Augmenting Subunit-Vaccine-Induced Immunity through a Dual Strategy of Gold Nanoparticle Conjugation and Chitosan Microneedle-Mediated Sustained Delivery

Subunit vaccines offer high safety but often exhibit low immunogenicity and rapid clearance and require adjuvants. In this study, we developed a dual strategy for augmenting subunit-vaccine-induced immune responses by integrating self-adjuvanting gold nanoparticle (GNP)-antigen conjugates with implantable chitosan (CS) microneedles (MNs) to achieve sustained intradermal antigen exposure. Conjugation of a model antigen, namely, ovalbumin (OVA), onto the GNP surface (GNP-OVA) resulted in virus-mimicking multivalent antigen display, which substantially enhanced dendritic cell maturation, as evidenced by the upregulation of CD86 and major histocompatibility complex class II. This conjugation strategy also enabled the efficient codelivery of the antigen and carrier into the same antigen-presenting cells, thereby facilitating improved antigen presentation. Furthermore, compared with free OVA and a physical GNP/OVA mixture, conjugated GNP-OVA exhibited considerably longer lymph node retention, primarily because of its nanovaccine properties, which facilitate its preferential trafficking into lymphatic vessels and its subsequent accumulation in lymph nodes. Encapsulation of GNP-OVA into CS MNs (i.e., GNP-OVA MNs) resulted in reliable skin implantation, sustained intradermal antigen exposure, and local immune cell recruitment. Rat immunization studies revealed that GNP-OVA MNs induced balanced T helper 1 and T helper 2 responses and elicited considerably higher and more durable OVA-specific immunoglobulin G levels than did subcutaneous vaccination with GNP-OVA or OVA alone. These responses persisted for at least 16 weeks, highlighting the potential of the developed platform for prolonged subunit vaccine immunization. This dual-strategy platform, combining virus-mimicking GNP-based nanovaccines with immunostimulatory CS MNs, reduces reliance on external adjuvants and enhances the potency and durability of subunit vaccines. Its modular and patient-friendly design underscores its high potential for advancing the development of next-generation vaccines against emerging infectious diseases.