Thymocyte-thymic epithelial cell interaction leads to high-level replication of human immunodeficiency virus exclusively in mature CD4(+) CD8(-) CD3(+) thymocytes: a critical role for tumor necrosis factor and interleukin-7

Loss of HIV candidate vaccine efficacy in male macaques by mucosal nanoparticle immunization rescued by V2-specific response

Systemic vaccination of macaques with V1-deleted (ΔV1) envelope immunogens reduce the risk of SIVmac251 acquisition by approximately 60%, with protective roles played by V2-specific ADCC and envelope-specific mucosal IL-17+NKp44+ innate lymphoid cells (ILCs). We investigated whether increased mucosal responses to V2 benefit vaccine efficacy by delivering oral nanoparticles (NPs) that release V2-scaffolded on Typhoid Toxin B (TTB) to the large intestine. Strikingly, mucosal immunization of male macaques abrogated vaccine efficacy with control TTB or empty NPs, but vaccine efficacy of up to 47.6% was preserved with V2-TTB NPs. The deleterious effects of NPs were linked to preferential recruitment of mucosal plasmacytoid dendritic cells (pDCs), reduction of protective mucosal NKp44+ ILCs, increased non-protective mucosal PMA/Ionomycin-induced IFN-γ+NKG2A-NKp44-ILCs, and increased levels of mucosal activated Ki67+CD4+ T cells, a potential target for virus infection. V2-TTB NP mucosal boosting rescued vaccine efficacy, likely via high avidity V2-specific antibodies mediating ADCC, and higher frequencies of mucosal NKp44+ ILCs and of ∆V1gp120 binding antibody-secreting B cells in the rectal mucosa. These findings emphasize the central role of systemic immunization and mucosal V2-specific antibodies in the protection afforded by ΔV1 envelope immunogens and encourage careful evaluation of vaccine delivery platforms to avoid inducing immune responses favorable to HIV transmission.

Immunomodulation with IL-7 and IL-15 in HIV-1 infection

Immunomodulating agents are substances that modify the host immune responses in diseases such as infections, autoimmune conditions and cancers. Immunomodulators can be divided into two main groups: 1) immunostimulators that activate the immune system such as cytokines, toll-like receptor agonists and immune checkpoint blockers; and 2) immunosuppressors that dampen an overactive immune system such as corticosteroids and cytokine-blocking antibodies. In this review, we have focussed on the two primarily T and natural killer (NK) cell homeostatic cytokines: interleukin-7 (IL-7) and -15 (IL-15). These cytokines are immunostimulators which act on immune cells independently of the presence or absence of antigen. In vivo studies have shown that IL-7 administration enhances proliferation of circulating T cells whereas IL-15 agonists enhance the proliferation and function of NK and CD8+ T cells. Both IL-7 and IL-15 therapies have been tested as single interventions in HIV-1 cure-related clinical trials. In this review, we explore whether IL-7 and IL-15 could be part of the therapeutic approaches towards HIV-1 remission.

Perspectives on interleukin-7 therapy in HIV infection

PURPOSE OF REVIEW

HIV infection is characterized by depletion of CD4 T cells and altered immune function, leading to severe immune deficiency. Mechanisms leading to this T-cell depletion are not completely understood. Potent antiretroviral therapy restores T-cell counts and improves prognosis. Apart from antiviral therapy for the infection, immunotherapies such as interleukin-7 that influence T-cell homeostatic mechanisms are undergoing clinical evaluation. Because of its pleiotropic effects on developing and mature T cells, interleukin-7 may help to restore immune function during HIV infection.

RECENT FINDINGS

Recent studies explored the therapeutic use of interleukin-7 in simian immunodeficiency virus models and in HIV-infected patients. Interleukin-7 can help to restore CD4 T-cell number and function.

SUMMARY

Numerous recent findings highlight the importance of interleukin-7 pathway impairment in the pathogenesis of HIV infection. Notably, interleukin-7 levels increased with advancing CD4 T-cell lymphopenia, whereas interleukin-7 receptor expression is downregulated mainly on CD8 T cells. Therapeutic trials conducted in monkeys and in humans (phase I) have provided evidence on the role of interleukin-7 in thymopoiesis and in restoration of T-cell functions. Interleukin-7 appeared to be well tolerated and to have no deleterious effects on viral load. These results should be confirmed in larger phase I/II studies.

Vaginal microbicides to prevent human immunodeficiency virus infection in women: perspectives on the female genital tract, sexual maturity and mucosal inflammation

Topically applied vaginal microbicides to protect against human immunodeficiency (HIV) virus infection offer an important female-controlled prevention strategy. Microbicides have been in development for more than 2 decades, and have included various agents that disrupt cellular and microbial membranes (surfactants), restore the natural acidic protective pH of the vagina (acid buffers), and those that interfere with interactions between HIV envelope proteins and cellular receptors (anionic polymers). Although none of these candidate microbicides have shown significant protection against HIV in clinical trials, a topical gel, including the antiretroviral drug tenofovir (TFV) 1% was the first microbicide to be tested to show some protection against HIV infection. This review explores the effect of female genital tract biology and anatomy, mucosal inflammation, and age on the effectiveness of microbicides to prevent HIV infection.

HIV infection of thymocytes inhibits IL-7 activity without altering CD127 expression

BACKGROUND

Thymic function is altered in HIV infection and characterized by dysregulation of the thymic epithelial network, reduced thymic output and ultimately an impaired naïve T-cell pool. The IL-7/IL-7 receptor (IL-7R) signalling pathway is critical for the maturation and differentiation of thymocytes. HIV infection is associated with a decrease in IL-7Rα (CD127) expression and impaired CD127 signalling in circulating CD8+ T-cells; however, little is known about the effect of HIV on CD127 expression and IL-7 activity in the thymus. Therefore, the effect of in vitro HIV infection on CD127 expression and IL-7-mediated function in thymocytes was investigated.

FINDINGS

In vitro HIV infection of thymocytes did not affect CD127 expression on either total thymocytes or on single positive CD4 or single positive CD8 subsets. However, HIV infection resulted in a decrease in the level of IL-7-induced STAT-5 phosphorylation and Bcl-2 expression in unfractionated thymocytes.

CONCLUSION

These findings indicate that HIV infection alters IL-7 responsiveness of thymocytes by a mechanism other than CD127 downregulation and potentially explain the disruption in thymopoiesis observed in HIV infection.

CD4⁺ recent thymic emigrants are infected by HIV in vivo, implication for pathogenesis

OBJECTIVE

The contribution of naive CD4⁺ T cells to the pool of HIV-infected cells remains poorly described. This study aimed at evaluating HIV infection in naive T-cell subsets in viremic and HAART-treated patients, together with various parameters implicated in naive T-cell homeostasis, in order to better understand infection in these subsets.

DESIGN AND METHODS

HIV provirus was quantified in various FACS-sorted CD4/CD8 T-cell subsets [recent thymic emigrants (RTEs), non-RTE naives and memory T cells] purified from peripheral blood cells of untreated viremic and HAART-treated aviremic HIV-infected patients. HIV proviral DNA was quantified using a highly sensitive real-time PCR assay allowing detection of one HIV copy in 10⁵ cells. Intrathymic precursor T-cell proliferation and circulating T-cell cycling were, respectively, evaluated through measurement of the sj/βTREC ratio (signal joint T-Cell Receptor Excision Circle frequency divided by DβJβTREC frequency) and Ki-67 expression. Plasma interleukin (IL)-7 concentrations were measured by ELISA.

RESULTS

RTEs and non-RTEs were equally HIV infected. Altogether, naive CD4⁺ T cells represented 0.24%-60% of the infected cells. In contrast, HIV DNA was undetectable in naive CD8⁺ T cells. RTE infection rate directly correlated with IL-7 plasma levels (r = 0.607, P = 0.0035) but was independent from plasma viral load, peripheral T-cell cycling and intrathymic precursor T-cell proliferation.

CONCLUSION

We demonstrated that RTEs are effectively HIV infected. The similar infection rate observed in RTEs and other naive T cells, its relationship with plasma IL-7 levels, together with the lack of correlation between RTE infection and either thymic or peripheral proliferation, strongly suggests that RTE infection occurs either late during thymopoiesis or early on during their extrathymic maturation.

Modulating T-cell homeostasis with IL-7: preclinical and clinical studies

Interleukin-7 (IL-7) is required for the development and survival of T cells and plays a critical role in modulating T-cell homeostasis. This review will address current understanding of IL-7 biology, review recent clinical experiences and discuss potential future clinical applications of IL-7, or IL-7 blockade, in the setting of disease.

Enhanced T cell recovery in HIV-1-infected adults through IL-7 treatment

HIV infection results in CD4+ T cell deficiency, but efficient combination antiretroviral therapy (c-ART) restores T cells and decreases morbidity and mortality. However, immune restoration by c-ART remains variable, and prolonged T cell deficiency remains in a substantial proportion of patients. In a prospective open-label phase I/IIa trial, we evaluated the safety and efficacy of administration of the T cell regulator IL-7. The trial included 13 c-ART-treated HIV-infected patients whose CD4+ cell counts were between 100 and 400 cells/microl and plasma HIV RNA levels were less than 50 copies/ml. Patients received a total of 8 subcutaneous injections of 2 different doses of recombinant human IL-7 (rhIL-7; 3 or 10 microg/kg) 3 times per week over a 16-day period. rhIL-7 was well tolerated and induced a sustained increase of naive and central memory CD4+ and CD8+ T cells. In the highest dose group, 4 patients experienced transient increases in viral replication. However, functional assays showed that the expanded T cells responded to HIV antigen by producing IFN-gamma and/or IL-2. In conclusion, in lymphopenic HIV-infected patients, rhIL-7 therapy induced substantial functional and quantitative changes in T cells for 48 weeks. Therefore, patients may benefit from intermittent therapy with IL-7 in combination with c-ART.

Low and undetectable breast milk interleukin-7 concentrations are associated with reduced risk of postnatal HIV transmission

OBJECTIVE

To investigate if breast milk interleukin [IL]-7 concentrations are associated with postnatal HIV transmission.

DESIGN

A case-control study nested within a cohort of women recruited in Lusaka, Zambia.

METHODS

IL-7 breast milk concentrations were measured in samples from 24 HIV-infected breast-feeding women who transmitted HIV to their child after the neonatal period and from 47 women who did not transmit. Samples were frequency-matched by the time of sample collection (1 week and 1 month postpartum). Logistic regression was used to adjust for possible confounders. For comparison, samples from 18 HIV-uninfected women from the same community were included in the analysis, and plasma IL-7 was determined.

RESULTS

Breast milk IL-7 concentrations were significantly higher than plasma IL-7 concentrations in all 3 groups. In contrast to levels among transmitters and HIV-uninfected women, breast milk IL-7 concentrations exhibited a bimodal distribution among nontransmitters. Breast milk IL-7 concentrations undetectable or less than 30 pg/mL were significantly associated with less HIV transmission (odds ratio = 0.13, 95% confidence interval: 0.03 to 0.64). The association remained strong after adjustment for breast milk viral load and sodium, maternal CD4 cell counts, parity, and time of sample collection.

CONCLUSION

Breast milk IL-7 may be necessary for effective HIV transmission.

Phylodynamics of HIV-1 in lymphoid and non-lymphoid tissues reveals a central role for the thymus in emergence of CXCR4-using quasispecies

Background

During HIV-1 infection coreceptor switch from CCR5- (R5)- to CXCR4 (X4)-using viruses is associated with disease progression. X4 strains of HIV-1 are highly cytopathic to immature thymocytes. Virtually no studies have evaluated the HIV-1 quasispecies present in vivo within thymic and lymphoid tissues or the evolutionary relationship between R5 and X4 viruses in tissues and peripheral blood.

Methodology/Principal Findings

High-resolution phylodynamic analysis was applied to virus envelope quasispecies in longitudinal peripheral blood mononuclear cells (PBMCs) and lymphoid and non-lymphoid tissues collected post mortem from therapy naïve children with AIDS. There were three major findings. First, continued evolution of R5 viruses in PBMCs, spleen and lymph nodes involved multiple bottlenecks, independent of coreceptor switch, resulting in fitter quasispecies driven by positive selection. Second, evolution of X4 strains appeared to be a sequential process requiring the initial fixation of positively selected mutations in V1-V2 and C2 domains of R5 variants before the emergence of high charge V3 X4 variants. Third, R5 viruses persisted after the emergence of CXCR4-using strains, which were found predominantly but not exclusively in the thymus.

Conclusions/Significance

Our data indicate that the evolution of X4 strains is a multi-step, temporally structured process and that the thymus may play an important role in the evolution/amplification of coreceptor variants. Development of new therapeutic protocols targeting virus in the thymus could be important to control HIV-1 infection prior to advanced disease.

Relationship between serum IL-7 concentrations and lymphopenia upon different levels of HIV immune control

Serum IL-7 levels correlate with T-cell depletion in HIV-infected individuals. In some patients, we observed that serum IL-7 decreases upon progression to AIDS, suggesting a role for IL-7 in T-cell maintenance in sporadic cases. Interestingly, IL-7 levels were significantly lower in stable long-term non-progressors (LTNP) than in patients who lost the LTNP status in a 3-year follow-up (P < 0.001), indicating that the serum IL-7 concentration might be a valuable marker for maintenance of the LTNP state.

Infection of CD127+ (interleukin-7 receptor+) CD4+ cells and overexpression of CTLA-4 are linked to loss of antigen-specific CD4 T cells during primary human immunodeficiency virus type 1 infection

We recently found that human immunodeficiency virus (HIV)-specific CD4+ T cells express coreceptor CCR5 and activation antigen CD38 during early primary HIV-1 infection (PHI) but then rapidly disappear from the circulation. This cell loss may be due to susceptibility to infection with HIV-1 but could also be due to inappropriate apoptosis, an expansion of T regulatory cells, trafficking out of the circulation, or dysfunction. We purified CD38+++CD4+ T cells from peripheral blood mononuclear cells, measured their level of HIV-1 DNA by PCR, and found that about 10% of this population was infected. However, a small subset of HIV-specific CD4+) T cells also expressed CD127, a marker of long-term memory cells. Purified CD127+CD4+ lymphocytes contained fivefold more copies of HIV-1 DNA per cell than did CD127-negative CD4+ cells, suggesting preferential infection of long-term memory cells. We observed no apoptosis of antigen-specific CD4+ T cells in vitro and only a small increase in CD45RO+CD25+CD127dimCD4+ T regulatory cells during PHI. However, 40% of CCR5+CD38+++ CD4+ T cells expressed gut-homing integrins, suggesting trafficking through gut-associated lymphoid tissue (GALT). Furthermore, 80% of HIV-specific CD4+ T cells expressed high levels of the negative regulator CTLA-4 in response to antigen stimulation in vitro, which was probably contributing to their inability to produce interleukin-2 and proliferate. Taken together, the loss of HIV-specific CD4+ T cells is associated with a combination of an infection of CCR5+ CD127+ memory CD4+ T cells, possibly in GALT, and a high expression of the inhibitory receptor CTLA-4.

IL-7 as a potential therapy for HIV-1-infected individuals

Highly active antiretroviral therapy (HAART), although effective in ameliorating the quality of life of HIV-1-infected individuals and their survival, has not been able to eradicate HIV-1. In fact, when HAART is interrupted, HIV-1 plasma viral load rebounds from viral reservoirs such as resting CD4+ T lymphocytes, monocytes and macrophages, remaining a major obstacle in attempting HIV eradication. Different therapeutic strategies have been attempted, such as structured treatment interruption (STI), immunotherapy (interleukin [IL]-2 and anti-CD3 antibodies [e.g., OKT3]), to try to stimulate HIV-1 out of latency along with antiretroviral intensification therapy. IL-7, a pleiotropic cytokine, bears diverse immune properties and plays a major role in T cell homeostasis. Moreover, IL-7 has recently been investigated as a possible immune adjuvant as well as a viral strain-specific inducer of HIV-1 replication. In fact, IL-7 was shown not only to be more effective than IL-2 in stimulating HIV-1 replication from resting CD4+ T lymphocytes ex vivo, but also to selectively induce a specific HIV-1 viral strain as compared with IL-2, suggesting the potential need for different viral inducers if complete eradication is to be achieved. In this present review, different immunological and virological properties of IL-7 are discussed, along with the possibility of its use as part of a combined antiretroviral-immune rationally based HIV-1 eradication approach.

Optimization of culture and storage conditions for an in vitro system to evaluate thymocyte phenotype and function

Studies on thymopoiesis are critical to the understanding of T-cell homeostasis as well as the host response to T-cell depletion. Various in vitro culture systems have been used in the study of thymocyte development; however it is unclear if current co-culture methods have been fully optimized. In this study in vitro suspension cultures have been re-evaluated and the optimal storage conditions for thymocytes have been established by evaluating various methods of storing/isolating thymic tissue and isolated thymocytes as well as the source of thymic epithelial cells (TEC). It was determined that thymocytes must be freshly isolated from whole thymic tissue and ideally stored at 4 degrees C prior to co-culture. Co-culture with either autologous or allogeneic TEC results in similar thymocyte subset distribution as well as interleukin-7 receptor-alpha (CD127) expression on these subsets. To evaluate the influence of the source of TEC on one aspect of thymocyte function the effect of IL-7 stimulation on the expression of CD127 was evaluated. IL-7 stimulation resulted in a downregulation of the expression of CD127 on all thymic subsets similar to that observed in circulating CD8+ T-cells. The effect of this was the same whether TEC were autologous or allogeneic. Optimizing culture techniques and facilitating the study of individual thymocyte subsets will lead to a better understanding of thymic function and development. It could also lead to therapeutic approaches that enhance immune recovery after T-cell depletion in HIV infection, bone marrow transplantation or following chemotherapy.

Differential susceptibility of human thymic dendritic cell subsets to X4 and R5 HIV-1 infection

OBJECTIVES

Human thymus can be infected by HIV-1 with potential consequences on immune regeneration and homeostasis. We previously showed that CD4 thymocytes preferentially replicate CXCR4 tropic (X4) HIV-1 dependently on interleukin (IL)-7. Here we addressed the susceptibility of thymic dendritic cells (DC) to HIV-1 infection.

METHODS

We investigated the replication ability of CXCR4 or CCR5 (R5) tropic HIV-1 in thymic micro-explants as well as in isolated thymic CD11clowCD14- DC, CD11chighCD14+ DC and plasmacytoid DC subsets.

RESULTS

Thymic tissue was productively infected by both X4 and R5 viruses. However, X4 but not R5 HIV-1 replication was enhanced by IL-7 in thymic micro-explants, suggesting that R5 virus replication occurred in cells other than thymocytes. Indeed, we found that R5 HIV-1 replicated efficiently in DC isolated from thymic tissue. The replicative capacity of X4 and R5 viruses differed according to the different DC subsets. R5 but not X4 HIV-1 efficiently replicated in CD11chighCD14+ DC. In contrast, no HIV-1 replication was detected in CD11clowCD14- DC. Both X4 and R5 viruses efficiently replicated in plasmacytoid DC, which secreted interferon-alpha upon HIV-1 exposure. Productive HIV-1 infection also caused DC loss, consistent with different permissivity of each DC subset.

CONCLUSIONS

Thymic DC sustain high levels of HIV-1 replication. DC might thus be the first target for R5 HIV-1 infection of thymus, acting as a Trojan horse for HIV-1 spread to thymocytes. Furthermore, DC death induced by HIV-1 infection may affect thymopoiesis.

Uncoupled anti-HIV and immune-enhancing effects when combining IFN-alpha and IL-7

Cytokine-based therapies have been examined for purging viral reservoirs and immunomodulation in HIV infection. However, single cytokines did not result in either HIV eradication or an efficient HIV-specific immune response. We hypothesize that cytokines with distinct biologic effects need to be combined for immunotherapy of HIV infection. In this study, we investigated the anti-HIV activity and immune-enhancing effects of the combination of IFN-alpha and IL-7. In human lymphocyte aggregate cultures infected ex vivo with the X4 HIV strain NL4-3, IFN-alpha/IL-7 potently inhibited HIV replication and preserved CD4(+) T cells, probably by up-regulating Bcl-2. IFN-alpha/IL-7 also strongly inhibited R5 HIV replication. Furthermore, in allogeneic MLRs, IFN-alpha/IL-7 increased T cell proliferation and IFN-gamma production. IFN-alpha alone also had strong anti-HIV activity, but neither preserved CD4(+) T cells nor increased T cell responses in MLRs. IL-7 alone maintained T cells and enhanced T cell activation in MLRs, but only moderately inhibited or increased HIV replication. Thus, coadministration of IFN-alpha/IL-7 combines the potent anti-HIV activity of IFN-alpha with the beneficial effects of IL-7 on T cell survival and function. We speculate that IFN-alpha will block viral replication, activate APCs, and up-regulate MHC molecules, thus allowing IL-7 to display its effects for generating an efficient immune response. In this scenario, the known reactivation of latent HIV by IL-7 may be advantageous.

Disease progression in macaques with low SIV replication levels: on the relevance of TREC counts

BACKGROUND

An attenuated immunodeficiency virus has been long considered innocuous. Nevertheless, converging data suggest that low levels of viral replication can still provoke AIDS. Pathogenesis of these attenuated infections is not understood.

OBJECTIVES

To determine the pathogenicity of a long-term attenuated infection and to delineate T-cell dynamics during such an infection.

METHODS

This is a cross-sectional study of 12 rhesus macaques infected with SIV Delta nef for 8 years. We evaluated apoptosis (annexin V), activation (HLA-DR, Ki67), and newly generated T cells (TCR excision circle: TREC).

RESULTS

Infection with SIV Delta nef induced pathological CD4 T-cell depletion after 8 years of infection. Virus replication and CD8 T-cell activation positively correlated with the rate of disease progression. The frequency of TREC within CD8+CD45RA+ cells increased in SIV Delta nef-infected animals compared to age-matched non-infected controls. Moreover, in the cohort of infected animals, TREC+CD45RA+CD4+ T-cell counts correlated strongly with non-progression to AIDS. The animal with the lowest rate of disease progression exhibited a 115-fold increase in TREC+CD45RA+CD4+ T-cell counts compared to age-matched non-infected controls. In contrast, the animal showing the fastest rate of progression to AIDS displayed 600-fold lower TREC+CD45RA+CD4+ T-cell counts compared to age-matched non-infected controls.

CONCLUSIONS

Our results suggest that the thymus plays a major role in the pathogenesis of an attenuated SIV infection and that a sustained thymic output could maintain CD4 T-cell homeostasis in the context of low viral loads.

HIV-1 clade promoters strongly influence spatial and temporal dynamics of viral replication in vivo

Although the primary determinant of cell tropism is the interaction of viral envelope or capsid proteins with cellular receptors, other viral elements can strongly modulate viral replication. While the HIV-1 promoter is polymorphic for a variety of transcription factor binding sites, the impact of these polymorphisms on viral replication in vivo is not known. To address this issue, we engineered isogenic SIVmac239 chimeras harboring the core promoter/enhancer from HIV-1 clades B, C, and E. Here it is shown that the clade C and E core promoters/enhancers bear a noncanonical activator protein-1 (AP-1) binding site, absent from the corresponding clade B region. Relative ex vivo replication of chimeras was strongly dependent on the tissue culture system used. Notably, in thymic histocultures, replication of the clade C chimera was favored by IL-7 enrichment, which suggests that the clade C polymorphism in the AP-1 and NF-kappaB binding sites is involved. Simultaneous infection of rhesus macaques with the 3 chimeras revealed a strong predominance of the clade C chimera during primary infection. Thereafter, the B chimera dominated in all tissues. These data show that the clade C promoter is particularly adapted to sustain viral replication in primary viremia and that clade-specific promoter polymorphisms constitute a major determinant for viral replication.

Impact of bone marrow hematopoiesis failure on T-cell generation during pathogenic simian immunodeficiency virus infection in macaques

Experimental infection of macaques with pathogenic strains of simian immunodeficiency virus (SIV) represents one of the most relevant animal models for studying HIV pathogenesis. In this study, we demonstrated a significant decrease in the generation of CD4+ T cells from bone marrow (BM) CD34+ progenitors in macaques infected with SIVmac251. This decrease appears to result from changes in the clonogenic potential of BM progenitors of both the myeloid and lymphoid lineages. We also demonstrated a significant decrease in the numbers of the most immature long-term culture-initiating cells (LTC-ICs). Hematopoietic failure occurred as early as primary infection, in the absence of CD34+ BM cell infection and was not related to plasma viral load. No major change was observed in the phenotype of BM CD34+ cells from infected macaques, including apoptosis markers such as annexin V staining and BcL-2 expression, but a significantly higher that normal proportion of cells were in the G0/G1 phase. This is the first demonstration that failure of BM hematopoiesis results in impaired T-cell production, which may contribute to the disruption of T-lymphocyte homeostasis characteristic of pathogenic lentiviral infections in primates.

IL-7 is a potent and proviral strain-specific inducer of latent HIV-1 cellular reservoirs of infected individuals on virally suppressive HAART

The persistence of HIV-1 in virally suppressed infected individuals on highly active antiretroviral therapy (HAART) remains a major therapeutic problem. The use of cytokines has been envisioned as an additional therapeutic strategy to stimulate latent proviruses in these individuals. Immune activation therapy using IL-2 has shown some promise. In the present study, we found that IL-7 was significantly more effective at enhancing HIV-1 proviral reactivation than either IL-2 alone or IL-2 combined with phytohemagglutinin (PHA) in CD8-depleted PBMCs. IL-7 also showed a positive trend for inducing proviral reactivation from resting CD4(+) T lymphocytes from HIV-1-infected patients on suppressive HAART. Moreover, the phylogenetic analyses of viral envelope gp120 genes from induced viruses indicated that distinct proviral quasispecies had been activated by IL-7, as compared with those activated by the PHA/IL-2 treatment. These studies thus demonstrate that different activators of proviral latency may perturb and potentially deplete only selected, specific portions of the proviral archive in virally suppressed individuals. The known immunomodulatory effects of IL-7 could be combined with its ability to stimulate HIV-1 replication from resting CD4(+) T lymphocytes, in addition to other moieties, to potentially deplete HIV-1 reservoirs and lead to the rational design of immune-antiretroviral approaches.

Homeostatic role of IL-7 in HIV-1 infected children on HAART: association with immunological and virological parameters

AIM

To investigate the role of IL-7 in HIV-infected children on highly active antiretroviral therapy (HAART) and its association with laboratory parameters related to disease progression.

PATIENTS AND METHODS

A cross-sectional study in 31 vertically HIV-infected children (median age 8.4 y) treated with HAART, and a longitudinal study in four of those same children was carried out. In both studies, viral load, CD4+ T-cell counts, thymic production of T cells by TCR rearrangement excision circles (TRECs), IL-7 plasma levels and viral phenotype were determined.

RESULTS

IL-7 levels were higher in HIV-infected children than in age-matched, uninfected controls. In addition, HIV children with CD4+ T cells between 200 and 500 T cells/mm3 had higher IL-7 levels and lower TREC values than HIV-infected children with CD4+ T cells >500 T cells/mm3. IL-7 levels were higher in children with syncytium-inducing (SI) phenotype than in those with non-syncytium-inducing (NSI) variants. During the follow-up of four HIV children, the decrease in viral load after HAART was always associated with a recovery of CD4+ T cells and TRECs, which was followed by a decrease in IL-7 returning to the levels present prior to the drop in CD4+ T cells. The four HIV-infected children had SI/X4 isolates in PBMC before HAART, and the viral phenotype switched to NSI/R5 after HAART.

CONCLUSION

Our data suggest that IL-7 plays a key role in the maintenance of T-cell homeostasis in HIV-infected children on HAART, both through peripheral expansion and through a thymus-dependent mechanism.

Role of cytokines and chemokines in the regulation of innate immunity and HIV infection

The earliest defense against microbial infection is represented by the responses of the innate (or natural) immune system, that also profoundly regulates the adaptive (or acquired) T- and B-cell immune responses. Activation of the innate immune system is primed by microbial invasion in response to conserved structures present in large groups of microorganisms (LPS, peptidoglycan, double-stranded RNA), and is finely tuned by different cell types (including dendritic cells, macrophages, natural killer cells, natural killer T cells, and gammadelta T cells). In addition, several soluble factors (complement components, defensins, mannose-binding lectins, interferons, cytokines and chemokines) can play a major role in the regulation of both the innate and adaptive immunity. In this review, we will briefly overview the regulation of some cellular subsets of the innate immune system particularly involved in human immunodeficiency virus (HIV) infection and then focus our attention on those cytokines and chemokines whose levels of expression are more profoundly affected by HIV infection and that, conversely, can modulate virus infection and replication.

Human immunodeficiency virus 1 favors the persistence of infection by activating macrophages through TNF

Macrophages play a major role in HIV-1 persistence. In the present paper, we demonstrate that the absence of apoptosis in HIV-1-infected primary human monocyte-differentiated macrophages (MDM) correlates with an increase in anti-apoptotic (Bcl-2 and Bcl-x(L)) and a decrease in pro-apoptotic (Bax and Bad) proteins. This is associated with macrophage activation as shown by tumor necrosis factor (TNF) production and NF-kappaB activation upon infection. TNF production was shown to be involved in the upregulation of Bcl-2 and Bcl-x(L) because this increase was abolished by an anti-TNF anti-serum or an inhibitor of TNF synthesis. In parallel, inhibition of TNF production induced an increase in the number of apoptotic cells. Furthermore, using an inhibitor of NF-kappaB activation, we demonstrated that TNF-induced upregulation of Bcl-x(L) and Bcl-2 occurs, respectively, through a NF-kappaB-dependent and an NF-kappaB-independent pathway.

Increased interleukin-7 plasma levels are associated with recovery of CD4+ T cells in HIV-infected children

To asses the role of interleukin 7 (IL-7) in the thymic reconstitution of CD4 T cells observed in children after successful antiretroviral therapy, a longitudinal study in five vertically HIV-1-infected children was carried out. Thymic function, IL-7 plasma levels, viral load, and T-lymphocytes subsets were determined every 2 or 3 months for about 90 months. In all the children, the drop in CD4+ T cells below 5-10% was associated with a marked increase in IL-7 plasma levels. The drastic decrease in viral load after treatment, led in all the cases to a recover of CD4 to levels higher than 30%, which was associated to an increase in thymic production of T cells and followed by a decrease in IL-7 to the normal levels. We conclude that the drop in CD4 in HIV children would induce an increase of IL-7 as part of a homeostatic mechanism. IL-7 would induce the thymus to produce new T cells to recover the normal levels of CD4 when the viral load was low and so the thymic function was not inhibited. The increase in the thymic production of new T cells recovers the CD4 population, and leads to a normalization of IL-7 levels.

Identification of T cell-signaling pathways that stimulate latent HIV in primary cells

Eradication of HIV infection depends on the elimination of a small, but stable population of latently infected T cells. After the discontinuation of therapy, activation of latent virus can rekindle infection. To purge this reservoir, it is necessary to define cellular signaling pathways that lead to activation of latent HIV. We used the SCID-hu (Thy/Liv) mouse model of HIV latency to analyze a broad array of T cell-signaling pathways and show in primary, quiescent cells that viral induction depends on the activation of two primary intracellular signaling pathways, protein kinase C or nuclear factor of activated T cells (NF-AT). In contrast, inhibition or activation of other important T cell stimulatory pathways (such as mitogen-activated protein kinase, calcium flux, or histone deacetylation) do not significantly induce virus expression. We found that the activation of NF-kappaB is critical to viral reactivation; however, all pathways that stimulate NF-kappaBdonot reactivate latent virus. Our studies further show that inhibition of NF-kappaB does not prevent activation of HIV by NF-AT, indicating that these pathways can function independently to activate the HIV LTR. Thus, we define several molecular pathways that trigger HIV reactivation from latency and provide evidence that latent HIV infection is maintained by the functional lack of particular transcription factors in quiescent cells.

IL-7 Stimulates T Cell Renewal Without Increasing Viral Replication in Simian Immunodeficiency Virus-Infected Macaques

The main failure of antiretroviral therapy is the lack of restoration of HIV-specific CD4(+) T cells. IL-7, which has been shown to be a crucial cytokine for thymopoiesis, has been envisaged as an additive therapeutic strategy. However, in vitro studies suggest that IL-7 might sustain HIV replication in thymocytes and T lymphocytes. Therefore, in the present study, we evaluated the effect of IL-7 on both T cell renewal and viral load in SIVmac-infected young macaques in the absence of antiretroviral therapy. This evaluation was conducted during the asymptomatic phase in view of a potential treatment of HIV patients. We show that IL-7 induces both a central renewal and a peripheral expansion of T lymphocytes associated with cell activation. No alarming modulation of the other hemopoietic cells was observed. No increase in the viral load was shown in blood or lymph nodes. These data strengthen the rationale for the use of IL-7 as an efficient immunotherapy in AIDS.

Cell-free HIV type 1 infection is restricted in the human trophoblast choriocarcinoma BeWo cell line, even with expression of CD4, CXCR4 and CCR5

The restriction of cell-free HIV-1 infection has been demonstrated in placental trophoblast choriocarcinoma BeWo cells. We tried to determine the level of the viral replication cycle at which this restriction occurs. BeWo cells produce infectious viruses after transfection with HIV-1 plasmids, independently of viral tropism. CCR5 and CXCR4, but not the CD4 molecule, were detected at the cell surface. We therefore derived CD4-expressing clones from transfected BeWo cells. Cell-free virus infection of these clones resulted in neither virus production nor viral sequence integration, indicating that the restriction occurs before integration of the virus. If we used luciferase reporter viruses pseudotyped with HIV-1 Env R5 and X4 for infection, no luciferase activity was detected, even in the BeWo-CD4+ clone, in contrast to what was observed in VSV-G pseudotyped virus infection. Our results show that infection of trophoblast-derived cells with cell-free virus is at least restricted at the level of entry. Thus, BeWo is an interesting human placental cell line that is resistant to HIV-1, even if CD4, CXCR4, and CCR5 are expressed.

T cell dynamics in HIV-1 infection

In the absence of antiretroviral treatment, HIV-1 establishes a chronic, progressive infection of the human immune system that invariably, over the course of years, leads to its destruction and fatal immunodeficiency. Paradoxically, while viral replication is extensive throughout the course of infection, deterioration of conventional measures of immunity is slow, including the characteristic loss of CD4(+) T cells that is thought to play a key role in the development of immunodeficiency. This conundrum suggests that CD4(+) T cell-directed viral cytopathicity alone cannot explain the course of disease. Indeed, recent advances now indicate that HIV-1 pathogenesis is likely to result from a complex interplay between the virus and the immune system, particularly the mechanisms responsible for T cell homeostasis and regeneration. We review these data and present a model of HIV-1 pathogenesis in which the protracted loss of CD4(+) T cells results from early viral destruction of selected memory T cell populations, followed by a combination of profound increases in overall memory T cell turnover, damage to the thymus and other lymphoid tissues, and physiological limitations in peripheral CD4(+) T cell renewal.

Effects of IL-7 on early human thymocyte progenitor cells in vitro and in SCID-hu Thy/Liv mice

IL-7 is a critical component of thymopoiesis in animals and has recently been shown to play an important role in T cell homeostasis. Although there is increasing interest in the use of IL-7 for the treatment of lymphopenia caused by the HIV type 1, evidence that IL-7 may accelerate HIV replication has raised concerns regarding its use in this setting. We sought to identify the effects of IL-7 on human thymocyte survival and to determine the impact of IL-7 administration on in vivo HIV infection of the human thymus. Using in vitro analysis, we show that IL-7 provides potent anti-apoptotic and proliferative signals to early thymocyte progenitors. Analysis of CD34(+) subpopulations demonstrates that surface IL-7 receptor is expressed on most CD34(high)CD5(+)CD1a(-) thymocytes and that this subpopulation appears to be one of the earliest maturation stages responsive to the effects of IL-7. Thus, IL-7 provides survival signals to human thymocytes before surface expression of CD1a. CD4(+)CD8(+) thymocytes are relatively unresponsive to IL-7, although IL-7 protects these cells from dexamethasone-induced apoptosis. IL-7 has a predominantly proliferative effect on mature CD4(+)CD3(+)CD8(-) and CD8(+)CD3(+)CD4(-) thymocytes. In contrast to the in vitro findings, we observe that in vivo administration of IL-7 to SCID-hu Thy/Liv mice does not appear to enhance thymocyte survival nor does it appear to accelerate HIV infection. Given the growing interest in the use of IL-7 for the treatment of human immunodeficiency, these findings support additional investigation into its in vivo effects on thymopoiesis and HIV infection.

Positive regulation of CXCR4 expression and signaling by interleukin-7 in CD4+ mature thymocytes correlates with their capacity to favor human immunodeficiency X4 virus replication

The emergence of X4 human immunodeficiency virus type 1 (HIV-1) variants in infected individuals is associated with poor prognosis. One of the possible causes of this emergence might be the selection of X4 variants in some specific tissue compartment. We demonstrate that the thymic microenvironment favors the replication of X4 variants by positively modulating the expression and signaling of CXCR4 in mature CD4(+) CD8(-) CD3(+) thymocytes. Here, we show that the interaction of thymic epithelial cells (TEC) with these thymocytes in culture induces an upregulation of CXCR4 expression. The cytokine secreted by TEC, interleukin-7 (IL-7), increases cell surface expression of CXCR4 and efficiently overcomes the downregulation induced by SDF-1 alpha, also produced by TEC. IL-7 also potentiates CXCR4 signaling, leading to actin polymerization, a process necessary for virus entry. In contrast, in intermediate CD4(+) CD8(-) CD3(-) thymocytes, the other subpopulation known to allow virus replication, TEC or IL-7 has little or no effect on CXCR4 expression and signaling. CCR5 is expressed at similarly low levels in the two thymocyte subpopulations, and neither its expression nor its signaling was modified by the cytokines tested. This positive regulation of CXCR4 by IL-7 in mature CD4(+) thymocytes correlates with their high capacity to favor X4 virus replication compared with intermediate thymocytes or peripheral blood mononuclear cells. Indeed, we observed an enrichment of X4 viruses after replication in thymocytes initially infected with a mixture of X4 (NL4-3) and R5 (NLAD8) HIV strains and after the emergence of X4 variants from an R5 primary isolate during culture in mature thymocytes.

The effects of different HIV type 1 strains on human thymic function

Studies of HIV-1-infected humans indicate that the thymus can be infected by HIV-1. In some of these patients, there is a significant CD4(+) T cell decline and a faster disease progression. This phenomenon is more evident in pediatric patients who depend heavily on their thymus for generation of new T cells. We hypothesize that HIV-1 causes T cell regenerative failure within the thymus, which has a profound impact on disease progression. Building on our established human thymopoiesis model, we include dynamic interactions between different HIV-1 strains (R5 and X4) and thymocytes. Our results predict that thymic infection with different HIV-1 strains induces thymic dysfunction to varying degrees, contributing to differences in disease progression as observed in both HIV-1-infected children and adults. Thymic infection in children is more severe than in adults, particularly during X4 infection. This outcome is likely due to both a higher viral load and a more active thymus in pediatric patients. Our results also indicate that a viral strain switch from R5 to X4 induces further deterioration in thymopoiesis. We predict that both viral and host factors play key roles in controlling thymic infection, including strain virulence and health status of the thymus.

Immune recovery in HIV disease: role of the thymus and T cell expansion in immune reconstitution strategies

While the progressive depletion of CD4(+) T cells is the hallmark of the impact of HIV on the immune system, considerable data also point to the loss of T cell function. The question is: Can the immune system recover from this insult and what are the therapeutic strategies available to us to mediate this immune recovery? This review will focus on our current knowledge of immune recovery following treatment with highly active antiretroviral therapy (HAART). Enhancement of thymic function in generating de novo T cell synthesis post-HAART has also emerged as a viable immune recovery strategy. Advances in molecular (T cell receptor excision circle assay) and conventional (computed tomography scans of the thymus) approaches to evaluate the role of the thymus in immune recovery as well as potential agents that might enhance thymic output (interleukin-7, IL-7) will contribute greatly to the assessment of the success of these approaches as immune recovery strategies. In this review, we will integrate this new information in the context of the current strategies for HIV therapy leading to long-term immune reconstitution.

Modalities of interleukin-7-induced human immunodeficiency virus permissiveness in quiescent T lymphocytes

The metabolic and cell cycle status of primary T lymphocytes conditions their susceptibility to human immunodeficiency virus (HIV) and HIV-derived vectors. While in fully quiescent T lymphocytes the reverse transcription and nuclear import of these retroelements are impaired, leading to an abortive infection, various stimuli can induce a state of virus permissiveness. Here, we studied the modalities by which interleukin-7 (IL-7), an important controller of T-cell homeostasis, exerts this effect. IL-7-exposed cord blood T lymphocytes proliferated and were efficiently transduced by HIV-derived vectors. In contrast, similarly treated adult peripheral blood (PB) T lymphocytes failed to divide, and only a subset of these cells became infectible. HIV-resistant and -sensitive subsets of IL-7-treated PB T lymphocytes differed in cell cycle status but not in naïve, memory, or activation phenotypes. Nuclear factor of activated T cells was not induced by IL-7, and cyclosporine did not prevent HIV-mediated gene transfer. Furthermore, the phosphatidylinositol 3-kinase (PI3K) inhibitor wortmannin blocked IL-7-induced cell survival and Bcl-2 synthesis but had no effect on the acquisition of HIV susceptibility, suggesting that IL-7-induced HIV type 1 permissiveness is not mediated by the PI-3 K pathway and that, perhaps, the Jak/STAT5 pathway, the other known mediator of IL-7-triggered signaling in T cells, governs this process.

Impact of cytokines on replication in the thymus of primary human immunodeficiency virus type 1 isolates from infants

Early infection of the thymus with the human immunodeficiency virus (HIV) may explain the more rapid disease progression among children infected in utero than in children infected intrapartum. Therefore, we analyzed infection of thymocytes in vitro by HIV type 1 primary isolates, obtained at or near birth, from 10 children with different disease outcomes. HIV isolates able to replicate in the thymus and impact thymopoiesis were present in all infants, regardless of the timing of viral transmission and the rate of disease progression. Isolates from newborns utilized CCR5, CXCR4, or both chemokine receptors to enter thymocytes. Viral expression was observed in discrete thymocyte subsets postinfection with HIV isolates using CXCR4 (X4) and isolates using CCR5 (R5), despite the wider distribution of CXCR4 in the thymus. In contrast to previous findings, the X4 primary isolates were not more cytopathic for thymocytes than were the R5 isolates. The cytokines interleukin-2 (IL-2), IL-4, and IL-7 increased HIV replication in the thymus by inducing differentiation and expansion of mature CD27(+) thymocytes expressing CXCR4 or CCR5. IL-2 and IL-4 together increased expression of CXCR4 and CCR5 in this population, whereas IL-4 and IL-7 increased CXCR4 but not CCR5 expression. IL-2 plus IL-4 increased the viral production of all pediatric isolates, but IL-4 and IL-7 had a significantly higher impact on the replication of X4 isolates compared to R5 isolates. Our studies suggest that coreceptor use by HIV primary isolates is important but is not the sole determinant of HIV pathogenesis in the thymus.

In vitro susceptibility to dexamethasone- and doxorubicin-induced apoptotic cell death in context of maturation stage, responsiveness to interleukin 7, and early cytoreduction in vivo in childhood T-cell acute lymphoblastic leukemia

Within childhood T-cell acute lymphoblastic leukemia (T-ALL), patients with a cortical (CD1a(+)) immunophenotype have been identified as a subgroup with favorable outcome in the acute lymphoblastic leukemia-Berlin-Frankfurt-Münster (ALL-BFM), Cooperative study group for childhood acute lymphoblastic leukemia (COALL) and Pediatric Oncology Group studies. We investigated in leukemic samples of children with T-ALL (n = 81) whether the different in vivo therapy response could be linked to differential in vitro susceptibility to apoptotic cell death. The extent of dexamethasone- as well as doxorubicin-induced apoptosis, detected by annexin V staining, positively correlated with the expression levels of CD1a (Spearman correlation coefficient, r(s) = 0.3 and 0.4, respectively; P <.01). When compared to cortical T-ALL, mature (CD1a(-), surface CD3(+)) T-ALL were significantly more resistant to doxorubicin, and immature, pro-/pre-T-ALL were more resistant to both drugs (P <.05). Apoptosis-related parameters (Bax, Bcl-2, CD95, and CD95-induced apoptosis) did not account for differential susceptibility to drug-induced apoptosis. By contrast, an interleukin 7-induced rescue of leukemic cells from spontaneous apoptosis, recently proposed to reflect distinct developmental stages and apoptotic programs in T-ALL, was highly associated with susceptibility to dexamethasone- but not doxorubicin-induced apoptosis (P <.001 versus P =.08). Analysis of clinical data showed that in vitro susceptibility to dexamethasone (but not to doxorubicin) closely correlated with early in vivo therapy response characterized by percentages of blast cells in bone marrow on day 15 (r(s) = -0.46, P =.001). Taken together, the in vitro assessment of drug-induced apoptosis revealed maturation-dependent differences within childhood T-ALL. The enhanced sensitivity to both drugs in cortical T-ALL might account for the better in vivo treatment response of this prognostically favorable T-ALL subgroup.

Interleukin-7-treated naive T cells can be productively infected by T-cell-adapted and primary isolates of human immunodeficiency virus 1

Although human immunodeficiency virus (HIV) gag/pol DNA can be detected in naive T cells, whether naive T cells can be productively infected by HIV is still questionable. Given that interleukin-7 (IL-7) is a prospective therapeutic immunomodulator for the treatment of HIV, we evaluated the effect of IL-7 on promoting naive T-cell infection of laboratory-adapted (IIIB), M-tropic, and primary isolates of HIV. Initially, we determined that the 3 cell surface markers widely used to identify naive T cells (CD45RA(+)CD45RO(-), CD45RA(+)CD62L(+), and CD45RO(-)CD27(+)CD95(low)) are all equivalent in T-cell receptor excision circle content, a marker for the replicative history of a cell as well as for de novo T cells. We therefore used CD45RA(+)CD45RO(-) expression to define naive T cells in this study. We demonstrate that although untreated or IL-2-treated naive T cells are not productively infected by HIV, IL-7 pretreatment mediated the productive infection of laboratory-adapted, M-tropic, and primary isolates of HIV as determined by p24 core antigen production. This up-regulation was between 8- and 58-fold, depending on the HIV isolate used. IL-7 pretreatment of naive T cells also potently up-regulated surface expression of CXCR4 but not CCR5 and mediated the expansion of naive T cells without the acquisition of the primed CD45RO phenotype. Collectively, these data indicate that IL-7 augments naive T-cell susceptibility to HIV and that under the appropriate environmental milieu, naive T cells may be a source of HIV productive infection. This information needs to be considered in evaluating IL-7 as an immunomodulator for HIV-infected patients.

Rapid and irreversible CD4+ T-cell depletion induced by the highly pathogenic simian/human immunodeficiency virus SHIV(DH12R) is systemic and synchronous

Highly pathogenic simian/human immunodeficiency virus chimeric viruses are known to induce a rapid, irreversible depletion of CD4+ T lymphocytes in the peripheral blood of acutely infected macaque monkeys. To more fully assess the systemic effects of this primary virus infection, specimens were collected serially between days 3 and 21 postinfection from variety of lymphoid tissues (lymph nodes, thymus, and spleen) and gastrointestinal tract and examined by DNA and RNA PCR, in situ hybridization, and immunohistochemical assays. In addition, the lymphoid tissues were evaluated by fluorescence-activated cell sorting. Virus infection was initially detected by DNA PCR on day 3 postinfection in lymph node samples and peaked on day 10 in the T-lymphocyte-rich areas of this tissue. CD4+ T-cell levels remained stable through day 10 in several lymphoid tissue specimens examined but fell precipitously between days 10 and 21. In situ terminal deoxynucleotidyltransferase-mediated dUTP-biotin nick end labeling (TUNEL) assays revealed the accumulation of apoptotic cells during the second week of infection in both lymph nodes and thymus, which colocalized, to a large extent, to sites of both virus replication and CD4+ T-lymphocyte loss.

Interleukin-7 in plasma correlates with CD4 T-cell depletion and may be associated with emergence of syncytium-inducing variants in human immunodeficiency virus type 1-positive individuals

Human immunodeficiency virus type 1 (HIV-1) primary infection is characterized by the use of CCR5 as a coreceptor for viral entry, which is associated with the non-syncytium-inducing (NSI) phenotype in lymphoid cells. Syncytium-inducing (SI) variants of HIV-1 appear in advanced stages of HIV-1 infection and are characterized by the use of CXCR4 as a coreceptor. The emergence of SI variants is accompanied by a rapid decrease in the number of T cells. However, it is unclear why SI variants emerge and what factors trigger the evolution of HIV from R5 to X4 variants. Interleukin-7 (IL-7), a cytokine produced by stromal cells of the thymus and bone marrow and by keratin, is known to play a key role in T-cell development. We evaluated IL-7 levels in plasma of healthy donors and HIV-positive patients and found significantly higher levels in HIV-positive patients. There was a negative correlation between circulating IL-7 levels and CD4(+) T-cell count in HIV-positive patients (r = -0.621; P < 0.001), suggesting that IL-7 may be involved in HIV-induced T-cell depletion and disease progression. IL-7 levels were higher in individuals who harbored SI variants and who had progressed to having CD4 cell counts of lower than 200 cells/microl than in individuals with NSI variants at a similar stage of disease. IL-7 induced T-cell proliferation and up-regulated CXCR4 expression in peripheral blood mononuclear cells in vitro. Taken together, our results suggest a role for IL-7 in the maintenance of T-cell regeneration and depletion by HIV in infected individuals and a possible relationship between IL-7 levels and the emergence of SI variants.

Interleukin-7 and infection itself by human immunodeficiency virus 1 favor virus persistence in mature CD4(+)CD8(-)CD3(+) thymocytes through sustained induction of Bcl-2

The sequence of events and the mechanisms leading to the destruction of the thymus during human immunodeficiency virus (HIV) infection are still poorly characterized. Investigated here are the survival capacity on HIV-1 infection of the mature single-positive CD4(+)CD8(-)CD3(+) (SP CD4(+)) and the intermediate CD4(+) CD8(-)CD3(-) thymocytes previously shown to be able to replicate the virus in the thymic microenvironment. It is demonstrated that the mature SP CD4(+) thymocytes exhibit a high survival capacity despite the production of a high yield of viruses. Interleukin-7, reported to be a crucial cofactor of tumor necrosis factor (TNF) to promote HIV replication, is shown here to counteract the apoptotic activity of TNF. Resistance to apoptosis of SP CD4(+) cells is conferred by a high expression of the IL-7 receptor (IL-7R) associated with the capacity of IL-7 to permanently up-regulate Bcl-2. In addition, this high Bcl-2 level is further enhanced by infection itself. In contrast, intermediate thymocytes, which replicate the virus at a lower level, are more sensitive to apoptosis, and their differentiation into double-positive CD4(+)CD8(+)CD3(-) (DP CD3(-)) cells strongly increases their death rate on infection. This sensitivity is related to a lower expression of IL-7R and Bcl-2 in intermediate thymocytes, which further decreases at the DP CD3(-) stage. In addition, a decreased level of Bcl-2 is observed in this subset during infection. Altogether these data suggest that in vivo, HIV infection might create a persistent virus reservoir within the SP CD4(+) thymocytes, whereas the later infection of intermediate cells might lead to thymopoiesis failure.

The dynamics of CD4+ T-cell depletion in HIV disease

The size and composition of the CD4+ T-cell population is regulated by balanced proliferation of progenitor cells and death of mature progeny. After infection with the human immunodeficiency virus, this homeostasis is often disturbed and CD4+ T cells are instead depleted. Such depletion cannot result simply from accelerated destruction of mature CD4+ T cells - sources of T-cell production must also fail. Ironically, this failure may be precipitated by physiological mechanisms designed to maintain homeostasis in the face of accelerated T-cell loss.

Increased production of IL-7 accompanies HIV-1-mediated T-cell depletion: implications for T-cell homeostasis

We hypothesized that HIV-1-mediated T-cell loss might induce the production of factors that are capable of stimulating lymphocyte development and expansion. Here we perform cross-sectional (n = 168) and longitudinal (n = 11) analyses showing that increased circulating levels of interleukin (IL)-7 are strongly associated with CD4+ T lymphopenia in HIV-1 disease. Using immunohistochemistry with quantitative image analysis, we demonstrate that IL-7 is produced by dendritic-like cells within peripheral lymphoid tissues and that IL-7 production by these cells is greatly increased in lymphocyte-depleted tissues. We propose that IL-7 production increases as part of a homeostatic response to T-cell depletion.