Toll-Like Receptors 2, -3 and -4 Prime Microglia but not Astrocytes Across Central Nervous System Regions for ATP-Dependent Interleukin-1β Release

Interleukin-1 beta (IL-1 beta) is a crucial mediator in the pathogenesis of inflammatory diseases at the periphery and in the central nervous system (CNS). Produced as an unprocessed and inactive pro-form which accumulates intracellularly, release of the processed cytokine is strongly promoted by ATP acting at the purinergic P2X(7) receptor (P2X(7)R) in cells primed with lipopolysaccharide (LPS), a Toll-like receptor (TLR) 4 ligand. Microglia are central to the inflammatory process and a major source of IL-1 beta when activated. Here we show that purified (> 99%) microglia cultured from rat cortex, spinal cord and cerebellum respond robustly to ATP-dependent IL-1 beta release, upon priming with a number of TLR isoform ligands (zymosan and Pam3CSK4 for TLR2, poly(I:C) for TLR3). Cytokine release was prevented by a P2X(7)R antagonist and inhibitors of stress-activated protein kinases. Enriched astrocytes (<= 5% microglia) from these CNS regions displayed responses qualitatively similar to microglia but became unresponsive upon eradication of residual microglia with the lysosomotropic agent Leu-Leu-OMe. Activation of multiple TLR isoforms in nervous system pathology, coupled with elevated extracellular ATP levels and subsequent P2X(7)R activation may represent an important route for microglia-derived IL-1 beta. This phenomenon may have important consequences for neuroinflammation and its position to the common pathology of CNS diseases.