Microglia protect neurons against ischemia by synthesis of tumor necrosis factor

Microglia and infiltrating leukocytes are considered major producers of tumor necrosis factor (TNF), which is a crucial player in cerebral ischemia and brain inflammation. We have identified a neuroprotective role for microglial-derived TNF in cerebral ischemia in mice. We show that cortical infarction and behavioral deficit are significantly exacerbated in TNF-knock-out (KO) mice compared with wild-type mice. By using in situ hybridization, immunohistochemistry, and green fluorescent protein bone marrow (BM)-chimeric mice, TNF was shown to be produced by microglia and infiltrating leukocytes. Additional analysis demonstrating that BM-chimeric TNF-KO mice grafted with wild-type BM cells developed larger infarcts than BM-chimeric wild-type mice grafted with TNF-KO BM cells provided evidence that the neuroprotective effect of TNF was attributable to microglial- not leukocyte-derived TNF. In addition, observation of increased infarction in TNF-p55 receptor (TNF-p55R)-KO mice compared with TNF-p75R and wild-type mice suggested that microglial-derived TNF exerts neuroprotective effects through TNF-p55R. We finally report that TNF deficiency is associated with reduced microglial population size and Toll-like receptor 2 expression in unmanipulated brain, which might also influence the neuronal response to injury. Our results identify microglia and microglial-derived TNF as playing a key role in determining the survival of endangered neurons in cerebral ischemia.