T cell-T cell killing is induced by specific epitopes: evidence for an apoptotic mechanism

Dominant-negative FADD rescues the in vivo fitness of a cytomegalovirus lacking an antiapoptotic viral gene

Genes that inhibit apoptosis have been described for many DNA viruses. Herpesviruses often contain even more than one gene to control cell death. Apoptosis inhibition by viral genes is postulated to contribute to viral fitness, although a formal proof is pending. To address this question, we studied the mouse cytomegalovirus (MCMV) protein M36, which binds to caspase-8 and blocks death receptor-induced apoptosis. The growth of MCMV recombinants lacking M36 (DeltaM36) was attenuated in vitro and in vivo. In vitro, caspase inhibition by zVAD-fmk blocked apoptosis in DeltaM36-infected macrophages and rescued the growth of the mutant. In vivo, DeltaM36 infection foci in liver tissue contained significantly more apoptotic hepatocytes and Kupffer cells than did revertant virus foci, and apoptosis occurred during the early phase of virus replication prior to virion assembly. To further delineate the mode of M36 function, we replaced the M36 gene with a dominant-negative FADD (FADD(DN)) in an MCMV recombinant. FADD(DN) was expressed in cells infected with the recombinant and blocked the death-receptor pathway, replacing the antiapoptotic function of M36. Most importantly, FADD(DN) rescued DeltaM36 virus replication, both in vitro and in vivo. These findings have identified the biological role of M36 and define apoptosis inhibition as a key determinant of viral fitness.

Quantitative measurement of pathogen-specific human memory T cell repertoire diversity using a CDR3 beta-specific microarray

Background

Providing quantitative microarray data that is sensitive to very small differences in target sequence would be a useful tool in any number of venues where a sample can consist of a multiple related sequences present in various abundances. Examples of such applications would include measurement of pseudo species in viral infections and the measurement of species of antibodies or T cell receptors that constitute immune repertoires. Difficulties that must be overcome in such a method would be to account for cross-hybridization and for differences in hybridization efficiencies between the arrayed probes and their corresponding targets. We have used the memory T cell repertoire to an influenza-derived peptide as a test case for developing such a method.

Results

The arrayed probes were corresponded to a 17 nucleotide TCR-specific region that distinguished sequences differing by as little as a single nucleotide. Hybridization efficiency between highly related Cy5-labeled subject sequences was normalized by including an equimolar mixture of Cy3-labeled synthetic targets representing all 108 arrayed probes. The same synthetic targets were used to measure the degree of cross hybridization between probes. Reconstitution studies found the system sensitive to input ratios as low as 0.5% and accurate in measuring known input percentages (R² = 0.81, R = 0.90, p < 0.0001). A data handling protocol was developed to incorporate the differences in hybridization efficiency. To validate the array in T cell repertoire analysis, it was used to analyze human recall responses to influenza in three human subjects and compared to traditional cloning and sequencing. When evaluating the rank order of clonotype abundance determined by each method, the approaches were not found significantly different (Wilcoxon rank-sum test, p > 0.05).

Conclusion

This novel strategy appears to be robust and can be adapted to any situation where complex mixtures of highly similar sequences need to be quantitatively resolved.

How TCRs bind MHCs, peptides, and coreceptors

Since the first crystal structure determinations of alphabeta T cell receptors (TCRs) bound to class I MHC-peptide (pMHC) antigens in 1996, a sizable database of 24 class I and class II TCR/pMHC complexes has been accumulated that now defines a substantial degree of structural variability in TCR/pMHC recognition. Recent determination of free and bound gammadelta TCR structures has enabled comparisons of the modes of antigen recognition by alphabeta and gammadelta T cells and antibodies. Crystal structures of TCR accessory (CD4, CD8) and coreceptor molecules (CD3epsilondelta, CD3epsilongamma) have further advanced our structural understanding of most of the components that constitute the TCR signaling complex. Despite all these efforts, the structural basis for MHC restriction and signaling remains elusive as no structural features that define a common binding mode or signaling mechanism have yet been gleaned from the current set of TCR/pMHC complexes. Notwithstanding, the impressive array of self, foreign (microbial), and autoimmune TCR complexes have uncovered the diverse ways in which antigens can be specifically recognized by TCRs.

Antigen-specific targeting of CD8+ T cells with receptor-modified T lymphocytes

Chimeric receptors that link ligand recognition domains, such as antibody Fv fragments, with TCR signaling domains can redirect T lymphocytes against MHC-unrestricted targets. Such receptor-modified T lymphocytes have shown promise in the treatment of infectious diseases and cancer. We hypothesized that receptor-modified T lymphocytes may also be designed to target antigen-specific T cells. We synthesized chimeric receptors consisting of the extracellular and transmembrane domains of the class I MHC H-2K(b) molecule linked to the signaling domains of either TCR-zeta, CD28 and zeta, or CD28, zeta, and lck. T lymphocytes modified to express these receptors and pulsed with antigenic peptide specifically killed precursor CTL. Cytolysis was efficient, even at effector:target ratios of less than one, and specific, selectively killing antigen-specific precursor CTL among a mixed population of T cells. Cytolysis required activation of the receptor-modified T cells, and did not occur with a signaling-deficient chimeric receptor. In contrast to precursor CTL, differentiated CTL proved resistant to lysis by the receptor-modified T cells. These data demonstrate the feasibility of redirecting T lymphocytes against antigen-specific T cells. Receptor-modified T cells expressing chimeric MHC receptors have potential application in autoimmune and alloimmune diseases.

Vesicles bearing MHC class II molecules mediate transfer of antigen from antigen-presenting cells to CD4+ T cells

Previous studies have provided evidence that myelin basic protein (MBP)-specific rat T cells acquire antigen via transfer of preformed peptide/MHC class II complexes from splenic antigen-presenting cells (APC). The purpose of the present study was to determine how T cells acquire peptide/MHC class II complexes from APC in vitro. Our results show that a MHC class II+ T cell line, R1-trans, released MHC class II-bearing vesicles that directly stimulated MBP-specific CD4+ T cells. Vesicles expressing complexes of MHC class II and MBP were also specifically cytotoxic to MBP-specific T cells. Surviving T cells acquired MHC class II/antigen complexes from these vesicles by a mechanism that did not require protein synthesis but depended on specific TCR interactions with peptide/self MHC complexes. Furthermore, MBP/MHC class II-bearing vesicles enabled T cells to present MBP to other T cell responders. These studies provide evidence that APC release vesicles expressing preformed peptide/MHC class II complexes that interact with clonotypic TCR, allowing MHC class II acquisition by T cells. Vesicular transport of antigen/MHC class II complexes from professional APC to T cells may represent an important mechanism of communication among cells of the immune system.

Self-veto mechanism of CD8+ cytotoxic effector T cells. Peptide-induced paralysis affects the peptide-MHC-recognizing cytotoxic T lymphocytes and is independent of Fas/Fas ligand interactions

The lytic activity of CD8+ cytotoxic T lymphocyte (CTL) cell lines or clones can be inhibited by addition of the peptide recognized by these cells. The mechanisms underlying this phenomenon are not fully understood. Here we have analyzed peptide-induced CTL paralysis using in vivo generated ovalbumin (OVA)-specific CTL. Lytic activity of OVA-specific CTL was inhibited by addition of the immunodominant OVA-peptide SIINFEKL in a dose-dependent manner. Paralysis was induced rapidly and binding of the peptide to MHC class I molecules was required. Using mixing experiments with CTL populations of different peptide specificities restricted to the same MHC class I molecule we identified a veto-like mechanism: the cytotoxic activity of the peptide-recognizing CTL was inhibited while the lytic activity of the peptide-presenting CTL was unaltered. Only CD8+ CTL but not CD4+ T cells or B+ cells induced paralysis. After removal of the peptide-presenting CTL by magnetic cell sorting, paralysis was maintained and paralyzed CTL showed no signs of apoptosis. Loss of cytotoxicity could be induced in CTL populations from Fas-deficient (lpr+/lpr+) or Fas ligand-deficient (gld+/gld+) mice and mixtures thereof, implying that Fas/Fas ligand interactions are not involved during induction of paralysis. Hence, peptide-induced paralysis of CTL is due to a self-veto mechanism rather than to mutual killing of CTL. These findings may have implications for in vivo immunization with peptides, viral escape and peripheral tolerance mechanisms.

T:T Antigen Presentation by Activated Murine CD8+ T Cells Induces Anergy and Apoptosis

Using an IL-2-secreting, noncytolytic, H-Y-specific, CD8+ T cell clone, the functional consequences of Ag presentation by T cells to T cells were investigated. Incubation of the T cells with H-Y-soluble peptide led to nonresponsiveness to Ag rechallenge. This was due to the simultaneous induction of apoptosis, involving approximately 40% of the T cells, and of anergy in the surviving cells. These effects were strictly dependent upon bidirectional T:T presentation, in that exposure of C6 cells to peptide-pulsed T cells from the same clone induced proliferation but not apoptosis or anergy. The inhibitory effects of T:T presentation were not due to a lack of costimulation, since the T cells expressed levels of CD80 and CD86 higher than those detected on cultured dendritic cells and equipped them to function as efficient APCs for primary CD8+ T cell responses. Following incubation with soluble peptide, CD80 expression increased, and high levels of CTLA-4 (CD152) expression were induced. Although addition of anti-CTLA-4 Ab augmented proliferation in response to soluble peptide, no protection from apoptosis or anergy was observed. Neither Fas nor TNF-α was expressed/produced by the C6 cells, and coligation of MHC class I molecules and TCR failed to reproduce the effects of T:T presentation. Taken together, these data suggest that T:T Ag presentation induces anergy and apoptosis in murine CD8+ T cells and may reflect the regulatory consequences of T:T interactions in the course of clonal expansion in vivo.

Resistance to Polyoma Virus-Induced Tumors Correlates with CTL Recognition of an Immunodominant H-2Dk-Restricted Epitope in the Middle T Protein

The natural mouse pathogen polyoma virus is highly oncogenic in H-2k mice carrying the endogenous superantigen encoded by the mouse mammary tumor provirus Mtv-7. This superantigen results in deletion of Vβ6 TCR-expressing polyoma-specific CD8+ CTL, which appear to be critical effectors against polyoma tumorigenesis. Here we have isolated cloned lines of CD8+ T cells from resistant (i.e., Mtv-7−) H-2k mice that specifically lyse syngeneic polyoma virus-infected cells and polyoma tumor cells. Nearly all these CTL clones express Vβ6 and are restricted in their recognition of virus-infected cells by H-2Dk. Screening a panel of synthetic peptides predicted to bind to Dk, for which no consensus peptide binding motif is known, we identified a peptide corresponding to a nine-amino acid sequence in the carboxyl-terminus of the middle T (MT) protein (amino acids 389–397) that was recognized by all the Vβ6+CD8+ CTL clones. The inability of MT389–397-reactive CTL to recognize cells infected with a mutant polyoma virus encoding a MT truncated just proximal to this sequence indicates that MT389-397 is a naturally processed peptide. The frequencies of precursor CTL specific for polyoma virus and MT389–397 peptide were similar, indicating that MT389–397 is the immunodominant epitope in H-2k mice. In addition, polyoma-infected resistant mice possess a 10- to 20-fold higher MT389-397-specific precursor CTL frequency than susceptible mice. This highly focused CTL response to polyoma virus provides a valuable animal model to investigate the in vivo activity of CTL against virus-induced neoplasia.

Antigen presentation by T cells: T cell receptor ligation promotes antigen acquisition from professional antigen-presenting cells

The purpose of this study was to determine whether the clonotypic specificity of the T cell receptor influences the specificity of T cell-mediated antigen presentation. We have previously shown that myelin basic protein (MBP)-specific Lewis rat GP2.E5/R1 (R1) T cells cultured with antigen, irradiated syngeneic splenocytes (IrrSPL) and tolerogenic monoclonal antibody become highly effective antigen-presenting cells (APC). In the current studies, we investigated the transfer of specific (MBP) and unrelated (conalbumin) antigens from antigen-pulsed SPL to R1 T cells. R1 T cells cultured with IrrSPL that were pulsed simultaneously with both MBP and conalbumin acquired and presented both antigens to the appropriate T cell responders in a secondary assay. These results suggested a physical transfer of major histocompatibility complex (MHC)/peptide complexes from professional APC to R1 T cells. Transfer of conalbumin from professional APC to R1 T cells required specific recognition of MBP and was optimal when both conalbumin and MBP were presented on the same group of professional APC. Antigens transfer did not occur when allogeneic SPL were used as APC. The anti-I-A mAb OX6 inhibited antigen transfer but only when added during the initiation of culture. OX6 also inhibited antigen acquisition by R1-trans, a variant of the R1 T cell line which constitutively synthesizes high levels of I-A, from MBP-pulsed IrrSPL but blockade of I-A did not inhibit antigen acquisition when soluble MBP was added directly to the culture. Despite constitutive synthesis of I-A, R1-trans T cells did not acquire guinea pig MBP from pulsed allogeneic APC. These studies demonstrate that although T cells of a particular specificity can present unrelated antigens, the cognate interaction of the T cell antigen receptor with the appropriate antigen/self-MHC complex strongly promotes acquisition of these complexes from professional APC.

Hierarchy of Epstein-Barr virus-specific cytotoxic T-cell responses in individuals carrying different subtypes of an HLA allele: implications for epitope-based antiviral vaccines

Major histocompatibility complex class I-restricted Epstein-Barr virus (EBV)-specific cytotoxic T lymphocytes (CTLs) in healthy virus carriers constitute a primary effector arm of the immune system in controlling the proliferation of virus-infected B cells in vivo. These CTLs generally recognize target epitopes included within the latent antigens of the virus. For example, CTLs from HLA B44+ healthy virus carriers often recognize peptide EENLLDFVRF [corrected] from EBV nuclear antigen 6. However, the strength of this response directly correlates with the HLA B44 subtype expressed by the individual donor. Indeed, HLA B*4405+ virus carriers consistently show a very high frequency of CTL precursors for the EENLLDFVRF [corrected] epitope, while a much weaker response is seen in HLA B*4403+ and HLA B*4402+ individuals. This disparity is not due to an intrinsic difference in the CTLs generated by individuals carrying different subtypes of HLA B44. In fact, virus-specific CTLs recognize EENLLDFVRF [corrected] peptide-sensitized HLA B*4405+ target cells more efficiently than B*4402+ or B*4403+ target cells irrespective of the HLA B44 subtype expressed by the donors from whom these effectors were isolated. This effect is evident whether the CTL epitope is endogenously processed or exogenously presented. In addition, a comparison of the intracellular transport kinetics of different B44 subtypes revealed that the B*4405 allele is rapidly assembled and arrives in the trans-Golgi compartment at a faster rate than B*4402 or B*4403. Based on these results, we propose that HLA class I alleles that are capable of binding peptides more efficiently from the intracellular pool, and are rapidly assembled and transported, may confer a protective advantage against viral infection.

Apoptotic index in ovarian carcinoma: correlation with clinicopathologic factors and prognosis

The apoptotic index (apoptotic cells/1000 tumor cells, AI) was evaluated in 71 ovarian carcinomas, all surgically resected. Apoptosis was examined by modified terminal deoxynucleotidyltransferase-mediated deoxyuridine triphosphate-biotin nick end-labeling (TUNEL) method in histologic sections. High AI (>/=2.8) significantly correlated with high mitotic index (P = 0.05), high histologic grade of the tumor (P = 0. 018), and short overall survival (P = 0.017). An inverse relationship between AI and bcl-2 protein expression was also observed (P = 0.007). In addition, AI was assessed in 5 ovarian epithelial tumors of borderline malignancy, and all were categorized as low AI (<2.8). No significant correlation was found between AI and other clinicopathologic factors, such as age, clinical stage, lymph node metastasis, tumor size, histology of the tumor, and expression of p53 protein. Multivariate survival analysis showed that only clinical stage (P = 0.0395) and mitotic index (P = 0.0387) had independent prognostic value, whereas AI did not. Our results suggest that counting apoptosis can be useful for predicting the patient survival in ovarian carcinoma, although AI is not an independent prognostic factor. It is also suggested that bcl-2 protein is an important regulator of apoptosis in ovarian carcinoma.

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.

Cytotoxic T lymphocyte (CTL) adherence assay (CAA): a non-radioactive assay for murine CTL recognition of peptide-MHC class I complexes

Cytotoxic T lymphocytes (CTL) form an important immune surveillance system against intracellular pathogens. Here we describe a simple, visual assay for identifying peptides specifically recognized by CTL, based on the discovery that CTL develop increased adhesive properties upon TCR triggering. Several CTL lines were shown to pellet to the bottom of a round bottom 96-well plate in the absence of peptide. In contrast, these same CTL lines incubated with their cognate peptide, allowing them to present peptide to each other, adhered to the sides of the well and were readily distinguished by macroscopic visual examination of the plate after 4-5 h or overnight incubation. This CTL adherence assay (CAA) demonstrated peptide specificity and MHC restriction, and was titratable with peptide concentration. With this technique, a minimal-sized, malaria CTL epitope was correctly identified from a panel of overlapping nonamers, although the adherence pattern of two mono-substituted, variant peptides was less predictive of lytic activity. Also, substitutions in an HIV-1 envelope CTL epitope that reduced lytic activity were correctly predicted. Inhibitors of RNA and protein synthesis, upon preincubation, abrogated the adherence, indicating, at minimum, a need for live cells. Wortmannin, a PI-3 kinase inhibitor, inhibited the peptide specific adherence, consistent with a role for TCR or integrin signal transduction in CAA. Other cytoskeletal and metabolic inhibitors had no effect. Adherence of the T cells may involve low affinity, nonspecific interactions since wells coated with FCS, BSA or milk powder all produced an effective CAA in the presence of peptide under serum free conditions. Consequently, CAA may represent a rapid, simple method for screening large numbers of peptides to find cytolytic epitopes for a given CTL line and may identify additional epitopes causing T cell activation and adherence but not cytolytic activity.

Resistance of actin to cleavage during apoptosis

A small number of cellular proteins present in the nucleus, cytosol, and membrane fraction are specifically cleaved by the interleukin-1beta-converting enzyme (ICE)-like family of proteases during apoptosis. Previous results have demonstrated that one of these, the cytoskeletal protein actin, is degraded in rat PC12 pheochromocytoma cells upon serum withdrawal. Extracts from etoposide-treated U937 cells are also capable of cleaving actin. It was assumed that cleavage of actin represented a general phenomenon, and a mechanism coordinating proteolytic, endonucleolytic, and morphological aspects of apoptosis was proposed. We demonstrate here that actin is resistant to degradation in several different human cells induced to undergo apoptosis in response to a variety of stimuli, including Fas ligation, serum withdrawal, cytotoxic T-cell killing, and DNA damage. On the other hand, cell-free extracts from these cells and the ICE-like protease CPP32 were capable of cleaving actin in vitro. We conclude that while actin contains cleavage sites for ICE-like proteases, it is not degraded in vivo in human cells either because of lack of access of these proteases to actin or due to the presence of other factors that prevent degradation.

An Epstein-Barr virus-associated superantigen

More than 90% of adults are latently infected with Epstein-Barr virus (EBV), the causative agent of infectious mononucleosis, a self-limiting lymphoproliferative disease characterized by extensive T cell activation. Reactivation of this herpesvirus during immunosuppression is often associated with oncogenesis. These considerations led us to analyze the early events that occur after exposure of the immune system to EBV. Strong major histocompatibility complex (MHC) class II-dependent but not MHC-restricted, T cell proliferation was observed in vitro in response to autologous, lytically infected EBV-transformed B cells. By measuring the appearance of the early activation marker CD69 on individual T cell V beta subsets, we could demonstrate selective activation of human V beta 13- T cells. This was confirmed with murine T cell hybridomas expressing various human BV genes. While EBV- Burkitt's lymphoma cells were nonstimulatory, they induced V beta-restricted T cell activation after EBV infection. EBV specific activation was also demonstrated in cord blood cells, excluding a recall-antigen response. Thus, all of the characteristics of a superantigen-stimulated response are seen, indicating that induction of the EBV lytic cycle is associated with the expression of a superantigen in B cells. A model is presented proposing a role for the superantigen in infection, latency, and oncogenesis.

Interleukin-1 beta-converting enzyme-like protease cleaves DNA-dependent protein kinase in cytotoxic T cell killing

Cytotoxic T cells (CTL) represent the major defense mechanism against the spread of virus infection. It is believed that the pore-forming protein, perforin, facilitates the entry of a series of serine proteases (particularly granzyme B) into the target cell which ultimately leads to DNA fragmentation and apoptosis. We demonstrate here that during CTL-mediated cytolysis the catalytic subunit of DNA-dependent protein kinase (DNA-PKcs), an enzyme implicated in the repair of double strand breaks in DNA, is specifically cleaved by an interleukin (IL)-1 beta-converting enzyme (ICE)-like protease. A serine protease inhibitor, 3,4-dichloroisocoumarin (DCl), which is known to block granzyme B activity, inhibited CTL-induced apoptosis and prevented the degradation of DNA-PKcs in cells but failed to prevent the degradation of purified DNA-PKcs by CTL extracts. However, Tyr-Val-Ala-Asp-CH2Cl (YVAD-CMK) and other cysteine protease inhibitors prevented the degradation of purified DNA-PKcs by CTL extracts. Furthermore, incubation of DNA-PKcs with granzyme B did not produce the same cleavage pattern observed in cells undergoing apoptosis and when this substrate was incubated with either CTL extracts or the ICE-like protease, CPP32. Sequence analysis revealed that the cleavage site in DNA-PKcs during CTL killing was the same as that when this substrate was exposed to CPP32. This study demonstrates for the first time that the cleavage of DNA-PKcs in this intact cell system is exclusively due to an ICE-like protease.

Flow cytometric detection of activation-induced cell death (apoptosis) in peripheral blood lymphocyte subpopulations from healthy cats

Human peripheral blood lymphocytes (PBLs) from healthy individuals are resistant to in vitro-induced apoptosis, but activated human lymphocytes can readily undergo apoptosis. The activation of human lymphocytes is accompanied by the upregulation of a cell surface antigen, the major histocompatibility complex (MHC) class II-antigen. Only a minority of PBLs are usually MHC class II-antigen-positive in healthy humans. In contrast, in healthy cats the majority of feline PBLs are MHC class II-antigen-positive. We have, therefore, investigated the sensitivity of feline peripheral blood lymphocytes obtained from specified pathogen free (SPF) cats to the induction of apoptosis. Feline PBLs from SPF cats (n = 16) and human PBLs from healthy donors (n = 2) were isolated. After short-term culture, cells were examined for the presence of fragmented DNA as a result of apoptosis by a DNA agarose gel electrophoresis method and for the presence of DNA double strand breaks by in situ 3' end labeling. In addition, relative DNA content per cell was flow cytometrically determined using propidium iodide (PI) or 7-actinomycin-D (7-AAD) and apoptotic cells were identified on the basis of a reduced DNA content. Cell surface antigens and cellular DNA were analyzed simultaneously by dual-color flow cytometric analyses in order to study lymphocyte subsets. Single- and dual-color analysis revealed that, in contrast to human lymphocytes, feline lymphocytes rapidly underwent apoptosis when cultured overnight in medium. Furthermore, the majority of apoptotic cells was found within the MHC II-positive cell subject.

Inactivation of human immunodeficiency virus (HIV)-1 envelope-specific CD8+ cytotoxic T lymphocytes by free antigenic peptide: a self-veto mechanism?

Free peptide has been found to inhibit cytotoxic T lymphocyte (CTL) activity, and veto cells bearing peptide-major histocompatibility complex (MHC) complexes have been found to inactivate CTL, but the two phenomena have not been connected. Here we show that a common mechanism may apply to both. CD8+ CTL lines or clones specific for a determinant of the human immunodeficiency virus (HIV) 1 IIIB envelope protein gp160, P18IIIB, are inhibited by as little as 10 min exposure to the minimal 10-mer peptide, I-10, within P18IIIB, free in solution, in contrast to peptide already bound to antigen-presenting cells (APC), which does not inhibit. Several lines of evidence suggest that the peptide must be processed and presented by H-2Dd on the CTL itself to the specific T cell receptor (TCR) to be inhibitory. The inhibition was not killing, in that CTL did not kill 51Cr-labeled sister CTL in the presence of free peptide, and in mixing experiments with CTL lines of different specificities restricted by the same MHC molecule, Dd, the presence of free peptide recognized by one CTL line did not inhibit the activity of the other CTL line that could present the peptide. Also, partial recovery of activity could be elicited by restimulation with cell-bound peptide, supporting the conclusion that neither fratricide nor suicide (apoptosis) was involved. The classic veto phenomenon was ruled out by failure of peptide-bearing CTL to inactivate others. Using pairs of CTL lines of differing specificity but similar MHC restriction, each pulsed with the peptide for which the other is specific, we showed that the minimal requirement is simultaneous engagement of the TCR and class I MHC molecules of the same cell. This could occur in single cells or pairs of cells presenting peptide to each other. Thus, mechanistically, the inhibition is analogous to veto, and might be called self-veto. As a clue to a possible mechanism, we found that free I-10 peptide induced apparent downregulation of expression of specific TCR as well as interleukin 2 receptor, CD69, lymphocyte function-associated antigen 1, and CD8. This self-veto effect also has implications for in vivo immunization and mechanisms of viral escape from CTL immunity.

Immune regulation in Epstein-Barr virus-associated diseases

Epstein-Barr virus (EBV) is a member of the human herpesvirus family and, like many other herpesviruses, maintains a lifelong latent association with B lymphocytes and a permissive association with stratified epithelium in the oropharynx. Clinical manifestations of primary EBV infection range from acute infectious mononucleosis to an asymptomatic persistent infection. EBV is also associated with a number of malignancies in humans. This review discusses features of the biology of the virus, both in cell culture systems and in the natural host, before turning to the role of the immune system in controlling EBV infection in healthy individuals and in individuals with EBV-associated diseases. Cytotoxic T cells that recognize virally determined epitopes on infected cells make up the major effector arm and control the persistent infection. In contrast, the options for immune control of EBV-associated malignancies are more restricted. Not only is antigen expression restricted to a single nuclear antigen, EBNA1, but also these tumor cells are unable to process EBV latent antigens, presumably because of a transcriptional defect in antigen-processing genes (such as TAP1 and TAP2). The likelihood of producing a vaccine capable of controlling the acute viral infection and EBV-associated malignancies is also discussed.

Perforin and lymphocyte-mediated cytolysis

We have discussed in the previous sections the recent progress made toward elucidating the regulatory mechanism of perforin gene transcription and the domain structure of the perforin molecule. It appears that the expression of perforin is, at least partially, controlled at the transcription level through the interaction between killer cell-specific cis- and trans- acting factors. One of such cognate pairs, NF-P motif (an EBS-homologous motif) and NF-P2 (a killer cell-specific DNA-binding protein), has been described. The regulatory mechanism of gene transcription, however, is likely to involve multiple factors which act in a coordinated fashion to bring about the most efficient expression of perforin limited strictly to activated killer lymphocytes. Through studies using synthetic peptides and recombinant perforins, it has been suggested that the N-terminal region of the perforin molecule is an important, though not the only, domain responsible for the lytic activity. Further studies are warranted to elucidate the role(s) of other potential amphiphilic structures located in the central portion of the perforin molecule in the overall pore-forming activity. The molecular basis underlying the resistance of killer lymphocytes to perforin-mediated lysis still remains an open question. Preliminary results, however, suggest that the surface protein(s) restricted to killer cells may account for their self-protection against perforin. Based on recent studies using perforin-deficient mice, the involvement of perforin in lymphocyte-mediated cytolysis both in vivo and in vitro has been confirmed. Two functional roles, a direct (lytic) and an indirect (endocytosis enhancer; conduit), both of which may contribute critically to the cell-killing event can be attributed to perforin. The fact that lymphocytes may also employ perforin-independent killing mechanism(s), e.g. Fas-dependent pathway, is beyond the scope of this review. There is, nevertheless, no doubt that these alternative cytolytic mechanisms may also play important roles in immune effector and/or regulatory responses associated with killer lymphocytes. Obviously, we are still a long way from concluding on the functional relevance of each individual cytolytic mechanism seen in different physiopathological situations. Suffice it to say, however, that a wealth of information on lymphocyte-mediated killing has already emerged through the multidisciplinary efforts conducted in our and other laboratories that promise to further dissect this complicated event in the years to come.

Viruses, virulence and pathogenicity

Pathogenicity is a complex process with stringent requirements of both the host cell and the infecting virion. Among these requirements are a port of entry into host cells, a means of replication for the virus, and a means by which infection damages host cells. Damage to the host can result from multiple mechanisms including transformation, suppression of cellular metabolism, apoptosis, autoimmune responses directed against infected or uninfected tissues, or by molecular mimicry. In the attempt to identify new associations between viral infection and disease, investigators should be mindful that variable host factors as well as viral infection may be required for pathogenesis. Efforts to associate specific viral infections with specific diseases may be obscured by final common pathways through which multiple agents damage host cells in similar ways.

The inhibitory effect of FK506 on cytotoxic T-lymphocyte killing

The immunosuppressant FK506 inhibits N-alpha-benzyloxylcarbonyl-L-lysine thiobenzyl ester (BLT) esterase release from cytotoxic T lymphocytes (CTL). In addition, serine esterase has been demonstrated to be strongly associated with CTL killing. In the present study, the effect of FK506 on the activity of CTL killing against target cells was examined. FK506 inhibited lysis of antigen (Ag)-treated CTL target cells by auto-CTL, but failed to inhibit lysis of conventional P815 target cells by CTL. Moreover, FK506 inhibited DNA fragmentation of CTL target cells lysed by auto-CTL. Killing of CTL target cells by FK506-pretreated auto-CTL was inhibited, but FK506-pretreated CTL target cells were killed by auto-CTL. Incubation of both FK506 and Ag-pretreated CTL target cells with untreated auto-CTL induced DNA fragmentation. These indicated that FK506 inhibited CTL killing by influencing effector, but not CTL target cells. These results suggest that FK506 may function by inhibiting alloreactive CTL killing in organ transplantation in addition to the interruption of the T-cell receptor signal transduction leading to cytokine production.

HLA A2.1-restricted cytotoxic T cells recognizing a range of Epstein-Barr virus isolates through a defined epitope in latent membrane protein LMP2

Cytotoxic T-lymphocyte (CTL) responses induced by persistent Epstein-Barr virus (EBV) infection in normal B-lymphoid tissues could potentially be directed against EBV-positive malignancies if expression of the relevant viral target proteins is maintained in tumor cells. For malignancies such as nasopharyngeal carcinoma and Hodgkin's disease, this will require CTL targeting against the nuclear antigen EBNA1 or the latent membrane proteins LMP1 and LMP2. Here we analyze in detail a B95.8 EBV-reactivated CTL response which is specific for LMP2 and restricted through a common HLA allele, A2.1. We found that in vitro-reactivated CTL preparations from several A2.1-positive virus-immune donors contained detectable reactivity against A2.1-bearing target cells expressing either LMP2A or the smaller LMP2B protein from recombinant vaccinia virus vectors. Peptide sensitization experiments then mapped the A2.1-restricted response to a single epitope, the nonamer CLGGLLTMV (LMP2A residues 426 to 434), whose sequence accords well with the proposed peptide binding motif for A2.1. Most Caucasian and African virus isolates (whether of type 1 or type 2) were identical in sequence to B95.8 across this LMP2 epitope region, although 2 of 12 such isolates encoded a Leu-->Ile change at epitope position 6. In contrast, most Southeast Asian and New Guinean isolates (whether of type 1 or type 2) constituted a different virus group with a Cys-->Ser mutation at epitope position 1. CTLs raised against the B95.8-encoded epitope were nevertheless able to recognize these variant epitope sequences in the context of A2.1 whether they were provided exogenously as synthetic peptides or generated endogenously in B cells transformed with the variant viruses. A CTL response of this kind could have therapeutic potential in that it is directed against a protein expressed in many EBV-positive malignancies, is reactive across a range of virus isolates, and is restricted through a relatively common HLA allele.

Human immunodeficiency virus type 1-specific cytotoxic T lymphocytes release gamma interferon, tumor necrosis factor alpha (TNF-alpha), and TNF-beta when they encounter their target antigens

Human immunodeficiency virus type 1 (HIV-1) infection is associated with elevated levels of inflammatory cytokines in the serum and cerebrospinal fluid of infected persons, but the sources of these proteins as well as the specific stimuli which trigger their production and release have not been fully defined. In this study, we evaluated the ability of HIV-1-specific cytotoxic T-lymphocyte (CTL) clones derived from seropositive persons to release gamma interferon (IFN-gamma), tumor necrosis factor alpha (TNF-alpha), and TNF-beta upon contact with target cells presenting viral antigen. Peripheral blood- and cerebrospinal fluid-derived HIV-1-specific CD3+ CD4- CD8+ CTL clones as well as freshly isolated peripheral blood mononuclear cells from infected persons were tested in parallel for HIV-1-specific cytotoxicity and cytokine release. Target cells consisted of autologous and allogeneic B-lymphoblastoid cell lines sensitized with synthetic HIV-1 peptides containing the epitopes recognized by these CTL. Cytokine production was measured by specific enzyme-linked immunosorbent assay of culture supernatant fluid. HIV-1-specific CTL clones directed at envelope, Gag, reverse transcriptase, and Nef epitopes specifically released IFN-gamma, TNF-alpha, and TNF-beta upon contact with their relevant target epitopes but not following contact with irrelevant epitopes. These cytokines were released in an HLA class I-restricted fashion, and release was detectable as early as 4 to 6 h of incubation and remained elevated at 48 h. Fresh peripheral blood mononuclear cells from a seropositive person likewise released IFN-gamma in an antigen-specific and HLA class I-restricted manner when incubated with target cells presenting a peptide containing a CTL epitope, paralleling the HIV-specific cytolytic activity of these cells. These studies indicate that in addition to mediating direct cytotoxicity, HIV-1-specific CTL may affect other immune responses by releasing IFN-gamma, TNF-alpha, and TNF-beta. Elevated levels of these cytokines which have been detected in serum and cerebrospinal fluid of infected persons may be due at least in part to the persistent HIV-1-specific CTL response.

Expression and function of HLA-B27 in lipid-linked form: implications for cytotoxic T lymphocyte-induced apoptosis signal transduction

Major histocompatibility complex (MHC) class I antigen-restricted cytotoxic T lymphocytes (CTL) kill their target cells not only by inducing irreversible membrane damage but also by triggering a programmed suicide cascade (apoptosis) in target cells. Recent evidence suggests that MHC class I antigens are involved in apoptosis signal transduction in T cells. Therefore, it is possible that MHC class I antigens are also responsible for CTL-induced signal transduction in target cells leading to apoptosis. To test this hypothesis, we have expressed HLA-B27 in Chinese hamster ovary (CHO) cells in a phosphatidyl inositol (PI) anchored form. The expressed Pl-anchored HLA-B27 (PI-B27), a 42-kDa molecule which can be cleaved off the cell surface by PI-specific phospholipase C, can function as an MHC restriction and antigen presentation element for specific CTL. Furthermore, PI-B27 transfectant CHO cells undergo rapid DNA fragmentation when pulsed with the appropriate peptide and treated with specific CTL, suggesting that the cytoplasmic and transmembrane domains of the heavy chain of class I MHC molecules are not required in CTL-induced apoptosis signal transduction in target cells.

Correlations between apoptotic and proliferative indices in malignant non-Hodgkin's lymphomas

Cell production versus cell loss rates were estimated, across the boundaries of histological classification, in 50 cases of malignant non-Hodgkin's lymphomas by use of mitotic indices, percentage of Ki-67+ cells and percentage of PC10+ cells as proliferative indices, and the relative number of apoptotic bodies (apoptotic indices, AIs) as parameters. Regression analysis revealed significant (P < 0.01) positive correlations between the AIs and the proliferative indices; among the immunohistochemically assessed proliferative indices; and between these, the mitotic indices and the AIs on the one hand and histological malignancy grades on the other hand. The cellular protein BCL-2, which counteracts apoptosis, was significantly (P < 0.01) more often expressed in lymphomas with low than in those with high AIs. Multivariate analysis of data showed that of all parameters tested in this series, only the AIs correlated significantly (P < 0.05) with overall lethality. The correlation between BCL-2 positivity of lymphoma cells and overall survival did not quite attain significance (P = 0.08). Results of the present study suggest that high AIs and lack of BCL-2 expression may be adverse prognostic factors, independent of histological grade.

Antiviral effect of TNF-alpha and IFN-gamma secreted from a CD8+ influenza virus-specific CTL clone

We observed that an influenza-specific cytotoxic T lymphocyte (CTL) clone (B7B7) stimulated with peptide-antigen could produce TNF-alpha and IFN-gamma simultaneously. The culture supernatant containing both TNF-alpha and IFN-gamma of antigen-stimulated CTL clone B7B7 significantly enhanced the lysis of influenza A/PR/8 virus-infected L-M2d6 cells or Meth A cells. Enhanced lysis of influenza virus-infected cells by the supernatants was inhibited by pretreatment of the supernatant with antimurine TNF-alpha antibody and antimurine IFN-gamma antibody. In addition to a single CTL clone, we observed that bulk-cultured CTLs were able to produce TNF and IFN when incubated with target cells. These results suggest that the protective mechanism mediated by TNF-alpha and IFN-gamma secreted from CTL may be possible in the course of an influenza infection.

Inhibition of apoptosis in human tumour cells by okadaic acid

Gamma-radiation, tetrandrine, bistratene A, and cisplatin were all found to induce pronounced morphological changes characteristic of apoptosis and extensive DNA fragmentation in the human BM13674 cell line 8 h after treatment. Apoptosis induced in BM13674 cells by these diverse agents was markedly inhibited by 1 microM okadaic acid, a tumour promoter that inhibits protein phosphatases 1 and 2A. This compound also inhibited the appearance of apoptosis in fresh human leukaemia cells that had been exposed to gamma-radiation. The inhibition of apoptosis was confirmed using fluorescence microscopy and DNA gel electrophoresis. Dephosphorylation of a limited number of proteins was shown to be associated with apoptosis and okadaic acid prevented these dephosphorylations. Previous studies on the BM13674 cell line showed that an inhibitor of protein synthesis failed to prevent apoptosis in these cells. The present data provides further support that posttranslational modification of proteins, in particular, phosphorylation/dephosphorylation status, plays an important role in inhibition/activation of programmed cell death in different human cells after exposure to several cytotoxic agents.

Mechanisms of self tolerance

The redundancy of biological systems minimizes the probability that isolated molecular and cellular defects entail deleterious consequences. This notion also applies to the establishment and maintenance of tolerance to self antigens. Thus, immune homeostasis is attributed to multiple distinct safety valves that are connected in series and intervene at defined control points of the life cycle of the developing lymphocyte to guarantee the physical elimination, functional inactivation, or regulated inhibition of self-reactive, potentially autoaggressive, B and T cells.

Rapid visual assay of cytotoxic T-cell specificity utilizing synthetic peptide induced T-cell-T-cell killing

Synthetic peptides are widely used to define the specificity of CD8+ cytotoxic T-lymphocyte (CTL) clones. When many peptides need to be tested by the standard chromium release assay large numbers of a CTL clone are required. Specific synthetic peptide epitopes induce CTL clones to kill each other. This phenomenon can be directly visualized using an inverted microscope and forms the basis for a convenient assay, which can be performed with as few as 100 CTL per peptide and does not require radiolabelled targets.

CD4+ T cell-mediated killing of major histocompatibility complex class II-positive antigen-presenting cells (APC) III. CD4+ cytotoxic T cells induce apoptosis of APC

A subset of CD4+ T cells, belonging to the T helper type 1 (Th1) cells, kills antigen-presenting cells (APC) in an antigen-specific and major histocompatibility (MHC) class II-restricted way. Evidence is presented that CD4+ cytotoxic T lymphocytes (CTL) induce apoptosis or programmed cell death within susceptible APC as witnessed by quantitative DNA fragmentation. Apoptosis is more reliable to determine cell death than the 51Cr-release assay, because some cells demonstrate resistance to CD4-mediated lysis in the 51Cr-release assay. Apoptosis becomes manifest after 2 to 4 h of incubation preceding the disintegration of the target cells which is detectable between 12 and 24 h as measured by the 51Cr-release assay. Unstimulated B cells, which are not killed, but function as APC, do not undergo apoptosis, whereas lipopolysaccharide or anti-mu-activated B cell blasts show apoptosis and are efficiently lysed. Several CD4+ Th2-type cells tested, which did not demonstrate killing of APC as measured by the 51Cr-release assay, are unable to mediate programmed cell death of appropriate APC. Actinomycin D or cycloheximide, inhibitors of transcription and translation, respectively, fail to prevent apoptosis of APC excluding the involvement of newly synthesized soluble products as mediators of killing. Pretreatment of CD4+ CTL, but not of APC with 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid, a specific inhibitor of the anion transport, efficiently prevents apoptosis of APC, although the secretion of interleukins is not affected. We propose, that upon contact of the CD4+ CTL with APC, molecules of yet undefined nature are activated and released in a polar fashion at the contact site and induce the endogenous pathway of programmed cell death.

BLT esterase activity as an alternative to chromium release in cytotoxic T cell assays

Granules released by cytotoxic T cells (CTL), during recognition and killing of target cells, contain granule enzyme A. This serine protease has an esterase activity, which is easily measured using the substrate benzyloxycarbonyl-L-lysine thiobenzyl ester (BLT). BLT activity, routinely used as an assay for granule release, provides an alternative to the standard chromium release assay as a measure of CTL-mediated killing. The two methods were highly comparable when either exogenous synthetic peptide or endogenously produced epitopes were used as targets and human CTL clones acted as effectors. The advantages of the BLT assay are that it uses inexpensive non-radioactive reagents, the assay can be run over any period between 4 and 30 h and can be performed with as few as 10(4) CTLs if synthetic peptide epitopes are used.

Lipopolysaccharide induces double-stranded DNA fragmentation in mouse thymus: protective effect of zinc pretreatment

Intraperitoneal injection of female NAW/W1 mice with 5 mg of Salmonella typhimurium lipopolysaccharide/kg results in decreased body and thymus weight. Reduced thymic weight is accompanied by fragmentation of DNA into multimers of about 200 bp size. This effect is consistent with the induction of intranucleosomal cleavage of double-stranded DNA in thymus. Maximal fragmentation of DNA occurs between 18 and 24 h after treatment; by 48 h post lipopolysaccharide treatment, there is little evidence of thymic DNA fragmentation. Pretreatment of mice with Zn protects against lipopolysaccharide-induced DNA fragmentation. This effect is maximal at about 72 h after Zn treatment (24 h after lipopolysaccharide treatment) and persists until about 96 h after Zn treatment. At 72 h after pretreatment, the antagonism of thymic DNA fragmentation by Zn is dose-dependent. To examine the role of the acute phase inflammatory response elicited by lipopolysaccharide treatment in the production of changes in thymic weight and DNA integrity, the effects of treatment with casein, a well-characterized inducer of the acute phase inflammatory response in mice, were examined. In contrast to the effect of lipopolysaccharide, casein treatment did not produce a similar pattern of DNA fragmentation in thymus. Taken together, these data suggest that lipopolysaccharide induces DNA fragmentation in thymus by a mechanism which does not occur during the pathophysiological changes which accompany the casein-induced acute phase response. Further, the antagonism by Zn of lipopolysaccharide-induced fragmentation of thymic DNA is consistent with earlier findings that Zn can prevent dexamethasone-induced DNA fragmentation in vitro.