Inclusion of a specific T cell epitope increases the protection conferred against foot-and-mouth disease virus in pigs by a linear peptide containing an immunodominant B cell site

Abstract

Background: Foot-and-mouth disease virus (FMDV) causes an economically important and highly contagious disease of cloven-hoofed animals. FMD control in endemic regions is implemented using chemically inactivated whole-virus vaccines. Currently, efforts are directed to the development of safe and marked vaccines. We have previously reported solid protection against FMDV conferred by branched structures (dendrimeric peptides) harbouring virus-specific B and T-cell epitopes. In order to gain insights into the factors determining a protective immune response against FMDV, in this report we sought to dissect the immunogenicity conferred by different peptide-based immunogens. Thus, we have assessed the immune response and protection elicited in pigs by linear peptides harbouring the same FMDV B-cell or B and T-cell epitopes (B and TB peptides, respectively).

Results: Pigs were twice immunized with either the B-cell epitope (site A) peptide or with TB, a peptide where the B-cell epitope was in tandem with the T-cell epitope [3A (21-35)]. Both, B and TB peptides were able to induce specific humoral (including neutralizing antibodies) and cellular immune responses against FMDV, but did not afford full protection in pigs. The data obtained showed that the T-cell epitope used is capable to induce efficient T-cell priming that contributes to improve the protection against FMDV. However, the IgA titres and IFNγ release elicited by these linear peptides were lower than those detected previously with the dendrimeric peptides.

Conclusions: We conclude that the incorporation of a FMDV specific T-cell epitope in the peptide formulation allows a significant reduction in virus excretion and clinical score after challenge. However, the linear TB peptide did not afford full protection in challenged pigs, as that previously reported using the dendrimeric construction indicating that, besides the inclusion of an adecuate T-cell epitope in the formulation, an efficient presentation of the B-cell epitope is crucial to elicit full protection by peptide vaccines.

Figure 1

A) Time course of clinical disease in challenged pigs immunized with peptide B (group 1), peptide TB (group2), and in non-immunized pigs (group 3), after challenge with FMDV C-S8c1. The mean body temperature (°C) [right, rhombs] and the score of the clinical signs (grey curve; calculated as indicated in Materials and Methods) are shown. B) Box plot showing the range of the maximum clinical scores (see Materials and Methods) recorded for the individual animals from groups 1, 2 and 3 after challenge. Statistically significant differences were found in the median values (line into the box) between groups 2 and 1 (* P = 0.026) and 2 and 3 (** P = 0.003).

Figure 2

Proliferative response against peptides (20 μg/ml per well) and virus (5·10⁵ pfu/ml) of PBMCs obtained at day 39 from peptide-immunized pigs (group 1 and 2). Results are expressed as SI and each bar represents the mean of triplicate cultures. The background cpm values (obtained with lymphocytes incubated with medium alone or mock-infected cells) were always ≤ 2300 cpm.