A functional CD8+ cell assay reveals individual variation in CD8+ cell antiviral efficacy and explains differences in human T-lymphotropic virus type 1 proviral load

High frequencies of functionally competent circulating Tax-specific CD8+ T cells in human T lymphotropic virus type 2 infection

Human T lymphotropic virus type 2 (HTLV-2) is characterized by a clinically asymptomatic persistent infection in the vast majority of infected individuals. In this study, we have characterized for the first time ex vivo specific CTL responses against the HTLV-2 Tax protein. We could detect CTL responses only against a single HLA-A*0201-restricted Tax2 epitope, comprising residues 11-19 (LLYGYPVYV), among three alleles screened. Virus-specific CTLs could be detected in most evaluated subjects, with frequencies as high as 24% of circulating CD8(+) T cells. The frequency of specific CTLs had a statistically significant positive correlation with proviral load levels. The majority of virus-specific CD8(+) T cells exhibited an effector memory/terminally differentiated phenotype, expressed high levels of cytotoxicity mediators, including perforin and granzyme B, and lysed in vitro target cells pulsed with Tax2((11-19)) synthetic peptide in a dose-dependent manner. Our findings suggest that a strong, effective CTL response may control HTLV-2 viral burden and that this may be a significant factor in maintaining persistent infection and in the prevention of disease in infected individuals.

The avidity and lytic efficiency of the CTL response to HTLV-1

In human T-lymphotropic virus type 1 (HTLV-1) infection, a high frequency of HTLV-1-specific CTLs can co-exist stably with a high proviral load and the proviral load is strongly correlated with the risk of HTLV-1-associated inflammatory diseases. These observations led to the hypothesis that HTLV-1 specific CTLs are ineffective in controlling HTLV-1 replication but contribute to the pathogenesis of the inflammatory diseases. But evidence from host and viral immunogenetics and gene expression microarrays suggests that a strong CTL response is associated with a low proviral load and a low risk of HAM/TSP. Here, we quantified the frequency, lytic activity and functional avidity of HTLV-1-specific CD8(+) cells in fresh, unstimulated PBMCs from individuals with natural HTLV-1 infection. The lytic efficiency of the CD8(+) T cell response-the fraction of autologous HTLV-1-expressing cells eliminated per CD8(+) cell per day-was inversely correlated with both the proviral load and the rate of spontaneous proviral expression. The functional avidity of HTLV-1-specific CD8(+) cells was strongly correlated with their lytic efficiency. We conclude that efficient control of HTLV-1 in vivo depends on the CTL lytic efficiency, which depends in turn on CTL avidity of Ag recognition. CTL quality determines the position of virus-host equilibrium in persistent HTLV-1 infection.

Impaired Tax-specific T-cell responses with insufficient control of HTLV-1 in a subgroup of individuals at asymptomatic and smoldering stages

Human T-cell leukemia virus type-1 (HTLV-1)-specific T-cell immunity, a potential antitumor surveillance system in vivo, is impaired in adult T-cell leukemia (ATL). In this study, we aimed to clarify whether the T-cell insufficiency in ATL is present before the disease onset or occurs as a consequence of the disease. We investigated T-cell responses against Tax protein in peripheral blood mononuclear cells (PBMCs) from individuals at earlier stages of HTLV-1-infection, including 21 asymptomatic HTLV-1 carriers (ACs) and four patients with smoldering-type ATL (sATL), whose peripheral lymphocyte count was in normal range. About 30% of samples tested showed clear Tax-specific interferon (IFN)-gamma producing responses. Proviral loads in this group were significantly lower than those in the other less-specific response group. The latter group was further divided to two subgroups with or without emergence of Tax-specific responses following depletion of CC chemokine receptor 4 (CCR4)(+) cells that contained HTLV-1-infected cells. In the PBMCs with Tax-specific responses, CD8(+) cells efficiently suppressed HTLV-1 p19 production in culture. The remaining group without the emergence of Tax-specific response after CCR4(+) cell-depletion included at least two sATL and one AC samples, which spontaneously produced HTLV-1 p19 in culture, where tetramer-binding, Tax-specific cytotoxic T-lymphocytes were either undetectable or unresponsive. Our results indicated that HTLV-1-specific T-cell responsiveness widely differed among HTLV-1 carriers, and that impairment of HTLV-1-specific T-cell responses was observed not only in advanced ATL patients but also in a subpopulation at earlier stages, which was associated with insufficient control of HTLV-1.

Treatment of HTLV-I-associated myelopathy/tropical spastic paraparesis: toward rational targeted therapy

The treatment of HAM/TSP is a challenge. No agent has shown to significantly modify the long-term disability associated with HAM/TSP. Advances in our understanding of the pathogenesis of HAM/TSP have led to the identification of several biomarkers and therapeutic targets. Clinical trials in HAM/TSP continue to be opportunities for further qualification and refinement of biomarkers and therapeutic targets. The validation of HAM/TSP relevant biomarkers and the identification of new targets remain key challenges in the development of effective targeted therapy in HAM/TSP.

Estimating in vivo death rates of targets due to CD8 T-cell-mediated killing

Despite recent advances in immunology, several key parameters determining virus dynamics in infected hosts remain largely unknown. For example, the rate at which specific effector and memory CD8 T cells clear virus-infected cells in vivo is hardly known for any viral infection. We propose a framework to quantify T-cell-mediated killing of infected or peptide-pulsed target cells using the widely used in vivo cytotoxicity assay. We have reanalyzed recently published data on killing of peptide-pulsed splenocytes by cytotoxic T lymphocytes and memory CD8 T cells specific to NP396 and GP276 epitopes of lymphocytic choriomeningitis virus (LCMV) in the mouse spleen. Because there are so many effector CD8 T cells in spleens of mice at the peak of the immune response, NP396- and GP276-pulsed targets are estimated to have very short half-lives of 2 and 14 min, respectively. After the effector numbers have diminished, i.e., in LCMV-immune mice, the half-lives become 48 min and 2.8 h for NP396- and GP276-expressing targets, respectively. Analysis of several alternative models demonstrates that the estimates of half-life times of peptide-pulsed targets are not affected when changes are made in the model assumptions. Our report provides a unifying framework to compare killing efficacies of CD8 T-cell responses specific to different viral and bacterial infections in vivo, which may be used to compare efficacies of various cytotoxic-T-lymphocyte-based vaccines.

HTLV-1 integration into transcriptionally active genomic regions is associated with proviral expression and with HAM/TSP

Human T-lymphotropic virus type 1 (HTLV-1) causes leukaemia or chronic inflammatory disease in ∼5% of infected hosts. The level of proviral expression of HTLV-1 differs significantly among infected people, even at the same proviral load (proportion of infected mononuclear cells in the circulation). A high level of expression of the HTLV-1 provirus is associated with a high proviral load and a high risk of the inflammatory disease of the central nervous system known as HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP). But the factors that control the rate of HTLV-1 proviral expression remain unknown. Here we show that proviral integration sites of HTLV-1 in vivo are not randomly distributed within the human genome but are associated with transcriptionally active regions. Comparison of proviral integration sites between individuals with high and low levels of proviral expression, and between provirus-expressing and provirus non-expressing cells from within an individual, demonstrated that frequent integration into transcription units was associated with an increased rate of proviral expression. An increased frequency of integration sites in transcription units in individuals with high proviral expression was also associated with the inflammatory disease HAM/TSP. By comparing the distribution of integration sites in human lymphocytes infected in short-term cell culture with those from persistent infection in vivo, we infer the action of two selective forces that shape the distribution of integration sites in vivo: positive selection for cells containing proviral integration sites in transcriptionally active regions of the genome, and negative selection against cells with proviral integration sites within transcription units.

The human leukaemia virus HTLV-1 causes a lifelong infection that cannot be cleared by the immune system. By integrating into the host's DNA, the virus can lie dormant within the cell. The virus can then be reactivated, by processes that are only partly understood, causing the infected cell to multiply and leading to an increase in the quantity of virus in the infected person. In some infected people, the virus is reactivated much faster than in others, and such people are more likely to develop HTLV-1-associated inflammatory diseases such as HAM/TSP, which results in paralysis of the legs. It is not understood what determines this rate of viral reactivation in each person. In this study, we found that integration of HTLV-1 in the host's DNA close to other genes was associated with faster viral reactivation and a higher probability of HAM/TSP. By comparing the viral integration site positions in samples from patients and in cells infected with HTLV-1 in the laboratory, we can identify some of the major forces that allow the virus to persist lifelong whilst avoiding eradication by the immune response.

High frequency of CD4+FoxP3+ cells in HTLV-1 infection: inverse correlation with HTLV-1-specific CTL response

Evidence from population genetics, gene expression microarrays, and assays of ex vivo T-cell function indicates that the cytotoxic T lymphocyte (CTL) response to human T-lymphotropic virus type 1 (HTLV-1) controls the level of HTLV-1 expression and the proviral load. The rate at which CTLs kill autologous HTLV-1–infected lymphocytes differs significantly among infected people, but the reasons for such variation are unknown. Here, we demonstrate a strong negative cor-relation between the frequency of CD4⁺FoxP3⁺ Tax⁻ regulatory T cells (T_regs) in the circulation and the rate of CTL-mediated lysis of autologous HTLV-1–infected cells ex vivo. We propose that the frequency of CD4⁺FoxP3⁺ Tax⁻ T_regs is one of the chief determinants of the efficiency of T cell–mediated immune control of HTLV-1.

How does HTLV-I persist despite a strong cell-mediated immune response?

Human T-lymphotropic virus type 1 (HTLV-1) is a pathogenic retrovirus that infects human CD4(+) T lymphocytes. Despite its presence in T cells, HTLV-1 causes little overt immunosuppression. This host-virus relationship has therefore been exploited as an excellent model system for studying the dynamic interaction between a persistent retrovirus and the normal human immune system. We use a combination of mathematical and experimental techniques to identify key factors on both sides of the in vivo host-virus interaction that significantly determine HTLV-I proviral load and disease risk. We develop a model to describe how these factors interact to enable viral persistence.

In vivo T lymphocyte dynamics in humans and the impact of human T-lymphotropic virus 1 infection

Human T-lymphotropic virus type 1 (HTLV-1) is a persistent CD4+ T-lymphotropic retrovirus. Most HTLV-1-infected individuals remain asymptomatic, but a proportion develop adult T cell leukemia or inflammatory disease. It is not fully understood how HTLV-1 persists despite a strong immune response or what determines the risk of HTLV-1-associated diseases. Until recently, it has been difficult to quantify lymphocyte kinetics in humans in vivo. Here, we used deuterated glucose labeling to quantify in vivo lymphocyte dynamics in HTLV-1-infected individuals. We then used these results to address four questions. (i) What is the impact of HTLV-1 infection on lymphocyte dynamics? (ii) How does HTLV-1 persist? (iii) What is the extent of HTLV-1 expression in vivo? (iv) What features of lymphocyte kinetics are associated with HTLV-1-associated myelopathy/tropical spastic paraparesis? We found that CD4+CD45RO+ and CD8+CD45RO+ T lymphocyte proliferation was elevated in HTLV-1-infected subjects compared with controls, with an extra 10(12) lymphocytes produced per year in an HTLV-1-infected subject. The in vivo proliferation rate of CD4+CD45RO+ cells also correlated with ex vivo viral expression. Finally, the inflammatory disease HTLV-1-associated myelopathy/tropical spastic paraparesis was associated with significantly increased CD4+CD45RO+ cell proliferation. We suggest that there is persistent viral gene expression in vivo, which is necessary for the maintenance of the proviral load and determines HTLV-1-associated myelopathy/tropical spastic paraparesis risk.

Quantifying HTLV-I dynamics

Despite significant advances in our understanding of the immune response to persistent viruses like human T-cell lymphotropic virus type I (HTLV-I), many important questions remain unanswered. Mathematical modelling enables us to interpret and synthesise diverse experimental data in new ways and thus can contribute to our understanding. Here, we review recent advances in mathematical modelling of HTLV-I infection and illustrate how mathematics has enabled us to identify factors that determine an individual's viral burden and risk of developing HTLV-I-associated diseases.

SYBR Green-based quantitation of human T-lymphotropic virus type 1 proviral load in Peruvian patients with neurological disease and asymptomatic carriers: influence of clinical status, sex, and familial relatedness

To evaluate the human T-lymphotropic virus type 1 (HTLV-1) proviral DNA load in patients with HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP) and asymptomatic HTLV-1 carriers, a SYBR Green-based real-time quantitative polymerase chain reaction (qPCR) assay was developed. HTLV-1 proviral DNA in peripheral blood mononuclear cells (PBMCs) was quantified using primers targeting the pX region and the HTLV-1 copy number normalized to the amount of ERV-3 (Endogenous Retrovirus 3) cellular DNA. Thirty-three asymptomatic HTLV-1 carriers (ACs) and 39 patients with HAM/TSP were enrolled. Some participants were relatives of HAM/TSP cases (16 ACs and 7 patients with HAM/TSP). On multiple linear regression analysis, the authors found a significant association between clinical status and HTLV-1 proviral load (P < .01), but only among women. ACs showed a median proviral load of 561 copies per 104 PBMCs (interquartile range: 251-1623). In HAM/TSP patients, the median proviral load was 1783 (1385-2914). ACs related to HAM/TSP patients presented a relatively high proviral load (median 1152); however, the association between relatedness to a HAM/TSP patient and proviral load was not significant (P = .1). In HAM/TSP patients, no association was found between proviral load and disease duration, progression or severity. The fact that the effect of HAM/TSP upon the HTLV-1 proviral load differed between sexes and the finding of a high proviral load among asymptomatic relatives of HAM/TSP patients suggest that not yet identified genetic or environmental factors influence the pathogenesis of HTLV-1 infection.

Understanding the failure of CD8+ T-cell vaccination against simian/human immunodeficiency virus

Although CD8+ T cells play an important role in controlling viral infections, boosting specific CD8+ T cells by prophylactic vaccination with simian immunodeficiency virus (SIV) epitopes fails to provide sterilizing immunity. Viral replication rates and viral contraction rates after the peak viremia hardly depend on the presence of memory CD8+ T cells. To study these paradoxical findings, we parameterize novel mathematical models for acute SIV and human immunodeficiency virus infection. These models explain that failure of vaccination is due to the fact that effector/target ratios are too low during the viral expansion phase. Because CD8+ T cells require cell-to-cell contacts, immune protection requires high effector/target ratios at the primary site of infection. Effector/target ratios become favorable for immune control at the time of the peak in the viral load when the virus becomes limited by other factors, such as the availability of uninfected target cells. At the viral set point, effector/target ratios are much higher, and perturbations of the number of CD8+ effector cells have a large impact on the viral load. Such protective effector/target ratios are difficult to achieve with nucleic acid- or protein-based vaccines.

Histone deacetylase inhibitors increase virus gene expression but decrease CD8+ cell antiviral function in HTLV-1 infection

The dynamics of human T-lymphotropic virus type-1 (HTLV-1) provirus expression in vivo are unknown. There is much evidence to suggest that HTLV-1 gene expression is restricted: this restricted gene expression may contribute to HTLV-1 persistence by limiting the ability of the HTLV-1–specific CD8+ cell immune response to clear infected cells. In this study, we tested the hypothesis that derepression of HTLV-1 gene expression would allow an increase in CD8+ cell–mediated lysis of HTLV-1–infected cells. Using histone deacetylase enzyme inhibitors (HDIs) to hyperacetylate histones and increase HTLV-1 gene expression, we found that HDIs doubled Tax expression in naturally infected lymphocytes after overnight culture. However, the rate of CD8+ cell–mediated lysis of Tax-expressing cells ex vivo was halved. HDIs appeared to inhibit the CD8+ cell–mediated lytic process itself, indicating a role for the microtubule-associated HDAC6 enzyme. These observations indicate that HDIs may reduce the efficiency of cytotoxic T-cell (CTL) surveillance of HTLV-1 in vivo. The impact of HDIs on HTLV-1 proviral load in vivo cannot be accurately predicted because of the widespread effects of these drugs on cellular processes; we therefore recommend caution in the use of HDIs in nonmalignant cases of HTLV-1 infection.

Proviral load and immune markers associated with human T-lymphotropic virus type 1 (HTLV-1)-associated myelopathy/tropical spastic paraparesis (HAM/TSP) in Peru

Human T-lymphotropic virus type 1 (HTLV-1) is the aetiological agent of HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP). The objective of this study is to identify which ex vivo and in vivo markers are associated independently with HAM/TSP in a Peruvian population. Eighty-one subjects (33 men/48 women) were enrolled: 35 presented with HAM/TSP, 33 were asymptomatic HTLV-1 carriers (ACs) and 13 were HTLV-1-seronegative controls (SCs). Ex vivo markers included T cell proliferation and Th1 [interferon (IFN)-gamma], Th2 [interleukin (IL)-4, IL-5], proinflammatory [tumour necrosis factor (TNF)-alpha] and anti-inflammatory (IL-10) cytokine production in non-stimulated peripheral blood mononuclear cell (PBMC) cultures. In vivo CD4(+) T cell count, markers of Th1 [interferon-inducible protein (IP)-10] and Th2 (sCD30) activity in plasma and HTLV-1 proviral load in PBMCs were also evaluated. In univariate analysis, several markers, including T cell proliferation, IFN-gamma, IP-10, sCD30 and proviral load were associated with HAM/TSP, but in a multiple logistic regression analysis only the proviral load remained associated significantly with disease manifestation [adjusted OR 9.10 (1.24-66.91)]. Our findings suggest that HAM/TSP is associated primarily with proviral load, whereas the observed association with some immune markers seems secondary.

Inefficient cytotoxic T lymphocyte-mediated killing of HIV-1-infected cells in vivo

Understanding the role of cytotoxic T lymphocytes (CTLs) in controlling HIV-1 infection is vital for vaccine design. However, it is difficult to assess the importance of CTLs in natural infection. Different human leukocyte antigen (HLA) class I alleles are associated with different rates of progression to AIDS, indicating that CTLs play a protective role. Yet virus clearance rates following antiretroviral therapy are not impaired in individuals with advanced HIV disease, suggesting that weakening of the CTL response is not the major underlying cause of disease progression and that CTLs do not have an important protective role. Here we reconcile these apparently conflicting studies. We estimate the selection pressure exerted by CTL responses that drive the emergence of immune escape variants, thereby directly quantifying the efficiency of HIV-1–specific CTLs in vivo. We estimate that only 2% of productively infected CD4 ⁺ cell death is attributable to CTLs recognising a single epitope. We suggest that CTLs kill a large number of infected cells (about 10 ⁷) per day but are not responsible for the majority of infected cell death.

Although cytotoxic T lymphocytes (CTLs) kill a large number of HIV-infected cells every day, they may not be responsible for the majority of infected cell death.

Quantification of the virus-host interaction in human T lymphotropic virus I infection

BACKGROUND

HTLV-I causes the disabling inflammatory disease HAM/TSP: there is no vaccine, no satisfactory treatment and no means of assessing the risk of disease or prognosis in infected people. Like many immunopathological diseases with a viral etiology the outcome of infection is thought to depend on the virus-host immunology interaction. However the dynamic virus-host interaction is complex and current models of HAM/TSP pathogenesis are conflicting. The CD8+ cell response is thought to be a determinant of both HTLV-I proviral load and disease status but its effects can obscure other factors.

RESULTS

We show here that in the absence of CD8+ cells, CD4+ lymphocytes from HAM/TSP patients expressed HTLV-I protein significantly more readily than lymphocytes from asymptomatic carriers of similar proviral load (P = 0.017). A high rate of viral protein expression was significantly associated with a large increase in the prevalence of HAM/TSP (P = 0.031, 89% of cases correctly classified). Additionally, a high rate of Tax expression and a low CD8+ cell efficiency were independently significantly associated with a high proviral load (P = 0.005, P = 0.003 respectively).

CONCLUSION

These results disentangle the complex relationship between immune surveillance, proviral load, inflammatory disease and viral protein expression and indicate that increased protein expression may play an important role in HAM/TSP pathogenesis. This has important implications for therapy since it suggests that interventions should aim to reduce Tax expression rather than proviral load per se.

Cellular immune response to HTLV-1

There is strong evidence at the individual level and the population level that an efficient cytotoxic T lymphocyte (CTL) response to HTLV-1 limits the proviral load and the risk of associated inflammatory diseases such as HAM/TSP. This evidence comes from host population genetics, viral genetics, DNA expression microarrays and assays of lymphocyte function. However, until now there has been no satisfactory and rigorous means to define or to measure the efficiency of an antiviral CTL response. Recently, methods have been developed to quantify lymphocyte turnover rates in vivo and the efficiency of anti-HTLV-1 CTLs ex vivo. Data from these new techniques appear to substantiate the conclusion that variation between individual hosts in the rate at which a single CTL kills HTLV-1-infected lymphocytes is an important determinant, perhaps the decisive determinant, of the proviral load and the risk of HAM/TSP. With these experimental data, it is becoming possible to refine, parameterize and test mathematical models of the immune control of HTLV-1, which are a necessary part of an understanding of this complex dynamic system.