The discriminative capacity of soluble Toll-like receptor (sTLR)2 and sTLR4 in inflammatory diseases

ten Oever, Jaap; Kox, Matthijs; van de Veerdonk, Frank L; Mothapo, Khutso M; Slavcovici, Adriana; Jansen, Tim L; Tweehuysen, Lieke; Giamarellos-Bourboulis, Evangelos J; Schneeberger, Peter M; Wever, Peter C; Stoffels, Monique; Simon, Anna; van der Meer, Jos WM; Johnson, Melissa D; Kullberg, Bart-Jan; Pickkers, Peter; Pachot, Alexandre; Joosten, Leo AB; Netea, Mihai G

In addition to the release of sTLR2 and sTLR4 from stimulated immune cells [4, 5] , constitutive release of sTLRs has been demonstrated in various biological fluids such as saliva, breast milk, and amniotic fluid [10, 23] . In plasma, sTLR2 represented by several polypeptides, has been found by others [4] , although the concentrations are low. To avoid both harmful or insufficient inflammatory responses, inhibition and activation of the immune system needs to be properly balanced. Various negative regulators of TLRs have been described [1] of which sTLR2 and sTLR4 constitute an important first-line negative regulatory mechanism [4] [5] [6] [23] [24] [25] . sTLR2 either interferes with CD14-mediated triggering of membranebound TLR2, dimerizes with TLR2 on the cell surface, or competes with cellular TLR2 for microbial ligands [4] . The complex formed by sTLR4 and MD-2 probably blocks the interaction between membrane-bound TLR4 and its ligand [25] . The rapid elevation of sTLR2 and sTLR4 in plasma upon LPS administrations, similar to that of pro-inflammatory cytokines, indicates that this feedback mechanism is rapidly activated. Consistent with our in-vitro data, the release of sTLRs in to the circulation demonstrates that immune modulation mediated by TLRs is not limited to the stimulation of the corresponding receptor on the cell membrane of immune cells. Since both sTLR2 and sTLR4 dampen inflammation by disrupting TLR-mediated pro-inflammatory responses [4] [5] [6] [23] [24] [25] , it might be possible that the counter regulatory mechanisms mediated by sTLRs extend to interference with endogenous TLR ligands. Although the kinetics of sTLR2 and sTLR4 concentrations parallel those of anti-inflammatory cytokines, quantitatively they appear to be differentially regulated. Plasma sTLR4 levels did not show any correlation with both IL-10 and IL-1Ra and for sTLR2, a negative correlation with IL-10 was found. Interestingly, while in-vitro release of sTLR2 and sTLR4 by immune cells is comparable, their in-vivo concentrations differ strongly, with much higher concentration in the circulation of sTLR2: this suggests a much more rapid clearance of sTLR4 from circulation. This may imply that these anti-inflammatory mechanisms are regulated at a different level and are potential complementary strategies to reduce inflammation.