Interleukin (IL)-12 has a pivotal profibrotic role in the development of idiopathic pulmonary fibrosis (IPF). Medical research trials based on IPF registry databases have actively recruited patients. Surfactant protein D (SP-D) is a useful biomarker in patients with IPF. SP-D binds to signal regulatory protein α (SIRPα), which acts as an inhibitory receptor, and this SP-D/SIRPα interaction may have an anti-inflammatory effect. Accordingly, the inhibitory effect of SP-D on IL-12p40 production by lipopolysaccharide (LPS)-stimulated macrophages was investigated.Materials and Methods
Human granulocyte-macrophage colony-stimulating factor (GM-CSF)-stimulated macrophages (day 9 of culture) was used to investigate IL-12p40 production after stimulation with SP-D.Results
GM-CSF was found to upregulate SIRPα expression by macrophages. PD98059 (an extracellular signal-regulated kinase [ERK] inhibitor) blunted induction of SIRPα expression by GM-CSF. SP-D significantly attenuated IL-12p40 production by macrophages after stimulation with LPS. Silencing of SIRPα/β/γ significantly reversed this inhibitory effect of SP-D. In contrast, neither SB023580 (a p38α/β MAPK inhibitor) nor BIRB796 (a p38γ/δ MAPK inhibitor) attenuated the inhibitory effect of SP-D on LPS-stimulated production of IL-12p40. Silencing of SHP also had no influence on this effect of SP-D. Interestingly, a Rho-associated protein kinase (ROCK) inhibitor (Y-27632) abolished the inhibition of LPS-stimulated IL-12p40 production by SP-D, whereas silencing of ERK 2 significantly blunted this effect of Y-27632.Conclusions
These findings suggest that SP-D inhibits LPS-stimulated production of IL-12p40 via the SIRPα/ROCK/ERK signaling pathway.