Analysis of Humoral Immune Responses to Surface and Virulence-Associated Chlamydia abortus Proteins in Ovine and Human Abortions by Use of a Newly Developed Line Immunoassay

The obligate intracellular bacterium Chlamydia abortus is the causative agent of enzootic abortion of ewes (EAE) and poses a significant zoonotic risk for pregnant women. Using proteomic analysis and gene expression library screening in a previous project, we identified potential virulence factors and candidates for serodiagnosis, of which nine have been scrutinised here by using a strip immunoassay. We have shown that aborting sheep exhibited a strong antibody response against surface (MOMP, MIP, Pmp13G) and virulence-associated (CPAF, TARP, SINC) antigens. While the latter disappeared within 18 weeks following abortion in the majority of animals, antibodies against surface proteins persisted beyond the duration of the study. In contrast, non-aborting experimentally infected sheep developed antibodies mainly against surface antigens (MOMP, MIP, Pmp13G), all of which did not persist. We were also able to detect antibodies against these surface antigens in C. abortus-infected women who had undergone septic abortion, whereas a group of shepherds and veterinarians with occupational exposure to C. abortus-infected sheep revealed only sporadic immune responses to the antigens selected. The most specific antigen for serodiagnosis of human C. abortus infections was Pmp13G, which showed no cross-reactivity with other chlamydiae infecting humans. We suggest that Pmp13G-based serodiagnosis accomplished by the detection of antibodies against virulence-associated antigens such as CPAF, TARP, and SINC, may improve laboratory diagnosis of human and animal C. abortus infections.