Activation and regulation of toll-like receptor 9: CpGs and beyond

Toll-like receptor 9 has been the focus of considerable research attention for the ability to modulate its activity, and subsequent innate immune responses, through DNA-based immunotherapeutics. Nucleic acids are attractive as therapeutics for their low cost, chemical stability and ease of production. While the ability for TLR9 to be differentially regulated by nucleic acids of varying sequences and structures offers flexibility for immunotherapeutic design, it also necessitates a more comprehensive characterization of these agonists in terms of how these structural parameters correlate with the activation of unique cellular responses. Despite the utilization of TLR9 agonists in human trials these issues have not been adequately addressed. While a wealth of cell stimulation experiments demonstrate the preferential ability for nucleic acids which contain unmethylated cytosine-phosphate-guanine (CpG) motifs to initiate innate immune responses this has not been supported by binding investigations from which largely contradictory information has emerged with respect to the ability of TLR9 to bind nucleic acids in a sequence-dependent fashion. Recent models help to reconcile this apparent contradiction by suggesting that while TLR9 activation is specific for CpG-containing nucleic acids, the receptor binds, and is functionally influenced by, nucleic acids in a sequence-independent fashion. We have proposed a model in which the absolute concentration of nucleic acids modulates the sensitivity of the receptor in a sequence-dependent fashion while activation is specifically achieved by CpG-containing ligands. In this review we reconsider the literature from the perspective of this new appreciation of the functional complexity of TLR9 ligand binding and higher-order regulation with discussion of the implications for immunotherapeutic targeting of TLR9.