Group B streptococcal β-hemolysin/cytolysin activates neutrophil signaling pathways in brain endothelium and contributes to development of meningitis

Abstract

Meningitis occurs when blood-borne pathogens cross the blood-brain barrier (BBB) in a complex interplay between endothelial cells and microbial gene products.We sought to understand the initial response of the BBB to the human meningeal pathogen group B Streptococcus (GBS) and the organism's major virulence factors, the exopolysaccharide capsule and the -hemolysin/cytolysin toxin (-h/c).Using oligonucleotide microarrays, we found that GBS infection of human brain microvascular endothelial cells (HBMEC) induced a highly specific and coordinate set of genes including IL-8, Gro, Gro, IL-6, GM-CSF, myeloid cell leukemia sequence-1 (Mcl-1), and ICAM-1, which act to orchestrate neutrophil recruitment, activation, and enhanced survival.Most strikingly, infection with a GBS strain lacking -h/c resulted in a marked reduction in expression of genes involved in the immune response, while the unencapsulated strain generally induced similar or greater expression levels for the same subset of genes.Cell-free bacterial supernatants containing -h/c activity induced IL-8 release, identifying this toxin as a principal provocative factor for BBB activation.These findings were further substantiated in vitro and in vivo.Neutrophil migration across polar HBMEC monolayers was stimulated by GBS and its -h/c through a process involving IL-8 and ICAM-1.In a murine model of hematogenous meningitis, mice infected with -h/c mutants exhibited lower mortality and decreased brain bacterial counts compared with mice infected with the corresponding WT GBS strains.