An in vitro model of the interaction of bacterial lipoproteins with the central nervous system
Description
Brain invasion by Borrelia burgdorferi, the agent of Lyme disease, results in an inflammatory and neurodegenerating disorder called Lyme neuroborreliosis. Rhesus monkeys infected with a neurotropic strain of B. burgdorferi showed immunohistochemical evidence of astrogliosis. We explored the hypothesis that mediators elicited by brain cells, particularly astrocytes and microglia, in response to bacterial lipoproteins, constitute a molecular basis for central nervous system (CNS) disease seen in neuroborreliosis. We established primary cultures of rhesus monkey astrocytes and stimulated the cells with recombinant lipidated outer surface protein A (L-OspA), a model B. burgdorferi lipoprotein. Pure astrocytic cultures produced the cytokines interleukin (IL-6) and tumor necrosis factor (TNF-alpha) in response to L-OspA. L-OspA could elicit a proliferative effect on astrocytes which was considerably reduced in astrocytes pretreated with anti IL-6 antibody prior to L-OspA stimulation. Interestingly, L-OspA could also induce apoptosis in astrocytes by virtue of its ability to induce TNF-alpha. Astrocyte proliferation with concomitant apoptotic cell death is commonly seen during gliosis. The synthetic lipopeptide Pam3hex, which mimics the lipid moiety present in all bacterial lipoproteins, also elicited both IL-6 and TNF-alpha in astrocytes L-OspA activated Extracellular Signal regulated Kinases Erk-1, Erk-2 and Mitogen Activated Protein Kinase (MAPK) p38 in astrocytes by virtue of its lipid moiety. Selective inhibition of these kinases with U0126 and SB 203580 respectively, indicated that TNF-alpha production in astrocytes as induced by bacterial lipoproteins is regulated by both Erk 1/2 and p38 MAP Kinases. IL-6 production in astrocytes as induced by L-OspA was unaffected by the blockage of either of the kinases individually but was diminished in the presence of both inhibitors. Further, the apoptotic mediator Fas was significantly upregulated (p = 0.004) in rhesus astrocytes when exposed to L-OspA To simulate a more in-vivo-like scenario we stimulated aggregate brain cultures and pure microglial cultures with L-OspA. We observed production of IL-1beta, and IL-12 in addition to IL-6 and TNF-alpha The findings of this in vitro model of the interaction of bacterial lipoproteins with the CNS indicate that bacterial lipoproteins have the potential to mount the inflammatory response seen in Lyme neuroborreliosis in the rhesus monkey