A cytotoxic T lymphocyte inhibits acquired immunodeficiency syndrome virus replication in peripheral blood lymphocytes

CD8+ (suppressor/cytotoxic) lymphocytes block replication of HIV-1 or the simian immunodeficiency virus of macaques (SIVmac) in PBL of infected individuals. We now show that these CD8+ lymphocytes undergo clonal expansion in vivo after AIDS virus infection of the individual, suggesting they may be antigen-specific T cells. These CD8+ cells block replication of virus in autologous but not MHC class I-mismatched PBL. The inhibitory lymphocytes express the integrin family molecule 4B4 and the CTL-associated S6F1 epitope of LFA-1. Finally, physical contact is required for the CD8+ lymphocyte-mediated inhibition of AIDS virus replication, since this inhibitory function is blocked by anti-LFA-1 and anti-CD8 mAbs. These studies suggest that the cell that inhibits AIDS virus replication in PBL of infected individuals is a CTL.     

CD8+ T cell subsets of cytotoxic T lymphocytes induced by Epstein-Barr virus infection in infectious mononucleosis

Mononuclear peripheral blood lymphocytes (PBL) from patient with infectious mononucleosis (IM) were tested in a 51Cr-release assay for cytotoxicity against autologous and allogeneic lymphoblastoid cell line (LCL), or Epstein-Barr virus (EBV)-genome positive and negative cell line. In acute phase, PBL lyse an autologous LCL as well as allogeneic LCL (Wa cells). High levels of cytotoxicity were observed in the combinations between effector and target cells sharing HLA-Class 1 product. EBV-genome positive Daudi and Raji cells which lack HLA-Class 1 antigen and have mismactched HLA-Class 1 antigen, respectively showed resistance to killing. EBV-genome negative tumor cells except NK sensitive K562 cells were not killed by IM lymphocytes. However, the IM lymphocytes without atypical form in convalescent phase failed to show killing activity against autologous and allogeneic LCL. These findings suggest that cell surface membrane antigen structure on EBV-infected LCL may be able to explain the recognition and triggering of lysis of target cells by HLA-Class 1 restricted cytotoxic T cells (CTL) from acute IM. Phenotypic analysis of PBL with atypical form from IM was made by two-color flow cytometry. The data demonstrate that CD8+ T cells quantitatively represent the major population of lymphocytes expanded during acute IM. Furthermore, approximately 70% of these CD8+ T cells express HLA-DR on these surface, suggesting that they have undergone activation. However, IL 2R (CD25 antigen) expression was not significantly elevated on activated T cells. The salient profile on cytofluorographs of an acute IM was the increased number of CD3+CD19-, CD8+CD11b-, CD8+CD28+ and CD8+S6F1+ cells. However, CD3-CD19+, CD8+CD11b+, CD8+S6F1-, CD4+Leu8- and CD25+HLA-DR+ antigens were little expressed. Increased number of CD8+CD11b-, CD8+CD28+ and CD8+S6F1+ cells, which are regarded as CTL were reduced according to the improvement of the clinical symptoms and laboratory findings. These results together with HLA typing analysis suggested a possibility HLA-Class 1 restriction of the CTL with surface phenotype of CD8+CD11b-, CD8+CD28+, and CD8+S6F1+.     

The cytotoxic activity of mouse T lymphocytes against allogeneic cytotoxic T cells.

Using a 51Cr cytotoxicity assay, the sensitivity of murine cytotoxic T cells to T cell mediated cytotoxicity has been tested. Two experimental approaches have been used. First, cytotoxic T-blast-lymphocytes (CTBL), enriched by the velocity sedimentation at 1 g were used both as cytotoxic effector cells and as 51Cr-labelled target cells. It was found that murine CTBL are capable to lyse directly and specifically allogeneic CTBL within 200 minutes. The second approach used was to incubate CTBL together with CTBL, the cytotoxic activity of which was directed against the transplantation antigens of the added allogeneic CTBL population. After 10 hours, the residual cytotoxic activity was tested. Again it was found that CTBL were capable of functionally inactivating allogeneic CTBL. Therefore the results obtained are incompatible with the concept that target cell lysis by cytotoxic T lymphocytes is mediated by a non specific "lymphotoxin", secreted by activated T cells after the antigen-recognition phase in the confined space between T cells and target cells.     

Accumulation of natural killer and cytotoxic T large granular lymphocytes in the liver during virus infection

The immunologic mechanisms involved in virus-induced hepatitis were examined by measuring the cytotoxic capabilities and the morphologic and antigenic phenotypes of leukocytes isolated from livers of virus-infected mice. Large granular lymphocytes (LGL) of both natural killer (NK) cell and cytotoxic T lymphocyte (CTL) phenotypes were found to accumulate in livers of mice infected with either the nonhepatotropic Armstrong strain of lymphocytic choriomeningitis virus (LCMV-ARM) or the hepatotropic WE strain (LCMV-WE). Between days 1 and 5 postinfection (p.i.), both viruses induced a three- to fourfold increase in NK cell lytic activity in the livers of C3H/St mice and a three- to fourfold increase in the number of LGL in the organ. These LGL were characterized as NK cells on the basis of cell surface antigens, kinetics of appearance, target cell range, and morphology. By day 7 p.i., virus-specific, H-2-restricted, Thy-1+, Lyt-2+, CTL activity was present in the liver, and its appearance correlated with a second wave of LGL accumulation. CTL activity, total leukocyte number, and CTL/LGL number were at least fivefold higher in the livers of mice infected with LCMV-WE than with LCMV-ARM. The dramatic LCMV-WE-induced day 7 increases in total leukocytes and LGL were absent in athymic nude (nu/nu) mice, suggesting that the increases were T cell-dependent. LCMV-ARM infection of C57BL/6 mice induced significant spleen CTL activity but little liver CTL activity, whereas LCMV-WE infection resulted in significant liver CTL activity but minimal spleen CTL activity. Mice infected with the cytopathic hepatotropic viruses, mouse hepatitis virus (MHV) and murine cytomegalovirus (MCMV), experienced much greater increases in liver NK/LGL by day 3 p.i. than did mice infected with LCMV or injected with the interferon-inducer poly(I-C). MHV-infected mice homozygous for the beige (bg/bg) mutation also exhibited significant increases in liver NK/LGL cell number and activity, although the activity was less than heterozygote controls, and the morphology of the LGL granules was aberrant. These data show that the LGL accumulate in virus-infected organs, in this case, the liver. An early NK/LGL influx is most pronounced during infection with cytopathic hepatotropic viruses. This initial influx of NK/LGL is followed later by an influx of CTL also possessing LGL morphology. The CTL/LGL response in the liver is significantly greater during hepatotropic virus infections, even when a strong CTL response in the spleen is lacking.     

Differentiation of memory T cells to virus plaque-forming cells and cytotoxic T lymphocytes

The aims of this study were to define the T-cell subpopulation(s) detected by the virus plaque assay, and particularly to determine whether the virus plaque assay could be used to enumerate cytotoxic T lymphocytes. In addition, studies were undertaken to ascertain whether cell proliferation was required for development of cytotoxic effector function and virus plaque formation by these subpopulations. The results of experiments with a secondary mouse mixed lymphocyte culture (MLC) model indicated that 70 percent of virus plaque-forming cells bore the Ly 1 phenotype and 30 percent the Ly 2,3 phenotype. Three lines of evidence suggested that cytotoxic T lymphocytes (CTL) can be detected by this assay: the fact that some virus plaque-forming cells (V-PFC) bear the same Ly phenotype as CTL; the use of an inhibitor of DNA synthesis indicated that proliferating cells could be eliminated with no effect on V-PFC production and cytotoxic activity of the Ly 2,3 cell population; and that infection of primed lymphocyteswith vesicular stomatitis virus before (MLC) stimulation eliminated cytotoxic activity. In primary MLC, development of V-PFC and CTL was completely abolished by cytosine arabinoside. In contrast, in secondary MLC, some CTL and V- PFC were generated by antigenic stimulation in the absence of proliferation. However, the development of both functions became progressively more susceptible to cytosine arabinoside as the time between primary immunization and in vitro boosting is increased. It is suggested that there may be a considerable disparity between the number of existing effector cells at any given time and the cytotoxic potential, i.e. the number of cells capable of being generated by antigenic stimulation.     

Tumor-specific cytotoxic T lymphocyte activity determines colorectal cancer patient prognosis

The composition of tumor-targeted T cell infiltrates is a major prognostic factor in colorectal cancer (CRC) outcome; however, the functional role of these populations in prolonging patient survival remains unclear. Here, we evaluated 190 patients with CRC for the presence of functionally active tumor-infiltrating lymphocytes (TILs), the tumor specificity of these TILs, and the correlation between patient TILs and long-term survival. Using intracytoplasmic cytokine staining in conjunction with HLA multimers loaded with tumor peptide and antigen-specific cytokine secretion assays, we determined that TNF-α expression delineates a population of tumor antigen–specific (TA-specific) cytotoxic T lymphocytes (CTLs) present within tumors from patients with CRC. Upregulation of TNF-α expression in TILs strongly correlated with an increase in the total amount of intratumoral TNF-α, which is indicative of tumor-specific CTL activity. Moreover, a retrospective multivariate analysis of 102 patients with CRC, which had multiple immune parameters evaluated, revealed that increased TNF-α concentration was an independent prognostic factor. Together, these results indicate that the prognostic impact of T cell infiltrates for CRC maybe largely based on subpopulations of active TA-specific T cells within the tumor, suggesting causal implication for these cells in patient survival. Additionally, these results support the use of intratumoral TNF-α, which is indicative of T cell function, as a prognostic parameter for CRC.     

Recognition of cloned vesicular stomatitis virus internal and external gene products by cytotoxic T lymphocytes

It has generally been assumed that most if not all CTL specific for vesicular stomatitis virus (VSV)-infected cells recognize the viral glycoprotein (G), an integral membrane protein abundantly expressed on infected cell surfaces. Using recombinant vaccinia viruses containing copies of cloned VSV genes to examine CTL recognition of VSV, we have confirmed that G is recognized by VSV-specific CTL. More interestingly, however, we have also found that nucleocapsid protein (N), an internal virion protein, can be detected on infected cell surfaces using mAb, and serves as a major target antigen for VSV-specific CTL. In contrast to the highly serotype-specific recognition of G, N is recognized by a major population of CTL able to lyse cells infected with either the Indiana or New Jersey VSV serotypes. Using target cells expressing a cloned MHC class I gene, we could directly show that CTL recognition of N occurs in the context of the MHC Ld molecule.     

Serine esterase and hemolytic activity in human cloned cytotoxic T lymphocytes

Target cell lysis by most murine cytotoxic T lymphocytes appears to be mediated by a complement (C9)-like protein called perforin, contained in high-density cytoplasmic granules. These granules also contain high levels of serine esterase activity, which may also play a role in cytolysis. Analysis of 17 cloned human cytotoxic T lymphocytes revealed the presence of serine esterase that is very similar to its murine counterpart in substrate and inhibitor specificities, pH optimum, and molecular mass; dot blot hybridization with synthetic oligonucleotides corresponding to the active sites of two known murine CTL esterases suggests homology to the murine enzyme HF. However, serine esterase was present at only approximately 10% of the level found in murine CTLs, and was not secreted during CTL-target cell interaction; moreover, hemolytic activity could not be detected in any of the seven cell lines tested. The results suggest that the human CTLs examined here kill their target cells by a mechanism different from that used by most cloned murine CTLs.     

Cooperation between cytotoxic and helper T lymphocytes in protection against lethal Sendai virus infection. Protection by T cells is MHC-restricted and MHC-regulated; a model for MHC-disease associations

The in vivo importance of class I MHC regulation of the Tc response to a natural pathogenic agent of high virulence was studied on the basis of our previous demonstration of a major difference in the capacity to generate a Sendai virus-specific Tc response between C57BL/6 (B6, H-2b) mice and H-2Kb mutant B6.C-H-2bm1 (bm 1) mice. These two mouse strains differ from each other only in three amino acids in the crucial H-2Kb restriction element for this response. bm 1 mice, in contrast to B6 mice, are Tc nonresponders against this virus, but show Sendai-specific T cell proliferation, antibody production, and DTH reactions, as well as NK cell activity, equal to those of B6 mice. B6, Sendai Tc-deficient bm 1 and T cell-deficient B6 nu/nu mice differ from each other in susceptibility to lethal pneumonia induced by i.n. inoculation of virulent Sendai virus. The lethal dose (LD50) in B6 mice averaged 152 TCID50, in bm 1 mice, 14 TCID50 and in B6 nu/nu mice 0.5 TCID50. The importance of Tc was also shown by the complete protection of B6 nu/nu mice against infection with a lethal virus dose by i.v. injection of a Sendai virus-specific, IL-2-dependent and H-2Kb-restricted B6 Tc clone. In vivo protection by this Tc clone was H-2Kb-restricted. Apart from Tc, an important role for virus-specific Th cells is evident from the difference in susceptibility between bm 1 and B6 nu/nu mice. This conclusion was supported by the demonstration that the mean survival time of B6 nu/nu and bm 1 nu/nu mice could be significantly prolonged, in an I-Ab-restricted manner, by the injection of in vitro-propagated, Sendai-specific B6 or bm 1 Th clones after a lethal dose of Sendai virus, and by the demonstration that inoculation of these Th clones provided help to virus-specific Tc by means of IL-2 production. Strikingly, Th and Tc cooperate in anti-Sendai virus immunity, since permanent survival of lethally infected nu/nu mice was only achieved by inoculation of a mixture of Tc and Th clones or a mixture of a Tc clone and rIL-2. This study provides a unique model for the study of MHC-disease associations.     

Quantitative analysis of the peripheral blood cytotoxic T lymphocyte response in patients with chronic hepatitis C virus infection.

Hepatitis C virus (HCV)-specific cytotoxic T lymphocytes (CTL) are present in the peripheral blood and liver of chronically infected patients. The current study was performed to study the relationship between the strength of the CTL response, liver disease severity, and viral load. The results may be summarized as follows: first, using CTL precursor frequency (CTLpf) analysis to quantitate the peripheral blood CTL response, chronically infected patients were less strongly sensitized to a panel of well-defined HCV epitopes than they were to an epitope within the influenza matrix protein. Second, HCV-specific CTLpf did not correlate with disease activity or viral load in the majority of patients on a cross-sectional basis, although it did increase in three patients concomitant with sharp increases in liver disease. Finally, interferon therapy did not enhance the CTLpf against the HCV epitopes studied in these patients, indicating that its antiviral effect is independent of the CTL response. Since the HCV-specific CTLpf in the blood is actually quite low, the CTL may contribute to ongoing liver disease in these patients while being quantitatively inadequate to destroy all of the infected hepatocytes, thereby facilitating HCV persistence and contributing to chronic liver disease.     

Modulation of susceptibility to HIV-1 infection by the cytotoxic T lymphocyte antigen 4 costimulatory molecule

CD4 T cells activated in vitro by anti-CD3/28-coated beads are resistant to infection by CC chemokine receptor 5 (CCR5)-dependent HIV-1 isolates. In vivo, antigen-presenting cells (APCs) activate CD4 T cells in part by signaling through the T cell receptor and CD28, yet cells stimulated in this manner are susceptible to HIV-1 infection. We show that cytotoxic T lymphocyte antigen 4 (CTLA-4) engagement counteracts the CD28 antiviral effects, and that the ratio of CTLA-4 to CD28 engagement determines the susceptibility of HIV-1 infection. Furthermore, unopposed CTLA-4 signaling provided by CD28 blockade promotes vigorous HIV-1 replication, despite minimal T cell proliferation. Finally, CTLA-4 antibodies decrease the susceptibility of antigen-activated CD4 T cells to HIV, suggesting a potential approach to prevent or limit viral spread in HIV-1-infected individuals.     

HLA B37 determines an influenza A virus nucleoprotein epitope recognized by cytotoxic T lymphocytes

Human influenza A virus-specific, cytotoxic T cells have been shown previously to recognize the virus nucleoprotein on infected cells. CTL preparations from four HLA B37-positive donors were shown to recognize a synthetic peptide that corresponded to amino acids 335-349 of the nucleoprotein sequence. Influenza-specific CTL from 10 donors of other HLA types failed to recognize this epitope. CD8+ CTL lines were derived from lymphocytes of two HLA B37-positive donors and used to show that the peptide was represented on virus-infected cells and to determine the probable boundaries of the epitope.     

Acquired constitutive expression of interferon beta after gene transduction enhances human immunodeficiency virus type 1-specific cytotoxic T lymphocyte activity by a RANTES-dependent mechanism.

CTL lines directed against HIV-1 antigens were generated from infected individuals and were transduced by the HMB-K(b)HuIFNbeta vector, resulting in low, constitutive expression of interferon beta (IFN-beta). The IFN-beta-transduced cells showed markedly increased HIV-1-specific, MHC class I-restricted CTL activity against HIV-1-LAI Gag, Pol, or Env antigens. This effect of IFN-beta was correlated with an overexpression of RANTES and completely abrogated by RANTES-blocking antibody. The present results provide the first evidence that IFN-beta transduction of CTL lines enhances HIV-specific cytotoxic activities through an upregulation of RANTES production. The efficient elimination of HIV-infected cells by IFN-beta-transduced CTL lines makes this gene therapy approach an attractive treatment for AIDS.     

Purification and characterization of a naturally processed hepatitis B virus peptide recognized by CD8+ cytotoxic T lymphocytes.

In vitro studies in patients with hepatitis B virus (HBV) infection have suggested that hepatocytolysis induced by CD8+ cytotoxic T lymphocytes (CTLs) is the most important effector pathway in eliminating infected cells. The recognition is implicated in the endogenously processed HBV antigens in the context of HLA class I molecules presented on the liver cell membrane. However, the naturally occurring HBV peptide antigens have not yet been demonstrated. We report here that a naturally processed peptide antigen P2 was isolated from HLA class I molecules of HBV-infected liver cell membrane. The P2 peptide exhibited the activity of sensitizing target cells for lysis by CD8+ CTLs. The P2 sequence (YVNVNMGLK) purified from liver tissue was in concordance with that encoded by the viral genome for the HBV nucleocapsid antigen or HBcAg 88-96. P2 peptide could also be isolated from the EBV-transformed B cells that were transfected by HBcAg-expressing vector. The P2 epitope, sharing the HLA-A11 binding motifs, was recognized by HLA-A11-restricted CD8+ CTLs. The data provided direct evidence that, in hepatitis B patients, antigenic peptides of HBV were processed by hepatocytes, presented with the class I MHC molecules, and recognized by CD8+ CTLs.     

Human infection with Trypanosoma cruzi induces parasite antigen-specific cytotoxic T lymphocyte responses.

Experimental models of Chagas' disease, an infection caused by the intracellular protozoan Trypanosoma cruzi, have demonstrated the crucial immunoprotective role played by CD8(+) T lymphocytes. These cells dominate inflammatory foci in parasitized tissues and their elimination from mice leads to uncontrolled parasite replication and subsequent death of the infected host. A trypomastigote surface antigen, TSA-1, and two amastigote surface molecules, ASP-1 and ASP-2, were recently identified as targets of CD8(+) cytotoxic T lymphocytes (CTL) in T. cruzi-infected mice. Until now, however, there was no evidence for the development of parasite-specific CTL in T. cruzi-infected humans. In this study, human CTL specific for TSA-1-, ASP-1-, and ASP-2-derived peptides were detected in the peripheral blood mononuclear cells from 21 of 24 HLA-A2(+) T. cruzi-infected patients. CTL recognition was antigen specific, A2-restricted, and CD8(+) T cell-dependent. Demonstration of human CTL against T. cruzi and against target molecules identified using the murine model provides important information for the optimal design and evaluation of vaccines to prevent or ameliorate Chagas' disease.     

Active immunization against virus infections due to antigenic drift by induction of crossreactive cytotoxic T lymphocytes

We have examined whether active immunization with c13 protein, a hybrid protein of the first 81 amino acids of the viral NS1 nonstructural protein and the HA2 subunit of A/PR/8 (H1N1) hemagglutinin, could protect BALB/c mice from challenge with A/PR/8 H1 subtype virus. Mice immunized with the c13 protein had a significant reduction of pulmonary virus titers with A/PR/8 (H1) virus, but failed to limit the replication of A/PC (H3) virus, which reflects the in vitro CTL activity of c13 immune spleen cells. We observed that the epitope recognized by HA2 specific CTL, which are induced by a derivative of c13 protein, is highly conserved among H1 and H2 subtype virus strains. This led us to test whether active immunization with c13 protein would also limit pulmonary virus replication in mice infected with the A/TW virus, a virus of the H1 subtype, which was isolated in 1986, and with a virus of the H2 subtype, A/Japan/305/57. Immunized mice had significantly lower lung virus titers than did control mice, and did not possess any neutralizing antibodies to the challenger viruses. These results indicate that active immunization with a fusion protein containing the cross-reactive CTL epitope protects mice from influenza infection by inducing CTL against influenza A H1 and H2 subtype virus strains, which markedly vary in their antibody binding sites on the HA1. The ability to induce active cross-reactive immunization with a fusion protein which contains a highly conserved CTL epitope offers a model for vaccine approaches against viruses which undergo significant variations in their antibody binding sites.     

In vivo administration of histoincompatible lymphocytes leads to rapid functional deletion of cytotoxic T lymphocyte precursors

It is well established that a single intravenous injection of F1 lymphocytes can rapidly and specifically reduce the ability of a parental recipient to generate CTL against donor alloantigens in a subsequent MLR. By fluorescently labeling the injected cells, we have been able to identify, and if desired, remove them in cell suspensions prepared from recipient spleen and lymph node. The injected cells, whether F1 or syngeneic, appeared to form part of the normal recirculating pool. Removal of injected F1 cells from responder lymph node or spleen cell suspensions had no effect on the response reduction observed in the 5-d in vitro MLR (typically 80% reduction for responder cells taken 2 d after injection of F1 cells). When the frequency of CTL precursors (CTLp) was measured by limiting dilution, it was reduced to the same degree as the MLR response, implying that response reduction is due to a reduction in the number of activatable CTL in the responder cell suspension. An equal mixture of responder cells from treated (i.e., F1 injected) and control mice gave a measured CTLp frequency equivalent to the average of the separate frequencies, implying the absence of suppressor cells active in vitro. Labeled F1 cells recovered from a first recipient could be used to induce response reduction in a second recipient. The results are discussed in terms of APCs that functionally delete rather than stimulate CTLp that recognize them (i.e., a "veto mechanism"). These experiments appear to rule out a role for in vivo-induced suppressor cells up to 8 d after injection of semiallogeneic cells but do not address the question of whether they are induced at later times.     

Efficient generation of a hepatitis B virus cytotoxic T lymphocyte epitope requires the structural features of immunoproteasomes

Interferon (IFN)-gamma-induced cells express the proteasome subunits low molecular weight protein (LMP)2, LMP7, and MECL-1 (multicatalytic endopeptidase complex-like 1), leading to the formation of immunoproteasomes. Although these subunits are thought to optimize MHC class I antigen processing, the extent of their role and the mechanistic aspects involved remain unclear. Herein, we study the proteolytic generation of an human histocompatibility leukocyte antigen (HLA)-Aw68-restricted hepatitis B virus core antigen (HBcAg) cytotoxic T lymphocyte (CTL) epitope that is recognized by peripheral blood lymphocytes from patients with acute self-limited but not chronic hepatitis B virus (HBV). Immunological data suggest that IFN-gamma-induced rather than uninduced HeLa cells process and present the HBV CTL epitope upon infection with HBcAg-expressing vaccinia viruses. Analyses of 20S proteasome digests of synthetic polypeptides covering the antigenic HBcAg peptide demonstrate that only immunoproteasomes efficiently perform the cleavages needed for the liberation of this HBV CTL epitope. Although the concerted presence of the three immunosubunits appears essential, we find that both catalytically active LMP7 and inactive LMP7 T1A support CTL epitope generation. We conclude that LMP7 influences the structural features of 20S proteasomes, thereby enhancing the activity of the LMP2 and MECL-1 catalytic sites, which provide cleavage specificity. Thus, LMP7 incorporation is of greater functional importance for the generation of an HBV CTL epitope than cleavage specificity.