Characteristic in vitro evolution pattern of foot and mouth disease virus A81/Castellanos/Arg/87.

The in vitro evolution of Foot and mouth disease virus (FMDV) A/81/Castellanos/Arg/87 (A/Castellanos/87) was studied by partial biological and biochemical characterization of viral populations selected after 25 passages on secondary fetal bovine kidney cell monolayers. These passages were performed in the presence or absence of immune pressure exerted in the form of antiviral polyclonal serum. While the viral populations passaged in the absence of immune pressure acquired characteristics such as antigenic heterogeneity, VP1 amino acid modification and plaque size reduction, the populations selected after immune pressure also presented both neutralizing resistance and attenuation for suckling mice. The comparison with other previously studied FMDV strains suggests that FMDV A/Castellanos/87 adopts a differential response to immunological pressure and other selective forces. In addition, the sequencing analysis of viral selected populations shows a restriction in the number and type of amino acid replacements tolerated by FMDV capsid proteins.

Response of foot-and-mouth disease virus C3 Resende to immunological pressure exerted in vitro by antiviral polyclonal sera.

The foot-and-mouth disease virus (FMDV) shows a remarkable antigenic variability. Like other RNA viruses, FMDV has a high mutation rate and it has been proposed that selection exerted by antibodies of the host could play a major role in its evolution. In this work, antiserum-resistant variants of FMDV (Nr variants) were selected upon 25 serial passages of a cloned C3 Resende strain on secondary monolayers of fetal bovine kidney (FBK-2) cells in the presence of subneutralizing levels of antiviral polyclonal sera (APS). After serial passage under immune selective pressure, the five Nr variant populations selected from five independent serial passages--their controls remaining unmodified--acquired the following characteristics: (i) increased resistance to neutralization by APS; (ii) five different antigenic specificities detected by enzyme-linked and neutralization assays using monoclonal antibodies; (iii) the same modification (residue 146, S to L) at the major antigenic site of VP1 (G-H loop, the 135-160 region); and (iv) specific changes for each Nr population outside the major antigenic site of VP1 at residues 46, 48 and 49 of the 40-60 region of VP1 (B-C loop). These results extend our previous work on selection of Nr variants using polyclonal sera, and add new information with regard to antigenic variation, mainly concerning the involvement of the 40-60 region of VP1 in the process of immune selection.

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.