Abstract To maintain the size of T cell pools throughout life, homeostatic mechanisms regulate both survival and proliferation through IL-7 and TCR signaling. IL-7 (through lymphopenia-induced proliferation/LIP) can also mediate expansion of autoreactive clones leading to the onset of autoimmune disease or enhance effector function against tumors and viruses. In the present study, we hypothesized that in a setting of chronic inflammatory environment, such as HIV infection, LIP could contribute to the immune activation leading to CD4 T cell depletion and CD8 T cell expansion. Type-I IFN is an essential cytokine in host defense during viral infection. In HIV infected patients, the genes associated with γc cytokines and Type-I IFN signaling were differentially expressed in CD4 and CD8 T cells. Using a lymphopenic murine model, we present evidence that, in vivo, IL-7 differentially regulated the expression of the total-Signal Transducer and Activator of Transcription 1 (t-STAT1) in CD4 and CD8 T cells undergoing LIP and in so doing enhanced CD4 T cell responsiveness to Type-I IFN. In this setting, chronic treatment with IFN-α led to decreased CD4 T cell counts and CD8 T cell expansion. This interplay between IL-7 and Type-I IFN might be advantageous in immunity against pathogens, however chronic stimulation of this pathway could be deleterious for CD4 T cell homeostasis and may contribute to the aberrant immune activation and eventual CD4 T cell depletion observed during HIV infection.
Abstract After acute viral infection a small population of effector CD8 T cells upregulate CD127 expression and develop into long lived resting memory T cells. During chronic infections, such as HIV, there is an accumulation of CD8 T cells with effector/memory-like phenotype. These observations led to the hypothesis that there is a developmental impairment in the long lived resting memory T cells within that setting. The transcription factors RUNX3 and Eomesodermin (EOMES) are important in the development of long lived memory CD8 T cells. We studied the dynamics of CD127 expression, RUNX3 and EOMES in patients with HIV infection and different levels of viremia. Consistent with other reports, CD127 expression was decreased in the memory CD8 T cell subset. The CD8 memory CD127low T cells showed increased expression of RUNX3 and EOMES mRNA, as well as effector molecules perforin and granzyme B. In patients with suppressed viremia, we could identify two populations of CD8 T cells expressing EOMEShigh and EOMESint. The EOMESint cells showed a short-lived effector phenotype (perforin+, granzyme B+, CD127low) and the EOMEShigh cells showed a memory-like phenotype (perforin-, CD127high). These results suggest that in human CD8 T cells, EOMES is necessary for the maintenance of long lived memory CD8 T cells and its expression was influenced by HIV-RNA levels and the associated inflammatory environment (R=0.4, p=0.01). These findings provide new insights into the pathogenesis of HIV infection.
Abstract HIV infection and its associated chronic immune activation alter T cell homeostasis leading to CD4 T cell depletion and CD8 T cell expansion. The mechanisms behind these outcomes are not totally defined and only partially explained by the direct cytopathic effect of the virus. We established a murine model of immunpoathogenesis in which adoptive T cell transfer into lymphopenic hosts chronically treated with IFN-α leads to CD4 T cell depletion and CD8 T cell expansion, recapitulating the alteration of the T cell pools observed in HIV infected patients. In these conditions, CD4 T cells undergoing IL-7/lymphopenia induced-proliferation upregulate their level of the Signal Transducer and Activator of Transcription 1 (STAT1) resulting in enhanced CD4 T cell responsiveness to IFN-α. In this setting, CD4 T cell depletion resulted from an indirect effect of IFN-α driven inflammatory environment, since animals with IFNAR deficient T cells underwent the same degree of CD4 T cell depletion than wild type animals. In contrast, direct IFN-α signaling through IFNAR on CD8 T cells was necessary for their expansion induced by chronic exposure to Type-I IFN under lymphopenic conditions. These findings suggest that direct and indirect effects of IFN-α signaling may play a role in the pathogenesis of HIV infection. Further analysis of this pathway may contribute to the development of new strategies to reverse the dysregulation of the T cell pools seen in patients with HIV infection.
Abstract HIV infection is associated with chronic immune activation and alteration of T cell homeostasis (CD4 T cell depletion and CD8 T cell expansion). While the acute CD4 depletion observed in the initial phase of HIV infection is likely due to direct cytopatic effects of the virus, the mechanism/s underlying the steady decline of the CD4 T cell pool during the chronic phase of infection are unclear and are felt to be associated with “immune activation”. In the current study, we addressed the impact of the combination of two distinct forces: homeostatic (CD4 T cell depletion) and inflammatory (HIV-driven IFN-α) on T cell homeostasis. We present evidences that IL-7 and lymphopenia enhanced CD4 T cell responsiveness to IFN-α by modulating expression of the Signal Transducers and Activators of Transcription (STAT) 1, 2 and 3. In a murine model, CD4 T cell depletion and CD8 T cell expansion were observed in a lymphopenic host chronically treated with IFN-α. These findings suggest that a synergistic interaction between lymphopenia and IFN-α may play a role in the pathogenesis of HIV infection. The analysis of this pathway may contribute to the development of new strategies to reverse the dysregulation of the T cell pools seen in patients with HIV infection.
The onset of autoimmunity in experimental rodent models and patients frequently correlates with a lymphopenic state. In this condition, the immune system has evolved compensatory homeostatic mechanisms that induce quiescent naive T cells to proliferate and differentiate into memory-like lymphocytes even in the apparent absence of antigenic stimulation. Because memory T cells have less stringent requirements for activation than naive cells, we hypothesized that autoreactive T cells that arrive to secondary lymphoid organs in a lymphopenic environment could differentiate and bypass the mechanisms of peripheral tolerance such as those mediated by self-antigen cross-presentation. Here, we show that lymphopenia-driven proliferation and differentiation of potentially autoreactive CD8(+) T cells into memory-like cells is not sufficient to induce self-reactivity against a pancreatic antigen. Induction of an organ-specific autoimmunity required antigen-specific CD4(+) T cell help. Notably, we found that this function could be accomplished by memory-like CD4(+) T cells generated in vivo through lymphopenia-induced proliferation. These helper cells promoted the further differentiation of memory-like CD8(+) T cells into effectors in response to antigen cross-presentation, resulting in their migration to the tissue of antigen expression where autoimmunity ensued. Thus, the cooperation of self-reactive memory-like CD4(+) and CD8(+) T cells under lymphopenic conditions overcomes cross-tolerance resulting in autoimmunity.
Lymphodepletion is currently used to enhance the efficacy of cytotoxic T lymphocyte adoptive transfer immunotherapy against cancer. This beneficial effect of conditioning regimens is due, at least in part, to promoting the breakdown of peripheral CD8+ T cell tolerance. Lymphodepletion by total body irradiation induces systemic translocation of commensal bacteria LPS from the gastrointestinal tract. Since LPS is a potent activator of the innate immune system, including antigen presenting dendritic cells, we hypothesized that LPS translocation could be required for the breakdown of peripheral tolerance observed in irradiated mice. To address this issue, we have treated irradiated mice with antibiotics in order to prevent LPS translocation and utilized them in T cell adoptive transfer experiments. Surprisingly, we found that despite of completely blocking LPS translocation into the bloodstream, antibiotic treatment did not prevent the breakdown of peripheral tolerance. Although irradiation induced the activation of cross-presenting CD8+ dendritic cells in the lymphoid tissue, LPS could not solely account for this effect. Activation of dendritic cells by mechanisms other than LPS translocation is sufficient to promote the differentiation of potentially autoreactive CD8+ T cells into effectors in irradiated mice. Our data indicate that LPS translocation is dispensable for the breakdown of CD8+ T cell tolerance in irradiated mice.
Le declenchement de l'auto-immunite correle frequemment avec un etat lymphopenique dans des modeles murins experimentaux, mais aussi chez des patients. Dans ces conditions, le systeme immunitaire a developpe des mecanismes homeostatiques de compensation qui induisent la proliferation des cellules T naives et leur differenciation en lymphocytes de type memoire, en absence apparente de stimulation antigenique. Comme les conditions d'activation requises par les cellules T memoires sont moins stringentes que celles des cellules T naives, nous avons pose l'hypothese que les cellules T auto-reactives, qui atteignent les organes lymphoides secondaires dans un environnement lymphopenique, pourraient se differencier et contourner les mecanismes de tolerance peripherique. L'utilisation d'un systeme murin transgenique specifique pour un antigene modele exprime dans le pancreas, nous a permis de montrer que les cellules T CD8+ de type memoire potentiellement auto-reactives, generees en conditions de lymphopenie, ne sont pas suffisantes pour induire de l'auto-immunite. Elles sont tolerisees dans les ganglions drainant le pancreas. L'induction de l'auto-reactivite necessite l'aide des cellules T CD4+ antigene-specifiques. Ces cellules « helper » entrainent la differenciation des cellules T CD8+ de type memoire en cellules effectrices suite a la cross-presentation antigenique, et leur migration dans le pancreas ou s'ensuit l'auto-immunite. De plus, nous avons trouve que l'IL-2, une cytokine principalement produite par les cellules T CD4+ « helper », joue un role majeur dans cet effet. Ainsi, en conditions de lymphopenie, la cooperation des cellules T CD4+ et CD8+ de type memoire auto-reactives contourne la cross-tolerance et aboutit au declenchement de l'auto-immunite. Ces etudes ouvrent de nouvelles perspectives, notamment pour le traitement des cancers par immunotherapie et le developpement de nouvelles strategies optimisant les reponses immunes anti-tumorales
Background Lymphopenia results in the proliferation and differentiation of naïve T cells into memory-like cells in the apparent absence of antigenic stimulation. This response, at least in part due to a greater availability of cytokines, is thought to promote anti-self responses. Although potentially autoreactive memory-like CD8+ T cells generated in a lymphopenic environment are subject to the mechanisms of peripheral tolerance, they can induce autoimmunity in the presence of antigen-specific memory-like CD4+ T helper cells. Methodology/Principal Findings Here, we studied the mechanisms underlying CD4 help under lymphopenic conditions in transgenic mice expressing a model antigen in the beta cells of the pancreas. Surprisingly, we found that the self-reactivity mediated by the cooperation of memory-like CD8+ and CD4+ T cells was not abrogated by CD40L blockade. In contrast, treatment with anti-IL-2 antibodies inhibited the onset of autoimmunity. IL-2 neutralization prevented the CD4-mediated differentiation of memory-like CD8+ T cells into pathogenic effectors in response to self-antigen cross-presentation. Furthermore, in the absence of helper cells, induction of IL-2 signaling by an IL-2 immune complex was sufficient to promote memory-like CD8+ T cell self-reactivity. Conclusions/Significance IL-2 mediates the cooperation of memory-like CD4+ and CD8+ T cells in the breakdown of cross-tolerance, resulting in effector cytotoxic T lymphocyte differentiation and the induction of autoimmune disease.
