Cranial irradiation acutely and persistently impairs injury-induced microglial proliferation

Abstract

Microglia, the resident immune cells of the central nervous system (CNS), play multiple roles in maintaining CNS homeostasis and mediating tissue repair, including proliferating in response to brain injury and disease. Cranial irradiation (CI), used for the treatment of brain tumors, has a long-lasting anti-proliferative effect on a number of cell types in the brain, including oligodendrocyte progenitor and neural progenitor cells; however, the effect of CI on CNS-resident microglial proliferation is not well characterized. Using a sterile cortical needle stab injury model in mice, we found that the ability of CNS-resident microglia to proliferate in response to injury was impaired by prior CI, in a dose-dependent manner, and was nearly abolished by a 20 ​Gy dose. Similarly, in a metastatic tumor model, prior CI (20 ​Gy) reduced microglial proliferation in response to tumor growth. The effect of irradiation was long-lasting; 20 ​Gy CI 6 months prior to stab injury significantly impaired microglial proliferation. We also investigated how stab and/or irradiation impacted levels of P2Y12R, CD68, CSF1, IL-34 and CSF1R, factors involved in the brain’s normal response to injury. P2Y12R, CD68, CSF1, and IL-34 expression were altered by stab similarly in irradiated mice and controls; however, CSF1R was differentially affected. qRT-PCR and flow cytometry analyses demonstrated that CI reduced overall Csf1r mRNA levels and microglial specific CSF1R protein expression, respectively. Interestingly, Csf1r mRNA levels increased after injury in unirradiated controls; however, Csf1r levels were persistently decreased in irradiated mice, and did not increase in response to stab. Together, our data demonstrate that CI leads to a significant and lasting impairment of microglial proliferation, possibly through a CSF1R-mediated mechanism.

Cranial irradiation disrupts homeostatic microglial dynamic behavior

“…As the resident immune cells of the central nervous system, microglia are primarily responsible for defending the brain against pathogens and responding to injury. In a cortical stab wound injury model, IR can impair microglial proliferation and colony stimulating factor 1 receptor expression [ 59 ], demonstrating that IR can reduce microglial responses to injury. This impaired injury response coupled with a reduced overall microglial number could therefore render the brain more vulnerable to outside insults.…”

Microglia replacement effectively attenuates the disease progress of ALSP in the mouse model and human patient

“…To achieve the efficient microglia, it is necessary to create a microglia-free niche and suppress the microglia proliferation in the recipient subjects 16 . The microglia-free niche can be achieved by CSF1R inhibition 3,16,43,44 whereas suppression of microglia proliferation can be achieved by busulfan of irradiation 16,24,45–47 . This is echoed by a parabiosis study that allogenic myeloid cells differentiated into microglia and replaced the recipient’s microglia through a long-term ablation of CX3CR1-CreER::CSF1R fl/fl microglia, by which simultaneously generated a microglia-free niche and inhibited the endogenous microglia without influencing donor cells 48 .…”

“…However, the CSF1R-deficient ALSP is a partially microglia-free condition with reduction of microglial cell number and less cell-cell competitive 24 . In addition, busulfan or irradiation during the tBMT can further reduce the microglial number [42][43][44] . Therefore, ALSP is a natural microglia-free condition upon tBMT.…”