Distinct Intensity of Host-Pathogen Interactions in Chlamydia psittaci- and Chlamydia abortus-Infected Chicken Embryos

Abstract

ABSTRACTFactors and mechanisms determining the differences in virulence and host specificity between the zoonotic agentsChlamydia psittaciandChlamydia abortusare still largely unknown. In the present study, two strains were compared for their invasiveness, virulence, and capability of eliciting an immune response in chicken embryos. On breeding day 10, embryonated chicken eggs were inoculated with 5 × 104inclusion-forming units. As shown by immunohistochemistry and quantitative real-time PCR,C. psittacidisplayed a significantly better capability of disseminating in the chorioallantoic membrane (CAM) and internal organs thanC. abortus. The higher infectious potential ofC. psittaciin birds was underlined by significantly higher mRNA expression rates of essential chlamydial genes, such asincA,groEL(in CAM, liver, and spleen),cpaf, andftsW(in CAM). Although the immune responses to both pathogens were similar,C. psittacielicited higher macrophage numbers and a stronger expression of a subset of immune-related proteins. The data imply that invasiveness ofChlamydiaspp. and propagation in the host are not solely dependent on the level of host immune response but, even to a greater extent, on the expression of bacterial factors related to virulence. The fact thatC. psittacihas coped far better thanC. abortuswith the avian embryo's response by upregulating essential genes may be a key to understanding the mechanisms underlying host adaptation and etiopathology.