Irradiation of Mouse Brain Leads to Neuroinflammation and Delayed Infiltration of Immune Cells: Role of Interleukin-1 and CCR2 Signaling

Abstract

Thesis (Ph.D.)--University of Rochester. School of Medicine & Dentistry. Dept. of Neurobiology and Anatomy, 2010.Cranial irradiation is a common therapy for cancers of the head and neck, and stereotactic radiosurgery is becoming increasingly utilized to treat CNS disorders. The use of radiotherapy is restricted by the sensitivity of the surrounding normal CNS tissues to radiation. Exceeding the radio-tolerance of normal tissues can result in a multitude of side effects that manifest as early as the cessation of therapy to several years later and contribute to patient morbidity and mortality. Data from animal studies and the treatment of patients suggest that neuroinflammation plays a role in the etiology of these side effects, although the exact inflammatory mechanisms remain unknown. This thesis characterizes a model of cranial irradiation in C57BL/6 mice. Consistent with previous reports, we observed a multi-phasic inflammatory response in the transcript levels of multiple inflammatory cytokines, along with astrocyte, endothelial, and microglial activation. Surprisingly, no overt pathology consistent with radiation-induced necrosis could be detected for up to a year. Despite the lack of pathologic changes, CNS radiation resulted in an acute infiltration of neutrophils and delayed increases in the numbers of T cells, MHCII positive cells, and CD11c positive cells. CD11c positive cells were found to express MHCII, suggesting that these are mature dendritic cells. Infiltrating dendritic cells and T cells were shown to have a preference for white matter. Moreover, these neuroinflammatory changes were limited to areas of the brain apparently exposed to the radiation beam. Radiosensitivity varies between animal strains and species, with a majority of the previous CNS radiation studies being conducted in C3H mice and rats. Using our mouse model of cranial irradiation, the effect of strain on the neuroinflammatory response to radiation was also investigated. There were significant differences in the levels of proinflammatory cytokines at both early (four hours) and late (six months) time points following radiation. The numbers of MHCII positive cells were different between strains, both at baseline and following radiation, with increased numbers present in the C57BL/6 mouse strain. Differences were also found in ICAM-1 expression, T cells, and dendritic cells with normal aging between strains, which may contribute to the disparity seen in their response to radiation. IL-1 has numerous functions in the normal CNS, as well as being a potent proinflammatory molecule. IL-1 has been shown to be a participant in neuroinflammation associated with numerous CNS injuries and disorders, in both acute and chronic settings. IL-1 has the ability to recruit leukocytes from the periphery into the CNS in response to injury or inflammation. One of the signaling pathways involved is that of CCL2/CCR2, which is important for the recruitment of myeloid cells following CNS injury. We show that radiation results in a delayed, dose-dependent increase in peripherally-derived immune cell recruitment that was present until the conclusion of the study (six months). Infiltrating cells were limited to areas of the brain that were exposed to radiation. It was shown that radiation-induced cellular recruitment does not lead to an increased total number of myeloid cells in the brain. This infiltration was found to be dependent on CCR2, as animals transplanted with CCR2-deficient bone marrow displayed decreased myeloid cell recruitment. IL-1 signaling in peripheral immune cells was also shown to be important for radiation-induced immune cell recruitment to the CNS, in the context of normal aging and following radiation exposure, although it was not essential for the neuroinflammatory response to radiation. The work presented here demonstrates that cranial irradiation leads to a state of chronic inflammation in the CNS. This thesis also suggests that peripheral immune cells may play a role in the development of late developing radiation injury. Importantly, these experiments elucidate some of the inflammatory mediators involved in the cellular recruitment following radiation exposure. The fact that infiltrating cells were only found in areas of the brain that were exposed to radiation, at doses considered safe in human patients, may provide a means to therapeutically deliver genes to the CNS. Further studies will be required to more fully characterize pathways contributing to radiation-induced neuroinflammation and cellular recruitment