Analysis of glial secretome: The long pentraxin PTX3 modulates phagocytic activity of microglia

Microglia, as the phagocytes of the central nervous system, play an important role in the recognition, engulfment, and clearance of apoptotic cells and invading microbes. Proteins secreted from activated glial cells may affect microglial phagocytic activity. Secreted proteins of mixed glial cells stimulated with lipopolysaccharide (LPS) and interferon-γ (IFN-γ) for 24 h were identified for the first time by liquid chromatography and tandem mass spectrometric analysis. Several proteins were newly identified as a glia-secreted protein. Among the proteins identified by the mixed glia secretome analysis, pentraxin 3 (PTX3) secretion was most highly induced by LPS/IFN-γ stimulation. Expression of PTX3 mRNA was detected in primary microglia and astrocyte cultures as well as glial cell lines. Glial secretion of PTX3 and its inflammatory induction was confirmed by Western blot analysis of conditioned media of mixed glial cultures. PTX3 did not influence LPS-induced nitric oxide production or neurotoxicity of BV-2 microglial cells. Most importantly, PTX3 selectively modulated microglial phagocytosis activity; it promoted engulfment of zymosan particles, while it inhibited uptake of apoptotic cells. Our results indicate that glia-derived PTX3 may modulate phagocytic functions of microglia, and this may have important implications in the regulation of microglial activity in health and disease.