Differential effects of macrophage- and granulocyte-macrophage colony-stimulating factors on cytokine gene expression during rat alveolar macrophage differentiation into multinucleated giant cells (MGC): role for IL-6 in type 2 MGC formation

Macrophage colony-stimulating factor (M-CSF) and granulocyte-macrophage (GM)-CSF stimulate the differentiation of rat alveolar macrophages (AM) into multinucleated giant cells (MGC) with distinct phenotypes (type 1 and type 2 MGC). In the present study, we analyzed the profile of cytokine gene expression induced respectively, by M-CSF and GM-CSF during rat AM differentiation using reverse transcription-PCR. Enhanced mRNA expression for IL-1alpha, TNF-alpha, and TGF-beta1 was observed 3 h after treatment with M-CSF (50 U/ml) or GM-CSF (50 U/ml). In contrast, IL-6 mRNA expression was increased by GM-CSF but not M-CSF. Kinetic analysis indicated that GM-CSF stimulated IL-6 expression early (1.5 h), with maximal effect observed at 24 h and up to 5 days thereafter. Increased mRNA levels for IL-6 were associated with higher IL-6 activity in the culture media of differentiating AM. IL-6 activity was stimulated 3 h after treatment with GM-CSF and increased with time (up to 5 days). Interestingly, addition of exogenous IL-6 (20-100 ng/ml) alone or in combination with GM-CSF to AM cultures increased slightly and selectively the formation of MGC with type 2 phenotype. Conversely, neutralization of endogenous IL-6 during AM differentiation into MGC inhibited significantly (up to 50%) the formation of type 2 MGC. These results suggest a role for IL-6 in the formation of type 2 MGC and provide some insights into the mechanisms of MGC formation and the processes that regulate it positively.