Department of Microbiology, Immunology, and Pathology, College of Veterinary Medicine and Biomedical Sciences, Colorado State University, Fort Collins, CO, USA.
Department of Molecular Diagnostics of Oncogenic Infections, Research Program Infection, Inflammation and Cancer, German Cancer Research Center, (Deutsches Krebsforschungszentrum Heidelberg, DKFZ), Im Neuenheimer Feld 242, 69120, Heidelberg, Germany.
Retrovirology. 2018 May 16;15(1):38. doi: 10.1186/s12977-018-0419-0.
Hosts are able to restrict viral replication to contain virus spread before adaptive immunity is fully initiated. Many viruses have acquired genes directly counteracting intrinsic restriction mechanisms. This phenomenon has led to a co-evolutionary signature for both the virus and host which often provides a barrier against interspecies transmission events. Through different mechanisms of action, but with similar consequences, spumaviral feline foamy virus (FFV) Bet and lentiviral feline immunodeficiency virus (FIV) Vif counteract feline APOBEC3 (feA3) restriction factors that lead to hypermutation and degradation of retroviral DNA genomes. Here we examine the capacity of vif to substitute for bet function in a chimeric FFV to assess the transferability of anti-feA3 factors to allow viral replication.
We show that vif can replace bet to yield replication-competent chimeric foamy viruses. An in vitro selection screen revealed that an engineered Bet-Vif fusion protein yields suboptimal protection against feA3. After multiple passages through feA3-expressing cells, however, variants with optimized replication competence emerged. In these variants, Vif was expressed independently from an N-terminal Bet moiety and was stably maintained. Experimental infection of immunocompetent domestic cats with one of the functional chimeras resulted in seroconversion against the FFV backbone and the heterologous FIV Vif protein, but virus could not be detected unambiguously by PCR. Inoculation with chimeric virus followed by wild-type FFV revealed that repeated administration of FVs allowed superinfections with enhanced antiviral antibody production and detection of low level viral genomes, indicating that chimeric virus did not induce protective immunity against wild-type FFV.
Unrelated viral antagonists of feA3 cellular restriction factors can be exchanged in FFV, resulting in replication competence in vitro that was attenuated in vivo. Bet therefore may have additional functions other than A3 antagonism that are essential for successful in vivo replication. Immune reactivity was mounted against the heterologous Vif protein. We conclude that Vif-expressing FV vaccine vectors may be an attractive tool to prevent or modulate lentivirus infections with the potential option to induce immunity against additional lentivirus antigens.
宿主能够限制病毒复制，以在适应性免疫完全启动之前控制病毒传播。许多病毒已经获得了直接对抗固有限制机制的基因。这种现象导致了病毒和宿主的共同进化特征，这往往是阻止种间传播事件的一个障碍。通过不同的作用机制，但产生相似的结果，泡沫病毒猫泡沫病毒（FFV）Bet 和慢病毒猫免疫缺陷病毒（FIV）Vif 对抗猫 APOBEC3（feA3）限制因子，导致逆转录病毒 DNA 基因组的高突变和降解。在这里，我们研究了 vif 取代 FFV 嵌合病毒中 bet 功能的能力，以评估将抗 feA3 因子转移以允许病毒复制的能力。
我们表明，vif 可以取代 bet 产生复制能力的嵌合泡沫病毒。体外选择筛选显示，工程化的 Bet-Vif 融合蛋白对 feA3 的保护作用不佳。然而，在经过 feA3 表达细胞的多次传代后，出现了具有优化复制能力的变体。在这些变体中，Vif 独立于 N 端 Bet 部分表达，并稳定保留。用其中一种功能性嵌合体对免疫功能正常的家猫进行实验性感染导致针对 FFV 骨架和异源 FIV Vif 蛋白的血清转化，但通过 PCR 不能明确检测到病毒。接种嵌合病毒后再接种野生型 FFV 表明，FV 的重复给药允许超感染，同时产生增强的抗病毒抗体产生和低水平病毒基因组的检测，表明嵌合病毒不能诱导针对野生型 FFV 的保护性免疫。
与 feA3 细胞限制因子无关的病毒拮抗剂可以在 FFV 中交换，导致体外复制能力，但在体内减弱。因此，Bet 可能具有除了对抗 A3 之外的其他功能，这些功能对体内成功复制是必不可少的。针对异源 Vif 蛋白产生免疫反应。我们得出结论，表达 Vif 的 FV 疫苗载体可能是预防或调节慢病毒感染的一种有吸引力的工具，具有诱导针对其他慢病毒抗原的免疫的潜在选择。
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