Development of a Novel Polyleucine-Based Immunopotentiator for Subunit Vaccines Against Group A Streptococcus

Mixture-based vaccine formulations offer distinct advantages by eliminating the need for chemical conjugation or complex encapsulations, thereby streamlining preparation, enhancing versatility, and reducing cost and production time. In this study, we report the design of a novel polypeptide-based self-assembling adjuvant where a water insoluble 15-mer polyleucine (L₁₅) is coupled with a polylysine unit (K₆). As a nano adjuvant, L₁₅K₆ markedly enhances antigen uptake by dendritic cells, promotes antigen-presenting cell maturation (CD40, CD80/86, and MHC-II), and upregulates Toll-like receptor expression, particularly Toll-like receptor 4 in dendritic cells. Bulk RNA sequencing further reveals robust NF-κB pathway activation by L₁₅K₆. When co-delivered with peptide or protein antigens derived from Group A Streptococcus, L₁₅K₆ induces robust antigen-specific IgG response with highly potent antibacterial activity against two clinical Group A Streptococcus strains in vitro. Beyond antigens derived from Group A Streptococcus, L₁₅K₆ also enhances immune responses against model protein antigens such as ovalbumin and bovine serum albumin, highlighting its broad applicability. Notably, a single immunization with L₁₅K₆ outperforms alum adjuvant in eliciting high levels of bovine serum albumin-specific IgG. These findings establish L₁₅K₆ as a simple yet powerful immunopotentiator with significant promise for the development of broadly applicable subunit vaccines.