ALTERED IMMUNOEXPRESSION OF MICROGLIA AND MACROPHAGES AFTER MILD HEAD-INJURY

In this study we examined the temporal response of microglia and macro phages to mild head injury in the rat. Microglia and macrophages were identified by their distinct morphology and by immunophenotype. With r egard to the latter, antibodies to OX42 and ED1 were used to define mi croglia and macrophages, respectively. Although there was no change in the morphology of brain macrophages after mild head injury, the morph ology of microglia was dramatically altered. Microglial cell bodies ap peared larger with a more elaborate arborization of cellular processes . After head injury certain populations of macrophages and microglia w ere more intensely immunostained. By 3 days postinjury these intensely stained cells exhibited a characteristic distribution in the brain. P rominently stained microglia were detected in the thalamus, hippocampu s, lateral and medial geniculate body, and the substantia nigra. Inten sely stained macrophages were located primarily in tbe cortex and suba rachnoid space adjacent to the site of impact. By 7 days postinjury in tensely immunostained macrophages and microglia were widespread throug hout the injured cortex. These results demonstrate that microglia and macrophages are sensitive to mild head injury. Early changes in the ma crophage population are more directly correlated with the most damaged tissue and may reflect migration of these cells from either the subar achnoid space or across the damaged blood-brain barrier. The early wid espread microglial response in regions exhibiting no overt neuronal ce ll damage suggests that these cells are responding to more subtle fact or(s) that are expressed in the mildly traumatized brain.