Shikata M, Nakagawa M, Sano Y, Matsumoto T, Hashimoto Y
Department of Applied Biology, Faculty of Textile Science, Kyoto Institute of Technology, Japan.
Acta Virol. 1998 Nov;42(5):285-92.
A temperature-sensitive (ts) mutant of bombyx mori nucleopolyhedrovirus (BmNPV), ts-S1, contains a mutation in a putative RNA polymerase gene, which is involved in late viral gene expression. When 4th-instar silkworm larvae were infected with 1.0 x 10(5) TCID50 of ts-S1 per larva and reared at 33 degrees C, the titre of budded virus (BV) and number of occlusion bodies (OBs) in the haemolymph of the infected larvae were very low in the early stage but markedly increased in the late stage of infection. In contrast, a rapid increase in both BV titre and OB number was detected in the haemolymph of infected larvae reared at 25 degrees C. LD50 values of ts-S1 and wild type BmNPV (wtBmNPV) for 4th-instar larvae were 2.41 and 0.96 TCID50 per larva at 25 degrees C, and > 1.0 x 10(6) and 1.70 TCID50 per larva at 33 degrees C, respectively. These results indicate that the virulence of ts-S1 for the larvae reared at 33 degrees C was markedly reduced. To examine further the reduction of ts-S1 virulence at the non-permissive temperature of 33 degrees C, silkworm larvae were infected with ts-S1 at the multiplicity of 1.0 x 10(2) - 1.0 x 10(6) TCID50 per larva, reared for various time at 33 degrees C and then shifted to 25 degrees C. Longer rearing periods at 33 degrees C resulted in better survival rates indicating that the reduction of virulence of ts-S1 was proportional to cumulative rearing time at 33 degrees C. When large virus inocula were used, a growth alteration of larvae was preferentially induced. However, when small virus inocula were used, the appearance of abortive infection due to the non-permissive temperature became more evident.
家蚕核型多角体病毒（BmNPV）的温度敏感（ts）突变体ts-S1，在一个推定的RNA聚合酶基因中存在突变，该基因参与病毒晚期基因表达。当用每头幼虫1.0×10⁵组织培养感染剂量50（TCID50）的ts-S1感染四龄家蚕幼虫，并在33℃饲养时，感染幼虫血淋巴中出芽病毒（BV）的滴度和多角体（OB）数量在感染早期非常低，但在感染后期显著增加。相比之下，在25℃饲养的感染幼虫血淋巴中，BV滴度和OB数量均迅速增加。ts-S1和野生型BmNPV（wtBmNPV）对四龄幼虫的半数致死剂量（LD50）在25℃时分别为每头幼虫2.41和0.96 TCID50，在33℃时分别大于每头幼虫1.0×10⁶和1.70 TCID50。这些结果表明，ts-S1对在33℃饲养的幼虫的毒力显著降低。为了进一步研究ts-S1在33℃非允许温度下毒力的降低情况，用每头幼虫1.0×10² - 1.0×10⁶ TCID50的ts-S1感染家蚕幼虫，在33℃饲养不同时间，然后转移到25℃。在33℃饲养时间越长，存活率越高，表明ts-S1毒力的降低与在33℃的累积饲养时间成正比。当使用大量病毒接种物时，优先诱导幼虫生长改变。然而，当使用少量病毒接种物时，由于非允许温度导致的流产感染现象变得更加明显。
