Biogenic gold nanoparticles synthesized using Streptomyces sp. M137-2 as potential vaccine adjuvant

In this study, we describe the adjuvanticity of biogenic AuNPs, previously synthesized as 40-50 nm spherical particles from cell-free fermentation broth of Streptomyces sp. M137-2. Bovine serum albumin (BSA), a model antigen, was conjugated to biogenic AuNPs to form AuNPs-BSA. The conjugates were characterize using UV-Vis spectroscopy, scanning electron microscopy (SEM), and Fourier-transform infrared spectroscopy (FT-IR). Both biogenic AuNPs and AuNPs-BSA showed high viability (over 70%) in macrophages derived from U937 cells. Biogenic AuNPs enhanced the uptake of conjugated BSA by macrophages. AuNPs-BSA-stimulated macrophages exhibited cytokine levels of TNF-α and IL-6 that were 4.5 times (p < 0.0001) and 4.6 times (p = 0.004) higher, respectively, as compared to BSA-stimulated cells. The adjuvanticity of the biogenic AuNPs is likely attributed to their antigen-carrier properties, which enhance antigen uptake by macrophages, as well as their immunostimulatory properties that induce strong pro-inflammatory responses. This study represents the first proposal of biogenic AuNPs as an adjuvant candidate, providing further studies in in vivo models.