REGULATION OF CYTOKINE EXPRESSION BY HIV

HIV-1在全身性病毒感染的早期感染大脑，但HIV相关的认知/运动复合体在病程后期发展，提示脑细胞介导的宿主反应可能抑制大脑早期生产性感染和神经系统疾病的发展。巨噬细胞在HIV感染的发病机制中起着关键作用，既是病毒复制的目标，也是多功能细胞因子的来源。我们先前的研究表明，单核细胞来源的ET-1在HIV感染过程中受到刺激，并提示其强大的血管活性特性可能导致与艾滋病痴呆综合征相关的脑血流模式的改变(JI 1993)。然而，我们对单核细胞来源的巨噬细胞(MDM)的研究表明，在体外，HIV-1感染既不能诱导ET-1的分泌，也不能通过已知的诱导剂LPS增强其产生。此外，在HIV感染的MDM制备的mRNA中，RT-PCR没有检测到ET-1和一氧化氮合酶(NOS)基因的表达，一氧化氮合酶是一种负责产生强大的血管扩张剂NO的酶。因此，只关注MDM的体外系统很可能无法反映体内发生的更复杂的细胞相互作用。由于脑内产生HIV-1的细胞，如血源性巨噬细胞和脑小胶质细胞，被星形胶质细胞包围，我们研究了星形胶质细胞对MDM中HIV-1表达的影响。HIV感染的MDM与原代人类星形胶质细胞共培养后，病毒复制减少。由于星形胶质细胞条件培养液可以抑制HIV复制MDM，而多聚甲醛固定的星形胶质细胞不能分泌细胞因子，因此这种作用部分是由一种未知的分泌因子介导的，因为星形胶质细胞条件培养液可以抑制HIV复制。相反，星形胶质细胞与细胞间的接触增加了MDM中HIV-1的复制，这与M-CSF的产生增加有关(AIDS，1999)。进一步的研究表明，当星形胶质细胞与HIV感染的MDM共同培养时，星形胶质细胞表达诱导型一氧化氮合酶(INOS)和产生NO，而不是MDM。这与艾滋病毒复制减少不谋而合。HIV感染的MDM和星形胶质细胞共培养上清液可刺激星形胶质细胞iNOS表达，但未检测到诱导iNOS表达的细胞因子，提示可能有一种新的因子介导了这一作用。竞争性一氧化氮合酶抑制剂部分逆转了星形胶质细胞的HIV-1抑制效应(血液1999)，这与星形胶质细胞介导的HIV抑制的双重机制一致。此外，利用具有可变半衰期和生产通量的NO供体化合物的研究表明，NO通过一种最可能涉及NFkB的机制直接抑制HIV-1复制。这表明星形胶质细胞通过产生HIV调节细胞因子和iNOS/NO的表达，在决定神经系统HIV疾病的进程中发挥关键作用。这也让我们推测，在HIV疾病中观察到的神经损伤可能是星形胶质细胞长期、高水平产生NO所致，这可能反映了宿主试图抑制病毒复制。

HIV-1 infects the brain in the early stage of systemic viral infection, but HIV-associated cognitive/motor complex develops later in the course of disease, suggesting that host responses mediated by brain cells may inhibit the early productive infection in the brain and the development of neurologic disease. Macrophages play a critical role in the pathogenesis of HIV infection, both as targets for viral replication and as a source of multifunctional cytokines. Our previous studies revealed that monocyte-derived ET-1 is stimulated during HIV infection and suggested that its potent vasoactive properties could cause alterations in the cerebral perfusion pattern associated with AIDS dementia complex (JI 1993). However, our studies with monocyte derived macrophages (MDM) showed that HIV-1 infection neither induces secretion of ET-1 nor potentiates its production by the known inducer, LPS, in vitro. In addition, expression of the genes for ET-1 and nitric oxide synthase (NOS), an enzyme responsible for production of the potent vasodilator, NO, are not detected by RT-PCR in mRNA prepared from HIV-infected MDM. Thus, the in vitro system looking only at MDM most likely fails to reflect the more complex cellular interactions occurring in vivo. Since HIV-1 producing cells in the brain, such as blood-derived macrophages and brain microglia, are surrounded by astrocytes, we studied the effect of astrocytes on HIV-1 expression in MDM. Co-culture of HIV-infected MDM with primary human astrocytes resulted in diminished virus replication. This effect was mediated in part by an unidentified secreted factor, since astrocyte conditioned medium could suppress HIV replication MDM and paraformaldehyde fixed astrocytes unable to secrete cytokines failed to inhibit HIV. In contrast, cell-to-cell contact with astrocytes augmented HIV-1 replication in MDM, which correlated with enhanced production of M-CSF (AIDS 1999). Additional studies revealed the expression of inducible NOS (iNOS)and production of NO by astrocytes, but not MDM, when astrocytes were cocultured with HIV-infected MDM. This coincided with decreased HIV replication. Supernatants from cocultures of HIV-infected MDM and astrocytes stimulated iNOS expression in astrocytes, but cytokines known to induce iNOS expression were not detected, suggesting a novel factor may mediate this effect. A competitive NOS inhibitor partially reversed the HIV-1 suppressive effect of astrocytes (BLOOD 1999), consistent with a dual mechanism for astrocyte-mediated HIV inhibition. Furthermore, studies utilizing NO donor compounds with variable half lives and flux of production reveal that NO inhibits HIV-1 replication directly via a mechanism most likely involving NFkB. This suggests that astrocytes play a pivotal role in determining the course of neurologic HIV disease via production of HIV modulating cytokines and expression of iNOS/NO. It also leads us to speculate that neurologic damage observed in HIV disease may ensue from prolonged, high level production of NO by astrocytes, which may reflect a host attempt to inhibit virus replication.