Abstract
In this study we describe the adaptive changes fixed on the capsid of several foot-and-mouth disease virus serotype A strains during propagation in cell monolayers. Viruses passaged extensively in three cell lines (BHK-21, LFBK and IB-RS-2) consistently gained positively charged amino acids in the putative heparin-sulfate-binding pocket (VP2 beta E-beta F loop, VP1 C-terminus and VP3 beta-B knob) surrounding the fivefold symmetry axis (VP1 beta-beta G loop) and at other discrete sites on the capsid (VP3 beta G-beta H loop, VP1 C-terminus, VP2 beta C strand and VP1 beta G-beta H loop). A lysine insertion in the VP1 beta F-beta G loop of two of the BHK-21-adapted viruses supports the biological advantage of positively charged residues acquired in cell culture. The charge transitions occurred irrespective of cell line, suggesting their possible role in ionic interaction with ubiquitous negatively charged cell-surface molecules such as glycosaminoglycans (GAG). This was supported by the ability of the cell-culture-adapted variants to replicate in the integrin-deficient, GAG-positive CHO-K1 cells and their superior fitness in competition assays compared with the lower passage viruses with WT genotypes. Substitutions fixed in the VP1 beta G-beta H loop (-3, 2 and +2 'RGD' positions) or in the structural element known to be juxtaposed against that loop (VP1 beta B-beta C loop) suggest their possible role in modulating the efficiency and specificity of interaction of the 'RGD' motif with alpha(v)-integrin receptors. The nature and location of the substitutions described in this study could be applied in the rapid cell culture adaptation of viral strains for vaccine production.