Xenoepitope substitution avoids deceptive imprinting and broadens the immune response to foot-and-mouth disease virus

Abstract

Many RNA viruses encode error-prone polymerases which introduce mutations into B and T cell epitopes, providing a mechanism for immunological escape. When regions of hypervariability are found within immunodominant epitopes with no known function, they are referred to as "decoy epitopes," which often deceptively imprint the host's immune response. In this work, a decoy epitope was identified in the foot-and-mouth disease virus (FMDV) serotype O VP1 G-H loop after multiple sequence alignment of 118 isolates. A series of chimeric cyclic peptides resembling the type O G-H loop were prepared, each bearing a defined "B cell xenoepitope" from another virus in place of the native decoy epitope. These sequences were derived from porcine respiratory and reproductive syndrome virus (PRRSV), from HIV, or from a presumptively tolerogenic sequence from murine albumin and were subsequently used as immunogens in BALB/c mice. Cross-reactive antibody responses against all peptides were compared to a wild-type peptide and ovalbumin (OVA). A broadened antibody response was generated in animals inoculated with the PRRSV chimeric peptide, in which virus binding of serum antibodies was also observed. A B cell epitope mapping experiment did not reveal recognition of any contiguous linear epitopes, raising the possibility that the refocused response was directed to a conformational epitope. Taken together, these results indicate that xenoepitope substitution is a novel method for immune refocusing against decoy epitopes of RNA viruses such as FMDV as part of the rational design of next-generation vaccines.

Fig 1

Identification of variable amino acids within the G-H loop and cyclic peptide design. The G-H loop sequence (top) is listed with variable amino acids in bold and underlined. The RGD integrin binding motif is denoted by italics and a surrounding box. A representative structure of the cyclic peptides (bottom) is shown with the xenoepitope substitutions underlined. aa, amino acid.

Fig 2

Cross-reactivity of sera from mice inoculated with chimeric cyclic peptides. Sera from mice immunized with chimeric cyclic peptides were probed against each cyclic peptide coated to a 96-well plate by ELISA. Mean group responses plus 1 standard deviation are represented. Significant differences were determined by ANOVA on ranks with multiple pairwise comparisons conducted using a Student-Newman-Keuls posthoc test (α = 0.05). +, different from OVA; *, different from the homologous peptide; MT, murine tolerogen.

Fig 3

B cell epitope mapping relative to the wild-type G-H loop. Sera from mice inoculated with WT or PRRSV chimeric cyclic peptides were probed against a library of short linear peptides (numbered 1 to 14) based on the wild-type sequence of type O1 BFS (shown just below the graph) by ELISA. Some peptides were used in a previous study and contain divergent amino acids (underlined). Mean group responses to each peptide (top) in the WT group (black bars) and PRRSV chimeric peptide (white bars) are shown.

Fig 4

Wild-type virus binding of sera from mice inoculated with chimeric peptides. Sera collected from mice inoculated with chimeric peptides were probed against type O virus in a sandwich virus capture ELISA. Mean group responses plus 1 standard deviation are represented. Significant differences were determined by ANOVA on ranks with multiple pairwise comparisons conducted using a Student-Newman-Keuls posthoc test (α = 0.05), and groupings are indicated by A or B.