Evaluation of recombinant type-specific antigens of Orientia tsutsugamushi which were expressed by baculovirus-insect cell system as antigens for indirect immunofluorescent assay in the serological diagnosis of scrub typhus

Abstract

Scrub typhus is a mite-borne rickettsiosis caused by the intracellular bacterium Orientia tsutsugamushi (OTS), which is classified as a biosafety level-3 (BSL-3) pathogen. For serological tests of scrub typhus, mouse fibroblast cells infected with the five prevalent serotypes of OTS in Japan are generally used as antigens for indirect immunofluorescent assay (IFA). In this study, sf-9 insect cells infected with the recombinant type-specific antigens (rTSA)-expressing baculovirus were applied for IFA. The paired sera samples of 15 scrub typhus-patients, 10 rickettsiosis-patients, and 10 control individuals were used. Both IgM and IgG IFA titers determined by the rTSA based IFA were correlated with those determined with the OTS-infected cell-based IFA (R ² = 0.7319 to 0.7956). Based on the criteria for serological diagnosis, such as the suitable cutoff for single serum (IgM ≥ 1:160) and/or significant increase in IgG titer between paired sera (≥ 4 times), all of the 15 scrub typhus patients diagnosed as positive with OTS-infected cell-based IFA were also diagnosed as positive by the rTSA-based IFA, whereas all 10 rickettsiosis patients and 10 control individuals were not. The rTSAs, which can be prepared in BSL-2 laboratories, are efficacious in the serological diagnosis of scrub typhus.