Radiation kills human peripheral T cells by a Fas-independent mechanism

PURPOSE: To investigate the incidence and clinical significance of primary or proposed secondary mitochondrial DNA (mtDNA) mutations in Japanese patients with Leber's hereditary optic neuropathy (LHON). METHODS: Blood samples from the 80 unrelated Japanese patients with bilateral optic atrophy were screened for primary LHON mutations. Patients found to have a primary LHON mutation were then tested for 9 proposed secondary LHON mutations. We investigated the association between these mutations and clinical characteristics. RESULTS: Primary mtDNA mutations were identified in 68 patients: at np 3460 in 3 (4%) of 68 patients, at np 11,778 in 59 patients (87%), and at np 14,484 in 6 patients (9%). We identified 5 secondary mtDNA mutations (at np 3394, 4216, 7444, 9438 or 13,708) in 10 (15%) of 68 LHON patients and 3 mutations (at np 3394, 4216 or 3708) in 6 (7%) of 90 healthy Japanese individuals. No patient was positive for more than one secondary mutation. The frequency of secondary mutations was similar in the 68 LHON patients and 90 controls. The clinical features of the Japanese patients with any of the 3 primary LHON mutations were similar to those of Caucasian patients, despite different mtDNA backgrounds in these populations. The percentage of patients with familial LHON harboring the 3460 or 14,484 mutations was lower in the Japanese population. CONCLUSIONS: Japanese patients with LHON exhibited a very high incidence (87%) of the 11,778 primary mutation. Most of the proposed secondary LHON mutations were rare in the Japanese population and they, except the 7444 mutation, may not influence the clinical features of LHON.     

Lysis of CD4+ T cells expressing HIV-1 gag peptides by gag-specific CD8+ cytotoxic T cells

The vast majority of in vitro experiments testing the cytotoxic T lymphocytes (CTL) activity in HIV infection has been performed with target cells consisting of autologous EBV-transformed B lymphoblastoid cell lines (B-LCLs) expressing Human immunodeficiency virus type I (HIV-1) proteins. However data concerning the lysis of primary CD4+ T lymphocytes expressing HIV-1 antigens by CTLs is still lacking. To study the CTL activity against such primary targets, we used a system involving PBMCs of an HIV+ asymptomatic patient (PT) as effector cells and the CD4+ lymphocytes or B-LCLs of his healthy HLA-identical twin brother (HTW) as target cells. These syngeneic targets were either infected with recombinant vaccinia virus containing HIV-1 gag gene (gag-vac), or coated with HIV-1 gag peptides. We demonstrate in this study that PT CTLs (which were CD3+, CD4-, CD8+, TCRalphabeta+, TCRgammadelta-, CD56-) specifically lysed both types of syngeneic target cells expressing gag-vac; however, CD4+ T cells expressing HIV gag proteins were lysed less efficiently than B-LCLs expressing the same HIV epitopes. On the other hand, no specific lysis was detected when the target cells were uninfected or infected by wild-type vaccinia virus.     

Fas-independent death of activated CD4(+) T lymphocytes induced by CTLA-4 crosslinking

The CTLA-4 receptor is a critical inhibitory regulator of T cell proliferation and effector function. However, the mechanisms through which CTLA-4 modulates the activation of T cells remain uncertain. Initial studies, using activated human T cells, have suggested that CTLA-4 crosslinking may induce apoptosis. However, more recent experiments have demonstrated that crosslinking of the CTLA-4 receptor on the surface of resting murine T cells blocks cell cycle progression without inducing apoptosis. Here we provide evidence that CTLA-4 crosslinking on the surface of activated murine CD4(+) T lymphocytes leads to death of a substantial fraction of the cells whereas in resting CD4(+) T cells the same stimulation conditions induce cell cycle arrest without apoptosis. Cell death induced by CTLA-4 stimulation occurs independently of Fas and therefore may involve a novel pathway. CTLA-4-mediated apoptosis may be a means of terminating the function of previously stimulated T cells. Exploitation of this mechanism also may provide a therapeutic strategy to eliminate alloreactive or autoreactive T cells.     

Identification and characterization of virus assembly intermediate complexes in HIV-1-infected CD4+ T cells

For type-C and lentiviruses, including human immunodeficiency virus type 1 (HIV-1), the pathway of virus assembly remains poorly defined, and the assembly and budding of capsids are believed to occur simultaneously at the plasma membrane of the infected cell. We have now identified two putative HIV-1 assembly intermediate complexes in infected CD4+ T cells. The first of these intermediates, a detergent-resistant complex (DRC), was identified as a large oligomer that had a density of 1.10-1.13 g/ml and was primarily composed of Pr55Gag and Pr160Gag-Pol precursors. The other putative intermediate was a detergent-sensitive complex (DSC) with a density of 1.15-1.17 g/ml, which apparently represented the products of extensive proteolytic processing of both the Pr55Gag and Pr160Gag-Pol precursors. Both complexes could be distinguished from released mature virions as well as immature viral particles. Surprisingly, the formation of DRC was not dependent upon the myristylation at the N-terminus of the Gag proteins, a signal required for plasma membrane targeting and virus production. However, the myristic acid modification was essential for the formation of DSC. These data suggest that interactions between individual Gag molecules and between Gag and Gag-Pol precursors may occur before their targeting to the plasma membrane during HIV-1 assembly. However, formation of the late virus assembly complex and productive processing of Pr55Gag and Pr160Gag-Pol precursors apparently do not occur until these precursors are targeted to the plasma membrane.     

MHC class II-transfected tumor cells directly present antigen to tumor-specific CD4+ T lymphocytes

We have developed and shown to be efficacious an immunotherapeutic strategy to enhance the generation of tumor-specific CD4+ T helper lymphocytes. The approach uses autologous tumor cells genetically modified to express syngeneic MHC class II genes as cell-based immunogens and is based on the hypothesis that tumor cells directly present tumor Ags to CD4+ T cells. Since the conventional pathway for CD4+ T cell activation is indirect via professional APC, induction of immunity following immunization with class II-transfected tumor cells was examined in bone marrow chimeric mice. Both tumor and host-derived cells are APC for tumor Ags, suggesting that the efficacy of tumor cell vaccines can be significantly improved by genetic modifications that enhance tumor cell Ag presentation.     

Rejection of cardiac xenografts by CD4+ or CD8+ T cells

We recently showed that brief complement inhibition induces accommodation of hamster cardiac transplants in nude rats. We have reconstituted nude rats carrying an accommodated xenograft with syngeneic CD4+ or CD8+ T cells to investigate the cellular mechanism of xenograft rejection. We show that CD4+ T cells can initiate xenograft rejection (10 +/- 1.7 days) by promoting production of IgG xenoreactive Abs (XAb). These XAb are able to activate complement as well as to mediate Ab-dependent cell-mediated cytotoxicity. Adoptive transfer of these XAb into naive nude rats provoked hyperacute xenograft rejection (38 +/- 13 min). The rejection was significantly (p < 0.001) delayed by cobra venom factor (CVF; 11 +/- 8 h in four of five cases) but was still more rapid than in control nude rats (3.3 +/- 0.5 days). CVF plus NK cell depletion further prolonged survival (>7 days in four of five cases; p < 0.01 vs CVF only). CD8+ T cell-reconstituted nude rats rejected their grafts later (19.4 +/- 5.8 days) and required a larger number of cells for transfer as compared with CD4+ T cell-reconstituted nude rats. However, second xenografts were rejected more rapidly than first xenografts in CD8+ T cell-reconstituted nude rats (9 +/- 2 days), indicating that the CD8+ T cells had been activated. This study demonstrates that CD4+ and CD8+ T cells can both reject xenografts. The CD4+ cells do so at least in part by generation of helper-dependent XAb that act by both complement-dependent and Ab-dependent cell-mediated cytotoxicity mechanisms; the CD8+ cells do so as helper-independent cytotoxic T cells.     

Productive infection of neonatal CD8+ T lymphocytes by HIV-1

CD8+ T lymphocytes confer significant but ultimately insufficient protection against HIV infection. Here we report that activated neonatal CD8+ T cells can be productively infected in vitro by macrophage-tropic (M-tropic) HIV-1 isolates, which are responsible for disease transmission, whereas they are resistant to T cell-tropic (T-tropic) HIV strains. Physiological activation of CD8-alpha/beta+ CD4- T cell receptor-alpha/beta+ neonatal T cells, including activation by allogeneic dendritic cells, induces the accumulation of CD4 messenger RNA and the expression of CD4 Ag on the cell surface. The large majority of anti-CD3/B7.1-activated cord blood CD8+ T cells coexpress CD4, the primary HIV receptor, as well as CCR5 and CXCR4, the coreceptors used by M- and T-tropic HIV-1 strains, respectively, to enter target cells. These findings are relevant to the rapid progression of neonatal HIV infection. Infection of primary HIV-specific CD8+ T cells may compromise their survival and thus significantly contribute to the failure of the immune system to control the infection. Furthermore, these results indicate a previously unsuspected level of plasticity in the neonatal immune system in the regulation of CD4 expression by costimulation.     

CD4+ T cells orchestrate both amplification and deletion of CD8+ T cells

This review focuses on the role of CD4+ T cells in regulating immune responses, orchestrating both the amplification and deletion of immune cells, particularly CD8+ T cells. These two functions, which represent only an apparent contradiction, appear to be two faces of the same process of regulation. In fact, because the immune response, once activated, needs to be carefully controlled or switched off when the antigenic stimulus is eliminated, the immune system has developed several strategies either to regulate clonal amplification or to avoid useless expansion of activated cells. In particular, we have reported many data demonstrating that CD4+ T cells may be indicated as the regulatory element in the activation as well as the deletion of CD8+ T cells. New data are also reported on the ability of anergic CD4+ T cells to suppress CD8+ T-cell activation through induction of apoptosis, and on the need for CD8+ T cells for antigen recognition in inducing cell death in CD4+ T cells. Moreover, the central role of CD4+ T cells in the maintenance of peripheral tolerance has been widely described.     

Cytokine modulation of TH1/TH2 phenotype differentiation in directly alloresponsive CD4+ human T cells

Organ graft rejection is caused by the recognition of allogeneic MHC molecules by recipient T cells by two different pathways. The indirect pathway of alloreactivity requires the presentation of MHC peptides from the graft by autologous APC, as with conventional antigen. The direct pathway, on the other hand, requires the recognition of foreign MHC on foreign cells. The regulatory mechanisms for this component of alloreactivity have not been extensively studied. We show here that the T cell response activated by alloantigens in the direct pathway is similarly constrained and modulated by cytokines, as has been shown for classic antigen presentation. Thus, the inclusion of IL-2 or TGF-beta in MLC performed with purified responder T cells resulted in outgrowth of cells secreting IL-2 and IFN-gamma, whereas addition of IL-4, IL-10, or anti-TGF-beta encouraged outgrowth of cells secreting IL-4 and IL-10. T cells alloactivated via the direct pathway and then cloned in IL-2 alone secreted IL-4 and IL-10 as well as IFN-gamma and IL-2 (Th0 phenotype). Established clones remained susceptible to cytokine modulation, such that IL-4 and IL-10 decreased their secretion of IL-2 and IFN-gamma, whereas TGF-beta suppressed IL-4 and IL-10 secretion. The first alterations of Th0 toward Th1 or Th2 phenotypes could already be observed after only a very brief exposure to cytokines of 48 hr, followed by extended culture with IL-2 alone. These results confirm that human T cells with Th1 and Th2 phenotypes, recognizing alloantigen via the direct pathway, derive from the same IL-2-secreting precursor and can be manipulated by cytokines in an analogous fashion to conventional antigen-reactive cells. These findings may have implications for manipulating the direct pathway of alloantigen recognition in human organ transplantation.     

Extracellular HIV-1 tat protein up-regulates the expression of surface CXC-chemokine receptor 4 in resting CD4+ T cells

Here we report that synthetic HIV-1 Tat protein, immobilized on a solid substrate, up-regulates the surface expression of the CXC-chemokine receptor 4 (CXCR4), but not of the CC-chemokine receptor 5 in purified populations of primary resting CD4+ T cells. The Tat-mediated increase of CXCR4 occurred in a well-defined range of concentrations (1-10 nM of immobilized Tat) and time period (4-8 h postincubation). Moreover, the increase of CXCR4 was accompanied by an increased entry of the HXB2 T cell line-tropic (X4-tropic), but not of the BaL macrophage-tropic strain of HIV-1. The ability of Tat to up-regulate CXCR4 expression was abrogated by the protein synthesis inhibitor cycloheximide, clearly indicating the requirement of de novo synthesis. As Tat protein is actively released by HIV-1 infected cells, our data indicate a potentially important role for extracellular Tat in rendering bystander CD4+ T cells more susceptible to infection with X4-tropic HIV-1 isolates.     

HIV-1 variation diminishes CD4 T lymphocyte recognition

Effective long-term antiviral immunity requires specific cytotoxic T lymphocytes and CD4(+) T lymphocyte help. Failure of these helper responses can be a principle cause of viral persistence. We sought evidence that variation in HIV-1 CD4(+) T helper epitopes might contribute to this phenomenon. To determine this, we assayed fresh peripheral blood mononuclear cells from 43 asymptomatic HIV-1(+) patients for proliferative responses to HIV-1 antigens. 12 (28%) showed a positive response, and we went on to map dominant epitopes in two individuals, to p24 Gag restricted by human histocompatibility leukocyte antigen (HLA)-DR1 and to p17 Gag restricted by HLA-DRB52c. Nine naturally occurring variants of the p24 Gag epitope were found in the proviral DNA of the individual in whom this response was detected. All variants bound to HLA-DR1, but three of these peptides failed to stimulate a CD4(+) T lymphocyte line which recognized the index sequence. Antigenic variation was also detected in the p17 Gag epitope; a dominant viral variant present in the patient was well recognized by a specific CD4(+) T lymphocyte line, whereas several natural mutants were not. Importantly, variants detected at both epitopes also failed to stimulate fresh uncultured cells while index peptide stimulated successfully. These results demonstrate that variant antigens arise in HIV-1(+) patients which fail to stimulate the T cell antigen receptor of HLA class II-restricted lymphocytes, although the peptide epitopes are capable of being presented on the cell surface. In HIV-1 infection, naturally occurring HLA class II-restricted altered peptide ligands that fail to stimulate the circulating T lymphocyte repertoire may curtail helper responses at sites where variant viruses predominate.     

Activation and cell cycle antigens in CD4+ and CD8+ T cells correlate with plasma human immunodeficiency virus (HIV-1) RNA level in HIV-1 infection

gamma-Sarcoglycan is a transmembrane, dystrophin-associated protein expressed in skeletal and cardiac muscle. The murine gamma-sarcoglycan gene was disrupted using homologous recombination. Mice lacking gamma-sarcoglycan showed pronounced dystrophic muscle changes in early life. By 20 wk of age, these mice developed cardiomyopathy and died prematurely. The loss of gamma-sarcoglycan produced secondary reduction of beta- and delta-sarcoglycan with partial retention of alpha- and epsilon-sarcoglycan, suggesting that beta-, gamma-, and delta-sarcoglycan function as a unit. Importantly, mice lacking gamma-sarco- glycan showed normal dystrophin content and local- ization, demonstrating that myofiber degeneration occurred independently of dystrophin alteration. Furthermore, beta-dystroglycan and laminin were left intact, implying that the dystrophin-dystroglycan-laminin mechanical link was unaffected by sarcoglycan deficiency. Apoptotic myonuclei were abundant in skeletal muscle lacking gamma-sarcoglycan, suggesting that programmed cell death contributes to myofiber degeneration. Vital staining with Evans blue dye revealed that muscle lacking gamma-sarcoglycan developed membrane disruptions like those seen in dystrophin-deficient muscle. Our data demonstrate that sarcoglycan loss was sufficient, and that dystrophin loss was not necessary to cause membrane defects and apoptosis. As a common molecular feature in a variety of muscular dystrophies, sarcoglycan loss is a likely mediator of pathology.     

Transfection of a retroviral construct carrying a non producer HIV-1 variant induces HIV-1 resistance in CD4+ CEMss cells

The phenotype of Human Immunodeficiency Virus-1 (HIV)-infected HUT-78 cell clone (F12) has been described (Federico et al, AIDS Res Hum Retrov 1989; 5: 365-96). Briefly, F12 cells are: i) CD4 down-regulated, ii) non producer and iii) fully resistant to homologous superinfection. We tested whether this phenotype was dependent upon the expression of the HIV-1 genome integrated therein. The SstI/SstI F12 provirus was cloned and inserted in the pLj retroviral vector bearing the neomycin (neo)-Geneticine resistance gene. CD4+ HIV-susceptible CEMss cells were transfected with this construct in the sense orientation. Neo-resistant clones exhibited an integrated viral DNA, low viral mRNA expression and (as in F12 cells) the presence of uncleaved gp160, no gp41 and a small amount of p55 gag precursor. Superinfection of the F12/HIV-DNA-transfected CEMss clones showed that these CD4+ cells had acquired a significant (0.7-1.5 logs) resistance towards superinfection with HIV-1. This was observed in all four transfected clones where the F12/HIV DNA was expressed, but not in the control clone that was transfected with the pLj vector alone. These results confirm those that were obtained with human CD4+ CEMss cells infected with a recombinant retrovirus bearing the same SstI/SstI F12/HIV genome (Federico et al, J Gen Virol, 1993, in press). Both sets of results indicate that the expression of this genome in bio-engineered CD4+ human cells results in their intracellular immunization against HIV-1.     

Disturbed CD4+ T cell homeostasis and in vitro HIV-1 susceptibility in transgenic mice expressing T cell line-tropic HIV-1 receptors

T cell line-tropic (T-tropic) HIV type 1 strains enter cells by interacting with the cell-surface molecules CD4 and CXCR4. We have generated transgenic mice predominantly expressing human CD4 and CXCR4 on their CD4-positive T lymphocytes (CD4+ T cells). Their primary thymocytes are susceptible to T-tropic but not to macrophage-tropic HIV-1 infection in vitro, albeit with a viral antigen production less efficient than human peripheral blood mononuclear cells. Interestingly, even without HIV infection, transgenic mice display a CD4+ T cell depletion profile of peripheral blood reminiscent of that seen in AIDS patients. We demonstrate that CD4+ T cell trafficking in transgenic mice is biased toward bone marrow essentially due to CXCR4 overexpression, resulting in the severe loss of CD4+ T cells from circulating blood. Our data suggest that CXCR4 plays an important role in lymphocyte trafficking through tissues, especially between peripheral blood and bone marrow, participating in the regulation of lymphocyte homeostasis in these compartments. Based on these findings, we propose a hypothetical model in which the dual function of CXCR4 in HIV-1 infection and in lymphocyte trafficking may cooperatively induce progressive HIV-1 infection and CD4+ T cell decline in patients.     

Vigorous HIV-1-specific CD4+ T cell responses associated with control of viremia

Virus-specific CD4+ T helper lymphocytes are critical to the maintenance of effective immunity in a number of chronic viral infections, but are characteristically undetectable in chronic human immunodeficiency virus-type 1 (HIV-1) infection. In individuals who control viremia in the absence of antiviral therapy, polyclonal, persistent, and vigorous HIV-1-specific CD4+ T cell proliferative responses were present, resulting in the elaboration of interferon-gamma and antiviral beta chemokines. In persons with chronic infection, HIV-1-specific proliferative responses to p24 were inversely related to viral load. Strong HIV-1-specific proliferative responses were also detected following treatment of acutely infected persons with potent antiviral therapy. The HIV-1-specific helper cells are likely to be important in immunotherapeutic interventions and vaccine development.     

B cells directly tolerize CD8(+) T cells

This report investigates the response of CD8(+) T cells to antigens presented by B cells. When C57BL/6 mice were injected with syngeneic B cells coated with the Kb-restricted ovalbumin (OVA) determinant OVA257-264, OVA-specific cytotoxic T lymphocyte (CTL) tolerance was observed. To investigate the mechanism of tolerance induction, in vitro-activated CD8(+) T cells from the Kb-restricted, OVA-specific T cell receptor transgenic line OT-I (OT-I cells) were cultured for 15 h with antigen-bearing B cells, and their survival was determined. Antigen recognition led to the killing of the B cells and, surprisingly, to the death of a large proportion of the OT-I CTLs. T cell death involved Fas (CD95), since OT-I cells deficient in CD95 molecules showed preferential survival after recognition of antigen on B cells. To investigate the tolerance mechanism in vivo, naive OT-I T cells were adoptively transferred into normal mice, and these mice were coinjected with antigen-bearing B cells. In this case, OT-I cells proliferated transiently and were then lost from the secondary lymphoid compartment. These data provide the first demonstration that B cells can directly tolerize CD8(+) T cells, and suggest that this occurs via CD95-mediated, activation-induced deletion.     

Increased rate of HIV-1 entry and its cytopathic effect in CD4+/CXCR4+ T cells expressing relatively high levels of CD26

The role of the T-cell activation antigen CD26 was evaluated in viral entry and infection of CD4(+)/CXCR4(+) cells by the lymphotropic HIV-1 Lai isolate. For this purpose, CEM T cells, which are permissive to HIV infection and express low levels of CD26, were used to establish by transfection four groups of cell clones expressing either low, high, and very high levels of CD26, or expressing the anti-sense RNA of CD26. Entry was monitored by the detection of proviral DNA synthesis and the kinetics of virus production, whereas the cytopathic effect was demonstrated by the occurrence of apoptosis. HIV entry and infection were consistently accelerated by at least 24 to 48 h in clones expressing high levels of CD26 compared to the parental cells or to the clones expressing low levels of CD26. Interestingly, infection of clones expressing very high levels of CD26 was not accelerated and showed a kinetics of infection similar to that of low CD26 expressing clones. Moreover, HIV infection was significantly reduced in the clones expressing CD26 anti-sense RNA. In the different clones, apoptosis was dependent on the severity of virus infection and occurred after the accumulation of HIV envelope glycoproteins. Our results demonstrate that with equivalently expressed levels of CD4 and CXCR4 in cell lines established from CEM cells, relatively high levels of CD26 contribute to an increased rate of HIV entry, infection, and apoptosis. Furthermore, they point out that overexpression of CD26 in a given cell line may lead to a negative effect on HIV infection. Consequently, CD26 appears to regulate HIV entry and apoptosis, processes which are critical for viral pathogenesis.     

Reciprocal regulation of CD30 expression on CD4+ T cells by IL-4 and IFN-gamma

Prior studies have implicated CD30 as a marker for Th2 cells, but the mechanism that underlies this correlation was unknown. We show here that CD30 was expressed on activated CD4+ T cells in the presence of IL-4. In the absence of endogenously produced IL-4, however, even Th2 lineage cells lost CD30 expression. Thus, CD30 is not an intrinsic marker of Th2 cells, but is inducible by IL-4. CD30 was also found to be down-regulated by IFN-gamma. Committed Th1 effector cells do not express CD30, although differentiating Th1 lineage cells temporarily express CD30. The transient expression of CD30 on differentiating Th1 lineage cells was mainly the result of endogenously produced IL-4 induced by IL-12. Culture of IL-12-primed cells under conditions that reverse the phenotype (Ag plus IL-4) resulted in two cell populations based upon their ability to express CD30. One population responded to IL-4 upon restimulation and became a CD30-positive, Th0-like cell population, while the other remained CD30 negative and synthesized only IFN-gamma. Thus, CD30 expressed on CD4+ T cells reflected the ability of CD4+ T cells to respond to IL-4.     

Cutting edge: CD4+ T cells kill CD8+ T cells via Fas/Fas ligand-mediated apoptosis

To explore the possibility that CD4+ T cells, described to mediate the elimination of themselves or B lymphocytes, could also mediate the elimination of CD8+ T cells, we analyzed apoptotic phenomena in cocultures of CD4+ and CD8+ autologous T cell lines. The data show that CD8+ T cells were lysed by activated CD4+ helper T cells by a Fas/FasL-mediated mechanism. CD4+ T cells were not lysed by activated CD8+ T cells, although Fas and FasL were equally expressed and anti-Fas Abs induced apoptosis in both CD4+ and CD8+ T cell populations. The results allowed us to speculate that CD4+ T cells not only help CD8+ T lymphocytes to mature into effector killer cells and to sustain this function but can also limit their growth.