Origin Of Microgliosis After Acute Ischemie Stroke In Mice

Microglia are the main immunocompetent cells in the central nervous system (CNS) which continuously monitor the microenvironment to maintain the homeostasis. Ischemic stroke induces rapid activation of microglia. Activated microglia alter their morphology, cell surface antigen expression and functional behaviour, and their numbers increase markedly at the site of ischemia. These responses of microglia are collectively termed "microgliosis". The main source of microgliosis remains controversial. Some studies suggest that microgliosis are maintained through self-expansion, but others suggest that bone marrow-derived progenitors may contribute to microgliosis. However, these reports are based on experiments in which animals suffered lethal irradiation and their bone marrow was artificially replaced with exogenous cells. It is unclear whether a similar phenomenon could be observed under physiological conditions.Here we investigated the origin and kinetics of reactive microglia using a photothrombosis stroke model, which is characterized by pathological disruption of blood brain barrier (BBB) permeability without intervention of irradiation or transplantation. To avoid the potential influence of irradiation or transplantation, we established a model of blood chimera using parabiotic animals and examined the source and dynamics of activated microglia in subacute phase after ischemic stroke by using in vivo two-photon microscopy. We found there was no infiltration of blood-derived cells with an intact BBB in healthy mice. However, a small population of blood-derived Cx3crlGFP/+cells were detected in the lesion sites of cerebral parenchyma when the BBB integrity was damaged after ischemia. Although these infiltrating cells also expressed Green fluorescence protein (GFP), they displayed different phenotypes and kinetics from reactive microglia. The number of infiltrating cells increased in the first5days after stroke and then subsequently decreased. We found these Cx3crlGFP/+infiltrating cells did not proliferate and were gradually lost through apoptosis. In contrast, immunohistology and in vivo imaging revealed that activated microglia renewed themselves and were recruited to ischemic area continuously, and their number increased in the first week after stroke. These results indicated that microglial proliferation rather than infiltration of Cx3crlGFP/+circulating cells is the main source of microgliosis after ischemic stroke. In addition, we found that the permeability of BBB was associated with the migration of Cx3crlGFP/+infiltrating cells and the activation and proliferation of resident microglia. Together, our data suggest that the Cx3crlGFP/+infiltrating cells and reactive microglia may represent two distinct populations of cells with different function and therapeutic potential for the treatment of stroke.