Efficacy of Two Chlamydia abortus Subcellular Vaccines in a Pregnant Ewe Challenge Model for Ovine Enzootic Abortion

Abstract

Chlamydia abortus, the aetiological agent of enzootic abortion of ewes, is a major cause of reproductive loss in small ruminants worldwide, accounting for significant economic losses to the farming industry. Disease can be managed through the use of commercial inactivated or live whole organism-based vaccines, although both have limitations particularly in terms of efficacy, safety and disease-associated outbreaks. Here we report a comparison of two experimental vaccines (chlamydial outer membrane complex (COMC) and octyl glucoside (OG)-COMC) based on detergent extracted outer membrane preparations of C. abortus and delivered as prime-boost immunisations, with the commercial live vaccine Cevac^® Chlamydia in a pregnant sheep challenge model. No abortions occurred in either experimental vaccine group, while a single abortion occurred in the commercial vaccine group. Bacterial shedding, as a measure of potential risk of transmission of infection to naïve animals, was lowest in the COMC vaccinated group, with reductions of 87.5%, 86.4% and 74% observed for the COMC, OG-COMC and live commercial vaccine groups, respectively, compared to the unvaccinated challenge control group. The results show that the COMC vaccine performed the best and is a safer efficacious alternative to the commercial vaccines. However, to improve commercial viability, future studies should optimise the antigen dose and number of inoculations required.

Keywords: Chlamydia abortus; cytokine analysis; enzootic abortion of ewes; quantitative real-time polymerase chain reaction (PCR); serological analysis; vaccine development; vaccine efficacy.

Figure 1

Experimental design. Numbers under bar indicate days prior to or post mating. V1 and V2 indicate timings for first and second vaccinations, respectively. CV indicates when the commercial vaccine was administered.

Figure 2

Serological responses following vaccination and challenge with Chlamydia abortus. Detection of C. abortus antibody in ewes vaccinated (V1 and CV/V2—see Figure 1) with commercial vaccine 1 (A) or experimental vaccines 2 (B) and 3 (C) and challenged at day 70 of gestation with C. abortus strain S26/3. Unvaccinated challenged (D) and unvaccinated non-challenged (E) ewes served as positive and negative control groups. Data are separated into lambed (blue lines) versus aborted (orange lines). Data points represent the arithmetic mean values for each cellular bleed and error bars represent the standard error of that mean (SEM). The 100% is equivalent to an OD450 nm of 2.25. The lambing/abortion period is indicated by the horizontal double-headed arrows.

Figure 3

Interferon-γ and IL-10 responses following vaccination and challenge with Chlamydia abortus. Peripheral blood mononuclear cells (PBMC) from the animals in the commercial (Vaccine 1) and experimental (Vaccines 2 and 3) vaccine groups were purified from whole blood (as described in Section 2.12 “Cellular Analysis”) collected pre-vaccination (A,E), post-vaccination/pre-challenge (B,F), post-challenge/pre-parturition (C,G) and post-parturition (D,H) (also see Figure 1). PBMC were set up in lymphocyte stimulation assays in vitro using medium only as an unstimulated cell control (blue bars), the mitogen Concanavalin A (ConA) as a positive control (orange bars) and UV-inactivated C. abortus EB antigen (grey bars) for measuring chlamydial antigen-specific stimulation. Antigen-specific recall responses were assessed by analysis of the culture supernatants for cytokines IFN-γ (A–D) and IL-10 (E–H). Data for lambed (L) and aborted (A) ewes are presented separately. Data points represent the mean values for each cellular bleed and error bars represent the standard error of that mean (SEM).