Examination of soluble integrin resistant mutants of foot-and-mouth disease virus

Foot-and-mouth disease virus (FMDV) initiates infection via recognition of one of at least four cell-surface integrin molecules α_vβ₁, α_vβ₃, α_vβ₆, or α_vβ₈ by a highly conserved Arg-Gly-Asp (RGD) amino acid sequence motif located in the G-H loop of VP1. Within the animal host, the α_vβ₆ interaction is believed to be the most relevant. Sub-neutralizing levels of soluble secreted α_vβ₆ (ssα_vβ₆) was used as a selective pressure during passages in vitro to explore the plasticity of that interaction.

Genetically stable soluble integrin resistant (SIR) FMDV mutants derived from A24 Cruzeiro were selected after just 3 passages in cell culture in the presence of sub-neutralizing levels of ssα_vβ₆. SIR mutants were characterized by: replication on selective cell lines, plaque morphology, relative sensitivity to ssα_vβ₆ neutralization, relative ability to utilize α_vβ₆ for infection, as well as sequence and structural changes. All SIR mutants maintained an affinity for α_vβ₆. Some developed the ability to attach to cells expressing heparan sulfate (HS) proteoglycan, while others appear to have developed affinity for a still unknown third receptor. Two classes of SIR mutants were selected that were highly or moderately resistant to neutralization by ssα_vβ₆. Highly resistant mutants displayed a G145D substitution (RGD to RDD), while moderately resistant viruses exhibited a L150P/R substitution at the conserved RGD + 4 position. VP1 G-H loop homology models for the A-type SIR mutants illustrated potential structural changes within the integrin-binding motif by these 2 groups of mutations. Treatment of O1 Campos with ssα_vβ₆ resulted in 3 SIR mutants with a positively charged VP3 mutation allowing for HS binding.