Role of IL-7 in Human Immunodeficiency Virus Infection

体内平衡细胞因子IL-7目前正在临床研究中，作为一种潜在的免疫重建剂，用于各种形式的免疫缺陷，包括HIV感染和癌症。我们之前已经证明，在没有任何伴随刺激的情况下，IL-7可以在CD4+和CD8+ T细胞中有效和选择性地诱导主要的肠归巢整合素a4b7的表达。我们发现这种效应：i)对T细胞是特异性的，并且在中亚群中更为明显；ii)经IL-7处理后迅速诱导；iii)需要超稳态的IL-7浓度（通常在淋巴细胞减少的情况下达到的浓度）；Iv)与经典细胞激活标记的表达解耦；v)与整合素的功能激活有关，其天然配体MAdCAM的结合活性增加。对IL-7诱导a4b7的分子机制的研究揭示了与IL-7受体刺激相关的两个主要信号通路，即JAK/STAT和PI3K/Akt通路。在体内也证实了IL-7对a4b7的诱导作用，包括hiv感染的人类受试者和用IL-7治疗的siv感染的猕猴。值得注意的是，我们发现IL-7对a4b7的诱导主要发生在表型幼稚的T细胞中，这伴随着获得记忆样表型，如CD95表达上调和TNF-a分泌；尽管CD45RA和CD45RO的表达没有改变，但在磷酯和离子霉素的刺激下，CD45RA和CD45RO的表达没有改变。T细胞的这种类似记忆的“伪装”与先前在体内从淋巴细胞减少症恢复的小鼠的初始T细胞中记录的相似。在没有任何伴随刺激的情况下，IL-7也可以诱导幼稚T细胞增殖，尽管与a4b7诱导相比，其动力学延迟。这些结果与宿主对淋巴细胞减少反应的新模型相一致，即IL-7的超稳态水平激活了幼稚T细胞中不寻常的表型调节程序，其特征是在诱导细胞循环和增殖之前获得肠道归巢和记忆样表型。肠道t细胞归巢在淋巴细胞减少宿主中耗尽t细胞库重建中的作用仍有待确定。为了正式证明IL-7诱导a4b7的生理相关性，我们进行了一项体内研究，在人源化NSG小鼠中注射了IL-7处理或未处理的自体T细胞。这些实验的结果清楚地证明了il -7处理的幼稚T细胞有优先的肠道归巢，而没有发现优先归巢到其他组织。这些现象在免疫重建过程中的生理学相关性目前正在研究中。

The homeostatic cytokine IL-7 is currently under clinical investigation as a potential immune-reconstitution agent in various forms of immunodeficiency, including HIV infection and cancer. We previously demonstrated that IL-7, in the absence of any concomitant stimulation, potently and selectively induces the expression of the principal gut-homing integrin, a4b7, in both CD4+ and CD8+ T cells. We found that this effect: i) is specific for T cells and more marked on the nave subpopulation; ii) is rapidly induced upon IL-7 treatment; iii) requires supra-homeostatic concentrations of IL-7 (those that are typically reached under conditions of lymphopenia); iv) is uncoupled from the expression of classic markers of cellular activation; and v) is associated with the functional activation of the integrin, as indicated by an increased binding activity for its natural ligand, MAdCAM. Investigation of the molecular mechanisms of a4b7 induction by IL-7 revealed the involvement of both major signaling pathways linked to stimulation of the IL-7 receptor, i.e., the JAK/STAT and PI3K/Akt pathways. Induction of a4b7 by IL-7 was also confirmed in vivo, both in HIV-infected human subjects and in SIV-infected macaques treated with IL-7. Of note, we found that induction of a4b7 by IL-7 occurs predominantly in phenotypically naive T cells, which concomitantly acquired a memory-like phenotype, as shown by upregulation of CD95 expression and secretion of TNF-a; upon stimulation with phorbol esters and ionomycin, despite an unaltered expression of CD45RA and CD45RO. This memory-like "masquerade" of T cells is similar to that previously documented in vivo in naive T cells of mice recovering from lymphopenia. Naive T cells were also induced to proliferate by IL-7, in the absence of any concomitant stimulation, albeit with delayed kinetics compared to a4b7 induction. These results are compatible with a new model of host response to lymphopenia whereby supra-homeostatic levels of IL-7 activate an unusual program of phenotypic modulation in naive T cells, characterized by the acquisition of a gut-homing and memory-like phenotype prior to the induction of cell cycling and proliferation. The role of intestinal T-cell homing in the reconstitution of the depleted T-cell pool in lymphopenic hosts remains to be defined. To formally demonstrate the physiological relevance of a4b7 induction by IL-7, we performed an in vivo study in which humanized NSG mice were injected with autologous T cells treated or not with IL-7. The results of these experiments clearly documented a preferential intestinal homing of IL-7-treated naive T cells, while no preferential homing to other tissues was detected. The physiological relevance of these phenomena in the processes of immunologic reconstitution is currently under investigation. We previously demonstrated that IL-7 treatment during the acute phase of SIV infection protected macaques from the dramatic loss of circulating naive and memory CD4(+) T cells that typically occurs upon SIV infection. This effect provides a rationale for the clinical evaluation of IL-7 in patients with acute HIV-1 infection. However, in contrast to the beneficial effects that we documented in acute SIV infection, subsequent studies an opposite effect of IL-7, which may paradoxically foster the progression of HIV-1 disease toward full-blown AIDS. Because endogenous levels of IL-7 naturally increase to supra-homeostatic concentrations in response to lymphopenia during the progression of HIV-1 disease, we hypothesized that the ability of IL-7 to redirect naive T cells to the intestinal compartment could occur in vivo in individuals with progressive HIV-1 disease. To investigate these phenomena and their relevance to AIDS in an in vivo model, we designed a new study in which 6 macaques chronically infected with either SIVmac251 or SIVsmE543 received a single injection of IL-7 (50 ug/kg, s.c.) and were sacrificed seven days later in order to specifically investigate the effects of IL-7 on T-cell homing and SIV replication in peripheral lymphoid tissues, particularly the GALT; three chronically SIV-infected animals received placebo and served as untreated controls. Detailed phenotypic analysis of circulating T cells documented a rapid upregulation of a4b7 in both CD4+ and CD8+ T cells. Comparison of pre-treatment and post-treatment intestinal tissues demonstrated that IL-7 administration resulted in increased numbers of infiltrating T cells within the GALT, associated with increased levels of SIV replication, predominantly in the Peyers patches. Enhanced SIV replication was also detected in lymph nodes. The increased levels of SIV replication in peripheral lymphoid tissues were mirrored by consistent increases in SIV plasma viremia. These in vivo data provide an initial validation of our hypothesis that the surge of endogenous IL-7 that occurs during the late stages of HIV infection may foster the terminal depletion of the CD4+ T-cell pool through the induction and activation of a4b7 leading to increased intestinal homing and HIV susceptibility. More recently, we also investigated whether exposure to HIV-1 can affect the expression of a4b7 in primary human T cells. Strikingly, we observed a marked upregulation of a4b7 on the surface of human T cells in the immediate aftermath of HIV-1 infection, which tends to decrease over time. The significance and mechanisms underlying this phenomenon are currently under investigation. Finally, we investigated the molecular mechanisms underlying the unique physiological effects of IL-7 on different T-cell populations, with particular emphasis on the ability of this cytokine to induce the expression of integrin a4b7 and the unconventional "cold" activation state in naive T cells. Thus, we performed extensive microarray analysis of mRNA transcripts from highly purified subpopulations of naive, memory a4b7high and memory a4b7low CD4+ T cells before and after stimulation with suprahomeostatic levels of IL-7. The results demonstrated upregulation of a panel of specific genes that regulate cytokine networks and cytokine receptor expression, which we are currently investigating, providing new mechanistic insights into the activity of IL-7. If these microarray results are validated by real-time PCR and siRNA knock-down, some of these IL-7-regulated genes may become suitable targets for new strategies of immunomodulation therapy.