Differentiation of Cross-Reacting Simian Cytomegalovirus Strains by Late 19S Rabbit Neutralizing Antibodies

Abstract The results of studies reported previously (1–3) demonstrated heterogeneity in the neutralizing properties of early (7-day) and late (hyperimmune) 7S and 19S antibodies prepared in rabbits against human herpes simplex virus (HSV) and the simian herpesvirus SA8. In the case of the late antibodies, we showed reciprocal cross-neutralization with the 7S antibodies, whereas neutralization by the 19S antibodies was limited essentially to the immunizing virus (3). Recent studies in this laboratory have been concerned with determining antigenic relationships between cytomegalovirus (CMV) strains using late immune rabbit sera (4). Evidence is presented here that the properties of late 7S and 19S rabbit antibodies prepared against simian CMV strains are similar to those reported previously for late 7S and 19S anti-HSV and anti-SA8 antibodies with respect to their specificity for the immunizing virus. Three simian CMV strains were employed. Strain SA6 was isolated from a naturally infected vervet monkey (5), and strains CV-207 and CV-209 were isolated from African green monkeys which had been inoculated with human CMV (6). Virus strains were classified as CMV using previously reported procedures (4). Each virus strain was further classified as being of simian rather than human origin based upon the criterion that the former replicate in both human and simian cells, while replication of the latter is limited to human cells (7). The isolation of CMV with properties characteristic of simian strains from monkeys inoculated with a human strain has been reported by others (8). Virus working pools were prepared in human fibroblast (WI-38) cultures following plaque-purification (4). Appropriate tests for possible contamination of virus working pools by bacteria, fungi, or pleuropneumonia-like organisms (PPLO) were negative. Late immune sera were prepared in rabbits against each CMV strain (4), and the 7S and 19S antibodies were isolated by gel filtration using Sephadex G-200 (1). Antibody neutralization titers were determined by plaque reduction assays in WI-38 cells (4), using fresh guinea pig serum as a source of complement (C).