The role of cyclic AMP, calcium and filamentous actin in adenosine modulation of Fc receptor-mediated phagocytosis in human neutrophils

Abstract

The role of cyclic AMP, calcium and filamentous actin (F-actin) content during adenosine modulation of Fc receptor (FcR)-mediated phagocytosis in adherent human neutrophils was investigated. Phagocytosis of IgG-opsonized yeast particles was found to be enhanced by pico- to nanomolar concentrations of adenosine or the A 1-agonist N 6-cyclopentyl-adenosine (CPA) but reduced by micromolar concentrations of adenosine or the A 2-agonist 5′- N-ethylcarboxamidoadenosine (NECA). NECA, in the presence of the cAMP-specific phosphodiesterase inhibitor Ro 20–1724, increased the intracellular content of cAMP during phagocytosis. Ro 20–1724 potentiated the NECA-induced reduction of the phagocytic capacity. These observations indicate that cAMP elevations are involved in A 2-receptor-mediated inhibition of phagocytosis. NECA, in the presence of Ro 20–1724, markedly enhanced the actin polymerization associated with adhesion to the substrate and contact with the phagocytic prey. During advanced phagocytosis, however, the F-actin content reached levels clearly below those observed in control cells. This prolonged depolymerization phase correlated with the A 2-receptor-induced cAMP elevation. Depletion of intracellular free calcium abolished the cAMP-elevating effects of NECA, and also completely abrogated the A 1- and A 2-receptor-mediated effects on phagocytosis. However, since NECA reduced the F-actin content even in Ca 2+-depleted cells, A 2-receptor-mediated inhibition of phagocytosis could not be directly coupled to changes in the overall content of F-actin. Our results indicate that adenosine modulates FcR-mediated phagocytosis in a calcium-dependent way, and does so through ‘stimulatory’ A 1 and ‘inhibitory’ A 2 receptors, and also that cAMP elevation is linked to the A 2-receptor-induced inhibition of phagocytosis.