A spleen-targeted vaccine for SARS-CoV-2 - Inducting neutralizing antibodies in mice.

The development of vaccines against infectious diseases is of the utmost importance to prevent global pandemics such as COVID-19. The application of antigens and adjuvants to efficiently target antigen presenting cells (APCs) is paramount for the development of efficient vaccines. In our previous study, we showed that splenic marginal zone-B (MZ-B) cells are promising APCs in addition to dendritic cells (DCs). In this study we achieved the targeted delivery of sufficient antigen to MZ-B cells by utilizing an intravenous (IV) immunization system we originally developed. This system involves the sequential injection of empty PEGylated liposomes (PEG-Lip) and antigen-containing PEG-Lip within a prescribed interval. Herein, we describe the application of this IV immunization system as a COVID-19 vaccine to induce specific antibodies against SARS-CoV-2. To establish efficacy, SARS-CoV-2 spike proteins were used as an antigen, and α-galactosylceramide (GC) was used as an adjuvant in this study. Three days after priming with empty PEG-Lip, we injected PEG-Lip containing spike protein and α-GC. Our IV immunization system successfully induced higher levels of anti-spike antibodies when spike protein derived from HEK-293, but not E. coli., was injected into mice. The levels were less produced using conventional immunization via subcutaneous (s.c.) injections of complete Freund's adjuvant without priming. Interestingly, a lower dose (0.2 µg) of spike protein antigen encapsulated into PEG-Lip induced a higher level of anti-spike antibodies than that produced using a significantly higher dose (5 µg). The induced anti-spike antibodies inhibited the interaction between the receptor binding domain of the spike protein and the angiotensin-converting enzyme 2. This indicates that the induced antibodies tend to neutralize SARS-CoV-2. Collectively, the specific delivery of spike proteins to spleen, probably MZ-B cells, via nano-carriers could be a promising approach for the development of global pandemic vaccines that require only minimum dosages of antigen.