Memory CD8+ T cells vary in differentiation phenotype in different persistent virus infections

Most antiviral CD8 T cells during chronic viral infection do not express high levels of perforin and are not directly cytotoxic

Despite the frequency of HIV-specific CD8 T cells, most HIV-infected patients do not control viral replication without antiviral drugs. Although CD8 T cells are important in containing acute HIV and simian immunodeficiency virus (SIV) infection, CD8 T-cell functions are compromised in chronic infection. To investigate whether functional deficits are specific to HIV, the phenotypic and functional properties of HIV, Epstein-Barr virus (EBV), and cytomegalovirus (CMV)-specific CD8 T cells, labeled with HLA A2.1 or B8 tetramers, were compared in 35 HIV-infected and 9 healthy donors. Cytotoxic T lymphocytes express the cytolytic molecules perforin and granzymes, and are thought to be CD45RA(+)CD27(-). Although most HIV- specific cells are antigen experienced and express granzyme A (median, 85%), few express high levels of perforin (median, 10%) or CD45RA (median, 14%) or have down-modulated CD27 (median, 12%). Perforin expression by HIV-specific cells is not significantly different from that of EBV- or CMV-specific cells in the same donors or in healthy donors. EBV- and CMV-specific cells, like HIV-specific cells, are often not cytotoxic when tested directly ex vivo. HIV-specific T-cell expression of other phenotypic markers is similar to that of EBV- and CMV-specific CD8 T cells in healthy donors. However, CMV-specific cells (and, to a lesser extent, EBV-specific cells) in HIV-infected donors are more likely to be CD27(-), CD45RA(+), and GzmA(+). These results suggest that the chance to eradicate an infection by T-cell-mediated lysis may be undermined once an infection becomes chronic. Impaired antiviral cytotoxicity during chronic infection is not specific to HIV but likely represents the immune response to chronic antigenic exposure.

HCV immunology--death and the maiden T cell

Cellular immune responses play an important role in the control of hepatitis C virus (HCV), although in the majority of cases they ultimately fail. We examine the mechanisms by which virus-specific T cells may interact with a cell that is infected with HCV and how this interaction may explain the success and failure of the immune response. As an infected cell presenting foreign antigen, the hepatocyte will interact with a large number of lymphocytes, both by direct cell to cell contact and by indirect means through the secretion of cytokines and chemokines. These interactions may lead on the one hand to the death of infected hepatocytes or suppression of viral replication and on the other hand to the death of T lymphocytes or down regulation of their function. We suggest that activation of lymphocytes in lymphoid organs leads to generation of effector T cells (positive loop), while at the same time presentation of antigen in the liver either on hepatocytes or other specialised antigen presenting cells depresses these responses (negative loop). This model helps to explain both the specific phenotype and low frequencies of HCV specific CTL in chronic infection, through early elimination of cells before expansion and maturation can occur. The outcome of HCV infection is likely to result from the early balance between these two simultaneous loops.

Regulatory CD8+CD28- T cells in heart transplant recipients

Human regulatory CD8+CD28- T cells (Ts) generated in vitro were demonstrated to suppress the activation and proliferation of T helper cells (Th) induced by allogeneic cells. This effect requires cell-to-cell contact, is antigen-specific, and results in Th anergy. To study the population of CD8+CD28- T cells present in vivo, flow cytometry was performed on whole blood specimens obtained from 25 heart transplant recipients and 12 normal controls. A significant expansion of CD8+CD28- T cells was found in transplant recipients as compared with normal individuals (p = 0.005). Expression of CD38, human leukocyte antigen-DR, and perforin positive cells within the CD8+CD28- subset was significantly higher in transplant patients than in normal controls, yet there was no correlation between the expression of these markers and acute rejection. Expression of the CD27 marker, however, was significantly higher within CD8+CD28- T cells from patients without rejection as compared with patients in rejection (p = 0.005), indicating that the memory-like CD8+CD28-CD27+ T-cell subset comprises regulatory cells, which play a protective role for the graft. CD8+CD28- T cells isolated from transplant patients did not display cytotoxic activity against donor cells and showed high expression of the killing inhibitory receptor CD94. This study identifies the phenotypic changes that occur in patients with heart transplants and opens new avenues for the induction of specific immunosuppression in transplantation.

Ablation of CD8 and CD4 T cell responses by high viral loads

To evaluate the impact of sustained viral loads on anti-viral T cell responses we compared responses that cleared acute lymphocytic choriomeningitis virus infection with those that were elicited but could not resolve chronic infection. During acute infection, as replicating virus was cleared, CD8 T cell responses were down-regulated, and a pool of resting memory cells developed. In chronically infected hosts, the failure to control the infection was associated with pronounced and prolonged activation of virus-specific CD8 T cells. Nevertheless, there was a progressive diminution of their effector activities as their capacity to produce first IL-2, then TNF-alpha, and finally IFN-gamma was lost. Chronic lymphocytic choriomeningitis virus infection was also associated with differential contraction of certain CD8 T cell responses, resulting in altered immunodominance. However, this altered immunodominance was not due to selective expansion of T cells expressing particular TCR Vbeta segments during chronic infection. High viral loads were not only associated with the ablation of CD8 T cell responses, but also with impaired production of IL-2 by virus-specific CD4 T cells. Taken together, our data show that sustained exposure to high viral loads results in the progressive functional inactivation of virus-specific T cell responses, which may further promote virus persistence.

Application of tetramer technology in studies on autoimmune diseases

Autoreactive T cells are thought to play a role in the immunopathogenesis of autoimmune diseases. Analysis of such cells had long been hampered by lack of suitable assays. Recently developed tetramer technology is based on the recognition of specific peptide-MHC complex by T cell receptor and on the increased binding affinity of multimerized peptide-MHC complex. MHC class I and class II tetramers can be used to detect autoreactive CD4(+) and CD8(+) T cells, while nonclassical MHC (such as CD1d) tetramer can be used to detect other T cell groups, for example natural killer T cells. Tetramer technologies enable direct quantitation of autoreactive T cells in blood and affected tissues. It is also possible to carry out phenotypic and functional characterization of specific T cells on a single cell basis by using tetramers. Of special interest, in situ tetramer staining has the great potential of analyzing autoreactive T cells in their cellular environments. Utilization of tetramers in studies of autoreactive T cells is expected to generate important information regarding the role of such cells in the underlying mechanisms of autoimmune diseases.

Prolonged presence of effector-memory CD8 T cells in the central nervous system after dengue virus encephalitis

Dengue virus infection in the central nervous system (CNS) of immunized mice results in a strong influx of CD8 T cells into the brain. Whereas the kinetics of the splenic antiviral response are conventional, i.e. expansion followed by a rapid drop in the frequency of specific CD8 T cells, dengue virus-specific CD8 T cells are retained in the CNS at a high frequency. These CD8 T cells display a partially activated phenotype (CD69(high), Ly-6A/E(high), CD62L(low)), characteristic for effector-memory T cells. CD43 expression, visualized by staining with the 1B11 mAb, decreased in time, suggesting that these persisting CD8 T cells differentiated into memory cells. These data add to the growing evidence implicating the CNS as a non-lymphoid tissue capable of supporting prolonged T cell survival/maintenance.

Distribution and functional analysis of memory antiviral CD8 T cell responses in HIV-1 and cytomegalovirus infections

In the present study, we have investigated the anatomic distribution and the function of different populations of HIV-1- and cytomegalovirus (CMV)-specific memory CD8 T cells. The different populations of virus-specific memory CD8 T cells were distinguished on the basis of the expression of CD45RA and CCR7, and the composition of HIV-1- and CMV-specific memory CD8 T cell pools were compared in subjects with chronic HIV-1 and CMV co-infection. The distribution of HIV-1-specific CD8 T cells was similar between blood and lymph node. However, CMV-specific CD8 T cells were accumulated predominantly in the blood away from the lymphoid tissue. The majority (>70%) of HIV-1- and CMV-specific CD8 T cells in both blood and lymph node had a phenotype, e.g. CCR7-, typical of effector T cells. HIV-1-specific memory CD8 T cells were mostly (>80%) pre-terminally differentiated cells, e.g. CD45RA-CCR7-, in both blood and lymph node while 30-50% of CMV-specific CD8 T cells were terminally differentiated, e.g. CD45RA+CCR7-. Therefore, consistently with studies in mice, antigen-specific effector memory CD8 T cells accumulate predominantly in the target organ of the pathogen in humans, and the differences in the composition of HIV-1- and CMV-specific CD8 T cell pools were also present in the lymphoid tissue. A substantial proportion (30-40%) of virus-specific CD8+CCR7+ T cells produced IFN-gamma. Thus, indicating that the expression of CCR7 does not provide a clear-cut separation of memory CD8 T cells with distinct functional capacities. Taken together, these results provide further advances in the characterization of human memory CD8 T cells.

Failing immune control as a result of impaired CD8+ T-cell maturation: CD27 might provide a clue

Despite readily detectable virus-specific CD8(+) T cells in most HIV-infected patients, immune surveillance is eventually lost, leading to progression to AIDS. Recently developed insights into human T-cell differentiation have been used to study the phenotype of virus-specific T cells in HIV-infected individuals. Based on these results, we propose that failing immune control in human viral infection could be a result of impaired cytotoxic T-lymphocyte (CTL) maturation into fully differentiated effector T cells. Impaired maturation is not confined to HIV-specific CD8(+) T cells but could also be involved in failing immunity to Epstein-Barr virus and other viral infections. We postulate that CD27(-) effector CD8(+) T cells might be required for adequate control of chronic viral infection and prevention of disease development.

Premature ageing of the immune system: the cause of AIDS?

The reasons for the failure of the immune system to control HIV-1 infection, and the resulting immunodeficiency, remain unclear. HIV-1 persists in its host despite vigorous immune responses, including a strong, and probably functional, HIV-specific cytotoxic T-lymphocyte response. Interestingly the immunological features of HIV-1-infected individuals show many similarities to those seen in elderly people without HIV infection. We propose that, through a process of continuous immune activation, HIV-1 infection leads to an acceleration of the adaptive immune system ageing process, resulting in premature exhaustion of immune resources, which participates in the onset of immunodeficiency. This hypothesis might shed new light on HIV-1 pathogenesis and could suggest the need to reconsider current immunotherapeutic strategies to fight the virus.

Proliferation requirements of cytomegalovirus-specific, effector-type human CD8+ T cells

Two prototypic types of virus-specific CD8(+) T cells can be found in latently infected individuals: CD45R0(+)CD27(+)CCR7(-) effector-memory, and CD45RA(+)CD27(-)CCR7(-) effector-type cells. It has recently been implied that CD45RA(+)CD27(-)CCR7(-) T cells are terminally differentiated effector cells and as such have lost all proliferative capacity. We show in this study, however, that stimulation of CMV-specific CD45RA(+)CD27(-)CCR7(-) T cells with their cognate peptide in concert with either CD4(+) help or IL-2, IL-15, or IL-21 in fact induces massive clonal expansion. Concurrently, these stimulated effector T cells change cell surface phenotype from CD45RA to CD45R0 and regain CCR7, while effector functions are maintained. Our data imply that CD45RA(+)CD27(-)CCR7(-) effector-type T cells contribute to immunity not only by direct execution of effector functions, but also by yielding progeny in situations of viral reinfection or reactivation.

HIV-specific CD8+ T cell proliferation is coupled to perforin expression and is maintained in nonprogressors

It is unclear why immunological control of HIV replication is incomplete in most infected individuals. We examined here the CD8+ T cell response to HIV-infected CD4+ T cells in rare patients with immunological control of HIV. Although high frequencies of HIV-specific CD8+ T cells were present in nonprogressors and progressors, only those of nonprogressors maintained a high proliferative capacity. This proliferation was coupled to increases in perforin expression. These results indicated that nonprogressors were differentiated by increased proliferative capacity of HIV-specific CD8+ T cells linked to enhanced effector function. In addition, the relative absence of these functions in progressors may represent a mechanism by which HIV avoids immunological control.

Beyond 51Cr release: New methods for assessing HIV-1-specific CD8+ T cell responses in peripheral blood and mucosal tissues

Much scientific effort has been directed towards elucidating the complexities of cell-mediated immune responses to HIV-1(reviewed in [1,2]). These studies have attempted to explain the immune system's ultimate failure to contain viral replication, leading to development of AIDS disease, and to identify immune responses that will be useful in developing immunomodulatory therapies and novel vaccine strategies. Although many of the complex interactions involved in AIDS pathogenesis remain unsolved, great progress has been made in characterizing the kinetics, specificity and functional dynamics of HIV-1-specific T cell responses. These investigations have come at a time when advances in virology, cellular immunology and molecular biology have converged to provide a variety of methodological approaches not available at the onset of the AIDS pandemic. Application of these tools to other infectious diseases and immunopathological conditions will provide a fertile area of research for future years. This review focuses on recent developments in the assessment of HIV-1-specific T cell responses in peripheral blood and tissues, with a particular emphasis on flow cytometry-based approaches.

Heterogeneity of CD4 and CD8+ memory T cells in localized and generalized Wegener's granulomatosis

Memory T cells display phenotypic heterogeneity. Surface antigens previously regarded as exclusive markers of naive T cells, such as L-selectin (CD62L), can also be detected on some memory T cells. Moreover, a fraction of CD45RO+ (positive for the short human isoform of CD45) memory T cells reverts to the CD45RA+ (positive for the long human isoform of CD45) phenotype. We analyzed patients with biopsy-proven localized Wegener's granulomatosis (WG) (n = 5), generalized WG (n = 16) and age- and sex-matched healthy controls (n = 13) to further characterize memory T cells in WG. The cell-surface expression of CD45RO, CD45RA, CD62L, CCR3, CCR5 and CXCR3 was determined on blood-derived T cells by four-color flow cytometric analysis. The fractions of CCR5+ and CCR3+ cells within the CD4+CD45RO+ and CD8+CD45RO+ memory T cell populations were significantly expanded in localized and generalized WG. The mean percentage of Th1-type CCR5 expression was higher in localized WG. Upregulated CCR5 and CCR3 expression could also be detected on a fraction of CD45RA+ T cells. CD62L expression was seen on approximately half of the memory T cell populations expressing chemokine receptors. This study demonstrates for the first time that expression of the inducible inflammatory chemokine receptors CCR5 and CCR3 on CD45RO+ memory T cells, as well as on CD45RA+ T cells ('revertants'), contributes to phenotypic heterogeneity in an autoimmune disease, namely WG. Upregulated CCR5 and CCR3 expression suggests that the cells belong to the effector memory T cell population. CCR5 and CCR3 expression on CD4+ and CD8+ memory T cells indicates a potential to respond to chemotactic gradients and might be important in T cell migration contributing to granuloma formation and vasculitis in WG.

Dominance of virus-specific CD8 T cells in human primary cytomegalovirus infection

Cellular immune responses are of high importance in initiating and maintaining immunity against virus infections. Whereas the cellular immune response during persistent cytomegalovirus (CMV) infection is well assessable, the individual contribution of CD4 and CD8 T cell responses during primary infection has not been described. A novel whole-blood assay, which relies on the flow-cytometric detection of antigen-induced cytokine expression, was used to characterize CMV-specific CD4 and CD8 T cell responses during primary infection of CMV seronegative recipients of a renal allograft from a CMV seropositive donor. These T cell responses were compared with long-term CMV-positive patients with known history of transplantation-related seroconversion. Results were further correlated to CMV load and serum IgG and IgM. The long-term seroconverted patients consistently showed a dominant CMV-specific CD4 T cell response (median frequencies: CD4, 1.12% [range, 0.35 to 8.10%] versus CD8 0.13% [range, <0.05 to 0.55%]). In contrast, during primary infection, the cellular immune response is strongly dominated by CMV-specific CD8 T cells (median peak frequencies: CD4, 1.24% [range, 0.21 to 1.60%] versus CD8, 2.47% [range, 1.34 to 6.67%]). Upon receipt of ganciclovir, viral load as well as CMV-specific CD8 responses decreased. The frequency of the respective CD4 T cells fluctuated during decrease of CMV load and became dominant over CMV-specific CD8 T cell responses. These results are consistent with the view of an effective direct antiviral activity of CD8 T cells, which is most critical during periods of high viremia. Later on during persistent infection, CD4 T cells dominate the immune response to support the state of antiviral immunity.

The Role of CD4(+) and CD8(+) T Cells in Controlling HIV Infection

Presently, it is thought that virus-specific T cells play a major role in restricting lentiviral replication and determining the rate of disease progression in humans. However, it remains unclear why this restriction fails in the majority of infected individuals. The major exception is a rare subgroup of HIV-infected long-term nonprogressors (LTNPs) who have been infected for approximately 20 years yet maintain normal CD4(+) T-cell counts and less than 50 copies of viral RNA/mL of plasma. Although virus-specific cellular (CD4(+) and CD8(+) T lymphocytes) immune responses have been shown to exert some degree of in vivo control of HIV replication, the precise correlates of protective immunity differentiating LTNPs from patients with progressive disease remain unknown. A greater understanding of the components and magnitude of an effective immune response to HIV is an important step toward the development of effective vaccines and immunotherapies.

NK cell activity during human cytomegalovirus infection is dominated by US2-11-mediated HLA class I down-regulation

A highly attractive approach to investigate the influence and hierarchical organization of viral proteins on cellular immune responses is to employ mutant viruses carrying deletions of various virus-encoded, immune-modulating genes. Here, we introduce a novel set of deletion mutants of the human CMV (HCMV) lacking the UL40 region either alone or on the background of a deletion mutant devoid of the entire US2-11 region. Deletion of UL40 had no significant effect on lysis of infected cells by NK cells, indicating that the expected enhancement of HLA-E expression by specific peptides derived from HCMV-encoded gpUL40 leader sequences was insufficient to confer target cell protection. Moreover, the kinetics of MHC class I down-regulation by US2-11 genes observed at early and late phases postinfection with wild-type virus correlated with increased susceptibility to NK lysis. Thus, the influence of HCMV genes on NK reactivity follows a hierarchy dominated by the US2-11 region, which encodes all viral genes capable of down-modulating expression of classical and non-classical MHC class I molecules. The insights gained from studies of such virus mutants may impact on future therapeutic strategies and vaccine development and incorporate NK cells in the line of defense mechanisms against HCMV infection.

Generation of CD3+CD8low thymocytes in the HIV type 1-infected thymus

Infection with the HIV type 1 (HIV-1) can result both in depletion of CD4(+) T cells and in the generation of dysfunctional CD8(+) T cells. In HIV-1-infected children, repopulation of the peripheral T cell pool is mediated by the thymus, which is itself susceptible to HIV-1 infection. Previous work has shown that MHC class I (MHC I) molecules are strongly up-regulated as result of IFN-alpha secretion in the HIV-1-infected thymus. We demonstrate in this study that increased MHC I up-regulation on thymic epithelial cells and double-positive CD3(-/int)CD4(+)CD8(+) thymocytes correlates with the generation of mature single-positive CD4(-)CD8(+) thymocytes that have low expression of CD8. Treatment of HIV-1-infected thymus with highly active antiretroviral therapy normalizes MHC I expression and surface CD8 expression on such CD4(-)CD8(+) thymocytes. In pediatric patients with possible HIV-1 infection of the thymus, a low CD3 percentage in the peripheral circulation is also associated with a CD8(low) phenotype on circulating CD3(+)CD8(+) T cells. Furthermore, CD8(low) peripheral T cells from these HIV-1(+) pediatric patients are less responsive to stimulation by Ags from CMV. These data indicate that IFN-alpha-mediated MHC I up-regulation on thymic epithelial cells may lead to high avidity interactions with developing double-positive thymocytes and drive the selection of dysfunctional CD3(+)CD8(low) T cells. We suggest that this HIV-1-initiated selection process may contribute to the generation of dysfunctional CD8(+) T cells in HIV-1-infected patients.

The dynamics of T-lymphocyte responses during combination therapy for chronic hepatitis C virus infection

Hepatitis C virus (HCV) readily sets up a persistent infection and is a major cause of liver disease worldwide. Interferon alfa and ribavirin therapy lead to sustained clearance of virus in 31% to 64% of patients with type 1 and non-type 1 genotypes, respectively. It is not clear to what extent these drugs act directly to reduce HCV replication, or indirectly via host immune responses, and what evoked immune responses are associated with clinical outcome. We have examined prospectively 15 patients with chronic HCV infection before, during, and after combination therapy. Quantitative assays for HCV antigen-specific CD4+ and CD8+ T-cell responses, and flow cytometric assays for analysis of the phenotype of T cells, in addition to viral sequencing of core protein, were performed throughout the treatment and follow-up period over 18 months. We found enhancement of proliferative T-cell responses during therapy. Proliferative responses are strikingly heterogeneous in terms of specificity, kinetics, and magnitude. Proliferative responses are often not associated with interferon-gamma release. T-cell responses are rarely sustained irrespective of treatment outcome and this is not due to the evolution of new immune escape variants. T-cell responses tend to peak late in the course of treatment. In conclusion, combination therapy for HCV has a transient effect on host virus-specific T cells in the blood. Induction of sustained T-cell responses may require additional immune modulation later in therapy.

HIV: current opinion in escapology

Much recent work strongly supports the hypothesis that CD8(+) T lymphocytes (CTLs) exert important immune control over HIV and so are a major selective force in its evolution. We analyse this host-pathogen interplay and focus on new data that describe the overall 'effectiveness' of CTL responses (strength, spread, specificity and 'stamina') and the mechanisms by which HIV may evade this suppressive activity. CTLs directed against HIV recognise very large numbers of distinct epitopes across the genome, are largely functional, turn over rapidly, and possess a phenotype that is distinct from CD8(+) lymphocytes specific for other viruses. Mutation of HIV epitopes that alters or abolishes CTL recognition altogether appears to be the most important immune escape mechanism, as the variation that HIV generates defies the limits of the T cell repertoire. However, this immune evasion is still only well-studied in a few patients. The rules that govern immune escape, and the ultimate limits of CTL capacity to deal with the variant epitopes that currently circulate, are not understood. This information will determine the feasibility of current vaccine approaches that, so far, make no provision for the enormous antigenic plasticity of HIV.

Avoiding the kiss of death: how HIV and other chronic viruses survive

Virus-specific CD8 T cells during chronic infection often exceed in numbers virus-replicating infected cells. Why then do antiviral CD8 T cells not do a better job of controlling infection? Although viral strategies for immune evasion are well known, this review will focus on changes in the CD8 T cell that interfere with cytolytic function. Most antiviral CD8 T cells in chronic infection do not express perforin, a molecule that is required for cytolysis. IL-2 and other costimulatory signals can restore cytotoxicity that has been impaired, suggesting a role for CD4 T cell anergy. The chance to eradicate an infection by T cell mediated lysis is undermined after an infection becomes established, in part because the effector immune response is impaired in the setting of chronic antigen.

Tracking T cells with tetramers: new tales from new tools

To understand the success or failure of immune responses against pathogens or tumours requires the direct measurement of specific lymphocytes. Recently, there has been an explosion of data in this field through the use of several new tools for measuring the number and function of T cells. This has allowed immunologists who study human disease and mouse models of infection and cancer to readily track specific T cells--in both time and space. Although there are common patterns, over time, each host-pathogen relationship seems to develop unique characteristics, as reflected in the quality of the T-cell response.