Species-specific BALB/c mouse antibodies to rickettsiae studied by western blotting

Serosurvey of Rickettsia spp. in cats from a Brazilian spotted fever-endemic area

Rickettsia spp. bacteria are responsible for tick-borne diseases worldwide, mostly maintained by rickettsial amplifiers capybaras in Brazilian endemic areas. The campus of the University of São Paulo, in southeastern Brazil, is an area endemic for Brazilian spotted fever (BSF), with high density of capybaras and Amblyomma spp., along with confirmed human cases. Besides capybaras, the university has also an in-campus high population of sheltered and free-roaming cats. Accordingly, the aim of this study was to determine the prevalence and characteristics associated with Rickettsia rickettsii, Rickettsia parkeri and Rickettsia felis exposure among cats in a BSF-endemic area. Out of 51 cats sampled, 23/35 shelter (65.7%) and 5/16 free-roaming (31.2%) were positive (titers ≥ 64) for at least one Rickettsia species. Ticks species were present in 3/16 free-roaming cats (18.8%), consisting of Amblyomma spp., nymphs of Amblyomma sculptum and adult Rhipicephalus sanguineus sensu lato. Despite sharing the capybaras environment, the seropositivity among the free-roaming and shelter cats was lower than owned cats in other endemic areas. Whether equally or less exposed to rickettsial infection, compared with owned cats in endemic areas, free-roaming and shelter cats may be used as environmental sentinels for human exposure to rickettsiae in such areas.

Survey of vector-borne agents in feral cats and first report of Babesia gibsoni in cats on St Kitts, West Indies

Background

As there is little data on vector-borne diseases of cats in the Caribbean region and even around the world, we tested feral cats from St Kitts by PCR to detect infections with Babesia, Ehrlichia and spotted fever group Rickettsia (SFGR) and surveyed them for antibodies to Rickettsia rickettsii and Ehrlichia canis.

Results

Whole blood was collected from apparently healthy feral cats during spay/ neuter campaigns on St Kitts in 2011 (N = 68) and 2014 (N = 52). Sera from the 52 cats from 2014 were used to detect antibodies to Ehrlichia canis and Rickettsia rickettsii using indirect fluorescent antibody tests and DNA extracted from whole blood of a total of 119 cats (68 from 2011, and 51 from 2014) was used for PCRs for Babesia, Ehrlichia and Rickettsia. We could not amplify DNA of SFG Rickettsia in any of the samples but found DNA of E. canis in 5% (6/119), Babesia vogeli in 13% (15/119), Babesia gibsoni in 4% (5/119), mixed infections with B. gibsoni and B. vogeli in 3% (3/119), and a poorly characterized Babesia sp. in 1% (1/119). Overall, 10% of the 52 cats we tested by IFA for E. canis were positive while 42% we tested by indirect fluorescent antibody (IFA) for R. rickettsii antigens were positive.

Conclusions

Our study provides the first evidence that cats can be infected with B. gibsoni and also indicates that cats in the Caribbean may be commonly exposed to other vector-borne agents including SFGR, E. canis and B. vogeli. Animal health workers should be alerted to the possibility of clinical infections in their patients while public health workers should be alerted to the possibility that zoonotic SFGR are likely circulating in the region.

Enhanced protection against Rickettsia rickettsii infection in C3H/HeN mice by immunization with a combination of a recombinant adhesin rAdr2 and a protein fragment rOmpB-4 derived from outer membrane protein B

BACKGROUND

Two surface proteins of Rickettsia rickettsii, outer membrane protein B (OmpB) and adhesion 2 (Adr2), have been recognized as protective antigens. Herein, the immunization with both OmpB and Adr2 was performed in mice so as to explore whether their combination could induce an enhanced immunoprotection against R. rickettsii infection.

METHODS

C3H/HeN mice were immunized with recombinant protein rAdr2 or/and rOmp-4, a fragment derived from OmpB, and then mice were challenged with R. rickettsii. After which rickettsial loads in mice were measured by quantitative PCR. The specific antibodies in mouse sera were determined by ELISA and antigen-specific cytokines secretion by mouse T cells were analyzed in vitro.

RESULTS

After challenge with R. rickettsii, the mice immunized with rAdr2 or/and rOmpB-4 had significant lower rickettsial load in livers, spleens, or lungs compared to PBS mock-immunized mice. Particularly, the load in lungs of mice immunized with both rAdr2 and rOmpB-4 was significantly lower than that with either of them. High levels of specific antibodies were detected in sera from mice immunized with rAdr2 or/and rOmpB-4, but the ratios of specific IgG2a to IgG1 induced by their combination were significantly higher than that by either rAdr2 or rOmpB-4. Following stimulation with rAdr2 or/and rOmpB-4, the INF-γ secreted by CD4(+) T cells from infected mice was significantly higher than that by cognate cells from uninfected mice. And the TNF-α secreted by CD4(+) or CD8(+) T cells from infected mice was markedly greater than that by cognate cells from uninfected mice after stimulation by their combination but not either of them.

CONCLUSION

The combination of rAdr2 and rOmpB-4 conferred an enhanced protection against R. rickettsii infection in mice, which was mainly dependent on a stronger Th1-oriented immunoresponse with greater INF-γ and TNF-α secretion by antigen-specific T cells and specific IgG2a elicited by the combination.

Protective immunity against Rickettsia heilongjiangensis in a C3H/HeN mouse model mediated by outer membrane protein B-pulsed dendritic cells

Rickettsia heilongjiangensis is an obligate intracellular bacterium that causes Far-Eastern tick-borne spotted fever. Outer membrane protein B (OmpB) is an important surface protein antigen of rickettsiae. In the present study, the ompB gene of R. heilongjiangensis was divided into four fragments, resulting in four recombinant proteins (OmpB-p1, OmpB-p2, OmpB-p3, and OmpB-p4). Each OmpB was used in vitro to stimulate murine bone marrow-derived dendritic cells (BMDCs) of C3H/HeN mice, and the OmpB-pulsed BMDCs were transferred to naïve C3H/HeN mice. On day 14 post-transfer of BMDCs, the mice were challenged with R. heilongjiangensis and the rickettsial loads in the mice were quantitatively determined on day 7 post-challenge. Mice receiving BMDCs pulsed with OmpB-p2, OmpB-p3, or OmpB-p4 exhibited significantly lower bacterial load compared with mice receiving OmpB-p1-pulsed BMDCs. CD4(+) and CD8(+) T cells isolated from the spleen of C3H/HeN mice receiving BMDCs pulsed with each OmpB were co-cultured with BMDCs pulsed with the respective cognate protein. In flow cytometric analysis, the expression level of CD69 on CD4(+) or CD8(+) T cells from mice receiving BMDCs pulsed with OmpB-p2, OmpB-p3, or OmpB-p4 was higher than that on cells from mice receiving OmpB-p1-pulsed BMDCs, while the expression level of tumor necrosis factor (TNF)-α on CD8(+) T cells and interferon (IFN)-γ on the CD4(+) and CD8(+) T cells from mice receiving OmpB-p2, -p3, or -p4 was significantly higher than on cells from mice receiving OmpB-p1-pulsed BMDCs. Our results suggest that the protective OmpBs could activate CD4(+) and CD8(+) T cells and drive their differentiation toward CD4(+) Th1 and CD8(+) Tcl cells, respectively, which produce greater amounts of TNF-α and, in particular, IFN-γ, to enhance rickettsicidal activity of host cells.

Evaluation of PCR-based assay for diagnosis of spotted fever group rickettsiosis in human serum samples

A nested PCR assay was developed for the detection of spotted fever group (SFG) rickettsiae in serum samples. The assay was based on specific primers derived from the rickettsial outer membrane protein B gene (rompB) of Rickettsia conorii. An SFG rickettsia-specific signal is obtained from R. akari, R. japonica, R. sibirica, and R. conorii. Other bacterial species tested did not generate any signal, attesting to the specificity of the assay. As few as seven copies of the rompB gene of R. conorii could be detected in 200 microl of serum sample. The assay was evaluated with a panel of sera obtained from patients with acute-phase febrile disease tested by immunofluorescent antibody assay (IFA). The SFG rickettsia-specific DNA fragment was detected in 71 out of 100 sera, which were proven to have immunoglobulin M antibodies against SFG rickettsial antigen by IFA. The results were further confirmed by restriction fragment length polymorphism and sequencing analysis of the DNA fragments. The results indicated that this PCR assay is suitable for the diagnosis of spotted fever group rickettsiosis in Korea.

Multispacer typing of Rickettsia prowazekii enabling epidemiological studies of epidemic typhus

Currently, there is no tool for typing Rickettsia prowazekii, the causative agent of epidemic typhus, currently considered a potential bioterrorism agent, at the strain level. To test if the multispacer typing (MST) method could differentiate strains of R. prowazekii, we amplified and sequenced the 25 most variable intergenic spacers between the R. prowazekii and R. conorii genomes in five strains and 10 body louse amplicons of R. prowazekii from various geographic origins. Two intergenic spacers, i.e., rpmE/tRNA(fMet) and serS/virB4, were variable among tested R. prowazekii isolates and allowed identification of three and two genotypes, respectively. When the genotypes obtained from the two spacers were combined, we identified four different genotypes. MST demonstrated that several R. prowazekii strains circulated in human body lice during an outbreak of epidemic typhus in Burundi. This may help to discriminate between natural and intentional outbreaks. Our study supports the usefulness of MST as a versatile method for rickettsial strain genotyping.

Serosurvey among Mediterranean spotted fever patients of a new spotted fever group rickettsial strain (Bar29)

Mediterranean spotted fever is an endemic disease in Catalonia, Spain. A new spotted fever group (SFG) rickettsial strain (Bar29) of unknown pathogenicity for humans was isolated by our group, in 1996, from the dog brown tick, Rhipicephalus sanguineus. Interestingly, Rickettsia conorii was not isolated in this study. The aim of the present study was to assess the possible pathogenic role of the Bar29 strain. To this purpose, serum samples from 15 patients with Mediterranean spotted fever were obtained and tested by immunofluorescence for antibodies against four related rickettsial strains (R. conorii, R. africae, R. massiliae, and Bar29). Eight of the studied sera reacted at high titers with only R. conorii and Bar29 antigens. For five of the eight sera, the titers against Bar29 were clearly higher than for R. conorii. Four of these sera were also studied by Western blot immunoassay to confirm a specific response. Two of these sera reacted with the high-molecular-mass specific proteins of Bar29 as well as with the low-molecular-mass region (LPS antigen) whereas their reactions with R. conorii were located only on bands of the LPS. This specific response would support the possible pathogenic role of the Bar29 strain for humans. According to this finding, spotted fever caused by R. conorii and rickettsial strain Bar29 may be present in our area. The epidemiological implications of spotted fever caused by R. conorii and by rickettsial strain Bar29 in the Catalonia deserve further studies with isolation and characterization of more rickettsial strains.

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.

Rickettsioses as paradigms of new or emerging infectious diseases

Rickettsioses are caused by species of Rickettsia, a genus comprising organisms characterized by their strictly intracellular location and their association with arthropods. Rickettsia species are difficult to cultivate in vitro and exhibit strong serological cross-reactions with each other. These technical difficulties long prohibited a detailed study of the rickettsiae, and it is only following the recent introduction of novel laboratory methods that progress in this field has been possible. In this review, we discuss the impact that these practical innovations have had on the study of rickettsiae. Prior to 1986, only eight rickettsioses were clinically recognized; however, in the last 10 years, an additional six have been discovered. We describe the different steps that resulted in the description of each new rickettsiosis and discuss the influence of factors as diverse as physicians' curiosity and the adoption of molecular biology-based identification in helping to recognize these new infections. We also assess the pathogenic potential of rickettsial strains that to date have been associated only with arthropods, and we discuss diseases of unknown etiology that may be rickettsioses.