Endotoxin preconditioning protects against the cytotoxic effects of TNFα after stroke:: a novel role for TNFα in LPS-ischemic tolerance

Lipopolysaccharide (LPS) preconditioning provides neuroprotection against subsequent cerebral ischemic injury. Tumor necrosis factor-a (TNF alpha) is protective in LPS-induced preconditioning yet exacerbates neuronal injury in ischemia. Here, we define dual roles of TNFa in LPS-induced ischemic tolerance in a murine model of stroke and in primary neuronal cultures in vitro, and show that the cytotoxic effects of TNF alpha are attenuated by LPS preconditioning. We show that LPS preconditioning significantly increases circulating levels of TNFa before middle cerebral artery occlusion in mice and show that TNF alpha is required to establish subsequent neuroprotection against ischemia, as mice lacking TNF alpha are not protected from ischemic injury by LPS preconditioning. After stroke, LPS preconditioned mice have a significant reduction in the levels of TNFa ( Bthreefold) and the proximal TNFa signaling molecules, neuronal TNF-receptor 1 (TNFR1), and TNFR-associated death domain ( TRADD). Soluble TNFR1 ( s-TNFR1) levels were significantly increased after stroke in LPS-preconditioned mice (B2.5-fold), which may neutralize the effect of TNFa and reduce TNF alpha-mediated injury in ischemia. Importantly, LPS-preconditioned mice show marked resistance to brain injury caused by intracerebral administration of exogenous TNFa after stroke. We establish an in vitro model of LPS preconditioning in primary cortical neuronal cultures and show that LPS preconditioning causes significant protection against injurious TNFa in the setting of ischemia. Our studies suggest that TNFa is a twin-edged sword in the setting of stroke: TNFa upregulation is needed to establish LPS-induced tolerance before ischemia, whereas suppression of TNFa signaling during ischemia confers neuroprotection after LPS preconditioning.