Identification of a neutralization epitope in the VP1 capsid protein of SV40

Three SV40 escape mutants were identified by selection in the presence of monoclonal antibodies with neutralizing activity. The VP1 amino acid alterations in these mutants were: (1) K73 → E (in loop BC); (2) D77 → E (in loop BC); (3) K171 → R (in loop EF); and (4) Q175 → H (in loop EF). These residues are clustered in close proximity to each other on the surface of the native capsid protein, strongly suggesting that they form a conformational epitope directly recognized by the neutralizing antibody. To our knowledge, the present study represents the first experimental mapping of a neutralization epitope of a polyomavirus family member. Structural information regarding the neutralization epitope should be useful for clarifying the extent of cross-reactivity exhibited by the humoral immune response towards related primate polyomaviruses (e.g., SV40, BKV, and JCV).