Regulation of OspE-related, OspF-related, and Elp lipoproteins of Borreliaburgdorferi strain 297 by mammalian host-specific signals

In previous studies we have characterized the cp32/18 loci in Borrelia burg dorferi 297 which encode OspE and OspF orthologs and a third group of lipop roteins which possess OspE/F-like leader peptides (Elps). To further these studies, we have comprehensively analyzed their patterns of expression thro ughout the borrelial enzootic cycle. Serial dilution reverse transcription- PCR analysis indicated that although a shift in temperature from 23 to 37 d egreesC induced transcription for all nine genes analyzed, this effect was often markedly enhanced in mammalian host-adapted organisms cultivated with in dialysis membrane chambers (DMCs) implanted within the peritoneal caviti es of rats. Indirect immunofluorescence assays performed on temperature-shi fted, in vitro-cultivated spirochetes and organisms in the midguts of unfed and fed ticks revealed distinct expression profiles for many of the OspE-r elated, OspF-related, and Elp proteins. Other than BbK2.10 and ElpA1, all w ere expressed by temperature-shifted organisms, while only OspE, ElpB1, Osp F, and BlbR2.11 were expressed in the midguts of fed ticks. Additionally, a lthough mRNA was detected for all nine lipoprotein-encoding genes, two of t hese proteins (BbK2.10 and ElpA1) were not expressed by spirochetes cultiva ted in vitro, within DMCs, or by spirochetes within tick midguts, However, the observation that B. burgdorferi-infected mice generated specific antibo dies against BbE2.10 and ElpA1 indicated that these antigens are expressed only in the mammalian host and that a form of posttranscriptional regulatio n is involved. Analysis of the upstream regions of these genes revealed sev eral differences between their promoter regions, the majority of which were found in the -10 and -35 hexamers and the spacer regions between them. Als o, rather than undergoing simultaneous upregulation during tick feeding, th ese genes and the corresponding lipoproteins appear to be subject to progre ssive recruitment or enhancement of expression as B. burgdoferi is transmit ted from its tick vector to the mammalian host. These findings underscore t he potential relevance of these molecules to the pathogenic events of early Lyme disease.