Importance of combined treatment with IL‐10 and IL‐4, but not IL‐13, for inhibition of monocyte release of the Ca²⁺‐binding protein MRP8/14

Expression of the two myeloic related proteins MRP8 and MRP14 is restricted to distinct stages of monocytic differentiation. Heterodimeric MRP8/14 complexes (27E10 antigen) have been shown to represent their biologically active forms. In this study, we investigated the effects of Th2-cytokines on release of these proteins from freshly obtained blood monocytes and monocytes cultured for 7 days in the presence of granulocyte-macrophage colony-stimulating factor (GM-CSF). Monocytes were stimulated with pokeweed mitogen (PWM) in the presence or absence of interleukin-13 (IL-13), IL-4 and IL-10, and secretion of MRP8, MRP14 and MRP8/14 was assessed by using a sandwich enzyme-linked immunosorbent assay system. Peripheral monocytes secreted significantly increased amounts of MRP14 and MRP8/14 but not MRP8 under stimulation with PWM. IL-10 and IL-4, but not IL-13, down-regulated the PWM-stimulated MRP8/14 secretion in a dose-dependent manner. Maximal inhibition required that IL-10 and IL-4 be added up to 1 h before or simultaneous with PWM. A combination of IL-10 and IL-4 even at suboptimal concentrations significantly suppressed protein secretion much more than using IL-10 or IL-4 at a doubled concentration alone. Peripheral monocytes cultured for 7 days in the presence of GM-CSF showed two-to threefold higher protein levels compared with freshly obtained blood monocytes but responded inefficiently to either IL-4, IL-13, or IL-10 alone. However, treatment with IL-10 in combination with IL-4 but not IL-13 strongly suppressed MRP14 and MRP8/14 release by these cells. The unresponsiveness of 7-day-cultured blood macrophages suggests that more differentiated and activated cells may lose their ability to respond to anti-inflammatory cytokines. Combined cytokine treatment may therefore more effectively control the progression of chronic inflammatory processes.