Wildum Steffen, Schindler Michael, Münch Jan, Kirchhoff Frank
Department of Virology, University of Ulm, Albert-Einstein-Allee 11, 89081 Ulm, Germany.
J Virol. 2006 Aug;80(16):8047-59. doi: 10.1128/JVI.00252-06.
Human immunodeficiency virus type 1 (HIV-1) utilizes Vpu, Env, and Nef to down-modulate its primary CD4 receptor from the cell surface, and this function seems to be critical for the pathogenesis of AIDS. The physiological relevance of CD4 down-modulation, however, is currently not well understood. In the present study, we analyzed the kinetics of CD4 down-modulation and the susceptibility of HIV-1-infected T cells to superinfection using proviral HIV-1 constructs containing individual and combined defects in vpu, env, and nef and expressing red or green fluorescent proteins. T cells infected with HIV-1 mutants containing functional nef genes expressed low surface levels of CD4 from the first moment that viral gene expression became detectable. In comparison, Vpu and Env had only minor to moderate effects on CD4 during later stages of infection. Consistent with these quantitative differences, Nef inhibited superinfection more efficiently than Vpu and Env. Notably, nef alleles from AIDS patients were more effective in preventing superinfection than those derived from a nonprogressor of HIV-1 infection. Our data suggest that protection against X4-tropic HIV-1 superinfection involves both CD4-independent and CD4-dependent mechanisms of HIV-1 Nef. X4 was effectively down-regulated by simian immunodeficiency virus and HIV-2 but not by HIV-1 Nef proteins. Thus, maximal protection seems to involve an as-yet-unknown mechanism that is independent of CD4 or coreceptor down-modulation. Finally, we demonstrate that superinfected primary T cells show enhanced levels of apoptosis. Accordingly, one reason that HIV-1 inhibits CD4 surface expression and superinfection is to prevent premature cell death in order to expand the period of effective virus production.
1型人类免疫缺陷病毒（HIV-1）利用Vpu、Env和Nef从细胞表面下调其主要CD4受体，这一功能似乎对艾滋病发病机制至关重要。然而，目前对CD4下调的生理相关性尚不清楚。在本研究中，我们使用在vpu、env和nef中存在单个和联合缺陷并表达红色或绿色荧光蛋白的前病毒HIV-1构建体，分析了CD4下调的动力学以及HIV-1感染的T细胞对超感染的易感性。感染含有功能性nef基因的HIV-1突变体的T细胞，从病毒基因表达可检测到的那一刻起，就表达低水平的表面CD4。相比之下，Vpu和Env在感染后期对CD4只有轻微到中等程度的影响。与这些定量差异一致，Nef比Vpu和Env更有效地抑制超感染。值得注意的是，来自艾滋病患者的nef等位基因在预防超感染方面比来自HIV-1感染非进展者的更有效。我们的数据表明，针对X4嗜性HIV-1超感染的保护涉及HIV-1 Nef的CD4非依赖性和CD4依赖性机制。X4被猿猴免疫缺陷病毒和HIV-2有效下调，但不被HIV-1 Nef蛋白下调。因此，最大程度的保护似乎涉及一种尚未明确的机制，该机制独立于CD4或共受体下调。最后，我们证明超感染的原代T细胞显示出更高水平的细胞凋亡。因此，HIV-1抑制CD4表面表达和超感染的一个原因是防止细胞过早死亡，以延长有效病毒产生的时间。
