Further characterization of a morphogenetic mutant of the foot-and-mouth disease virus

Modification of foot-and-mouth disease virus O1 Caseros after serial passages in the presence of antiviral polyclonal sera

Foot-and-mouth disease virus (FMDV) shows a remarkable antigenic variability and, like other RNA viruses, presents a high rate of mutation. It has been proposed that selection exerted by antibodies of the host could play a major role in the rapid evolution of FMDV. The present work reports the selection of FMDV antibody-resistant (Nr) populations after serial passages of a cloned FMDV O1 Caseros strain on secondary monolayers of bovine kidney cells in the presence of subneutralizing antiviral polyclonal sera (APS). After a limited number of passages, i.e., 29, under selective pressure, the virus population showed the following characteristics: (i) increased resistance to neutralization by APS (Nr), (ii) altered electrophoretic mobility of its structural viral proteins (VP1), and (iii) alterations at the RNA nucleotide sequence that codes for the major antigenic site of VP1. These acquired characteristics were detected at passage 15 and remained unmodified throughout successive passages. These results document a rapid selection and fixation of specific mutations in response to immunological pressure. In addition, the findings that (i) mutations not related to APS selection were not detected and (ii) after 29 passages at a high multiplicity of infection without immunological pressure, the RNA sequence that codes for VP1 remained unmodified clearly demonstrated that FMDV O1 Caseros presents in vitro a remarkable unexpected genetic stability.

Modification of foot-and-mouth disease virus after serial passages in the presence of antiviral polyclonal sera

Foot-and-mouth disease virus (FMDV) shows a remarkable antigenic variability. Like other RNA viruses, this virus has a high rate of mutation. It has been proposed that selection exerted by the host's antibodies could play a major role in the rapid evolution of FMDV. The present work reports the selection of FMDV antibody-resistant populations (Nr), after serial passages of cloned FMDV A24 Cruzeiro strain on secondary monolayers of bovine fetal kidney cells in the presence of subneutralizing antiviral polyclonal sera (APS). After a limited number of passages under selective pressure, the virus population showed the following characteristics: (1) increased resistance to neutralization by APS; (2) altered electrophoretic mobility of structural viral proteins (VP1); (3) remarkable plaque size reduction, (4) a pronounced thermosensitivity (ts); and (5) decreased pathogenicity for mice, in both uncloned and cloned small plaque size populations. This indicates that FMDV populations under antibody pressure in vitro, have acquired, in addition to expected characteristics of natural FMDV variants (resistance to neutralization and altered viral structural proteins), phenotypic markers which correspond to attenuated, less virulent variants.

Further characterization of an RNA defective mutant of the foot-and-mouth disease virus

In this paper a further characterization of a foot-and-mouth disease virus (FMDV) temperature-sensitive mutant, ts 6, is described. This mutant presents a defective RNA synthesis at non-permissive temperature (NPT) by comparison to the wt capacity. However, a low level of viral RNA synthesis (below 10%) was sufficient to achieve an almost normal protein synthesis including a normal pattern of protein cleavage. In addition, morphogenetic precursor particles, 14S and 75S, are formed, indicating that the structural proteins VP0, VP1 and VP3 provide the necessary signal for self-assembly, and that the RNA is not necessary for such assemblage. Finally, although an almost normal synthesis of viral proteins and morphogenetic precursors (14S and 75S) occurs, the induction of cellular protein shut-off was not evident, indicating that this viral effect appears late in the viral cycle once complete viral replication has occurred.