Neuroprotective effect of monophosphoryl lipid A, a detoxified lipid A derivative, in photothrombotic model of unilateral selective hippocampal ischemia in rat

Finding a neuroprotective strategy to rescue patients suffering from acute brain damage is of great interest. Monophosphoryl lipid A (MPL) is a derivative of lipopolysaccharide (LPS) that lacks many of the endotoxic properties of the parent molecule, and yet has similar protective effect. Here, we report the first evidence that MPL preconditioning, similar to LPS preconditioning, can induce neuroprotection against cerebral ischemia. MPL (0.5, 1, 5 μg/rat) was injected unilaterally into the left cerebral ventricle of male rats, and 48 h later, rats were subjected to ipsilateral selective hippocampal ischemia using a modified version of the photothrombotic method. The neuroprotective effects of MPL and LPS were evaluated by measuring infarct size and assessing cognitive function. The expression level of some inflammatory and anti-inflammatory cytokines involving in TLR4 signaling pathway was also measured. Cognitive impairment and infarct size were obvious in control group receiving normal saline intracerebroventricularly and then selective hippocampal ischemia, compared to the sham group. Immunologic preconditioning with MPL or LPS significantly reduced infarct size and improved cognitive function. Additionally, immunologic preconditioning resulted in inflammatory mediators, NF-κB and TNF-α, down-regulation but anti-inflammatory mediators, IRF3, IFN-β, and TGF-β, up-regulation. Our data showed that both MPL and LPS preconditioning may reprogram the TLR4 signaling pathway to produce a cytokine profile which eventually leads to neuroprotection against ischemia injury. MPL, unlike LPS, is safe and well tolerated in clinic, thus it could be considered as a new approach in prevention or even treatment of cerebral ischemic insult consequences.