Evaluation of the Protective Efficacy of Different Doses of a Chlamydia abortus Subcellular Vaccine in a Pregnant Sheep Challenge Model for Ovine Enzootic Abortion

Abstract

Chlamydia abortus causes the disease ovine enzootic abortion, which is one of the most infectious causes of foetal death in small ruminants worldwide. While the disease can be controlled using live and inactivated commercial vaccines, there is scope for improvements in safety for both sheep and human handlers of the vaccines. We have previously reported the development of a new prototype vaccine based on a detergent-extracted outer membrane protein preparation of C. abortus that was determined to be more efficacious and safer than the commercial vaccines when administered in two inoculations three weeks apart. In this new study, we have developed this vaccine further by comparing its efficacy when delivered in one or two (1 × 20 µg and 2 × 10 µg) doses, as well as also comparing the effect of reducing the antigen content of the vaccine by 50% (2 × 5 µg and 1 × 10 µg). All vaccine formulations performed well in comparison to the unvaccinated challenge control group, with no significant differences observed between vaccine groups, demonstrating that the vaccine can be administered as a single inoculation and at a lower dose without compromising efficacy. Future studies should focus on further defining the optimal antigen dose to increase the commercial viability of the vaccine.

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 above and below the bar indicate the number of days prior to or post-mating. Animals received one (V1) or two (V1 and V2) vaccine doses at seven (V1) and four (V2) weeks prior to mating. Bleed dates (weeks prior to and post-mating) for serological (solid arrows) and cellular (dotted arrows) analysis are as indicated.

Figure 2

Serological responses following vaccination and challenge with C. abortus. Detection of C. abortus antibody in ewes vaccinated with one (V1) or two (V1 and V2) doses of the experimental COMC antigen preparation (2 × 10 μg (A), 20 μg (B), 2 × 5 μg (C) and 10 μg (D)) and challenged (Ch) on day 70 of gestation with C. abortus strain S26/3. Unvaccinated challenged (E) and unvaccinated non-challenged (F) ewes served as positive and negative control groups. Data are separated into lambed (solid lines) versus aborted (dotted lines). Data points represent the arithmetic mean values for each cellular bleed and error bars represent the standard error of that mean (SEM). A value of 100% is equivalent to an OD450nm of 2.25. The lambing/abortion period for each group is indicated by the horizontal double-headed arrows.

Figure 3

Interferon-γ responses following vaccination and challenge with C. abortus. Peripheral blood mononuclear cells (PBMCs) from the vaccinated challenge control and negative control groups were purified from whole blood (as described in Materials and Methods) collected pre-vaccination (A), post-vaccination (B), post-vaccination/pre-challenge (C), post-challenge/pre-parturition (D) and post-parturition (E) (also see Figure 1). PBMCs were set up in lymphocyte stimulation assays in vitro using medium only as an unstimulated cell control (black bars), the mitogen Concanavalin A (ConA) as a positive control (panel A only; grey bars) and UV-inactivated C. abortus EB antigen (no fill bars) to measure chlamydial antigen-specific stimulation. Antigen-specific IFN-γ recall responses were assessed by analysis of the culture supernatants. Data points represent the mean values for each cellular bleed and error bars represent the standard error of that mean (SEM). L, lambed ewes; A, aborted ewes.