3.5 VLPs plus ISPA or CA induce increased levels of anti-FMDV Abs in cattle
After promising results obtained in the murine model, the immune efficacy of the VLPs-ISPA and VLPs-CA vaccines was studied in cattle, a natural host of the virus.
FMDV-serologically negative calves (per group) were inoculated only once with VLPs (n=4), VLPs-ISPA (n=4), VLPs-CA (n=4) or iFMDV-CA (n=2) at day 0. The same amount of VLPs or iFMDV (25 µg) was used in all formulations.
When Abs were measured by Lp ELISA, calves vaccinated with VLPs-ISPA and VLPs-CA displayed an increment in the elicited anti-FMDV humoral response as compared to animals vaccinated with VLPs alone (p<0.01) at 15, 30 and 45 dpv (Figures 5A). Moreover, the FMDV humoral response achieved in VLPs-ISPA and VLPs-CA vaccinated animals was similar to that achieved in iFMDV-CA vaccinated animals.
Analysis of IgG1 isotype at 45 dpv showed that, in cattle vaccinated with VLPs, VLPs-ISPA or VLPs-CA, no significant differences were detected between groups, although while in the group vaccinated with VLPs, the mean titer of IgG1 Abs was 2.5, the mean titers in the other groups were ≥ 3. (Figure 5B). Moreover, the VLPs-ISPA and VLPs-CA vaccines achieved higher IgG2 levels than the VLPs vaccine (p<0.01).
FMDV-neutralizing Abs elicited in bovines by the different vaccines were next studied due to their connection with protection against viral challenge.
At 45 dpv, VNT results (Table 2) showed a significant increase (p<0.001) in mean VNT titers in the VLPs-ISPA and VLPs-CA groups (2.1±0.1 and 2.4±0.2 respectively) compared with the VLPs group. Animals vaccinated with iFMDV-CA also presented high levels of neutralizing Abs (2.45±0.07). Remarkably, these values have been reported to be associated with 90% Expected Percentage of Protection (EPP) (Maradei et al., 2008), which estimates the likelihood that cattle would be protected against a challenge of 10.000 bovine infective doses after vaccination (OIE - World Organisation for Animal Health, 2012).