Perivascular macrophages create an intravascular niche for CD8(+) T cell localisation prior to the onset of fatal experimental cerebral malaria.

OBJECTIVES: The immunologic events that build up to the fatal neurological stage of experimental cerebral malaria (ECM) are incompletely understood. Here, we dissect immune cell behaviour occurring in the central nervous system (CNS) when Plasmodium berghei ANKA (PbA)-infected mice show only minor clinical signs. METHODS: A 2-photon intravital microscopy (2P-IVM) brain imaging model was used to study the spatiotemporal context of early immunological events in situ during ECM. RESULTS: Early in the disease course, antigen-specific CD8(+) T cells came in contact and arrested on the endothelium of post-capillary venules. CD8(+) T cells typically adhered adjacent to, or were in the near vicinity of, perivascular macrophages (PVMs) that line post-capillary venules. Closer examination revealed that CD8(+) T cells crawled along the inner vessel wall towards PVMs that lay on the abluminal side of large post-capillary venules. 'Activity hotspots' in large post-capillary venules were characterised by T-cell localisation, activated morphology and clustering of PVM, increased abutting of post-capillary venules by PVM and augmented monocyte accumulation. In the later stages of infection, when mice exhibited neurological signs, intravascular CD8(+) T cells increased in number and changed their behaviour, actively crawling along the endothelium and displaying frequent, short-term interactions with the inner vessel wall at hotspots. CONCLUSION: Our study suggests an active interaction between PVM and CD8(+) T cells occurs across the blood-brain barrier (BBB) in early ECM, which may be the initiating event in the inflammatory cascade leading to BBB alteration and neuropathology.