To study the mutants of a highly pathogenic porcine reproductive and respiratory syndrome virus (PRRSV), HPPRRSV-JXA1 strain was continuously passed in Marc-145 cell cultures with or without anti-JXA1 pig sera for 30 passages and named JXAl-Abf30 and JXAl-f30 respectively. Infected cells were collected for extraction of viral RNA every 10 passages, ORF3 and ORF5 genes were amplified,cloned and sequenced. Sequence analysis indicated that numbers of nucleotide mutations on ORF3 and ORF5 genes in JXAl-Abf30 and JXAl-f30 strains were 18 and 9, numbers of amino acids mutations were 15 and 3 respectively. The ratio of nucleotide mutative speed between the two strains was 186%. The ratio of amino acid mutative speed of two strains was 500%, non-synonymous VS synonymous mutation ratios (NS/S) of ORF3 and ORF5 genes in two strains were 5.0 and 0. 5. Besides,there were 4 stable NS mutations in ORF5 gene and two of them (related to AA31 and AA190) were related to the known epitopes during passages with anti-PRRSV sera, which were not detected in passages without specific antibody. It indicated that mutations in PRRSV under antibody immune selection pressure were significantly faster than PRRSV without antibody immune selection pressure, anti-PRRSV sera had strong immune selective pressures on viral e-volution in their ORF5 genes, but for ORF3 genes, mutation influence was less than ORF5 genes.%本试验旨在对猪繁殖与呼吸综合征病毒(PRRSV)在抗体免疫选择压力下分子变异情况进行研究.将HPPRRSVJXA1株在添加适量抗体或不添加抗体的Marc-145细胞上连续传代各30代,获得毒株JXA1-Abf30和JXA1-f30.对传代前后毒株的ORF3和OR F5基因进行扩增、克隆和序列分析,研究结果显示,JXA1-Abf30和JXA1 f30在ORF3和ORF5基因上核苷酸发生突变位点数累计分别为18和9个,氨基酸发生突变位点数累计分别为15和3个.JXA1-Abf30和JXA1-f30的核苷酸突变速率比为186％,氨基酸突变速率比为500％,非同义突变(NS)与同义突变(S)比值(NS/S)分别为5.0和0.5.此外,抗体压力下传代的毒株ORF5基因有4个碱基位点发生稳定的非同义突变,其中2个(AA31和AA190)与已知抗原表位有关.由研究结果可知,抗体压力下传代的PRRSV的突变速率和非同义突变数目明显高于无抗体压力下的传代毒株；抗体免疫选择压能显著影响PRRSV的ORF5基因变异,并呈现明显的压力选择性突变,对ORF3基因变异的影响小于ORF5基因.
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