Differential effects of CpG-DNA in Toll-like receptor-2/-4/-9 tolerance and cross-tolerance

Lipopolysaccharide (LPS) tolerance is a state of refractoriness towards a second stimulation by LPS after a preceding stimulation. LPS is recognized by Toll-like receptor-4 (TLR-4), which belongs to a group of pattern recognition receptors mediating activation of innate immunity by microbial components. To date, it is not known in detail to what extent other TLR-dependent stimuli also induce tolerance and whether preceding and challenging stimuli are interchangeable. We have examined tolerance induction in detail for lipoteichoic acid (LTA), LPS and CpG-DNA, which are recognized by TLR-2, -4 and -9, respectively. In RAW264.7 macrophages, all three stimuli induced tolerance towards a subsequent challenge with the same stimulus used for priming, as well as cross-tolerance towards subsequent challenge with other stimuli signalling via different TLRs. However, whereas LPS/LTA cross-tolerance was also functional in an in vivo model of galactosamine (GalN)-primed liver damage, pretreatment with CpG only protected against GalN/CpG challenge and failed to induce cross-tolerance for LPS and LTA. CpG-DNA pretreatment even enhanced tumour necrosis factor (TNF)-alpha production and liver damage upon subsequent challenge with LPS or LTA. Stimulation with CpG-DNA resulted in a peculiar sensitization for interferon (IFN)-gamma secretion. The data indicate that, in contrast to in vitro macrophage desensitization, the in vivo consequences of repeated TLR stimulation greatly differ amongst different TLR ligands.