gms | German Medical Science

Objective: Inflammatory mechanisms in early and delayed brain injury after SAH have become an interesting research target. In previous studies we could show an intraparenchymal inflammatory (Iba-1-positive cells) response after SAH. This cellular infiltration started on day 4, peaked around day 14 after the bleeding and correlated with signs of axonal injury (extracellular amyloid precursor protein) and clinical deficits. To determine, whether these inflammatory cells are peripheral immune cells, that are recruited into the brain secondarily or primary brain residential microglia, a chimeric mouse model was tested.

Methods: Mice were transplanted with green fluorescent protein(GFP) bone marrow, after sublethal irradiation. After reconstitution experimental SAH was induced using filament perforation technique. Inflammatory cell invasion into the brain parenchyma was documented immunofluorescently by Ionized Calcium-binding adaptor molecule-1 (Iba-1). In these chimeric mice, all primary brain cells were GFP negative, while all cells that derived from the peripheral bone marrow were GFP positive, and could thus be differentiated fluorescently. To exclude pre-sensitiving of the blood brain barrier by irradiation, a head-protected (lead helmet) group served as controls. Sham-operated animals after irradiation allowed for exclusion of blood brain barrier breakdown as irradiation sequelae.

Results: In total body irradiated mice, a large fraction of all intraparenchymal Iba-1-positive cells did also show GFP-positivity (Day 4: 10 ± 4.9 / 5 ± 5; day 9: 16 ± 3.5 / 12 ± 5.1; day 14: 17 ± 3.8 / 12 ± 5.1; day 28: 11 ± 1.5 / 2 ± 1.7; Iba-1 positive / Iba-1-GFP double positive cells per hpf, app 64% total). In the head-protected animals, however, no GFP-positive cells could be detected inside the CNS, although they could be found in the subarachnoid space after the bleeding. In total body irradiated sham operated animals, no inflammatory cell invasion could be documented.

Conclusions: 1. Brain residential microglia cells – as verified by Iba-1 positivity – and not secondarily invaded peripheral monocytes are responsible for the intraparenchymal inflammatory response and are co-localized with signs of axonal injury in the early phase after aSAH. 2. Irradiation does not lead to a breakdown of the blood brain barrier, but it leads to a pre-sensitization even weeks after the irradiation.