A cGAMP-containing hydrogel for prolonged SARS-CoV-2 RBD subunit vaccine exposure induces a broad and potent humoral response

The SARS-CoV-2 virus spike protein, specifically its receptor binding domain (RBD), has emerged as a promising target for generation of neutralizing antibodies. Although the RBD peptide subunit is easily manufactured and highly stable, RBD-based subunit vaccines have been hampered by its poor inherent immunogenicity. We hypothesize that this limitation can be overcome by sustained co-administration alongside a potent and optimized adjuvant. The innate immune second messenger, cGAMP, holds promise as it activates the potent anti-viral STING pathway, but has exhibited poor performance as a therapeutic due to its nonspecific pharmacodynamic profiles when administered systemically and its poor pharmacokinetics arising from rapid excretion and degradation by its hydrolase ENPP1. To overcome these limitations, we sought to mimic the natural scenario of viral infections by creating an artificial immunological niche that enables slow release of cGAMP and the RBD antigen. Specifically, we co-encapsulated cGAMP and RBD in an injectable polymer-nanoparticle (PNP) hydrogel system. This cGAMP-adjuvanted hydrogel vaccine elicited more potent, durable, and broad antibody responses and improved neutralization than both dose-matched bolus controls and a hydrogel-based vaccine lacking cGAMP. The cGAMP-adjuvanted hydrogel platform developed is suitable for delivery of other antigens and may provide enhanced immunity against a broad range of pathogens.

Hydrogel-based slow release of a receptor-binding domain subunit vaccine elicits neutralizing antibody responses against SARS-CoV-2

The development of effective vaccines that can be rapidly manufactured and distributed worldwide is necessary to mitigate the devastating health and economic impacts of pandemics like COVID-19. The receptor-binding domain (RBD) of the SARS-CoV-2 spike protein, which mediates host cell entry of the virus, is an appealing antigen for subunit vaccines because it is efficient to manufacture, highly stable, and a target for neutralizing antibodies. Unfortunately, RBD is poorly immunogenic. While most subunit vaccines are commonly formulated with adjuvants to enhance their immunogenicity, we found that clinically-relevant adjuvants Alum, AddaVax, and CpG/Alum were unable to elicit neutralizing responses following a prime-boost immunization. Here we show that sustained delivery of an RBD subunit vaccine comprising CpG/Alum adjuvant in an injectable polymer-nanoparticle (PNP) hydrogel elicited potent anti-RBD and anti-spike antibody titers, providing broader protection against SARS-CoV-2 variants of concern compared to bolus administration of the same vaccine and vaccines comprising other clinically-relevant adjuvant systems. Notably, a SARS-CoV-2 spike-pseudotyped lentivirus neutralization assay revealed that hydrogel-based vaccines elicited potent neutralizing responses when bolus vaccines did not. Together, these results suggest that slow delivery of RBD subunit vaccines with PNP hydrogels can significantly enhance the immunogenicity of RBD and induce neutralizing humoral immunity.