Invasion of brain microvascular endothelial cells by group B streptococci

Group B streptococci (GBS) are the leading cause of meningitis in newborns. Although meningitis develops following bacteremia, the precise mechanism or mechanisms whereby GBS leave the bloodstream and gain access to the central nervous system (CNS) are not known. We hypothesized that GBS produce meningitis because of a unique capacity to invade human brain microvascular endothelial cells (BMEC), the single-cell layer which constitutes the blood-brain barrier. In order to test this hypothesis, we developed an in vitro model with BMEC isolated from a human, immortalized by simian virus 40 transformation, and propagated in tissue culture monolayers. GBS invasion of BMEC monolayers was demonstrated by electron microscopy. Intracellular GBS were found within membrane-bound vacuoles, suggesting the organism induced its own endocytic uptake. GBS invasion of BMEC was quantified with a gentamicin protection assay. Serotype III strains, which account for the majority of CNS isolates, invaded BMEC more efficiently than strains from other common GBS serotypes. GBS survived within BMEC for up to 20 h without significant intracellular replication. GBS invasion of BMEC required active bacterial DNA, RNA, and protein synthesis, as well as microfilament and microtubule elements of the eukaryotic cytoskeleton. The polysaccharide capsule of GBS attenuated the invasive ability of the organism. At high bacterial densities, GBS invasion of BMEC was accompanied by evidence of cellular injury; this cytotoxicity was correlated to beta-hemolysin production by the bacterium. Finally, GBS demonstrated transcytosis across intact, polar BMEC monolayers grown on Transwell membranes. GBS invasion of BMEC may be a primary step in the pathogenesis of meningitis, allowing bacteria access to the CNS by transcytosis or by injury and disruption of the endothelial blood-brain barrier.