Immune Adjuvant Effect of Molecularly-defined Toll-Like Receptor Ligands

Abstract

Vaccine efficacy is optimized by addition of immune adjuvants. However, although adjuvants have been used for over a century, to date, only few adjuvants are approved for human use, mostly aimed at improving vaccine efficacy and antigen-specific protective antibody production. The mechanism of action of immune adjuvants is diverse, depending on their chemical and molecular nature, ranging from non-specific effects (i.e., antigen depot at the immunization site) to specific activation of immune cells leading to improved host innate and adaptive responses. Although the detailed molecular mechanism of action of many adjuvants is still elusive, the discovery of Toll-like receptors (TLRs) has provided new critical information on immunostimulatory effect of numerous bacterial components that engage TLRs. These ligands have been shown to improve both the quality and the quantity of host adaptive immune responses when used in vaccine formulations targeted to infectious diseases and cancer that require both humoral and cell-mediated immunity. The potential of such TLR adjuvants in improving the design and the outcomes of several vaccines is continuously evolving, as new agonists are discovered and tested in experimental and clinical models of vaccination. In this review, a summary of the recent progress in development of TLR adjuvants is presented.

Keywords: TLR; cancer; immune response; microbial pathogens; vaccine adjuvant.

Figure 1

Schematic cartoon of Toll-like receptor (TLR) signaling [17,24,27,28,29,30]. Extracellular TLR homodimers (TLR4 and TLR5) are represented in black; heterodimers of TLR2 and TLR1, TLR6 or TLR10 are indicated in black/green. Intracellular homodimers (TLR3, TLR7, TLR8 and TLR9) are indicated in gray.