IL-7 is currently under clinical investigation as a potential immune-reconstitution agent for various forms of immuno-deficiency, including HIV infection and cancer. However, its effects have never been evaluated during acute HIV-1 (or SIV) infection, a critical phase of the disease in which the most dramatic depletion of CD4(+) T cells is believed to occur. To investigate the effects of IL-7 in an in vivo model of HIV-1 infection, we performed a study of IL-7 treatment during the acute phase of SIV infection in a well-established nonhuman primate model of AIDS. Our study included 12 juvenile rhesus macaques divided into two groups: Group 1 (n=6) was infected with a pathogenic SIV isolate, SIVmac251, and received weekly injections of placebo; group 2 (n=6) was infected with SIV and concomitantly treated with fully glycosylated recombinant macaque IL-7 s.c., once per week for 7 weeks, at a dose of 50 ug/kg. They received no antiretroviral treatment. Multiple clinical, immunological and virological parameters were monitored in all study animals throughout the acute phase of SIV infection. Treatment with IL-7 did not cause clinically detectable side effects and, despite the absence of concomitant ART, did not induce significant increases in the levels of SIV replication except at the earliest time point tested (day 4 post-infection). Strikingly, animals treated with IL-7 were protected from the dramatic decline of circulating nave and memory CD4(+) T cells that occurred in untreated animals. Treatment with IL-7 induced only transient T-cell proliferation, but it was associated with sustained increase in the expression of the anti-apoptotic protein Bcl-2 on both CD4(+) and CD8(+) T cells, persistent expansion of all circulating CD8(+) T-cell subsets, and development of earlier and stronger SIV Tat-specific T-cell responses. However, the beneficial effects of IL-7 were not sustained after treatment interruption. These data demonstrate that IL-7 administration is effective in protecting the CD4(+) T-cell pool during the acute phase of SIV infection in macaques, providing a rationale for the clinical evaluation of this cytokine in patients with acute HIV-1 infection. Next, we focused our attention on the role of IL-7 in T-cell peripheral homing. Increasing attention is being focused on the gut-associated lymphoid tissue (GALT) as a major target tissue for HIV infection, where critical events for pathogenesis take place. Specifically, the GALT is a primary anatomical site for virus 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 peripheral T-cell homing, we investigated the ex vivo effects of exogenous IL-7 on the expression of a large panel of tissue-homing integrins and chemokine receptors. We observed 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; 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 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 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 nave T cells, which concomitantly acquire 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 is similar to that previously documented in vivo in nave T cells of mice recovering from lymphopenia. Nave 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 nave 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. During the course of HIV-1 infection, the endogenous levels of IL-7 naturally increase to supra-homeostatic concentrations in response to lymphopenia during the progression of the disease. Thus, 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.