Peripheral clonal deletion of antiviral memory CD8+ T cells

Comparison of activation versus induction of unresponsiveness of virus-specific CD4+ and CD8+ T cells upon acute versus persistent viral infection

The functional status of CD4+ T cells during establishment of persistent infection with the noncytopathic lymphocytic choriomeningitis virus was assessed and compared to that of cytotoxic CD8+ T cells. Functionality of virus-specific CD4+ T cells was measured by proliferative responses, cytokine secretion, cognate help, and IFNgamma-mediated protection against challenge infection with recombinant vaccinia virus. Functional CD4+ T cells were induced early after infection and remained measurable up to 6 weeks but then were rendered unresponsive. In contrast, CD8+ T cells were functionally inactivated within 10-15 days. Importantly, functional inactivation of virus-specific CD4+ T cells during persistent viral infection seemed to be critical for the survival of the host.

Dynamics of cytotoxic T-lymphocyte exhaustion

We examine simple mathematical models to investigate the circumstances under which the dynamics of cytotoxic T-lymphocyte (CTL) activation and differentiation may result in the loss of virus specific CD8+ cells, a process known as CTL exhaustion. We distinguish between two general classes of viruses: (i) viruses infecting cells that are not involved in the immune response; and (ii) viruses infecting antigen presenting cells (APCs) and helper cells. The models specify host and viral properties that lead to CTL exhaustion and indicate that this phenomenon is only likely to be observed with viruses infecting APCs and helper cells. Moreover, it is found that for such viruses, a high rate of replication and a low degree of cytopathogenicity promote the exhaustion of the CTL response. In addition, a high initial virus load and a low CD4+ cell count promote the occurrence of CTL exhaustion. These conclusions are discussed with reference to empirical data on lymphocytic choriomeningitis virus and on human immunodeficiency virus.

Selection of virus variants and emergence of virus escape mutants after immunization with an epitope vaccine

In this report, we assessed the evolution of the cytotoxic T-lymphocyte (CTL) response induced by an epitope vaccine. In two macaques immunized with a mixture of lipopeptides derived from simian immunodeficiency virus (SIV) Nef and Gag proteins, CTL responses were directed against the same, single epitope of the Nef protein (amino acids 128 to 137) presenting an alanine at position 136 (Nef 128-137/136A). However, after 5 months of SIV infection, peripheral blood mononuclear cells from both macaques lost their ability to be stimulated by autologous SIV-infected cells while still retaining their capacity to generate cytotoxic responses after specific Nef 128-137/136A peptide stimulation. The sequences of the pathogenic viral isolate used for the challenge showed a mixture of several variants. Within the Nef epitopic sequence from amino acids 128 to 137, 82% of viral variants expressed the epitopic peptide Nef 128-137/136A; the remaining variants presented a threonine at position 136 (Nef 128-137/136T). In contrast, sequence analysis of cloned proviral DNA obtained from both macaques 5 months after SIV challenge showed a different pattern of quasi-species variants; 100% of clones presented a threonine at position 136 (Nef 128-137/136T), suggesting the disappearance of viral variants with an alanine at this position under antiviral pressure exerted by Nef 128-137/136A-specific CTLs. In addition, 12 months after SIV challenge, six of eight clones from one macaque presented a glutamic acid at position 131 (Nef 128-137/131E+136T), which was not found in the infecting isolate. Furthermore, CTLs generated very early after SIV challenge were able to lyse cells sensitized with the Nef 128-137/136A epitope. In contrast, lysis was significantly less effective or even did not occur when either the selected peptide Nef 128-137/136T or the escape variant peptide Nef 128-137/131E+136T was used in a target cell sensitization assay. Dose analysis of peptides used to sensitize target cells as well as a major histocompatibility complex (MHC)-peptide stability assay suggested that the selected peptide Nef 128-137/136T has an altered capacity to bind to the MHC. These data suggest that CTL pressure leads to the selection of viral variants and to the emergence of escape mutants and supports the fact that immunization should elicit broad CTL responses.

Interleukin 10 secretion and impaired effector function of major histocompatibility complex class II-restricted T cells anergized in vivo

Continuous antigenic stimulation in vivo can result in the generation of so-called "anergic" CD4(+) or CD8(+) T cells that fail to proliferate upon antigenic stimulation and fail to develop cytolytic effector functions. Here we show that class II major histocompatibility complex-restricted T cells specific for influenza hemagglutinin (HA) that become anergic in mice expressing HA under control of the immunoglobulin kappa promoter exhibit an impaired effector function in causing diabetes in vivo, as compared to their naive counterparts, when transferred into immunodeficient recipients expressing HA under the control of the insulin promoter. Furthermore, HA-specific T cells anergized in vivo contain higher levels of interleukin (IL)-4 messenger RNA (mRNA) than naive and recently activated T cells with the same specificity and more than a 100-fold higher levels of IL-10 mRNA. The higher expression of the IL-10 gene is also evident at the protein level. These findings raise the interesting possibility that T cells rendered anergic in vivo have in fact become regulatory T cells that may influence neighboring immune responses through the release of IL-10.

Virus-specific MHC-class II-restricted TCR-transgenic mice: effects on humoral and cellular immune responses after viral infection

A transgenic mouse expressing MHC class II-restricted TCR with specificity for a lymphocytic choriomeningitis virus (LCMV) glycoprotein-derived T helper cell epitope was developed to study the role of LCMV-specific CD4+ T cells in virus infection in vivo. The majority of CD4+ T cells in TCR transgenic mice expressed the transgenic receptor, and LCMV glycoprotein-specific TCR transgenic CD4+ T cells efficiently mediated help for the production of LCMV glycoprotein-specific isotype-switched antibodies. In contrast, LCMV glycoprotein-specific TCR transgenic mice exhibited a drastically reduced ability to provide help for the generation of antibody responses specific for the virus-internal nucleoprotein, indicating that intramolecular/intrastructural help is limited to antigens that are accessible to B cells on the viral surface. Antiviral cellular immunity was studied with noncytopathic LCMV and recombinant cytopathic vaccinia virus expressing the LCMV glycoprotein. TCR transgenic mice failed to efficiently control LCMV infection, demonstrating that functional LCMV-specific CD4+ T cells--even if activated and present at extremely high frequencies--cannot directly mediate protective immunity against LCMV. Despite the fact that LCMV-primed CD4+ T cells from TCR transgenic mice as well as from control mice showed low MHC class II-restricted cytotoxic activity in vivo, this did not correlate with protection against LCMV replication in vivo. In contrast, CD4+ T cells from TCR-transgenic mice mediated efficient protection against infection with recombinant vaccinia virus. These results further support the need for different immune effector functions for protective immunity against different viral infections.

What can we learn about human immunodeficiency virus infection from a study of lymphocytic choriomeningitis virus?

The role of cytotoxic T lymphocytes (CTL) in human immunodeficiency virus (HIV) infection remains elusive. Since the discovery 10 years ago of high levels of specific CTL in this disease, some have argued that they play an important role in virus control, others that they drive disease progression through destruction of T helper cells, and others still that they play no obvious role at all. By contrast, the central role of CTL in murine lymphocytic choriomeningitis virus (LCMV) infection has been very clearly worked out through the use of in vivo depletion and adoptive transfer experiments, as well as knockout and transgenic mice. To interpret the possible roles for CTL in HIV, we have therefore made a comparison between what is known about CTL and their interaction with virus-infected cells in these two infections. This illustrates a potential critical role for these cells in both control of HIV replication and immune-mediated pathology, but one that is highly dependent on virus dose, distribution and dynamics.

Antigenic cancer cells grow progressively in immune hosts without evidence for T cell exhaustion or systemic anergy

One enigma in tumor immunology is why animals bearing malignant grafts can reject normal grafts that express the same nonself-antigen. An explanation for this phenomenon could be that different T cell clones react to the normal graft and the malignant cells, respectively, and only the tumor-reactive clonotypes may be affected by the growing tumor. To test this hypothesis, we used a T cell receptor transgenic mouse in which essentially all CD8(+) T cells are specific for a closely related set of self-peptides presented on the MHC class I molecule Ld. We find that the tumor expressed Ld in the T cell receptor transgenic mice but grew, while the Ld-positive skin was rejected. Thus, despite an abundance of antigen-specific T cells, the malignant tissue grew while normal tissue expressing the same epitopes was rejected. Therefore, systemic T cell exhaustion or anergy was not responsible for the growth of the antigenic cancer cells. Expression of costimulatory molecules on the tumor cells after transfection and preimmunization by full-thickness skin grafts was required for rejection of a subsequent tumor challenge, but there was no detectable effect of active immunization once the tumor was established. Thus, the failure of established tumors to attract and activate tumor-specific T cells at the tumor site may be a major obstacle for preventive or therapeutic vaccination against antigenic cancer.

A critical role for neutralizing-antibody-producing B cells, CD4(+) T cells, and interferons in persistent and acute infections of mice with lymphocytic choriomeningitis virus: implications for adoptive immunotherapy of virus carriers

This study demonstrates that neutralizing-antibody-producing B cells, CD4(+) T cells, and interferons (IFNs) are of key importance in virus control both in adoptive immunotherapy of persistent infection and in the late phase of acute infection with the WE strain of lymphocytic choriomeningitis virus (LCMV). We report the following results. (i) Clearance of LCMV-WE from C57BL/6 carrier mice by adoptive transfer of memory spleen cells requires B cells and CD4(+) T cells but not necessarily CD8(+) T cells. (ii) At the doses examined, CD8(+) T cells contribute to the initial reduction of viral titers but are alone not sufficient to clear the virus because they are exhausted. (iii) In the presence of functional IFN-gamma, virus clearance correlates well with the generation of neutralizing antibodies in the treated carrier mice. (iv) In the absence of receptors for IFN-gamma, virus clearance is not achieved. (v) Adoptive immunotherapy of mice persistently infected with a distinct virus isolate, LCMV-Armstrong, revealed only low levels of neutralizing antibodies; in this case, CD8(+) T cells were needed for virus clearance in addition to B and CD4(+) T cells. (vi) After low dose infection of C57BL/6 mice with LCMV-WE, virus is eliminated below detectable levels by CD8(+) T cells, but long-term (>2 months) virus control is usually not achieved in the absence of B cells or CD4(+) T cells; reappearance of the virus is paralleled either by exhaustion of virus-specific cytotoxic T lymphocytes or lethal immunopathology. These findings are of importance for adoptive immunotherapy strategies against persistent virus infections in humans.

Peptide antigen treatment of naive and virus-immune mice: antigen-specific tolerance versus immunopathology

Peptide-specific down-regulation of T cell responses may represent a powerful tool to intervene in autoimmune diseases or graft rejections. It is therefore important to know whether peptide treatment tolerizes both naive and antigen-experienced memory T lymphocytes. Here we show that a major histocompatibility complex class I binding peptide, derived from the glycoprotein (GP33 peptide) of lymphocytic choriomeningitis virus (LCMV), specifically tolerized naive cytotoxic T lymphocytes (CTL) when administered three times intraperitoneally in incomplete Freund's adjuvants. However, in the presence of GP33-specific memory CTL in LCMV-primed mice, the same treatment had a general immunosuppressive effect on unrelated third-party antigen-specific T cell responses and caused severe immunopathological damage to the spleen.

Antigen localisation regulates immune responses in a dose- and time-dependent fashion: a geographical view of immune reactivity

This review summarises experimental evidence to illustrate that induction of immune reactivity depends upon antigen reaching and being available in lymphoid organs in a dose- and time-dependent manner. If antigen reaches lymph organs in a localised staggered manner and with a concentration gradient, a response is induced in the draining lymph node. Antigen-presenting cells are of critical importance to transport antigen from the periphery to local organised lymphoid tissue. If antigen is all over the lymphoid system, then it deletes all specific cells in the thymus or induces them within a few days; because of their limited life-span they then die off, leaving the repertoire depleted of this specificity. If antigen does not reach lymphoid organs it is ignored by immune cells. Once a response is induced, activated but not resting T cells will reach antigen outside lymphoid organs, whereas activated B cells differentiate into plasma cells in an inducing environment, mostly in lymphoid tissue including bone marrow, but also in chronic lymphoid-like infiltrations in peripheral organs. In immunopathology (when the infectious agent is known) or in autoimmunity (when the triggering infectious agent is not known or not recognised) lymphoid tissue may become organised close to the antigen (e.g. in organ-specific autoimmune diseases) and may thereby maintain an autoantigen-driven disease-causing immune response for a long time. The notion that native T cells get induced or silenced in the periphery may be questioned because induction can only occur in lymphoid organs providing anatomical structures where critical cell-cell interactions are properly guided and where, therefore, cells are likely to meet sufficiently frequently and in a critical milieu. Since overall immune reactivity critically depends upon the localisation of antigens in a dose- and time-dependent manner, it seems more likely-but this remains to be shown-that activated T cells may get exhausted in non-lymphoid peripheral tissues, whereas they are usually maintained in lymphoid organs. The critical role of antigen in regulating immune responses also has relevance for our understanding of immunological defence against epithelial and mesenchymal tumours, against many infectious diseases and for understanding autoimmunity and immunological memory. Collectively the data indicate that antigen, dependent upon localisation, dose and time, seems to be the simplest regulator of immune responses.

Immunology and immunity studied with viruses

Immunity to viruses is used to define important biological parameters of immunology. Specificity, tolerance and T and B cell memory were analysed with murine model infections. The key parameters of antigen kinetics, localization and patterns of T and B cell response induction in maintaining memory and in causing deletion of reactive lymphocytes were compared for self and for viral foreign antigens. Evidence is reviewed that suggests that B cells essentially recognize antigen patterns, whereas T cells react against antigens newly brought into lymphoid tissues; antigens outside lymphoid tissues are ignored, and antigens always present in, or spreading too fast throughout, lymphoid tissues exhaust and delete T cell responses. Finally, effector mechanisms of antiviral immunity are summarized, as they vary with different viruses. On this basis immunological T and B cell memory against viruses is reviewed. Memory studies suggest that increased precursor frequencies of B and T cells appear to remain in the host independent of antigen persistence. However, in order to protect against cytopathic viruses, memory B cells have to produce antibody to maintain protective elevated levels of antibody: B cell differentiation into plasma cells is driven by persisting antigen. Similarly, to protect against infection with a non-cytopathic virus, cytotoxic T cells have to recirculate through peripheral organs. Activation and capacity to emigrate into solid tissues as well as cytolytic effector function are also dependent upon, and driven by, persisting antigen. Because no convincing evidence is yet available of the existence of identifiable B or T cells with specialized memory characteristics, the phenotype of protective immunological memory correlates best with antigen-driven activation of low frequency effector T cells and plasma cells.

Large clonal expansions of CD8+ T cells in acute infectious mononucleosis

Primary infection with Epstein-Barr virus often results in the clinical syndrome of acute infectious mononucleosis (glandular fever). This illness is characterized by a striking lymphocytosis, the nature of which has been controversial. We show that large monoclonal or oligoclonal populations of CD8+ T cells account for a significant proportion of the lymphocytosis and provide molecular evidence that these populations have been driven by antigen. The results suggest that the selective and massive expansion of a few dominant clones of CD8+ T cells is an important feature of the primary response to this virus.

Intrathymic and extrathymic clonal deletion of T cells

Clonal elimination accounts for self-tolerance induction in the thymus and also affects mature T cells responding to exogenous antigens in the periphery. Recent evidence on the microenvironments, cell-cell interactions and signalling requirements for clonal deletion of immature and mature T cells is discussed.

Parameters controlling the programmed death of mature mouse T lymphocytes in high-dose suppression

We have characterized parameters of both T cells and antigen-presenting cells (APCs) that influence high-dose suppression due to apoptosis. Blockade of interleukin-2 (IL-2) utilization is shown to inhibit both proliferation and the ensuing death. An analysis of sublines of a mature T cell clone demonstrates a correlation between IL-2 receptor alpha chain (IL-2R) induction, increased proliferation, and greater suppression at high antigen doses. Profound loss of cells at high antigen dose was found to require at least 48 to 72 hr to develop. Antigen add-back experiments showed that strong T cell receptor reengagement of activated, cycling cells was essential for proliferative suppression and cell loss. Increasing the ratio of APC:T lymphocytes to 50:1 augmented cell death. For antigen-induced death of lymph node T cells, fresh T-depleted splenocytes were more effective than splenocytes that had been irradiated or treated with mitomycin C. Thus, T lymphocyte apoptosis at high antigen doses is a function of the activation response of the T lymphocyte as well as the efficiency of antigen presentation by the APC. These results strengthen the theory that apoptosis takes part in a feedback regulatory mechanism that has been called propriocidal regulation, which limits T cell expansion at high antigen doses.

Autocrine feedback death and the regulation of mature T lymphocyte antigen responses

Antigen-induced T cell death is an important regulatory mechanism in the peripheral immune system. Evidence suggests that this process depends on T cell growth-inducing lymphokines such as IL-2 and occurs in proportion to the degree of T cell receptor occupancy. Strong T cell receptor stimulation leads to the synthesis of death molecules such as Fas ligand and tumor necrosis factor that cause T cell suicide. We propose that T cell death under these circumstances is the culmination of a feedback control mechanism termed propriocidal regulation or autocrine feedback death that regulates the expansion of specific T cell clones under conditions of high lymphokine and antigen load. In a quasi-stochastic system such as the antigen receptor repertoire, feedback information may be essential for the appropriate regulation of peripheral immune responses. Our understanding of this feedback mechanism affords a means to manipulate antigen-specific T cell death in vivo. The application of this approach to the therapy of T cell-medicated immunological diseases is discussed.

Enhanced establishment of a virus carrier state in adult CD4+ T-cell-deficient mice

CD4+ T cells play an important role in regulating the immune response; their contribution to virus clearance is variable. Mice that lack CD4+ T cells (CD4-/- mice) and are therefore unable to produce neutralizing antibodies cleared viscero-lymphotropic lymphocytic choriomeningitis virus (LCMV) strain WE when infected intravenously with a low dose (2 x 10(2) PFU) because of an effective CD8+ cytotoxic T-cell (CTL) response. In contrast, infection with a high dose (2 x 10(6) PFU) of LCMV strain WE led to expansion of antiviral CTL, which disappeared in CD4-/- mice; in contrast, CD4+ T-cell-competent mice developed antiviral memory CTL. This exhaustion of specific CTL caused viral persistence in CD4-/- mice, whereas CD4+ T-cell-competent mice eliminated the virus. After infection of CD4-/- mice with the faster-replicating LCMV strain DOCILE, abrogation of CTL response and establishment of viral persistence developed after infection with a low dose (5 x 10(2) PFU), i.e., an about 100-fold lower dose than in CD(4+)-competent control mice. These results show that absence of T help enhances establishment of an LCMV carrier state in selected situations.