Pathogenesis of Lyme neuroborreliosis: Borrelia burgdorferi lipoproteins induce both proliferation and apoptosis in rhesus monkey astrocytes.

Brain invasion by Borrelia burgdorferi, the agent of Lyme disease, results in an inflammatory and neurodegenerative disorder called neuroborreliosis. In humans, neuroborreliosis has been correlated with enhanced concentration of glial fibrillary acidic protein in the cerebrospinal fluid, a sign of astrogliosis. Rhesus monkeys infected by us with B. burgdorferi showed evidence of astrogliosis, namely astrocyte proliferation and apoptosis. We formulated the hypothesis that astrogliosis could be caused by spirochetal lipoproteins. 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, and tripalmitoyl-S-glyceryl-Cys-Ser-Lys(4)-OH (Pam(3)Cys), a synthetic lipopeptide that mimics the structure of the lipoprotein lipid moiety. L-OspA elicited not only astrocyte proliferation but also apoptosis, two features observed during astrogliosis. Astrocytes produced both IL-6 and TNF-alpha in response to L-OspA and Pam(3)Cys. Proliferation induced by L-OspA was diminished in the presence of an excess of anti-IL-6 antibody, and apoptosis induced by this lipoprotein was completely suppressed with anti-TNF-alpha antibody. Hence, IL-6 contributes to, and TNF-alpha determines, astrocyte proliferation and apoptosis, respectively, as elicited by lipoproteins. Our results provide proof of the principle that spirochetal lipoproteins could be key virulence factors in Lyme neuroborreliosis, and that astrogliosis might contribute to neuroborreliosis pathogenesis.

Autocrine and exocrine regulation of interleukin-10 production in THP-1 cells stimulated with Borrelia burgdorferi lipoproteins.

We have recently demonstrated that interleukin-10 (IL-10), produced by THP-1 monocytes in response to Borrelia burgdorferi lipoproteins, dampens the production of concomitantly elicited inflammatory cytokines. Thus, IL-10 could potentially down-regulate inflammatory and microbicidal effector mechanisms of the innate immune response to a B. burgdorferi infection, facilitating the establishment of the spirochete. To understand the mechanism(s) implicated in the regulation of the synthesis and release of IL-10 during early infection, we investigated the autocrine effects of IL-6, IL-12, tumor necrosis factor alpha (TNF-alpha), and IL-10 itself, as well as the exocrine effect of IFN-gamma on the production of macrophage-derived IL-10 with lipoprotein as a stimulant. In addition, in view of the differences in the receptor and signal transduction pathways of lipopolysaccharide (LPS) and bacterial lipoproteins, we also investigated the effects described above with LPS as a stimulant. The THP-1 human monocytic cell line and purified recombinant lipidated OspA (L-OspA) were used as the model target cell and stimulant, respectively. TNF-alpha increased the production of IL-10, as elicited by lipoproteins. The production of IL-10 by THP-1 cells stimulated with L-OspA was autoregulated by a negative feedback mechanism involving the IL-10 receptor (IL-10R). Exogenous IFN-gamma significantly inhibited the production of IL-10. Both autocrine (IL-10) and exocrine (IFN-gamma) inhibition of IL-10 production resulted in an increase in the production of the proinflammatory cytokines IL-6 and IL-12. The same results were obtained when the stimulant was LPS. The results further illustrate that IL-10 may play a pivotal role in Lyme disease pathogenesis. Moreover, the regulation of its production with lipoprotein as a stimulant is indistinguishable from that observed when LPS acts as a stimulant.