TLR4, the classical receptor for lipopolysaccharide on Gram-negative bacteria, has also been implicated as a sensor for an unidentified, heat-sensitive mycobacterial ligand (Quesniaux et al., 2004; Lahiri et al., 2008). Other important innate immune receptors are the cytosolic nucleotide-binding and oligomerization domain-like receptors or NOD-like receptors (NLRs), which are TLR-related proteins responsible for recognition of intracellular pathogens, including mycobacteria. NOD1 and NOD2 specifically bind diaminopimelic acid and the peptidoglycan breakdown
product muramyl dipeptide, triggering the production of proinflammatory cytokines. This suggests a synergistic effect between NLRs and TLR2 in tuberculosis (Korbel et al., 2008). As mentioned earlier, both TLR and NLR ligands promote inflammation by triggering the release of chemokines and proinflammatory cytokines, expression of adhesion molecules and recruitment of macrophages, DCs PARP activation and polymorphonuclear neutrophils (Korbel et al., 2008). After antigen processing and expression of epitopes in an MHC-restricted manner, mature DCs can stimulate naive T cells to differentiate into effector cells. Depending on the ligand, the immune response may thus be skewed toward CTL responses or toward a particular Th response (Boog, 2008). Based on increasing evidence for the contribution of CD1-restricted immune responses to protection against tuberculosis, CD1-restricted,
nonproteinaceous ligands, such as glycolipids, are also being considered as potential candidates for new tuberculosis vaccines (Hamasur et al., 2003). In conclusion, mycobacterial ligands have great potential as adjuvants due to their ability to activate the innate immune buy PS-341 response, ultimately leading to cellular and humoral responses against coadministered antigens (Mills, 2009). In this context, synthetic ligands capable of targeting TLRs more precisely and safely than pathogen-derived ligands are being designed (Guy, 2007). However, a great deal of work is still required, because the success
of vaccination is related to the route of administration, the delivery method used and the APC population stimulated by the adjuvant. On the other hand, TLR overstimulation can also generate unwanted toxic effects, and so adjuvant dose and mechanism of action must be carefully considered and potential toxicities should be investigated and Ribonucleotide reductase characterized (Boog, 2008). Despite the limited number of adjuvant systems approved for clinical applications, several vaccine delivery and adjuvant combinations have been evaluated, resulting in promising preliminary formulations. Currently, four leading adjuvants for tuberculosis subunit vaccines are being investigated: CAF01 (LipoVac), developed by the Statens Serum Institute, is a novel tuberculosis vaccine adjuvant utilizing N,N′-dimethyl-N,N′-dioctadecylammonium (DDA) liposomes with the synthetic mycobacterial immunomodulator α, α′-trehalose 6,6′-dibehenate (TDB) inserted into the lipid bilayer.