Novel adjuvants are highly desired to improve immune responses of SARS-CoV-2 vaccines. This work reports the potential of the stimulator of interferon genes (STING) agonist adjuvant, the cyclic di-adenosine monophosphate (c-di-AMP), in a SARS-CoV-2 vaccine based on the receptor binding domain (RBD). Here, mice immunized with two doses of monomeric RBD adjuvanted with c-di-AMP intramuscularly were found to exhibit stronger immune responses compared to mice vaccinated with RBD adjuvanted with aluminum hydroxide (Al(OH)3 ) or without adjuvant. After two immunizations, consistent enhancements in the magnitude of RBD-specific immunoglobulin G (IgG) antibody response were observed by RBD + c-di-AMP (mean: 15360) compared to RBD + Al(OH)3 (mean: 3280) and RBD alone (n.d.). Analysis of IgG subtypes indicated a predominantly Th1-biased immune response (IgG2c, mean: 14480; IgG2b, mean: 1040, IgG1, mean: 470) in mice vaccinated with RBD + c-di-AMP compared to a Th2-biased response in those vaccinated with RBD + Al(OH)3 (IgG2c, mean: 60; IgG2b: n.d.; IgG1, mean: 16660). In addition, the RBD + c-di-AMP group showed better neutralizing antibody responses as determined by pseudovirus neutralization assay and by plaque reduction neutralization assay with SARS-CoV-2 wild type. Moreover, the RBD + c-di-AMP vaccine promoted interferon-γ secretion of spleen cell cultures after RBD stimulation. Furthermore, evaluation of IgG-antibody titers in aged mice showed that di-AMP was able to improve RBD-immunogenicity at old age after 3 doses (mean: 4000). These data suggest that c-di-AMP improves immune responses of a SARS-CoV-2 vaccine based on RBD, and would be considered a promising option for future COVID-19 vaccines.
Keywords: SARS-CoV-2; adjuvant; c-di-AMP; immune response; vaccines.
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