The effect of adenosine receptor agonists on cytokine release by human mononuclear cells depends on the specific Toll-like receptor subtype used for stimulation.
In the present study, we determined whether the immunomodulatory effect of adenosine receptor stimulation depends on the Toll-like Receptor (TLR) used for stimulation of cytokine release. Therefore, human mononuclear cells were stimulated by different TLR agonists in the absence and presence of A1 (CPA), A2a (CGS21680), and A3 (Cl-IB-MECA) adenosine receptor agonists. Effects of these agonists on Il-6, Il-10, IFN-gamma, TNF-alpha, and Il-1beta production were expressed as percentage inhibition/stimulation after TLR stimulation. CGS21680 inhibited TLR4-mediated TNF-alpha release and potentiated TLR3- and TLR5-mediated IL-6 release. Cl-IB-MECA inhibited TLR4-agonist-induced IFN-gamma release. Interestingly, CPA en Cl-IB-MECA tended to inhibit cytokine release only after TLR4 stimulation. In more detail, CPA potentiated TLR5-mediated IL-6 production, TLR3-mediated IFN-gamma production and TLR3-mediated Il-1beta-production compared to TLR4-mediated stimulation. Cl-IB-MECA potentiated TLR5-mediated IL-6 and Il-1beta formation as compared to TLR4-mediated stimulation. Finally, CGS21680 potentiated TLR5-mediated IL-6 production compared to TLR1-2 stimulation, and potentiated TLR3- and TLR5-mediated IL-10 production compared to TLR1-2-mediated stimulation. In conclusion, the effect of adenosine agonists on cytokine production depends on the specific TLR agonist used for stimulation. These findings suggest that well-known anti-inflammatory effects of adenosine agonists on LPS-induced inflammation cannot be extrapolated to situations in which stimulation of other TLR subtypes is involved.