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Infection and Immunity, October 1999, p. 5470-5472, Vol. 67, No. 10
Department of Pathology,
Received 31 March 1999/Returned for modification 11 May
1999/Accepted 21 July 1999
A murine monoclonal antibody directed against Borrelia
burgdorferi B31 outer surface protein C (OspC) antigen was
generated by a method whereby borreliae were inoculated into the mouse
via the natural transmission mode of tick feeding. Passive immunization with this antibody resulted in protection of C3H/HeJ and outbred mice
from a tick-transmitted challenge infection. Immunofluorescence staining of borrelia cells indicated surface exposure of the OspC epitope reactive with the monoclonal antibody.
The outer surface protein C (OspC)
of Borrelia burgdorferi, the causative agent of Lyme
disease, elicits a protective immune response against infection in
animals which have been actively immunized with the recombinant antigen
(4, 9, 10). Anti-OspC activity in polyclonal serum samples
from B. burgdorferi-infected, immune hosts has been
implicated in protection (6, 7). Also, a polyclonal,
monospecific anti-recombinant OspC has shown therapeutic properties
when passively administered to chronically infected subjects
(14). To our knowledge, however, no monoclonal antibodies (MAbs) against OspC have been documented to demonstrate protection upon
passive transfer. This laboratory generated a panel of MAbs derived
from a tick-borne B. burgdorferi antigen introduction method, rather than using an inoculum of borrelial antigens obtained from in vitro-cultivated organisms (5). One of these MAbs
was directed against OspC. The present study was performed to determine if this anti-OspC MAb would passively protect experimental mice.
The generation of the MAb panel has been described elsewhere
(5). Briefly, B. burgdorferi-infected nymphal
ticks were allowed to feed on 8- to 12-week-old female BALB/cByJ mice.
Infection of the mice with B. burgdorferi was confirmed by
positive cultures derived from ear biopsy specimens (12).
The mice were reinfested with B. burgdorferi-infected ticks
1 month later. At the end of this immunization schedule, serum samples
were analyzed by enzyme-linked immunosorbent assay (ELISA) against
B. burgdorferi whole-cell lysate, and the mouse with the
highest titer was selected for hybridoma production. Three days before
the cell fusion procedure, 105 strain B31
low-passage-number organisms (passage 1, cultured from ticks) were
injected intravenously. This boost served to enhance existing, primed
B-cell polyclonal populations prior to spleen harvesting, but only
those populations common to antigens expressed in both cultured and
tick-transmitted B. burgdorferi. Spleen cells were harvested
and fused with cP3x63-Ag8.653 myeloma cells by use of polyethylene
glycol 1000. The spleen cell/myeloma cell ratio was approximately 5:1.
Fused cells were selected by using medium containing
hypoxanthine-aminopterin-thymidine. Wells were screened by ELISA and
Western blotting with low-passage-number B. burgdorferi B31
as the antigen. Cells from positive wells were expanded and cloned by
limited dilution. The OspC specificity of one of the MAbs was
determined by its immunoblot reactivity against recombinant OspC, and
it was designated B5 (Fig. 1). MAb B5 was
isotyped as an immunoglobulin G2a (IgG2a).
Anti-OspC IgG was purified from ascitic fluid by ammonium sulfate
precipitation. Groups of test mice were injected intravenously with 100 µl containing either 200 to 300 µg of the anti-OspC antibody or the
same amount of normal mouse IgG 1 day prior to tick infestation. Inbred
mice (C3H/HeJ) and outbred mice (specific-pathogen-free mice maintained
at the Division of Vector-Borne Infectious Diseases) were used in this
study. One day following passive transfer of the antibody, each mouse
was infested with 10 B. burgdorferi-infected nymphal
Ixodes scapularis ticks, which were allowed to feed to repletion. The B31 strain-infected tick colony has been described previously (8). To assay for infectivity, ear skin biopsy
specimens were cultured 4 weeks post-tick feeding as described
previously (12); also, serological bleedings were taken
between 2 and 4 weeks after tick drop-off and samples were assayed by
Western blotting. One positive-control mouse was passively immunized
with a polyclonal anti-OspA antibody, which was known to be protective (2, 3).
All mice passively immunized with the anti-OspC MAb were protected from
infection (11 of 11), whereas each mouse inoculated with control
antibody was not protected (Table 1).
Following feeding, replete ticks were randomly collected from the
OspC-immunized inbred mice and were surface sterilized (by serial
washes in 70% ethanol for 2 min, 3% hydrogen peroxide for 2 min, and
sterile water for 2 min), crushed, and inoculated into
Barbour-Stoenner-Kelley modified culture medium (BSK II medium). Seven
of nine ticks yielded viable borreliae in culture, reflecting the 70 to
80% infection rate of the tick colony. This result ensured that the
ticks placed upon the mice were indeed harboring B. burgdorferi. It also corroborated a result previously seen in this
laboratory, that a population of borreliae survives within the tick
following feeding on OspC-immunized mice (4), unlike
borreliae in ticks that feed on OspA-immunized mice (3).
0019-9567/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.
An OspC-Specific Monoclonal Antibody Passively
Protects Mice from Tick-Transmitted Infection by Borrelia
burgdorferi B31
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FIG. 1.
Western blot demonstrating MAb B5 reactivity against
OspC antigens. Lanes: 1, B. burgdorferi B31 lysate; 2, lysate from Escherichia coli harboring the plasmid
expression vector pBluescript (Stratagene, La Jolla, Calif.) only; 3, E. coli lysate expressing OspC from pBluescript. Molecular
mass markers, in kilodaltons, are indicated on the left. The
recombinant OspC migrates higher than the B. burgdorferi
OspC due to the addition of a vector-encoded fusion partner. MAb B5 was
used at a dilution of 1:5,000.
TABLE 1.
Results of infectivity assay 4 weeks postchallenge
To assess the cell surface accessibility of the antibody, indirect immunofluorescence was performed on cultured B31 organisms as follows. Five microliters of a stationary-phase borrelial culture grown in BSK II medium was allowed to air dry on a microscope slide. The cells were heat fixed by a slight warming of the slide but were not acetone or methanol fixed. The organisms were overlaid with a 1:100 dilution of the anti-OspC B5 antibody and incubated for at least 1 h at room temperature. Following the primary incubation, the slide was washed three times in phosphate-buffered saline (PBS) and allowed to air dry, followed by a secondary incubation with a 1:50 dilution of fluorescein isothiocyanate-conjugated goat anti-mouse IgG (Kirkegaard & Perry Laboratories, Gaithersburg, Md.) for 1 h at room temperature. The slide was washed three times with PBS, allowed to air dry, and overlaid with mounting medium (Kirkegaard & Perry Laboratories) and a coverslip. The finished slide was viewed under a dark-field microscope, and by epifluorescence with a fluorescein filter, at magnification of ×100 and ×400. The nonfixed borreliae fluoresced, indicating surface exposure of the OspC epitope reactive with this antibody (Fig. 2B). In addition, an indirect immunofluorescence assay using a MAb directed against flagellin, a non-surface-exposed antigen, gave a negative signal when nonfixed B. burgdorferi were used (data not shown). Normal mouse serum failed to cause the borrelia cells to fluoresce (Fig. 2D). Surface exposure of OspC epitopes in some B. burgdorferi strains has been speculated to correlate with protective capability (1), and the surface accessibility of the OspC epitope reactive with MAb B5 shown in Fig. 2B is consistent with that observation.
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This study has demonstrated the protective efficacy of a MAb directed against the B. burgdorferi B31 OspC antigen. Because the protective properties of active immunization with OspC, and the fact that OspC expression is upregulated on the borrelial surface during tick feeding (11), have been well documented it is logical that a MAb generated by antigen inoculation via the natural route of tick bites would mirror the protective ability of the immunogen. Unexplained differences in the therapeutic effects of passively transferred and actively induced anti-OspC antibodies have been observed (13, 14). This MAb could be used to examine whether recognition of different epitopes is required for protection versus therapeutic clearance of infection.
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ACKNOWLEDGMENTS |
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We gratefully acknowledge the contributions of Rendi Murphree, Sarah Sullivan, and Steve Sviat. We express our thanks to Marc Dolan and Joe Piesman for providing ticks.
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FOOTNOTES |
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* Corresponding author. Mailing address: Department of Pathology, College of Veterinary Medicine and Biomedical Sciences, Colorado State University, Fort Collins, CO 80523-1671. Phone: (970) 491-1607. Fax: (970) 491-0603. E-mail: lmbow{at}lamar.colostate.edu.
Editor: R. N. Moore
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