Down-regulation of IRAK-4 is a component of LPS- and CpG DNA-induced tolerance in macrophages

Macrophages are important mediators of the immune response to infection by virtue of, amongst other things, their ability to secrete cytokines (e.g. TNF) that trigger inflammation. However, excessive systemic release of inflammatory cytokines can cause septic shock and ultimately death. Tolerance is an adaptive mechanism that prevents macrophage activation and inflammatory cytokine production. The activation of macrophages by pathogens is largely mediated by Toll-like receptors (TLRs). IRAK-4 and IRAK-1 are proximal protein kinases in TLR signalling pathways; IRAK-1 is activated via its phosphorylation by IRAK-4. The rapid degradation of IRAK-1 following its TLR-induced activation has been proposed to represent a major mechanism for tolerance. Here, we established that IRAK-1 degradation is insufficient to cause tolerance; in the absence of IRAK-1, IRAK-4 likely activates downstream signalling proteins (e.g. NF-κB) via IRAK-2. Significantly, tolerance coincided with IRAK-4 down-regulation, which occurred at the protein level via proteolytic degradation as well as at the mRNA level. Gene silencing experiments confirmed the importance of IRAK-4 for the regulation of TNF expression. The different kinetics of IRAK-4 and IRAK-1 down-regulation may result in both quantitative and qualitative differences in TLR signalling and potentially allow macrophages to temporally modify their inflammatory responses. Furthermore, differences in the kinetics and extent of IRAK-4 down-regulation by TLR ligands may provide a mechanism whereby macrophages can tailor their inflammatory response according to the location and/or type of pathogen detected.