Olfactory ensheathing cells as putative host cells for Streptococcus pneumoniae: evidence of bacterial invasion via mannose receptor-mediated endocytosis

Olfactory ensheathing cells (OECs) are a special glia that ensheath olfactory receptor axons that enter the brain via olfactory phila, thus, providing a potential route for access of pathogens. Streptococcus pneumoniae (Sp), that has a capsule rich in mannosyl residues, is the most common cause of rhinosinusitis that may evolve to meningitis. We have tested whether OECs in vitro express the mannose receptor (MR), and could internalize Sp via MR. Cultures were infected by a suspension of Sp (ATCC 49619), recognized by an anti-Sp antibody, in a 100:1 bacteria:cells ratio. Competition assays, by means of mannan, showed around a 15-fold reduction in the number of internalized bacteria. To verify whether MR could be involved in Sp uptake, OECs were reacted with an antibody against the MR C-terminal peptide (anti-cMR) and bacteria were visualized with Sytox Green. Selective cMR-immunoreaction was seen in perinuclear compartments containing bacteria whereas mannan-treated cultures showed an extremely low percentage of internalized bacteria and only occasional adhered bacteria. Our data suggest the involvement of MR in adhesion of bacteria to OEC surface, and in their internalization. Data are also coherent with a role of OECs as a host cell prior to (and during) bacterial invasion of the brain.