Inflammation in ischemic brain injury: timing is important.

Inflammation is a defense reaction against diverse insults that serves to remove noxious agents and to limit their detrimental effects. There is increasing evidence that post-ischemic inflammation plays an important role in brain ischemia. However, whether inflammatory processes are deleterious or beneficial to recovery is presently a matter of debate and controversy. Experimentally and clinically, stroke is followed by an acute and a prolonged inflammatory response characterized by the production of inflammatory cytokines, leukocyte and monocyte infiltration in the brain, and the activation of resident glial cells. These events may contribute to ischemic brain injury. Several groups report conflicting results regarding the role of inflammation and effects of anti-inflammatory treatments in cerebral ischemia. Experimental studies employing knockout mice for different cytokines and chemokines provide only partial answers. This highlights the importance of clarifying the role of the immune response in pathological changes at the site of ischemic lesions in the brain. Here, we describe dual effects of the brain's inflammatory response and new evidence for a neuroprotective role of proliferating microglial cells in ischemia. In addition, we discuss a potential role of post-ischemic inflammation in brain regeneration and modulation of synaptic plasticity.