Rate of Increase in Circulating IL-7 and Loss of IL-7Rα Expression Differ in HIV-1 and HIV-2 Infections: Two Lymphopenic Diseases with Similar Hyperimmune Activation but Distinct Outcomes

Human naïve regulatory T-cells feature high steady-state turnover and are maintained by IL-7

Naïve FoxP3-expressing regulatory T-cells (Tregs) are essential to control immune responses via continuous replenishment of the activated-Treg pool with thymus-committed suppressor cells. The mechanisms underlying naïve-Treg maintenance throughout life in face of the age-associated thymic involution remain unclear. We found that in adults thymectomized early in infancy the naïve-Treg pool is remarkably well preserved, in contrast to conventional naïve CD4 T-cells. Naïve-Tregs featured high levels of cycling and pro-survival markers, even in healthy individuals, and contrasted with other circulating naïve/memory CD4 T-cell subsets in terms of their strong γc-cytokine-dependent signaling, particularly in response to IL-7. Accordingly, ex-vivo stimulation of naïve-Tregs with IL-7 induced robust cytokine-dependent signaling, Bcl-2 expression, and phosphatidylinositol 3-kinase (PI3K)-dependent proliferation, whilst preserving naïve phenotype and suppressive capacity. Altogether, our data strongly implicate IL-7 in the thymus-independent long-term survival of functional naïve-Tregs, and highlight the potential of targeting the IL-7 pathway to modulate Tregs in different clinical settings.

Major CD4 T-Cell Depletion and Immune Senescence in a Patient with Chronic Granulomatous Disease

Chronic granulomatous disease (CGD) results from primary defects in phagocytic reactive oxygen species (ROS) production. T-cell evaluation is usually neglected during patients’ follow-up, although T-cell depletion has been reported in CGD through unknown mechanisms. We describe here a 36-year-old patient with X-linked CGD with severe CD4 T-cell depletion <200 CD4 T-cells/μl, providing insights into the mechanisms that underlie T-cell loss in the context of oxidative burst defects. In addition to the typical infections, the patient featured a progressive T-cell loss associated with persistent lymphocyte activation, expansion of interleukin (IL)-17-producing CD4 T-cells, and impaired thymic activity, leading to a reduced replenishment of the T-cell pool. A relative CD4 depletion was also found at the gut mucosal level, although no bias to IL-17-production was documented. This immunological pattern of exhaustion of immune resources favors prompt, potentially curative, therapeutic interventions in CGD patients, namely, stem-cell transplantation or gene therapy. Moreover, this clinical case raises new research questions on the interplay of ROS production and T-cell homeostasis and immune senescence.

Expression of the IL-7 receptor alpha-chain is down regulated on the surface of CD4 T-cells by the HIV-1 Tat protein

HIV infection elicits defects in CD4 T-cell homeostasis in both a quantitative and qualitative manner. Interleukin-7 (IL-7) is essential to T-cell homeostasis and several groups have shown reduced levels of the IL-7 receptor alpha-chain (CD127) on both CD4 and CD8 T-cells in viremic HIV+ patients. We have shown previously that soluble HIV Tat protein specifically down regulates cell surface expression of CD127 on human CD8 T-cells in a paracrine fashion. The effects of Tat on CD127 expression in CD4 T-cells has yet to be described. To explore this effect, CD4 T-cells were isolated from healthy individuals and expression levels of CD127 were examined on cells incubated in media alone or treated with Tat protein. We show here that, similar to CD8 T-cells, the HIV-1 Tat protein specifically down regulates CD127 on primary human CD4 T-cells and directs the receptor to the proteasome for degradation. Down regulation of CD127 in response to Tat was seen on both memory and naive CD4 T-cell subsets and was blocked using either heparin or anti-Tat antibodies. Tat did not induce apoptosis in cultured primary CD4 T-cells over 72 hours as determined by Annexin V and PI staining. Pre-incubation of CD4 T-cells with HIV-1 Tat protein did however reduce the ability of IL-7 to up regulate Bcl-2 expression. Similar to exogenous Tat, endogenously expressed HIV Tat protein also suppressed CD127 expression on primary CD4 T-cells. In view of the important role IL-7 plays in lymphocyte proliferation, homeostasis and survival, down regulation of CD127 by Tat likely plays a central role in immune dysregulation and CD4 T-cell decline. Understanding this effect could lead to new approaches to mitigate the CD4 T-cell loss evident in HIV infection.

Preserved CD4 T-cell telomere length during long-lasting HIV-2 infection

HIV-2 infection features a much slower course than HIV-1 infection, often asymptomatic for over 20 years, without antiretroviral therapy (ART). Nevertheless, CD4 T cells progressively decline, in direct correlation with immune activation and cell cycling. We report, for the first time, preserved telomere length within naive and memory CD4 subsets in prolonged HIV-2 infection despite the increased CD4 turnover.

Rapid T cell repopulation after rabbit anti-thymocyte globulin (rATG) treatment is driven mainly by cytomegalovirus

Rabbit anti-thymocyte globulin (rATG) induces a long-lasting lymphocytopenia. CD4(+) T cells remain depleted for up to 2 years, whereas the CD8(+) T cell compartment is refilled rapidly by highly differentiated CD27(-) CD45RA(+) CD57(+) effector-type cells. Because the presence of these highly differentiated CD8(+) T cells has been associated with cytomegalovirus (CMV) infection, we questioned to what extent restoration of CMV T cell immunity contributes to the re-emergence of T cells following rATG treatment. We compared T cell repopulation in six CMV-seropositive patients with CMV reactivation (reactivating CMV(+) ) to that in three CMV(+) patients without reactivation (non-reactivating CMV(+) ), and to that in three CMV-seronegative recipients receiving a kidney from a CMV-seronegative donor (CMV(-/-) ). All patients received rATG because of acute allograft rejection. Total CD4 and CD8 counts, frequency and phenotype of virus-specific CD8(+) T cells were determined. In reactivating CMV(+) patients, total CD8(+) T cells reappeared rapidly, whereas in non-reactivating CMV(+) patients they lagged behind. In CMV(-/-) patients, CD8(+) T cell counts had not yet reached pretransplant levels after 2 years. CMV reactivation was indeed followed by a progressive accumulation of CMV-specific CD8(+) T cells. During lymphocytopenia following rATG treatment, serum interleukin (IL)-7 levels were elevated. Although this was most prominent in the CMV-seronegative patients, it did not result in an advantage in T cell repopulation in these patients. Repopulated CD8(+) T cells showed increased skewing in their Vβ repertoire in both CMV(-/-) and reactivating CMV-seropositive patients. We conclude that rapid T cell repopulation following rATG treatment is driven mainly by CMV.

Memory B-cell depletion is a feature of HIV-2 infection even in the absence of detectable viremia

OBJECTIVE

Memory B-cell loss has long been recognized as an important contributor to HIV immunodeficiency. HIV-2 infection, which is characterized by a slow rate of progression to AIDS and reduced to undetectable viremia, provides a unique model to investigate B-cell disturbances.

DESIGN AND METHODS

B-cell subsets were evaluated in 38 HIV-2-infected individuals, along with markers of T-cell activation and serum levels of immunoglobulins and a major B-cell homeostatic cytokine, B-cell activating factor (BAFF). Untreated HIV-1-infected and seronegative control individuals were studied in parallel. Statistical analysis was performed using Mann-Whitney tests and Spearman's correlations.

RESULTS

We found that HIV-2 was associated with significant depletion of both unswitched (CD27(+)IgD(+)) and switched (CD27(+)IgD(neg)) memory B-cells that directly correlated with T-cell activation, even in individuals with undetectable plasma viral load. Nevertheless, the presence of detectable viremia, even at low levels, was associated with significant memory B-cell loss and higher BAFF levels. Moreover, these alterations were not recovered by antiretroviral-therapy, as treated HIV-2-infected patients showed more pronounced B-cell disturbances, possibly related to their extended length of infection.

CONCLUSION

These first data regarding B-cell imbalances during HIV-2 infection show that, irrespective of viremia, prolonged HIV infection leads to irreversible damage of memory B-cell homeostasis.

Longitudinal assessment of interleukin 7 plasma levels in HIV-infected patients in the absence of and under antiretroviral therapy

BACKGROUND

Cross-sectional studies in HIV-positive patients have suggested that interleukin 7 (IL-7) may increase in parallel to CD4 decline during the natural course of HIV infection. We tested this hypothesis in a longitudinal study examining the evolution of IL-7 and CD4 counts in 2 different scenarios.

METHODS

IL-7 and CD4 counts were regularly monitored in 30 drug-naive patients during a follow-up period of 46 ± 14 months in the absence of therapy and in 42 patients who started highly active antiretroviral therapy and maintained undetectable viremia for 2 years. Multivariate linear regression analysis was used to ascertain what factors were associated with IL-7 variations during follow-up.

RESULTS

In antiretroviral therapy-naive patients, CD4 counts significantly decreased (P < 0.0001), whereas plasma HIV-RNA and IL-7 levels remained fairly stable. In patients on highly active antiretroviral therapy, CD4 counts significantly increased (P < 0.0001) and IL-7 tended to decrease (P = 0.1). There was no correlation between CD4 and IL-7 variations either in the naive or in the treated population. The only parameter significantly associated with IL-7 variation during follow-up was its baseline level that showed a negative correlation.

CONCLUSIONS

In HIV patients with low or moderate degree of immunodeficiency, CD4 counts and plasma IL-7 levels do not evolve in parallel, suggesting that other factors different from CD4 counts must be involved in the upregulation of IL-7 observed in HIV infection.

Memory and naive-like regulatory CD4+ T cells expand during HIV-2 infection in direct association with CD4+ T-cell depletion irrespectively of viremia

OBJECTIVE

The dynamics of CD4(+) regulatory T cells (Treg) during HIV-1 infection remains unclear. To further investigate Treg in this context, we characterized, for the first time, this population in HIV-2-positive individuals. Although both HIV infections are associated with hyperimmune activation and CD4(+) T-cell lymphopenia, most HIV-2-positive individuals display slower disease progression and low-to-undetectable viremia.

DESIGN/METHODS

Samples were obtained from cohorts of untreated HIV-2-positive and HIV-1-positive, treated HIV-1-positive and seronegative individuals. The proportion of CD4(+) T cells bearing a Treg phenotype, defined in terms of high-level CD25 or Foxp3 expression, was assessed by flow cytometry and correlated with markers of disease progression. The proportions of naive and memory-like subsets as well as cycling cells were determined.

RESULTS

We observed an increased proportion of Treg, associated with disease progression, as well as increased proportions of cycling (Ki67(+)) memory Treg, in untreated HIV-2-positive and HIV-1-positive individuals. We also noted an expansion of Treg that persisted over time in treated, immunologically discordant HIV-1-positive individuals, who, similarly to HIV-2-positive patients, present undetectable viremia and low CD4 T-cell count.

CONCLUSION

Overall, we demonstrated that Treg frequency was increased in all lymphopenic HIV-2-positive and HIV-1-positive individuals irrespective of the presence or absence of viremia or antiretroviral treatment. This, in turn, suggests that the observed alterations in Treg frequency in HIV/AIDS are more directly related to the degree of CD4 depletion than to viremia.

Stem Cell Transplantation for Lymphoma Patients with HIV Infection

The advent of Highly Active Antiretroviral Therapy (HAART) has radically changed incidence characteristics and prognosis of HIV-positive patients affected by lymphomas. At this time there is consensus in the literature that, in first line, HIV-positive patients should always be treated with curative intent preferentially following the same approach used in the HIV-negative counterpart. On the contrary, an approach of salvage therapy in HIV-positive lymphomas is still a matter of debate given that for a wide range of relapsed or resistant HIV-negative Hodgkin's disease (HD) and non-Hodgkin lymphoma (NHL) patients, autologous peripheral or allogeneic stem cell transplantation are among the established options. In the pre-HAART era, therapeutic options derived from pioneering experiences gave only anecdotal success, either when transplantation was used to cure lymphomas or to improve HIV infection itself. Concerns relating to the entity, quality, and kinetics of early and late immune reconstitutions and the possible worsening of underlying viroimmunological conditions were additional obstacles. Currently, around 100 relapsed or resistant HIV-positive lymphomas have been treated with an autologous peripheral stem cell transplantation (APSCT) in the HAART era. Published data compared favorably with any previous salvage attempt showing a percentage of complete remission ranging from 48% to 90%, and overall survival ranging from 36% to 85% at median follow-up approaching 3 years. However, experiences are still limited and have given somewhat confounding indications, especially concerning timing and patients' selection for APSCT and feasibility and outcome for allogeneic stem cell transplant. Moreover, little data exist on the kinetics of immunological reconstitution after APSCT or relevant to the outcome of HIV infection. The aim of this review is to discuss current knowledge of the role of allogeneic and autologous stem cell transplantation as a modality in the cure of HIV and hemopoietic cancer patients. Several topics dealing with practical aspects concerning the management of APSCT in HIV-positive patients, including patient selection, timing of transplant, conditioning regimen, and relapse or nonrelapse mortality, are discussed. Data relating to the effects of mobilization and transplantation on virological parameters and pre- and posttransplant immune reconstitution are reviewed. Finally, in this review, we examine several ethical and legal issues relative to banking infected or potentially infected peripheral blood stem cells and we describe our experience and strategies to protect positive and negative donors/recipients and the health of caretakers.

The interleukin-7 receptor α chain contributes to altered homeostasis of regulatory T cells in multiple sclerosis

Treg homeostasis is disturbed in multiple sclerosis (MS). Frequencies of recent thymic emigrant (RTE)-Treg are reduced and the disparity between RTE-Treg and long-lived memory Treg coincides with the MS-associated Treg defect, as shown previously. Recent studies demonstrate that IL-7 and thymic stromal lymphopoietin (TSLP) are critical for Treg maturation. Therefore, altered signaling through their receptors (IL-7R, TSLP receptor (TSLPR)), sharing the IL-7Rα-chain (IL-7Rα), might contribute to impaired Treg development. Using blood samples from 56 patients with MS and 33 healthy controls, we assessed IL-7Rα-expression on conventional T cells; frequencies, phenotypes and suppressive activities of Treg, plasma levels of IL-7 and soluble IL-7Rα; and screened for MS-associated IL-7RA gene polymorphism rs6897932. Moreover, we determined Treg expressing two different TCR Vα-chains designating thymus-originated cells. As TSLP/TSLPR signaling in thymic myeloid dendritic cells (MDCs) promotes Treg differentiation, we measured TSLPR expression on peripheral MDCs to indirectly test whether altered TSLPR expression might add to compromised Treg neogenesis. We found reduced IL-7Rα expression on conventional T cells and upregulated IL-7 plasma levels together with reduction of RTE-Treg frequencies and Treg function in MS, without clear genetic influence. Decreased IL-7Rα expression in MS correlated with declined dual-receptor-Treg and reduced MDC TSLPR expression, indicating contracted thymic Treg output. We suggest that altered IL-7R/TSLPR signaling contributes to impaired Treg neogenesis in MS, which is compensated by expanded memory-Treg and finally results in dysfunctional Treg.

The role of IL-1beta in reduced IL-7 production by stromal and epithelial cells: a model for impaired T-cell numbers in the gut during HIV-1 infection

OBJECTIVES

Interleukin (IL)-7 is a key cytokine in T-cell homeostasis. Stromal cells, intestinal epithelial cells and keratinocytes are known to produce this cytokine. The mechanisms and cellular factors regulating IL-7 production are still unclear. We assessed whether IL-1beta and interferon (IFN)-gamma, cytokines produced during inflammatory conditions, may impact on IL-7 production.

DESIGN

We used human intestinal epithelial cells (DLD-1 cell line) and bone marrow stromal cells (HS27 cell line), known to produce IL-7; IL-7 production was evaluated at the mRNA and protein levels. To assess whether treatment of HS27 cells with IL-1beta and/or IFN-gamma leads to changes in the gene expression of cytokines, Toll-like receptors (TLRs) and chemokines, we analysed gene expression profiles using the whole-genome microarray Human Gene 1.0 ST.

RESULTS

We found that IFN-gamma enhanced the expression of IL-7 mRNA (P < 0.001) in both cell lines. IL-1beta treatment led to a significant down-regulation (P < 0.001) of IL-7 mRNA expression in both cell lines. The IL-7 concentration in supernatants collected from treated DLD-1 and HS27 cell cultures reflected the trend of IL-7 mRNA levels. The gene profiles revealed dramatic changes in expression of cytokines and their receptors (IL-7/IL-7R alpha; IL-1alpha,IL-1beta/IL-1R1; IFN-gamma/IFN-gammaR1), of IFN regulatory factors (IRF-1 and 2), of TLRs and of important chemo-attractants for T cells. The microarray results were verified by additional methods.

CONCLUSIONS

Our results are discussed in the setting of inflammation and T-cell survival in the gut compartment during HIV-1 infection where stromal and epithelial cells may produce factors that contribute to impaired IL-7 homeostasis and homing of T cells.

Dual mechanism of impairment of interleukin-7 (IL-7) responses in human immunodeficiency virus infection: decreased IL-7 binding and abnormal activation of the JAK/STAT5 pathway

Interleukin-7 (IL-7) plays a central role in controlling the homeostasis of both naive and long-term-memory CD4(+) T cells. To better understand how human immunodeficiency virus (HIV) perturbs CD4(+) T-cell homeostasis, we performed a detailed analysis of IL-7R expression, IL-7 binding, and IL-7-dependent early and late signaling events in CD4(+) T-cell subsets from viremic and efficiently treated patients. HIV infection differentially affected the expression of IL-7 receptor (IL-7R) chains, with decreases in IL-7Ralpha/CD127 expression in the memory subset and increases in gammac/CD132 expression in all CD4(+) T cells. This resulted in preserved IL-7 binding in the naive compartment and decreased IL-7 binding in the memory compartment of viremic patients. Accordingly, the percentages of cells signaling in response to IL-7, as measured by pSTAT5 induction, were decreased in memory subsets, including conventional CD4(+) T cells and regulatory T cells. However, the levels of pSTAT5 induction per responding cell, as measured by pSTAT5 fluorescence intensity, were increased within all naive and memory CD4(+) T-cell subsets of viremic patients. The basal level of pSTAT5 was also increased, indicating a constitutive activation of the JAK/STAT5 pathway. IL-7 functional responses, as measured by Bcl-2, CD25, and Foxp3 induction, were impaired in viremic patient CD4(+) T cells, suggesting that chronic activation led to downstream defects in the STAT5 signaling pathway. Thus, HIV infection perturbs IL-7 responses at both receptor binding and signaling steps, which likely compromises the regenerative capacity of the CD4(+) T-cell pool and may contribute to CD4(+) T-cell depletion.

Inverse association of repressor growth factor independent-1 with CD8 T cell interleukin (IL)-7 receptor [alpha] expression and limited signal transducers and activators of transcription signaling in response to IL-7 among [gamma]-chain cytokines in HIV patients

BACKGROUND

CD8 T lymphocytes from chronically infected HIV-positive patients degenerate into a preapoptotic state and exhibit impaired functionality. Particularly in viremic patients, this was associated with an increased proportion of interleukin-7 receptor-alpha low-expressing (IL-7Ralpha(low)) effector-like CD8 T cells. As cytokine signaling through signal transducers and activators of transcription (STAT) is essential for cellular function, we hypothesized that activation of this pathway may be impaired in these cells.

OBJECTIVES

To evaluate cytokine-induced STAT activation in IL-7Ralpha(low) and IL-7Ralpha(high) CD8 T cells from chronically infected HIV-positive patients and investigate the potential molecular mechanism involved in the reduced IL-7Ralpha expression.

METHODS

CD8 T cells from HIV-positive patients on and off antiretroviral therapy were assayed respectively for STAT activation, cytokine receptor, and transcription factor expression by flow cytometry and real-time PCR.

RESULTS

IL-7 stimulation failed to activate STAT5 in a substantial proportion of patient CD8 T cells. This correlated with reduced IL-7Ralpha mRNA and surface protein expression. Interestingly, IL-7Ralpha(low) cells appeared to be fully capable of recruiting the STAT pathway in response to IL-2, IL-4, IL-10, and IL-15. mRNA expression suggested a potential role for growth factor independent (Gfi)-1 as an IL-7Ralpha transcriptional repressor, but not that of other transcriptional regulators studied, including Gfi-1B and GA-binding protein alpha. Programmed death-1 inhibitory receptor, though upregulated in CD8 T cells from HIV-positive patients, appeared unrelated to IL-7Ralpha expression and STAT signaling capacity.

Dendritic cell differentiation and maturation in the presence of HIV type 2 envelope

Dendritic cells (DCs) are fundamental for the initiation of immune responses and are important players in AIDS immunopathogenesis. Impairment of DC function may result from bystander effects of HIV-1 envelope proteins independently of direct HIV-1 infection. HIV-2 envelope proteins are thought to interact with a broader range of receptors than those of HIV-1, and have been shown to have T cell immunosuppressive properties mediated by monocytes. The effects of HIV-2 envelope on DC differentiation and maturation were investigated. The modulatory properties of the HIV-2 envelope on DC generated from monocytes were assessed using both recombinant proteins (HIV-2(ROD) and HIV-2(ALI)) and whole chemically inactivated virus (aldrithiol-2-treated HIV-2(ROD)). DC phenotype was assessed by flow cytometry and DC function by their ability to stimulate allogeneic T cells and to produce cytokines. We demonstrate that HIV-2 Env had no effects upon DC differentiation and maturation despite its broad receptor usage and ability to modulate monocyte function. It is plausible to speculate that a reduced ability of the HIV-2 Env to impair myeloid DC function could represent a contributory factor to the relatively benign course of HIV-2 disease.

IL-7 sustains CD31 expression in human naive CD4+ T cells and preferentially expands the CD31+ subset in a PI3K-dependent manner

The CD31(+) subset of human naive CD4(+) T cells is thought to contain the population of cells that have recently emigrated from the thymus, while their CD31(-) counterparts have been proposed to originate from CD31(+) cells after homeostatic cell division. Naive T-cell maintenance is known to involve homeostatic cytokines such as interleukin-7 (IL-7). It remains to be investigated what role this cytokine has in the homeostasis of naive CD4(+) T-cell subsets defined by CD31 expression. We provide evidence that IL-7 exerts a preferential proliferative effect on CD31(+) naive CD4(+) T cells from adult peripheral blood compared with the CD31(-) subset. IL-7-driven proliferation did not result in loss of CD31 expression, suggesting that CD31(+) naive CD4(+) T cells can undergo cytokine-driven homeostatic proliferation while preserving CD31. Furthermore, IL-7 sustained or increased CD31 expression even in nonproliferating cells. Both proliferation and CD31 maintenance were dependent on the activation of phosphoinositide 3-kinase (PI3K) signaling. Taken together, our data suggest that during adulthood CD31(+) naive CD4(+) T cells are maintained by IL-7 and that IL-7-based therapies may exert a preferential effect on this population.

HIV-2: the forgotten AIDS virus

HIV type 2 (HIV-2), a closely related retrovirus discovered a few years after HIV type 1, causes AIDS in only a minority of infected individuals. Determining why HIV-2 causes asymptomatic infection in most patients could further our understanding of HIV immunopathogenesis. Studies to date have suggested that both enhanced immune responses and lower viral replication could play a role. We summarize the important findings to date and highlight areas that warrant further exploration.

Loss of CD127 expression links immune activation and CD4(+) T cell loss in HIV infection

Although chronic immune activation correlates with CD4(+) T cell loss in HIV infection, an understanding of the factors mediating T cell depletion remains incomplete. We propose that reduced expression of CD127 (IL-7 receptor alpha chain, IL-7Ralpha), induced by immune activation, contributes to CD4(+) T cell loss in HIV infection. In particular, loss of CD127 on central memory CD4(+) T cells (T(CM)) severely restrains the regenerative capacity of the memory component of the immune system, resulting in a limited ability to control T cell homeostasis. Studies from both pathogenic and controlled HIV infection indicate that the containment of immune activation and preservation of CD127 expression are critical to the stability of CD4(+) T cells in infection. A better understanding of the factors regulating CD127 expression in HIV disease, particularly on T(CM) cells, might unveil new approaches exploiting the IL-7/IL-7R receptor pathway to restore T cell homeostasis and promote immune reconstitution in HIV infection.

Central memory CD8+ T cells appear to have a shorter lifespan and reduced abundance as a function of HIV disease progression

Progressive HIV disease has been associated with loss of memory T cell responses to Ag. To better characterize and quantify long-lived memory T cells in vivo, we have refined an in vivo labeling technique to study the kinetics of phenotypically distinct, low-frequency CD8(+) T cell subpopulations in humans. HIV-negative subjects and antiretroviral-untreated HIV-infected subjects in varying stages of HIV disease were studied. After labeling the DNA of dividing cells with deuterated water ((2)H(2)O), (2)H-label incorporation and die-away kinetics were quantified using a highly sensitive FACS/mass spectrometric method. Two different populations of long-lived memory CD8(+) T cells were identified in HIV-negative subjects: CD8(+)CD45RA(-)CCR7(+)CD28(+) central memory (T(CM)) cells expressing IL-7Ralpha and CD8(+)CD45RA(+)CCR7(-)CD28(-) RA effector memory (T(EMRA)) cells expressing CD57. In pilot studies in HIV-infected subjects, T(CM) cells appeared to have a shorter half-life and reduced abundance, particularly in those with high viral loads; T(EMRA) cells, by contrast, retained a long half-life and accumulated in the face of progressive HIV disease. These data are consistent with the hypothesis that IL-7Ralpha(+) T(CM) cells represent true memory CD8(+) T cells, the loss of which may be responsible in part for the progressive loss of T cell memory function during progressive HIV infection.

Tenets of protection from progression to AIDS: lessons from the immune responses to HIV-2 infection

In the past 25 years, life survival curves of many countries have been remodeled owing to HIV infection. Both HIV-1 and HIV-2 can cause AIDS, yet patients infected with HIV-2 fare much better clinically and most will never experience detrimental effects of the infection. Despite over two decades of comprehensive research into vaccine development, a prophylactic vaccine is not yet realized. An essential missing link in the innovation of a successful vaccine strategy is the description of a favorable immune response that abolishes virus replication. Lessons learned from studying the role of the immune system in the long-term nonprogression characteristic of HIV-2 infection will offer insight into how a balanced immune response can protect from the destruction of the immune system associated with chronic HIV-1 infection.

Disruption of the gamma c cytokine network in T cells during HIV infection

The common gamma chain (gammac)-sharing cytokines (IL's-2, 4, 7, 9, 15, and 21) play a vital role in the survival, proliferation, differentiation and function of T lymphocytes. As such, disruption of their signaling pathways would be expected to have severe consequences on the integrity of the immune system. Indeed, it appears that the signaling network of these cytokines is both disrupted and exploited by HIV at various stages of infection. IL-2 secretion and signaling downstream of its receptor are impaired in T cells from chronically-infected HIV+ patients. Elevated plasma IL-7 levels and decreased IL-7Ralpha expression in patient T cells results in significantly decreased responsiveness to this critical cytokine. Interestingly, IL-2 and IL-15 are also able to render CD4+ T cells permissive to HIV infection through their influence on the activity of the APOBEC3G deaminase enzyme. Herein, we describe the current state of knowledge on how the gammac cytokine network is affected during HIV infection, with a focus on how this impairs CD4+ and CD8+ T cell function while also benefiting the virus itself. We also address the use of cytokines as adjuncts to highly active antiretroviral therapy to bolster immune reconstitution in infected patients.

Monocyte-mediated T cell suppression by HIV-2 envelope proteins

HIV-2 is associated with an attenuated form of HIV disease. We investigate here the immunosuppressive effects of the HIV-2 envelope protein, gp105. We found that gp105 suppresses activation of T cells through a monocyte-mediated mechanism. Suppression of T cell activation by gp105 depends on contact between monocytes and T cells, but not on CD4+CD25+ T cells. The TLR4 pathway is likely involved, since gp105 activates TLR4 signaling and induces TNF-alpha production by monocytes. Immunosuppression is viewed as the main pathophysiologic consequence of infection by HIV. However, the main immunologic defect caused by HIV, depletion of T cells, requires T cell activation. Our findings are consistent with a new concept that HIV-2 envelope proteins act on monocytes to suppress T cell activation and that this property may contribute to the benign course of HIV-2. We hypothesize that the HIV-2 envelope immunosuppressive properties limit bursts of T cell activation, thus reducing viremia and contributing to the slow rate of disease progression that characterizes HIV-2 disease.

Low CD4 T-cell counts despite low levels of circulating HIV: Insights from the comparison of HIV-1 infected patients with a discordant response to antiretroviral therapy to patients with untreated advanced HIV-2 disease

A significant proportion of HIV-1+ patients with suppression of viremia under antiretroviral therapy fail to recover CD4(+) T-cell counts (ART-Discordants). Similarly, untreated HIV-2+ patients can also exhibit major CD4 depletion in spite of undetectable viremia. We characterize here the immunological disturbances associated with major CD4-lymphopenia in these two scenarios as compared to untreated viremic HIV-1+ patients with similar CD4-lymphopenia and HIV-1+ patients with successful immunological and virological responses under ART. Low CD4 counts were associated with major naive CD4 and CD8 depletion, irrespective of type of infection or ART-exposure. However, ART-Discordants exhibited lower levels of T-cell activation as compared to both untreated HIV-2 and HIV-1 cohorts, and a less marked increase in circulating IL-7 despite similar CD4 depletion. Nevertheless, ART-Discordants showed a preserved Bcl-2 expression, suggesting increased IL-7 consumption, which in conjunction with the relatively lower T-cell activation may contribute to their CD4 count stability and low rate of opportunistic infections.

Efficient thymopoiesis contributes to the maintenance of peripheral CD4 T cells during chronic human immunodeficiency virus type 2 infection

Human immunodeficiency virus type 2 (HIV-2) infection leads to a lifelong asymptomatic period in the majority of patients. Even in patients with progressive disease, a slow CD4 count decline characterizes the chronic phase of HIV-2 infection, suggesting that peripheral T-cell homeostasis is controlled better following HIV-2 infection than following HIV-1 infection. Herein we showed that, in contrast to HIV-1-infected patients, HIV-2-infected patients demonstrate enhanced thymic function compared to age-matched healthy individuals. The correlation between higher thymic production and lower CD4 T-cell loss in these patients suggests that efficient thymopoiesis is implicated in the long-lasting maintenance of CD4 T-cell counts in HIV-2 disease.