Suppression of Leukotriene B₄ Biosynthesis by Endogenous Adenosine in Ligand-activated Human Neutrophils

Adenosine (Ado) has been shown to suppress several functional responses of human polymorphonuclear leukocytes (PMNs). The current study investigated whether endogenous Ado regulates the biosynthesis of leukotriene (LT)B₄ in ligand-stimulated PMNs. Measurements of Ado in PMN resuspended in Hanks' buffered salt solution (HBSS) or plasma showed a cell concentration– and time–dependent accumulation of the nucleoside. The removal of endogenous Ado with either Ado deaminase or the blockade of its action by the Ado A_2a receptor antagonist, 8-(3-chlorostyryl) caffeine, markedly increased LTB₄ biosynthesis upon ligand stimulation in HBSS. Similarly, LTB₄ synthesis by ligand-stimulated PMNs in plasma (containing recombinant LTA₄ hydrolase to allow the conversion of protein-bound LTA₄) was strongly enhanced by addition of Ado deaminase. Addition of red blood cells to suspensions of PMNs in plasma mimicked the effect of adding Ado deaminase and LTA₄ hydrolase in enhancing LTB₄ biosynthesis upon ligand stimulation. This effect of red blood cells on LTB₄ biosynthesis was blocked by dipyridamole, an inhibitor of Ado transport, or captopril, an inhibitor of LTA₄ hydrolase. These results demonstrate that endogenous Ado efficiently downregulates ligand-stimulated LTB₄ biosynthesis in PMN suspensions, pointing out a potentially important regulatory function of Ado in inflammatory exudates. These results also unveil a dual role for red blood cells in upregulating LTB₄ biosynthesis, namely, the removal of endogenous Ado and the conversion of LTA₄ released by activated PMNs.