Analysis of T-cell-dependent and -independent antigens of Rickettsia conorii with monoclonal antibodies

A Vaccine for Canine Rocky Mountain Spotted Fever: An Unmet One Health Need

Outbreaks of life-threatening Rocky Mountain spotted fever in humans and dogs associated with a canine-tick maintenance cycle constitute an important One Health opportunity. The reality of the problem has been observed strikingly in Mexico, Brazil, Colombia, and Native American tribal lands in Arizona. The brown dog tick, Rhipicephalus sanguineus sensu lato, acquires the rickettsia from bacteremic dogs and can maintain the bacterium transtadially to the next tick stage. The subsequent adult tick can then transmit infection to a new host, as shown by guinea pig models. These brown dog ticks maintain spotted fever group rickettsiae transovarially through many generations, thus serving as both vector and reservoir. Vaccine containing whole-killed R. rickettsii does not stimulate sufficient immunity. Studies of Rickettsia subunit antigens have demonstrated that conformationally preserved outer-membrane autotransporter proteins A and B are the leading vaccine candidates. The possibility of a potentially safe and effective live attenuated vaccine has only begun to be explored as gene knockout methods are applied to these obligately intracellular pathogens.

The realities of biodefense vaccines against Rickettsia

Rickettsia prowazekii, R. rickettsii, R. conorii, and R. typhi are serious biologic weapon threats because of high infectivity of low dose aerosols, stable small particle aerosol infectivity, virulence causing severe disease, difficulty in establishing a timely diagnosis, ineffectiveness of usual empiric treatments, potential for engineered complete antimicrobial resistance, lower level of immunity, availability of the agents in nature, and feasibility of propagation, stabilization, and dispersal. Infection induces long-term immunity, killed rickettsial vaccines stimulate incomplete protection, and a live attenuated mutant stimulates strong immunity but reverts to virulence. Prospects for rational development of a safe, effective live attenuated vaccine are excellent.

Distribution of immunogenic epitopes on the two major immunodominant proteins (rOmpA and rOmpB) of Rickettsia conorii among the other rickettsiae of the spotted fever group

Forty-four monoclonal antibodies were raised against strain Seven, the type strain of Rickettsia conorii. Of these 44 monoclonal antibodies, 13, 27, and 4 were demonstrated to be directed against the 116-kDa protein (rOmpA), the 124-kDa protein (rOmpB), and lipopolysaccharide-like antigen, respectively. The antiprotein monoclonal antibodies were found to be directed against 29 distinct epitopes, which were located on the two major immunodominant proteins discussed above. Further analysis showed that strain-specific epitopes were located on the rOmpA protein and species- and subgroup-specific epitopes were located on the rOmpB protein. R. conorii Manuel, Indian tick typhus rickettsia, and Kenya tick typhus rickettsia also possessed all 29 epitopes, whereas the other rickettsiae of the spotted fever group (SFG) expressed between 3 and 25 epitopes, with the exception of Rickettsia helvetica, R. akari, and R. australis which did not possess any epitopes. Additional analyses by Western immunoblotting confirmed that the epitopes shared among the SFG rickettsiae were located on the same two high-molecular-mass proteins as on R. conorii. However, although epitopes on the R. conorii rOmpB protein were expressed on the rOmpB proteins of most other SFG rickettsiae, some were found on the rOmpA proteins of R. aeschlimannii, R. rickettsii, and R. rhipicephali. Both proteins possessing the common epitopes were found to have different sizes in the SFG rickettsial species. The different distributions of common epitopes in the SFG rickettsiae were also used to build a taxonomic dendrogram, which demonstrated that all the R. conorii strains formed a relatively independent cluster within the SFG rickettsiae and was generally consistent with previously proposed taxonomies.

Production of monoclonal antibodies against Rickettsia massiliae and their use in antigenic and epidemiological studies

Rickettsiae are gram-negative, obligate intracellular bacteria which have historically been divided into three groups: the typhus group, the scrub typhus group, and the spotted fever group (SFG). Recently, several new SFG rickettsiae have been characterized, and most of these species are associated with ticks and have, as yet, no known pathogenicity toward humans. Rickettsia massiliae, which is widely distributed in Europe and Africa, is one such rickettsia. In order to investigate the antigenic relationships between R. massiliae and other rickettsial species and to develop a more convenient methodology for identifying R. massiliae, we produced monoclonal antibodies against the type strain (Mtu1T) of R. massiliae by fusing immunized splenocytes with SP2/0-Ag14 myeloma cells. A panel of 16 representatives were selected from the 163 positive hybridomas identified on initial screening, and their secreted monoclonal antibodies were further characterized. The reactivities of these 16 monoclonal antibodies with a large panel of rickettsial species were assessed by the microimmunofluorescence assay. All species of the SFG rickettsiae reacted with the monoclonal antibodies directed against epitopes on lipopolysaccharide, which is the common antigen among the SFG rickettsiae. Some closely related species of the SFG, such as Bar29, "R. aeschlimanni," and R. rhipicephali, showed strong cross-reactivities with the monoclonal antibodies directed against epitopes on the two major high-molecular-mass heat-labile proteins (106 and 120 kDa). In addition, species-specific monoclonal antibodies demonstrated that R. massiliae is antigenically different from other rickettsial species. Moreover, these species-specific monoclonal antibodies were successfully used for identifying R. massiliae in the ticks collected from southern France, and are therefore potentially useful tools in the identification and investigation of R. massiliae in ticks in large-scale field work.

Characterization of and application of monoclonal antibodies against Rickettsia africae, a newly recognized species of spotted fever group rickettsia

Rickettsia africae is a newly described species which causes African tick bite fever. Mediterranean spotted fever caused by R. conorii is endemic in the same regions of Africa as tick bite fever, and differentiation of the two syndromes by characterization of their etiological agents is important for epidemiological studies. R. africae and R. conorii are, however, difficult to distinguish, and therefore, our aim was to produce monoclonal antibodies to address this problem. Monoclonal antibodies were produced against R. africae by fusing splenocytes from BALB/C mice immunized with purified rickettsial organisms and SP2/0-Ag14 myeloma cells. A total of 355 hybridomas producing monoclonal antibodies to R. africae were identified by initial screening with six different antigens by microimmunofluorescence assay. A panel of 23 representative monoclonal antibodies were selected and subcloned. This panel was screened with a further 17 different spotted fever group (SFG) rickettsial reference antigens. Of these 23 monoclonal antibodies, 1 cross-reacted with only R. parkeri, whereas the others cross-reacted with more than two different antigens. Immunoblotting indicated that all the monoclonal antibodies were directed against the epitopes on two major high-molecular-mass heat-labile proteins, of which the molecular masses were 128 and 135 kDa, respectively. This monoclonal antibody panel was used successfully to identify R. africae in the blood culture of an infected patient, in infected cells within shell vials, and in infected ticks collected from Africa. Furthermore, the cross-reactivity of each SFG rickettsia with each of these 23 monoclonal antibodies was scored and was used to build a dendrogram of taxonomic relatedness between R. africae and the other SFG rickettsiae on the basis of Jaccard coefficients and unweighted pair group method with arithmetic mean analysis. The relatedness was generally consistent with that obtained by other methods of comparison.

Cloning, sequencing, and expression of the gene coding for an antigenic 120-kilodalton protein of Rickettsia conorii

Several high-molecular-mass (above 100 kDa) antigens are recognized by sera from humans infected with spotted fever group rickettsiae and may be important stimulators of the host immune response. Molecular cloning techniques were used to make genomic Rickettsia conorii (Malish 7 strain) libraries in expression vector lambda gt11. The 120-kDa R. conorii antigen was identified by monospecific antibodies to the recombinant protein expressed on construct lambda 4-7. The entire gene DNA sequence was obtained by using this construct and two other overlapping constructs. An open reading frame of 3,068 bp with a calculated molecular mass of approximately 112 kDa was identified. Promoters and a ribosome-binding site were identified on the basis of their DNA sequence homology to other rickettsial genes and their relative positions in the sequence. The DNA coding region shares no significant homology with other spotted fever group rickettsial antigen genes (i.e., the R. rickettsii 190-, 135-, and 17-kDa antigen-encoding genes). The PCR technique was used to amplify the gene from eight species of spotted fever group rickettsiae. A 75-kDa portion of the 120-kDa antigen was overexpressed in and purified from Escherichia coli. This polypeptide was recognized by antirickettsial antibodies and may be a useful diagnostic reagent for spotted fever group rickettsioses.

Rickettsia australis infection: a murine model of a highly invasive vasculopathic rickettsiosis

A mouse model of spotted fever group rickettsiosis, in which disease results from disseminated rickettsial infection of endothelial cells and vascular damage, was developed by intravenous inoculation of 6- to 8-week-old, male, Balb/c mice with Rickettsia australis. Animals developed progressively severe vasculitis, interstitial pneumonia, and multifocal hepatic necrosis. These lesions correlated with early disseminated infection of endothelial cells followed by growth and invasion of rickettsiae into perivascular cells. The dose of 2 x 10(6) organisms was uniformly lethal. Serum interleukin- (IL) 1, IL-6, and interferon (IFN) increased by day 3 and tumor necrosis factor (TNF) on day 5. TNF, IL-6, and IFN declined on day 7. Spleen cells responded to Rickettsia australis antigen by producing IFN, TNF, IL-1, and IL-6 on day 5, followed by lower quantities of these cytokines on day 7. Despite the production of antibodies, IFN, TNF, IL-1, and IL-6, a lethal outcome occurred frequently. A decreased ability to secrete IL-2 suggests an element of infection-associated immunosuppression.

Antigens of Rickettsia conorii recognized by seropositive healthy people from Salamanca (central-west Spain)

The antigenic reactivity in Western immunoblotting assay of individual Rickettsia conorii components with sera of healthy people living in Salamanca Province, an endemic zone of Mediterranean spotted fever, is evaluated. Polypeptides of molecular weights 100 kDa (92.7%), 135 kDa (75.6%), 160 kDa (70.7%) and 115 kDa (48.8%) were recognized by a higher proportion of sera with indirect immunofluorescent antibody test titers > or = 1:80. Reaction with apparent rickettsial lipopolysaccharide was found in 15 (36.6%) of these samples. The involvement of different rickettsial strains, atypical routes of inoculation, varying content of the inoculum, and host factors may be determinants of the clinical expression of the spotted fever group rickettsial infection in people who produce antibodies reactive with Rickettsia conorii antigens.

Use of western blot to analyze the reactivity of sera from patients with Mediterranean spotted fever

The reactivity of antigenic components of Rickettsia conorii with sequentially obtained sera from 20 adult Spanish patients with Mediterranean spotted fever was analyzed by Western blot. The major rickettsial antigens reacting with the serum samples corresponded to molecular weights of 135 and 115 kDa. These antigens constantly exhibited higher staining intensity than the other antigens, and reacted with 100% and 86.7%, respectively, of acute sera and with 100% of convalescent phase samples. Rickettsial lipopolysaccharide antigens reacted with 94.7% of sera collected in the fourth and fifth week after onset of symptoms. Other major antigens reactive in the blots had molecular sizes of 160, 100, 90 and 60 kDa, and a relatively frequent humoral immune response was also seen to antigens of 80, 73 and 55 kDa.

A comparative view of Rickettsia tsutsugamushi and the other groups of rickettsiae

Recent researches on the rickettsial group microorganisms are summarized in their comparative aspects of morphology, cultivation and multiplication, susceptibility to chemotherapeutics, chemical structure of envelopes, nucleic acid, protein constitution, and gene structures. From this overview, Rickettsia tsutsugamushi seems to have different properties from the others and should be reclassified into a new genus, and a new species name as Orientia tsutsugamushi is proposed.

Species-specific monoclonal antibodies to Rickettsia japonica, a newly identified spotted fever group rickettsia

A total of 192 hybridomas were developed from mice immunized with Rickettsia japonica, a newly identified spotted fever group rickettsia pathogenic for humans. Of these hybridomas, 101 were species specific, 37 were spotted fever group reactive, and the other 54 were also reactive with one or more of the other pathogenic species of spotted fever group rickettsiae, Rickettsia akari, Rickettsia australis, Rickettsia conorii, Rickettsia rickettsii, and Rickettsia sibrica. Seven of the species-specific monoclonal antibodies were characterized. These monoclonal antibodies all belong to the immunoglobulin G class and react with all five strains of R. japonica at the same immunofluorescence titers, indicating that the five strains all belong to a single species. The species-specific epitopes reactive with these monoclonal antibodies are located on the surface proteins of the organisms demonstrated as 145- and 120-kilodalton bands on Western immunoblots. These two antigenic bands were shown to be proteins, because treatment with proteinase K completely destroyed the reactivity of the bands with the monoclonal antibodies.

Line blot and western blot immunoassays for diagnosis of Mediterranean spotted fever

The line blot, a new immunoassay in which antigens are placed on nitrocellulose as narrow lines, was evaluated for its sensitivity and specificity relative to the microimmunofluorescence assay for the diagnosis of Mediterranean spotted fever (MSF). The line blot assay was only slightly less sensitive and less specific than the microimmunofluorescence assay for detection of immunoglobulin M (IgM) or IgG in 100 serum specimens from 42 patients with MSF. No line blot reactions were observed among 50 control serum specimens from febrile patients with other illnesses. The line blot assay was largely group reactive for spotted fever rickettsiae, but 26% of the positive serum specimens also cross-reacted by IgM with Rickettsia typhi. Western immunoblotting was used to characterize the antigenic components recognized by 19 MSF serum specimens. For both IgM and IgG, lipopolysaccharide was the cross-reactive group antigen, whereas the high-molecular-weight species-specific protein antigens (SPAs) were the only reactive proteins. Relative to the other nine rickettsiae, Rickettsia bellii was unique both in exhibiting no SPA reactions and in having a lipopolysaccharide with a predominantly high-molecular-weight distribution. Although most of the 19 MSF serum specimens examined by Western blotting exhibited preferential reactivity to SPAs of two strains of R. conorii and weaker reactions to the other rickettsiae, 2 serum specimens exhibited SPA reactions consistent with typhus infections. In comparison with other assays, the line blot and Western blot immunoassays have advantages which may permit an improvement in the general availability and commercialization of assays for the serodiagnosis of rickettsial infections.

Rocky Mountain spotted fever: a disease in need of microbiological concern

Rocky Mountain spotted fever, a life-threatening tick-transmitted infection, is the most prevalent rickettsiosis in the United States. This zoonosis is firmly entrenched in the tick host, which maintains the rickettsiae in nature by transovarian transmission. Although the incidence of disease fluctuates in various regions and nationwide, the problems of a deceptively difficult clinical diagnosis and little microbiologic diagnostic effort persist. Many empiric antibiotic regimens lack antirickettsial activity. There is neither an effective vaccine nor a generally available assay that is diagnostic during the early stages of illness, when treatment is most effective. Microbiology laboratories that offer only the archaic retrospective Weil-Felix serologic tests should review the needs of their patients. Research microbiologists who tackle these challenging organisms have an array of questions to address regarding rickettsial surface composition, structure-function analysis, and pathogenic and immune mechanisms, as well as laboratory diagnosis.

Characterization of Sicilian strains of spotted fever group rickettsiae by using monoclonal antibodies

Twenty-two hybridomas producing anti-Rickettsia conorii monoclonal antibodies were obtained by nine fusion experiments. The strain chosen for immunization of mice was MAVI, an R. conorii strain isolated from a Sicilian patient with Boutonneuse fever. When tested for immunoglobulin isotype by an indirect immunofluorescence (IIF) assay, 46.6% of supernatants from the 22 hybridomas were immunoglobulin M. The supernatants were tested in the IIF assay for binding to the MAVI strain and four spotted fever group rickettsia strains isolated from Sicilian ticks (two virulent and two nonpathogenic when inoculated intraperitoneally in male guinea pigs). Only five of the supernatants showed a positive IIF result on all tested strains, although they produced different titers to the various strains, possibly an indication that they recognized an antigen common to spotted fever group rickettsiae. Immunodominant epitopes for humans were determined by using patient sera to analyze inhibition of binding to the MAVI strain. Although a limited number of serum samples were screened, a high percentage of Boutonneuse fever patients produced antibodies recognizing the same epitopes as were recognized by the mouse monoclonal antibodies. A striking heterogeneity was found both in the expression of mouse-recognized epitopes on the five rickettsial strains and in the serum antibody responses of Boutonneuse fever patients to these epitopes.

Protective monoclonal antibodies recognize heat-labile epitopes on surface proteins of spotted fever group rickettsiae

Thirty-eight monoclonal antibodies that have not been reported previously were developed from mice immunized with Rickettsia rickettsii, R. conorii, and R. sibirica. Western immunoblotting showed that these monoclonal antibodies are directed against heat-sensitive epitopes which are located on two major surface polypeptides with molecular sizes ranging from 115 to 150 kilodaltons. The detection of the two bands did not depend on the presence of 2-mercaptoethanol. Both bands were destroyed by treatment with proteinase K. Monoclonal antibodies examined by immunofluorescence assay reacted with epitopes that are species specific, group reactive, or shared among a smaller subset of species of spotted fever group rickettsiae. Nine of the monoclonal antibodies were evaluated for their ability to neutralize rickettsial infection and thus protect animals against disease caused by homologous species of rickettsiae. Treatment of rickettsiae with monoclonal antibodies F3-12, F3-14, and F3-36 completely protected guinea pigs against illness caused by the homologous organism R. rickettsii. Monoclonal antibodies F9-5G11 and F15-5B12, derived from mice immunized with R. sibirica, conferred partial protection by delaying the onset and shortening the duration of fever in guinea pigs inoculated with R. sibirica. Monoclonal antibodies F2-15, F2-31, F2-53, and F3-12 protected mice from a lethal infection with R. conorii. Heat-labile epitopes of spotted fever group rickettsial surface proteins are important candidate antigens for development of vaccines to confer protective immunity.

Gamma interferon as a crucial host defense against Rickettsia conorii in vivo

Gamma interferon (IFN-gamma) plays an important role as a host defense in rickettsial infection. Swiss Webster mice, which are resistant to Rickettsia conorii (Malish 7 strain) infection, were treated with a monoclonal antibody against mouse IFN-gamma. When the antibody-treated mice were inoculated with 12 50% tissue culture infective doses of R. conorii, the mortality was 47% and the morbidity was 100%. None of the control mice, which received the same dose of R. conorii, died or became ill. The enumeration of rickettsiae in organs by direct immunofluorescence in paraffin sections demonstrated higher quantities of rickettsiae in the spleen had liver of IFN-gamma-depleted mice as compared with those of the infected controls. The kinetic analysis of IFN-gamma levels in sera showed depletion in the treated mice. These results indicate that IFN-gamma plays an important role as a host defense in the early stage of rickettsial infection. Survival of some mice despite continued treatment with antibody to IFN-gamma suggests that other immune mechanisms may also be important.