A team of UK-based scientists has developed and validated a DNA vaccine against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2)  that encodes both spike receptor-binding domain (RBD) and nucleocapsid of the virus as vaccine immunogens.

The vaccine demonstrates high potency in stimulating antibody-mediated and cell-mediated immune responses against SARS-CoV-2 and its variants. The study is currently available on the bioRxiv* preprint server.

Study: A novel bivalent DNA vaccine encoding both spike protein receptor-binding domain and nucleocapsid protein of SARS-CoV-2 to elicit T cell and neutralising antibody responses that cross react with variants. Image Credit: NIAID

Background

The SARS-CoV-2 pandemic prompted a global response that saw hundreds of laboratories across the globe developing and testing potential vaccination candidates to prevent infection and COVID-19 disease. Among currently available vaccines, mRNA-based and adenovirus vector-based vaccines have shown high efficacy in preventing both SARS-CoV-2 infection and symptomatic COVID-19 disease. However, since these vaccines have been developed against the original, previously circulating strain of SARS-CoV-2, there remains a global concern about the efficacy and longevity of vaccine-induced protective immunity against newly emerging viral variants with multiple spike mutations, including B.1.1.7, B.1.351, P1, and B.1.617.2.

In the current study, the scientists have evaluated the anti-SARS-CoV-2 efficacy of a DNA vaccine that contains both SARS-CoV-2 spike RBD and nucleocapsid as immunogens.

Because of high immunogenicity, the spike protein has been incorporated in most of the vaccine development platforms to induce strong neutralizing antibody and T cell responses against SARS-CoV-2. However, in many COVID-19 recovered patients, a robust immune response against viral nucleocapsid protein has also been observed. The nucleocapsid protein, which is highly conserved between coronaviruses, plays a vital role in viral replication and packaging of the viral genome into virions.

Given the importance of nucleocapsid in inducing robust natural immunity, scientists have incorporated it in the DNA vaccine to induce a broad range of immune responses. They have linked the nucleocapsid sequence to a modified fragment crystallizable (Fc) domain of human immunoglobulin G1 (IgG1) to increase the binding strength between antibody and antigen.

Important observations

Multiple DNA constructs were produced by fusing spike RBD sequence with IgG1 Fc domain, fibritin timer domain, or disulfide bridge motif, as well as fusing nucleocapsid sequence with IgG1 Fc or modified Fc domain. The characterization of these DNA constructs revealed that functional RBD and nucleocapsid proteins are produced as both secreted and extracellular forms. Both proteins are capable of inducing robust humoral and cellular immunity against SARS-CoV-2.      

Immunization of mice with DNA constructs revealed that nucleocapsid linked to modified Fc generated the strongest CD4+ and CD8+ T cell responses specific to nucleocapsid. Importantly, these T cells demonstrated cross-reactivity to both SARS-CoV and SARS-CoV-2.

The comparison of RBD-specific immunity revealed that DNA constructs containing RBD monomer induce higher frequency T cell response than full-length spike-containing constructs.

To determine the immunogenicity of bivalent DNA construct, mice were immunized with DNA constructs containing RBD alongside modified Fc-linked nucleocapsid.

A high-frequency CD4+ and CD8+ T cell responses specific to RBD and nucleocapsid was observed in all immunized mice. Regarding humoral immunity, the bivalent DNA construct exhibited high potency in stimulating robust neutralizing antibody titers against spike RBD. However, comparatively lower antibody titers were observed against the nucleocapsid protein.

Importantly, vaccine-induced neutralizing antibodies exhibited cross-reactivity to spike variants containing mutations commonly observed in B.1.1.7 and B.1.351. However, the reactivity to spike protein of B.1.351 variant was significantly lower than that observed against wild-type spike. In contrast, a comparable reactivity was observed between wild-type and B.1.1.7 spike variants.

To further emphasize the significance of a DNA-based vaccine platform, a vaccine variant was developed by incorporating B.1.315 RBD sequence alongside modified Fc-linked nucleocapsid protein.

The characterization of the vaccine variant revealed higher neutralizing antibody titers against the B.1.315 spike than that against the wild-type spike. Interestingly, comparable antibody titers against wild-type and mutated nucleocapsid were observed in response to the vaccine variant. Moreover, the vaccine variant showed high potency in inhibiting the interaction of wild-type RBD or B.1.351 RBD with human ACE2.

Study significance

The study highlights the importance of DNA-based platforms for the cost-effective and rapid development of potential vaccines. Moreover, the study reveals that bivalent DNA vaccines containing both spike and nucleocapsid proteins can induce broad-spectrum antibody and T cell responses against both wild-type and mutated SARS-CoV-2.

A relatively straightforward process of incorporating mutated immunogens makes DNA platforms a valuable strategy to target newly emerging viral variants.

*Important Notice

bioRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.



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