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Infection and Immunity, January 2001, p. 593-598, Vol. 69, No. 1
Department of Microbiology, Immunology and
Molecular Genetics1 and Division of
Infectious Diseases, Department of Medicine,2
University of California School of Medicine, Los Angeles, California
90095
Received 10 August 2000/Returned for modification 20 September
2000/Accepted 20 October 2000
We have recently found that strain B31 infection-immune rabbits are
completely protected against homologous challenge with large numbers
(>106) of host-adapted Borrelia burgdorferi
(HAB) (E. S. Shang, C. I. Champion, X. Wu, J. T. Skare,
D. B. Blanco, J. N. Miller, and M. A. Lovett, Infect.
Immun. 68:4189-4199, 2000). In this study, we have extended these
findings to determine whether B31 strain infection-immune rabbits are
also protected against heterologous HAB challenge. Infection-immune
rabbits challenged with large numbers (>106) of homologous
HAB strain B31 were completely protected from erythema migrans (EM) and
skin and disseminated infection. In contrast, infection-immune rabbits
challenged with heterologous HAB strains N40 and Sh-2-82 were
completely susceptible to EM and skin and disseminated infection;
challenge with strain 297 also resulted in EM and infection of the skin
and viscera, but clearance of infection occurred 3 weeks postchallenge.
These findings confirm that immunity elicited in rabbits by B31 strain
infection confers complete protection against large-dose homologous HAB challenge but not against a heterologous strain.
Lyme disease in humans, caused by
tick transmission of the spirochete Borrelia burgdorferi, is
known to result from different strains of B. burgdorferi
sensu stricto and sensu lato found in different geographic regions of
the world. While outer surface protein A (OspA) of B. burgdorferi, a surface protein expressed during tick adaptation,
is currently being used for human vaccination (39, 43), it
has been shown to possess antigenic heterogeneity among many strains
(26, 29, 35, 48). Moreover, it is now recognized that OspA
is downregulated during mammalian infection and that its mechanism of
protective immunity works by inhibiting B. burgdorferi
growth in the tick following a blood meal from a vaccinated individual
(15, 37). Furthermore, organisms that have become host
adapted and no longer express OspA are not susceptible to killing by
OspA-specific antibodies (7). Thus, the search for
additional cross-protective immunogens has attracted considerable interest for future adjuncts to the OspA vaccine.
Several experimental animal models for the study of Lyme disease
pathogenesis and immunity have been utilized, including the rat
(8), hamster (25), mouse (5),
rabbit (18, 47), guinea pig (41), dog
(10, 21), and monkey (33). With the exception
of rabbits, mammalian infection with B. burgdorferi results
in chronic infection in these animals (3, 6, 16, 31, 34).
In the mouse model, Barthold has demonstrated that chronic infection
elicits complete protection against homologous challenge and partial
protection against heterologous challenge using cultivated organisms
(4). However, protection studies in mice challenged with
organisms acquired from infected tissues which no longer express OspA,
called host adapted by Barthold and which we now refer to as
host-adapted Borrelia (HAB), show only partial protection
against homologous challenge and little to no protection against
heterologous challenge (4). Recently, Hansen and coworkers
have demonstrated that immunization of mice with decorin-binding
protein A (DbpA) from strain 297, a surface protein which has been
implicated in spirochetal adhesion and which is upregulated during
mammalian infection, protected against homologous challenge with in
vitro-cultivated B. burgdorferi but not against heterologous
challenge (23). Further, mice immunized with DbpA were
only partially protected against homologous HAB challenge
(11). Thus, in experimental mouse protection studies, only
limited partial protection has been achievable when using host-adapted
organisms for homologous and heterologous challenge.
We have previously demonstrated that the rabbit model of Lyme disease
has unique features relevant to the immunobiology of B. burgdorferi, including the highly reproducible development of
erythema migrans (EM) (18). Of particular significance,
infected rabbits clear the infection after several months, resulting in complete protection against challenge reinfection using large numbers
(>107) of cultivated organisms (18). Passive
immunization of rabbits with infection-immune rabbit serum results in
complete protection against homologous challenge with cultivated
organisms (13), suggesting that antibody is the major
factor in this protection. We have also previously reported that
infection-immune rabbits were resistent to challenge using numbers of
organisms that were manyfold greater than that which could be
demonstrated following immunization with either OspA (19,
38) or outer membrane vesicles derived from cultivated, virulent
B. burgdorferi (38). We have also recently
found that B31 infection-immune rabbits are completely protected
against homologous HAB challenge using greater than 106
organisms (38).
In this study, we have tested whether the high degree of homologous
protective immunity that occurs following infection in rabbits also
extends to heterologous challenge using HAB. HAB organisms were also
used for challenge in order to assess protective immunity against
Borrelia antigens other than OspA, which is present in both
tick-infecting and cultivated organisms. The results show that B31
infection-immune rabbits are completely protected against homologous
challenge with HAB but not heterologous challenge with HAB strains N40
and Sh-2-82. While B31 infection-immune rabbits challenged with HAB
strain 297 also developed EM and skin infection, these animals
completely cleared the infection 3 weeks postchallenge, as assessed by
the absence of organisms in skin and internal tissues cultured in BSKII
medium. These findings indicate that the high degree of complete
protective immunity against high-dose homologous HAB challenge does not
extend to heterologous HAB strain challenge.
Homologous and heterologous challenge of B. burgdorferi
B31 infection-immune rabbits with HAB.
To generate B. burgdorferi B31 infection-immune rabbits, 20 New Zealand White
rabbits were inoculated intradermally at eight sites with
107 in vitro-cultivated B. burgdorferi strain
B31 passage 1 for a total of 8 × 107 organisms per
rabbit, as previously described (38). All rabbits developed typical EM lesions and were shown to have culture-positive skin infection in BSKII medium at 7 days postinoculation and clearance of skin infection 4 months later, as previously reported
(18). Infection-immune rabbits were challenged 5 months
after the initial inoculation as described below.
0019-9567/01/$04.00+0 DOI: 10.1128/IAI.69.1.593-598.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.
Homologous and Heterologous Borrelia
burgdorferi Challenge of Infection-Derived Immune Rabbits Using
Host-Adapted Organisms
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ABSTRACT
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QPCR of donor skin implants.
In order to demonstrate that
differences in protection between the strains were not due to variable
numbers of HAB in the skin punches used for implant, quantitative PCR
(QPCR) was performed as previously described on the donor skin punches
using B. burgdorferi gyrase B (gyrB) as the
target DNA and a nonhomologous internal standard (PCR MIMIC) as the
competitor (38). To first determine if the gene copy
number of gyrB was the same in B. burgdorferi strains B31, N40, 297, and Sh-282, QPCR was performed on DNA extracted from 108 in vitro-cultivated B. burgdorferi of
each strain enumerated by dark-field microscopy. As shown in Table
2, each strain of 108
organisms was found to have 1.25 × 109 to 2.5 × 109 copies of gyrB. The discrepancy between the
number of organisms and the gyrB copy number determined by
QPCR is likely due to multiple copies of the chromosome
(38). Although a recent study using PCR analysis
of cultured B. burgdorferi showed that the chromosomal gene
recA was consistent with a single recA per
bacterium (32), our findings are more consistent with
those for Borrelia hermsii, in that organisms isolated from
mice have 13 to 18 chromosome copies per cell (27). Thus,
the comparison of the number of organisms determined by QPCR to that
enumerated by dark-field microscopy (Table 2) indicates that in
vitro-cultivated B. burgdorferi has 12 to 25 chromosome
copies per cell. Our findings also demonstrate that the four B. burgdorferi strains have relatively comparable copy numbers of
gyrB, validating the use of this gene to determine relative
copy numbers of organisms in each strain present in infected tissue.
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Antigenic comparison of cultivated B. burgdorferi strains by two-dimensional Western blot analysis of Triton X-100-solubilized outer membrane proteins. In order to determine if antigenic differences in outer membrane proteins between the different strains could account for the lack of HAB heterologous cross-protection, two-dimensional Western blot analysis using B31 strain-immune rabbit serum was performed. In vitro-cultivated B. burgdorferi strains B31, N40, 297, and SH-2-82 (4 × 108 organisms) were incubated in 2% Triton X-100 for 2 h at 4°C, which has been shown previously to solubilize the spirochetal outer membrane and enrich for outer membrane-associated proteins (14). Following detergent treatment, samples were centrifuged at 20,000 × g for 15 min to remove protoplasmic cylinders, and the outer membrane-enriched soluble supernatants were precipitated with 10% trichloroacetic acid, washed with acetone, and suspended in isoelectric focusing sample buffer as previously described (40). The solubilized proteins were separated by two-dimensional nonequilibrium pH gel electrophoresis and transferred to a polyvinylidene difluoride (Immobilon P) membrane as previously described (40). The blots from each strain were probed with pooled prechallenge immune rabbit serum from the 20 B31 infection-immune rabbits at a dilution of 1:1,000, and antibody-antigen interactions were identified with the enhanced chemiluminescence system (Amersham Corp., Arlington Heights, Ill.) as previously described (40).
Analysis of the amido black-stained protein profile of each blot showed that all four strains were almost identical with the exception of strain N40, which did not possess the major 32-kDa OspB protein migrating just above OspA, and strain 297, which possessed a prominent 67-kDa protein which is likely albumin contamination (Fig. 1A). Western blot analysis demonstrated that B31 strain-immune rabbit serum reacted strongly with many similar migrating proteins among the different strains, although some differences were apparent which may be relevant to the lack of heterologous cross-protection (Fig. 1B). In addition, a strong anti-OspA reaction was present in Western blots of all four strains (Fig. 1B). Taken together, these findings suggest that many of the highly cross-reactive outer membrane antigens between these different strains cultivated in vitro, including OspA, do not provide cross-immunity when challenged with HAB. This finding for OspA is not surprising given the downregulation of this protein following mammalian host adaptation of B. burgdorferi.
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1,280 homologous HAB organisms resulted in protection against disseminated infection but only partial protection against skin infection
(4). In studies by Cassatt et al., mice immunized with
DbpA and challenged with
10 homologous B31 and N40 HAB resulted in
infection of 3 of 10 and 1 of 10 mice, respectively, indicating only
partial protection in both cases (11). Of particular
consideration is the small numbers of HAB organisms used for challenge
in these studies. The dose of
1,280 HAB organisms used for challenge
was determined by limiting-dilution QPCR and did not consider the potential for multiple chromosome copy numbers (7). Due to the inability to isolate sufficient numbers of host-adapted organisms, the HAB chromosome copy number is not known at this time. However, given a potential chromosome copy number of 25 per cell, based on the
findings in this study, it is conceivable that considerably fewer HAB
organisms were used for challenge of these mice. Thus, the small
numbers of HAB used for challenge in these mouse studies (7,
11) were 100- to 1,000-fold less than in vitro-cultivated organisms used for standard challenge inoculation (103 to
104 organisms), yet mice were only partially protected
against homologous HAB challenge. By comparison, homologous HAB
challenge using infected rabbit skin allows for markedly greater
numbers of organisms to be used for challenge (5.8 × 105 [Sh-2-82] to 2.7 × 106 [NP40]
organisms per rabbit; 1.5 × 105 to 6.8 × 105 organisms at each of four sites). Moreover,
infection-immune rabbits challenged with high-dose homologous HAB
challenge are completely protected. The number of HAB organisms used
for this challenge was at least 10 times greater than the standard
challenge of rabbits using 6 × 104 in
vitro-cultivated organisms (38). This was also at least 400 times more HAB than used for the challenge of mice. Thus, the
relative greater degree of protection in immune rabbits against HAB
compared to chronically infected and treated mice may reflect a greater
quantitative and/or qualitative response to Borrelia antigens expressed during rabbit infection or, alternatively, the
expression during rabbit infection of novel Borrelia
antigens which are potent protective immunogens.
We have speculated that proteins specific to or upregulated in HAB are
responsible for the high level of homologous protective immunity
observed in infection-immune rabbits (38). Several proteins have been previously identified in the mouse model as being
upregulated in vivo, including DbpA (11), OspC
(37), and OspE and OspF (44). Proteins
identified as uniquely expressed in vivo include EppA
(12), p35 and p37 (17), the OspE-F homologue p21 (45), the OspF homologues bbk2.1 (2) and
pG (46), and the proteins encoded on operon 2.9-71pB
(1). While the concept that antigens unique to HAB are
essential targets for protective immunity is appealing, many of the
molecules currently known to be upregulated or uniquely expressed
during mouse infection have not shown great potential as protective
immunogens. OspC has been found to be highly variable among strains of
B. burgdorferi and not protective against some homologous
strains (9, 24). Recent findings also suggest that the
protective OspC epitopes are conformation dependent (20).
Active and passive immunizations with combined antisera to p35 and p37
were protective following challenge with 100 but not with 10,000 cultivated organisms (17). Recently, it has been reported
that mice immunized with recombinant DbpA showed no protection
following challenge with infected ticks (22). Thus, it is
likely that the B. burgdorferi molecules that confer the
high degree of complete protection observed following infection are HAB
proteins that as yet have not been identified.
In this study we investigated whether this high degree of protection
against large numbers of HAB also extends to heterologous challenge.
Although B31 infection-immune rabbits were completely protected against
homologous HAB challenge, they were not protected against HAB challenge
using the heterologous strains N40, Sh-2-82, and 297. Interestingly,
B31 strain-immune animals did show rapid clearance of infection
following heterologous challenge with HAB strain 297. This difference
following challenge between strain 297 and the other strains was not
due to variations in the number of organisms used for challenge or to
major differences in antibody recognition of in vitro-expressed
membrane antigens. We have recently observed a similar rapid clearance
of infection from skin in rabbits immunized with purified outer
membrane derived from virulent B31 or B313 strains and challenged with
homologous in vitro-cultivated organisms (38). This rapid
clearance of infection may represent a degree of partial immunity and
is impressive in view of the large numbers of HAB used for the
heterologous challenge. However, this potential cross-protection
against heterologous HAB challenge using strain 297 must be considered
in the context of previous reports showing that cultivated organisms of
these two strains are susceptible to killing by antibody raised against
the other strain (29). Thus, whether the rapid clearance
of infection observed in this study following heterologous HAB
challenge is the result of common immunogens on host-adapted organisms,
on both host-adapted and cultivated organisms, or on solely cultivated organisms is not known at this time.
Of further interest is the comparison of the strains used for this
study. Strains B31, N40 (8), and Sh-2-82 (36)
are all Ixodes scapularis isolates from New York State,
while strain 297 is a human cerebrospinal fluid isolate from
Connecticut (42). Despite isolation from the same species
of tick and geographic location, no cross-protection was observed.
Strains B31 and N40 have been shown by Mathiesen and coworkers, using
genomic macrorestriction neighbor-joining analysis, to be closely
related based on their OspA and 23S rDNA gene sequences
(30). However, differences between strains B31 and N40
were noted by restriction fragment length polymorphism analysis
(30), which in other studies have been associated with
differences in human disease manifestations (49). It is
therefore conceivable that the lack of protection observed among these
strains translates to differences in upregulated HAB antigens that are
strain specific.
In summary, we have corroborated our recent findings showing that
infection-immune rabbits are completely protected against high-dose
homologous challenge using HAB. However, this high degree of
infection-derived protective immunity does not extend to other host-adapted strains used for heterologous challenge. This finding was
somewhat surprising given the high degree of protection against high-dose homologous challenge and suggests that strain-specific molecules are the basis for infection-derived immunity. In this regard,
we have recently isolated several novel HAB proteins from infected
rabbit tissue that we have found to be upregulated relative to the
amounts detected in cultivated organisms. We are currently investigating whether these proteins represent strain-specific or
common antigens. It is hoped that determining the molecular basis of
homologous infection-derived immunity will provide insights into the
potential for heterologous cross-protection.
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ACKNOWLEDGMENTS |
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This work was supported by National Institutes of Health (NIH) grant AI-37312 to J. N. Miller and NIH grant AI-29733 to M. A. Lovett.
We thank Maurice M. Exner and Cheryl Champion for helpful suggestions.
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FOOTNOTES |
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* Corresponding authors. Mailing address: Department of Microbiology and Immunology, UCLA School of Medicine, 10833 Le Conte Ave. CHS 43-239, Los Angeles, CA 90095. Phone: (310) 825-4188 or (310) 825-1979. Fax: (310) 267-2265. E-mail: eshang{at}mednet.ucla.edu or jmiller{at}microimmun.medsch.ucla.edu.
Editor: D. L. Burns
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