Foamy matters: an update on Quillaja saponins and their use as immunoadjuvants.
Immunoadjuvant Quillaja spp. tree saponins stimulate both cellular and humoral responses, significantly widening vaccine target pathogen spectra. Host toxicity of specific saponins, fractions and extracts may be rather low and further reduced using lipid-based delivery systems. Saponins contain a hydrophobic central aglycone decorated with several sugar residues, posing a challenge for viable chemical synthesis. These, however, may provide simpler analogs. Saponin chemistry affords characteristic interactions with cell membranes, which are essential for its mechanism of action. Natural sources include Quillaja saponaria barks and, more recently, Quillaja brasiliensis leaves. Sustainable large-scale supply can use young plants grown in clonal gardens and elicitation treatments. Quillaja genomic studies will most likely buttress future synthetic biology-based saponin production efforts.