On the dilution behaviour of immuno-stimulating complexes (ISCOMs).

Immunostimulating complexes (ISCOMs) are used as potent vaccine delivery systems. However, the mechanisms by which ISCOMs work are mostly unknown. The immunological potency of ISCOMs has often been associated with their characteristic cage-like structure. Since ISCOMs can be regarded as composite mixed micelles that contain Quillaja saponin as the surfactant, we postulated a micelle to vesicle transition of the particles upon dilution with aqueous solutions. The dilution behaviour of ISCOMs is not only an important preparative aspect, but may also be of high relevance for both cell culture and in vivo applications in which dilutions occur. Crude and purified preparations of ISCOM matrices were prepared by dialysis. Methods used to analyse the micelle to vesicle transitions were transmission electron microscopy and dynamic light scattering. Significant morphological changes occurred upon dilution with TRIS buffered saline and non-buffered saline, and a step-wise transition from the typical cage-like structure via both less well defined ISCOM-like structures and helical micelles to small lipidic particles was observed. Aggregation of the resulting small lipidic particles was noted. The results obtained by dynamic light scattering complemented these findings. With increasing dilution factors, increase in particle size and polydispersity was observed. Zeta-potentials showed a trend towards less negative values upon dilution, indicating that saponins were not retained within the lipid matrix. The results indicated that at least partial separation of Quillaja saponins from the remaining colloidal species occurred upon dilution with aqueous solutions. The release of saponins upon dilution may be an important aspect of how ISCOMs work.