Pollen metabolome analysis reveals adenosine as a major regulator of dendritic cell-primed T_H cell responses

Background: Water-soluble components from pollen modulate dendritic cell (DC) functions, such as IL-12 secretion and 3′-5′-cyclic adenosine monophosphate (cAMP) signaling and migration, possibly contributing to the establishment of a T_H2-dominated immune response against pollen. Because these effects could not solely be attributed to the previously identified pollen-associated lipid mediators, the pollen metabolome was analyzed for candidate immunomodulatory substances. Objective: We sought to perform an analysis of the effect of pollen-associated adenosine on DC function and T _H cell differentiation. Methods: Fractions of aqueous pollen extracts (APEs) were generated by means of ultrafiltration and were subjected simultaneously to biological tests and metabolome analysis (ultra-high- resolution mass spectrometry) and ultraperformance liquid chromatography. Effects of pollen-derived adenosine on monocyte-derived DC cAMP signaling, cytokine response, and capacity to differentiate T_H cells were studied. Results: The less than 3-kd fraction of APEs comprised thousands of substances, including adenosine in micromolar concentrations. Pollen-derived adenosine mediated A₂ receptor-dependent induction of cAMP and inhibition of IL-12p70 in DCs. APEs digested with adenosine deaminase failed to mediate IL-12 inhibition. DCs of nonatopic donors exposed to APEs showed an adenosine-dependent reduced capacity to differentiate T_H1 cells and an enhanced capacity to induce regulatory T cells and IL-10. DCs of atopic donors failed to induce IL-10 but instead induced IL-5 and IL-13. Conclusion: This study identifies adenosine out of thousands of metabolites as a potent immunoregulatory substance in pollen. It acts on the level of the DC, with differential effects in atopic and nonatopic donors.