Antigenic variation in Lyme disease spirochetes by segmental recombination of VMP-like sequence cassettes

Lyme disease is a multisystemic disorder caused by tick-borne infection of humans or other mammalian hosts with Borrelia burgdorferi. If untreated, the spirochetes can persist in the mammalian host for months or years. The mechanisms by which Lyme disease spirochetes evade the immune response have not been determined. In this study, we have identified and characterized an elaborate genetic system in the Lyme disease spirochete B. burgdorferi that promotes extensive antigenic variation of a 34-kDa surface-exposed lipoprotein, VlsE. A 28-kilobase linear plasmid of B. burgdorferi B31 (lp28-1) was found to contain a vmp-like sequence (vls) locus that closely resembles the variable major protein (vmp) system for antigenic variation of relapsing fever organisms. The presence of lp28-1 correlates with the high-infectivity phenotype in B. burgdorferi strains tested. Segments of the 15 non-expressed (silent) vls cassette sequences located upstream of vlsE are able to recombine into the centra vlsE cassette region during infection of C3H/HeN mice, resulting in antigenic variation of the expressed lipoprotein. When compared to parental VlsE, VlsE variants progressively accumulate sequence changes during the period of 4, 7, 14, 21, and 28 days post infection in C3H/HeN mice. However, no recombination was detected during the period of 28-day in vitro culture, suggesting in vivo induction of VlsE antigenic variation. Adaptive immune responses do not appear to play a significant role in this induction, since similar recombination events were also observed in immunodeficient SCID mice. The $5\sp\prime$ and $3\sp\prime$ noncassette regions of vlsE are apparently not subject to recombination and sequence variation. The structure and sequence of the silent vls cassette locus is preserved during the process of the VlsE antigenic variation, consistent with a nonreciprocal recombination mechanism. This combinatorial form of antigenic variation could potentially yield millions of VlsE variants in the mammalian host, and thereby contribute to immune evasion, long-term survival, and pathogenesis of B. burgdorferi. ^