Cellular Receptors in HIV Infection/Acute Phase Response

HIV-1包膜蛋白的两种组分gp 120和gp 41被发现有效地抑制人单核细胞上的趋化因子受体的表达和功能，包括趋化因子受体以及细菌趋化肽fMLF。HIV-1包膜蛋白的抑制作用不是由于与其他化学引诱物直接竞争受体，而是通过涉及蛋白激酶C激活的“异源脱敏”机制。Gp 120还抑制人CD 4 + T淋巴细胞中趋化因子受体CXCR 4的表达和功能，这需要酪氨酸激酶（p56 lck）激活介导的信号事件。我们进一步研究了人白细胞与HIV-1包膜蛋白中在HIV-1融合中重要的肽结构域的相互作用，发现人吞噬细胞上的两个7跨膜受体FPR和FPRL 1被一些HIV-1包膜结构域激活。这些受体最初被鉴定为趋化性N-甲酰化肽的活化位点，这些肽是合成的或由某些细菌和细胞质线粒体产生的。这些观察结果表明，虽然HIV-1包膜蛋白在介导病毒入侵中是至关重要的，但它们的结构域也可能引起宿主免疫应答，可能导致宿主先天免疫应答的激活。然而，免疫细胞的有效和长期激活也可能导致细胞对其他刺激物的无反应性，这是“脱敏”的结果，这可能有利于艾滋病相关的机会性感染。 由于趋化因子受体CCR 5和CXCR 4是HIV-1感染人类细胞的关键共受体，我们研究了利用受体脱敏作为开发新型抗HIV-1药物的手段的可能性。我们已经发现，细菌fMLF激活FPR后，由蛋白激酶C介导的CCR 5的快速丝氨酸磷酸化，与CCR 5的衰减有关，包括其作为HIV-1辅助受体的能力。我们还鉴定了一种随机合成的肽，W肽，其激活FPR和FPRL 1并抑制HIV-1感染受体转染的细胞系以及人外周血单核细胞。W肽在其序列中含有D-氨基酸，因此可以抵抗酶降解，并且更适合于体内施用以测试其抗HIV-1功效。 血清淀粉样蛋白A（SAA）是一种急性期蛋白，通常以0.1 mM水平存在于血清中，但在全身性炎症条件下增加1000倍。血清SAA升高的慢性炎性病症可最终导致淀粉样变性，其特征在于“淀粉样”原纤维在组织中的沉积并与器官功能的进行性破坏相关。我们发现重组人（rh）SAA在体外对人白细胞表现出相当大的化学吸引活性，并诱导这些细胞在小鼠注射部位的浸润，这表明SAA在局部存在时可能发挥促炎症作用。为了确定SAA的细胞受体，我们仔细评估了SAA诱导的细胞信号传导模式和其他趋化因子的交叉脱敏作用。我们发现，高浓度的细菌衍生的趋化肽fMLP能够抑制细胞对随后的SAA刺激的反应，这表明SAA可能使用fMLP具有低亲和力的白细胞受体。通过使用细胞系工程表达基因编码的各种七跨膜，G-蛋白偶联受体，我们确定了一个受体FPRL 1特异性结合和激活SAA。SAA所用受体的鉴定将为进一步研究SAA在急性时相反应和组织淀粉样变中的作用奠定基础。 由于SAA是一种与慢性炎症相关的淀粉样蛋白，我们研究了其他淀粉样蛋白，如Ab 42，可能使用类似的受体进行细胞相互作用的可能性。Ab 42是B淀粉样肽的42个氨基酸形式，并且是阿尔茨海默病（AD）的关键致病因子。除了报道的对神经细胞的直接毒性作用之外，已经发现Ab 42激活脑中的单核吞噬细胞，这导致炎症和神经毒性介质的释放。我们已经证明FPRL 1被Ab 42用于诱导单核吞噬细胞的迁移和活化。此外，我们已经检测到高水平的表达FPRL 1基因的CD 11b阳性吞噬细胞周围和浸润的病变在AD患者的脑组织。这些结果表明，这种受体在AD病变的促炎方面的积极参与。有趣的是，我们小组最近的研究也揭示了FPRL 1是人类朊病毒蛋白肽片段的受体，其同种型导致“库鲁”或“疯牛病”。肽片段Prp 10 -126在体外形成聚集体并激活单核吞噬细胞释放对神经元有毒的介质。正在进行研究以阐明FPRL 1在神经退行性疾病发病机制中的作用及其作为治疗靶点的潜力。

Two components of the HIV-1 envelope protein, gp120 and gp41, were found to potently inhibit the expression and function of chemoattractant receptors on human monocytes, including the receptors for chemokines as well as for bacterial chemotactic peptide fMLF. The inhibitory effects of HIV-1 envelope proteins was not due to a direct competition for the receptors with other chemoattractants, but rather, through a "heterologous desensitization" mechanism involving the activation of protein kinase C. Gp120 also inhibited the expression and function of chemokine receptor CXCR4 in human CD4+ T lymphocytes, and this required signaling events mediated by tyrosine kinase(p56lck) activation. We further investigated the interaction of human leukocytes with peptide domains in HIV-1 envelope proteins that are important in HIV-1 fusion and found that two 7 transmembrane receptors on human phagocytes, FPR and FPRL1, are activated by some HIV-1 envelope domains. These receptors were originally identified as activation sites for chemotactic N-formylated peptides, either synthetic or produced by certain bacteria and cytoplasmic mitochondria. These observations suggest that while HIV-1 envelope proteins are crucial in mediating viral invasion, their domains may also elicit host immune responses possibly resulting in activation of host innate immune responses. However, a potent and prolonged activation of immune cells may alsoculminate in unresponsiveness of the cells to other stimulants, a consequence of "desensitization" which could be beneficial for AIDS-related opportunistic infection. Since the chemokine receptors CCR5 and CXCR4 are crucial co-receptors of HIV-1 infection of human cells, we investigated the possibility to utilize receptor desensitization as a means of develop novel anti-HIV-1 agents. We have revealed that activation of FPR by the bacterial fMLF was followed by a rapid serine phosphorylation of CCR5 mediated by protein kinase C, in association with the attenuation of CCR5 including its capacity to act as an HIV-1 coreceptor. We also identified a random synthetic peptide, W peptide, that activates both FPR and FPRL1 and inhibits HIV-1 infection of receptor transfected cell lines as well as human peripheral blood mononuclear cells. W peptide contains a D-amino acid in its sequence and thus may be resistant to enzymatic degradation and more suitable for in vivo administration to test its anti-HIV-1 efficacy. Serum amyloid A (SAA), an acute phase protein, is normally present in serum at 0.1 mM levels, but increases by 1000 fold in systemic inflammatory conditions. Chronic inflammatory conditions with elevated serum SAA may culminate in amyloidosis, characterized by deposition of "amyloid" fibrils in tissues and associated with progressive destruction of organ function. We found that recombinant human (rh) SAA exhibited considerable chemoattractant activity for human leukocytes in vitro and induced infiltration of these cells at injection sites in mice, suggesting that SAA, when present locally, may play a proinflammatory role. In an effort to identify the cell receptor for SAA, we carefully evaluated the pattern of cell signaling induced by SAA and cross-desensitization by other chemotactic factors. We found that the bacterial derived chemotactic peptide fMLP at high concentrations was able to inhibit the cell response to subsequent stimulation with SAA, suggesting that SAA might use a leukocyte receptor for which fMLP has low affinity. By using cell lines engineered to express genes encoding various seven-transmembrane, G-protein coupled receptors, we identified a receptor termed FPRL1 to be specifically bound and activated by SAA. The identification of the receptor used by SAA will greatly facilitate the further research on the role of SAA in acute phase responses and tissue amyloidosis. Since SAA is an amyloidogenic protein associated with chronic inflammation, we examined the possibility that other amyloidogenic proteins, such as Ab42, may use similar receptors for cell interaction. Ab42 is a 42 amino acid form of the b amyloid peptide and is a key causative factor of Alzheimer's disease (AD). In addition to its reported direct toxic effect on nerve cells, Ab42 has been found to activate mononuclear phagocytic cells in the brain which results in inflammation and release of neurotoxic mediators. We have demonstrated FPRL1 to be used by Ab42 to induce migration and activation of mononuclear phagocytes. In addition, we have detected a high level expression of the FPRL1 gene in CD11b positive phagocytic cells surrounding and infiltrating the lesions in the brain tissues of AD patients. These results suggest an active involvement of this receptor in the proinflammatory aspects of AD lesions. Interestingly, recent research by our group also revealed FPRL1 to be a receptor for a peptide fragment of the human prion protein, which, with its isoforms, causes "Kuru" or "Mad Cow Disease". The peptide fragment Prp10-126 forms aggregates and activates mononuclear phagocytes to release mediators toxic to neurons in vitro. Studies are being conducted to elucidate the role of FPRL1 in the pathogenesis of neurodegenerative diseases and its potential as a therapeutic target.