Inhibition of IL-10 receptor function in alveolar macrophages by Toll-like receptor agonists

Despite an immunosuppressive lung environment, alveolar macrophages (AM) retain the capacity to respond to microorganisms. This report demonstrates that IL-10, constitutively produced by normal alveolar epithelium, stimulates signal transduction through the IL-10R on AM and that IL-10R function can be inhibited by stimulation of Toll-like receptor (TLR) on AM. IL-10 mRNA and protein were constitutively expressed in normal alveolar epithelium of mice, and IL-10R were constitutively expressed on normal murine AM. Stimulation of AM through TLR2, TLR4, or TLR9 was sufficient to inhibit IL-10R signal transduction, including phosphorylation and nuclear translocation of STAT3 transcription factor. Inhibition of IL-10R function by TLRs was not associated with a decrease in IL-10R expression, but did require expression of the myeloid differentiation factor 88 adaptor protein. Continuous exposure of macrophages to IL-10 caused sustained expression of the chemokine receptors CCR1 and CCR5. However, the addition of TLR ligands inhibited IL-10-induced expression of CCR1 and CCR5. Finally, exposure of macrophages to TLR ligands blocked the ability of IL-10 to inhibit the induction of TNF-alpha by C2-ceramide. These findings demonstrate a novel regulatory mechanism that may allow AM to overcome inhibitory effects of constitutive IL-10 in the lungs that may permit a more effective response to pulmonary infections.