Dissecting the mechanism of action of the adjuvant system AS01 in human immune cells: synergy of QS-21 and monophosphoryl lipid A and role of type I interferons

The adjuvant system AS01 comprised of the TLR4 agonist monophosphoryl lipid A (MPLA) and the saponin QS-21 embedded in liposomes is currently used in recombinant vaccines against shingles, respiratory syncytial virus infection, and malaria. The efficacy of these vaccines is attributed to the strong immunostimulatory effect of AS01. While preclinical studies have shed light on the mechanisms by which AS01 enhances immune responses, few studies have been conducted with human immune cells to dissect how the two immunostimulants work individually and in combination. We investigated the effect of AS01 on human THP-1 cells and peripheral blood mononuclear cells (PBMCs) and found that it induced secretion of a broad panel of chemokines and a modest number of cytokines. Among the chemokines, CCL3 and CCL4 were found to have the greatest fold increase. Inhibition of TLR4 signaling revealed variable dependence on TLR4, while experiments with liposomes formulated with a TLR4 agonist, QS-21, or both demonstrated synergistic effects on the induction of many chemokines and TNF. AS01 induced strong expression of interferon-stimulated genes; this was greatly reduced by treatment with an antibody that blocks the type I interferon (IFN) receptor, indicating that AS01 induced a type I IFN-associated transcriptional signature that is partially dependent on signaling via the type I IFN receptor. AS01 selectively increased the expression of CD83 on THP-1 cells and this effect was found to be mostly driven by QS-21. This work provides novel insights into the mechanisms by which AS01 stimulates human immune cells.