LPS and IL-1 differentially activate mouse and human astrocytes: role of CD14

Treatment of cultures with toll-like receptor (TLR) ligands or cytokines has become a popular approach to investigate astrocyte neuroinflammatory responses and to simulate the neural environment in various CNS disorders. However, despite much effort, the mechanism of astrocyte activation such as their responses to the TLR ligands and IL-1 remain highly debated. We compared highly pure primary mouse and human astrocyte cultures in their ability to produce proinflammatory mediators (termed "A1") and immunoregulatory mediators (termed "A2") in response to LPS, poly IC, and IL-1 stimulation. In human astrocytes, IL-1 induced both A1 and A2 responses, poly IC induced mostly A2, and LPS induced neither. In mouse astrocytes, LPS induced mostly an A1-predominant response, poly IC induced both A1 and A2, and IL-1 neither. In addition, mouse astrocytes produce abundant IL-1 protein, whereas human astrocytes did not, despite robust IL-1 mRNA expression. Of the TLR4 receptor complex proteins, human astrocytes expressed TLR4 and MD2 but not CD14, whereas mouse astrocytes expressed all three. Mouse astrocyte CD14 (cell-associated and soluble) was potently upregulated by LPS. Silencing TLR4 or CD14 by siRNA suppressed LPS responses in mouse astrocytes. In vivo, astrocytes in LPS-injected mouse brains also expressed CD14. Our results show striking differences between human and mouse astrocytes in the use of TLR/IL-1R and subsequent downstream signaling and immune activation. IL-1 translational block in human astrocytes may be a built-in mechanism to prevent autocrine and paracrine cell activation and neuroinflammation. These results have important implications for translational research of human CNS diseases.