HA-ppy immunity: unveiling the power of hyaluronan as an adjuvant in SARS-CoV-2 vaccination

The COVID-19 pandemic highlighted the urgent need for the development of highly effective and scalable vaccines. This research explores the development of a novel protein-based vaccine utilizing hyaluronic acid (HA) as an adjuvant, in the context of SARS-CoV-2. HA has been widely recognized for its exceptional physicochemical properties such as biodegradability, biocompatibility and nontoxicity, and therefore, is already implied in several medical applications. As previously reported, HA also displayed immunomodulatory properties suitable for an ideal vaccine adjuvant. Nonetheless, its role in this context remains largely underexplored. In this study, we validated a protein-based vaccine formulated through the conjugation of the receptor binding domain (RBD) of the SARS-CoV-2 Spike protein to HA (HA-RBD). Using appropriate preclinical models, such as BALB/c and K18-hACE2 mice, we assessed the immune response elicited by this bioconjugate following a two-dose schedule (prime + booster). HA-RBD successfully induced a robust antigen-specific IgG response, including remarkable levels of the IgG1 and IgG2c subclasses. Additionally, antibodies elicited by HA-RBD displayed neutralizing activity in vitro, a critical feature for counteracting SARS-CoV-2 infection. The bioconjugate outperformed other commercial adjuvants, such as Quil-A and AddaVax, and due to HA’s biocompatibility, it demonstrated non-reactogenicity at the injection site, avoiding local side effects commonly associated with other adjuvants. HA-RBD also promoted long-term immunological memory, as indicated by sustained antibody production over time, even with only two doses. This aspect was further corroborated by the observation, several months after the administration, of long-lived plasma cells, a cellular subpopulation essential to establish durable immunity. Finally, HA-RBD demonstrated protective efficacy by guaranteeing the survival of K18-hACE2 mice, a murine model susceptible to SARS-CoV-2, following viral challenge shortly after the vaccination. Moreover, HA-RBD also proved effective against sequential infections with different viral variants and long after the last administration. Despite these promising results, certain limitations were identified. For instance, the ability of HA-RBD to elicit an effective cellular response is yet to be determined and the precise mechanisms underlying HA’s adjuvant properties require further elucidation. Further work 2 will focus on investigating these aspects, nevertheless, the observed efficacy of HA in enhancing the immune response and promoting long-term protection indicates that this vaccine platform holds significant promise. In conclusion, this thesis provides strong evidence that HA is an effective adjuvant for protein-based vaccines, paving the way for its further investigation in the development of durable vaccines for infectious diseases.