Dendritic cells in the healthy and inflamed brain: A two photon microscopy investigation.

Dendritic cells (DCs) are professional antigen-presenting cells, derived from a bone marrow progenitor, which modulate the balance between suppression and induction of the immune response. DCs have been widely characterized in peripheral organs, especially in lymph nodes where antigen presentation mostly occurs. DCs have also been identified in the meninges and choroid plexus, as well as within the brain parenchyma. Several key aspects of DCs function in the brain are, however, still unexplored. Transgenic mice expressing fluorescent proteins in cell subsets provide valuable tools for in vivo investigations by two-photon fluorescence (TPF) microscopy. We recently demonstrated that in thy1GFP-M mice, engineered for green fluorescent protein (GFP) expression in neurons, also DCs express GFP. This murine line is, therefore, suited for the visualization of brain DCs. We here analyzed with TPF microscopy GFP-tagged brain DCs in healthy thy1GFP-M mice and during infection with Trypanosoma brucei (Tb). This parasite is the etiological agent of human African trypanosomiasis or sleeping sickness, whose encephalitic stage is fatal if untreated, and in which pathogenetic mechanisms of the neuroimmune response remain to be clarified. Our in vivo observations showed, in normal conditions, GFP-DCs in the subarachnoid space and meninges, where these cells were mainly static and occasionally in a probing-like motion. A motile behavior of GFP-DCs was also observed in the upper cortical layers, supporting a role of immunosurveillance of DCs in the healthy brain. Striking changes of motility and quantity of GFP-DCs were observed in the brain of Tb-infected thyGFP-M mice. During the early meningoencephalitic stage, GFP-DCs invaded the parenchyma with rapid and wide displacements, and also occurred in static adhesion to or crawling on the inflamed endothelium. With disease progression, a drastic decrease in the number of GFP-DCs was observed at the brain surface, and GFP-DCs appeared arranged in static clusters of cells exhibiting numerous processes, likely to increase the cell membrane surface on which antigens are exposed. Preliminary in vivo observations of thy1GFP-M mice infected with transgenic fluorescent Tb have shown direct interactions between GFP-DCs and the parasites; further analyses are ongoing. Taken together, the present in vivo investigations not only reveal a motile behavior of DCs at the brain surface and in the upper cortical layers, but also suggest a relevant role of brain DCs in African trypanosome infection. In particular, these cells could play a role in the transition from immune resistance to immune tolerance during this severe brain infection.