Cell therapy centered on IL-1Ra is neuroprotective in experimental stroke.

Cell-based therapies are emerging as new promising treatments in stroke. However, their functional mechanism and therapeutic potential during early infarct maturation has so far received little attention. Here, we asked if cell-based delivery of the interleukin-1 receptor antagonist (IL-1Ra), a known neuroprotectant in stroke, can promote neuroprotection, by modulating the detrimental inflammatory response in the tissue at risk. We show by the use of IL-1Ra-overexpressing and IL-1Ra-deficient mice that IL-1Ra is neuroprotective in stroke. Characterization of the cellular and spatiotemporal production of IL-1Ra and IL-1α/ß identifies microglia, not infiltrating leukocytes, as the major sources of IL-1Ra after experimental stroke, and shows IL-1Ra and IL-1ß to be produced by segregated subsets of microglia with a small proportion of these cells co-expressing IL-1α. Reconstitution of whole body irradiated mice with IL-1Ra-producing bone marrow cells is associated with neuroprotection and recruitment of IL-1Ra-producing leukocytes after stroke. Neuroprotection is also achieved by therapeutic injection of IL-1Ra-producing bone marrow cells 30 min after stroke onset, additionally improving the functional outcome in two different stroke models. The IL-1Ra-producing bone marrow cells increase the number of IL-1Ra-producing microglia, reduce the availability of IL-1ß, and modulate mitogen-activated protein kinase (MAPK) signaling in the ischemic cortex. The importance of these results is underlined by demonstration of IL-1Ra-producing cells in the human cortex early after ischemic stroke. Taken together, our results attribute distinct neuroprotective or neurotoxic functions to segregated subsets of microglia and suggest that treatment strategies increasing the production of IL-1Ra by infiltrating leukocytes or microglia may also be neuroprotective if applied early after stroke onset in patients.

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.

Reactive microgliosis engages distinct responses by microglial subpopulations after minor central nervous system injury.

Microglia are bone marrow-derived cells that constitute a facultative macrophage population when activated by trauma or pathology in the CNS. Endogenous CNS-resident microglia as well as exogenous (immigrant) bone marrow-derived cells contribute to reactive microgliosis, raising fundamental questions about the cellular composition, kinetics, and functional characteristics of the reactive microglial cell population. Bone marrow chimeric mice reconstituted with green fluorescent protein-expressing (GFP(+)) donor bone marrow cells were subjected to entorhinal cortex lesion, resulting in selective axonal degeneration and a localized microglial reaction in the hippocampus. Flow cytometric evaluation of individually dissected hippocampi differentiated immigrant GFP(+) microglia from resident GFP(-) microglia (CD11b(+)CD45(dim)) and identified a subset of mainly resident CD11b(+) microglia that was induced to express CD34. The proportion of immigrant GFP(+) microglia (CD11b(+)CD45(dim)) increased signficantly by 3 and 5 days postlesion and reached a maximum of 13% by 7 days. These cells expressed lower CD11b levels than resident microglia, forming a distinct subpopulation on CD11b/CD45 profiles. The proportion of CD34(+)CD11b(+) microglia was significantly increased at 3 days postlesion but had normalized by 5 and 7 days, when the microglial reaction is known to be at its maximum. Our results show that distinct subpopulations of microglia respond to minor CNS injury. The heterogeneity in microglial response may have functional consequences for repair and possibly therapy.