Adjuvant System AS01 is a liposome-based vaccine adjuvant containing 3-O-desacyl-4′-monophosphoryl lipid A and the saponin QS-21. AS01 has been selected for the clinical development of several candidate vaccines including the RTS,S malaria vaccine and the subunit glycoprotein E varicella zoster vaccine (both currently in phase III). Given the known immunostimulatory properties of MPL and QS-21, the objective of this study was to describe the early immune response parameters after immunization with an AS01-adjuvanted vaccine and to identify relationships with the vaccine-specific adaptive immune response. Cytokine production and innate immune cell recruitment occurred rapidly and transiently at the muscle injection site and draining lymph node postinjection, consistent with the rapid drainage of the vaccine components to the draining lymph node. The induction of Ag-specific Ab and T cell responses was dependent on the Ag being injected at the same time or within 24 h after AS01, suggesting that the early events occurring postinjection were required for these elevated adaptive responses. In the draining lymph node, after 24 h, the numbers of activated and Ag-loaded monocytes and MHCIIhigh dendritic cells were higher after the injection of the AS01-adjuvanted vaccine than after Ag alone. However, only MHCIIhigh dendritic cells appeared efficient at and necessary for direct Ag presentation to T cells. These data suggest that the ability of AS01 to improve adaptive immune responses, as has been demonstrated in clinical trials, is linked to a transient stimulation of the innate immune system leading to the generation of high number of efficient Ag-presenting dendritic cells.

Alum has been the most widely used adjuvant for over 80 years. Although there have been searches for alternative adjuvants, aluminium-containing adjuvants will continue to be used for many years due to their good track record of safety, low cost and adjuvanticity with a variety of antigens. For infections that can be prevented by induction of serum antibodies, aluminium-containing adjuvants formulated under optimal conditions are the adjuvants of choice. There are also some limitations of aluminium-containing adjuvants, which include local reactions, augmentation of IgE antibody responses, ineffectiveness for some antigens and inability to augment cell-mediated immune responses, especially cytotoxic T-cell responses. In this review, we describe the current knowledge regarding the mechanisms (both cellular and molecular) by which alum employs its adjuvant effect, although the final mechanism is not yet well-defined. Furthermore, we discuss how alum's adjuvanticity could be improved.

B cells are a major part of the adaptive immune response to inhaled HDM allergen, particularly when the amount of inhaled allergen is low, by expanding allergen-specific T cells.