TLR-Induced IL-12 and CCL2 Production by Myeloid Cells Is Dependent on Adenosine A₃ Receptor-Mediated Signaling

TLR-induced signaling potently activates cells of the innate immune system and is subject to regulation at different levels. Inflammatory conditions are associated with increased levels of extracellular adenosine, which can modulate TLR-induced production of cytokines through adenosine receptor-mediated signaling. There are four adenosine receptor subtypes that induce different signaling cascades. In this study, we demonstrate a pivotal contribution of adenosine A₃ receptor (A₃R)-mediated signaling to the TLR4-induced expression of IL-12 in different types of human myeloid APC. In dendritic cells, IL-12 and CCL2 responses as evoked by TLR2, 3, 4, 5, and 8, as well as IL-12 responses evoked by whole pathogens, were all reduced when A₃R-mediated signaling was blocked. As a result, concomitant production of IFN-γ and IL-17 by T cells was significantly inhibited. We further show that selective inhibition of A₃R-mediated signaling reduced TLR-induced phosphorylation of the transcription factor STAT1 at tyrosine 701. Next-generation sequencing revealed that A₃R-mediated signaling controls the expression of metallothioneins, known inhibitors of STAT1 phosphorylation. Together our results reveal a novel regulatory layer of innate immune responses, with a central role for metallothioneins and autocrine/paracrine signaling via A₃Rs.