Toll-like receptor (TLR) and saponin adjuvants each have improved vaccine potency and safety. Now, researchers at Stanford University have shown that combining them in a nanoparticle form not only improves potency, but also durability, target breadth and degree of virus neutralization.
A modular approach enables fine-tuning of adjuvants by mixing saponin nanoparticles (SNPs) and TLR adjuvants into a single nanostructure. Paper Eric Appel, PhD, associate professor of materials science and engineering at Stanford University, is senior author and the first author to have a postdoctoral fellow in the Appel lab, Ben O.
The TLRa-SNP cohorts „expressed different levels of IgG isotypes,” the scientists said. Some, for example, produced IgG1 responses comparable to those produced by SNPs, while one produced an order of magnitude higher IgG1 response. However, all TLRa-SNP associates exhibited at least fivefold higher IgG2c titers than SNPs.
The scientists developed four types of TLRs: Pam3CSK4, Monophosphoryl-Lipid A (MPLA), an imidazoquinoline derivative, and a combination of imidazoquinoline derivative and MPLA. Each was conjugated to a saponin and used as an adjuvant in SARS-CoV-2 and HIV vaccines. In mice, each formulation elicited significantly stronger humoral responses than saponin nanoparticles (ISCOMATRIX™) or aluminum hydroxide/alum adjuvants without TLRs.
A broad, lasting bond
„We expected an increase in broadly neutralizing antibodies and a better breadth of responses,” Oh says. „However, we were pleasantly surprised to find that our nanoparticle complement could bind strongly to even the most immune-evasive variants, such as beta and omicron variants. [of SARS-CoV-2].
Compared to SNP adjuvants, TLRa-SNPs adjuvants caused approximately 50% more antigen accumulation in draining lymph nodes. Additionally, immunized mice “showed significantly higher anti-RBD IgG endpoint titers at all time points. [the boost]with minor variations among animals.
Even after one year, the mean titer concentration values (EC50) in mice immunized using TLRa-SNP adjuvants was an order of magnitude higher than SNP or aluminum hydroxide/alum adjuvants. For context, titers for aluminum/alum adducts decreased 24-fold and saponin nanoparticles decreased 13-fold. Titers of TLRa-SNPs remained high, however, only decreased five- to sevenfold.
Importantly, the researchers found zero infection at week five among mice receiving the TLRa-SNPs vaccines. Infectivity of human convalescent plasma, in contrast, averaged 15%.
„Nanoparticle-based vaccine adjuvants are already on the market, so adoption of our adjuvant should not affect vaccine production,” says Ou. They fit into the normal pharmaceutical temperature supply chain, being stable at -20°C for several months, and at 4°C for at least six weeks.