Ultra-High-Dose-Rate FLASH Irradiation Limits Reactive Gliosis in the Brain

Encephalic radiation therapy delivered at a conventional dose rate (CONV, 0.1-2.0 Gy/min) elicits a variety of temporally distinct damage signatures that invariably involve persistent indications of neuroinflammation. Past work has shown an involvement of both the innate and adaptive immune systems in modulating the central nervous system (CNS) radiation injury response, where elevations in astrogliosis, microgliosis and cytokine signaling define a complex pattern of normal tissue toxicities that never completely resolve. These side effects constitute a major limitation in the management of CNS malignancies in both adult and pediatric patients. The advent of a novel ultra-high dose-rate irradiation modality termed FLASH radiotherapy (FLASH-RT, instantaneous dose rates ≥106 Gy/s; 10 Gy delivered in 1-10 pulses of 1.8 µs) has been reported to minimize a range of normal tissue toxicities typically concurrent with CONV exposures, an effect that has been coined the "FLASH effect." Since the FLASH effect has now been found to significantly limit persistent inflammatory signatures in the brain, we sought to further elucidate whether changes in astrogliosis might account for the differential dose-rate response of the irradiated brain. Here we report that markers selected for activated astrogliosis and immune signaling in the brain (glial fibrillary acidic protein, GFAP; toll-like receptor 4, TLR4) are expressed at reduced levels after FLASH irradiation compared to CONV-irradiated animals. Interestingly, while FLASH-RT did not induce astrogliosis and TLR4, the expression level of complement C1q and C3 were found to be elevated in both FLASH and CONV irradiation modalities compared to the control. Although functional outcomes in the CNS remain to be cross-validated in response to the specific changes in protein expression reported, the data provide compelling evidence that distinguishes the dose-rate response of normal tissue injury in the irradiated brain.

Glia-Selective Deletion of Complement C1q Prevents Radiation-Induced Cognitive Deficits and Neuroinflammation

The adverse neurocognitive sequelae following clinical radiotherapy (RT) for central nervous system (CNS) malignancies are often long-lasting without any clinical recourse. Despite recent progress, the cellular mechanisms mediating RT-induced cognitive deficits (RICD) are poorly understood. The complement system is an immediate sensor of a disturbed inflammatory environment and a potent mediator of gliosis with a range of nonimmune functions in the CNS, including synaptic pruning, which is detrimental if dysregulated. We hypothesize that complement-mediated changes in glial cell function significantly contribute to RICD. The underlying alterations in CNS complement cascade proteins (C1q, C3), TLR4, and colabeling with glia (IBA1, GFAP) were examined using gene expression, immunofluorescence, and in silico modeling approaches in the adult mouse brain following 9 Gy cranial RT. Three-dimensional volumetric quantification showed elevated molecular signatures of gliosis at short- and long-term post-RT times. We found significant elevations in complement C1q, C3, and TLR4 post-RT accompanied by increased colabeling of astrocytes and microglia. To address the mechanism of RT-induced complement cascade activation, neuroinflammation, and cognitive dysfunction, we used a genetic approach-conditional, microglia-selective C1q (Flox) knockdown mice-to determine whether a glia-specific, upstream complement cascade contributes to RICD. C1q-Flox mice exposed to cranial RT showed no cognitive deficits compared with irradiated WT mice. Further, irradiated C1q-Flox mice were protected from RT-induced microglial activation and synaptic loss, elevation of anaphylatoxin C5a receptor, astrocytic-C3, and microglial-TLR4 expression in the brain. Our findings demonstrate for the first time a microglia-specific mechanism of RICD involving an upstream complement cascade component, C1q. SIGNIFICANCE: Clinically-relevant radiotherapy induces aberrant complement activation, leading to brain injury. Microglia-selective genetic deletion of CNS complement C1q ameliorates radiation-induced cognitive impairments, synaptic loss, and neuroinflammation, highlighting the potential for C1q as a novel therapeutic target.See related commentary by Korimerla and Wahl, p. 1635.