Cationic liposomes containing a TLR4 agonist promote the efficient development of cellular immunity against SARS-CoV-2 Spike protein in a subunit vaccine.

Abstract

Abstract Subunit vaccines require an adjuvant, as protein antigens alone provoke a very poor immune response. Liposomes can be used to deliver an adjuvant, particularly when it is an amphipathic molecule such as a toll-like receptor (TLR) 4 agonist. TLR4 engagement causes the release of cytokines known to skew the immune response towards Th1-mediated cellular immunity, making it attractive for use in viral vaccines. Additionally, liposomes formulated with a TLR4 agonist can be neutral, positive, or negative in charge, further affecting the immune response. We tested the effect of liposome composition and charge on the immune response against the SARS-CoV-2 Spike protein following vaccination with recombinant Spike trimers and liposomes containing a synthetic TLR4 agonist in mice. Anti-Spike antibody titers were modestly boosted by neutral liposomes, but charged liposomes increased it further. Control groups given liposomes without the TLR4 agonist demonstrated that cationic liposomes alone boosted antibody titers, but gave a strikingly different T cell response. These liposomes evoked a strongly Th2-associated immune profile, with high IgG1 levels and T cells that released IL-5 when stimulated ex vivo. In contrast, the TLR4-stimulating cationic liposomes resulted in high IgG2c levels, along with strong production of IFNγ, some IL-17, and no IL-5 by restimulated T cells. Interestingly, despite similar antibody titers, serum from groups given the TLR4-stimulating liposomes more effectively prevented Spike binding to ACE-2 than that of the control groups in pseudoneutralization assays. These data show a beneficial effect of TLR4-stimulating cationic liposomes on the anti-viral cellular immune response in subunit vaccines. Supported by a supplement to NIH Adjuvant Discovery Contract 75N93019C00045