The Lyme Disease Spirochete Borrelia Burgdorferi Induces Increased Expression of Multiple Neuroinflammatory Factors in Human Microglial Cells In Vitro

Abstract

Many microorganisms can cause inflammation in the central nervous system (CNS) by damaging or penetrating the protective blood‐brain and blood‐cerebral spinal fluid barriers that sequester the brain from the periphery. Breakdown of the blood‐brain barrier (BBB) or blood‐cerebral spinal fluid (blood‐CSF) barrier could facilitate entry of both B. burgdorferi and activated immune cells into the CNS leading to multiple neurological complications, collectively termed neuroborreliosis. Indeed, B. burgdorferi has an affinity for the CNS and can be isolated from the CSF of humans as early as 18 days after the bite of an infected tick. However, the mechanism by which the bacterium initiates neuroinflammation leading to neural disfunction remains unknown. The ensuing neurological symptoms are likely a result of a glial‐driven, host inflammatory response to the bacterium. However, the specific contributions of human microglial cells to the pathogenesis of neuroborreliosis have not yet been elucidated.We hypothesize that dead B. burgdorferi and debris persist in the CNS after antibiotic treatment and contribute to an ongoing inflammatory response in the CNS. To begin characterization of this response, highly pure cultures of primary human microglia were incubated with live virulent B. burgdorferi and the membrane lipoprotein Pam3Cys to elucidate the cell‐ and lipoprotein‐specific responses to the bacterium. Cultures were analyzed by immuncytochemistry, real time PCR array and ELISA analysis. Array results were confirmed with individual sets of PCR primers. Our results demonstrate a robust increase in numerous inflammatory chemokines, cytokines and related genes in all treatment conditions examined. Most prominent was an increase in the neurophil chemoattractants IL‐8, IL‐6, CXCL1, CXCL10 and LIF. These data suggest that, in addition to B. burgdorferi, infiltrating phagocytic cells, particularly potentially neurotoxic neutrophils, may be important additional contributors to the early inflammatory events associated with the onset of neuroborreliosis.Our continuing studies are now focused on elucidating microglial phagocytosis of B. burgdorferi using real time PCR arrays and culture monolayers coupled with confocal microscopy and trans emission microscopy. Interestingly, initial data has demonstrated that although no there is no phagocytosis of live bugs in vitro, primary human microglia highly up regulate numerous genes associated with phagocytosis and associated cell signaling when exposed to dead and/or dead‐sonicated forms of B. burgdorferi. More specifically, in microglia exposed to dead and/or dead‐sonicated B. burgdorferi, there was a significant increase in expression of genes known to be involved in recognition of Bb (TLRs, NOD1, CD14); Fc receptors; kinases (LYN, MAPK); and things involved in uptake/phagolysosomes (GULP‐1, members of the Rac/Rho family, phospholipases, VAMP, syntaxin). Taken together, these results suggest that primary human microglia continue to mount a pro‐inflammatory response in the absence of viable Borrelia burgdorferi.Support or Funding InformationThis study was funded by a grant from the University of North Dakota Office of Research Development and Compliance (2014).