Previous immunological studies indicated that the Lyme disease spirochete, Borrelia burgdorferi, expresses Erp outer surface proteins during mammalian infection. We conducted analyses of Erp expression throughout the entire tick-mammal infectious cycle, which revealed that the bacteria regulate Erp production in vivo. Bacteria within unfed nymphal ticks expressed little to no Erp proteins. However, as infected ticks fed on mice, B. burgdorferi increased production of Erp proteins, with essentially all transmitted bacteria expressing these proteins. Mice infected with B. burgdorferi mounted rapid IgM responses to all tested Erp proteins, followed by strong immunoglobulin G responses that generally increased in intensity throughout 11 months of infection, suggesting continued exposure of Erp proteins to the host immune system throughout chronic infection. As naive tick larvae acquired B. burgdorferi by feeding on infected mice, essentially all transmitted bacteria produced Erp proteins, also suggestive of continual Erp expression during mammalian infection. Shortly after the larvae acquired bacteria, Erp production was drastically downregulated. The expression of Erp proteins on B. burgdorferi throughout mammalian infection is consistent with their hypothesized function as factor Hbinding proteins that protect the bacteria from host innate immune responses.The causative agent of Lyme disease, Borrelia burgdorferi, is transmitted to humans and other warm-blooded hosts via the bites of infected ixodid ticks. There are three postembryonic stages of these ticks: larva, nymph, and adult, each of which takes only one blood meal. There is essentially no transovarial transmission of B. burgdorferi, so emergent larvae are uninfected and acquire the bacteria by feeding on infected hosts. Fully engorged larvae molt to nymphs, which can transmit B. burgdorferi to the hosts on which they feed. Fed nymphs molt to adults, the females of which feed, lay eggs, and then die. In most areas where Lyme disease is endemic, both the larval and nymphal stages feed on the same host species, and B. burgdorferi perpetuates through a cycle between these tick stages and their warm-blooded hosts. To complete this infectious cycle, the bacteria must interact with many different host and vector tissues, as well as evade clearance by the host's immune system. To do so, B. burgdorferi apparently senses its environment and coordinates the synthesis of numerous proteins.Among the bacterial proteins known to be expressed during mammalian infection are the Erp lipoproteins. These outer surface proteins are encoded by allelic genes located on the cp32 plasmids of B. burgdorferi (57). A potential function for these proteins is suggested by observations that they bind the complement inhibitory factor H proteins of numerous different vertebrates (5,6,26,(29)(30)(31)54). It is hypothesized that a bacterium needs to express a wide repertoire of Erp proteins so that it can bind the factor H molecules from a wide variety of mammalian hosts, allowing the bacte...

Many Borrelia burgdorferi Erp outer surface proteins have been demonstrated to bind the host complement regulator factor H, which likely contributes to the ability of these organisms to evade the host innate immune system. B. burgdorferi controls Erp protein synthesis throughout the bacterial infectious cycle, producing the proteins during mammalian infections but repressing their synthesis during tick infections. Defining the mechanism by which B. burgdorferi regulates the expression of these virulence determinants will provide important insight into the biological and pathogenic properties of the Lyme disease spirochete. The present study demonstrates that two highly conserved DNA sequences located 5 of erp operons specifically bind bacterial proteins. Analyses with B. burgdorferi of transcriptional fusions between erp promoter/operator DNAs and the gene for green fluorescent protein indicated that the expression of these operons is regulated at the level of transcriptional initiation. These analyses also indicated significant differences in the promoter strengths of various erp operons, which likely accounts for reported variations in expression levels of different Erp proteins. Mutagenesis of promoter-gfp fusions demonstrated that at least one of the proteins which bind erp operator DNA functions as a repressor of transcription.The spirochete Borrelia burgdorferi is maintained in nature through an infectious cycle involving warm-blooded vertebrates and Ixodes sp. ticks. These bacteria regulate the expression of a number of different proteins during the infectious cycle, among which are the Erp outer surface lipoproteins. Individual bacteria encode multiple members of the Erp family, with the type strain, B31, being known to carry 17 erp genes at 10 separate loci (12,13,50). Each locus occupies an allelic position on a different plasmid of the cp32 family. Although cp32 plasmids are largely identical in their sequences, many are compatible with each other, with 12 apparent incompatibility groups identified to date (16,47). Many Erp proteins have been demonstrated to bind host factor H, a regulator of complement activation, and may thereby help protect the bacteria from the alternative pathway of complement-mediated killing (4, 5, 25-29, 34, 46). Consistent with that function, B. burgdorferi expresses Erp proteins during transmission between the tick vector and the vertebrate host, times when the bacterium is exposed to host serum and the innate immune system (22-24, 36, 37). For unknown reasons, B. burgdorferi down-regulates Erp expression during infections of ticks (22,36).A number of in vitro studies have provided insights into the regulatory mechanisms by which B. burgdorferi controls Erp synthesis. The culture temperature significantly impacts Erp expression levels, with bacteria grown at 34°C producing higher levels of Erp proteins than do bacteria cultivated at 23°C (24,44,48). These temperatures mimic those experienced by feeding and unfed ticks, respectively (40). The bacteria also respond to quorum s...

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