MHC-restricted cytotoxic T cells: studies on the biological role of polymorphic major transplantation antigens determining T-cell restriction-specificity, function, and responsiveness

Leflunomide-mediated suppression of antiviral antibody and Tcell responses: differential restoration by uridine

BACKGROUND

Leflunomide is an isoxazol derivative with immunosuppressive capacities in various experimental allo- and xenotransplantation models. Two main mechanisms of action have been described: Inhibition of pyrimidine de novo synthesis and impairment of tyrosine phosphorylation of different tyrosine kinases involved in receptor signaling via B cell and cytokine receptors.

MATERIALS AND METHODS

Interference of Leflunomide with the IgM antibody responses to vesicular stomatitis virus (VSV, T-independent type 1), IgM to recombinant VSV glycoprotein (T-independent type 2), and IgG to lymphocytic choriomeningitis virus (LCMV, T-dependent) were analyzed whereas the cytotoxic T cell (CTL) response was examined after LCMV infection. Interference with the CD8+ T cell-mediated autoimmune diabetes in a transgenic mouse expressing the LCMV-glycoprotein in the pancreatic islets was studied as a model for T cell-mediated autoimmune diseases. Uridine substitution experiments were performed to differentiate between the above mentioned two mechanisms of action on different functions of the immune system in vivo.

RESULTS

Leflunomide at 35 mg/kg/day suppressed the humoral immune response against all antigens tested. Similar effects on T-independent compared to T-dependent antibody responses required two to four times higher drug doses. CTL responses to LCMV were considerably impaired. Uridine substitution prevented lethal VSV encephalitis under Leflunomide treatment by restoring the neutralizing IgM and IgG responses. However, the inhibition of LCMV specific CTLs and suppression of CD8+ T cell-mediated autoimmune diabetes remained unaffected by additional uridine treatment.

CONCLUSIONS

Leflunomide-mediated suppression of B cell and T helper cell activity but not of CTLs largely depends on inhibition of pyrimidine de novo synthesis.

Differential regulation of antiviral T-cell immunity results in stable CD8+ but declining CD4+ T-cell memory

Emerging evidence indicates that CD8+ and CD4+ T-cell immunity is differentially regulated. Here we have delineated differences and commonalities among antiviral T-cell responses by enumeration and functional profiling of eight specific CD8+ and CD4+ T-cell populations during primary, memory and recall responses. A high degree of coordinate regulation among all specific T-cell populations stood out against an approximately 20-fold lower peak expansion and prolonged contraction phase of specific CD4+ T-cell populations. Surprisingly, although CD8+ T-cell memory was stably maintained for life, levels of specific CD4+ memory T cells gradually declined. However, this decay, which seemed to result from less efficient rescue from apoptosis, did not affect functionality of surviving virus-specific CD4+ T cells. Our results indicate that CD4+ T-cell memory might become limiting under physiological conditions and that conditions precipitating CD4+ T-cell loss might compromise protective immunity even in the presence of unimpaired CD8+ T-cell responses.

Exploiting genetic alterations to design novel therapies for cancer

In the 3 decades since the signing of the National Cancer Act, there has been tremendous progress in the elucidation of the molecular underpinnings of cancer. Molecular genetic studies have been particularly insightful, revealing genetic rearrangements, such as chromosomal translocations, which may be the seminal event leading to deregulated cell growth for many childhood and adult cancers. These findings have led to new diagnostic and prognostic tools but have been slow to be translated into new therapeutic modalities. This article reviews a variety of methods now under development to exploit genetic changes in cancer to develop specific anticancer agents using gene therapy, viral therapy, and immunotherapy approaches. As many of these strategies inevitably enter the clinic, it will be imperative for health care professionals to be familiar with these novel approaches as they help patients navigate the likely broad array of treatment options.

IFN-gamma affects both the stability and the intracellular transport of class I MHC complexes

In accordance with the key role of major histocompatibility complex (MHC) class I molecules in the adaptive immune response against viruses, their expression can be enhanced by the potent cytokine interferon-gamma (IFN-gamma), which upregulates the expression of multiple components in the pathway of class I-restricted antigen presentation. In this study, we analyzed the effect of IFN-gamma treatment on class I formation, peptide editing, trafficking, and cell surface expression. We show that IFN-gamma treatment promotes significantly the assembly and cell surface expression of stable class I complexes. Yet the existence of large intracellular pools of both free class I heavy chains and suboptimal class I complexes indicates that the optimal peptide supply limits cell surface expression levels of class I complexes. Unexpectedly, we found that IFN-gamma appears generally to slow the maturation rates of both class I complexes and transferrin receptors. Apparently, IFN-gamma causes prolonged retention of molecules in the endoplasmic reticulum (ER) because it regulates the expression of ER-residing proteins that participate in protein maturation. Consequently, it induces more rigorous ER quality control. The significance of these effects of IFN-gamma for in vivo immune responses is discussed.

Common antiviral cytotoxic t-lymphocyte epitope for diverse arenaviruses

Members of the Arenaviridae family have been isolated from mammalian hosts in disparate geographic locations, leading to their grouping as Old World types (i.e., lymphocytic choriomeningitis virus [LCMV], Lassa fever virus [LFV], Mopeia virus, and Mobala virus) and New World types (i.e., Junin, Machupo, Tacaribe, and Sabia viruses) (C. J. Peters, M. J. Buchmeier, P. E. Rollin, and T. G. Ksiazek, p. 1521-1551, in B. N. Fields, D. M. Knipe, and P. M. Howley [ed.], Fields virology, 3rd ed., 1996; P. J. Southern, p. 1505-1519, in B. N. Fields, D. M. Knipe, and P. M. Howley [ed.], Fields virology, 3rd ed., 1996). Several types in both groups-LFV, Junin, Machupo, and Sabia viruses-cause severe and often lethal human diseases. By sequence comparison, we noted that eight Old World and New World arenaviruses share several amino acids with the nucleoprotein (NP) that consists of amino acids (aa) 118 to 126 (NP 118-126) (RPQASGVYM) of LCMV that comprise the immunodominant cytotoxic T-lymphocyte (CTL) epitope for H-2(d) mice (32). This L(d)-restricted epitope constituted >97% of the total bulk CTLs produced in the specific antiviral or clonal responses of H-2(d) BALB mice. NP 118-126 of the Old World arenaviruses LFV, Mopeia virus, and LCMV and the New World arenavirus Sabia virus bound at high affinity to L(d). The primary H-2(d) CTL anti-LCMV response as well as that of a CTL clone responsive to LCMV NP 118-126 recognized target cells coated with NP 118-126 peptides derived from LCMV, LFV, and Mopeia virus but not Sabia virus, indicating that a common functional NP epitope exists among Old World arenaviruses. Use of site-specific amino acid exchanges in the NP CTL epitope among these arenaviruses identified amino acids involved in major histocompatibility complex binding and CTL recognition.

Familial CD8 deficiency due to a mutation in the CD8 alpha gene

CD8 glycoproteins play an important role in both the maturation and function of MHC class I-restricted T lymphocytes. A 25-year-old man, from a consanguineous family, with recurrent bacterial infections and total absence of CD8(+) cells, was studied. Ab deficiencies and ZAP-70 and TAP defects were ruled out. A missense mutation (gly90-->ser) in both alleles of the immunoglobulin domain of the CD8 alpha gene was shown to correlate with the absence of CD8 expression found in the patient and two sisters. Conversely, high percentages of CD4(-)CD8(-)TCR alpha beta(+) T cells were found in the three siblings. A novel autosomal recessive immunologic defect characterized by absence of CD8(+) cells is described. These findings may help to further understanding of the role of CD8 molecules in human immune response.

TAP-independent presentation of CTL epitopes by Trojan antigens

The majority of CTL epitopes are derived from intracellular proteins that are degraded in the cytoplasm by proteasomes into peptides that are transported into the endoplasmic reticulum by the TAP complex. These peptides can be further processed into the optimal size (8-10 residues) for binding with nascent MHC class I molecules, generating complexes that are exported to the cell surface. Proteins or peptides containing CTL epitopes can be introduced into the cytoplasm of APCs by linking them to membrane-translocating Trojan carriers allowing their incorporation into the MHC class I Ag-processing pathway. The present findings suggest that these "Trojan" Ags can be transported into the endoplasmic reticulum in a TAP-independent way where they are processed and trimmed into CTL epitopes. Furthermore, processing of Trojan Ags can also occur in the trans-Golgi compartment, with the participation of the endopeptidase furin and possibly with the additional participation of a carboxypeptidase. We believe that these findings will be of value for the design of CTL-inducing vaccines for the treatment or prevention of infectious and malignant diseases.

Failure to expand influenza and tetanus toxoid memory T cells in vitro correlates with disease course in SIV infected rhesus macaques

Marked decreases in influenza (flu) and tetanus toxoid (T.T.) antigen specific CD8(+) and CD4(+) T cell memory responses were noted shortly after SIV infection in monkeys that go on to develop clinical disease within 18 months (normal progressor, NP) following SIV infection but not in monkeys that remain asymptomatic >3 years post SIV infection (long-term nonprogressor, LTNP). While PBMCs from NP and LTNP monkeys demonstrate both low and high avidity flu and T.T. specific CD8(+) and CD4(+)T cell immune responses prior to SIV infection, the PBMCs from NP but not LTNP fail to generate high avidity T cell responses post SIV infection. This failure to generate high avidity T cell responses in vitro correlated with increased apoptotic cell death in PBMC cultures from NP animals. Since high avidity antigen specific CTLs have been shown to be most efficient in eliminating viral infections, the present finding has important implications for the evaluation of the level of immune reconstitution following various modalities of therapy in HIV-1 infected patients.

A novel cytotoxic T-cell epitope presented by HLA-A24 molecule in hepatitis C virus infection

BACKGROUND/AIMS

It has been suggested that cytotoxic T lymphocytes (CTL) have crucial roles for the hepatocellular damage in hepatitis C virus (HCV) infection. A series of CTL epitopes located in the HCV protein have been identified. However, no CTL epitopes restricted by HLA-A24, a common HLA allele in humans, has been identified.

METHODS

Peripheral blood and liver infiltrating mononuclear cells from the patients with hepatitis C virus infection and healthy controls were stimulated with a series of peptides containing HLA-A24 binding motifs located in HCV protein.

RESULTS

An immunodominant HLA-A24 restricted CTL epitope (A24-4; AYSQQTRGL, amino acids 1031-1039) presented by HLA-A24 molecule was identified using a series of synthetic peptides containing the HLA-A24 binding motifs. The CTL activity against this peptide was induced both in peripheral blood and liver infiltrating mononuclear cells from HLA-A24-positive chronic hepatitis C patients, not from HLA-A24-negative patients and HLA-A24-positive healthy controls. CTL activity was blocked by anti-HLA-A24 and anti-CD8 antibodies, not by anti-CD4 antibody. Furthermore, the A24-4-specific CTL recognized the HCV gene transfected target cells.

CONCLUSIONS

Because this peptide is presented by a common HLA class I molecule, it might be useful for protection against hepatocellular damage and vaccine development in large population of the HCV-infected patients.

A naturally processed rat major histocompatibility complex class I-associated viral peptide as target structure of borna disease virus-specific CD8+ T cells

The first naturally processed peptide synthesized by a virus and recognized by classical CD8(+) T cells in association with the RT1.A(l) major histocompatibility complex class I molecule of the Lewis rat is reported. Borna disease virus-specific CD8(+) T cells recognize syngeneic target cells pulsed with peptides extracted from Borna disease virus-infected cells. The predicted peptide sequence ASYAQMTTY from the viral p40 protein coeluted with the cytotoxic T-lymphocyte-reactive fraction was identified among natural ligands by tandem mass spectrometry. Numerous naturally processed peptides derived from intracellular bacteria, viruses, or tumors and recognized by CD8(+) T cells of man and mice are known, leading to a better understanding of cellular immune mechanisms against pathogens in these two species. In contrast, for the rat little information exists with regard to the function and role of CD8(+) T cells as part of their cellular immune defense system. This first naturally processed viral epitope in the rat contributes to the understanding of the rat cellular immune response and might trigger the identification of more cytotoxic T-lymphocyte epitopes in this animal.

Challenges and opportunities for development of an AIDS vaccine

Among the devastating consequences of AIDS has been its epidemic spread in the developing world. The disease has caused unprecedented suffering, debilitation, loss of life and disruption of family, social and economic stability. Because of the considerable expense and logistical difficulty in providing antiviral drugs to populations infected with the human immunodeficiency virus throughout the world, the biomedical community is looking towards vaccines to help solve this compelling problem.

Induction of cytotoxic T lymphocyte and antibody responses to enhanced green fluorescent protein following transplantation of transduced CD34(+) hematopoietic cells

Genetic modification of hematopoietic stem cells often results in the expression of foreign proteins in pluripotent progenitor cells and their progeny. However, the potential for products of foreign genes introduced into hematopoietic stem cells to induce host immune responses is not well understood. Gene marking and induction of immune responses to enhanced green fluorescent protein (eGFP) were examined in rhesus macaques that underwent nonmyeloablative irradiation followed by infusions of CD34(+) bone marrow cells transduced with a retroviral vector expressing eGFP. CD34(+) cells were obtained from untreated animals or from animals treated with recombinant human granulocyte colony-stimulating factor (G-CSF) alone or G-CSF and recombinant human stem cell factor. Levels of eGFP-expressing cells detected by flow cytometry peaked at 0.1% to 0.5% of all leukocytes 1 to 4 weeks after transplantation. Proviral DNA was detected in 0% to 17% of bone marrow--derived colony-forming units at periods of 5 to 18 weeks after transplantation. However, 5 of 6 animals studied demonstrated a vigorous eGFP-specific cytotoxic T lymphocyte (CTL) response that was associated with a loss of genetically modified cells in peripheral blood, as demonstrated by both flow cytometry and polymerase chain reaction. The eGFP-specific CTL responses were MHC-restricted, mediated by CD8(+) lymphocytes, and directed against multiple epitopes. eGFP-specific CTLs were able to efficiently lyse autologous CD34(+) cells expressing eGFP. Antibody responses to eGFP were detected in 3 of 6 animals. These data document the potential for foreign proteins expressed in CD34(+) hematopoietic cells and their progeny to induce antibody and CTL responses in the setting of a clinically applicable transplantation protocol. (Blood. 2001;97:1951-1959)

A 3'-transcribed region of the HLA-A2 gene mediates posttranscriptional stimulation by IFN-gamma

The expression of several MHC class I genes is up-regulated at the transcriptional level by IFN-gamma. Posttranscriptional mechanisms also have been implicated, but not well characterized. To investigate the mechanism of IFN-gamma stimulation of the human MHC class I gene HLA-A2, several human tumor cell lines were transfected with reporter gene constructs driven by the HLA-A2 promoter. We have previously shown that the extended 525-bp HLA-A2 promoter alone, which includes a 5' IFN-stimulated response element consensus sequence, is not sufficient for IFN-gamma response in either K562 or Jurkat cells. In the current study, stable transfection of a genomic HLA-A2 gene construct, containing both 5'- and 3'-flanking sequences, resulted in stimulation of the gene by IFN-gamma. Nuclear run-on assays revealed that, unlike other class I genes, IFN-gamma stimulation of HLA-A mRNA accumulation occurs almost entirely through posttranscriptional mechanisms. RNA stability assays showed that the effect is not mediated by alteration of the half-life of the HLA-A2 mRNA. Formation of the 3' end was unaffected by IFN-gamma treatment. Sequences that mediate the majority of IFN-gamma induction of HLA-A2 mRNA reside in a 127-bp 3'-transcribed region of the gene. This region contains the terminal splice site, the usage of which is not affected by IFN-gamma treatment. These results demonstrate a novel posttranscriptional mechanism of regulation of MHC class I genes by IFN-gamma.

Embryonic genes in cancer

Over the past several years, great strides have been made in our understanding of the mechanisms and functions of many signalling pathways involved in both development and oncogenesis. It is now clear that some proteins/genes essential for cell differentiation can also provide signals for growth control, suggesting that these genes may be mutated in tumours; this knowledge can help further the search for new targets for chemotherapy. However, many questions about how dysregulation of signalling pathways leads to neoplasia are still unanswered. With the dramatic increase in the number of researchers examining those questions, answers will not be far away.

Mixed allogeneic chimerism and tolerance to composite tissue allografts

The development of effective immunosuppressive drugs has made solid organ allotransplantation the preferred approach for treatment of end-organ failure. The benefits of these immunosuppressants outweigh their risks in preventing rejection of lifesaving solid-organ allografts. On the contrary, composite tissue allotransplants are non-lifesaving and whether the risks of immunosuppressants justify their benefits is a subject of debate. Hence, composite tissue allografts (CTA) have not enjoyed widespread clinical application for reconstruction of large tissue defects. Therefore, a method of preventing rejection that would eliminate the need for toxic immunosuppressants is of particular importance in CTA. Bone marrow transplantation (BMT) to establish mixed chimerism induces tolerance to a variety of allografts in animal models. This article reviews mixed chimerism-based tolerance protocols. Their limitations and their relevance to CTA are discussed, highlighting some unique characteristics (high antigenicity and the presence of active bone marrow) that make CTAs different from solid organ allografts.

A novel technique for the fluorometric assessment of T lymphocyte antigen specific lysis

The 51Cr release assay has traditionally been used to investigate effector cell cytotoxic function against labeled targets, but this method has inherent problems that include hazards associated with radioactivity, cell labeling and high spontaneous release. Here we describe a novel flow cytometric assay which addresses and improves upon the problems currently encountered with the 51Cr release assay. The fluorometric assessment of T lymphocyte antigen specific lysis (FATAL) assay employs dual staining (PKH-26 and CFSE) to identify and evaluate the target population. We found that the PKH-26/CFSE combination efficiently labeled target cells. Evaluation of the spontaneous leakage from dye labeled target cells was forty fold lower than the spontaneous leakage seen with the 51Cr release assay. The FATAL assay permitted a more accurate assessment of the effector: target ratio, and detected low levels of cytotoxic T lymphocyte (CTL) mediated lysis. There was a strong correlation between the 51Cr release and FATAL assays, when performed in parallel with identical effector and target cells (r(2)=0.998, P=<0.0001). This novel method of detecting cytolysis represents a qualitative and quantitative improvement over standard 51Cr release analysis. The FATAL assay will be of value to further investigate mechanisms of cytolysis by effector cell populations.

Immunotherapy for colorectal cancer

Immunologic approaches to therapy for colorectal cancer have evolved substantially. In the past, patients were treated with nonspecific immune stimulants such as bacillus Calmette-Guérin (BCG). The current focus lies in targeting tumor-associated antigens. This is done either through passive immune therapy, with antibodies targeted directly to tumor cells, or by active immune therapy through vaccination with tumor cells, tumor cell lysates, peptides, carbohydrates, gene constructs encoding proteins, or anti-idiotype antibodies that mimic tumor-associated antigens. These different approaches to immunotherapy are reviewed.

Cross-reactive, cell-mediated immunity and protection of chickens from lethal H5N1 influenza virus infection in Hong Kong poultry markets

In 1997, avian H5N1 influenza virus transmitted from chickens to humans resulted in 18 confirmed infections. Despite harboring lethal H5N1 influenza viruses, most chickens in the Hong Kong poultry markets showed no disease signs. At this time, H9N2 influenza viruses were cocirculating in the markets. We investigated the role of H9N2 influenza viruses in protecting chickens from lethal H5N1 influenza virus infections. Sera from chickens infected with an H9N2 influenza virus did not cross-react with an H5N1 influenza virus in neutralization or hemagglutination inhibition assays. Most chickens primed with an H9N2 influenza virus 3 to 70 days earlier survived the lethal challenge of an H5N1 influenza virus, but infected birds shed H5N1 influenza virus in their feces. Adoptive transfer of T lymphocytes or CD8(+) T cells from inbred chickens (B(2)/B(2)) infected with an H9N2 influenza virus to naive inbred chickens (B(2)/B(2)) protected them from lethal H5N1 influenza virus. In vitro cytotoxicity assays showed that T lymphocytes or CD8(+) T cells from chickens infected with an H9N2 influenza virus recognized target cells infected with either an H5N1 or H9N2 influenza virus in a dose-dependent manner. Our findings indicate that cross-reactive cellular immunity induced by H9N2 influenza viruses protected chickens from lethal infection with H5N1 influenza viruses in the Hong Kong markets in 1997 but permitted virus shedding in the feces. Our findings are the first to suggest that cross-reactive cellular immunity can change the outcome of avian influenza virus infection in birds in live markets and create a situation for the perpetuation of H5N1 influenza viruses.

Acute graft-vs-host disease: pathobiology and management

Acute graft-vs-host disease (GVHD) is a major obstacle to safe allogeneic hematopoietic stem cell transplantation (HSCT), leading to a significant morbidity and mortality. GVHD occurs when transplanted donor T lymphocytes react to foreign host cells. It causes a wide variety of host tissue injuries. This review focuses on the pathobiological basis, clinical aspects, and current management strategies of acute GVHD. Afferent phase of acute GVHD starts with myeloablative conditioning, i.e., before the infusion of the graft. Total-body irradiation (TBI) or high-dose chemotherapy regimens cause extensive damage and activation in host tissues, which release inflammatory cytokines and enhance recipient major histocompatibility complex (MHC) antigens. Recognition of the foreign host antigens by donor T cells and activation, stimulation, and proliferation of T cells is crucial in the afferent phase. Effector phase of acute GVHD results in direct and indirect damage to host cells. The skin, gastrointestinal tract, and liver are major target organs of acute GVHD. Combination drug prophylaxis in GVHD is essential in all patients undergoing allogeneic HSCT. Steroids have remained the standard for the treatment of acute GVHD. Several clinical trials have evaluated monoclonal antibodies or receptor antagonist therapy for steroid-resistant acute GVHD, with different successes in a variety of settings. There are some newer promising agents like mycophenolate mofetil, glutamic acid-lysine-alanine-tyrosine (GLAT), rapamycin, and trimetrexate currently entering in the clinical studies, and other agents are in development. Future experimental and clinical studies on GVHD will shed further light on the better understanding of the disease pathobiology and generate the tools to treat malignant disorders with allogeneic HSCT with specific graft-vs-tumor effects devoid of GVHD.

Cellular immunity against Theileria parva and its influence on parasite diversity

Theileria parva, a tick-borne parasite of African cattle, causes a fatal disease known as East Coast fever. Cattle that recover from the disease develop strong parasite-specific MHC-class I-restricted cytotoxic T-lymphocyte responses. Protection can be transferred between immune and naïve calves in the CD8+ T cell fraction emanating from a responding lymph node. In vitro studies suggest that this response requires input from activated CD4+ T cells. The T parva life cycle involves developmental stages in mammalian and tick hosts and can lead to a number of different endemic scenarios for the disease. These range from a stable situation with high prevalence of herd infection, but low fatality rates, to a low prevalence/high fatality scenario. The impact on endemic stability is an important consideration for the design of vaccine implementation strategies. For subunit vaccines targeted at T parva schizonts, the principal issue in this regard is whether development of the piroplasm stage is blocked by immunity.

Transplantation tolerance and autoimmunity after xenogeneic thymus transplantation

Successful grafting of vascularized xenografts (Xgs) depends on the ability to reliably induce both T cell-independent and -dependent immune tolerance. After temporary NK cell depletion, B cell suppression, and pretransplant infusion of donor Ags, athymic rats simultaneously transplanted with hamster heart and thymus Xgs developed immunocompetent rat-derived T cells that tolerated the hamster Xgs but provoked multiple-organ autoimmunity. The autoimmune syndrome was probably due to an insufficient development of tolerance for some rat organs; for example, it led to thyroiditis in the recipient rat thyroid, but not in simultaneously transplanted donor hamster thyroid. Moreover, grafting a mixed hamster/rat thymic epithelial cell graft could prevent the autoimmune syndrome. These experiments indicate that host-type thymic epithelial cells may be essential for the establishment of complete self-tolerance and that mixed host/donor thymus grafts may induce T cell xenotolerance while maintaining self-tolerance in the recipient.

Flow cytometric analysis of peptide binding to major histocampatibility complex class I for hepatitis C virus core T-cell epitopes

BACKGROUND/METHODS

To characterize the repertoire of T-cell epitopes on the hepatitis C virus (HCV) core protein, we studied major histocompatibility complex (MHC) class I binding of 75 decapeptides on 20 human B-cell lines and murine spleen cells using a flow cytometric assay. The results were compared with MHC class I stabilization on T2 cells, the SYFPEITHI algorithm, and known T-cell epitopes from the literature.

RESULTS

Binding of peptides proved to be specific for MHC class I molecules. We observed peak fluorescence signals at positions amino acids (aa) 35-44, aa 87-96, aa 131-140, and aa 167-176 in virtually all HLA-A2-positive cell lines. These sites corresponded to T-cell epitopes predicted by SYFPEITHI and the positions of known T-cell epitopes, whereas T2 stabilization was at variance for two peptides. The assay was applied to HLA-A2-negative cells and murine spleen cells without further modification, and identified additional peptides, corresponding to known T-cell epitopes.

CONCLUSIONS

Peptide binding to different MHC class I alleles can be mapped rapidly by a flow cytometric assay and enables a first orientation on the sites of possible T-cell epitopes. Application of this assay to HCV core suggests a rather limited repertoire of epitopes in the Caucasoid population.

Coordinate downregulation of multiple MHC class I antigen processing genes in chemical-induced murine tumor cell lines of distinct origin

In murine tumor cell lines, downregulation of MHC class I surface expression has been frequently detected, but the underlying molecular mechanisms of such deficiencies have not been defined. In this study, murine tumor cell lines of different histology derived from spontaneous or from chemical-induced tumors were analyzed for the expression of multiple components of the major histocompatibility complex (MHC) class I antigen-processing machinery (APM), including the peptide transporter TAP, the interferon (IFN)-gamma inducible proteasome subunits and several chaperones. The tumor cell lines analyzed demonstrated a heterogeneous expression pattern of various APM components. In comparison to control cells an impaired coordinated expression of at least three APM components was detected. In particular, extensive APM deficiencies were found in cell lines derived from chemical-induced tumors. A strong coordinated downregulation of expression and/or function of TAP, the low molecular weight proteins (LMP) subunits, the proteasome activator PA28 and/or tapasin was found in 5 of 10 tumor cells, which was associated with impaired MHC class I surface expression. In contrast, the expression of beta2-microglobulin (beta2-m), PA28beta, the constitutive proteasome subunits X, Y, Z and of the chaperones calnexin, calreticulin, ER60 and phospho disulfide isomerase (PDI) was unaltered or only weakly decreased. The deficient expression of APM components could be corrected by IFN-gamma treatment, which also reconstituted MHC class I surface expression. However, impaired expression of APM molecules appears not to be the only cause of abnormal MHC class I expression, since it could neither be corrected by the addition of exogeneous MHC class I binding peptides nor by incubation at low temperature. These results suggest that one major mechanism of murine tumor cells, in particular chemical-induced tumors, to evade the immune system is the combined dysregulation of various APM components and other factors, which still have to be identified.

Antigen-processing machinery breakdown and tumor growth

Defects in the major histocompatibility complex (MHC) class I antigen-processing machinery (APM) have been described in tumors of different histology. Murine data suggest that defects in the MHC class II APM might also be associated with malignant transformation of human cells. This article describes the pathophysiology of the MHC class I and II APM, reviews APM abnormalities in tumor cells and discusses their role in the escape of tumor cells from in vitro recognition by T cells.

Vigorous peripheral blood cytotoxic T cell response during the acute phase of hepatitis C virus infection

After infection by hepatitis C virus (HCV), a minority of patients develop acute symptomatic disease and some of them are able to clear the virus. In this study, we analyzed peripheral blood mononuclear cells from nine patients with acute symptomatic disease with respect to their cytotoxic T lymphocyte (CTL) response using a panel of HCV-derived peptides in a semiquantitative secondary in vitro culture system. We could detect early CTL responses in 67% of these patients. The CTL responses were directed against multiple viral epitopes, in particular within the structural (core 2-9, core 35-44, core 131-140, and core 178-187) and nonstructural regions of the virus (NS3 1073-1081, NS3 1406-1415, NS4 1807-1816, NS5 2252-2260, and NS5B 2794-2802). We compared the CTL responses displayed by recently and chronically infected HLA-A2-positive patients. Virus-specific CTLs were detectable in chronic carriers but the percentage of positive peptide-specific CTL responses was significantly higher in recently infected patients (P = 0.002). Follow-up of recently infected patients during subsequent disease development showed a significant decrease in the values and proportions of positive peptide-specific CTL responses (P = 0.002 and 0.013, respectively). Patients with limited viral replication exhibited significantly more vigorous early responses (P = 0.024). These data suggest a protective role for the early antiviral CTL response in HCV infection.

Accessing complexity: the dynamics of virus-specific T cell responses

The cellular dynamics of the immune system are complex and difficult to measure. Access to this problematic area has been greatly enhanced by the recent development of tetrameric complexes of MHC class I glycoprotein + peptide (tetramers) for the direct staining of freshly isolated, antigen-specific CD8(+ )T cells. Analysis to date with both naturally acquired and experimentally induced infections has established that the numbers of virus-specific CD8(+) T cells present during both the acute and memory phases of the host response are more than tenfold in excess of previously suspected values. The levels are such that the virus-specific CD8(+) set is readily detected in the human peripheral blood lymphocyte compartment, particularly during persistent infections. Experimentally, it is now possible to measure the extent of cycling for tetramer (+)CD8(+) T cells during the acute and memory phases of the host response to viruses. Dissection of the phenotypic, functional, and molecular diversity of CD8(+) T cell populations has been greatly facilitated. It is hoped it will also soon be possible to analyze CD4(+) T cell populations in this way. Though these are early days and there is an enormous amount to be done, our perceptions of the shape of virus-specific cell-mediated immunity are changing rapidly.

Human papillomavirus type 33 E7 peptides presented by HLA-DR*0402 to tumor-infiltrating T cells in cervical cancer

Several characteristics make human papillomavirus (HPV) amenable to vaccination. Anti-HPV-directed vaccines are based on the observation that HPV E6 and E7 oncoproteins are constitutively expressed in HPV-positive cervical cancer and may serve as tumor rejection antigens. Five HPV types (16, 18, 31, 33, and 45) account for 80% of cervical cancer. Until now, the type of immune response capable of mediating an effective antitumor response has not been defined. In order to define the anticancer-directed immune response in situ, we characterized CD4(+) and CD8(+) sorted T cells from peripheral blood lymphocytes, freshly harvested tumor tissue, and tumor-infiltrating lymphocytes (TIL) from a patient with cervical cancer. The HLA-DR-restricted CD4(+) T-cell receptor VB16-, VA10-, VA21-, and VA22-positive CD4(+) T-cell line derived from TIL recognizes autologous HLA-DR*0402(+) (HPV33(+)) cervical cancer cells, as determined by gamma interferon secretion. Testing of different peptides spanning the E7 gene revealed that the HPV33(73-87) peptide ASDLRTIQQLLMGTV represents the immunodominant epitope which can also be presented by the DR*0401 allele to TIL. Such major histocompatibility complex class II-presented peptides represent attractive candidates to augment T-cell responses directed against autologous tumor cells.

There is only one immune system! The view from immunopathology

The generators of B and T cell diversity produce specificities for both autochthonous and exogenous paratopes. A wide variety of positive and negative, central and peripheral mechanisms has evolved to regulate the immune response. All potential immunogens are recognized by the system using the same set of 'rules', without discrimination between 'self' and 'nonself' or between the 'toxic' and the 'benign'. In every response, whether positive or negative, the factors mobilized and the balance between protection and damage depend upon the quality, quantity, location, and timing of immunogen presentation, as well as upon properties of the host.

FTY720 immunosuppression impairs effector T cell peripheral homing without affecting induction, expansion, and memory

FTY720 (2-amino-2-(2-[4-octylphenyl]ethyl)-1,3-propanediol hydrochloride) prolongs survival of solid organ allografts in animal models. Mechanisms of FTY720 immunomodulation were studied in mice infected with lymphocytic choriomeningitis virus (LCMV) to assess T cell responses or with vesicular stomatitis virus to evaluate Ab responses. Oral FTY720 (0.3 mg/kg/day) did not affect LCMV replication and specific CTL and B cells were induced and expanded normally. Moreover, the anti-viral humoral immune responses were normal. However, FTY720 treatment showed first a shift of overall distribution of CTL from the spleen to peripheral lymph nodes and lymphocytopenia was observed. This effect was reversible within 7-21 days. Together with unimpaired T and B cell memory after FTY720 treatment, this finding rendered enhancement of lymphocyte apoptosis by FTY720 in vivo unlikely. Secondly, the delayed-type hypersensitivity reaction to a viral MHC class I-presented peptide was markedly reduced by FTY720. These results were supported by impaired circulation of LCMV specific TCR transgenic effector lymphocytes in the peripheral blood and reduced numbers of tissue infiltrating CTL in response to delayed-type hypersensitivity reaction. Thirdly, in a CD8+ T cell-mediated diabetes model in a transgenic mouse expressing the LCMV glycoprotein in the islets of the pancreas, FTY720 delayed or prevented disease by reducing islet-infiltrating CTL. Thus, FTY720 effectively reduced recirculation of CD8+ effector T cells and their recruitment to peripheral lesions without affecting the induction and expansion of immune responses in secondary lymphoid organs. These properties may offer the potential to treat ongoing organ-specific T cell-mediated immunopathologic disease.

Prospects for vaccination in prostate cancer

Prostate cancer continues to be a major cause of morbidity and mortality. Surgical and medical management of the disease has improved but there is little effect upon overall survival. Novel therapies such as immunotherapy offer the potential of treating the disease by stimulating the immune system to mount a response to the cancer systemically. This review highlights the pitfalls and progress in the understanding of the role of the immune system in malignancy and also explains the theory behind prostate cancer vaccine trials.

Defining parameters for successful immunocytotherapy of persistent viral infection

Persistent infections with viruses such as HIV, Epstein-Barr virus, cytomelagovirus, and hepatitis B and C viruses continue to be major human health problems. Immunocytotherapy for persistent viral infections has proven successful in animal models but less effective in humans. While the requirement of antigen-specific CD8(+) T cells is known, the precise role of CD4(+) T cells as regards specific priming, numbers needed, and interaction with CD8(+) T cells is less clear. To address these issues, we used a mouse model of persistent virus infection in which adoptive transfer of T cells effectively purges virus from all tissues. We demonstrate that (1) inclusion of antigen-specific CD4(+) in addition to CD8(+) T cells is mandatory for efficient and long-term virus control. Neither naive nor CD4(+) T cells with specificity for a different virus are sufficient. (2) The minimal numbers of virus-specific T cells required for virus clearance from sera and tissues are 350,000 virus-specific CD8(+) and 7000 virus-specific CD4(+) T cells or approximately 5 x 10(7) CD8(+) and as few as 1 x 10(6) CD4(+) T cells per square meter of body surface area, a CD8:CD4 ratio of 50:1. (3) Production of interferon-gamma, obligatory for resolution of persistent infection, is dependent on the interaction of virus-specific CD4(+) and CD8(+) T cells. (4) Maintenance of CD8(+) T cell effector functions after adoptive transfer is directly proportional to the amount of cotransferred, virus-specific CD4(+) T cells.

Liver transplant recipient sera derived soluble HLA mediates allele specific CTL apoptosis

BACKGROUND

Significant levels of donor soluble human leukocyte antigen (HLA) class I (sHLA) are present in patients after transplants. We investigated the possibility that sHLA may inhibit cytolytic T lymphocyte (CTL) activity by inducing apoptosis of the CTL, thereby serving as a mechanism for specific tolerance.

METHODS

sHLA-A2 and A3 were isolated from the sera of liver transplant recipients by affinity chromatography. T cell bulk lines directed against HLA-A2 and HLA-A3 were generated by stimulation with HLA-A2, A3+ peripheral blood leukocytes and B-lymphoblastoid cells. Induction of T cell apoptosis by sHLA was analyzed by adding sHLA to allospecific CTL 4 or for 24 hr before flow cytometric analysis of propidium iodide and fluorescein isothiocyanate-conjugated annexin V stained cells. T cell receptor (TCR) engagement by sHLA was demonstrated using a monoclonal antibody specific for the TCR.

RESULTS

sHLA-A3 inhibited CTL activity of a HLA-A3 T cell line by 53%, whereas sHLA-A2 had no effect. sHLA-A3 also increased T cell death by 77% over the control, whereas sHLA-A2 had no significant effect. However, sHLA-A2 induced 21% apoptosis of an anti-HLA-A2 T cell line, whereas sHLA-A3 caused only 3% apoptosis. The antibody complexed form of sHLA was ineffective in the induction of apoptosis. Preincubation of the T cells with anti-T cell receptor monoclonal antibody protected the T cells from sHLA-induced apoptosis, indicating that sHLA-TCR engagement is necessary for this process to occur.

CONCLUSION

TCR-mediated apoptosis of alloreactive CTL may serve as a mechanism by which sHLA can modulate the immune response.

A CTL assay requiring only 150 microliter of mouse blood

In this paper, we present a method for measuring antigen specific cytotoxic T lymphocyte (CTL) activity from individual mouse peripheral blood samples without animal sacrifice. Peripheral blood cells are stimulated in vitro with a cocktail of antigen, cytokines, costimulatory molecules and irradiated feeder cells resulting, 7 days later, in a readily detectable antigen specific signal from a well plated under limiting dilution conditions. This highly sensitive and antigen specific assay is more efficient than conventional CTL assays and thus increases the number of mice that can be tested in a single assay. Since blood samples can be assayed from an individual mouse at multiple times during the course of an in vivo study, the assay can facilitate and strengthen correlative studies on CTL responses and in vivo results.

Activated and memory CD8+ T cells can be distinguished by their cytokine profiles and phenotypic markers

Dissecting the mechanisms of T cell-mediated immunity requires the identification of functional characteristics and surface markers that distinguish between activated and memory T lymphocytes. In this study, we compared the rates of cytokine production by virus-specific primary and memory CD8+ T cells directly ex vivo. Ag-specific IFN-gamma and TNF-alpha production by both primary and long-term memory T cells was observed in </=60 min after peptide stimulation. Although the on-rate kinetics of cytokine production were nearly identical, activated T cells produced more IFN-gamma, but less TNF-alpha, than memory T cells. Ag-specific cytokine synthesis was not a constitutive process and terminated immediately following disruption of contact with peptide-coated cells, demonstrating that continuous antigenic stimulation was required by both T cell populations to maintain steady-state cytokine production. Upon re-exposure to Ag, activated T cells resumed cytokine production whereas only a subpopulation of memory T cells reinitiated cytokine synthesis. Analysis of cytokine profiles and levels of CD8, LFA-1, and CTLA-4 together revealed a pattern of expression that clearly distinguished in vivo-activated T cells from memory T cells. Surprisingly, CTLA-4 expression was highest at the early stages of the immune response but fell to background levels soon after viral clearance. This study is the first to show that memory T cells have the same Ag-specific on/off regulation of cytokine production as activated T cells and demonstrates that memory T cells can be clearly discriminated from activated T cells directly ex vivo by their cytokine profiles and the differential expression of three well-characterized T cell markers.

T(h)1 but not T(h)0 cell help is efficient to induce cytotoxic T lymphocytes by immunization with short synthetic peptides

Immunization of BALB/c mice with peptide HVSGHRMAWDMMMNWA, encompassing residues 121-135 from hepatitis C virus E1 protein, induced CD4(+) T(h)1 cells as well as a long-lasting CD8(+) cytotoxic T lymphocyte (CTL) response in vivo when the peptide was administered s.c. with or without incomplete Freund's adjuvant. Using truncated peptides from this sequence it was shown that the determinant recognized by cytotoxic T cells was encompassed by residues SGHRMAWDM. Deletion of residues from the N-terminus or the C-terminus of the wild-type peptide abrogated its helper character. When Val122 of the wild peptide was replaced by Ala, the ability to induce a cytotoxic response was lost concomitantly with the loss of the T(h)1 pattern of cytokine production. Interestingly, the Ala-modified peptide, when co-immunized with a peptide encompassing residues 323-329 from ovalbumin (OVA), which is able to induce a T(h)1 response in BALB/c mice, restored the capacity of the modified peptide to induce CTL. However, co-immunization of the Ala-modified peptide with a peptide encompassing residues 106-118 from sperm whale myoglobin, which induces a T(h)0 cytokine profile in BALB/c mice, was much less efficient than the OVA peptide to restore CTL induction. These results demonstrate that CTL induction with a short synthetic peptide requires that this peptide contains domains recognized by T(c) cells as well as by T(h)1 cells. For those peptides that do not contain this type of T(h) domain, competent T cell help can be provided by co-immunization with a distinct peptide that is able to stimulate a T(h)1 response.

Cell-mediated graft rejection observed in two lines of human histocompatibility leukocyte antigen class I transgenic mice

BACKGROUND

Human histocompatibility leukocyte antigen (HLA) class I molecules are essential for graft rejection. However, to determine the specific role of these molecules in clinical situations is difficult. We investigated the applicability of HLA class I transgenic mice (C3H.B35 and C3H.B51) for elucidation of the role of HLA class I molecules.

METHODS

Skin or heart grafts were transplanted. Cytotoxic T cells (CTL) of C3H.B51 against C3H.B35 were generated and their cytotoxicity against various transfectant cell lines was determined.

RESULTS

C3H.B35 skin and heart grafted to C3H.B51 were rejected within 17 and 28 days, respectively. Cytotoxic T cells generated from C3H.B51 showed cytotoxicity against a HLA-B*3501-transfectant cell line that did not express H-2 molecule, which indicates that these cytotoxic T cells recognize HLA-B35 molecules directly without H-2 restriction.

CONCLUSION

Our results suggest that C3H.B51 recognize C3H.B35 grafts as allo-MHC class I-incompatible grafts, and these mice are valuable to elucidate the role of HLA class I molecules in transplantation.

Immune response enhancement by in vivo administration of B7.2Ig, a soluble costimulatory protein

The identification of both class I- and class II-restricted tumor-associated peptides recognized by T cells has led to the test of these peptides as immunogens in experimental immunotherapy for cancer patients. However, optimal T cell activation requires signaling both through the T cell receptor for antigen and through costimulatory pathways. B7.1 and B7.2 are powerful costimulatory molecules expressed on the surface of antigen-presenting cells. Using a mouse model, we have sought to optimize costimulatory signals during antipeptide responses by administering a soluble form of B7.2 at the time of peptide immunization. Administration of B7. 2Ig fusion protein significantly enhanced T helper cell and CTL responses. These findings suggest that soluble forms of human B7.2 protein may provide a straightforward and practical method of supplying optimal costimulation during clinical immunotherapy.

The viral clearance in interferon-treated chronic hepatitis C is associated with increased cytotoxic T cell frequencies

BACKGROUND/AIMS

Cytotoxic T lymphocytes have been demonstrated in peripheral blood and liver tissue of patients with chronic hepatitis C virus infection, but their significance for viral clearance is unknown. Therefore, we analyzed hepatitis C virus-specific cytotoxic T lymphocyte precursor frequencies in chronic hepatitis C virus carriers during interferon-alpha treatment.

METHODS

Blood mononuclear cells or CD8+ T cells from HLA-A2 positive and negative patients and controls were analyzed in chromium-release assays using a panel of 18 synthetic peptides from the HCV core, E1 and NS4 antigens bearing HLA-A2 binding motifs. Specific cytotoxic T lymphocyte precursor frequencies were studied within CD8+ T cells derived from interferon-alpha-treated patients using a TNF-alpha-based ELISPOT assay and compared to viremia levels.

RESULTS

T cells from 16 of 24 HLA-A2+ but none of the six HLA-A2- patients with chronic hepatitis C and six HLA-A2+ healthy controls lysed targets pulsed with peptide cocktails. Fine specificity revealed four very immunogenic epitopes in the core (C36-44, C132-140) and the envelope regions (E332-340, E363-372). Cytotoxic T lymphocyte precursor frequencies were prospectively analyzed in 11 interferon-alpha-treated HLA-A2+ hepatitis C virus patients. Four sustained and two transient therapy responders showed lower pretreatment viremia levels and significantly higher specific cytotoxic T lymphocyte precursor frequencies during viral clearance compared to five therapy non-responders and untreated controls.

CONCLUSIONS

The quantitative induction of HLA-class I restricted responses by interferon-alpha could contribute to a beneficial outcome of hepatitis C virus infections. Furthermore, it appears that the balance between viral load and specific cellular immune responses is critical for successful viral clearance.

Positive and negative selection of alphabetaTCR+ T cells in thymectomized adult radiation bone marrow chimeras

BACKGROUND

The mature T-cell repertoire is characterized by the negative selection of potentially autoreactive T cells and the positive selection of T cells restricted to antigen-recognition in the context of self-MHC molecules. It is currently believed that the thymus is critical for these selection events. Although alpha(beta)T cell receptor (TCR)+ T cells have been reported in thymectomized recipients, whether this represents clonal expansion of residual T cells or de novo generation of new T cells in the absence of a thymus has not been definitively evaluated.

METHODS

In the current study, development of the T cell repertoire was evaluated in adult radiation bone marrow chimeras prepared after complete surgical thymectomy.

RESULTS

CD4+ and CD8+ T cells were present and exhibited donor-specific TCR-Vbeta expression and self-tolerance, indicative of negative selection. Positive selection was confirmed with the demonstration of host MHC restriction and the presence of donor-derived CD8+ T cells after the transplantation of marrow from Class I deficient donors into normal recipients.

CONCLUSIONS

These data provide evidence, for the first time, that the development of a functional T-cell repertoire can occur in adult recipients without the thymic microenvironment.

Hypervariable epitope construct: a synthetic immunogen that overcomes MHC restriction of antigen presentation

Vaccines are not universal in their ability to induce favorable immune responses in all individuals because the major histocompatibility complex (MHC) molecules needed for presentation of vaccine components to T cells are limited in the peptides they recognize and bind. A heterogeneous cocktail of related peptides synthesized simultaneously and representing amino acids 414-434 of the SIV envelope protein was used to induce immune responses stronger than those induced by a single T cell peptide synthesized conventionally and representing the same region of the viral envelope. The heterogeneous peptide mixture called a hypervariable epitope construct (HEC) was capable of overcoming MHC restriction in peptide presentation in four different inbred mouse strains, including a strain that was a poor responder to the AA 414-434 single sequence peptide (SSP). HEC induced proliferation responses 15 times better than those induced by SSP. Antibodies elicited by HEC but not SSP immunization effectively bind viral antigen. The 414-434 HEC and the 414-434 SSP were also tested for their ability to upregulate the expression of MHC class I molecules on the surface of the mutant RMA-S murine cell line. Surface display of MHC molecules was measured by confocal microscopy followed by calculation of fluorescence intensity of images. HECs upregulated expression of MHC molecules 30% more than SSP peptides. Our findings suggest that HEC cocktails could be effective components of subunit vaccines to help overcome the unresponsiveness observed in outbred animals and in humans as a result of MHC-restricted antigen presentation.

Poxvirus-Encoded Serpins Do Not Prevent Cytolytic T Cell-Mediated Recovery from Primary Infections

Previous observations that the highly conserved poxvirus-encoded serpins inhibit cytotoxic activities of alloreactive CTL via granule and/or Fas-mediated pathways was taken to indicate their involvement in immune evasion by poxviruses. We now show that interference with 51Cr release from target cells by ectromelia and cowpoxvirus is limited to alloreactive but not MHC-restricted CTL. The data are in support of the paramount importance of CTL and its effector molecule perforin in the recovery from primary ectromelia virus infection and question the role of serpins in the evasion of poxviruses from killing by CTL. Further analysis of poxvirus interference with target cell lysis by alloreactive CTL revealed that suppression primarily affects the Fas-mediated, and to a lesser extent, the granule exocytosis pathway. Serpin-2 is the main contributor to suppression for both killing pathways. In addition, inhibition of lysis was shown to be both target cell type- and MHC allotype-dependent. We hypothesize that differences in TCR affinities and/or state of activation between alloreactive and MHC-restricted CTL as well as the quality (origin) of target cells are responsible for the observed phenomenon.

Selection and Long-Term Persistence of Reactive CTL Clones During an EBV Chronic Response Are Determined by Avidity, CD8 Variable Contribution Compensating for Differences in TCR Affinities

Recent studies have suggested that the diversity of TCR repertoire after primary immunization is conserved in memory T cells and that a progressive narrowing of this repertoire may take place during recall infections. It now remains to be investigated which parameters determine the repertoire of the memory response and possibly restrict its diversity after subsequent antigenic challenges. To address this question, we took advantage of a panel of CD8+ T cell clones from the joint of a rheumatoid arthritis patient and selected for their reactivity against a single MHC/peptide complex. Characterization of both TCR chains documented a great diversity among those clones and the persistence of clonotypes over a 2-yr period. Strikingly, despite the observed repertoire heterogeneity, all clones displayed a narrow range of MHC/peptide density requirements in cytotoxicity assays (ED50 between 9 and 36 nM). TCR affinities were then indirectly estimated by blocking CD8 interaction with an anti-CD8 mAb. We found a wide range of TCR affinities among the different clonotypes that segregated with Vβ usage. We thus propose that during an in vivo chronic response, a narrow range of avidity of the TCR-CD8 complex conditions long-term clonotype persistence, and that the level of CD8 contribution is adjusted to keep clonotypes with variable TCR affinities within this avidity window.