Intranasal administration of a single dose of a candidate live attenuated vaccine derived from an NSP16-deficient SARS-CoV-2 strain confers sterilizing immunity in animals

Live attenuated vaccines might elicit mucosal and sterilizing immunity against SARS-CoV-2 that the existing mRNA,adenoviral vector and inactivated vaccines fail to induce.Here,we describe a candidate live attenuated vaccine strain of SARS-CoV-2 in which the NSP16 gene,which encodes 2′-O-methyltransferase,is catalytically disrupted by a point mutation.This virus,designated d16,was severely attenuated in hamsters and transgenic mice,causing only asymptomatic and nonpathogenic infection.A single dose of d16 administered intranasally resulted in sterilizing immunity in both the upper and lower respiratory tracts of hamsters,thus preventing viral spread in a contact-based transmission model.It also robustly stimulated humoral and cell-mediated immune responses,thus conferring full protection against lethal challenge with SARS-CoV-2 in a transgenic mouse model.The neutralizing antibodies elicited by d16 effectively cross-reacted with several SARS-CoV-2 variants.Secretory immunoglobulin A was detected in the blood and nasal wash of vaccinated mice.Our work provides proof-of-principle evidence for harnessing NSP16-deficient SARS-CoV-2 for the development of live attenuated vaccines and paves the way for further preclinical studies of d16 as a prototypic vaccine strain,to which new features might be introduced to improve safety,transmissibility,immunogenicity and efficacy.

Naturally occurring spike mutations influence the infectivity and immunogenicity of SARS-CoV-2

Mutations in SARS-CoV-2 variants of concern(VOCs)have enhanced transmissibility and immune evasion with respect to current vaccines and neutralizing antibodies(NAbs).How naturally occurring spike mutations affect the infectivity and antigenicity of VOCs remains to be investigated.The entry efficiency of individual spike mutations was determined in vitro using pseudotyped viruses.BALB/c mice were immunized with 2-dose DNA vaccines encoding B.1.1.7,B.1.351,B.1.1.529 and their single mutations.Cellular and humoral immune responses were then compared to determine the impact of individual mutations on immunogenicity.In the B.1.1.7 lineage,Del69–70 and Del 144 in NTD,A570D and P681H in SD1 and S982A and D1118H in S2 significantly increased viral entry,whereas T716I resulted in a decrease.In the B.1.351 lineage,L18F and Del 242–244 in the NTD,K417N in the RBD and A701V in S2 also increased viral entry.S982A weakened the generation of binding antibodies.All sera showed reduced cross-neutralization activity against B.1.351,B.1.617.2(Delta)and B.1.1.529(Omicron BA.1).S982A,L18F,and Del 242–244 hindered the induction of cross-NAbs,whereas Del 69–70,Del144,R246I,and K417N showed the opposite effects.B.1.351 elicited adequate broad cross-NAbs against both B.1.351 and B.1.617.2.All immunogens tested,however,showed low neutralization against circulating B.1.1.529.In addition,T-cell responses were unlikely affected by mutations tested in the spike.We conclude that individual spike mutations influence viral infectivity and vaccine immunogenicity.Designing VOC-targeted vaccines is likely necessary to overcome immune evasion from current vaccines and neutralizing antibodies.