Granulocyte–macrophage colony‐stimulating factor modulates lipopolysaccharide (LPS)‐binding and LPS‐response of human macrophages: inverse regulation of tumour necrosis factor‐α and interleukin‐10

Summary Granulocyte–macrophage colony‐stimulating factor (GM‐CSF) is a well‐known stimulus for the activation, differentiation and survival of monocytes (MO). Up to now most investigations focused on the short‐term effects of GM‐CSF. In this study we investigated the effects of GM‐CSF on the long‐term differentiation of human MO in the presence of serum. We found that MO‐derived macrophages (Mφ) cultured with serum plus GM‐CSF (GM‐Mφ) were different from control Mφ (SER‐Mφ) in terms of lipopolysaccharide (LPS)‐stimulated cytokine release: GM‐Mφ showed an increased tumour necrosis factor‐α (TNF‐α) and interleukin‐6 (IL‐6) production, especially at lower LPS concentrations, but the secretion of IL‐10 was diminished. In addition, GM‐Mφ secreted TNF‐α but not IL‐6 and IL‐10, spontaneously. The spontaneous TNF‐α production was not due to LPS contamination as it could not be blocked by anti‐CD14 antibody. Flow cytometry revealed, however, that the receptor for LPS, CD14, was up‐regulated on GM‐Mφ and those Mφ released twice as much soluble CD14 into the supernatant as compared with SER‐Mφ. The higher CD14 expression also resulted in an enhanced LPS‐binding capacity of GM‐Mφ. Furthermore, the LPS‐response of GM‐Mφ could only be blocked by about fourfold higher concentration of anti‐CD14 antibody compared with SER‐Mφ. In summary, GM‐CSF promotes the generation of a pro‐inflammatory type of Mφ in two different ways: first, the down‐regulation of autocrine IL‐10 production increases the release of cytokines such as IL‐6 and TNF‐α and second, the up‐regulation of membrane and soluble CD14 expression leads to a higher sensitivity towards LPS‐stimulation.