gms | German Medical Science

Objective: The tumor mass of glioblastoma multiforme consists of up to one third of microglia and/or macrophages. Distinguishing between both cell types is of current interest as studies in various CNS diseases suggested different effector roles. Due to the absence of specific markers, bone marrow chimeras are often generated to discriminate between radio-resistant GFP^- microglia and infiltrated GFP⁺ macrophages. In this study we used irradiation of the total body (TBI) as well as with a head protection (HP) and compared both methods in the syngeneic glioblastoma mouse model.

Method: Mice were irradiated either with or without head protection and subsequently reconstituted by GFP-expressing bone marrow cells. After reconstitution tumor cells were injected into mouse brains followed by three weeks of growth. The percentage of GFP⁺ cells in the blood and infiltration of immune cells into the brain were analyzed by FACS (Fluorescence Activated Cell Sorting). Tumor volume was monitored by the use of MRI and the localization of GFP⁺ cells in the brain was evaluated via immunofluorescence staining.

Results: In head protected mice (HP), the reconstitution of the T- and B-cell population with GFP⁺ cells was diminished while the myeloid lineage reached 80% replacement similar to total body irradiation (TBI). Nevertheless, infiltration and localization of these cells into the tumor hemisphere was significantly different. Less than 30% of the CD11b⁺CD45⁺ cells were also GFP⁺ in HP mice, contrasting with 65% in TBI. In both irradiation groups this GFP⁺CD11b⁺CD45⁺ cell fraction was at least 90 % positive for Iba-1 meaning macrophages. Infiltrated cells localized after TBI intra- and extra-tumoral, while in HP the GFP+ cells were only restricted to the tumor tissue. Surprisingly, TBI also appeared to change the tumor progression and enlarged the volume by four times. On the contrary, HP tumor bearing mice evolved the same tumor size as the non-irradiated control group.

Conclusions: Head protection is a meaningful tool to analyze the infiltration of myeloid cells. It avoids unspecific infiltration and non-specific side effects as increased tumor volumes and over all localization of peripheral immune cells. Thereby, head protection reflects more the physiological situation than total body irradiation suggesting that resident microglia and not macrophages are the main immune cell population in glioblastoma multiforme and could be effective targets in tumor therapy.