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- § 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- § antibody. Hence, IL-6 contributes to, and TNF- § 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.

Antigenic variation is an effective strategy evolved by pathogenic microbes to avoid immune destruction. Variable Ags such as the variable major protein of Borrelia hermsii, the variant surface glycoprotein of African trypanosomes, and the pilin of Neisseria gonorrhoeae include an immunodominant variable domain and one or more invariable domains that are not antigenic. Short, nonantigenic, invariable regions also may be present within the variable domain. VlsE (variable major protein-like sequence, expressed), the variable surface Ag of Borrelia burgdorferi, the Lyme disease spirochete, also contains both variable and invariable domains. In addition, interspersed within the VlsE variable domain there are six invariable regions (IR1–6) that together amount to half of this portion’s primary structure. We show here that these IRs are conserved among strains and genospecies of the B. burgdorferi sensu lato complex. Surprisingly, unlike the invariable regions of variable major protein, variant surface glycoprotein, and pilin, which are not antigenic in natural infections, the most conserved of the IRs, IR6, is immunodominant in Lyme disease patients and in monkeys infected with B. burgdorferi. IR6 is exposed on the surface of VlsE, as assessed by immunoprecipitation experiments, but is inaccessible to Ab on the spirochete’s outer membrane, as demonstrated by immunofluorescence and in vitro killing assays. VlsE thus significantly departs from the antigenic variation paradigm, whereby immunodominance is only manifest in variable portions. We submit that IR6 may act as a decoy epitope(s) and contribute to divert the Ab response from other, perhaps protective regions of VlsE.