Evidence of Combined Effect of Amino Acid Substitutions within G-H and B-C Loops of VP1 Conferring Serological Heterogeneity in Foot-and-Mouth Disease Virus Serotype A.

Foot-and-mouth disease virus (FMDV) serotype A exhibits a high degree of genetic and antigenic diversity that results in frequent vaccine failure due to serological mismatch between the vaccine and heterologous strains even from the same serotype. Currently, knowledge on the molecular basis of antigenic relationships among the FMDVs is limited, though intratype antigenic variation due to evolutionary mutation(s) is widely considered as the main hurdle to appropriate FMD vaccine development. Here, we studied genetic and antigenic variations of four FMDV serotype A isolates, BAN/GA/Sa-197/2013 (BAN-197), BAN/CH/Sa-304/2016 (BAN-304), BAN/DH/Sa-307/2016 (BAN-307) and BAN/DH/Sa-310/2017 (BAN-310) circulating in Bangladesh during 2012-2017. Antigenic relationships (r1- values) of the field isolates were evaluated by the two-dimensional microneutralization test (2D-MNT) using the hyperimmune antisera raised in cattle against the vaccine strain, BAN-304. The results showed good serological cross-reactivity (r1 -values ˃ 0.4) between the vaccine strain and two of the field isolates, BAN-307 and BAN-310, except BAN-197 that substantially mismatched (r1 = 0.129±0.043) with the vaccine virus, BAN-304. Nevertheless, VP1-based phylogeny grouped all the isolates within the same sublineage C under the lineage A/ASIA/G-VII. Computational analyses of the viral capsid proteins demonstrated significant deviation at the VP1 G-H loop of BAN-197 from the vaccine strain; while VP(2-4) of both isolates were structurally conserved. In silico combinatorial substitutions of the mutant amino acid (aa) residues within the VP1 of BAN-197 with the respective residues in BAN-304 explicitly explained how the distortion of the G-H loop and antigenic heterogeneity occurred in BAN-197. It revealed that two substitutions of distantly located aa at B-C (T48I:threonine→isoleucine) and G-H (A143V:alanine→valine) loops, in combination, distorted the VP1 G-H loop. Overall, this work contributes to understanding the molecular basis of antigenic relationships within FMDV serotype A and the selection of suitable vaccine strain(s) for effective prophylaxis of FMD using VP1 based analyses.