Selective activation of Fas/Fas ligand-mediated cytotoxicity by a self peptide

Semiallogenic fusions of MSI(+) tumor cells and activated B cells induce MSI-specific T cell responses

BACKGROUND

Various strategies have been developed to transfer tumor-specific antigens into antigen presenting cells in order to induce cytotoxic T cell responses against tumor cells. One approach uses cellular vaccines based on fusions of autologous antigen presenting cells and allogeneic tumor cells. The fusion cells combine antigenicity of the tumor cell with optimal immunostimulatory capacity of the antigen presenting cells.Microsatellite instability caused by mutational inactivation of DNA mismatch repair genes results in translational frameshifts when affecting coding regions. It has been shown by us and others that these mutant proteins lead to the presentation of immunogenic frameshift peptides that are - in principle - recognized by a multiplicity of effector T cells.

METHODS

We chose microsatellite instability-induced frameshift antigens as ideal to test for induction of tumor specific T cell responses by semiallogenic fusions of microsatellite instable carcinoma cells with CD40-activated B cells. Two fusion clones of HCT116 with activated B cells were selected for stimulation of T cells autologous to the B cell fusion partner. Outgrowing T cells were phenotyped and tested in functional assays.

RESULTS

The fusion clones expressed frameshift antigens as well as high amounts of MHC and costimulatory molecules. Autologous T cells stimulated with these fusions were predominantly CD4(+), activated, and reacted specifically against the fusion clones and also against the tumor cell fusion partner. Interestingly, a response toward 6 frameshift-derived peptides (of 14 tested) could be observed.

CONCLUSION

Cellular fusions of MSI(+) carcinoma cells and activated B cells combine the antigen-presenting capacity of the B cell with the antigenic repertoire of the carcinoma cell. They present frameshift-derived peptides and can induce specific and fully functional T cells recognizing not only fusion cells but also the carcinoma cells. These hybrid cells may have great potential for cellular immunotherapy and this approach should be further analyzed in preclinical as well as clinical trials. Moreover, this is the first report on the induction of frameshift-specific T cell responses without the use of synthetic peptides.

Antigen presented by tumors in vivo determines the nature of CD8+ T-cell cytotoxicity

The biological relevance of the perforin and Fas ligand (FasL) cytolytic pathways of CD8(+) T lymphocytes (CTL) for cancer immunotherapy is controversial. We investigated the importance of these pathways in a murine renal cell carcinoma expressing influenza viral hemagglutinin as a defined surrogate antigen (Renca-HA). Following Renca-HA injection, all FasL-dysfunctional FasL(gld/gld) mice (n = 54) died from Renca-HA tumors by day 62. By contrast, perforin(-/-) (51%; n = 45) and Fas(lpr/lpr) (55%; n = 51) mice remained tumor-free at day 360. Blocking FasL in vivo inhibited tumor rejection in these mice. Moreover, established Renca-HA tumors were cleared more efficiently by adoptively transferred HA(518-526)-specific T-cell receptor-transgenic CTL using FasL rather than perforin. Strikingly, a range of mouse tumor cells presenting low concentrations of immunogenic peptide were all preferentially lysed by the FasL but not the Pfp-mediated effector pathway of CTL, whereas at higher peptide concentrations, the preference in effector pathway usage by CTL was lost. Interestingly, a number of human renal cancer lines were also susceptible to FasL-mediated cytotoxicity. Therefore, the FasL cytolytic pathway may be particularly important for eradicating Fas-sensitive tumors presenting low levels of MHC class I-associated antigens following adoptive T-cell therapy.

Distinct requirements for activation-induced cell surface expression of preformed Fas/CD95 ligand and cytolytic granule markers in T cells

Fas (CD95/Apo-1) ligand is a potent inducer of apoptosis and one of the major killing effector mechanisms of cytotoxic T cells. Thus, Fas ligand activity has to be tightly regulated, involving various transcriptional and post-transcriptional processes. For example, preformed Fas ligand is stored in secretory lysosomes of activated T cells, and rapidly released by degranulation upon reactivation. In this study, we analyzed the minimal requirements for activation-induced degranulation of Fas ligand. T cell receptor activation can be mimicked by calcium ionophore and phorbol ester. Unexpectedly, we found that stimulation with phorbol ester alone is sufficient to trigger Fas ligand release, whereas calcium ionophore is neither sufficient nor necessary. The relevance of this process was confirmed in primary CD4(+) and CD8(+) T cells and NK cells. Although the activation of protein kinase(s) was absolutely required for Fas ligand degranulation, protein kinase C or A were not involved. Previous reports have shown that preformed Fas ligand co-localizes with other markers of cytolytic granules. We found, however, that the activation-induced degranulation of Fas ligand has distinct requirements and involves different mechanisms than those of the granule markers CD63 and CD107a/Lamp-1. We conclude that activation-induced degranulation of Fas ligand in cytotoxic lymphocytes is differently regulated than other classical cytotoxic granule proteins.

Tumor necrosis factor alpha is not a pathogenic determinant in acute lethal encephalitis induced by a highly neurovirulent strain of mouse hepatitis virus

To investigate the role of tumor necrosis factor alpha (TNFalpha) in the pathogenesis of acute viral encephalitis, TNFalpha-deficient mice were infected with a highly neurovirulent strain of mouse hepatitis virus, JHM, and compared with JHM-infected C57BL/6 mice as controls. All the JHM-infected mice had succumbed to infection by 6 days postinfection. The virus replication kinetics, histopathological changes and mRNA expression levels of proinflammatory cytokines in the brain did not differ between TNFalpha-deficient and control C57BL/6 mice. These results suggest that TNFalpha is not a pathogenic determinant in JHM-induced acute lethal encephalitis.

NK cells use NKG2D to recognize a mouse renal cancer (Renca), yet require intercellular adhesion molecule-1 expression on the tumor cells for optimal perforin-dependent effector function

The NKG2D receptor on NK cells can recognize a variety of ligands on the tumor cell surface. Using a mouse renal cancer (Renca), we show that NKG2D recognition by NK cells was crucial for their ability to limit tumor metastases in vivo in both liver and lungs using perforin-dependent effector mechanisms. However, for the R331 cell line established from Renca, NKG2D recognition and perforin-dependent lysis played no role in controlling liver metastases. R331 cells were also more resistant to perforin-dependent lysis by NK cells in vitro. We therefore used these phenotypic differences between Renca and R331 to further investigate the crucial receptor:ligand interactions required for triggering lytic effector functions of NK cells. Reconstitution of R331 cells with ICAM-1, but not Rae-1gamma, restored NKG2D-mediated, perforin-dependent lysis. Interestingly, R331 cells were efficiently lysed by NK cells using death ligand-mediated apoptosis. This death ligand-mediated killing did not depend on NKG2D recognition of its ligands on tumor cells. This result suggests that the intracellular signaling in NK cells required for perforin and death ligand-mediated lysis of tumor target cell are quite distinct, and activation of both of these antitumor lytic effector functions of NK cells could improve therapeutic benefits for certain tumors.

Fas ligand-mediated apoptosis in degenerative disorders of the brain

While defective apoptosis predisposes to neoplasia, inappropriate apoptosis in the brain leads to permanent neurological deficits. Disregulated apoptosis has been implicated in several neurodegenerative disorders including Alzheimer's, Parkinson's, and Huntington's diseases. Recent reports have suggested that the key apoptosis regulator Fas ligand (FasL) may participate in both neuronal and immune cell apoptosis in Alzheimer's disease. FasL has also been implicated as a negative regulator for the inflammatory component of the demyelinating brain disorder multiple sclerosis (MS). Here we discuss how FasL-mediated apoptosis may balance immune cell access to the brain with Alzheimer's disease and MS representing extremes of too little and too much immune access, respectively.

Response of established human breast tumors to vaccination with mammaglobin-A cDNA

BACKGROUND

A novel breast cancer-associated antigen, mammaglobin-A, is expressed in 80% of primary breast tumors. The characterization of immune responses against this highly expressed breast cancer-specific antigen would be of value in the development of new therapeutic strategies for breast cancer.

METHODS

We developed an in vivo model using human leukocyte antigen-A*0201/human CD8+ (HLA-A2+/hCD8+) double-transgenic mice to define the epitopes and to study the level of protection acquired by mammaglobin-A cDNA vaccination toward mammaglobin-A+/HLA-A2+ breast cancer cell lines. Mammaglobin-A epitopes were identified using an HLA class I peptide binding prediction computer program, and their activity was verified using gamma interferon ELISPOT and cytotoxicity assays.

RESULTS

We identified seven mammaglobin-A-derived candidate epitopes that bind the HLA-A*0201 molecule (Mam-A2.1-7). CD8+ cytotoxic T lymphocytes (CTLs) from HLA-A2+/hCD8+ mice reacted to the Mam-A2.1 (amino acids [aa] 83-92, LIYDSSLCDL), Mam-A2.2 (aa 2-10, KLLMVLMLA), Mam-A2.4 (aa 66-74, FLNQTDETL), and Mam-A2.6 (aa 32-40, MQLIYDSSL) epitopes. CD8+ CTLs from breast cancer patients also recognized a similar epitope pattern as did those in the HLA-A2+/hCD8 mice and reacted to the Mam-A2.1, Mam-A2.2, Mam-A2.3, Mam-A2.4, and Mam-A2.7 epitopes. Passive transfer of mammaglobin-A-reactive CTLs into SCID (severe combined immunodeficient) beige mice with actively growing mammaglobin-A+ tumors resulted in statistically significant regression (P<.001) in the growth of the tumors.

CONCLUSIONS

The HLA-A2+/hCD8+ mouse represents a valuable animal model to characterize the HLA-A*0201-restricted CD8+ CTL immune response to mammaglobin-A in vivo, and the data reported here demonstrate the immunotherapeutic potential of mammaglobin-A for the treatment and/or prevention of breast cancer.

Fas ligand-mediated apoptosis in degenerative disorders of the brain

While defective apoptosis predisposes to neoplasia, inappropriate apoptosis in the brain leads to permanent neurological deficits. Disregulated apoptosis has been implicated in several neurodegenerative disorders including Alzheimer's, Parkinson's, and Huntington's diseases. Recent reports have suggested that the key apoptosis regulator Fas ligand (FasL) may participate in both neuronal and immune cell apoptosis in Alzheimer's disease. FasL has also been implicated as a negative regulator for the inflammatory component of the demyelinating brain disorder multiple sclerosis (MS). Here, we discuss how FasL-mediated apoptosis may balance immune cell access to the brain with Alzheimer's disease and MS representing extremes of too little and too much immune access, respectively.

Identification of an HLA-A0201-restricted CTL epitope generated by a tumor-specific frameshift mutation in a coding microsatellite of the OGT gene

Deficient DNA mismatch repair results in microsatellite instability and might induce shifts of translational reading frames of genes encompassing coding microsatellites. These may be translated in truncated proteins, including neo-peptide tails functioning as tumor rejection antigens, when presented in the context of MHC class I. Recently, others and we identified a frameshift mutation in the coding T(10) microsatellite of the O-linked N-acetylglucosamine transferase gene (OGT) occuring in up to 41% of microsatellite unstable colorectal cancers. Here we describe a novel HLA-A0201-restricted cytotoxic T lymphocyte (CTL)-epitope (28-SLYKFSPFPL; FSP06) derived from this mutant OGT-protein. FSP06-specific CTL-clones killed peptide-sensitized target cells and tumor cell lines expressing both HLA-A0201 and mutant OGT proteins. This demonstrates that FSP06 is endogenously expressed and represents a CD8(+)-T cell epitope. Our data corroborate the concept of frameshift peptides constituting a novel subset of tumor-associated antigens specifically encountered in cancer cells with deficient mismatch repair.

Identification of an HLA-A*0201-restricted CD8+ T-cell epitope SSp-1 of SARS-CoV spike protein

A novel coronavirus, severe acute respiratory syndrome (SARS)-associated coronavirus (SARS-CoV), has been identified as the causal agent of SARS. Spike (S) protein is a major structural glycoprotein of the SARS virus and a potential target for SARS-specific cell-mediated immune responses. A panel of S protein-derived peptides was tested for their binding affinity to HLA-A*0201 molecules. Peptides with high affinity for HLA-A*0201 were then assessed for their capacity to elicit specific immune responses mediated by cytotoxic T lymphocytes (CTLs) both in vivo, in HLA-A2.1/K(b) transgenic mice, and in vitro, from peripheral blood lymphocytes (PBLs) harvested from healthy HLA-A2.1(+) donors. SARS-CoV protein-derived peptide-1 (SSp-1 RLNEVAKNL), induced peptide-specific CTLs both in vivo (transgenic mice) and in vitro (human PBLs), which specifically released interferon-gamma (IFN-gamma) upon stimulation with SSp-1-pulsed autologous dendritic cells (DCs) or T2 cells. SSp-1-specific CTLs also lysed major histocompatibility complex (MHC)-matched tumor cell lines engineered to express S proteins. HLA-A*0201-SSp-1 tetramer staining revealed the presence of significant populations of SSp-1-specific CTLs in SSp-1-induced CD8(+) T cells. We propose that the newly identified epitope SSp-1 will help in the characterization of virus control mechanisms and immunopathology in SARS-CoV infection, and may be relevant to the development of immunotherapeutic approaches for SARS.

WT1 is a tumor-associated antigen in colon cancer that can be recognized by in vitro stimulated cytotoxic T cells

The Wilms' tumor suppressor gene (WT1) has been shown to be overexpressed in acute and chronic leukemias and in a variety of solid human malignancies, including cancers of the breast and lung. In our present study, we investigated the potential role of WT1 gene in human colon cancer. WT1 mRNA and protein expression was analyzed in a panel of human colon cancer cell lines and primary colon carcinomas by RT-PCR and Western blot analysis, respectively. A mutational screen of WT1' zinc-finger region was carried out by sequence analysis. Finally, using peptide-stimulated cytotoxic T cells it was investigated whether WT1-expressing colon tumor cells are a potential target for antigen-specific immunotherapy. Medium to high abundant levels of WT1 mRNA were detected by RT-PCR in 10 of 12 (83%) colon cell lines and by quantitative, real-time RT-PCR in 13 of 15 (87%) primary tumors, whereas only very low levels of expression were found in 2 primary tumors. Interestingly, however, low levels of WT1 mRNA were also detected in all samples derived from normal colon mucosa. When RT-PCR products were examined by sequence analysis, both +KTS and -KTS splice isoforms but no zinc-finger mutations were found, suggesting that the wild-type form of the WT1 gene is expressed. To determine whether the WT1 protein can serve as a target antigen for immunotherapy, 2 HLA-A2.1-restricted WT1 peptides (Db126 and WH187) were used for the in vitro induction of WT1-specific cytotoxic T lymphocytes (CTLs). The WH187-specific CTLs not only lysed target cells pulsed exogenously with cognate peptide but also WT1-expressing colon tumor cells in a HLA-restricted manner. These findings identify the WT1 protein as an attractive target for the development of antigen-specific immunotherapy in human colon cancer.

Specific donor Vβ-associated CD4+ T-cell responses correlate with severe acute graft-versus-host disease directed to multiple minor histocompatibility antigens

CXB-2/By (CXB-2) recombinant inbred mice express a subset of the minor histocompatibility antigen (miHA) repertoire expressed by C.B10-H2(b)/LiMcdJ (BALB.B) mice. On lethal irradiation and the transplantation of H2(b)-matched C57BL/6 (B6) T cell-depleted bone marrow cells, along with naive unfractionated T cells, both strains succumb to acute graft-versus-host disease (GVHD). Although alloreactive B6 CD4(+) T cells are a necessary source of T-cell help for the B6 CD8(+) component of the GVHD response in both recipient strains, they are capable of mediating severe GVHD by themselves only in BALB.B mice. Previous CD4(+) T-cell receptor repertoire analysis demonstrated overlapping oligoclonal Vbeta use between the CD4(+) B6 anti-BALB.B and B6 anti-CXB-2 responses, with indications of additional BALB.B unique T-cell responses (Vbeta2 and Vbeta11). We report here that the more severe B6 anti-BALB.B response is not due to a quantitative difference in the responding cells, because the frequency of alloreactive donor CD4(+) T cells over time was equivalent in the spleens of BALB.B versus CXB-2 recipients. The responses were also similar in the number of infiltrating B6 CD4(+) T cells in the lingual epithelium of the 2 recipients. In contrast, a significantly greater degree of infiltration and injury of BALB.B intestinal epithelium correlated with the increased level of clinical GVHD severity. Of most significance, despite the involvement of at least 11 Vbeta-associated CD4(+) T-cell families in the overall B6 anti-BALB.B response, the development of severe GVHD correlated with the presence of Vbeta2- and Vbeta11-positive donor T cells. Transplantation of donor CD4(+) T cells from Vbeta-associated families that were shared between the B6 anti-BALB.B and anti-CXB-2 responses resulted in minimal GVHD potential. These data suggest that severe GVHD across miHA barriers depends on the involvement of a restricted number of potent T-cell specificities and implies that there are only a limited number of corresponding responsible miHAs.

Imatinib mesylate affects the development and function of dendritic cells generated from CD34+ peripheral blood progenitor cells

Imatinib mesylate (STI571) is a competitive Bcr-Abl tyrosine kinase inhibitor and has yielded encouraging results in treatment of chronic myelogenous leukemia (CML) and gastrointestinal stroma tumors (GISTs). Apart from inhibition of the Abl protein tyrosine kinases, it also shows activity against platelet-derived growth factor receptor (PDGF-R), c-Kit, Abl-related gene (ARG), and their fusion proteins while sparing other kinases. In vitro studies have revealed that imatinib mesylate can inhibit growth of cell lines and primitive malignant progenitor cells in CML expressing Bcr-Abl. However, little is known about the effects of imatinib mesylate on nonmalignant hematopoietic cells. In the current study we demonstrate that in vitro exposure of mobilized human CD34+ progenitors to therapeutic concentrations of imatinib mesylate (1-5 microM) inhibits their differentiation into dendritic cells (DCs). DCs obtained after 10 to 16 days of culture in the presence of imatinib mesylate showed concentration-dependent reduced expression levels of CD1a and costimulatory molecules such as CD80 and CD40. Furthermore, exposure to imatinib mesylate inhibited the induction of primary cytotoxic T-lymphocyte (CTL) responses. The inhibitory effects of imatinib mesylate were accompanied by down-regulation of nuclear localized RelB protein. Our results demonstrate that imatinib mesylate can act on normal hematopoietic cells and inhibits the differentiation and function of DCs, which is in part mediated via the nuclear factor kappaB signal transduction pathway.

Allelic variation of MHC structure alters peptide ligands to induce atypical partial agonistic CD8+ T cell function

T cell receptors recognize small changes in peptide ligands leading to different T cell responses. Here, we analyzed a panel of HLA-A2-Tax11-19 reactive T cell clones to examine how small allelic variations of MHC molecules could alter the functional outcome of antigen recognition. Similar to the effects induced by antigenic altered peptide ligands, weak or partial agonistic T cell functions were identified in individual T cell clones with the recognition of MHC-altered peptide ligands (MAPLs). Interestingly, one subtype of HLA-A2 molecules induced an unusual type of partial agonistic function; proliferation without cytotoxicity. Modeling of crystallographic data indicated that polymorphic amino acids in the HLA-A2 peptide binding groove, especially the D-pocket, were responsible for this partial agonism. Reciprocal mutations of the Tax peptide side chain engaging the D-pocket indeed restored the agonist functions of the MHC-peptide complex. Whereas early intracellular signaling events were not efficiently induced by these MAPLs, phosphorylated c-Jun slowly accumulated with sustained long-term expression. These data indicate that MAPLs can induce atypical partial agonistic T cell function through structural and biochemical mechanisms similar to altered peptide ligands.

Distinct thresholds for CD8 T cell activation lead to functional heterogeneity: CD8 T cell priming can occur independently of cell division

To examine the bases for CD8 T cell functional heterogeneity, we analyzed responses to partial vs full agonist Ag. An extended period of interaction with APCs was required to set the threshold required for cell division in response to partial as compared with full agonist Ag. Acquisition of cytolytic function was restricted to the divided T cell population. In contrast, the threshold for commitment to produce IFN-gamma and express some activation markers appeared lower and independent of cell division. Indeed, we characterized a T cell population stimulated in response to the partial agonist that was committed to produce IFN-gamma, but failed to divide or secrete IL-2. Importantly, this activated nondivided population behaved as "primed" rather than "anergized," indicating 1) that priming of CD8 T cells may be induced by suboptimal stimulation independent of cell division and 2) that encounter with Ag does not always induce a complete differentiation program in naive CD8 T cells, as previously reported.

Dendritic cells transfected with tumor RNA for the induction of antitumor CTL in colorectal cancer

Dendritic cells (DC) are the most potent antigen-presenting cells known, currently tested for vaccination studies in cancer patients. The use of tumor-derived RNA to load DC overcomes the requirement of defined HLA types and the identification of tumor antigens expressed by the tumors. Here, we show that human monocyte-derived DC generated under serum-free conditions by GM-CSF, IL-4 and TNF-alpha acquire a mature phenotype and expression of the chemokine receptor CCR-7, which plays a pivotal role in DC migration to the afferent lymph nodes. We demonstrate the feasibility of total RNA transfection into such DC using the renal cell carcinoma (RCC) cell line N43-EGFP, which was stably transfected with an EGFP-encoding vector. Moreover, we show that DC transfected with RNA from colorectal cancer cells present HLA class I-restricted antigenic epitopes to induce a primary antitumor CTL response in vitro. Interestingly, the CTL induced by SW480 RNA also recognized another colon cancer line, HCT116, and the RCC line A498. Our results confirm the feasibility of total RNA transfection of serum-free generated DC for the induction of CTL against colon cancer and RCC cells, and support the relevance of shared tumor rejection epitopes between colorectal cancer and RCC.

Epstein-Barr nuclear antigen 1-specific CD4(+) Th1 cells kill Burkitt's lymphoma cells

The gamma-herpesvirus, EBV, is reliably found in a latent state in endemic Burkitt's lymphoma. A single EBV gene product, Epstein-Barr nuclear Ag 1 (EBNA1), is expressed at the protein level. Several mechanisms prevent immune recognition of these tumor cells, including a block in EBNA1 presentation to CD8(+) killer T cells. Therefore, no EBV-specific immune response has yet been found to target Burkitt's lymphoma. We now find that EBNA1-specific, Th1 CD4(+) cytotoxic T cells recognize Burkitt's lymphoma lines. CD4(+) T cell epitopes of EBNA1 are predominantly found in the C-terminal, episome-binding domain of EBNA1, and approximately 0.5% of peripheral blood CD4(+) T cells are specific for EBNA1. Therefore, adaptive immunity can be directed against Burkitt's lymphoma, and perhaps this role for CD4(+) Th1 cells extends to other tumors that escape MHC class I presentation.

Dendritic cell immunogenicity is regulated by peroxisome proliferator-activated receptor gamma

Dendritic cells (DC) are the most potent APCs known that play a key role for the initiation of immune responses. Ag presentation to T lymphocytes is likely a constitutive function of DC that continues during the steady state. This raises the question of which mechanism(s) determines whether the final outcome of Ag presentation will be induction of immunity or of tolerance. In this regard, the mechanisms controlling DC immunogenicity still remain largely uncharacterized. In this paper we report that the nuclear receptor peroxisome proliferator-activated receptor gamma (PPAR-gamma), which has anti-inflammatory properties, redirects DC toward a less stimulatory mode. We show that activation of PPAR-gamma during DC differentiation profoundly affects the expression of costimulatory molecules and of the DC hallmarker CD1a. PPAR-gamma activation in DC resulted in a reduced capacity to activate lymphocyte proliferation and to prime Ag-specific CTL responses. This effect might depend on the decreased expression of costimulatory molecules and on the impaired cytokine secretion, but not on increased IL-10 production, because this was reduced by PPAR-gamma activators. Moreover, activation of PPAR-gamma in DC inhibited the expression of EBI1 ligand chemokine and CCR7, both playing a pivotal role for DC migration to the lymph nodes. These effects were accompanied by down-regulation of LPS-induced nuclear localized RelB protein, which was shown to be important for DC differentiation and function. Our results suggest a novel regulatory pathway for DC function that could contribute to the regulated balance between immunity induction and self-tolerance maintenance.

Tumor-specific CTL kill murine renal cancer cells using both perforin and Fas ligand-mediated lysis in vitro, but cause tumor regression in vivo in the absence of perforin

Kidney cancer is a devastating disease; however, biological therapies have achieved some limited success. The murine renal cancer Renca has been used as a model for developing new preclinical approaches to the treatment of renal cell carcinoma. Successful cytokine-based approaches require CD8(+) T cells, but the exact mechanisms by which T cells mediate therapeutic benefit have not been completely identified. After successful biological therapy of Renca in BALB/c mice, we generated CTLs in vitro using mixed lymphocyte tumor cultures. These CTL mediated tumor-specific H-2K(d)-restricted lysis and production of IFN-gamma, TNF-alpha, and Fas ligand (FasL) in response to Renca. CTL used both granule- and FasL-mediated mechanisms to lyse Renca, although granule-mediated killing was the predominant lytic mechanism in vitro. The cytokines IFN-gamma and TNF-alpha increased the sensitivity of Renca cells to CTL lysis by both granule- and FasL-mediated death pathways. Adoptive transfer of these anti-Renca CTL into tumor-bearing mice cured most mice of established experimental pulmonary metastases, and successfully treated mice were immune to tumor rechallenge. Interestingly, we were able to establish Renca-specific CTL from mice gene targeted for perforin (pfp(-/-)) mice. Although these pfp(-/-) CTL showed reduced cytotoxic activity against Renca, their IFN-gamma production in the presence of Renca targets was equivalent to that of wild-type CTL, and adoptive transfer of pfp(-/-) CTL was as efficient as wild-type CTL in causing regression of established Renca pulmonary metastases. Therefore, although granule-mediated killing is of paramount importance for CTL-mediated lysis in vitro, some major in vivo effector mechanisms clearly are independent of perforin.

T cell receptor-mediated signal transduction controlled by the beta chain transmembrane domain: apoptosis-deficient cells display unbalanced mitogen-activated protein kinases activities upon T cell receptor engagement

The bases that support the versatility of the T cell receptor (TCR) to generate distinct T cell responses remain unclear. We have previously shown that mutant cells in the transmembrane domain of TCRbeta chain are impaired in TCR-induced apoptosis but are not affected in other functions. Here we describe the biochemical mechanisms by which this mutant receptor supports some T cell responses but fails to induce apoptosis. Extracellular signal-regulated protein kinase (ERK) is activated at higher and more sustained levels in TCRbeta-mutated than in wild type cells. Conversely, activation of both c-Jun N-terminal kinase and p38 mitogen-activated protein kinase is severely reduced in mutant cells. By attempting to link this unbalanced induction to altered upstream events, we found that ZAP-70 is normally activated. However, although SLP-76 phosphorylation is normally induced, TCR engagement of mutant cells results in lower tyrosine phosphorylation of LAT but in higher tyrosine phosphorylation of Vav than in wild type cells. The results suggest that an altered signaling cascade leading to an imbalance in mitogen-activated protein kinase activities is involved in the selective impairment of apoptosis in these mutant cells. Furthermore, they also provide new insights in the contribution of TCR to decipher the signals that mediate apoptosis distinctly from proliferation.

Type II pneumocyte-CD8+ T-cell interactions. Relationship between target cell cytotoxicity and activation

CD8+ T-cell responses play an important role in the clearance of respiratory virus infection, but may also contribute to lung injury in the process. The effector mechanisms involved in viral clearance and associated lung injury include both cytolytic and noncytolytic effector functions. Previously we have shown that CD8+ T-cell recognition of alveolar epithelial cells triggers chemokine expression by the epithelial cell and that this plays an important role in the inflammatory infiltration that ensues in the context of T cell-mediated injury (Zhao and colleagues, J. Clin. Invest. 2000;106:R49-R58). In the present study we sought to understand the relationship between alveolar cell cytotoxicity and chemokine expression, both of which occur as a result of CD8+ T-cell antigen recognition. Alveolar epithelial cells efficiently process and present overlapping viral epitopes, and CD8+ T-cell recognition of these class I major histocompatibility complex-restricted epitopes resulted in cytotoxicity of the alveolar cells by both wild-type and perforin-deficient T cells. However, the contribution of perforin-mediated lysis to the total cytotoxicity of alveolar cells by CD8+ T cells was minimal, and the majority of the lysis was attributable to tumor necrosis factor-alpha expressed by the T cell. CD8+ T-cell recognition also led to activation of nuclear factor-kappaB in the alveolar epithelial target cells, at levels inversely proportional to the effector/target (E:T) ratio. Finally, at varying E:T ratios, we demonstrated an inverse relationship between alveolar cell cytotoxicity and monocyte chemotactic protein-1 expression, both of which occur as a result of T-cell recognition. These findings may have important ramifications in understanding the relationship between viral clearance and lung injury.

Frameshift peptide-derived T-cell epitopes: a source of novel tumor-specific antigens

Microsatellite instability (MSI) caused by defective DNA mismatch repair (MMR) is a hallmark of hereditary nonpolyposis colorectal cancers (HNPCC) but also occurs in about 15% of sporadic tumors. If instability affects microsatellites in coding regions, translational frameshifts lead to truncated proteins often marked by unique frameshift peptide sequences at their C-terminus. Since MSI tumors show enhanced lymphocytic infiltration and our previous analysis identified numerous coding mono- and dinucleotide repeat-bearing candidate genes as targets of genetic instability, we examined the role of frameshift peptides in triggering cellular immune responses. Using peptide pulsed autologous CD40-activated B cells, we have generated cytotoxic T lymphocytes (CTL) that specifically recognize HLA-A2.1-restricted peptides derived from frameshift sequences. Among 16 frameshift peptides predicted from mutations in 8 different genes, 3 peptides conferred specific lysis of target cells exogenously loaded with cognate peptide. One peptide derived from a (-1) frameshift mutation in the TGFbetaIIR gene gave rise to a CTL bulk culture capable of lysing the MSI colorectal cancer cell line HCT116 carrying this frameshift mutation. Given the huge number of human coding microsatellites and assuming only a fraction being mutated and encoding immunologically relevant peptides in MSI tumors, frameshift protein sequences represent a novel subclass of tumor-specific antigens. It is tempting to speculate that a frameshift peptide-directed vaccination approach not only could offer new treatment modalities for existing MSI tumors but also might benefit asymptomatic at-risk individuals in HNPCC families by a prophylactic vaccination strategy.

Human lymphocyte apoptosis after exposure to influenza A virus

Infection of humans with influenza A virus (IAV) results in a severe transient leukopenia. The goal of these studies was to analyze possible mechanisms behind this IAV-induced leukopenia with emphasis on the potential induction of apoptosis of lymphocytes by the virus. Analysis of lymphocyte subpopulations after exposure to IAV showed that a portion of CD3(+), CD4(+), CD8(+), and CD19(+) lymphocytes became apoptotic (terminal deoxynucleotidyltransferase-mediated dUTP-biotin nick end labeling positive). The percentage of cells that are infected was shown to be less than the percentage of apoptotic cells, suggesting that direct effects of cell infection by the virus cannot account fully for the high level of cell death. Removal of monocytes-macrophages after IAV exposure reduced the percent of lymphocytes that were apoptotic. Treatment of virus-exposed cultures with anti-tumor necrosis factor alpha did not reduce the percentage of lymphocytes that were apoptotic. In virus-exposed cultures treated with anti-FasL antibody, recombinant soluble human Fas, Ac-DEVD-CHO (caspase-3 inhibitor), or Z-VAD-FMK (general caspase inhibitor), apoptosis and production of the active form of caspase-3 was reduced. The apoptotic cells were Fas-high-density cells while the nonapoptotic cells expressed a low density of Fas. The present studies showed that Fas-FasL signaling plays a major role in the induction of apoptosis in lymphocytes after exposure to IAV. Since the host response to influenza virus commonly results in recovery from the infection, with residual disease uncommon, lymphocyte apoptosis likely represents a part of an overall beneficial immune response but could be a possible mechanism of disease pathogenesis.

Differential survival of transferred CD8 T cells and host reconstitution depending on TCR avidity for host-expressed alloantigen

We transferred naive alloreactive CD8 T cells from TCR transgenic mice to irradiated recipients expressing a partial (H-2Kbm8) or a full (H-2Kb) agonist alloantigen (alloAg). The consequences were strikingly distinct, resulting in acceleration of host lymphopoiesis in the former group, but in strong graft-vs-host reaction, preventing host lymphocyte reconstitution in the latter group. This was correlated, respectively, with long-term persistence and with rapid disappearance of the transferred CD8 T cells. Analysis of transferred T cells showed that initial T cell expansion and modulation of expression of activation markers CD44 and CD62L, as well as induction of cytotoxic function, were similar in both groups. However, IL-2 production and subsequent up-regulation of CD25, early perforin-independent cytolysis, and early down-regulation of Bcl-2 expression were detected only in T cells transferred in hosts expressing full agonist alloAg. Expansion of transferred CD8 T cells was not dependent on either IL-2 or CD25 expression. This expansion could lead to either accelerated host reconstitution or to strong graft-vs-host, depending on the nature of the alloAg. Thus, the extent of Ag stimulation may be a crucial parameter in protocols of alloreactive T cell immunotherapy.

Modulation of CD8+ T cell response to antigen by the levels of self MHC class I

The response of H-Y-specific TCR-transgenic CD8(+) T cells to Ag is characterized by poor proliferation, cytolytic activity, and IFN-gamma secretion. IFN-gamma secretion, but not cytotoxic function, can be rescued by the B7.1 molecule, suggesting that costimulation can selectively enhance some, but not all, effector CD8(+) T cell responses. Although the H-Y epitope binds H-2D(b) relatively less well than some other epitopes, it can induce potent CTL responses in nontransgenic mice, suggesting that the observed poor responsiveness of transgenic CD8(+) T cells cannot be ascribed to the epitope itself. Previously reported reactivity of this TCR to H-2A(b) is also not the cause of the poor responsiveness of the H-Y-specific CD8(+) T cells, as H-Y-specific CD8(+) T cells obtained from genetic backgrounds lacking H-2A(b) also responded poorly. Rather, reducing the levels of H-2(b) class I molecules by breeding the mice to (C57BL/6 x B10.D2)F(1) or TAP1(+/-) backgrounds partially restored cytotoxic activity and enhanced proliferative responses. These findings demonstrate that the self MHC class I gene dosage may regulate the extent of CD8(+) T cell responsiveness to Ag.

Therapy of human tumors in NOD/SCID mice with patient-derived reactivated memory T cells from bone marrow

In an analysis of 84 primary-operated breast cancer patients and 11 healthy donors, we found that the bone marrow of most patients contained memory T cells with specificity for tumor-associated antigens. Patients' bone marrow and peripheral blood contained CD8+ T cells that specifically bound HLA/peptide tetramers. In short-term culture with autologous dendritic cells pre-pulsed with tumor lysates, patients' memory T cells from bone marrow (but not peripheral blood) could be specifically reactivated to interferon-gamma-producing and cytotoxic effector cells. A single transfer of restimulated bone-marrow T cells into NOD/SCID mice caused regression of autologous tumor xenotransplants associated with infiltration by human T cells and tumor-cell apoptosis and necrosis. T cells from peripheral blood showed much lower anti-tumor reactivity. Our findings reveal an innate, specific recognition of breast cancer antigens and point to a possible novel cancer therapy using patients' bone-marrow-derived memory T cells.

Immunosensitization of melanoma tumor cells to non-MHC Fas-mediated killing by MART-1-specific CTL cultures

The discovery of human melanoma rejection Ags has allowed the rational design of immunotherapeutic strategies. One such Ag, MART-1, is expressed on >90% of human melanomas, and CTL generated against MART-1(27-35) kill most HLA A2.1(+) melanoma cells. However, variant tumor cells, which do not express MART-1, down-regulate MHC, or become resistant to apoptosis, will escape killing. Cytotoxic lymphocytes kill by two main mechanisms, the perforin/granzyme degranulation pathway and the TNF/Fas/TNF-related apoptosis-inducing ligand superfamily of apoptosis-inducing ligands. In this study, we examined whether cis-diaminedichloroplatinum (II) cisplatin (CDDP) sensitizes MART-1/HLA A2.1(+) melanoma and melanoma variant tumor cells to non-MHC-restricted, Fas ligand (FasL)-mediated killing by CTL. MART-1(27-35)-specific bulk CTL cultures were generated by pulsing normal PBL with MART-1(27-35) peptide. These CTL cultures specifically kill M202 melanoma cells (MART-1(+), HLA A2.1(+), FasR(-)), and MART-1(27-35) peptide-pulsed T2 cells (FasR(+)), but not M207 melanoma cells (MART-1(+), HLA A2.1(-), FasR(-)), FLU(58-66) peptide-pulsed T2 cells, or DU145 and PC-3 prostate cells (MART-1(-), HLA A2.1(-), FasR(+)). CDDP (0.1-10 microg/ml) sensitized non-MART-1(27-35) peptide-pulsed T2 to the CD8(+) subset of bulk MART-1-specific CTL, and killing was abolished by neutralizing anti-Fas Ab. Furthermore, CDDP up-regulated FasR expression and FasL-mediated killing of M202, and sensitized PC-3 and DU145 to killing by bulk MART-1-specific CTL cultures. These findings demonstrate that drug-mediated sensitization can potentiate FasL-mediated killing by MHC-restricted CTL cell lines, independent of MHC and MART-1 expression on tumor cells. This represents a novel approach for potentially controlling tumor cell variants found in primary heterogeneous melanoma tumor cell populations that would normally escape killing by MART-1-specific immunotherapy.

The role of Fas-FasL in CD8+ T-cell-mediated insulin-dependent diabetes mellitus (IDDM)

In the past few years a number of studies have evaluated the contributions of different cytolytic pathways in the autoimmune destruction of pancreatic beta cells, which results in insulin-dependent (type I) diabetes mellitus. Conflicting results continue to emerge regarding the role of Fas-mediated apoptosis in beta-cell destruction. This is likely to reflect differences inherent to the model systems under investigation, as well as the pleiotropic nature of the genes that are involved in cytotoxicity. Despite these complications, it may be possible to reconcile some of these apparently conflicting results by considering that T-cell-mediated cytotoxicity can occur simultaneously by several mechanisms and that variables such as the cytokine milieu and the strength of the signal to the T cell received through the T-cell receptor complex may alter the relative contribution of each cytolytic pathway to beta-cell destruction.

Induction of cytotoxic T-lymphocyte responses in vivo after vaccinations with peptide-pulsed dendritic cells

Vaccination of patients with cancer using dendritic cells (DCs) was shown to be effective for B-cell lymphoma and malignant melanoma. Here we provide evidence that patients with advanced breast and ovarian cancer can be efficiently vaccinated with autologous DCs pulsed with HER-2/neu– or MUC1-derived peptides. Ten patients were included in this pilot study. The DC vaccinations were well tolerated with no side effects. In 5 of 10 patients, peptide-specific cytotoxic T lymphocytes (CTLs) could be detected in the peripheral blood using both intracellular IFN-γ staining and 51Cr-release assays. The major CTL response in vivo was induced with the HER-2/neu–derived E75 and the MUC1-derived M1.2 peptide, which lasted for more than 6 months, suggesting that these peptides might be immunodominant. In addition, in one patient vaccinated with the MUC1-derived peptides, CEA- and MAGE-3 peptide-specific T-cell responses were detected after several vaccinations. In a second patient immunized with the HER-2/neu peptides, MUC1-specific T lymphocytes were induced after 7 immunizations, suggesting that antigen spreading in vivo might occur after successful immunization with a single tumor antigen. Our results show that vaccination of DCs pulsed with a single tumor antigen may induce immunologic responses in patients with breast and ovarian cancer. This study may be relevant to the design of future clinical trials of other peptide-based vaccines.

Induction of cytotoxic T-lymphocyte responses in vivo after vaccinations with peptide-pulsed dendritic cells

Vaccination of patients with cancer using dendritic cells (DCs) was shown to be effective for B-cell lymphoma and malignant melanoma. Here we provide evidence that patients with advanced breast and ovarian cancer can be efficiently vaccinated with autologous DCs pulsed with HER-2/neu– or MUC1-derived peptides. Ten patients were included in this pilot study. The DC vaccinations were well tolerated with no side effects. In 5 of 10 patients, peptide-specific cytotoxic T lymphocytes (CTLs) could be detected in the peripheral blood using both intracellular IFN-γ staining and 51Cr-release assays. The major CTL response in vivo was induced with the HER-2/neu–derived E75 and the MUC1-derived M1.2 peptide, which lasted for more than 6 months, suggesting that these peptides might be immunodominant. In addition, in one patient vaccinated with the MUC1-derived peptides, CEA- and MAGE-3 peptide-specific T-cell responses were detected after several vaccinations. In a second patient immunized with the HER-2/neu peptides, MUC1-specific T lymphocytes were induced after 7 immunizations, suggesting that antigen spreading in vivo might occur after successful immunization with a single tumor antigen. Our results show that vaccination of DCs pulsed with a single tumor antigen may induce immunologic responses in patients with breast and ovarian cancer. This study may be relevant to the design of future clinical trials of other peptide-based vaccines.

Differential use of FasL- and perforin-mediated cytolytic mechanisms by T-cell subsets involved in graft-versus-myeloid leukemia responses

In graft-versus-leukemia (GVL) responses, the cellular subsets and effector mechanisms responsible for cytotoxicity against leukemic cells in vivo remain poorly characterized. A murine model of syngeneic GVL that features CD4+ and CD8+T-cell responses against the MMB3.19 myeloid leukemia cell line has been previously described. MMB3.19 expresses high levels of functional Fas and tumor necrosis factor (TNF) receptors that do not transduce proapoptotic signals. Through the use of perforin- and Fas ligand (FasL)-deficient mice, it was demonstrated that CD4+ T cells mediate anti-MMB3.19 effects in vivo primarily through the use of FasL and secondarily through perforin mechanisms. Conversely, CD8+ T cells induce GVL effects primarily through the use of perforin and minimally through FasL mechanisms. Although the in vivo observations of CD8+ T cells were reflective of their in vitro cytotoxic T lymphocyte (CTL) activity, for CD4+ T cells, in vitro responses were dominated by the perforin pathway. In addition, the diminished capacity of T cells from perforin- and FasL-deficient mice to lyse MMB3.19 target cells appeared directly related to their deficient cytotoxic functions rather than to defects in activation because these cells were fully capable of mounting proliferative responses to the tumor cells. These findings demonstrate that GVL responses of T-cell subsets can involve preferential use of different cytotoxic mechanisms. In particular, these findings identify a role for both FasL-employing CD4+CTLs and the more novel perforin-utilizing CD4+ T-cell subset in responses against a myeloid leukemia.

Differential use of FasL- and perforin-mediated cytolytic mechanisms by T-cell subsets involved in graft-versus-myeloid leukemia responses

In graft-versus-leukemia (GVL) responses, the cellular subsets and effector mechanisms responsible for cytotoxicity against leukemic cells in vivo remain poorly characterized. A murine model of syngeneic GVL that features CD4+ and CD8+T-cell responses against the MMB3.19 myeloid leukemia cell line has been previously described. MMB3.19 expresses high levels of functional Fas and tumor necrosis factor (TNF) receptors that do not transduce proapoptotic signals. Through the use of perforin- and Fas ligand (FasL)-deficient mice, it was demonstrated that CD4+ T cells mediate anti-MMB3.19 effects in vivo primarily through the use of FasL and secondarily through perforin mechanisms. Conversely, CD8+ T cells induce GVL effects primarily through the use of perforin and minimally through FasL mechanisms. Although the in vivo observations of CD8+ T cells were reflective of their in vitro cytotoxic T lymphocyte (CTL) activity, for CD4+ T cells, in vitro responses were dominated by the perforin pathway. In addition, the diminished capacity of T cells from perforin- and FasL-deficient mice to lyse MMB3.19 target cells appeared directly related to their deficient cytotoxic functions rather than to defects in activation because these cells were fully capable of mounting proliferative responses to the tumor cells. These findings demonstrate that GVL responses of T-cell subsets can involve preferential use of different cytotoxic mechanisms. In particular, these findings identify a role for both FasL-employing CD4+CTLs and the more novel perforin-utilizing CD4+ T-cell subset in responses against a myeloid leukemia.

Quantification of tumor-specific T lymphocytes with the ELISPOT assay

The characterization of tumor-associated antigens and of human leukocyte antigen (HLA) class I molecule-binding peptide epitopes derived from these antigens has prompted the initiation of various vaccination trials aimed at inducing tumor-specific CD8+ T cells in persons with cancer. Sensitive and easy-to-perform T-cell assays that assess the frequency of tumor-reactive T cells are crucial for the evaluation and further development of vaccination approaches. This review focuses on a novel ELISPOT technique that allows quantification of tumor-specific T lymphocytes from peripheral blood by detecting antigen-induced cytokine secretion. Various ELISPOT methods using different antigen-presenting cells and different cytokines as read-out are described. T-cell analyses performed using the standard chromium release assay and the ELISPOT assay are also compared. Results from various clinical trials, including peptide and whole tumor cell vaccination and cytokine treatment, are now available and show the suitability of the ELISPOT assay for monitoring T-cell responses. To establish a basis for standardization and to further improve this technique, the first comparative quality assurance studies analyzing T-cell frequencies in different laboratories with the ELISPOT assay are being performed.

Comparison of Fas- versus perforin-mediated pathways of cytotoxicity in TCR- and Thy-1-activated murine T cells

T cell-mediated cytotoxicity can be triggered by cross-linking of TCR or Thy-1 surface proteins. While the TCR-triggered signaling initiates both perforin- and Fas ligand (FasL)-Fas-mediated mechanisms of cytotoxicity, it was not clear which mechanism was utilized by Thy-1-triggered signals and which pathway of cytotoxicity was triggered at low levels of antigen expression. It is shown that glycophosphatidylinositol-linked surface glycoprotein Thy-1 preferentially activates FasL-Fas- but not perforin-mediated cytotoxicity. This is explained by the lesser intensity of Thy-1-mediated signaling in T cells. The data suggest that Thy-1-triggered Fas-mediated cytotoxicity is completely dependent on cross-talk between Thy-1 and TCR signals since mutations in TCR-CD3 complex molecules or inhibition of tyrosine kinases or calcineurin abolished or strongly inhibited Thy-1-triggered FasL-Fas-mediated cytotoxicity. Lower concentrations of antigenic peptide or levels of cross-linking with anti-TCR-CD3 mAb are required to trigger Fas-mediated than perforin-mediated cytotoxicity by different cytotoxic T lymphocyte (CTL) lines and clones, and it is shown that cross-linking of Thy-1 is much less efficient in triggering accumulation of second messengers (intracellular Ca(2+)) than cross-linking of TCR on CTL. Taken together, these data reflect the possibility of differential activation of FasL and/or perforin pathways of cytotoxicity depending on the nature of activating stimuli and surface receptor.

Characterization of CD8+ T lymphocytes that persist after peripheral tolerance to a self antigen expressed in the pancreas

As a result of expression of the influenza hemagglutinin (HA) in the pancreatic islets, the repertoire of HA-specific CD8+ T lymphocytes in InsHA transgenic mice (D2 mice expressing the HA transgene under control of the rat insulin promoter) is comprised of cells that are less responsive to cognate Ag than are HA-specific CD8+ T lymphocytes from conventional mice. Previous studies of tolerance induction involving TCR transgenic T lymphocytes suggested that a variety of different mechanisms can reduce avidity for Ag, including altered cell surface expression of molecules involved in Ag recognition and a deficiency in signaling through the TCR complex. To determine which, if any, of these mechanisms pertain to CD8+ T lymphocytes within a conventional repertoire, HA-specific CD8+ T lymphocytes from B10.D2 mice and B10.D2 InsHA transgenic mice were compared with respect to expression of cell surface molecules, TCR gene utilization, binding of tetrameric KdHA complexes, lytic mechanisms, and diabetogenic potential. No evidence was found for reduced expression of TCR or CD8 by InsHA-derived CTL, nor was there evidence for a defect in triggering lytic activity. However, avidity differences between CD8+ clones correlated with their ability to bind KdHA tetramers. These results argue that most of the KdHA-specific T lymphocytes in InsHA mice are not intrinsically different from KdHA-specific T lymphocytes isolated from conventional animals. They simply express TCRs that are less avid in their binding to KdHA.

Differential Requirements for NF-κB and AP-1 trans-Activation in Response to Minimal TCR Engagement by a Partial Agonist in Naive CD8 T Cells

We investigated the basis for partial reactivity of naive CD8 T cells expressing an alloreactive transgenic TCR in response to a mutant alloantigen. When unstimulated APCs were used, IFN-γ as well as IL-2 and cell proliferation were observed in response to wild-type Ag, whereas mutant Ag induced only IFN-γ. DNA binding and reporter gene assays showed that the response to mutant Ag involved NF-κB, but not AP-1 activation, whereas wild-type Ag activated both transcription factors. Increasing the contribution of costimulatory signals by using LPS-activated APCs partially corrected the activation by mutant Ag, because proliferation and weak IL-2 production could be measured. This also led to AP-1 activation, albeit with delayed kinetics, in response to mutant Ag. To explain how engagement of the same TCR by distinct ligands results in different T cell responses, it may be proposed, in line with models stressing the importance of the kinetics of Ag/TCR interaction, that two types of signals be distinguished: a “fast” short-lived signal is sufficient to activate NF-κB; whereas a “slow” signal obtained after prolonged TCR engagement is required for AP-1 activation. Failure to activate AP-1 in limiting conditions (unstimulated mutant APC) was partially corrected by increasing costimulation.

Comparing the Relative Role of Perforin/Granzyme Versus Fas/Fas Ligand Cytotoxic Pathways in CD8+ T Cell-Mediated Insulin-Dependent Diabetes Mellitus

CD8+ cytotoxic T cells play a critical role in initiating insulin-dependent diabetes mellitus. The relative contribution of each of the major cytotoxic pathways, perforin/granzyme and Fas/Fas ligand (FasL), in the induction of autoimmune diabetes remains controversial. To evaluate the role of each lytic pathway in β cell lysis and induction of diabetes, we have used a transgenic mouse model in which β cells expressing the influenza virus hemagglutinin (HA) are destroyed by HA-specific CD8+ T cells from clone-4 TCR-transgenic mice. Upon adoptive transfer of CD8+ T cells from perforin-deficient clone-4 TCR mice, there was a 30-fold increase in the number of T cells required to induce diabetes. In contrast, elimination of the Fas/FasL pathway of cytotoxicity had little consequence. When both pathways of cytolysis were eliminated, mice did not become diabetic. Using a model of spontaneous diabetes, which occurs in double transgenic neonates that express both clone-4 TCR and Ins-HA transgenes, mice deficient in either the perforin or FasL/Fas lytic pathway become diabetic soon after birth. This indicates that, in the neonate, large numbers of autoreactive CD8+ T cells can lead to destruction of islet β cells by either pathway.

Immune escape of tumors in vivo by expression of cellular FLICE-inhibitory protein

The antiapoptotic protein cellular FLICE (Fas-associated death domain-like IL-1beta-converting enzyme) inhibitory protein (cFLIP) protects cells from CD95(APO-1/Fas)-induced apoptosis in vitro and was found to be overexpressed in human melanomas. However, cytotoxic T cell-induced apoptosis, which is critically involved in tumor control in vivo, is not inhibited by cFLIP in vitro, as only CD95- and not perforin-dependent lysis is affected. This calls into question whether cFLIP is sufficient to allow escape from T cell-dependent immunity. Using two murine tumors, we directly demonstrate that cFLIP does result in escape from T cell immunity in vivo. Moreover, tumor cells are selected in vivo for elevated cFLIP expression. Therefore, our data indicate that CD95-dependent apoptosis constitutes a more prominent mechanism for tumor clearance than has so far been anticipated and that blockade of this pathway can result in tumor escape even when the perforin pathway is operational.

Involvement of the N-methyl-D-aspartate receptor in neuronal cell death induced by cytotoxic T cell-derived secretory granules

The mechanisms underlying neurotoxicity mediated by cytotoxic T lymphocytes (CTL) and their secretory granule proteins perforin and granzymes remain unclear. We evaluated the possible role of the neurotransmitter glutamate in cell death observed in differentiated neurons exposed to CTL-derived granules. Excitotoxicity induced by excessive concentrations of extracellular glutamate is associated with a rise in intracellular calcium and can lead to generation of NO through the activation of glutamatergic N-methyl-D-aspartate (NMDA) receptors. Consistent with an involvement of glutamate, we found that cell death in mature cerebral granule cells was inhibited by 65-80% by two NMDA receptor blockers (MK-801 and D-2-amino-5-phosphonovaleric acid) or a NO synthase blocker (N(G)-nitro-L-arginine methylester). Furthermore, neurons treated with secretory granules responded with a biphasic rise in the intracellular calcium concentration ([Ca2+]i). Whereas MK-801 did not interfere with the immediate rise of [Ca2+]i, the second wave of calcium accumulation starting at 40 min was delayed by 20 min and reduced in amplitude in the presence of MK-801. In immature, NMDA receptor-negative neurons, MK-801 prevented neither the cytotoxicity nor the calcium influx observed 5 min after addition of cytotoxic granules. The demonstration that NMDA receptors and NO are involved in granule-mediated killing of mature neurons opens new avenues in the treatment of neuronal cell death in CTL-mediated diseases such as viral encephalitis.

Differential Coupling of Second Signals for Cytotoxicity and Proliferation in CD8+ T Cell Effectors: Amplification of the Lytic Potential by B7

The role of second signals delivered through B7/CD28 interactions in T cell activation is well documented. However, once CTLs are elicited, TCR-mediated cytotoxicity appears to be uncoupled from the requirement for costimulatory signals. In this study, we show an uncoupling across a broad range of concentrations of peptide, thus demonstrating that cytolysis is a TCR-mediated response that is fully independent of costimulatory signals. However, the same T cell effectors remain fully responsive to B7 engagement, which is able to amplify Ag-mediated proliferation and cytolytic capacity. B7 expression by targets results in an IL-2-mediated proliferative expansion of the effectors concurrent with the elimination of the targets. Thus, costimulation of effectors results in a vast expansion in lytic units over time, which does not occur in the absence of IL-2 or B7. Both TCR-derived and second signals appear to be necessary to achieve this result. These results suggest that B7-expressing APC or a cohort of IL-2-producing helper cells would functionally extend the duration and effectiveness of the cytotoxic response occurring in localized immune responses.

A Partially Agonistic Peptide Acts as a Selective Inducer of Apoptosis in CD8+ CTLs

We have analyzed the effect of partially agonistic peptides on the activation and survival of CTL clones specific for a highly immunogenic HLA A11-restricted peptide epitope derived from the EBV nuclear Ag-4. Several analogues with substitutions of TCR contact residues were able to trigger cytotoxic activity without induction of IL-2 mRNA and protein or T cell proliferation. Triggering with these partial agonists in the absence of exogenous IL-2 resulted in down-regulation of the cytotoxic potential of the specific CTLs. One analogue selectively triggered apoptosis as efficiently as the original epitope, subdividing the partial agonists into apoptosis-inducing and noninducing ligands. Analysis of early T cell activation events, induction of Ca2+ influx, and acid release did not reveal significant differences between the two types of analogue peptides. These results demonstrate that some partial agonists can dissociate the induction of CTL death from CTL activation. Peptides with such properties may serve as useful tools to study signal transduction pathways in CD8+ lymphocytes and as therapeutic agents modulating natural immune responses.

Identification of HLA-A2–Restricted T-Cell Epitopes Derived From the MUC1 Tumor Antigen for Broadly Applicable Vaccine Therapies

The tumor-associated antigen MUC1 is overexpressed on various hematological and epithelial malignancies and is therefore a suitable candidate for broadly applicable vaccine therapies. It was demonstrated that major histocompatibility complex (MHC)-unrestricted cytotoxic T cells can recognize epitopes of the MUC1 protein core localized in the tandem repeat domain. There is increasing evidence now that MHC-restricted T cells can also be induced after immunization with the MUC1 protein or segments of the core tandem repeat. Using a computer analysis of the MUC1 amino acid sequence, we identified two novel peptides with a high binding probability to the HLA-A2 molecule. One of the peptides is derived from the tandem repeat region and the other is derived from the leader sequence of the MUC1 protein, suggesting that, in contrast to previous reports, the MUC1-directed immune responses are not limited to the extracellular tandem repeat domain. Cytotoxic T cells (CTL) were generated from several healthy donors by primary in vitro immunization using peptide-pulsed dendritic cells. The addition of a Pan-HLA-DR binding peptide PADRE as a T-helper epitope during the in vitro priming resulted in an increased cytotoxic activity of the MUC1-specific CTL and a higher production of cytokines such as interleukin-12 and interferon-γ in the cell cultures, demonstrating the importance of CD4 cells for an efficient CTL priming. The peptide induced CTL lysed tumors endogenously expressing MUC1 in an antigen-specific and HLA-A2–restricted fashion, including breast and pancreatic tumor cells as well as renal cell carcinoma cells, showing that these peptides are shared among many tumors. The use of MUC1-derived peptides could provide a broadly applicable approach for the development of dendritic cell-based vaccination therapies.

Identification of HLA-A2–Restricted T-Cell Epitopes Derived From the MUC1 Tumor Antigen for Broadly Applicable Vaccine Therapies

The tumor-associated antigen MUC1 is overexpressed on various hematological and epithelial malignancies and is therefore a suitable candidate for broadly applicable vaccine therapies. It was demonstrated that major histocompatibility complex (MHC)-unrestricted cytotoxic T cells can recognize epitopes of the MUC1 protein core localized in the tandem repeat domain. There is increasing evidence now that MHC-restricted T cells can also be induced after immunization with the MUC1 protein or segments of the core tandem repeat. Using a computer analysis of the MUC1 amino acid sequence, we identified two novel peptides with a high binding probability to the HLA-A2 molecule. One of the peptides is derived from the tandem repeat region and the other is derived from the leader sequence of the MUC1 protein, suggesting that, in contrast to previous reports, the MUC1-directed immune responses are not limited to the extracellular tandem repeat domain. Cytotoxic T cells (CTL) were generated from several healthy donors by primary in vitro immunization using peptide-pulsed dendritic cells. The addition of a Pan-HLA-DR binding peptide PADRE as a T-helper epitope during the in vitro priming resulted in an increased cytotoxic activity of the MUC1-specific CTL and a higher production of cytokines such as interleukin-12 and interferon-γ in the cell cultures, demonstrating the importance of CD4 cells for an efficient CTL priming. The peptide induced CTL lysed tumors endogenously expressing MUC1 in an antigen-specific and HLA-A2–restricted fashion, including breast and pancreatic tumor cells as well as renal cell carcinoma cells, showing that these peptides are shared among many tumors. The use of MUC1-derived peptides could provide a broadly applicable approach for the development of dendritic cell-based vaccination therapies.

Autoreactive cytotoxic T cells in mice are induced by immunization with a conserved mitochondrial enzyme in Freund's complete adjuvant

Standard methods to generate autoimmune reactions in mice, by immunization with antigens emulsified with adjuvants, stimulate strong helper (CD4) T-cell and antibody responses but are not reported to induce cytolytic CD8 T cells. The aim of this study was to assess whether specific autoreactive CD8 T cells could be readily generated after immunization with a 'weak' autoantigen in adjuvant. Mice were immunized intraperitoneally three times with the E3 subunit of the mitochondrial 2-oxoacid dehydrogenase enzyme complexes (dihydrolipoamide dehydrogenase) emulsified with Freund's complete adjuvant. Splenic and lymph node lymphocytes were harvested after 14 days for in vitro functional studies. T lymphocytes were tested for proliferative responses and cytotoxicity against antigen-loaded isogeneic target cells. An autoreactive cytolytic T lymphocyte (CTL) response was detectable only after the in vitro restimulation of lymphocytes with E3 antigen-loaded syngeneic splenocytes. These CTL were identified as H-2-restricted CD8+ T cells. A proliferative response to E3 was demonstrable against antigen-pulsed syngeneic splenocytes. Immunized mice also generated strong antibody responses to E3. Liver histology showed portal infiltrates interpreted as a response of the liver to a non-specific immunological stimulus. It is concluded that autoreactive cytolytic T cells can be generated experimentally upon appropriate stimulation of the immune system, and can be identified in vitro upon release from the controlling mechanisms that are likely to regulate them in vivo.

Evidence of Alternative or Concomitant Use of Perforin- and Fas-Dependent Pathways in a T Cell-Mediated Negative Regulation of Ig Production

To study the possible involvement of perforin (Pfp)- and/or Fas-dependent cytotoxicity pathways in a T cell-mediated negative regulation of Ig production, we used the T cell-induced Ig-allotype suppression model. T splenocytes from Igha/a mice, when neonatally transferred into histocompatible Igha/b F1 or Ighb/b congenic hosts, are intrinsically able to totally, specifically, and chronically suppress the production of IgG2a of the Ighb haplotype (IgG2ab). It has not been established whether the suppression effectors, which are anti-IgG2ab MHC class I-restricted CD8+ T cells, cytolyse IgG2ab+ B targets or whether they only silence Ig production. In this study, using T cells from Igha/a Pfp+/+ or Pfpo/o mice, the latter obtained by crossbreeding, and B cells from Ighb/b Fas+/+ or Faslpr/lpr (lymphoproliferation) mice in appropriate adoptive transfer models, we demonstrated that: 1) under blockage of the Pfp-mediated pathway, Igha/a T cells were still able to induce suppression against wild-type IgG2ab+ B cells, 2) IgG2ab+ B cells with impaired Fas expression were also subjected to suppression by WT Igha/a T splenocytes, and 3) the suppression establishment was totally inhibited when both Pfp- and Fas-dependent mechanisms were simultaneously blocked, i.e., when Igha/a Pfpo/o T cells were used to induce suppression against Ighb/b Faslpr/lpr B cells. These results provide the first demonstration of the existence of alternative or simultaneous use of the major cytotoxic mechanisms in a T cell-mediated down-regulation of an Ig production.

CD4+ cells play a major role in xenogeneic human anti-pig cytotoxicity through the Fas/Fas ligand lytic pathway

BACKGROUND

In this study, the role of cell-mediated cytotoxicity by human leukocytes against pig endothelial cells was examined in vitro. The aim was to determine which cell subsets were responsible for this phenomenon and which pathways were involved in cell lysis.

METHODS

Primed human peripheral blood mononuclear cells (PBMC) or purified CD4+ or CD8+ T cells were used in a cell-mediated cytotoxicity assay in which cytotoxicity of an SV40 transformed porcine endothelial cell (EC) line (SVAP) was determined by Annexin V binding.

RESULTS

Human PBMC demonstrated specific lysis of porcine EC that was proportional to the effector: target ratio. CD4+ T cells accounted for >60% of this lysis, whereas CD8+ T cells accounted for <20%. CD4+ T cell-mediated lysis depended on direct recognition of porcine major histocompatibility complex class II molecules as inhibition of swine leukocyte antigen class II on porcine EC-inhibited CD4+ T cell cytotoxicity. This lysis was mediated through the Fas/FasL pathway as addition of anti-Fas and/or anti-FasL antibody profoundly inhibited antiporcine lysis. In addition, FasL gene expression was detected in primed PBMC and CD4+ T cells by RT-PCR, whereas granzyme B gene expression was not. Primed CD4+ T cells demonstrated high level FasL protein by Western blotting and two-color FACS analysis, whereas NK cells and CD8+ T cells did not. Finally, recombinant human FasL induced apoptosis in Fas expressing porcine EC cells, demonstrating that human FasL interacted with and activated Fas on porcine EC cells.

CONCLUSIONS

In conclusion, human to pig cell-mediated cytotoxicity was mediated predominantly by CD4+ T cells through the Fas/FasL pathway of apoptosis. These results suggest that direct cytotoxicity by xenoreactive CD4+ T cells may be one of several effector mechanisms involved in cellular xenograft rejection.