Immunostimulating Effect of Inactivated Parapoxvirus Ovis on the Serological Response to Equine Influenza Booster Vaccination

Abstract

Equine influenza virus (EIV) is responsible for recurring outbreaks that are detrimental to the equine industry. Vaccination is key for prevention, but the effectiveness and duration of protection provided by existing vaccines is often insufficient. In order to improve vaccine efficacy, we evaluated the benefit of immune stimulation with inactivated Parapoxvirus ovis (iPPVO) on the antibody response induced by a vaccine boost against EIV. A whole inactivated ISCOMatrix-adjuvanted equine influenza vaccine was administered alone (n = 10) or combined with iPPVO injections at D0, D2 and D4 post vaccination (n = 10) to adult horses that required a vaccine boost 6 months after the last immunization, as now recommended by the WOAH. Antibody levels were measured with the single radial haemolysis (SRH) assay at 1, 3 and 6 months post-vaccination. Results revealed that horses that received iPPVO had higher antibody levels than the control group injected with the EI vaccine alone. Although the vaccine used contains only a clade 1 and European lineage strain, the increase in protective antibodies was also observed against a clade 2 strain. Thus, immune stimulation with iPPVO, a substance already marketed as an immunostimulant, could be used to improve vaccination protocols in horses and potentially other species.

Keywords: equine influenza viruses; horse; immunomodulator; inactivated Parapoxvirus ovis; vaccine.

Figure 1

Horse selection and EIV vaccination protocol with or without iPPVO^®.

Figure 2

Mean H3N8 single radial haemolysis (SRH) antibody response measured before and after booster vaccination. The values correspond to sera isolated from blood samples that were collected in the control and iPPVO groups before the annual vaccination (AV), 1 and 3 months after the AV and before the semi-annual boost (SAB), and 1, 3 months and 6 months after the SAB. The red circles correspond to the mean SRH values in the control group (n = 10), and the blue squares correspond to the mean SRH values in the iPPVO group (n = 10) for (a) A/equine/Paris/1/2018 strain clade 1 strain and (b) A/equine/Jouars/4/06 EIV clade 2 strain. The diamonds represent the time of the AV injection and the 6 months SAB injections with or without iPPVO. Error bars represent standard error of the mean. Black stars correspond to a significant increase in the haemolysis area for the iPPVO group compared to the control group (two-factor ANOVA test; * and ** for p < 0.05 and p < 0.005, respectively). A red star represents the significant increase in the haemolysis area for the control group over time, and the blue stars represent the significant increase in the haemolysis area for the iPPVO group over time.

Figure 2

Mean H3N8 single radial haemolysis (SRH) antibody response measured before and after booster vaccination. The values correspond to sera isolated from blood samples that were collected in the control and iPPVO groups before the annual vaccination (AV), 1 and 3 months after the AV and before the semi-annual boost (SAB), and 1, 3 months and 6 months after the SAB. The red circles correspond to the mean SRH values in the control group (n = 10), and the blue squares correspond to the mean SRH values in the iPPVO group (n = 10) for (a) A/equine/Paris/1/2018 strain clade 1 strain and (b) A/equine/Jouars/4/06 EIV clade 2 strain. The diamonds represent the time of the AV injection and the 6 months SAB injections with or without iPPVO. Error bars represent standard error of the mean. Black stars correspond to a significant increase in the haemolysis area for the iPPVO group compared to the control group (two-factor ANOVA test; * and ** for p < 0.05 and p < 0.005, respectively). A red star represents the significant increase in the haemolysis area for the control group over time, and the blue stars represent the significant increase in the haemolysis area for the iPPVO group over time.

Figure 3

Evolution of H3N8 antibody response to simple radial haemolysis (SRH) measured before and after annual vaccination (AV) and semi-annual vaccination (SAB) with or without iPPVO. Values are for sera isolated from blood samples that were collected for 1 year from AV to 6 months after SAB. Values correspond to sera isolated from blood samples collected in the control and iPPVO groups before AV, 1 and 3 months after AV; before the booster, 1, 3 and 6 months after the semi-annual booster (SAB). Cumulative histograms represent the distribution of the number of horses according to the different levels of protection (white: unprotected; grey: clinical protection and black: virological protection) for the different sampling points for horses in the control group (a) and horses in the iPPVO group (b) for A/equine/Paris/1/2018 strain and in the control group (e) and horses in the iPPVO group (f) for A/equine/Jouars/4/06 EIV strain. Each horse is represented with a specific colour in the different graphs (c, d, g and h). The different colours correspond to the SRH values of the horses in the control group (n = 10) (c) and iPPVO group (n = 10) (d) for A/equine/Paris/1/2018 strain and the SRH values of the horses in the control group (n = 10) (g) and iPPVO group (n = 10) (h) for A/equine/Jouars/4/06 EIV strain. The diamond represents the timing of the AV injection and the 6 month SAB injections with or without iPPVO. * and ** indicate significant difference (χ² tests; p < 0.05 and 0.005, respectively) in the frequency of horses reaching the protection threshold between the control group and the iPPVO group. Only similar time points post immunisation were compared.