Mechanisms of Mixed Th1/Th2 Responses in Mice Induced by Albizia julibrissin Saponin Active Fraction by in Silico Analysis

The purified active fraction of Albizia julibrissin saponin (AJSAF) is an ideal adjuvant candidate that improves antigen-specific both cellular and humoral immune responses and elicits mixed Th1/Th2 responses, but its mechanisms remain unclear. The key features of action of AJSAF were investigated in mice immunized with Newcastle disease virus-based recombinant influenza vaccine (rL-H5) and AJSAF at the same leg (AJSAF+rL-H5) or different legs (AJSAF/rL-H5). The adjuvant activity of AJSAF on rL-H5 is strictly dependent on their spatial colocalization. Serum H5 antigen (H5Ag)-specific IgG, IgG1, IgG2a, and IgG2b antibody titers in AJSAF+rL-H5 group were significantly higher than those in AJSAF/rL-H5 group. The mechanisms of selectivity of Th1 or Th2 in mice induced by AJSAF was explored by the transcriptomic and proteomic profiles of H5Ag-stimulated splenocytes from the immunized mice using gene microarray and two-dimensional difference gel electrophoresis coupled with matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. Compared to rL-H5 alone, AJSAF/rL-H5 induced more differentially expressed genes (DEGs) than AJSAF+rL-H5, whereas AJSAF+rL-H5 upregulated higher mRNA expression of Th1 (T-bet, IFN-γ, TNF-α, IL-12β, and IL-12Rβ1) and Th2 (IL-10 and AICDA) immune response genes. The neutrophil response and its derived S100A8 and S100A9 might be involved in the AJSAF-mediated Th1 response. Meanwhile, AJSAF might induce the adaptive immune responses by improving a local innate immune microenvironment. These findings expanded the current knowledge on the mechanisms of action of saponin-based adjuvants, and provided new insights into how adjuvants shape adaptive immune responses.