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Infection and Immunity, August 2003, p. 4818-4822, Vol. 71, No. 8
0019-9567/03/$08.00+0 DOI: 10.1128/IAI.71.8.4818-4822.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.
Molecular Bacteriology Section,1 Plague Section, Bacterial Zoonoses Branch, Division of Vector-Borne Infectious Diseases, National Center for Infectious Diseases, Centers for Disease Control and Prevention, Public Health Service, U.S. Department of Health and Human Services, Fort Collins, Colorado2
Received 27 November 2002/ Returned for modification 5 March 2003/ Accepted 7 May 2003
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Bartonella infections are associated with arthropod vector transmission. B. bacilliformis is transmitted by sand flies (21), and B. henselae has been demonstrated in cat fleas (18, 22), with the human body louse (Pediculus humanus) instrumental in the transmission of B. quintana (43). Domestic cats are considered a reservoir host for B. henselae (14, 27), but some Bartonella are carried asymptomatically in a variety of wild rodents worldwide (5, 7, 31). Additionally, there is serological and molecular evidence of California coyotes serving as reservoir hosts (10, 13), and PCR data have implicated Ixodid ticks in harboring Bartonella (11, 12).
The current state of diagnostics for the determination of infection is underdeveloped, but serology by the indirect fluorescence assay and enzyme immunoassay against whole cells are the predominant methodologies being applied, mostly for CSD (15, 41). However, problems with cross-reactivity among Bartonella species and variable sensitivities and specificities observed among laboratories have led to caution when interpreting the serologic-based results (1, 6, 19, 23, 32). Furthermore, diagnostic assays for bartonelloses caused by organisms other than B. henselae or B. quintana are underdeveloped.
Little is known regarding antigens that induce an antibody response following Bartonella infection. Several immunogenic proteins associated with Bartonella infections have been noted by Western blot banding patterns (20, 35, 36-39), but only the B. henselae 17-kDa antigen and HtrA stress response protein and the B. bacilliformis Bb65 antigen have been characterized (2, 3, 25). Bartonella-specific monoclonal antibodies have been described, but the molecular identities of the corresponding antigens have not yet been elucidated (33, 34). The goal of this project was to identify immunogens associated with Bartonella infections. As a first step, we screened Bartonella genomic libraries with polyclonal antiserum against whole-cell lysates of various Bartonella isolates. In this report, we describe an immunoreactive Bartonella gene product as being dihydrolipoamide succinyltransferase expressed by the sucB gene, which is part of the
-ketoglutarate dehydrogenase complex that has been described in several prokaryotes.
Identification of the sucB gene from genomic libraries. Bartonella strains used in this study for DNA manipulations, immunoblotting, and antibody production are listed in Table 1. B. quintana and B. vinsonii subsp. berkhoffii were cultivated on brain heart infusion agar medium supplemented with 5% rabbit blood (BBL Becton Dickinson Microbiology Systems, Cockeysville, Md.) and harvested as previously described (31). Genomic DNA was purified from thawed cell suspensions by a phenol-chloroform extraction procedure followed by ethanol precipitation according to standard procedures. For genomic cloning, purified Bartonella DNA was subjected to partial Sau3AI restriction enzyme digestion and was ligated into the ZapExpress BamHI-predigested bacteriophage lambda cloning vector (Stratagene, La Jolla, Calif.) with subsequent packaging of the ligated DNA with the GigaPack III Gold packaging extract (Stratagene) as directed by the manufacturer. Recombinant lambda plaques were plated, titrated, and amplified according to the manufacturer's instruction manual.
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-ketoglutarate dehydrogenase operon complex present in several prokaryotes: sucA, sucB, and lpdA, which encode the
-ketoglutarate dehydrogenase (E1o), dihydolipoamide succinyltransferase (E2o), and dihydrolipoamide dehydrogenase enzymes, respectively. The sucA and lpdA genes are truncated within this insert, and sucB is represented in its entirety. The sucB coding sequence consists of 1,233 bp with a calculated molecular mass of 43.8 kDa from the deduced amino acid sequence. The sucB gene coding sequence was subcloned into a plasmid expression vector and transformed into Escherichia coli, and the gene product was synthesized. The recombinant SucB immunoblotted positively with the anti-Bartonella antibodies used to screen the library (Fig. 1A, lanes 1 to 3).
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SucB reactivity was tested against polyclonal antibodies raised against various Bartonella species and strains. Cross-reactivity was observed against all anti-Bartonella antibodies tested (Fig. 1A). Although reactivity against anti-B. quintana and anti-B. henselae was weaker, this may simply reflect the strength of the antiserum used. Four samples of preimmunized mouse serum showed no reactivity to SucB (Fig. 1B, lanes 11 to 14). Significantly, there was robust reactivity with the recombinant SucB against antibodies from an experimentally infected mouse injected with live Bartonella isolated from P. maniculatus and boosted with an S. beecheyi strain (Fig. 1A, lane 10). This result demonstrated that an antibody response is mounted against SucB in response to a Bartonella infection, as well as showing the strong cross-reactivity between B. vinsonii subsp. berkhoffii and rodent-isolated Bartonella.
Antibodies against other bacterial pathogens were tested for cross-reactivity against SucB. Polyclonal antisera against various bacterial pathogens were obtained from the following sources: rabbit anti-Legionella pneumophila from Yousef Abu Kwaik (University of Kentucky); rabbit anti-Coxiella burnetti phase II from Bob Heinzen (University of Wyoming); rabbit anti-Rickettsia rickettsii, Rickettsia typhi, and Rickettsia prowazekii generated at the Rocky Mountain Laboratories, Hamilton, Mont.; rabbit anti-Francisella tularensis and human anti-Yersinia pestis from the Diagnostic and Reference Section, Division of Vector-Borne Infectious Diseases (DVBID), Centers for Disease Control and Prevention (CDC); human anti-Borrelia burgdorferi, Leptospira spp., and Treponema pallidum from the Molecular Bacteriology Section, DVBID, CDC. SucB seroreactivity was seen with antisera specific to C. burnetti and F. tularensis, while weaker reactivity was observed against R. typhi (Fig. 1B). No cross-reactivity was observed with antibodies against the other microbes tested. Anti-Brucella spp. antibodies were unavailable for testing.
Recent serological testing of patients with a febrile illness of unexplained origin from New Mexico by this laboratory had suggested possible infections with rodent-associated Bartonella (M. Y. Kosoy et al., Abstr. Am. Soc. Rickettsiology-Bartonella Emerg. Pathogen Group 2001 Joint Conf., abstr. 108, 2001; F. Koster et al., Abstr. Am. Soc. Rickettsiology-Bartonella Emerg. Pathogen Group 2001 Joint Conf., abstr. 133, 2001). This observation and the discovery that rodent species in the western United States harbor Bartonella led us to investigate whether these organisms could be the causative agents of illnesses in humans having exposure to wild rodents. Concomitantly, this laboratory recently discovered that Bartonella isolates obtained from ground squirrels in Nevada had gltA (citrate synthase), 16S rRNA, and groEL gene sequences identical to those of B. washoensis isolated from a cardiac patient from the same area, providing evidence of Bartonella rodent-to-human transmission (30). In conjunction with these observations, we sought to expand our understanding of Bartonella infection-associated immunogens, particularly those putatively causing non-CSD illnesses, by screening genomic libraries using antibodies generated against rodent-isolated Bartonella.
B. vinsonii subsp. berkhoffii and B. quintana were initially chosen as genomic library representatives for Bartonella, as we were interested in investigating antigens from other species besides B. henselae. Although B. vinsonii subsp. berkhoffii is associated with infection in dogs (8, 29), coyotes and ticks have been implicated as reservoir and vector hosts, respectively, in the western United States (11, 13), and there has been one documented human case infection (42). In addition, a related organism, B. vinsonii subsp. arupensis, was recently isolated from a human patient in Wyoming (45).
The genomic libraries were screened initially with antibodies specific to rodent isolates. The rationale was to recognize any putative gene products reactive against antibodies to rodent-borne Bartonella and subsequently to use comparative genomics to determine the extent of cross-reactive, homologous genes between genus and species. Although in this study we screened genomic libraries from only two Bartonella species, we have purified genomic DNA from several Bartonella strains and generated libraries to other rodent Bartonella isolates for future genetic comparisons. We have indeed been successful in amplifying sucB by PCR from several Bartonella spp. and isolates, indicating the presence of this gene as expected (data not shown).
At the time this study began, Bartonella isolates from clinically defined human cases in the western United States were not available and, accordingly, neither were antiserum samples from culture-confirmed patients. In addition, we could not utilize antiserum from the rodent reservoir hosts from which the Bartonella organisms were isolated, as these naturally infected animals do not seem to mount a detectable antibody response (31). Antigens identified by polyclonal antibodies prepared against killed whole-cell lysates do not necessarily correlate with immunogens associated with Bartonella infections; however, this approach does identify candidate antigens that can be assayed for their reactivity against antiserum raised in an infected host. Indeed, SucB proved to be an infection-associated immunogen, as it was detected by antiserum from the experimentally infected mouse shown in Fig. 1A.
Molecular analysis of the expression library clone indicated that the gene encoding the protein reactive against anti-Bartonella antibodies was sucB, which encodes dihydrolipoamide succinyltransferase (E2o), an enzyme that is one part of three components forming the
-ketoglutarate dehydrogenase complex found in several eukaryotes and prokaryotes. This enzyme complex is also composed of
-ketoglutarate dehydrogenase (E1o) and dihydrolipoamide dehydrogenase (E3) and is encoded by the genes sucA and lpd, respectively. The DNA sequence of the B. vinsonii subsp. berkhoffii cloned insert showed a similar gene arrangement as that described in Rhodobacter capsulatus and E. coli (17, 44).
To our knowledge, only three other immunoreactive Bartonella antigens have been molecularly characterized. They are the 17-kDa antigen of B. henselae that elicits a strong humoral response in patients with CSD (2), the B. henselae HtrA stress response protein (3), and a GroEL class of heat shock protein from B. bacilliformis termed Bb65 (25). Other Bartonella antigens have been identified by Western blot banding patterns, but they have not been cloned and sequenced. Several groups have shown that anti-Bartonella serum samples are reactive on immunoblots with proteins of molecular masses of 45 to 50 kDa. These bands may correspond to the SucB antigen (20, 35, 39).
Antigenic cross-reactivity was seen when recombinant SucB was assayed with antibodies raised against various Bartonella strains. Significantly, the SucB was reactive against antiserum from a mouse experimentally infected with live Bartonella. The antigenic cross-reactivity was evidenced by the fact that this antibody was directed against Bartonella obtained from deer mice (P. maniculatus) and California ground squirrels (S. beecheyi). This result suggested that SucB can be a broad indicator of infection against different Bartonella species. Importantly, antigenic cross-reactivity was observed when SucB was immunoblotted against antibodies to C. burnetti and F. tularensis, with slight reactivity against anti-R. typhi. SucB may therefore be one of the antigens responsible for the serological cross-reactions that have been noted in Western blotting and indirect fluorescence assays by other researchers (32, 39).
SucB has been shown to be an immunogenic protein during infections by two other intracellular pathogens, B. melitensis, which causes ovine and caprine brucellosis and can be transmitted to humans (46), and C. burnetti, the causative agent of Q fever (40). Noting the immunogenicity of SucB from these organisms that are close phylogenetic relatives of Bartonella and the cross-reactivity observed in serological assays between them, one must be cautious when correlating antibody reactivity against SucB with a Bartonella infection and vice versa. Also confounding is that the clinical manifestations of bartonelloses, Q fever, and brucelloses are similar and could be confused. However, a broadly cross-reactive antigen such as SucB could potentially be used as an identifier of these diseases, with more specific diagnostic tools to differentiate the infectious agents. Moreover, as serologic diagnostic assays for these diseases are explored and become more developed, researchers should be aware of the potential cross-reactivity of Bartonella SucB with antibodies against other organisms.
Nucleotide sequence accession numbers. The DNA sequence of the B. vinsonii subsp. berkhoffii clone, which includes the sequence encoding the sucB gene and the partial sequence encoding the sucA and lpdA genes, has been submitted to GenBank under accession number AY160679. The DNA sequence of the B. quintana sucB gene has been submitted to GenBank under accession number AY160680.
| ACKNOWLEDGMENTS |
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