Role of IFN-α signaling in T cell pool dysregulation during chronic exposure to type-I IFN under lymphopenic conditions: impact on HIV pathogenesis (VIR9P.1145)

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.

IL-7/Lymphopenia enhanced Type-I IFN response by modulating STAT-1 levels: its impact in CD4 T cell homeostasis (HUM8P.338)

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.

Chronic exposure to type-I IFN under lymphopenic conditions alters CD4 T cell homeostasis

HIV infection and the 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. In this manuscript, we addressed the impact of lymphopenia and chronic exposure to IFN-α on T cell homeostasis. In a lymphopenic murine model, this interaction led to decreased CD4 counts and CD8 T cell expansion in association with an increase in the Signal Transducer and Activator of Transcription 1 (STAT1) levels resulting in enhanced CD4 T cell responsiveness to IFN-α. Thus, in the setting of HIV infection, chronic stimulation of this pathway could be detrimental for CD4 T cell homeostasis.

While the acute CD4 depletion observed in the initial phase of HIV infection is likely due to direct cytopathic 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.” We hypothesized that the combination of two distinct forces: homeostatic (CD4 T cell depletion) and inflammatory (HIV-driven IFN-α), lead to a form of T cell activation that results in a decline in the CD4 T cell pool and an increase in the CD8 T cells. IL-7 and lymphopenia enhanced CD4 T cell responsiveness to IFN-α by modulating expression of the Signal Transducers and Activators of Transcription 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.

Lymphopenia modulates levels of STAT1 expression leading to enhanced CD4 T cell responsiveness to Type-I IFN (P6272)

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.

CD4 and CD8 T cell immune activation during chronic HIV infection: roles of homeostasis, HIV, type I IFN, and IL-7

Immune activation plays an important role in the pathogenesis of HIV disease. Although the causes are not fully understood, the forces that lead to immune dysfunction differ for CD4 and CD8 T cells. In this study, we report that the molecular pathways that drive immune activation during chronic HIV infection are influenced by differences in the homeostatic regulation of the CD4 and CD8 T cell pools. Proliferation of CD4 T cells is controlled more tightly by CD4 T cell numbers than is CD8 T cell proliferation. This difference reflects the importance of maintaining a polyclonal CD4 T cell pool in host surveillance. Both pools of T cells were found to be driven by viral load and its associated state of inflammation. In the setting of HIV-induced lymphopenia, naive CD4 T cells were recruited mainly into the proliferating pool in response to CD4 T cell depletion, whereas naive CD8 T cell proliferation was driven mainly by levels of HIV RNA. RNA analysis revealed increased expression of genes associated with type I IFN and common γ chain cytokine signaling in CD4 T cell subsets and only type I IFN-associated genes in CD8 T cell subsets. In vitro studies demonstrated enhanced STAT1 phosphorylation in response to IFN-α and increased expression of the IFNAR1 transcripts in naive and memory CD4 T cells compared with that observed in CD8 T cells. CD4 T cell subsets also showed enhanced STAT1 phosphorylation in response to exogenous IL-7.