Protective monoclonal antibodies define maturational and pH-dependent antigenic changes in Sindbis virus E1...

Monoclonal (MC) antibodies specific for either the EI or E2 glycoproteins of Sindbis virus (SIN) were used to probe for differences in the surface topography of SIN epitopes between infected cells and mature virions. Employing an enzyme-linked immunosorbent assay (ELISA) in which binding of individual peroxidase-labeled MC antibodies to immobilized (solid-phase) detergent-disrupted SIN was inhibited specifically by one or more unlabeled antibodies, viral epitopes could be grouped into six spatially distinct antigenic sites--five on E1, designated a through e, and one site on E2. All six sites were represented on the surfaces of SIN-infected cells as shown by the complement (C')-dependent lysis mediated by antibodies of the corresponding epitope specificities. In contrast, virus-neutralizing (NT) activity was restricted to antibodies specific for epitopes on E2 and on site c of E1, irrespective of the presence of added C' and an antiserum against mouse immunoglobulins. That E1 sites a, b, d, and e became inaccessible to antibody binding was shown by a competitive-inhibition ELISA. Whereas all MC antibodies were inhibited from binding to solid-phase SIN when premixed with detergent-treated virions, only those having NT activity could be competitively inhibited by intact virions. Sites E1-d and E1-e could be exposed not only by detergent disruption but also by lowering the virion pH from 7.2 to 6.0. These collective results indicate that a majority of immunologically relevant E1 epitopes present on SIN-infected cell surfaces become cryptic during SIN maturation and, except at low pH, remain undetectable on virion surfaces.