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Clinical and Diagnostic Laboratory Immunology, August 2005, p. 935-940, Vol. 12, No. 8
1071-412X/05/$08.00+0 doi:10.1128/CDLI.12.8.935-940.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.
Departments of Microbiology and Immunology,1 Pathology,2 Medicine, New York Medical College, Valhalla, New York 105953
Received 8 March 2005/ Returned for modification 9 May 2005/ Accepted 18 May 2005
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BmpA is a member of the paralogous Bmp protein family, encoded by the tandemly located bmp genes on the linear B. burgdorferi chromosome (9). The bmp genes are conserved in the DNA sequence and genetic structure in all B. burgdorferi sensu lato strains (10) and are constitutively expressed in vitro (13). Expression of bmpD is modulated during coculture with tick cells (3); expression of bmpA (but not the other bmp genes) is modulated during infection in mice (12). The Bmp proteins have putative lipidation sites at their N termini, 37 to 52% amino acid identities to each other, and similar predicted molecular weights and immunogenicities (9). Their functions are unknown.
It is also not known if Bmp protein expression is modulated during human infections. Unfortunately, the need for at least 1 µg of borrelial RNA for current microarray analyses makes application of this technology difficult for diseases such as human Lyme disease where the number of organisms in specimens is low. The similar predicted molecular weights of the individual Bmp proteins would make it difficult to distinguish between them by one-dimensional immunoblotting of B. burgdorferi whole-cell lysates. The presumptively low levels of BmpB, BmpC, and BmpD in B. burgdorferi whole-cell lysates (8) also hinder the collection of purified native materials for analysis of the antibody responses to and the specificities for individual Bmp proteins. In order to provide evidence for the possible expression of multiple Bmp proteins by B. burgdorferi during human infections, we have used recombinant Bmp (rBmp) proteins, immunoglobulin G (IgG) immunoblotting and immunodotting, and a competitive IgG enzyme-linked immunosorbent assay (ELISA) to determine whether patients with Lyme disease produce antibodies specific for individual members of the Bmp protein family.
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Cloning of bmp genes. Complete bmp genes (9) were amplified by PCR from B. burgdorferi B31 (ATCC 35210) DNA by using the forward and reverse primers listed in Table 1. The amplified fragments were cloned into pQE40 (QIAGEN, Valencia, Calif.) by using SphI-SmaI restriction sites (bmpA) or into pET30 Xa/LIC (Novagen, Madison, Wi.) (bmpB, bmpC, bmpD) by using ligation-independent cloning, according to the manufacturer's instructions. Escherichia coli M15 (QIAGEN) was used for expression of rBmpA fused with dihyrofolate reductase and the six-His tag; E. coli BL21-CodonPlus-RIL (Strategene, Austin, Tex.) was used for rBmpB, rBmpC, and rBmpD fused with the six-His tag (16). Calcium chloride-competent E. coli M15(pREP4) cells were transformed with purified pQE40 (QIAGEN) bmpA, while calcium chloride-competent E. coli BL21-CodonPlus-RIL were transformed with each of the purified pET Xa/LIC derivatives. Colonies of E. coli transformants were screened for recombinant protein with mouse anti-five-His tag monoclonal antibodies (QIAGEN), according to the manufacturer's instructions, and colonies expressing high levels of recombinant protein were selected. To confirm the sequence of bmp genes in the transformed bacterial clones used to produce the rBmp proteins, plasmid DNA from each of three E. coli colonies expressing high levels of each rBmp protein was purified and sequenced. No variations in amino acid sequence from those in GenBank were obtained with the recombinant plasmids for bmpA, bmpB, or bmpC. In one of the three bmpD plasmids examined, the DNA sequence had a transversion from T to C that resulted in an amino acid substitution from Glu to Lys. Because it was present in only one of three subclones, this substitution would appear to represent a mutation unique to this particular subclone that was not present in the original transformed parental strain and thus be unlikely to affect the antigenicity of the expressed recombinant protein that was used in this study.
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TABLE 1. Primers and conditions used for production of rBmp proteins
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Immunoblotting and dot immunobinding. For IgG immunoblotting, purified rBmp proteins were electrophoresed on a 10 to 20% gradient SDS-polyacrylamide gel and transferred to nitrocellulose by semidry blotting, and replicate lanes were either silver stained or blocked and incubated with patient sera. Blots were developed with horseradish peroxidase-conjugated anti-human IgG antibodies (Jackson ImmunoResearch, West Grove, Pa.) and tetramethylbenzidine blotting (Pierce Chemical, Rockford, Ill.). For IgG immunodotting (dot immunobinding), recombinant protein (500 ng) was dissolved in SDS-PAGE loading buffer; adsorbed to nitrocellulose; and after it was blocked, incubated with patient sera. Immunodots were developed with alkaline phosphatase-conjugated anti-human IgG antibodies (Jackson ImmunoResearch) and ECF technology (Amersham Biosciences) (11). Each blot contained positive and negative control dots. The last serum dot (patient or control) that gave a visual signal greater than that of negative control dot was scored as positive.
Competitive indirect fluorescent IgG ELISA. Competitive indirect fluorescent ELISA was performed in duplicate by using 96-well plates containing adsorbed 100-ng aliquots of rBmp proteins or control protein fractions from E. coli containing the plasmid vector with or without the dihydrofolate reductase gene insertion (13). Diluted sera were mixed with serial dilutions of rBmp competitor, bovine serum albumin (BSA; Sigma), or E. coli lysate (final concentrations of competitors or controls, 1 to 64 µg/ml) (7). The mixtures were incubated at 23°C for 2 h and were then added to wells coated with one of the rBmp proteins for a further 4 h at 23°C. After the wells were washed, the wells were developed with alkaline phosphatase-conjugated goat anti-human IgG and 4-methylumbelliferyl phosphate (ICN Biomedicals, Irvine, Calif.), and the fluorescence intensity was measured (Fluorescence Concentration Analyzer; IDEXX, Westbrook, Maine). The competition in wells containing BSA and E. coli lysate never exceeded 10%; the competition seen with BSA was subtracted from the fluorescence intensity obtained with rBmp proteins. All sera showed 100% inhibition of ELISA reactivity after incubation with an excess of the homologous rBmp protein. Values from individual assays were normalized by comparison to the antigen control wells (7). The results are reported as percent inhibition of binding by heterologous competitors at the 50% inhibitory dose (IC50) for binding to the adsorbed homologous rBmp protein. Competitive inhibition of IgG anti-rBmp ELISA activity was analyzed statistically by analysis of variance with a Tukey-Kramer multiple comparison posttest; the level of significance was set at a P value of <0.01. There were two situations in which a response was defined as cross-reactive, depending on whether the inhibition of reactivity of serum by a heterologous Bmp ligand used at the IC50 of the homologous Bmp ligand was >50% or <50%. A response was considered cross-reactive if inhibition of reactivity by heterologous ligand was >50% (heteroclitic cross-reactivity) (7). A response was also considered cross-reactive if inhibition of reactivity by heterologous ligand was <50%, but the inhibition was not significant (P < 0.01) (7). The competitive ELISA was done in duplicate on two independent occasions. The results from these separate experiments were reproducible and were combined.
Statistical analysis. The concordance of the results between immunoblotting and dot immunobinding assays was analyzed by McNemar tests; the level of significance was set at a P value of <0.05. The differences in the reactivities of patient and control sera with rBmp proteins were analyzed by Fisher's exact test; the level of significance was set at a P value <0.01. Confidence limit intervals were computed by the modified Wald method (1).
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FIG. 1. Purified B. burgdorferi rBmpA (lane A), rBmpB (lane B), rBmpC (lane C), and rBmpD (lane D) proteins (1.5 µg) were separated by 10 to 20% SDS-PAGE, electrolytically transferred to nitrocellulose, and (A) silver stained or analyzed for IgG antibodies to Bmp proteins by immunoblotting and competitive ELISA with sera from (B) patient 23 (early Lyme disease) or (C) patient 32 (IgG clinical laboratory immunoblot positive). For the competitive IgG ELISA, the sera were first incubated with the indicated soluble rBmp protein shown by the bars (rBmpA, striped bars; rBmpB, white bars; rBmpC, gray bars; rBmpD, black bars), and their ELISA reactivities with the adsorbed rBmp protein (horizontal axis) were then determined. The results are shown as percent inhibition of ELISA reactivity at 50% inhibition (dashed line) with the soluble homologous rBmp protein ( ). *, reactivity significantly less than reactivity at 50% inhibition with the homologous rBmp protein (P < 0.01, analysis of variance with Tukey-Kramer multiple comparison posttest). The molecular weight of rBmpA is much higher than those of the other rBmp proteins because of fusion of the BmpA sequence to dihydrofolate reductase and the six-His tag. The other rBmp proteins contained only the six-His tag fused to their respective Bmp sequences, and their molecular weights are therefore closer to those of the native proteins. See Materials and Methods for details of the assays.
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TABLE 2. Immunodot reactivities to rBmp proteins of 39 serum samples from 15 patients with early Lyme disease, 4 PGLI serum samples, and 24 control serum samples
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TABLE 3. Antibody fine specificity by competitive ELISA to rBmp proteins in 18 serum samples from 10 patients with early Lyme disease and 3 PGLI serum samples
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The reasons for the differences in patient reactivity to the various Bmp proteins are not clear. They could reflect differences in the levels of expression of the Bmp proteins in the human host analogous to the differences in their levels of expression in vitro (3, 8). They could also reflect differences in exposure of the individual Bmp proteins on B. burgdorferi. In this context, the lack of detection of specific antibodies against BmpC might be the result of the low levels of expression of this protein and its lack of exposure on the outer surface of B. burgdorferi (16; J. J. Shin, A. V. Bryksin, H. P. Godfrey, and F. C. Cabello, unpublished data). The small numbers of patients and patient sera examined and the genetic heterogeneity of the human population could also be expected to contribute to the apparent variability of the antibody response to the Bmp proteins.
Approximately 70% of rBmpA, rBmpB, and rBmpD and 50% of rBmpC were solubilized after purification and refolding. The possibility exists that refolding of modified rBmp fusion proteins might affect their three-dimensional structures and therefore the accessibility of Bmp epitopes in ELISA. This in turn might affect the observed reactivity of patient sera to the individual members of this paralogous protein family in the present study. However, it seems more likely that binding of the protein to polystyrene would be a more significant factor in determining epitope availability than the modifications introduced by the fusion partner. Adsorption of proteins to polystyrene is associated with the loss or alteration of antigenic epitopes, the generation of new epitopes, and the loss of antibody affinity compared to those of the same molecules in solution for 90 to 94% of molecules in a sample. However, 6 to 10% of absorbed molecules do not undergo these changes during the process of absorption (4, 5), and it is this small number of unchanged molecules that permits the use of soluble proteins for the analysis of antibody specificity by competitive solid-phase ELISA (4). The complete inhibition of ELISA reactivity following the incubation of patient sera with an excess of the homologous rBmp protein suggests that these sera contained no antibodies able to react with "novel" epitopes caused by binding to polystyrene.
Detection of antibodies specific for BmpA, BmpB, or BmpD suggests that these proteins are expressed in patients infected with B. burgdorferi. It thus complements the results of other studies that have demonstrated bmp gene expression in infected mice (12). Experimental analysis of the modulation of expression of many B. burgdorferi lipoproteins has been done in vivo by the use of genomic methods (14). The relatively simple techniques used in the present study to analyze patient samples are equally valid. They have the further advantage of being directly applicable to conditions such as Lyme disease, where the numbers of bacteria in the tissues are low. They could therefore be used to study the expression of other paralogous protein families of B. burgdorferi in human patients. These studies would clearly be relevant for serologic diagnosis, understanding of Lyme disease pathogenesis, and the design of new vaccines.
This work was supported by NIH grant R01 AI43063 (to F.C.C.).
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