Human T cell leukemia/lymphoma virus-infected antigen-specific T cell clones: indiscriminant helper function and lymphokine production

The ability of human T cell leukemia/lymphoma virus (HTLV)-I to alter the function of infected T lymphocytes was examined directly by investigating the properties of an antigen-specific T cell clone before and after transformation with HTLV-I. Following infection, the T4 antigen-specific clone manifested a tenfold increase in its surface interleukin 2 (IL 2) receptor (Tac) density and acquired the viral determinants p19, p24, and 4D12 not present in the uninfected clone. Prior to infection, the T cell clone responded to antigen stimulation in the presence of histocompatible antigen-presenting cells with proliferation and secretion of multiple lymphokines, including IL 2, B cell growth factor (BCGF), B cell differentiation factor (BCDF), and interferon-gamma (IFN-gamma). Following infection, the T cell clone both proliferated and produced constitutively three of these lymphokines (BCGF, BCDF, and IFN-gamma) in the absence of accessory cells or antigen. Co-cultivation with any accessory cells regardless of histocompatibility resulted in increased proliferation and lymphokine production. IL 2 production by the HTLV-I-transformed cell, however, could not be detected. Similarly, the uninfected clone was able to provide B cell help for Ig production only when stimulated with both histocompatible cells and antigen. In contrast, the infected cell provided T cell help to B cells in an unregulated manner, independent of antigen or histocompatibility. Thus, functions such as the induction of proliferation, B cell help, and lymphokine production, which are finely regulated in uninfected antigen-specific T cell clones, became indiscriminant after HTLV-I infection.     

Infection of human natural killer (NK) cells with replication-defective human T cell leukemia virus type I provirus. Increased proliferative capacity and prolonged survival of functionally competent NK cells.

Human T-cell leukemia virus type I (HTLV-I) can infect a variety of human cell types, but only T lymphocytes are efficiently immortalized after HTLV-I infection. This study reports an attempt to infect and to immortalize NK cells with HTLV-I. Co-cultivation of freshly isolated NK cells with a HTLV-I-producing T cell line did not result in NK cell infection. However, NK cells activated with an anti-CD16 mAb and co-cultivated with a HTLV-I-producing T cell line were reproducibly infected by HTLV-I. HTLV-I infection was documented in NK cell lines and clones by the detection of defective integrated provirus by both Southern blot and polymerase chain reaction analysis. Although HTLV-I-infected NK cells produced viral proteins, they did not produce infectious viral particles. HTLV-I-infected NK cells were phenotypically indistinguishable from their uninfected counterparts (CD16+, CD2+, CD56+, CD3-). They also retained the ability to mediate both natural and antibody-dependent cell cytotoxicity. The IL-2-dependent proliferation of HTLV-I-infected NK cells was significantly greater than that of uninfected NK cells. The doubling time of this infected population was reduced from 9 days to 3 days, and the overall survival of the culture in the absence of restimulation was extended from 5 wk to 18 wk. Unlike T lymphocytes, HTLV-I-infected NK cells were not immortal, implying a fundamental difference between these two lymphocyte populations.     

The soluble viral glycoprotein of vesicular stomatitis virus efficiently sensitizes target cells for lysis by CD4+ T lymphocytes

The soluble glycoprotein Gs of vesicular stomatitis virus (VSV), at approximately 10(4) molecules per cell, sensitized target cells for lysis by clones of CD4+ cytolytic T lymphocytes (CTL). In addition to lysis, the clones responded by proliferation and interleukin-2 release. Targets sensitized by Gs competed effectively with VSV-infected cells for recognition. Immune cytolysis by these CD4+ CTLs was restricted by class II major histocompatibility complex (MHC) antigens and was specific to VSV. The specific class II MHC antigen which was restricting for each clone remained the same whether the targets were sensitized by infection with VSV or by exogenously added soluble antigen. Sensitization by Gs appeared to require prior processing because the antigen-presenting cells that were fixed prior to exposure to Gs failed to be recognized by the CTL clones. The high efficiency of this uptake and processing was suggested by the inability of Gs at concentrations up to 10(7) per cell to block superinfection by VSV or to effect the RNA-synthetic machinery of uninfected cells. Also, Gs failed to hemolyze sheep erythrocytes when there was hemolysis by virions or an amino-terminal peptide of the VSV glycoprotein. Extrapolation of these results to viral diseases was possible because soluble viral glycoproteins were naturally synthesized during many viral infections and class II MHC antigens were inducible in cells of nonlymphoid origin. Therefore, CD4+ CTLs may be important participants in increasing virus-induced pathology, especially among adjacent uninfected cells.     

Human T cell leukemia/lymphoma virus type I infection of a CD4+ proliferative/cytotoxic T cell clone progresses in at least two distinct phases based on changes in function and phenotype of the infected cells

The effect of human T cell leukemia/lymphoma virus type I (HTLV-I) infection on the function and the phenotype of a human proliferating/cytotoxic T cell clone, specific for tetanus toxin, was investigated. During the period after infection, two distinct phases were observed, based on growth properties, phenotype, and functional activity of the infected cells. Phase I HTLV-I infected cells (0 to about 150 days after infection) proliferated in an IL-2-dependent way, but without the requirement for repetitive antigenic stimulation. No differences in expression of the CD2, CD3, CD4, Tp103, and CD28 Ag between these cells and the parental cells could be demonstrated, with the exception of the expression of IL-R p55 and HLA-DR Ag, which were constitutively expressed on the phase I cells. The phase I HTLV-I-infected cells, as well as the parental 827 cells reacted with a mAb specific for an epitope on the variable part of the TCR beta-chain, indicating that the TCR was not altered after HTLV-I infection. Like the parental clone, the phase I cells proliferated in response to tetanus toxin, but the tetanus toxin-specific response of the phase I cells did not require the presence of APC. Results of experiments, in which the levels of intracellular Ca2+ were measured, indicated that HTLV-I cells can acquire the capability to process Ag and present that to themselves. Phase I HTLV-I-infected T cells had lost their cytotoxic activity which was likely to be due to an effect on the lytic machinery rather than on Ag recognition by the TCR, inasmuch as it was found that phase I HTLV-I-infected T cells did no longer contain N-alpha-benzyloxy-L-lysine thiobenzylester-serine esterase activity. Furthermore, it was found that phase I HTLV-I-infected T cells had a diminished capacity to form conjugates with target cells. From a period of about 200 days after HTLV-I infection, phase II cells emerged that proliferated strongly in the absence of IL-2 and that had lost all functional activity. These cells did not express the CD3/T cell receptor complex on their surface. Phase I as well as phase II HTLV-I-infected cells were targets for CTL raised in the autologous donor.     

Human T-cell leukemia virus type I-induced proliferation of human immature CD2+CD3- thymocytes.

The mitogenic activity of human T-cell leukemia virus type I (HTLV-I) is triggering the proliferation of human resting T lymphocytes through the induction of the interleukin-2 (IL-2)/IL-2 receptor autocrine loop. This HTLV-I-induced proliferation was found to be mainly mediated by the CD2 T-cell antigen, which is first expressed on double-negative lymphoid precursors after colonization of the thymus. Thus, immature thymocytes express the CD2 antigen before that of the CD3-TCR complex. We therefore investigated the responsiveness of these CD2+CD3- immature thymocytes and compared it with that of unseparated thymocytes, containing a majority of the CD2+CD3+ mature thymocytes, and that of the CD2-CD3- prothymocytes. Both immature and unseparated thymocytes were incorporating [3H]thymidine in response to the virus, provided that they were cultivated in the presence of submitogenic doses of phytohemagglutinin. In contrast, the prothymocytes did not proliferate. Downmodulation of the CD2 molecule by incubating unseparated and immature thymocytes with a single anti-CD2 monoclonal antibody inhibited the proliferative response to HTLV-I. These results clearly underline that the expression of the CD2 molecule is exclusively required in mediating the proliferative response to the synergistic effect of phytohemagglutinin and HTLV-I. Immature thymocytes treated with a pair of anti-CD2 monoclonal antibodies were shown to proliferate in response to HTLV-I, even in the absence of exogenous IL-2. We further verified that the proliferation of human thymocytes is consecutive to the expression of IL-2 receptors and the synthesis of IL-2. These observations provide evidence that the mitogenic stimulus delivered by HTLV-I is more efficient than that provided by other conventional mitogenic stimuli, which are unable to trigger the synthesis of endogenous IL-2. Collectively, these results show that the mitogenic activity of HTLV-I is able to trigger the proliferation of cells which are at an early stage of T-cell development. They might therefore represent target cells in which HTLV-I infection could favor the initiation of the multistep lymphoproliferative process leading to adult T-cell leukemia.     

A kinetic model of CD4+ lymphocytes with the human immunodeficiency virus (HIV).

This report describes a kinetic model of in vitro cytopathology involving interactions of human immunodeficiency virus (HIV) with CD4+ helper T lymphocytes. The model uses nonlinearly coupled, ordinary differential equations to simulate the dynamics of infected and uninfected cells and free virions. It is assumed that resting cells are more readily infected than activated cells, but once infected, only activated cells produce more virus. Resting cells can be activated by some appropriate stimulus (e.g. phytohemagglutinin, soluble antigen). The model predicts that the initial inoculum of virus is taken up by resting cells and without stimulation the system comes to a steady state of two populations, namely infected and uninfected cells. Stimulation of this system produces two additional populations, namely infected and uninfected activated cells which, along with the previous populations, exhibit cyclic behavior of growth, viral expression/release, and death. Additional stimuli enhance or diminish the cyclic behavior depending upon their occurrence in time. These simulations suggest a similar dynamics in human HIV infection and may explain a major factor responsible for the widely varying depletion rate of (CD4+) helper T cells in AIDS patients.     

The role of CD4+ cells in sustaining lymphocyte proliferation during lymphocytic choriomeningitis virus infection.

The murine immune response to lymphocytic choriomeningitis virus (LCMV) infection involves the activation of CD8+, class I MHC-restricted and virus-specific CTL. At times coinciding with CTL activation, high levels of IL-2 gene expression and production occur, the IL-2R is expressed, and T cell blastogenesis and proliferation are induced. We have previously found that, although both CD4+ and CD8+ T cell subsets transcribe IL-2, the CD4+ subset appears to be the major producer of IL-2 whereas the CD8+ subset appears to be the major proliferating population when the subsets are separated after activation in vivo. The studies presented here were undertaken to examine the contribution made by the CD4+ subset to lymphocyte proliferation in vivo. Responses to LCMV infection were examined in intact mice and in mice depleted of CD4+ or CD8+ subsets by antibody treatments in vivo. Protocols were such that in vivo treatments with anti-CD4 or anti-CD8 depleted the respective subset by greater than 90%. In situ hybridizations demonstrated that the IL-2 gene was expressed in non-B lymphocytes isolated from either CD4+ cell-depleted or CD8+ cell-depleted mice on day 7 post-infection with LCMV. When placed in culture, however, cells from CD8+ cell-depleted mice produced significantly higher levels of detectable IL-2 than did cells isolated from CD4+ cell-depleted mice on day 7 post-infection. IL-2 was apparently produced in vivo in mice depleted of either CD4+ or CD8+ cells, as expression of the gene for the p55 chain of the IL-2R, IL-2 responsiveness, and lymphocyte proliferation were observed with cells isolated from both sets of mice. Lymphocyte proliferation was shown to be sustained in mice depleted of CD4+ cells in vivo by three criteria: 1) non-B lymphocytes isolated from infected mice depleted of CD4+ cells underwent more DNA synthesis than did those isolated from uninfected mice or from infected mice depleted of CD8+ cells; 2) leukocyte yields were expanded during infection of CD4+ cell-depleted mice; and 3) CD8+ cell numbers were increased during infection of CD4+ cell-depleted mice. The majority of non-B lymphocytes having the characteristics of blast lymphocytes was recovered in the CD8+ populations isolated from infected CD4+ cell-depleted mice. These findings suggest that the requirement for the CD4+ subset to sustain CD8+ lymphocyte proliferation in vivo is limited, and that CD4+ and CD8+ cell types can function independently in many aspects of their responses to viral infections.     

Functional alterations of herpes simplex virus-specific CD4+ multifunctional T cell clones following infection with human T lymphotropic virus type I

In an attempt to understand the mechanisms of immunodeficiency induced by human T lymphotropic virus type I (HTLV-I), HSV-specific CD4+ human multifunctional T cell clones were infected with HTLV-I in vitro. Early after HTLV-I infection, when their growth was still IL-2-dependent, clones were found to have almost completely lost their cytotoxic activity. At that time, their HSV-Ag-induced proliferative response and helper function for anti-HSV antibody production by B cells were only partially impaired. After this initial phase, the HTLV-I-infected clone became IL-2-independent, and the helper function was also completely lost. IL-2-dependent HTLV-I-infected clones showed degrees of proliferative response and elevation of intracellular free Ca2+ concentration induced by anti-CD3 mAb equivalent to those of HTLV-I-uninfected clones. On the other hand, during the IL-2-independent stage, expression of CD3-TCR complex on the cell surface was markedly decreased, and no significant elevation of intracellular free Ca2+ concentration was detected in response to anti-CD3 mAb. These data indicated that the loss of cytotoxic activity of HSV-specific T cell clones observed early after HTLV-I infection was not the result of impaired antigen recognition via the CD3-TCR complex, but might be due to dysfunction in the effector phase. On the other hand, the dysfunction of helper activity found late after HTLV-I infection might have mainly occurred in the recognition phase due to the decreased expression of CD3-TCR complex. The present data appear to suggest certain aspects of the pathogenesis of the immunodeficiency occurring in HTLV-I infection.     

Vaccinia virus-specific human CD4+ cytotoxic T-lymphocyte clones.

Vaccinia virus-specific cytotoxic T-lymphocyte (CTL) clones were established from a healthy donor, who had been immunized with vaccinia virus vaccine, by stimulation of peripheral blood lymphocytes with UV-inactivated vaccinia virus antigen. The phenotype of all of the clones established was CD3+ CD4+ CD8- Leu11-. We used a panel of allogenic vaccinia virus-infected B-lymphoblastoid cell lines and demonstrated that some of the clones recognized vaccinia virus epitopes presented by human leukocyte antigen (HLA) class II molecules. Monoclonal antibodies specific for either HLA-DP or HLA-DR determinant reduced the cytotoxicity of specific clones. The HLA-restricted cytotoxicity of the clones is vaccinia virus specific, because vaccinia virus-infected but not influenza virus-infected autologous target cells were lysed. Using vaccinia virus deletion mutants, we found that some of the CTL clones recognize an epitope(s) that lies within the HindIII KF regions of the vaccinia virus genome. These results indicate that heterogeneous CD4+ CTL clones specific for vaccinia virus are induced in response to infection and may be important in recovery from and protection against poxvirus infections.     

Examination of CD8+ T cell function in humans using MHC class I tetramers: similar cytotoxicity but variable proliferation and cytokine production among different clonal CD8+ T cells specific to a single viral epitope

Following infection by human T cell lymphotrophic virus-I (HTLV-I), high frequencies of polyclonal Tax11-19-reactive CD8(+) T cells can be detected in the peripheral blood. To investigate whether there are differences in the effector functions of these cells, we generated a panel of Tax11-19-reactive T cell clones by single cell sorting of HLA-A2/Tax11-19 tetramer binding CD8(+) T cells followed by repeated stimulation with PHA and IL-2. Examination of the TCRs revealed 17 different T cell clones with unique clonal origins. Nine representative CD8(+) T cell clones showed a similar cytotoxic dose-response activity against Ag-pulsed target cells, even though they express different TCRs. This cytotoxic effector function was not influenced by the engagement of either CD28 or CD2 costimulatory molecules. In contrast to the cytotoxic activity, qualitatively different degrees of proliferative response and cytokine secretion were observed among T cell clones of different clonal origin. The induction of proliferation and cytokine secretion required the engagement of costimulatory molecules, particularly CD2-LFA-3 interaction. These results indicate that functionally diverse, polyclonal CTL populations can be activated specific to a single immunodominant viral epitope; they can manifest virtually identical cytotoxic effector function but have marked differences in proliferation and cytokine secretion.     

Central nervous system pathology caused by autoreactive CD8+ T-cell clones following virus infection

Theiler's murine encephalomyelitis virus (TMEV) causes a demyelinating disease in infected mice which has similarities to multiple sclerosis. Spleen cells from TMEV-infected SJL/J mice stimulated with antigen-presenting cells infected with TMEV resulted in a population of autoreactive CD8+ cytotoxic T cells that kill uninfected syngeneic cells. We established CD8+ T cell clones that could kill both TMEV-infected and uninfected syngeneic targets, although infected target cells were killed more efficiently. The CD8+ T-cell clones produced gamma interferon when incubated with either infected or uninfected syngeneic target cells. Intracerebral injection of the clones into na?ve mice induced degeneration, not only in the brain, but also in the spinal cord. This suggests that CD8+ Tc1 cells could play a pathogenic role in central nervous system inflammation.     

CD4+ T cells are required to sustain CD8+ cytotoxic T-cell responses during chronic viral infection.

In this study, we have examined the relative contributions of CD4+ and CD8+ T cells in controlling an acute or chronic lymphocytic choriomeningitis virus (LCMV) infection. To study acute infection, we used the LCMV Armstrong strain, which is cleared by adult mice in 8 to 10 days, and to analyze chronic infection, we used a panel of lymphocyte-tropic and macrophage-tropic variants of LCMV that persist in adult mice for several months. We show that CD4+ T cells are not necessary for resolving an acute LCMV infection. CD4+ T-cell-depleted mice were capable of generating an LCMV-specific CD8+ cytotoxic T-lymphocyte (CTL) response and eliminated virus with kinetics similar to those for control mice. The CD8+ CTL response was critical for resolving this infection, since beta 2-microglobulin knockout (CD8-deficient) mice were unable to control the LCMV Armstrong infection and became persistently infected. In striking contrast to the acute infection, even a transient depletion of CD4+ T cells profoundly affected the outcome of infection with the macrophage- and lymphocyte-tropic LCMV variants. Adult mice given a single injection of anti-CD4 monoclonal antibody (GK1.5) at the time of virus challenge became lifelong carriers with high levels of virus in most tissues. Unmanipulated adult mice infected with the different LCMV variants contained virus for prolonged periods (> 3 months) but eventually eliminated infection from most tissues, and all of these mice had LCMV-specific CD8+ CTL responses. Although the level of CTL activity was quite low, it was consistently present in all of the chronically infected mice that eventually resolved the infection. These results clearly show that even in the presence of an overwhelming viral infection of the immune system, CD8+ CTL can remain active for long periods and eventually resolve and/or keep the virus infection in check. In contrast, LCMV-specific CTL responses were completely lost in chronically infected CD4-depleted mice. Taken together, these results show that CD4+ T cells are dispensable for short-term acute infection in which CD8+ CTL activity does not need to be sustained for more than 2 weeks. However, under conditions of chronic infection, in which CD8+ CTLs take several months or longer to clear the infection, CD4+ T-cell function is critical. Thus, CD4+ T cells play an important role in sustaining virus-specific CD8+ CTL during chronic LCMV infection. These findings have implications for chronic viral infections in general and may provide a possible explanation for the loss of human immunodeficiency virus-specific CD8+ CTL activity that is seen during the late stages of AIDS, when CD4+ T cells become limiting.     

Activated peripheral CD8 lymphocytes express CD4 in vivo and are targets for infection by human immunodeficiency virus type 1.

There is increasing evidence that CD8 lymphocytes may represent targets for infection by human immunodeficiency virus type 1 (HIV-1) in vivo whose destruction may contribute to the loss of immune function underlying AIDS. HIV-1 may infect thymic precursor cells destined to become CD4 and CD8 lymphocytes and contribute to the numerical decline in both subsets on disease progression. There is also evidence for the induction of CD4 expression and susceptibility to infection by HIV-1 of CD8 lymphocytes activated in vitro. To investigate the relationship between CD8 activation and infection by HIV-1 in vivo, activated subsets of CD8 lymphocytes in peripheral blood mononuclear cells (PBMCs) of HIV-seropositive individuals were investigated for CD4 expression and HIV infection. Activated CD8 lymphocytes were identified by expression of CD69, CD71, and the human leukocyte antigen (HLA) class II, the beta-chain of CD8, and the RO isoform of CD45. CD4(+) and CD4(-) CD8 lymphocytes, CD4 lymphocytes, other T cells, and non-T cells were purified using paramagnetic beads, and proviral sequences were quantified by PCR using primers from the long terminal repeat region. Frequencies of activated CD8 lymphocytes were higher in HIV-infected study subjects than in seronegative controls, and they frequently coexpressed CD4 (mean frequencies on CD69(+), CD71(+), and HLA class II(+) cells of 23, 37, and 8%, respectively, compared with 1 to 2% for nonactivated CD8 lymphocytes). The level of CD4 expression of the double-positive population approached that of mature CD4 lymphocytes. That CD4 expression renders CD8 cell susceptible to infection was indicated by their high frequency of infection in vivo; infected CD4(+) CD8 lymphocytes accounted for between 3 and 72% of the total proviral load in PBMCs from five of the eight study subjects investigated, despite these cells representing a small component of the PBMC population (<3%). Combined, these findings provide evidence that antigenic stimulation of CD8 lymphocytes in vivo induces CD4 expression that renders them susceptible to HIV infection and destruction. The specific targeting of responding CD8 lymphocytes may provide a functional explanation for the previously observed impairment of cytotoxic T-lymphocyte (CTL) function disproportionate to their numerical decline in AIDS and for the deletion of specific clones of CTLs responding to HIV antigens.     

Modeling the T-cell dynamics and pathogenesis of HTLV-I infection

Human T-cell lymphotropic virus type I (HTLV-I) infection in humans causes a chronic infection of CD4⁺ T cells, and is associated with various disease outcomes, among them with the development of adult T-cell leukemia (ATL). The T-cell dynamics after HTLV-I infection can be described in a mathematical model with coupled differential equations. The infection process is modeled assuming cell-to-cell infection of CD4⁺ T cells. The model allows for CD4⁺ T cell subsets of susceptible, latently infected and actively infected cells as well as for leukemia cells. Latently infected T cells may harbor the virus for several years until they become activated and able to infect susceptible T cells. Uncontrolled proliferation of CD4⁺ T cells with monoclonal DNA-integration of HTLV-I results in the development of ATL. The model describes basic features that characterize HTLV-I infection; the chronic infection of CD4⁺ T cells, the increasing number of abnormal cells and the possible progression to ATL.     

Lysis of tumor cells by CD3+4-8-16+ T cell receptor alpha beta- clones, regulated via CD3 and CD16 activation sites, recombinant interleukin 2, and interferon beta 1

A small subpopulation (about 2%) of normal CD3+ human T lymphocytes lacks both CD4 and CD8 antigens. We have cloned these cells from peripheral blood lymphocytes (PBL) obtained from healthy individuals and from a patient with severe combined immunodeficiency. Six out of seven CD3+4-8-clones exert strong cytolytic activity against a variety of so-called NK-susceptible and -nonsusceptible tumor target cells. Their target cell specificity spectrum can virtually be as wide as that of CD3-NK cell-derived clones, with strong lytic capacity. Some of these clones also exert antibody-dependent cellular cytotoxicity (ADCC), a characteristic of NK cell-derived clones but not of CD3+4+ or CD8+ mature T cell-derived clones. Such CD3+ T cell clones do not express the CD16 (IgG Fc receptor) antigen, but as we demonstrate here, the CD16 antigen can be identified on CD3+4-8-clones. Both ADCC activity and CD16 antigen expression are lower in CD3+4-8- than in CD3- NK cell clones. Lytic activity of mature CD3+4+ or CD8+ and CD3- NK cell clones can be augmented, respectively, by anti-CD3 or anti-CD16 monoclonal antibodies (MAb), but that of CD3+4-8- clones are augmented by both MAb. Lytic activity of CD3+4+ or CD8+ clones is considerably enhanced after 3 hr of incubation with recombinant IL 2, as found for CD3- NK cells. Enhancement of lytic activity of allospecific CD3+4+ or CD8+ clones requires 18 hr of incubation. Thus, CD3+4-8-16+ cells share several features with CD3- NK cells. However, they express the CD3 antigen, which is characteristic for CD4+ or CD8+ mature T cells. Our results also indicate that although CD3+4-8- clones react with five preparations of anti-CD3 MAb tested, these clones do not express a classical CD3+/Ti alpha, beta antigen receptor complex. This is suggested by the finding that the CD3+4-8- clones do virtually not express the common epitope of the T cell receptor alpha, beta-chains as identified by the WT31 MAb. These CD3+4-8- lymphocytes may represent functionally mature lymphocytes of a distinct T cell subpopulation having a particular immune function.     

Murine CD8+ cytotoxic T lymphocytes lyse Toxoplasma gondii-infected cells

Studies were performed to determine whether CTL against Toxoplasma gondii-infected cells could be induced in a murine model of T. gondii infection in which CD8+ T lymphocytes have been shown to play a major role in resistance against this parasite. In 51Cr release assays, nylon wool nonadherent spleen cells from BALB/c (H-2d) mice immunized with the temperature-sensitive (ts-4) mutant strain of T. gondii were cytotoxic for T. gondii-infected P815 (H-2d) mastocytoma cells but not for uninfected cells. This cytotoxic activity was remarkably increased after in vitro stimulation with T. gondii-infected syngeneic spleen cells. The effector cells were shown to be CD8+ T lymphocytes, because the cytotoxicity was significantly inhibited by depletion of CD8+ T lymphocytes but not by depletion of CD4+ T lymphocytes. This cytotoxic activity was genetically restricted. Spleen cells from T. gondii-immune BALB/c mice were not cytotoxic for T. gondii-infected EL4 (H-2b) thymoma cells, whereas spleen cells from T. gondii-immune C57B1/6 (H-2b) mice were cytotoxic for T. gondii-infected EL4 cells but not for T. gondii-infected P815 cells. The cytolytic activity of CD8+ T lymphocytes against T. gondii-infected cells might be a mechanism whereby these cells confer resistance against T. gondii.     

The antiviral activity of HIV-specific CD8+ CTL clones is limited by elimination due to encounter with HIV-infected targets.

Adoptive immunotherapy of virus infection with viral-specific CTL has shown promise in animal models and human virus infections and is being evaluated as a therapy for established HIV-1 infection. Defining the individual obstacles for success is difficult in human trials. We have therefore examined the localization, persistence, and antiviral activity of HIV-1 gag-specific CTL clones in both HIV-1-infected and uninfected haplotype-matched human (hu)-PBL-SCID mice. Injection of gag-specific clones but not control CTL into HIV-1-infected hosts reduced plasma viremia by >10-fold but failed to eliminate the virus infection from most treated animals. The failure to eradicate virus did not reflect selection of escape variants because the gag epitope remained unmutated in virus isolates obtained after CTL therapy. Injection of carboxyfluorescein diacetate succinimide ester-labeled CTL demonstrated markedly different fates for gag-specific CTL in the presence or absence of HIV-1 infection. HIV-1-specific CTL rapidly disappeared in infected recipients, whereas they were maintained at high numbers in uninfected mice. By contrast, control CTL were long lived in both infected and uninfected recipients. Thus, interaction of CTL with virus-infected target cells in vivo leads not only to target destruction but also to the rapid disappearance of the infused CTL, and it limits the capacity of CTL therapy to eliminate HIV-1 infection.     

Impaired calcium mobilization in CD4+ and CD8+ T cells in a retrovirus-induced immunodeficiency syndrome, murine AIDS.

After infection with LP-BM5 murine leukemia viruses, susceptible strains of mice develop a severe and progressive immunodeficiency disease, termed murine AIDS (MAIDS), features of which include markedly impaired T cell response to mitogens or specific Ag stimulation and decreased production of IL-2. Since an elevation of intracellular calcium concentration resulting from binding of Ag to the TCR is associated with IL-2 production, T cells from mice either uninfected or infected with LP-BM5 murine leukemia viruses were examined by a calcium mobilization assay. Both CD4+ and CD8+ T cells from infected mice manifested impaired calcium mobilization responses upon in vitro stimulation with anti-CD3 mAb or Con A. The abnormalities appeared early after virus inoculation and showed no difference in time course between subsets of T cells. Frequencies of prestimulation calcium-positive cells among both CD4+ and CD8+ cells in mice with MAIDS were significantly higher than those for uninfected mice. These abnormalities were associated with presence of the MAIDS-inducing defective virus genome, but were not induced by infection of mice genetically resistant to development of MAIDS or with nonpathogenic helper murine leukemia virus, a virus component that induces high spontaneous proliferation of T cells, even in MAIDS-resistant mice.