KINETICS AND IN-VIVO INDUCTION OF GENETIC-VARIATION OF VLSE IN BORRELIA-BURGDORFERI

The Lyme disease agent, Borrelia burgdorferi, is able to persistently infect humans and animals for months or years in the presence of an ac tive immune response. It is not known how the organisms survive immune attack in the mammalian host. vlsE, a gene localized near one end of linear plasmid lp28-1 and encoding a surface-exposed lipoprotein in B, burgdorferi B31, was shown recently to undergo extensive genetic and antigenic variation within 28 days of initial infection in C3H/HeN mic e. In this study, we examined the kinetics of vlsE sequence variation in C3H/HeN mice at 4, 7, 14, 21, and 28 days and at 7 and 12 months po stinfection, Sequence changes,were detected by PCR amplification and s equence analysis as early as 4 days postinfection and accumulated prog ressively in both C3H/HeN and CB-17 severe combined immunodeficient (S CID) mice throughout the course of infection. The sequence changes wer e consistent with sequential recombination of segments from multiple s ilent vis cassette sites into the vlsE expression site. No vlsE sequen ce changes were detected in organisms cultured in vitro for up to 84 d ays, These results indicate that vlsE recombination is induced by a fa ctor(s) present in the mammalian host, independent of adaptive immune responses. The possible inducing conditions appear to be present in va rious tissue sites because isolates from multiple tissues showed simil ar degrees of sequence variation, The rate of accumulation of predicte d amino acid changes was higher in the immunologically intact C3H/HeN mice than in SCID mice, a finding consistent with immune selection of VlsE variants.