ABSTRACT
Development of effective vaccines for infectious diseases has been one of the most successful global health interventions in history. Though, while ideal subunit vaccines strongly rely on antigen and adjuvant(s) selection, the mode and timescale of exposure to the immune system has often been overlooked. Unfortunately, poor control over the delivery of many adjuvants, which play a key role in enhancing the quality and potency of immune responses, can limit their efficacy and cause off-target toxicities. There is critical need for new adjuvant delivery technologies to enhance their efficacy and boost vaccine performance. Nanoparticles (NPs) have been shown to be ideal carriers for improving antigen delivery due to their shape and size, which mimic viral structures, but have been generally less explored for adjuvant delivery. Here, we describe the design of self-assembled poly(ethylene glycol)-b-poly(lactic acid) nanoparticles decorated with CpG, a potent TLR9 agonist, to increase adjuvanticity in COVID-19 vaccines. By controlling the surface density of CpG, we show that intermediate valency is a key factor for TLR9 activation of immune cells. When delivered with the SARS-CoV-2 spike protein, CpG NP adjuvants greatly improve the magnitude and duration of antibody responses when compared to free CpG, and result in overall greater breadth of immunity against variants of concern. Moreover, encapsulation of CpG NPs into injectable polymeric-nanoparticle (PNP) hydrogels enhance the spatiotemporal control over co-delivery of CpG NP adjuvant and spike protein antigen such that a single immunization of hydrogel-based vaccines generates comparable humoral responses as a typical prime-boost regimen of soluble vaccines. These delivery technologies can potentially reduce the costs and burden of clinical vaccination, both of which are key elements in fighting a pandemic.