Characterization of monoclonal antibody reacting exclusively against intracellular Orientia tsutsugamushi

Characterization of rickettsial adhesin Adr2 belonging to a new group of adhesins in α-proteobacteria

BACKGROUND

Rickettsia prowazekii is the etiological agent of epidemic typhus and is an obligate intracellular bacterium that grows as a parasite freely within the cytoplasm of a eukaryotic host cell. Previous studies have shown that rOmpA and rOmpB which belong to the family of rickettsial cell surface antigens are involved in vitro in the adhesion of Rickettsiae to epithelial cells. Recently, two putative rickettsial adhesins have been identified using high resolution 2D-PAGE coupled with mass spectrometry. In this study, we further characterize and describe the adhesin Adr2 from R. prowazekii.

METHODOLOGY/PRINCIPAL FINDINGS

Using an overlay assay coupled with mass spectrometry two adhesins, Adr1 (RP827) and Adr2 (RP828), were identified from the R. prowazekii proteome Recombinant R. prowazekii Adr2 was expressed through fusion with Dsbc in Escherichia coli, purified and concentrated, thus allowing production of specific monoclonal antibodies, as confirmed by western blot assays. Finally, inhibition of rickettsiae-induced cytotoxicity with monoclonal anti-Adr2 antibody has showed a greatest impact on bacterial cell entry at 8 h post-infection (ca50%) and then decreased progressively to attempt 18% of inhibition at day 7. These, correlated to the inhibition of rickettsiae-induced cytotoxicity with monoclonal anti-rOmpB antibody. Thus, Adr2 is sufficient to mediate R. prowazekii entry into the cell at early stage of mammalian cell infection.

CONCLUSIONS

Our results suggest that R. prowazekii Adr2 could be the main actor promoting the entry of rickettsiae into the host cells. The present study opens the framework for future investigations for better understanding of the Adr2 -mediated mechanisms involved in adhesion/invasion or intracellular survival of R. prowazekii.

Improved antibiotic susceptibility test of Orientia tsutsugamushi by flow cytometry using monoclonal antibody

Orientia tsutsugamushi causes scrub typhus, which is endemic in many countries in the Asia-Pacific region including Korea. Recent emergence of doxycycline-resistant strains from Thailand has underlined the importance of the susceptibility tests of O. tsutsugamushi to antibiotics. To improve the flow cytometric technique for the susceptibility test, we applied a monoclonal antibody (MAb) in the quantification of O. tsutsugamushi. With using MAb FS15, we determined the doxycycline susceptibility of two strains, Boryong and AFSC-4 strain which is reported to be doxycycline-sensitive and resistant, respectively. The growth of both strains was inhibited to below 10% of the control in the presence of 0.1 microg/mL or higher concentrations of doxycycline. We suggest that our approach is more quantitative and reproducible than the conventional microscopic methods.

Application of monoclonal antibody, specific for intracellular Orientia tsutsugamushi, to immunofluorescent antibody test for determining antibiotic susceptibility

The simple quantification of viable intracellular bacteria is important for the study of an obligate intracellular bacterium, Orientia tsutsugamushi. We applied a novel monoclonal antibody (M686-13)--specific for intracellular Orientia--to an immunofluorescent antibody (IFA) test for determining antibiotic susceptibility of O. tsutsugamushi. M686-13 did not react with Orientia that was inhibited by doxycycline, although bacterial particles still remained in the cells. This preferential staining of proliferating bacteria made the IFA test rapid and precise. Using this method, we could successfully measure the minimal inhibitory concentration (MIC) of a Korean strain of O. tsutsugamushi to doxycycline and clindamycin. This method may be used in other procedures to evaluate the growth of Orientia.

Scrub typhus and tropical rickettsioses

PURPOSE OF REVIEW

Recent developments in molecular taxonomic methods have led to a reclassification of rickettsial diseases. The agent responsible for scrub typhus (Orientia tsutsugamushi ) has been removed from the genus Rickettsia and a bewildering array of new rickettsial pathogens have been described. An update of recent research findings is therefore particularly timely for the nonspecialist physician.

RECENT FINDINGS

An estimated one billion people are at risk for scrub typhus and an estimated one million cases occur annually. The disease appears to be re-emerging in Japan, with seasonal transmission. O. tsutsugamushi has evolved a variety of mechanisms to remain viable in its intracellular habitat. Slowing the release of intracellular calcium inhibits apoptosis of macrophages. Subsets of chemokine genes are induced in infected cells, some in response to transcription factor activator protein 1. Cardiac involvement is uncommon and clinical complications are predominantly pulmonary. Serious pneumonitis occurred in 22% of Chinese patients. Dual infections with leptospirosis have been reported. Standardized diagnostic tests are being developed and attempts to improve treatment of women and children are being made. Of the numerous tick-borne rickettsioses identified in recent years, African tick-bite fever appears to be of particular importance to travellers. The newly described flea-borne spotted fever caused by Rickettsia felis may be global in distribution.

SUMMARY

Rash and fever in a returning traveler could be rickettsial and presumptive doxycycline treatment can be curative. Recent research findings raise more questions than answers and should stimulate much needed research.