Bovine Leukemia Virus-Induced Persistent Lymphocytosis in Cattle Does Not Correlate with Increased Ex Vivo Survival of B Lymphocytes

Bovine leukemia virus (BLV) is an oncogenic retrovirus associated with B-cell lymphocytosis, leukemia, and lymphosarcoma in the ovine and bovine species. We have recently reported that in sheep, BLV protects the total population of peripheral blood mononuclear cells (PBMCs) from ex vivo spontaneous apoptosis. This global decrease in the apoptosis rates resulted from both direct and indirect mechanisms which allow extension of cell survival. Although sheep are not natural hosts for BLV, these animals are prone to develop virus-induced leukemia at very high frequencies. Most infected cattle, however, remain clinically healthy. This difference in the susceptibilities to development of leukemia in these two species might be related to alterations of the apoptotic processes. Therefore, we designed this study to unravel the mechanisms of programmed cell death in cattle. We have observed that PBMCs from persistently lymphocytotic BLV-infected cows were more susceptible to spontaneous ex vivo apoptosis than cells from uninfected or aleukemic animals. These higher apoptosis rates were the consequence of an increased proportion of B cells exhibiting lower survival abilities. About one-third of the BLV-expressing cells did not survive the ex vivo culture conditions, demonstrating that viral expression is not strictly associated with cell survival in cattle. Surprisingly, culture supernatants from persistently lymphocytotic cows exhibited efficient antiapoptotic properties on both uninfected bovine and uninfected ovine cells. It thus appears that indirect inhibition of cell death can occur even in the presence of high apoptosis rates. Together, these results demonstrate that the protection against spontaneous apoptosis associated with BLV is different in cattle and in sheep. The higher levels of ex vivo apoptosis occurring in cattle might indicate a decreased susceptibility to development of leukemia in vivo.

牛白血病病毒（Bovine leukemia virus, BLV）是一种致癌性逆转录病毒，与绵羊和牛的B细胞淋巴细胞增多症、白血病及淋巴肉瘤相关。我们近期曾报道，在绵羊体内，BLV可保护外周血单个核细胞（peripheral blood mononuclear cells, PBMCs）免受体外自发性细胞凋亡的影响。这种细胞凋亡率的整体下降，源于能够延长细胞存活时间的直接与间接双重机制。尽管绵羊并非BLV的自然宿主，但该物种极易以极高的发病率出现病毒诱导型白血病。然而，大多数受感染的奶牛仍保持临床健康状态。这两个物种在白血病易感性上的差异，可能与细胞凋亡过程的改变有关。
因此，本研究旨在阐明牛体内的程序性细胞死亡机制。我们观察到，与未感染或无白血病病变的动物相比，持续淋巴细胞增多的BLV感染奶牛的PBMCs对体外自发性细胞凋亡的易感性更高。这种更高的细胞凋亡率，是B细胞比例升高且存活能力下降所导致的结果。约三分之一表达BLV的细胞无法在体外培养条件下存活，这表明在牛体内，病毒表达并非严格与细胞存活挂钩。
令人意外的是，持续淋巴细胞增多的奶牛的细胞培养上清液，对未感染的牛源和绵羊源细胞均展现出有效的抗凋亡特性。由此可见，即便细胞凋亡率较高，间接抑制细胞死亡的过程仍可发生。综上，本研究结果表明，BLV相关的自发性细胞凋亡防护作用在牛与绵羊体内存在差异。奶牛体内更高的体外细胞凋亡水平，或许提示其在体内对白血病的易感性有所降低。