Immune mechanisms of protection: can adjuvants rise to the challenge?

Table 2 Proposed mechanisms of adjuvant activity of major adjuvant components

Adjuvant	Composition	Adjuvant activity	Mechanism of adjuvant action	References
Aluminum salts	AlOH or AlPO₄ non-crystalline gels	Antibody and T_H2 cells	↑ Chemokine/cytokine production; ↑ recruitment of monocytes and differentiation to DC; ↑ antigen uptake by DC	[31, 34, 46, 106, 107]
MF59	Oil (squalene)-in-water emulsion	↑ Ab titre; ↑ Ab cross-reactivity; drives T_H2 cells	↑ Chemokine/cytokine production; recruitment of myeloid DCs to injection site; ↑ antigen uptake by DCs	[34, 108–110]
TLR ligands	MPL, GpG, imiquimod, resiquimod (both imidazoquinolinamines) or poly(I:C)	Drives T_H1 and CTL cells; ↑ T cell memory	TLR signaling in DCs promotes antigen presentation on MHC I and MHC II, enhanced migration of DCs to lymph nodes and DCs cytokine production; may have direct impacts on lymphocytes	[111]
QS21	Purified fraction of Quil A that has lower toxicity and retains adjuvant effects	Antibody, T_H1 and CTL responses	Enhances protective responses through poorly understood mechanisms; has lytic capacity and local reactogenicity	[97]

Abbreviations: Ab, antibody; CTL, cytotoxic T lymphocyte; MPL, monophosphoryl lipid A; poly(I:C), a synthetic analog of double-stranded RNA; Th, T helper cell; TLR, Toll-like receptor.

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