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Clinical and Diagnostic Laboratory Immunology, June 2005, p. 746-751, Vol. 12, No. 6
1071-412X/05/$08.00+0 doi:10.1128/CDLI.12.6.746-751.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.
Microbiology Research Laboratory,1 Section of Infectious Diseases, Gundersen Lutheran Medical Center, La Crosse, Wisconsin 54601,3 Wisconsin State Laboratory of Hygiene and Department of Medical Microbiology and Immunology, Madison, Wisconsin 537062
Received 8 February 2005/ Returned for modification 10 March 2005/ Accepted 30 March 2005
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Despite the propensity for antigenic variation, however, the complement-dependent borreliacidal (killing) antibody response detected in a human infection is remarkably predictable. Several B. burgdorferi proteins, including OspA, OspB, OspC, DbpA, Oms66, and the 39-kDa periplasmic protein, induce borreliacidal antibodies (7, 9, 10, 17, 19, 26, 28, 31), but the response in early human Lyme disease is specific primarily for OspC (5, 6, 16, 17, 25). In addition, borreliacidal antibodies specific for OspA and OspB may be produced, but the response is almost exclusively seen in patients with Lyme arthritis (7, 10, 26). Moreover, traditional laboratory isolates that express OspA and OspB in vitro can be used to detect OspA and OspB borreliacidal antibodies, because the antibodies can attach to the outer surface proteins (Osps). However, OspC borreliacidal antibodies have not been detected by isolates that also express OspA and OspB, likely because the concurrent expression of these Osps hinders the attachment of OspC antibodies. Detecting OspC borreliacidal antibodies is therefore dependent on isolates such as B. burgdorferi 50772, because the spirochetes lack the plasmid containing ospA and ospB (2), and the absence of the Osps enables borreliacidal OspC antibodies to bind (6, 25). A borreliacidal antibody test that uses each type of isolate reliably provides sensitive and specific serodiagnostic confirmation of Lyme disease (5, 6, 8).
We previously exploited the susceptibility of B. burgdorferi 50772 to OspC borreliacidal antibodies (25) to demonstrate that the human antibody response was specific for the 50 amino acids (aa) nearest the carboxy (C) terminus of OspC (16). In this study, we examined smaller fragments of the OspC C terminus and again used borreliacidal activity specific for B. burgdorferi 50772 to more precisely identify the location of the borreliacidal epitope. We also determined whether the response was induced after the infection of laboratory mice or hamsters. The results confirm that the OspC borreliacidal antibody response is highly conserved in human Lyme disease and provide compelling evidence that the epitope responsible will be an extremely valuable serodiagnostic antigen and may provide the basis for an effective Lyme disease vaccine. Additionally, the results demonstrate that the production of OspC borreliacidal antibodies is unique to human infection.
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Animals. Ten-week-old female C3H/Hej mice (Jackson Laboratories, Bar Harbor, Minn.) or 6-week-old LVG hamsters (Charles River Laboratories, Kingston, N.Y.) were housed three or four per cage at ambient temperature. Food and water were available ad libitum. Animal experiments were reviewed and approved by the Gundersen Lutheran Animal Care and Use Committee (Institutional Animal Care and Use Committees).
Sera. Serum samples that contained significant concentrations of immunoglobulin M (IgM) and/or IgG OspC antibodies were obtained from patients with early Lyme disease characterized by single or multiple erythema migrans lesions. In addition, immune sera specific for host-adapted (3) Lyme disease spirochetes were obtained from hamsters and mice. Groups of donor animals were infected by injecting 100-µl volumes of Barbour-Stoenner-Kelly (BSK) medium containing 106 B. burgdorferi 297 cells subcutaneously in the lower back. Five weeks postchallenge, 12 approximately 2- by 2-mm pieces of infected ear tissue were removed from each animal by punch biopsy and implanted subcutaneously into the lower back of a naïve recipient animal. After 15 weeks, the recipient animals were bled to exsanguination and killed by inhalation of CO2, and the bladder, kidney, spleen, heart, and ear tissues were cultured in BSK medium. Cultures were then examined weekly for 3 weeks by dark-field microscopy.
Cloning of OspC fragments. Recombinant OspC and OspC-Dra (50 C-terminal amino acids) were recovered from the previously described (16) E. coli JM109 containing either pX3-22 (OspC) or pX2-Dra (OspC-Dra). A recombinant protein containing the fragment with the final 16 OspC C-terminal amino acids (C16) was constructed using forward primer 5'-AGTTAAAGAACTTACAAGTCCTGTTGTGG-3', reverse primer 5'-GGTTAAGGTTTTTTTGGACTTTCTG-3', and pX3-22 (15) as templates. The amplified C16 fragment was precipitated using Pellet paint (Novagen, Madison, Wis.) and ligated (pXT16) into a Pinpoint Xa-1 T-vector (Promega, Madison, Wis.). A fragment containing the final seven C-terminal amino acids (C7) was similarly made by annealing primers 5'-GGCAGAAAGTCCAAAAAAACCTTAACCA-3' and 5'-GGTTAAGGTTTTTTTGGACTTTCTTTCTTCTGCCAA-3' and also ligated (pXT7) into a PinPoint Xa-1 T-vector (Promega). Both pX16 and pX7 were then transformed into E. coli JM109, and proper orientations were confirmed by DNA sequencing (BigDye ABI, Foster City, Calif.).
Recovery of OspC or OspC fragments. The recombinant OspC or OspC fragments were recovered by culturing the E. coli in a 100-ml volume of 2x tryptone-yeast extract (TY) broth containing 100 µg/ml of ampicillin (Sigma Chemical Co., St. Louis, Mo.) for 12 h at 37°C, diluting tenfold with additional 2x TY broth, and incubating for another hour. Isopropyl-ß-D-thiogalactosidase (IPTG) (Sigma) was then added to a final concentration of 0.1 µM, and the cultures were reincubated for 4 h at 37°C. The suspensions were pelleted by centrifugation at 10,000 x g for 15 min at 4°C; resuspended in purification buffer containing 50 mM Tris (pH 8.0), 50 mM NaCl, 2 mM EDTA, and 0.1% Triton X-100; and lysed with a sonicator (model W350; Branson Sonic Power, Danbury, Conn.). Sonicated E. coli cells were pelleted by centrifugation at 10,000 x g for 20 min, and the supernatants were passed over columns containing SoftLink resin (Promega) at a rate of 0.5 ml/min at 4°C. The fusion proteins contained a 16-kDa biotinylated purification tag on the amino terminus of the protein that binds to SoftLink resin. Columns containing protein were then washed with 5 column volumes of purification buffer, and the bound proteins were eluted with purification buffer that also contained 5 mM biotin (Sigma). The proteins were then examined by using sodium dodecyl sulfate-polyacrylamide gel electrophoresis and Western blotting with mouse monoclonal antibodies specific for OspC to confirm their purity.
Adsorption with OspC or OspC fragments. One-milliliter volumes of Tetralink tetrameric avidin resin (Promega) were washed and suspended in 40 ml of phosphate-buffered saline (PBS; pH 7.2) and loaded into separate 10- by 70-mm polypropylene columns. Amounts (0.5 mg) of OspC, OspC-Dra (18), C16, or C7 were solubilized in 1-ml volumes of PBS and passed over separate columns. Complete binding was confirmed by protein assay (Bio-Rad). One-milliliter volumes of sera diluted 1:10 in PBS were then passed over the column four times, and the amounts of antibodies removed were confirmed by using OspC-Dra, C16, or C7 enzyme-linked immunosorbent assays (ELISAs).
Indirect ELISA. An OspC ELISA and ELISAs containing the C16 or C7 OspC fragments were prepared as described previously (16). Serum from a person not exposed to B. burgdorferi was used as a normal control.
Detection of borreliacidal antibodies.
Borreliacidal antibodies were detected by a flow cytometric procedure (6, 8). Viable B. burgdorferi 297 or 50772 organisms in logarithmic growth phase were enumerated with a Petroff-Hausser counting chamber and diluted with fresh BSK medium to a concentration of approximately 5 x 105 organisms/ml. Concomitantly, serum samples were diluted 1:20 with BSK and filter sterilized by passage through a 0.2-µm microfuge filter (Costar, Cambridge, Mass.). The filtered serum samples were then transferred to sterile 1.5-ml screw-cap microfuge tubes (Sarstedt, Newton, N.C.) and diluted serially (1:40 to 1:20,480) with BSK. Serum samples were heat inactivated at 56°C for 10 min, and a 100-µl aliquot of the spirochetes and 10 µl of sterile guinea pig serum (50% hemolytic component
200 units/ml) were added. The assay mixtures were mixed thoroughly and incubated for 16 to 24 h at 35°C.
Following incubation, 100 µl of each assay suspension was transferred to a 12- by 75-mm polystyrene tube (Becton-Dickinson, Franklin Lakes, N.J.) containing 400 µl of PBS and 1 µg of acridine orange (Sigma) per ml. A FACScan flow cytometer (Becton Dickinson Immunocytometry Systems, San Jose, CA) was then used to detect borreliacidal activity. Spirochetes were isolated by gating (CellQuest software; Becton Dickinson) and analyzed for 1 to 2 min with the flow rate set at low. Borreliacidal antibodies kill the spirochete by inducing a complement cascade that disrupts the outer membrane and causes the membrane to bleb. Borreliacidal antibodies were detected indirectly by monitoring the increased fluorescence intensity that occurs when the acridine orange intercalates into blebbed, nonviable spirochetes. A
13% shift in the mean fluorescence intensity compared to that of a normal serum control was considered positive (5, 6). The presence of blebbed, nonmotile B. burgdorferi was then confirmed by dark-field microscopy. The undiluted and adsorbed sera were assayed concurrently to eliminate the variability of titers caused by interassay variation in the concentrations of spirochetes.
Western blotting. Western blotting was performed using standard procedures. Briefly, 225 µg of B. burgdorferi 50772 protein was loaded into the preparative wells of 0.1% sodium dodecyl sulfate-12% polyacrylamide gels, and the proteins were separated by running in an electrophoresis unit (PROTEAN IIxi; Bio-Rad Laboratories, Hercules, Calif.) at 24 mA for 4 h. The proteins were transferred from the gels to polyvinylidene difluoride membrane (Perkin-Elmer Life Sciences Inc., Boston, Mass.) by electrophoresing overnight at 10 V. The polyvinylidene difluoride was cut into strips and blocked with 1% bovine serum albumin in PBS (pH 7.2)-0.1% Tween 20 for 1 h at 22°C. Strips were incubated for 1 h at 22°C with serum diluted to a ratio of 1:100 and washed four times with PBS-0.1% Tween 20. Horseradish peroxidase-labeled anti-human IgM or IgG heavy and light chains (Kirkegaard and Perry Laboratories, Gaithersburg, Md.) were added, and the strips were incubated for 1 h at 22°C. Strips were washed and developed using the TMB membrane peroxidase substrate system (Kirkegaard & Perry).
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4-fold reduction in titer) or by Western blotting (Fig. 1). In contrast, the reactivity on the Western blots for the other B. burgdorferi proteins appeared unaffected. |
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TABLE 1. Removal of OspC antibodies from early Lyme disease sera by adsorption with OspC-Dra, C16, or C7
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FIG. 1. Representative Western blots of antibodies in an early Lyme disease serum reactive against B. burgdorferi 50772 before adsorptions (lanes 1) and after adsorptions with OspC (lanes 2), C16 (lanes 3), or C7 (lanes 4). Panels A and B are IgM and IgG Western blots, respectively. Note that the effects of the adsorptions on the detection of OspC antibodies in the other four sera were identical.
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4-fold reduction) of OspC antibodies. |
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TABLE 2. Removal of borreliacidal antibodies from Lyme disease sera after adsorption with OspC-Dra, C16, and C7
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TABLE 3. Effect of removal of OspC antibodies on borreliacidal activity in mouse or hamster immune serum samples
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TABLE 4. Recovery of Borrelia burgdorferi from mice and hamsters challenged with spirochete-infected ear tissues
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FIG. 2. Western blots of pooled (n = 5) hamster immune serum before (lane B) and after (lane C) adsorptions with OspC or pooled (n = 5) mouse immune serum before (lane D) and after (lane E) adsorptions with OspC. Lane A shows reactivity by polyclonal antibodies specific for OspC.
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This finding is remarkable. As an infected tick begins feeding on a human, OspC is expressed (27) and becomes the major outer membrane protein expressed in early infection, regardless of the Borrelia genospecies infecting the patient (11, 22, 24, 35). Correspondingly, OspC antibodies quickly become the dominant humoral immune response (22, 35); however, OspC ELISAs have lacked accuracy for confirming Lyme disease (12, 20, 24, 30, 33). The shortcomings were explained most often by the heterogeneity of ospC (15, 18, 29, 30), even among isolates within the same genospecies (32). The antibody responses induced by the highly variable regions would have widely variable specificities. Additionally, antibodies induced by other infections may also react to these regions. However, previous studies that characterized the ability of OspC antibodies to kill the spirochetes independent of scavenging by phagocytic cells have provided compelling evidence that the borreliacidal antibodies are specific for a conserved region of the protein. For example, a flow cytometric test that detects borreliacidal OspC antibodies by monitoring the ability of the antibodies to kill only B. burgdorferi sensu stricto 50772 detected the response in >70% of early Lyme disease sera collected from throughout the United States (5).
In this study, we extend these findings by demonstrating that the dominant OspC borreliacidal antibody epitope is located within the surface-exposed (22) seven amino acid residues of the C-terminal end of the protein. More importantly, a BLAST search (1) confirmed the region is highly conserved among infectious B. burgdorferi sensu stricto, B. garinii, and B. afzelii isolates. The results therefore provide explanation for the increased accuracy (5, 6) of the flow cytometric borreliacidal antibody test. Moreover, the findings provide strong evidence that an ELISA comprised of the epitope would correlate directly with the highly specific borreliacidal antibody response. This should provide a sensitive and highly specific serodiagnostic confirmation of Lyme disease without the technical complexity and live organisms necessary to detect borreliacidal activity.
Mathiesen et al. (22) also reported that the OspC antibodies in sera from patients with neuroborreliosis caused by infection with B. garinii were specific predominantly for the identical sequence of 10 C-terminal amino acid residues of the protein. However, an ELISA with a synthetic peptide (21), while more reactive than the rOspC ELISA, detected OspC antibodies in European sera significantly less often (36%) than in sera from U.S. patients (11, 16, 20, 33). The authors noted the distinct absence of IgG OspC antibodies in sera from European patients (34) and suggested the failure of the response to reach maturity was because the epitope induced antibodies in a T-cell-independent fashion (22). However, this is clearly not the case after infection with B. burgdorferi sensu stricto. In this study, we detected high titers of borreliacidal IgG OspC antibodies, and numerous other investigators (5, 7, 11, 20, 25) have also confirmed that IgG OspC antibodies are commonly present in all stages of the illness. It seems unlikely the identical epitope would induce both a T-cell-independent and a T-cell-dependent response.
Rather, we suspect the failure of the OspC antibodies to switch classes in the European Lyme disease patients is more likely due to the predilection of the spirochetes to colonize the central nervous system, which may inhibit the maturation of the antibody response. In support, European patients are often infected with B. garinii or B. afzelii, which commonly cause neuroborreliosis (23). These genospecies are not found in the United States, where the patients are instead infected with B. burgdorferi sensu stricto, which most often colonizes the skin or joints. Additional studies to evaluate this hypothesis remain necessary, but the collective results still highlight the immunodominance of the OspC antibody response specific for the C terminus of the protein and provide support for the development of serodiagnostic procedures to exploit this finding.
In addition, the results provide compelling evidence that the C terminus of OspC is a good human vaccine candidate. However, we failed to detect borreliacidal OspC antibodies in sera from the laboratory mice or hamsters, despite vigorous antibody responses against many other B. burgdorferi proteins. The sera instead contained only small concentrations of OspC antibodies detected by ELISA or Western blotting. More significantly, the OspC antibodies were not borreliacidal antibodies. The findings therefore confirmed a significant difference in the antigenic reactivities of B. burgdorferi sensu stricto spirochetes that infect human or rodent hosts. More importantly, the results cast considerable doubt on the ability of rodent animal models to provide experimental data that accurately predict the ability of vaccination with OspC to provide protection against human infection.
In conclusion, the immunodominant IgM and IgG OspC antibody response in human Lyme disease is specific for the highly conserved seven C-terminal amino acid residues. This finding provides valuable information for future efforts to develop effective serodiagnostic tests and vaccines, but an accurate assessment of their effectiveness cannot be obtained by using laboratory mice or hamsters.
Financial support was provided by the Gundersen Lutheran Medical Foundation, La Crosse, Wisconsin.
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