The rationale for the present study was to determine how different species of bacteria interact with cells of the human meninges in order to gain information that would have broad relevance to understanding aspects of the innate immune response in the brain. Neisseria lactamica is an occasional cause of meningitis in humans, and in this study we investigated the in vitro interactions between N. lactamica and cells derived from the leptomeninges in comparison with the closely related organism Neisseria meningitidis, a major cause of meningitis worldwide. N. lactamica adhered specifically to meningioma cells, but the levels of adherence were generally lower than those with N. meningitidis. Meningioma cells challenged with N. lactamica and N. meningitidis secreted significant amounts of the proinflammatory cytokine interleukin-6 (IL-6), the C-X-C chemokine IL-8, and the C-C chemokines monocyte chemoattractant protein 1 (MCP-1) and RANTES, but it secreted very low levels of the cytokine growth factor granulocyte-macrophage colony-stimulating factor (GM-CSF). Thus, meningeal cells are involved in the innate host response to Neisseria species that are capable of entering the cerebrospinal fluid. The levels of IL-8 and MCP-1 secretion induced by both bacteria were essentially similar. By contrast, N. lactamica induced significantly lower levels of IL-6 than N. meningitidis. Challenge with the highest concentration of N. lactamica (10 8 CFU) induced a small but significant down-regulation of RANTES secretion, which was not observed with lower concentrations of bacteria. N. meningitidis (10 6 to 10 8 CFU) also down-regulated RANTES secretion, but this effect was significantly greater than that observed with N. lactamica. Although both bacteria were unable to invade meningeal cells directly, host cells remained viable on prolonged challenge with N. lactamica, whereas N. meningitidis induced death; the mechanism was overwhelming necrosis with no significant apoptosis. It is likely that differential expression of modulins between N. lactamica and N. meningitidis contributes to these observed differences in pathogenic potential.Meningitis, inflammation of the meninges that surround the brain and the spinal cord (63), is the most common serious infection of the central nervous system. In humans, the meninges comprise the pachymenix (dura mater) and the leptomeninges, which consists of the arachnoid and pia mater together with the trabeculae that traverse the cerebrospinal fluid (CSF)-filled subarachnoid space (SAS) (4, 25). The bacterial etiology of meningitis is broad, and susceptibility to the causative organisms varies with age, with different groups of organisms affecting neonates, children, and adults (28). Neonatal bacterial meningitis is due mainly to transmission of bacteria from the maternal genital tract to the newborn infant, and the main organisms that cause disease are Streptococcus agalactiae (group B hemolytic streptococcus) and Escherichia coli K1 (28, 47). After the neonatal period, the most common bacterial ...