potential for using the S1 subunit vaccine as a booster to enhance cross-neutralizing antibodies against emerging variants of concern.

Trivalent SARS-CoV-2 S1 Subunit Protein Vaccination Induces Broad Humoral Responses in BALB/c Mice

This paper presents a novel approach for improving the efficacy of COVID-19 vaccines against emergent SARS-CoV-2 variants. We have evaluated the immunogenicity of unadjuvanted wild-type (WU S1-RS09cg) and variant-specific (Delta S1-RS09cg and OM S1-RS09cg) S1 subunit protein vaccines delivered either as a monovalent or a trivalent antigen in BALB/c mice. Our results show that a trivalent approach induced a broader humoral response with more coverage against antigenically distinct variants, especially when compared to monovalent Omicron-specific S1. This trivalent approach was also found to have increased or equivalent ACE2 binding inhibition, and increased S1 IgG endpoint titer at early timepoints, against SARS-CoV-2 spike variants when compared monovalent Wuhan, Delta, or Omicron S1. Our results demonstrate the utility of protein subunit vaccines against COVID-19 and provide insights into the impact of variant-specific COVID-19 vaccine approaches on the immune response in the current SARS-CoV-2 variant landscape. Particularly, our study provides insight into effects of further increasing valency of currently approved SARS-CoV-2 vaccines, a promising approach for improving protection to curtail emerging viral variants.

Salmonella-mediated oral delivery of multiple-target vaccine constructs with conserved and variable regions of SARS-CoV-2 protect against the Delta and Omicron variants in hamster

Since the emergence of the pandemic COVID19 caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the development of vaccines has been the prime strategy to control the disease transmission. Most of the developed vaccines target the spike protein, however, the emerging variants have alterations, particularly at the same region which may pose resistance to neutralizing antibodies. In this study, we explored the variable and conserved regions of SARS-CoV-2 as a potential inclusion in a multiple-target vaccine with the exploitation of Salmonella-based vector for oral mRNA vaccine against Delta and Omicron variants. Increased IgG and IgA levels imply the induction of humoral response and the CD4⁺, CD8⁺ and IFN-γ⁺ sub-population level exhibits cell-mediated immune responses. The degree of CD44⁺ cells indicates the induction of memory cells corresponding to long-term immune responses. Furthermore, we assessed the protective efficacy of the vaccines against the Delta and Omicron variants in the hamster model. The vaccine constructs induced neutralizing antibodies and protected the viral-challenged hamsters with significant decrease in lung viral load and reduced histopathological lesions. These results reinforce the use of the conserved and variable regions as potential antigen targets of SARS-CoV-2 as well as the exploitation of bacteria-mediated delivery for oral mRNA vaccine development.

Relationship between Humoral Response in COVID-19 and Seasonal Influenza Vaccination

There is evidence that vaccination against seasonal influenza can improve innate immune responses to COVID-19 and decrease disease severity. However, less is known about whether it could also impact the humoral immunity in SARS-CoV-2 infected patients. The present study aimed to compare the SARS-CoV-2 specific humoral responses (IgG antibodies against nucleocapsid; anti-N, receptor binding domain; anti-RBD, subunit S2; anti-S2, and envelope protein; anti-E) between non-hospitalized, COVID-19 unvaccinated, and mild COVID-19 convalescent patients who were and were not vaccinated against influenza during the 2019/2020 epidemic season (n = 489 and n = 292, respectively). The influenza-vaccinated group had significantly higher frequency and titers of anti-N antibodies (75 vs. 66%; mean 559 vs. 520 U/mL) and anti-RBD antibodies (85 vs. 76%; mean 580 vs. 540 U/mL). The prevalence and concentrations of anti-S2 and anti-E antibodies did not differ between groups (40-43%; mean 370-375 U/mL and 1.4-1.7%; mean 261-294 U/mL) and were significantly lower compared to those of anti-RBD and anti-N. In both groups, age, comorbidities, and gender did not affect the prevalence and concentrations of studied antibodies. The results indicate that influenza vaccination can improve serum antibody levels produced in response to SARS-CoV-2 infection.

Immunogenicity of an AAV-Based COVID-19 Vaccine in Murine Models of Obesity and Aging

The SARS-CoV-2 pandemic has had a disastrous impact on global health. Although some vaccine candidates have been effective in combating SARS-CoV-2, logistical, economical, and sociological aspects still limit vaccine access globally. Recently, we reported on two room-temperature stable AAV-based COVID-19 vaccines that induced potent and protective immunogenicity following a single injection in murine and primate models. Obesity and old age are associated with increased mortality in COVID-19, as well as reduced immunogenicity and efficacy of vaccines. Here, we investigated the effectiveness of the AAVCOVID vaccine candidates in murine models of obesity and aging. Results demonstrate that obesity did not significantly alter the immunogenicity of either vaccine candidate. In aged mice, vaccine immunogenicity was impaired. These results suggest that AAV-based vaccines may have limitations in older populations and may be equally applicable in obese and non-obese populations.