Patrolling monocytes play a critical role in CX3CR1-mediated neuroprotection during excitotoxicity

Excitotoxicity underlies neuronal death in many neuropathological disorders, such as Alzheimer's disease and multiple sclerosis. In murine models of these diseases, disruption of CX3CR1 signaling has thus far generated data either in favor or against a neuroprotective role of this crucial regulator of microglia and monocyte functions. In this study, we investigated the recruitment of circulating PU.1-expressing cells following sterile excitotoxicity and delineated the CX3CR1-dependent neuroprotective functions of circulating monocytes versus that of microglia in this context. WT, Cx3cr1-deficient and chimeric mice were subjected to a sterile excitotoxic insult via an intrastriatal injection of kainic acid (KA), a conformational analog of glutamate. Following KA administration, circulating monocytes physiologically engrafted the brain and selectively accumulated in the vicinity of excitotoxic lesions where they gave rise to activated macrophages depicting strong Iba1 and CD68 immunoreactivity 7 days post-injury. Monocyte-derived macrophages completely vanished upon recovery and did thus not permanently seed the brain. Furthermore, Cx3cr1 deletion significantly exacerbated neuronal death, behavioral deficits and activation of microglia cells following sterile excitotoxicity. Cx3cr1 disruption also markedly altered the blood levels of patrolling monocytes 24 h after KA administration. The specific elimination of patrolling monocytes using Nr4a1(-/-) chimeric mice conditioned with chemotherapy provided direct evidence that these circulating monocytes are essential for neuroprotection. Taken together, these data support a beneficial role of CX3CR1 signaling during excitotoxicity and highlight a novel and pivotal role of patrolling monocytes in neuroprotection. These findings open new research and therapeutic avenues for neuropathological disorders implicating excitotoxicity.