Immunogenicity evaluation of next-generation vaccine platforms for prevention of Influenza and COVID-19

Background: Over the past hundred years, humans have experienced a long list of microbial threats to health. Among them, pandemics represent one of the major burdens for public health, economy and society. Influenza and most recently Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2), being two pandemic-risk viruses both transmitted through the respiratory tracts, share many common features, and the constant emergence of new viral variants brings difficulties to permanently control the disease they cause [1]. Development of new, safer, cheaper, high-throughput vaccine platforms (such as DNA- or vector-based vaccines) targeting most conserved and immunodominant viral epitopes, might be the turning point to ensure control of infectious diseases and prevention of pandemics [2]. Aim: The aim of this thesis is to report immunogenicity findings of new vaccine platforms, which may contribute to their development as future vaccination strategies against pandemic-causing infectious diseases, such as Influenza and COVID-19. Methods: In Project 1, serum samples from BALB/c mice immunized intramuscularly with two different DNA-based Influenza vaccine constructs expressing the Influenza Neuraminidase (NA) protein have been tested in a pseudotype-based Enzyme-Linked Lectin Assay (pELLA) assay, in order to evaluate the presence of post-vaccination NA-inhibiting antibodies. In Project 2, we evaluated the potential of a Leishmania tarentolae (Lt)-purified SARS-CoV-2 recombinant RBD-SD1 antigen (“Lt-RBD”) and of a Lt-based vaccine platform expressing SARS-CoV-2 S protein (“Lt-spike”) combined with the Lt-RBD (LeCoVax-2) in inducing antigen-specific T cell mediated responses when administered to BALB/c mice via the mucosal (rectal, R) or systemic (subcutaneous, SC) route of immunization. Both Lt-RBD and LeCoVax-2 were used either adjuvanted or not. T cell responses (release of IFN-γ, IL-4, TNF-α) raised after vaccine administration were assessed by Enzyme-Linked ImmunoSpot (ELISpot) assay upon isolation of splenocytes. Results and Conclusions: Results from both Projects highlight an elicitation of immune responses after BALB/c mice immunization with either DNA-based or Lt-based vaccines. In Project 1, a positive titer of NA-inhibiting antibodies was especially detected in serum samples belonging to the mice group immunized with a high dose (10 μg) of the two different dbDNA™ vaccines, “Construct 1” and “Construct 2”, but an overall positive result was also detected in the group treated with a low dose (1μg) of “Construct 2”. These results support the use of the innovative dbDNATM as DNA-based vaccine platform for Influenza NA and the suitability of the pELLA assay for the immunogenicity assessment of this kind of NA-expressing vaccines. In Project 2, promising data have been obtained upon evaluation of antigen-specific cytokines-producing T-cell capacity of both the Lt-based platform LeCoVax-2 and the Lt-purified antigen RBD-SD1. When administered via the R route, the purified adjuvanted RBD-SD1 did not induce any detectable T-cell-mediated immune responses, in comparison with the relatively high production of Th1/Th2 cytokines observed after immunization with LeCoVax-2 (especially when adjuvanted). LeCoVax-2, however, was also effective when administered enterally without adjuvant. These results show immunogenicity of this innovative Lt-based platform also by mucosal immunization and pose the basis for further investigations. In conclusion, our results support further development of the two novel vaccine platforms evaluated. These studies are worthy to be conducted as they might give us a clue not only about the most immunogenic but also affordable and scalable vaccination strategy to use in order to promptly react to the next pandemic [4].