Telomeres and HIV-1 infection: in search of exhaustion

Cancer and HIV-1 Infection: Patterns of Chronic Antigen Exposure

The main role of the human immune system is to eliminate cells presenting foreign antigens and abnormal patterns, while maintaining self-tolerance. However, when facing highly variable pathogens or antigens very similar to self-antigens, this system can fail in completely eliminating the anomalies, leading to the establishment of chronic pathologies. Prototypical examples of immune system defeat are cancer and Human Immunodeficiency Virus-1 (HIV-1) infection. In both conditions, the immune system is persistently exposed to antigens leading to systemic inflammation, lack of generation of long-term memory and exhaustion of effector cells. This triggers a negative feedback loop where effector cells are unable to resolve the pathology and cannot be replaced due to the lack of a pool of undifferentiated, self-renewing memory T cells. In addition, in an attempt to reduce tissue damage due to chronic inflammation, antigen presenting cells and myeloid components of the immune system activate systemic regulatory and tolerogenic programs. Beside these homologies shared between cancer and HIV-1 infection, the immune system can be shaped differently depending on the type and distribution of the eliciting antigens with ultimate consequences at the phenotypic and functional level of immune exhaustion. T cell differentiation, functionality, cytotoxic potential and proliferation reserve, immune-cell polarization, upregulation of negative regulators (immune checkpoint molecules) are indeed directly linked to the quantitative and qualitative differences in priming and recalling conditions. Better understanding of distinct mechanisms and functional consequences underlying disease-specific immune cell dysfunction will contribute to further improve and personalize immunotherapy. In the present review, we describe relevant players of immune cell exhaustion in cancer and HIV-1 infection, and enumerate the best-defined hallmarks of T cell dysfunction. Moreover, we highlight shared and divergent aspects of T cell exhaustion and T cell activation to the best of current knowledge.

Coinhibitory receptors and CD8 T cell exhaustion in chronic infections

PURPOSE OF REVIEW

To describe the recent data on the role of coinhibitory receptors, such as PD-1, Tim-3, CD160, as mediators of the 'exhaustion' of virus-specific CD8 T cells in chronic infections and particularly in HIV.

RECENT FINDINGS

Exhaustion of chronic virus-specific CD8 T cells is a dynamic process characterized by altered differentiation, impaired function, and compromised proliferation/survival profile of these cells. This process is mediated by coinhibitory receptors expressed on the surface of virus-specific CD8 T cells and an orchestrated function of centrally connected pathways. Coexpression of several coinhibitory receptors characterizes severely exhausted virus-specific CD8 T cells. Several studies suggest a synergistic action, instead of a redundant role, of the different receptors. In-vivo manipulation of the coinhibitory network can rejuvenate exhausted virus-specific CD8 T cell responses and constrain replication of chronic viruses, including HIV.

SUMMARY

Revealing the molecular basis of virus-specific CD8 T cell exhaustion in chronic infections is critical for the understanding of the disease pathogenesis and the designing of novel vaccines aiming to enhance the cytolytic arm of the immune system. This is of particular interest for the development of immunotherapies in the context of a functional cure for HIV.

Regulatory T cells and HIV-1 infection

Recently, it has been emphasized that chronic generalized immune activation is a leading event in the pathogenesis of HIV-1 infection. Supporting evidence comes from observations that in cases of lack of activation, infected subjects maintain a high number of T cells and do not develop AIDS-related events. Despite intensive studies, the exact mechanisms of T-cell activation are still not well understood and options for their control are limited. Very promising in this direction is a recently described T-cell subpopulation--regulatory T cells. Their functional activity and vitality are strongly dependent on the presence of IL-2. Better understanding of the mechanisms of T-cell activation, as well as the contribution of regulatory T cells to its control will increase therapeutic options for HIV-1-infected subjects. The application of immune-based therapy together with highly active antiretroviral therapy will lend a helping hand to the natural regulatory mechanisms in the control of infection.

Telomerase activity of HIV-1-specific CD8+ T cells: constitutive up-regulation in controllers and selective increase by blockade of PD ligand 1 in progressors

Exhaustion of virus-specific T cells may play an important role in the pathophysiology of chronic viral infections. Here, we analyzed telomere length and telomerase activity in HIV-1-specific CD8+ T cells from progressors or controllers to determine underlying molecular pathways of T-cell exhaustion and senescence. Telomere lengths of HIV-1-specific CD8+ T cells from progressors were significantly shorter compared with autologous cytomegalovirus (CMV)/Epstein-Barr virus (EBV)-specific CD8+ T cells or bulk CD8+ T cells, while telomere lengths from controllers significantly exceeded those of autologous bulk CD8+ T cells and reached a similar level as HIV-1-specific CD8+ T cells collected during primary HIV-1 infection. Telomere length stabilization in controllers corresponded to high levels of constitutive telomerase activity, which was associated with preservation of cytotoxic and proliferative properties. Conversely, limited constitutive telomerase activity was observed in HIV-1-specific CD8+ T cells from progressors, although an increase in both telomere length and telomerase activity was achieved in antigenic-peptide-stimulated cells from progressors after blocking the PD-1/PD ligand 1 (PD-L1) pathway. Collectively, these data suggest a causal role of telomere shortening for the functional deficiencies of HIV-1-specific CD8+ T cells in chronic progressive infection, while high constitutive telomerase activities appears to contribute to maintenance of polyfunctional HIV-1-specific CD8+ T cells from HIV-1 controllers.

The HIV-1 HLA-A2-SLYNTVATL is a help-independent CTL epitope

The CTL response to the HLA-A*0201-restricted, HIV-1 p17 Gag(77-85) epitope (SLYNTVATL; SL9) has been extensively studied in patients. Although this reactivity is exceptionally prominent in chronically infected patients and inversely correlated to viral load, SL9-specific CTLs (SL9-CTLs) are rarely detected in acute infection. To explore the cellular basis for this unusual manifestation, SL9-CTLs primed ex vivo from naive circulating CD8(+) T cells of healthy, seronegative donors were generated and characterized. SL9 appeared to differ from other well-studied A*0201-restricted epitopes in several significant respects. In contrast to published reports for influenza and melanoma peptides and the HIV gag IV9 epitope studied here in parallel, SL9-CTLs were primed by immature but not mature autologous dendritic cells. Highly activated SL9-CTLs produce sufficient autocrine mediators to sustain clonal expansion and CTL differentiation for months without CD4(+) T cells or exogenous IL-2. Moreover, SL9-CTLs were sensitive to paracrine IL-2-induced apoptosis. IL-2 independence and sensitivity to paracrine IL-2 were also characteristic of SL9-CTLs immunized by dendritic cells transduced by a nonreplicating lentiviral vector encoding full-length Gag. In vitro-primed SL9-CTLs resembled those derived from patients in degeneracy of recognition and functional avidities for both SL9 and its natural mutations. Together, these data show that SL9 is a highly immunogenic, help-independent HIV epitope. The scarcity of SL9-CTLs in acute infection may result from cytokine-induced apoptosis with the intense activation of the innate immunity. In contrast, SL9-CTLs that constitutively produce autocrine help would predominate during CD4-diminished chronic infection.

Factors related to loss of HIV-specific cytotoxic T lymphocyte activity

OBJECTIVE

To identify factors associated with loss of in vitro stimulated anti-HIV cytotoxic T lymphocyte (CTL) activity.

METHODS

Immunological, virological and other characteristics of individuals who sustained anti-HIV CTL activity for prolonged periods with viral replication suppressed below detectable levels were compared with those that lost anti-HIV CTL activity under the same circumstances. Forty-four individuals, all but one receiving highly active antiretroviral therapy or combination therapy, were followed for 56 months. Virus load, lymphocyte counts, CD28 expression on CD8 T cells, in vitro restimulated HIV-specific CTL and T cell proliferation were assessed at regular intervals.

RESULTS

Anti-HIV CTL responses were maintained throughout by 20 individuals with consistently detectable HIV replication and in 17 of 24 individuals with sustained suppression of HIV replication. As a group, the seven who lost anti-HIV CTL were older, had weaker baseline anti-HIV CTL activity, higher historical virus loads, lower historical and contemporary CD4 T cell counts and a lower percentage of CD8 T cells expressing CD28. Multivariate analysis suggested that CD4 T cell counts and anti-HIV CTL amplitude at study onset were independently associated with CTL loss in these individuals, as was percentage of CD8 T cells expressing CD28 at study's end. There was a significant direct correlation between nadir CD4 T cell counts and duration of anti-HIV CTL persistence after suppression of viral replication.

CONCLUSIONS

Most HIV-infected individuals retain CD8 anti-HIV CTL with in vitro proliferative potential, even when antigen is limited. Those who lose HIV-specific CTL responses generally show past or current evidence of severe disease progression or activity.

Expression of CD57 defines replicative senescence and antigen-induced apoptotic death of CD8+ T cells

Virus-specific CD8(+) T-cell responses play a pivotal role in limiting viral replication. Alterations in these responses, such as decreased cytolytic function, inappropriate maturation, and limited proliferative ability could reduce their ability to control viral replication. Here, we report on the capacity of HIV-specific CD8(+) T cells to secrete cytokines and proliferate in response to HIV antigen stimulation. We find that a large proportion of HIV-specific CD8(+) T cells that produce cytokines in response to cognate antigen are unable to divide and die during a 48-hour in vitro culture. This lack of proliferative ability of HIV-specific CD8(+) T cells is defined by surface expression of CD57 but not by absence of CD28 or CCR7. This inability to proliferate in response to antigen cannot be overcome by exogenous interleukin-2 (IL-2) or IL-15. Furthermore, CD57 expression on CD8(+) T cells, CD4(+) T cells, and NK cells is a general marker of proliferative inability, a history of more cell divisions, and short telomeres. We suggest, therefore, that the increase in CD57(+) HIV-specific CD8(+) T cells results from chronic antigen stimulation that is a hallmark of HIV infection. Thus, our studies define a phenotype associated with replicative senescence in HIV-specific CD8(+) T cells, which may have broad implications to other conditions associated with chronic antigenic stimulation.

Proliferative arrest and cell cycle regulation in CD8(+)CD28(-) versus CD8(+)CD28(+) T cells

CD8(+)CD28(-) T cells have been characterized by oligoclonal expansions, impaired proliferative responses, but preserved cytotoxicity and reduced telomeres. To examine this subset further and define the underlying mechanisms of proliferation arrest, we investigated several features of this cell type compared with CD8(+)CD28(+) controls. We analyzed expression of various activation markers, thymidine incorporation upon activation, T-cell receptor (TCR) zeta-chain phosphorylation, cell cycle characteristics, and cell cycle related gene expression. Flow cytometry revealed higher expression of CD11b, CD29, CD57, and CD94, and lower expression of CD25 in CD8(+)CD28(-) compared with CD8(+)CD28(+) T cells. Sorted CD8(+)CD16(-)CD28(-) cells exhibited decreased phosphorylation of the TCR zeta-chain in three of four probands. Proliferation of these T cells was impaired, even when activated with mitogens that bypass TCR signaling. Cell cycle profiles demonstrated a lower percentage of cycling cells and significantly higher levels of cyclin dependent kinase inhibitor p16(INK4a) in the CD28(-) subset compared with the CD28(+) control. These observations suggest that expanded CD8(+)CD28(-) T cells in normal elderly individuals have reduced proliferation concomitant with increased p16(INK4a) expression. Defects in TCR signaling were associated with altered TCR zeta-chain phosphorylation.

Proliferation and telomere length in acutely mobilized blood mononuclear cells in HIV infected patients

The aim of the study was to investigate the mobilization of T cells in response to a stressful challenge (adrenalin stimulation), and to access T cells resided in the peripheral lymphoid organs in HIV infected patients. Seventeen patients and eight HIV seronegative controls received an adrenalin infusion for 1 h. Blood was sampled before, during and 1 h after adrenalin infusion. Proliferation and mean telomere restriction fragment length (telomeres) of blood mononuclear cells (BMNC) and purified CD8+ and CD4+ cells were investigated at all time points. In patients, the proliferation to pokeweed mitogens (PWM) was lower and decreased more during adrenalin infusion. After adrenalin infusion the proliferation to PWM was restored only in the controls. In all subjects telomeres in CD4+ cells declined during adrenalin infusion. Additionally, the patients had shortened telomeres in their CD8+ cells, and particularly HAART treated patients had shortened telomeres in all cell-subtypes. The finding that patients mobilized cells with an impaired proliferation to PWM during and after adrenalin infusion has possible clinical relevance for HIV infected patients during pathological stressful conditions, such as sepsis, surgery and burns. However, this study did not find a correlation between impaired proliferation and telomeres. It is concluded that physiological stress further aggravates the HIV-induced immune deficiency.

TCRBV CDR3 diversity of CD4+ and CD8+ T-lymphocytes in HIV-infected individuals

TCRBV CDR3 repertoire diversity was analyzed in a cross-sectional study of HIV-infected individuals by CDR3 fingerprinting/spectratyping and single strand conformation polymorphism (SSCP). Most TCRBV families were detected in CD4+ cells of HIV-infected patients with CD4 counts ranging from 35 to 1103. In patients with CD4 counts >500, CD4+ TCRBV CDR3 fingerprinting profiles contained subtle variations with generally gaussian-distributed sizes. Lower CD4 counts coincided with more fragmented TCRBV CDR3 repertoires, containing dominant bands and bands missing from the CDR3 profiles. The CD8+ population of the same patients exhibited skewed CDR3 profiles of the majority of TCR BV families at CD4 counts >500. Irregularity of CD8+ CDR3 size distribution was most profound at low CD4 counts and suggested domination of the CD8+ TCRBV repertoire by a limited number of clones. Skewed patterns of CDR3 diversity probably reflect (oligo)clonal expansion of particular CD4+ and CD8+ cell populations during chronic infection with HIV. In addition, irregular CDR3 profiles of CD4+ and CD8+ at low CD4 counts suggest diminished TCR repertoire diversity, which may contribute to immunodeficiency.

Effects of subcutaneous IL-2 therapy on telomere lengths in PBMC in HIV-infected patients

In this study we investigated the effect of interleukin-2 (IL-2) on mean terminal restriction fragment (TRF) lengths in peripheral blood mononuclear cells (PBMC). Ten human immunodeficiency virus (HIV)-infected individuals were included and IL-2 was administered subcutaneously with 3 x 106 IU three times a week for 24 weeks. Mean TRF length was decreased on average by 267 bp at week 4 (P = 0.03) and 286 bp at week 8 (P = 0.09). Individual TRF changes at weeks 12, 16, 20 and 24 were highly variable. However, in the 12 weeks following therapy, TRF lengths generally increased reaching baseline levels by the end of the study. At baseline, mean TRF lengths were positively correlated to the ratio of naïve and memory phenotype within both CD4+ and CD8+ cells. This study shows that IL-2 treatment induces transient shortened mean TRF lengths in PBMC from HIV-infected individuals, indicating that IL-2 enhances the lymphocyte count by peripheral proliferation or recruitment of memory T cells into the blood.

Effects of G-CSF on telomere lengths in PBMCs from human immunodeficiency virus-infected patients: results from a randomized, placebo-controlled trial

Telomeres are unique terminal chromosomal structures, the length of which has been shown to decrease with cell division in vitro and with increased age in vivo for human somatic cells. In human immunodeficiency virus (HIV)-1 infection, decrease of telomere length is primarily found in CD8+ T cells, and not in CD4+ T cells. In this double-blind placebo-controlled study, we investigated the effect of granulocyte colony stimulating factor (G-CSF) treatment combined with highly active antiretroviral therapy (HAART) on mean telomere length in peripheral blood mononuclear cells (PBMC). The terminal restriction fragment (TRF) length showed no changes during G-CSF treatment although the number of lymphocytes increased significantly. The mean TRF length correlated positively (R = 0.552, P = 0.009) and negatively (R = -0.503, P = 0.02) to the proportion of CD4+ memory and naïve cells, respectively. Our data suggest that during G-CSF treatment lymphocytes are recruited by a combination of central and peripheral proliferation.

Functional and genetic integrity of the CD8 T-cell repertoire in advanced HIV infection

BACKGROUND

HIV-specific cytotoxic T lymphocytes (CTL) can restrict HIV replication in acute and chronic infection, but disease progression occurs in parallel with declining CTL activity. An understanding of why CTL fail to control HIV replication might reveal important mechanisms of disease progression and enhance prospects for developing effective CTL-based immunotherapies.

OBJECTIVES

To investigate the functional integrity, T-cell repertoire diversity, and HIV reactivity of CD8 T lymphocytes in individuals with advanced HIV infection.

METHODS

Individuals were considered to have progressed to advanced HIV infection if their total T-cell count was < 500 x 10(6) cells/(l) on at least two successive clinic visits. CD8 T cells from these individuals were analyzed for CTL function, HIV reactivity and T-cell receptor (TCR) diversity by chromium release assays and reverse transcriptase polymerase chain reaction.

RESULTS

CD8 T cells from all individuals with advanced HIV infection proliferated and differentiated into functional CTL in vitro. Despite extremely low T-cell counts and previous AIDS-defining illnesses, six individuals had inducible anti-HIV CTL responses. In two additional cases, HIV-specific CTL activity became detectable following significant treatment-associated remission of T-cell lymphopenia. Assessment of TCRbetaV gene family representation and betaV gene intrafamily diversity indicated CD8 T-cell repertoire diversity is maintained through advanced HIV infection.

CONCLUSIONS

These data suggest that HIV-specific CTL activity can be selectively compromised while the functional and genetic integrity of the CD8 population as a whole remains intact. A substantial fraction of individuals retain inducible anti-HIV CTL activity through advanced HIV infection and, in at least some cases, effective treatment can restore HIV-specific CTL responses even at this late stage of disease.

Postthymic Development of CD28−CD8+ T Cell Subset: Age-Associated Expansion and Shift from Memory to Naive Phenotype

During human aging, one of the major changes in the T cell repertoire is a dramatic expansion of T cells with the atypical CD28−CD8+ phenotype. In this study, we show that this increase is a consequence not only of an expansion in the CD28−CD8+ population but also of a decrease in the number of CD28+CD8+ T cells. The decrease in circulating CD28+CD8+ T cells is dramatically accelerated after the age of 50 and is not accompanied by an equivalent reduction in the CD28+CD4+ subset. Our findings confirm that aging leads to an accumulation of CD45RO+ T cells within the CD28+CD8+ subset as previously observed. Surprisingly, we found an increase in CD45RA+ expression with age in the CD28−CD8+ subset. Immune-phenotyping for activation markers, measurement of telomere DNA content, and cytokine production analysis indicate that the large majority of CD28−CD8+ T cells are Ag-experienced, despite their CD45RA+ phenotype. Our study further demonstrates that the poor proliferative response displayed by CD28−CD8+ T cells is not a consequence of telomere shortening. Also, analysis of cytokine production at the single cell level revealed that the proportions of IFN-γ+, IL-4+, and IL-10+ T cells are considerably higher among the CD28−CD8+ than the CD28+CD8+ subset. In summary, these data explain the presence of CD45RA+ T cells in the elderly, shed light on the phylogenetic origin of CD28−CD8+ T cells, and suggest a role for these cells in the immune senescence process.

Telomere dynamics in monkeys: increased cell turnover in macaques infected with chimeric simian-human immunodeficiency viruses

To address the question of how cell turnover is affected by retroviral infections, we used the telomeric terminal restriction fragments (TRFs) as markers of cell replicative history and measured their length in macaques infected with chimeric simian-human immunodeficiency viruses (SHIVs). The TRF lengths of mononuclear cells in 104 samples, including longitudinal samples from nine cynomolgus and ten pig-tailed macaques infected with SHIV, and in samples from 26 uninfected macaques, were quantitated by an improved method, based on two-dimensional calibration of DNA sizes, pulsed field electrophoresis, and high-resolution Southern blot images. The average TRF lengths of peripheral blood mononuclear cells (PBMCs) from uninfected pig-tailed (14.9+/-1.6 kbp) and cynomolgus (14.1+/-1.8 kbp) macaques were about 3 and 5 kbp longer than those of human infants and 30-year-old adults, respectively. The rate of TRF length shortening in infected pig-tailed macaques was significantly (P = 0.035) higher (2.2-fold) than in uninfected monkeys. The TRFs in SHIV-infected cynomolgus monkeys, which, in general, had lower viral loads than pig-tailed macaques, shortened on average more rapidly (1.6-fold) than in uninfected animals, but the difference was not statistically significant. The TRFs of mononuclear cells from the lymph nodes of two rapidly progressing SHIV-infected macaques that developed AIDS and died also shortened in parallel but somewhat more rapidly than in the PBMCs. These results suggest that the rate of PBMC turnover in macaques could be increased several-fold during infections by immunodeficiency viruses, likely due to immune activation by SHIV antigens.

Telomere dynamics in HIV-1 infected and uninfected chimpanzees measured by an improved method based on high-resolution two-dimensional calibration of DNA sizes

We developed an improved method for accurately measuring telomere lengths based on two-dimensional calibration of DNA sizes combined with pulsed field electrophoresis and quantitative analysis of high-resolution gel images. This method was used to quantify the length of telomeres in longitudinal samples of peripheral blood mononuclear cells (PBMCs) from five chimpanzees infected with human immunodeficiency virus type 1 (HIV-1) and three uninfected animals, 14 to 27 years of age. The average length of the telomere restriction fragments (TRF) of infected and uninfected chimpanzees were 11.7 +/- 0.25 kbp, and 11.6 +/- 0.61 kbp, respectively, and were about 1 kbp and 3 kbp longer than those of human infants and 30 year old adults, respectively. There was a trend of a slight decrease (30-60 bp per year) in the TRF of two HIV infected chimpanzees over 30-35 months, while the TRF of one naive chimpanzee slightly increased over 20 months. Although the number of chimpanzees in this study is small and no statistically significant linear dependencies on time were observed, it appears that in chimpanzees, rates of shortening of the TRF are comparable or smaller than in adult humans and are not significantly affected by HIV-1 infection, which may be related to the inability of HIV-1 to cause disease in these animals.