The gut-associated lymphoid tissue (GALT) is a primary anatomical site for HIV-1 replication, particularly during the early stages of HIV infection, leading to extensive depletion of CD4+ T cells. Since IL-7 plays an important role in T-cell homeostasis and is a potent regulatory factor for intestinal mucosal lymphocytes, we investigated the effects of IL-7 on the expression of a large panel of tissue-homing integrins and chemokine receptors. We found that IL-7, in the absence of concomitant stimulations, potently and selectively induces the expression of alpha4beta7, the principal gut-homing integrin, in both CD4+ and CD8+ T cells. This effect: i) is specific for T cells; ii) is rapidly induced upon IL-7 treatment; iii) requires supra-homeostatic concentrations of IL-7 (those 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 alpha4beta7 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 alpha4beta7 by IL-7 was also confirmed in vivo, both in HIV-infected subjects and in SIV-infected macaques treated with IL-7. Of note, we found that induction of alpha4beta7 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 alpha4beta7 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. 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 gut homing of IL-7-treated naive T cells, while no preferential homing to other tissues was detected. 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 treatment during the acute phase of SIV infection protected macaques from the dramatic loss of circulating naive and memory CD4+ T cells induced by SIV infection. More recently, however, we demonstrated that IL-7 may have opposite effects during chronic SIV infection, paradoxically fostering the progression toward full-blown AIDS. At the basis of these studies was our observationBecause 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 7 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. To elucidate the molecular mechanisms underlying the diverse biological effects of IL-7, particularly its ability to induce α4β7, we performed an extensive mRNA and microRNA (miRNA) expression profiling both in vitro and in vivo. In vitro, we analyzed FACS-sorted populations of nave, memory α4β7high and memory α4β7low primary T cells derived from healthy blood and stimulated ex vivo with suprahomeostatic concentrations of IL-7. A large number of differentially expressed genes were common to the 3 subpopulations, while distinct groups of genes were selectively expressed in individual subpopulations. Shared genes include several regulators of apoptosis (eg, Bcl-2, Mib-1) as well as two regulators of cytokine signaling, SOCS2 and CISH, not previously linked to IL-7. Detailed analysis of a group of 90 genes selectively expressed by nave and α4β7high memory T cells (which selectively upregulate α4β7 in response to IL-7) identified 5 specific pathways activated by IL-7, primarily controlled by a specific isoform of CISH acting as a master regulator. Notably, neither α4 nor β7 mRNAs were upregulated, suggesting that their rapid induction by IL-7 occurs primarily at the post-transcriptional level. Analysis of miRNA expression profiles also identified miRNAs specifically upregulated in nave and α4β7high memory T cells. Strikingly, the most upregulated genes identified in human cells treated with IL-7 ex vivo, most notably SOCS2 and CISH, were also strongly upregulated in vivo in IL-7-inoculated macaques. These results start to delineate the molecular basis for the unique biological effects of IL-7 and provide new leads for elucidating the molecular mechanisms of α4β7 induction by IL-7.
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