Brain macrophage activation in murine cerebral malaria precedes accumulation of leukocytes and CD8⁺ T cell proliferation

Although the activation of brain macrophages is associated with both human and mouse cerebral malaria (CM) the relative contributions of the heterogeneous populations of brain macrophages to the disease are unknown. In this work, we dissociate for the first time inflammatory monocytes from resident brain macrophages in mice developing CM when infected with Plasmodium berghei. Based on the differential expression of CD45 in brain macrophage cell populations and by using bone-marrow (BM) chimeras reconstituted with GFP cells we clearly distinguish between blood-derived cells and resident brain cells of hematopoietic origin. FACS and histological analysis reveal that although inflammatory monocytes and CD8⁺ T cells invade the brain during CM, parenchymal macrophages also undergo morphological changes and over express MHC class I and Sca-1. In addition to the leukocyte sequestration in the brain, in situ proliferation contributes to the expansion of CD8⁺ T cells during CM. Finally, kinetic analysis of brain cells during infection with P. berghei demonstrates that activation of parenchymal macrophages precedes leukocyte sequestration in the brain vasculature. Thus, our results reveal the phenotype of activation of brain macrophages during CM showing that parenchymal brain macrophages are activated before overwhelming brain inflammation. These results further suggest that brain macrophages may contribute to the local proliferation of CD8⁺ T cells that culminate in death of mice with CM syndrome.