CX3CR1 deficiency induces an early protective inflammatory environment in ischemic mice

The studies on fractalkine and its unique receptor CX3CR1 in neurological disorders yielded contrasting results. We have explored the consequences of CX3CR1 deletion in ischemic (30' MCAo) mice on: (1) brain infarct size; (2) microglia dynamism and morphology; (3) expression of markers of microglia/macrophages (M/M) activation and polarization. We observed smaller infarcts in cx3cr1(-/-) (26.42 ± 7.41 mm(3) , mean ± sd) compared to wild type (36.29 ± 11.57) and cx3cr1(-/+) (34.49 ± 8.91) mice. We longitudinally analyzed microglia by in vivo two-photon microscopy before, 1 and 24 h after transient ischemia. Microglia were stationary in both cx3cr1(-/-) and cx3cr1(-/+) mice throughout the study. In cx3cr1(-/-) mice, they displayed a significantly higher number of ramifications >10 μm at baseline and at 24 h after ischemia compared to cx3cr1(-/+) mice, indicating that CX3CR1 deficiency impaired the development of microglia hypertrophic/amoeboid morphology. At 24 h after ischemia, we performed post mortem quantitative immunohistochemistry for different M/M markers. In cx3cr1(-/-) immunoreactivity for CD11b (M/M activation) and for CD68 (associated with phagocytosis) were decreased, while that for CD45(high) (macrophage and leukocyte recruitment) was increased. In addition, immunoreactivity for Ym1 (M2 polarization) was enhanced, while that for iNOS (M1) was decreased. Our data show that in cx3cr1(-/-) mice protection from ischemia at early time points after injury is associated with a protective inflammatory milieu, characterized by the promotion of M2 polarization markers.