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Clinical and Vaccine Immunology, May 2007, p. 628-634, Vol. 14, No. 5
1071-412X/07/$08.00+0 doi:10.1128/CVI.00409-06
Copyright © 2007, American Society for Microbiology. All Rights Reserved.
OspC Phylogenetic Analyses Support the Feasibility of a Broadly Protective Polyvalent Chimeric Lyme Disease Vaccine
Christopher G. Earnhart1 and
Richard T. Marconi1,2*
Department of Microbiology and Immunology,1
Center for the Study of Biological Complexity, Medical College of Virginia at Virginia Commonwealth University, Richmond, Virginia 23298-06782
Received 1 November 2006/
Returned for modification 18 January 2007/
Accepted 5 March 2007

ABSTRACT
Using available
Borrelia outer surface protein C (OspC) sequences,
a phylogenetic analysis was undertaken to delineate the number
of antigenic domains required for inclusion in a broadly protective,
chimeric, OspC-based Lyme disease vaccine. The data indicate
that approximately 34 would be required and that an OspC-based
vaccinogen is feasible.

TEXT
Lyme disease is the leading arthropod-borne disease in North
America and Europe. It is caused by the spirochete species
Borrelia burgdorferi (North America and Europe),
Borrelia garinii (Europe),
and
Borrelia afzelii (Europe). A Lyme disease vaccine is not
currently available for use in humans. The outer surface protein
A (OspA)-based LymeRix vaccine, introduced in 1998, was pulled
from the market in 2002 due to poor sales, possibly traced to
concerns over vaccine-induced arthritis (
21). There is a pressing
need for the development of an effective, safe, and broadly
protective Lyme disease vaccine (
29,
40).
The expression profile (38), immunodominance (11, 42), and critical functional role of OspC during transmission from the tick (34) and during mammalian infection (18, 30, 43-45) have made it an attractive candidate vaccinogen (12). OspC is an
22-kDa protein encoded on a 26-kb circular plasmid that is apparently essential, since it is universal among Lyme disease spirochete isolates (9, 26). While ospC exhibits significant sequence diversity, it is genetically stable during infection (20, 41). Phylogenetic analyses conducted to date have delineated phyletic clusters that have been termed OspC types, which are designated by a letter (23, 39, 48). OspC sequence identity is typically greater than 97% within a type but less than 80% between types. It is evident that OspC types have not arisen as a result of geographic isolation, since within tight geographic regions, strains have been identified that express different OspC types (1, 2, 11, 33, 48). The multiplicity of OspC types appears to be maintained by balancing selection, where maintenance of multiple alleles in the population is more advantageous than the directional selection of a single, most fit allele (5, 48). The selective force is not known with certainty but may be related to immune evasion or to an association between OspC type and infectivity in various mammalian hosts (4, 5).
Although vaccination of mice with OspC protein has been demonstrated to be protective, the protection is strain specific (6, 16, 17, 32, 37, 46). This observation suggests that the protective epitopes likely reside within OspC type-specific domains. Consistent with this, we previously demonstrated that the early murine humoral response is type specific and that the immunodominant epitopes of OspC that are presented during early infection in mice and humans are located within hypervariable domains presented at the surface of the OspC dimer (7, 11, 15, 22). By necessity, a broadly protective Lyme disease vaccine must be polyvalent. It has been reported that a polyvalent vaccine formulation consisting of full-length OspC proteins of 14 types has been tested; however, the formulation was too reactogenic and a balance in the antibody response to each OspC type was not obtained (19). The goal of our efforts is to develop a chimeric polyvalent vaccine construct that consists of the type-specific, protective epitopes of those OspC types associated with invasive infection in humans. As proof of principle, we previously developed a construct that incorporates epitopes from OspC types A, B, K, and D (11-13). This construct elicited bactericidal antibody against spirochetes expressing each of the OspC types incorporated into the vaccinogen in a complement-dependent manner (12).
To facilitate the further development of a broadly protective chimeric construct, we have conducted phylogenetic analyses of OspC sequences available in the NCBI databases. The segment of OspC analyzed spanned residues 20 through 200 (using numbering for the B31MI sequence). Shorter sequences were excluded from these analyses, leaving sequences from 280 Borrelia strains available for analysis. The OspC type designation of each sequence was determined through alignment (PAM40 scoring matrix) and pairwise identity matrix analysis. Consistent with earlier studies, sequences that exhibited 95% or greater sequence identity were considered to belong to the same OspC type (3, 48) (Fig. 1). A clear bimodal distribution of sequence comparisons, with a mean sequence identity of 65% between differing OspC type sequences and >97% identity within types was observed. In addition to the 21 types described by Wang et al. (48), 17 additional clusters were defined. We did not assign OspC type designations to clusters that included fewer than 3 sequences, nor did we include in our analyses previously defined OspC types with fewer than three sequences (i.e., types C, G, J, and O to T). OspC types not previously assigned a letter designation were named based on a prototype strain contained within the cluster.
Of 280 analyzed sequences, 202 were assigned to OspC types,
all of which were from Lyme disease-causing species. The remaining
78 sequences were not assigned to an OspC type, as they did
not occur in a cluster of three or more sequences of >95%
identity. Twenty-seven of the unassigned sequences were from
Borrelia species not typically associated with Lyme disease,
including
B. bissettii,
B. japonica,
B. andersonii,
B. tanukii,
and
B. valaisiana. A summary of the geographic and biological
origin of the isolate from which each OspC sequence was obtained
is indicated in Table
1. In an analysis of type-assigned sequences,
the majority of
B. burgdorferi isolates were from North America
(80%), with lesser numbers from Europe (16%) and Asia (4%).
Fifty-three percent of
B. burgdorferi, 48% of
B. afzelii, and
79% of
B. garinii OspC sequences originated from isolates collected
from humans. It is noteworthy that the
B. garinii human isolates
were predominantly of cerebrospinal fluid (CSF) origin (68%),
whereas
B. afzelii isolates were predominantly from the skin
(83%). In contrast,
B. burgdorferi isolates were recovered from
human skin (51%), plasma (30%), and CSF (19%). These findings
are in agreement with the known patterns of disease caused by
these organisms and indicate that the sample of OspC sequences
assessed in this report is representative of the true population
of Lyme disease spirochetes.
To facilitate further phylogenetic analyses, the set of sequences
analyzed was reduced to 74 by eliminating identical sequences.
These sequences were then aligned and analyzed using the Phylip
(v. 3.66) phylogenetics package with bootstrapping (
n = 1,000).
Distances were calculated for the regions spanning amino acids
20 to 200, 20 to 130, and 131 to 200 using the Dayhoff PAM matrix,
and trees were created by neighbor joining. The
B. hermsii OspC
ortholog (Vtp) sequence served as an out-group (
27,
31). A consensus
tree was generated by majority rule (50% cutoff for group inclusion).
Distances were recalculated for the consensus tree by the maximum
likelihood method under the Dayhoff PAM model (Fig.
2). The
consensus trees generated with the amino acids 20 to 200 segment
of OspC were well supported at the terminal nodes, with all
determined OspC types clustering as expected. While several
of the deeper branches were less supported by the bootstrap
analyses (Fig.
2A), this is not unexpected since the extended
regions of identity among the sequences makes their phylogenetic
differentiation subtle. Consensus trees generated using the
amino acids 20 to 200 and 20 to 130 segments of OspC exhibited
similar phylogenetic clustering (Fig.
2A and B) with clustering
patterns consistent with species identity. However, the consensus
tree generated using amino acids 131 to 200 (Fig.
2C) yielded
significantly different clustering patterns that were not strongly
supported by bootstrap analyses. This observation is consistent
with the hypothesis that recombination between short segments
of the
ospC gene has occurred between strains of differing OspC
types. Evidence for recombination of short segments of
ospC between OspC types can be seen in specific sequences. For example,
sequences of the
B. afzelii OspC type, PLj7, have regions within
the amino acid 20 to 130 domain that are identical to that seen
in
B. garinii OspC sequences that form the Pki cluster. In the
amino acids 131 to 200 region of PLj7, the hypervariable loop
5 and loop 6 regions have motifs identical to those seen in
B. burgdorferi OspC types F and M, respectively. Further evidence
for recombination was noted during a broader assessment of regional
similarities in DNA sequence by bootscanning (SimPlot v. 3.5.1)
(
25). In bootscanning, potential recombination is assessed by
generation of phylogenetic trees (Kimura model, Ts/Tv ratio
= 2.0, neighbor joining) using sequence segments contained within
a sliding window (40-base window, 10-base step interval). The
trees are bootstrapped (
n = 100), and the number of permuted
trees supporting sequence grouping within that window is reported.
Evidence of recombination is typically considered to be supported
when >70% of permuted trees support a particular grouping
of sequences. In the case presented in Fig.
3, the PLj7 sequence
was scanned for possible areas of recombination with OspC types
Pki, F, and M, as described above. Peaks representing possible
recombination between types PLj7 and Pki occur in regions predicted
to encode the unstructured N terminus, alpha helices 1 and 4,
and loop 4. There was evidence of possible recombination with
type M at alpha helix 3 and at the junction of loop 6 and alpha
helix 5, and with type F at alpha helices 3 and 5. Possible
recombination was also found between numerous other OspC types
(data not shown). The mechanisms associated for genetic transfer
in the
Borrelia, particularly involving the
ospC-bearing cp26
plasmid, have not been clearly delineated (
3,
8,
10,
14,
24,
28,
35,
36,
47).
The evidence that OspC variability occurs by exchange between
existing OspC types rather than by hypermutation provides evidence
that there is a limit to the number of OspC type-specific epitopes
required for inclusion in a broadly protective vaccinogen. Since
currently mapped linear epitopes are all contained in the C-terminal
region of OspC (amino acids 131 to 200), it is possible to define
a theoretical number of antigenic domains required for a chimeric
vaccinogen. By inspecting this region in the 74 representative
sequences described above, the number of unique epitope-containing
regions can be reduced to 34 by elimination of sequences that
are either identical or have only a single amino acid change
(Fig.
4). It is likely that this number can be further restricted
by epitope mapping since some epitopes may convey protection
against two or more OspC types. Further reduction in the required
number of antigenic domains could also come from consideration
of only those OspC types associated with human disease or, more
specifically, with invasive human disease (
1,
11,
39). One theoretical
concern with vaccination against a subset of OspC epitopes is
the potential to drive selection toward types not included in
the vaccinogen, thus increasing the fraction of the population
bearing those rare types. However, as humans are incidental
hosts, it is unlikely that vaccination will significantly alter
the population distribution of strains expressing specific OspC
types in the tick vector or mammalian reservoirs.
In summary, the extensive nature of the OspC database has allowed
thorough analyses to be conducted which have defined new OspC
types and provided information regarding their frequency of
isolation and association with human disease. The data suggest
that the number of OspC epitope-containing sequences required
for inclusion in a broadly protective chimeric vaccinogen is
limited and that the development of a chimeric vaccinogen is
feasible.

ACKNOWLEDGMENTS
This work was supported in part by grants from the National
Institutes of Health (NIAID) (to R.T.M.) and the American Heart
Association (to C.G.E.).

FOOTNOTES
* Corresponding author. Mailing address: Department of Microbiology and Immunology, Medical College of Virginia at Virginia Commonwealth University, Richmond, VA 23298-0678. Phone: (804) 828-3888. Fax: (804) 827-1548. E-mail:
rmarconi{at}vcu.edu 
Published ahead of print on 14 March 2007. 

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Clinical and Vaccine Immunology, May 2007, p. 628-634, Vol. 14, No. 5
1071-412X/07/$08.00+0 doi:10.1128/CVI.00409-06
Copyright © 2007, American Society for Microbiology. All Rights Reserved.
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