Interleukin-18 expression after focal ischemia of the rat brain: Association with the late-stage inflammatory response

Interleukin-18, previously designated interferon gamma -inducing factor, is a proinflammatory cytokine structurally related to interleukin-1 beta and is therefore considered a member of the growing family of interleukin-1-like cytokines. Both interleukin-18 and -1 beta are synthesized as inactive precursors that necessitate cleavage by caspase-1 for functional activity. In this study, the authors analyzed the expression pattern of interleukin-18, -1 beta, and caspase-1 in focal brain ischemia induced in rats either by permanent middle cerebral artery occlusion or by photothrombosis of cortical microvessels. Using reverse transcriptase-polymerase chain reaction, they found a delayed increase of interleukin-18 mRNA starting at 48 hours and reaching its peak between 7 and 14 days after ischemia. In contrast, interleukin-1 beta mRNA peaked within 16 hours and was downregulated thereafter. The time course of caspase-1 mRNA expression paralleled that of interleukin-18, but not of interleukin-1 beta mRNA. Immunocytochemically, interleukin-18 expression was localized to EDI-positive phagocytic microglia/macrophages infiltrating the necrotic lesion between 3 and 6 days after ischemia. In contrast, interleukin-1 beta immunoreactivity was expressed by ramified microglia in the infarct border zone and remote ipsilateral cortex during the first 16 hours postlesion. Induction of interleukin-18 was not accompanied by detectable expression of interferon-gamma mRNA. Their data show spatial and temporal diversity in interieukin-1 and -18 cytokine family expression in brain ischemia, and suggest a role of the interteukin-18/caspase-1 pathway in late-stage inflammatory responses to focal brain ischemia.