Baculoviral COVID-19 Delta DNA vaccine cross-protects against SARS-CoV2 variants in K18-ACE2 transgenic mice

After severe acute respiratory syndrome coronavirus-2 (SARS-CoV2) made the world tremble with a global pandemic, SARS-CoV2 vaccines were developed. However, due to the coronavirus’s intrinsic nature, new variants emerged, such as Delta and Omicron, refractory to the vaccines derived using the original Wuhan strain. We developed an HERV-enveloped recombinant baculoviral DNA vaccine against SARS-CoV2 (AcHERV-COVID19S). A non-replicating recombinant baculovirus that delivers the SARS-CoV2 spike gene showed a protective effect against the homologous challenge in a K18-hACE2 Tg mice model; however, it offered only a 50% survival rate against the SARS-CoV2 Delta variant. Therefore, we further developed the AcHERV-COVID19 Delta vaccine (AcHERV-COVID19D). Cross-protection experiments revealed that mice vaccinated with the AcHERV-COVID19D showed 100% survival upon challenge with Delta and Omicron variants and 71.4% survival against prototype SARS-CoV2. These results support the potential of the viral vector vaccine, AcHERV-COVID19D, in preventing the spread of coronavirus variants such as Omicron and SARS-CoV2 variants. Author Summary After the SARS-CoV2 pandemic, it is known that the existing vaccine has diminished efficacy against the emerging variants. We developed a baculoviral COVID19 DNA vaccine for the Delta variant (AcHERV-COVIS19D). Compared to AcHERV-COVID19S, designed to protect from the prototype of SARS-CoV2, AcHERV-COVID19D elicited higher humoral and cellular immunity and showed perfect protection against SARS-CoV2 delta strain and Omicron challenge. The broad and robust cellular immunity of the AcHERV-COVID19D vaccine appears to have played a significant role in the cross-protection of the Omicron variant. Our AcHERV-COVID19D can be a potential vaccine against emerging SARS-CoV2 variants.

93 But the level of neutralising antibodies was not as high as seen in the serum of an 94 individual who had received commercial vaccines (positive control) (Fig. 2C).

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To test the prophylactic effect of the AcHERV-COVID19S, mice were 96 challenged with a lethal dose of SARS-CoV2 (prototype). After SARS-CoV2 infection, 97 the mice from the non-treatment control (NTC) group lost more than 17.4% of their 98 initial body weight, and all died on the 9th day. The AcHERV-COVID19S vaccine 99 group showed mild weight loss and symptoms but recovered rapidly over 13 days and 100 exhibited 100% survival with good health (Fig. 2D-E).

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The results of histopathological analysis of mouse lung tissues are shown in 102 Figure 2F-G. Seven days after infection, lung tissues of the AcHERV-COVID19S 103 vaccine group showed a significantly milder pathology upon SARS-CoV2 infection.
104 Unlike the vaccine group, NTC mice showed severe lesions, perivascular and alveolar 105 infiltration, and severe inflammation compared to the lungs of normal mice. On day 13 106 post-challenge, viral titres were measured from the lung tissues of all surviving mice 107 using qRT-PCR. The virus was weakly detected in mice at this point but was below the 108 lower limit of detection in some cases (Fig. 2H) 123 To measure the cellular immunity-inducing effects of the vaccine, an additional 124 experiment was performed using C57BL/6 mice. An ELISPOT assay revealed that the 125 level of IFN-γ-secreting splenocytes was significantly higher in AcHERV-COVID19S 126 vaccinated mice compared to control mice (*: P<0.05, S1 Fig A). We also measured the 127 induction of Th1-and Th2-type cytokines by qRT-PCR. TNF-α and IL-2 were used to 128 assess Th1-type levels, while IL-4 was used to assess Th2-type cytokines. We found 129 that the mRNA expression levels of TNF-α, IL-2, and IL-4 were higher in the To examine the cross-reactivity to the AcHERV-COVID19D vaccine, 112 mice 193 were divided into 13 groups, as shown in Table 1, and an experiment was carried out.
194 Groups 2, 7, and 11 (total n=28) were immunized with the AcHERV-COVID19D delta   (15, 16). We speculate that the cross-256 protection ability of AcHERV-COVID19D may be related to the virulence of a given 257 virus. The ability of the AcHERV-COVID19D to cross-protect against Omicron is 258 beyond the level expected for the observed neutralizing antibody activity (Fig. 4A), 259 indicating that cellular immunity is involved (Fig. 4B).

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Although weight loss was observed, the adenoviral delta vaccine (Ad-261 COVID19D) group also showed 100% survival following the Omicron challenge. The 298 The scheme for constructing the recombinant baculoviruses, AcHERV-COVID19S, and 299 AcHERV-COVID19D, is shown in Fig. 1A.

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A recombinant adenovirus delivering the full-length SARS-CoV2 S gene was 301 constructed by inserting the relevant sequence into pAdenoX-CMV (S2 Fig 2).