Protein-specific cytotoxic T lymphocytes. Recognition of transfectants expressing intracellular, membrane-associated or secreted forms of beta-galactosidase

Granulocyte-Macrophage Colony-Stimulating Factor-Based Melanoma Cell Vaccines Immunize Syngeneic and Allogeneic Recipients via Host Dendritic Cells

Subcutaneous injection of GM-CSF-expressing cancer cells into experimental animals results in protective cancer immunity. To delineate the mode of action of such vaccines, we used trinitrophenyl, the antigenic moiety of the contact allergen trinitrochlorobenzene, as surrogate Ag. Trinitrophenyl-derivatized bone marrow-derived dendritic cells were found to elicit a contact hypersensitivity response in syngeneic, but not in allogeneic recipients, compatible with their expected mode of direct Ag presentation. When expressing GM-CSF, haptenized M3 melanoma cells were also able to induce a contact hypersensitivity response but, in contrast to bone marrow-derived dendritic cells, not only in syngeneic but also in allogeneic recipients. This argues for a critical role of host APC. To identify their nature, we introduced the beta-galactosidase (betagal) gene into M3-GM cells. Their administration activated betagal-specific, L(d)-restricted CTL in syngeneic BALB/c mice. Evaluation of lymph nodes draining M3-GM-betagal injection sites revealed the presence of cells presenting the respective L(d)-binding betagal peptide epitope. Based on their capacity to activate betagal-specific CTL, they were identified as being CD11c(+) dendritic cells. These experiments provide a rational basis for the use of GM-CSF-based melanoma cell vaccines in an allogeneic setting.

A novel reporter strain to follow Cre-mediated recombination in T and NK cells

The Cre-loxP system permits the generation of mouse models in which the fate of a cell can be followed through time. Such approach is of great value in immunology because it may allow lineage studies and the dissection of the contribution of specific effector T cells to long-term memory responses or autoimmune responses. An essential component of such a strategy is the development of appropriate reporter strains of mice in which the inducible reporter molecule is not immunogenic and is well expressed at the cell surface of T cells. We describe here a novel reporter strain of mice that is designed to fulfill these criteria and show that this strain permits the monitoring of Cre-mediated recombination in both T cells and NK cells.

Foreign antigenic peptides delivered to the tumor as targets of cytotoxic T cells

Cytotoxic T cells (CTL) are readily activated by immunogenic peptides and they exert potent anti-tumor activity if the same peptides are displayed on class I major histocompatibility complex (MHC) of the tumor cells. A handful of tumor-associated antigens have been identified and many of them are weak antigens. As an alternative strategy, strongly antigenic foreign peptides are delivered to the tumor, marking them for CTL recognition. To establish the principle of this new strategy, in vitro and in vivo tumor destruction was tested with BALB/c CTL to L(d)-associated beta-galactosidase (beta-gal) peptide p876. In vitro, anti-p876 CTL destroyed tumor cells in a single-cell suspension or in 3-D tumor boluses when exogenous p876 was added. Exogenous IL-2 was required to sustain CTL activity for complete destruction of tumor boluses. In vivo, BALB/c mice were immunized with p876 and a CD4 activating Pan DR reactive epitope (PADRE). PADRE, which binds to several different MHC class II antigen and activates CD4 T cells, induced delayed-type hypersensitivity and stimulated T cell proliferation. Immunized mice were injected with tumor cells loaded with p876 and mixed with PADRE. Starting from the day after tumor injection, mice received five rounds of peptide injection at the tumor sites and all tumors were rejected. Injection with saline had no effect. Injection with PADRE had minor anti-tumor activity. Immunization and treatment with p876 alone was not protective. Therefore, by delivering CD4 and CD8 reactive foreign peptides to the tumor, peptide-specific T cells rejected the tumors as demonstrated by the in vitro and in vivo tests.

Measles virus and canine distemper virus target proteins into a TAP-independent MHC class I-restricted antigen-processing pathway

After infection of CEM174.T2 cells [deficient for the transporter of antigen presentation (TAP)] with measles virus (MV) the nucleocapsid protein is recognized by L(d)-restricted cytotoxic T cells in a TAP-independent, chloroquine-sensitive fashion. Presentation via the TAP-independent pathway requires virus replication. During MV infection of the cell the nucleocapsid as well as the matrix protein enter the endolysosomal compartment as indicated by colocalization with the lysosomal-associated membrane protein 1 (LAMP-1). Similarly, the nucleocapsid protein of canine distemper virus (CDV) is recognized in a TAP-independent fashion. In addition, a recombinant MV expressing bacterial beta-galactosidase protein is able to introduce the recombinant antigen into the TAP-independent pathway whereas a vaccinia virus expressing beta-galactosidase is not. These data and a report about TAP-independent recognition of parainfluenza virus type 1 suggest that members of the Paramyxoviridae family regularly introduce viral proteins into the TAP-independent antigen-processing pathway.

Definition of a novel binding site on CD8 cells for a conserved region of the MHC class Ib molecule Qa-1 that regulates IFN-gamma expression

Natural killer (NK) cells and activated CD8 cells both express cytotoxic activity and produce substantial levels of IFN-gamma in response to viral and bacterial infections. In the case of NK cells, cellular activation and IFN-gamma expression are regulated by an interaction between NK receptors and MHC class Ib molecules, including HLA-E/Qa-1. We have used soluble tetrameric complexes of the murine class Ib molecule Qa-1 to define the significance of this interaction for CD8 cells. We find that all CD8 cells express a receptor for Qa-1 and that ligation of this receptor by Qa-1 results in up-regulation of IFN-gamma production.

Oral delivery of DNA vaccines using attenuated Salmonella typhimurium as carrier

The efficacious delivery of eukaryotic expression plasmids to inductive cells of the immune system constitutes a key prerequisite for the generation of effective DNA vaccines. Here, we have explored the use of bacteria as vehicles to orally deliver expression plasmids. Attenuated Salmonella typhimurium aroA harbouring eukaryotic expression plasmids that encoded virulence factors of Listeria monocytogenes were administered orally to BALB/c mice. Strong cytotoxic and helper T cell responses as well as antibody production were elicited even after a single administration. Mice immunised four times with Salmonella that carried a eukaryotic expression plasmid encoding the secretory listerial protein listeriolysin were protected against a subsequent lethal challenge with this pathogen. A single dose was already partially protective. The efficiency of this vaccination procedure was due to transfer of the expression plasmid from the bacterial carrier to the mammalian host. Evidence for such an event could be obtained in vivo and in vitro. Expression of the desired antigen in various lymphoid tissues was already detectable 1 day after administration of the DNA vaccine and persisted for at least 1 month in spleen and mesenteric lymph nodes. Induction of cytotoxic and helper T cell responses was observed in all mouse strains tested including outbred strains whereas antibodies were mainly detected in BALB/c. Furthermore, we could show that immunogenicity could be improved by increasing the invasiveness of the bacterial carrier.

Transient expression of bacterial gene fragments in eukaryotic cells: implications for CD8(+) T cell epitope analysis

CD8(+) T cells are potent effectors of acquired immunity against some viruses and intracellular bacterial pathogens. Antigens recognized by CD8(+) T cells are small, 8-9 amino acid peptides derived from proteins produced by the pathogen. These peptides are presented by MHC class I molecules on the surface of the infected cell. When characterizing the CD8(+) T cell response to a bacterial or viral pathogen, it is often necessary to express an antigenic protein in a eukaryotic host cell that is capable of processing and presenting peptide epitopes to antigen-specific CD8(+) T cells. We describe a system designed to transiently express bacterial polypeptides and MHC class I molecules in eukaryotic cells. Recognition of these peptide-MHC complexes stimulates TNF production by antigen-specific CD8(+) T cell lines. This system should be useful for analysis of CD8(+) T cell epitope-containing bacterial gene fragments when expression of the entire bacterial protein is detrimental to the eukaryotic cell, or when overexpression of the bacterial gene is detrimental to the bacterial cloning strain. Furthermore, this system can be used for the rapid mapping of CD8(+) T cell epitopes within a protein.

Characterization and manipulation of T cell immunity to skin grafts expressing a transgenic minor antigen

BACKGROUND

Minor histocompatibility antigens play a significant role in allograft rejection when donor and recipient are matched at MHC loci. An improved understanding of T cell immunity directed toward a model minor antigen may provide new approaches for preventing graft rejection.

METHODS

C57BL/6 (B6) recipient mice were engrafted with skin from B6 beta-galactosidase transgenic (beta-gal tg) donors and the induced T cell immune responses were characterized by cytokine ELISA spot assay. beta-gal-specific immunity was manipulated prior to transplant through preinjection with beta-gal in complete Freund's adjuvant (CFA) or through preinjection with soluble beta-gal i.v.

RESULTS

B6 mice rejected beta-gal tg skin by day 25. Rejection was associated with a low frequency of predominantly CD8+, interferon-gamma-producing T cells capable of directly recognizing both beta-gal tg cells and an immunodominant major histocompatibility complex I-restricted peptide derived from the beta-gal protein. Rejection of multiple minor antigen disparate skin and major histocompatibility complex-disparate skin occurred significantly faster, and was associated with a 10- to 30-fold higher frequency of alloreactive T cells, than rejection of beta-gal tg skin. Prepriming of recipients with beta-gal in complete Freund's adjuvant resulted in an increased frequency of beta-gal-specific T cells and accelerated rejection of beta-gal tg skin. Intravenous injection of soluble beta-gal-induced graft tolerance and a lack of detectable beta-gal-specific immunity.

CONCLUSIONS

The findings reveal that transgenically expressed beta-gal behaves as a minor transplantation antigen and that manipulation of the beta-gal-specific T cell repertoire can dramatically affect rejection of beta-gal tg skin grafts. The work provides the foundation for mechanistic studies of tolerogenesis to minor antigenic determinants.

Intracellular Rate-Limiting Steps in MHC Class I Antigen Processing

Quantitative aspects of the endogenous pathway of Ag processing and presentation by MHC class I molecules to CD8+ CTL were analyzed over a wide range of Ag expression in recombinant vaccinia virus-infected cells expressing β-galactosidase as model Ag. Only the amount of starting Ag was varied, leaving other factors unaltered. Below a certain level of Ag synthesis, increasing protein amounts led to a sharp rise in recognition by CTL. Higher levels of Ag expression led to a saturation point, which intracellularly limited the number of naturally processed peptides bound to MHC and thereby also CTL recognition. The rate-limiting step was located at the binding of the antigenic peptide to MHC inside the vaccinia virus-infected cell or before this event.

Reduction in mitochondrial membrane potential is an early event in Fas-independent CTL-mediated apoptosis

Cytotoxic T lymphocytes (CTL) can destroy target cells via the Fas-mediated pathway or the granule-mediated pathway. We used Fas-negative target cells to examine for target-cell reduction in mitochondrial membrane potential (DeltaPsi(m)) induced by intact CTL via the granule-mediated pathway. We find that reduction in DeltaPsi(m) is an early step in Fas-independent CTL killing of target cells that precedes phosphatidyl serine translocation, cytosolic protein release, or loss of plasma membrane integrity. Target-cell reduction in DeltaPsi(m) and cytoplasmic protein release in Fas-independent CTL killing were inhibited by N-carbobenzoxy-Ala-Pro-Phe chloromethyl ketone, but not by caspase inhibitors N-carbobenzoxy-Val-Ala-Asp fluoromethyl ketone (z-VAD-fmk) or N-carbobenzoxy-Asp-Glu-Val-Asp fluoromethyl ketone (z-DEVD-fmk). This contrasts with Fas-mediated apoptosis, in which the reduction in DeltaPsi(m) can be inhibited by z-VAD-fmk or z-DEVD-fmk. Assessing the changes in target-cell DeltaPsi(m) can provide for a sensitive and rapid means with which to monitor CTL activity.

Sialoadhesin-Positive Host Macrophages Play an Essential Role in Graft-Versus-Leukemia Reactivity in Mice

We recently established an effective immune T-cell–mediated graft-versus-leukemia (GVL) murine model system in which complete tumor remissions were achievable even in advanced metastasized cancer. We now describe that this T-cell–mediated therapy is dependent on host macrophages expressing the lymphocyte adhesion molecule sialoadhesin (Sn). Depletion of Kupffer cells in tumor-bearing mice during adoptive immunotherapy (ADI) or the treatment of these animals with anti-Sn monoclonal antibodies led to complete or partial inhibition of the immune T-cell–mediated therapeutic effect. Furthermore, Sn+ host macrophages in livers formed clusters during ADI with donor CD8 T cells. To test for a possible antigen presentation function of these macrophages, we used as an in vitro model the antigen β-galactosidase for which a dominant major histocompatibility complex (MHC) class I Ld-restricted peptide epitope is known to be recognized by specific CD8 cytotoxic T lymphocytes (CTL). We demonstrate that purified Sn+ macrophages can process exogenous β-galactosidase and stimulate MHC class I peptide-restricted CTL responses. Thus, Sn+ macrophages, which are significantly increased in the liver after ADI, may process tumor-derived proteins via the MHC class I pathway as well as via the MHC class II pathway, as shown previously, and present respective peptide epitopes to CD8 as well as to CD4 immune T cells, respectively. The synergistic interactions observed before between immune CD4 and CD8 T cells during ADI could thus occur in the observed clusters with Sn+ host macrophages.

Sialoadhesin-Positive Host Macrophages Play an Essential Role in Graft-Versus-Leukemia Reactivity in Mice

We recently established an effective immune T-cell–mediated graft-versus-leukemia (GVL) murine model system in which complete tumor remissions were achievable even in advanced metastasized cancer. We now describe that this T-cell–mediated therapy is dependent on host macrophages expressing the lymphocyte adhesion molecule sialoadhesin (Sn). Depletion of Kupffer cells in tumor-bearing mice during adoptive immunotherapy (ADI) or the treatment of these animals with anti-Sn monoclonal antibodies led to complete or partial inhibition of the immune T-cell–mediated therapeutic effect. Furthermore, Sn+ host macrophages in livers formed clusters during ADI with donor CD8 T cells. To test for a possible antigen presentation function of these macrophages, we used as an in vitro model the antigen β-galactosidase for which a dominant major histocompatibility complex (MHC) class I Ld-restricted peptide epitope is known to be recognized by specific CD8 cytotoxic T lymphocytes (CTL). We demonstrate that purified Sn+ macrophages can process exogenous β-galactosidase and stimulate MHC class I peptide-restricted CTL responses. Thus, Sn+ macrophages, which are significantly increased in the liver after ADI, may process tumor-derived proteins via the MHC class I pathway as well as via the MHC class II pathway, as shown previously, and present respective peptide epitopes to CD8 as well as to CD4 immune T cells, respectively. The synergistic interactions observed before between immune CD4 and CD8 T cells during ADI could thus occur in the observed clusters with Sn+ host macrophages.

Protection against mammary tumor growth by vaccination with full-length, modified human ErbB-2 DNA

ErbB-2 is overexpressed in several human cancers and conveys a transforming activity that is dependent on tyrosine kinase activity. Antibodies and T cells to ErbB-2 have been isolated from cancer patients, indicating ErbB-2 as a potential target of active vaccination. In this study, 3 mutant ErbB-2 DNA constructs encoding full-length, ErbB-2 proteins were tested as tumor vaccines. To eliminate tyrosine kinase activity, the ATP binding lysine residue 753 was substituted with alanine by replacing codon AAA with GCA in mutant ErbB-2A. To direct recombinant ErbB-2 to the cytoplasm where major histocompatibility complex (MHC) I peptide processing takes place, the endoplasmic reticulum (ER) signal sequence was deleted in cyt ErbB-2. The third construct cyt ErbB-2A contained cytoplasmic ErbB-2 with the K to A mutation. Expression of recombinant proteins was measured by flow cytometry in transfected murine mammary tumor cell line D2F2. Transmembrane ErbB-2 and ErbB-2A were readily detected. Cytoplasmic ErbB-2 and ErbB-2A were detected only after the transfected cells were incubated overnight with a proteasome inhibitor, indicating prompt degradation upon synthesis. ErbB-2 autophosphorylation was eliminated by the K to A mutation as demonstrated by Western blot analysis. Growth of ErbB-2-positive tumor in BALB/c mice was inhibited after vaccination with ErbB-2 or ErbB-2A, but not with cyt ErbB-2 or cyt ErbB-2A. ErbB-2A that is free of tyrosine kinase activity is a potential candidate for anticancer vaccination. The 3 mutant constructs should be useful tools to delineate the role of individual immune effector cell in ErbB-2-specific antitumor immunity and to develop strategies for enhancing such immunity.

Pathogenicity island 2 mutants of Salmonella typhimurium are efficient carriers for heterologous antigens and enable modulation of immune responses

The potential use as vaccine delivery system of Salmonella typhimurium strains harboring defined mutations in the sseC (HH104) and sseD (MvP101) genes, which encode putative effector proteins of the type III secretion system of Salmonella pathogenicity island 2, was evaluated and compared with that of the well-characterized aroA mutant strain SL7207 by using beta-galactosidase (beta-Gal) as a model antigen. When orally administered to immune-competent or gamma interferon-deficient (IFN-gamma-/-) BALB/c mice, both mutants were found to be highly attenuated (50% lethal dose, >10(9) bacteria). Both strains were also able to efficiently colonize and persist in Peyer's patches. Immunization with HH104 and MvP101 triggered beta-Gal-specific serum and mucosal antibody responses equivalent to or stronger than those observed in SL7207-immunized mice. Although immunoglobulin G2 (IgG2) serum antibodies were dominant in all groups, IgG1 was also significantly increased in mice vaccinated with MvP101 and SL7207. Comparable beta-Gal-specific IgA and IgG antibodies were detected in intestinal lavages from mice immunized with the different strains. Antigen-specific CD4(+) T-helper cells were generated after vaccination with all vaccine prototypes; however, responses were significantly more efficient when HH104 and MvP101 were used (P < 0.05). Significantly higher levels of IFN-gamma were produced by restimulated spleen cells from mice immunized with HH104 than from those vaccinated with the MvP101 or SL7207 derivatives (P </= 0.05). Interestingly, the three strains induced major histocompatibility complex class I-restricted CD8(+) cytotoxic T cells against beta-Gal; however, cytotoxic T-lymphocyte responses were significantly stronger after immunization with HH104 (P < 0.05). These novel S. typhimurium attenuated strains constitute promising delivery systems for vaccine antigens. The qualitative differences observed in the obtained responses with different carriers may be useful for those applications in which a targeted immunomodulation is required.

Salmonella vaccine carrier strains: effective delivery system to trigger anti-tumor immunity by oral route

Recombinant Salmonella strains expressing heterologous antigens can be delivered by oral route triggering the elicitation of efficient antigen-specific humoral, T helper and cytotoxic responses. The potential of attenuated Salmonella spp. to trigger anti-tumor immunity was evaluated for the first time by using beta-galactosidase (beta-gal) as a model tumor-associated antigen (TAA). Beta-gal was expressed in a Salmonella typhimurium aroA vaccine carrier strain either constitutively or under the control of a promoter activated upon infection. Oral immunization with both vaccine prototypes resulted in the elicitation of beta-gal-specific humoral and cell-mediated immunity. Although both strains were able to trigger antigen-specific CTL, responses were more efficient when the expression was controlled by the promoter activated upon infection. The anti-tumor efficacy of the stimulated response was validated by challenging vaccinated animals with an aggressive fibrosarcoma transfected with beta-gal, which operationally acts as a TAA. Both groups of vaccinated mice exhibited a significant reduction in tumor take and growth with respect to animals vaccinated with plasmidless carrier (p < 0.05). However, the overall efficiency was better in the group in which beta-gal was controlled by the in vivo-activated promoter (85% versus 54%; p < 0.05).

Generation of the vesicular stomatitis virus nucleoprotein cytotoxic T lymphocyte epitope requires proteasome-dependent and -independent proteolytic activities

The proteasome is involved in the generation of most of the MHC class I antigenic epitopes. However, it is not known if the proteasome generates the exact cytotoxic T lymphocyte (CTL) epitope or only epitope precursors which require further modification by additional proteases. Digestion of the extended vesicular stomatitis virus nucleoprotein epitope 52-59 (RGYVYQGL) by the 20S proteasome in vitro shows that the proteasome is capable of generating the correct C terminus but not the exact N terminus of the CTL epitope. This finding suggests that proteolytic activity in addition to the proteasome is required for generation of the CTL epitope. By using the proteasome inhibitor lactacystin we were able to confirm this finding in vivo. Lactacystin prevented the processing of N- and C-terminally extended epitopes, whereas the processing of only N-terminally extended epitopes was unaffected. Thus, the proteasome is necessary and sufficient for the generation of the exact C terminus of this CTL epitope, whereas the exact N terminus seems to be generated by a different protease.

Allo- and self-restricted cytotoxic T lymphocytes against a peptide library: evidence for a functionally diverse allorestricted T cell repertoire

BALB/c-derived spleen cells were depleted of cytotoxic T lymphocytes (CTL) recognizing allogeneic (H2b) and TAP-negative cells followed by stimulation with the same cells loaded with a synthetic library binding to H2-Kb. The resulting CTL lines were found to differ widely in peptide specificity and to exhibit an avidity towards the library as that demonstrated for syngeneic CTL. These results demonstrate that positive selection in the context of a certain MHC molecule does not seem to be required for generating high-avidity TCR that are restricted by the same molecule. However, positive selection increases the frequency of such CTL. By raising T cell lines from a repertoire which did not undergo negative selection by the restriction element in question, it becomes possible to produce effective self-peptide/ MHC as well as nonself-peptide/MHC-specific CTL as tools for adoptive tumor immunotherapy.

Oral somatic transgene vaccination using attenuated S. typhimurium

An attenuated strain of S. typhimurium has been used as a vehicle for oral genetic immunization. Eukaryotic expression vectors containing truncated genes of ActA and listeriolysin--two virulence factors of Listeria monocytogenes--have been used to transform S. typhimurium aroA. Multiple or even single oral immunizations with such transformants induced excellent cellular and humoral responses. In addition, protective immunity was induced with listeriolysin transformants. The quality of the responses suggested a transfer of plasmid DNA from the bacterial carrier to the host. Such transfer was unequivocally shown in vitro with primary peritoneal macrophages. We describe a highly versatile system for antigen delivery, identification of protective antigens for vaccination, and efficient generation of antibodies against the product of open reading frames present on virtually any DNA segment.

Trypanosoma cruzi infection does not impair major histocompatibility complex class I presentation of antigen to cytotoxic T lymphocytes

Trypanosoma cruzi (T. cruzi), the etiological agent of Chagas' disease, lives free within the cytoplasm of infected host cells. This intracellular niche suggests that parasite antigens may be processed and presented on major histocompatibility complex (MHC) class I molecules for recognition by CD8+ T cells. However, the parasite persists indefinitely in the mammalian host, indicating its success at evading immune clearance. It has been shown that T. cruzi interferes with processing and presentation of antigenic peptides in the MHC class II pathway. This investigation sought to determine whether interference in MHC class I processing and presentation occurs with T. cruzi infection. Surface expression of MHC class I molecules was found to be unaffected or up-regulated by T. cruzi infection in vitro. A model system employing a beta-galactosidase (beta-gal)-specific murine cytotoxic T lymphocyte (CTL) line (0805B) showed: (i) in vitro infection of mouse peritoneal macrophages or J774 cells with T. cruzi did not inhibit MHC class I presentation of exogenous peptide (a nine-amino acid epitope of beta-gal) to the CTL line, (ii) in vitro infection of a beta-gal-expressing 3T3 cell line (LZEJ) with T. cruzi did not inhibit MHC class I presentation of the endogenous protein to the CTL line and (iii) mouse renal adenocarcinoma cells infected with T. cruzi and subsequently infected with adenovirus expressing beta-gal were able to present antigen to the beta-gal-specific CTL line. These findings indicate that the failure of the immune response to clear T. cruzi does not result from global interference by the parasite with MHC class I processing and presentation. Parasites engineered to express beta-gal were unable to sensitize infected antigen-presenting cells in vitro to lysis by the CTL 0805B line. This was probably due to the intracellular localization of the beta-gal within the parasite and its inaccessibility to the host cell cytoplasm.

The defined attenuated Listeria monocytogenes delta mp12 mutant is an effective oral vaccine carrier to trigger a long-lasting immune response against a mouse fibrosarcoma

Listeria monocytogenes has been proposed as a carrier to elicit major histocompatibility complex class-I restricted immune responses able to protect against tumor challenge. In this study the properties of the attenuated L. monocytogenes delta mp12 mutant has been evaluated in vivo against a highly aggressive mouse fibrosarcoma which expresses beta-galactosidase (beta-gal) as a tumor-associated antigen (TAA). Immunization with the vaccine prototypes resulted in both elicitation of specific antibodies and generation of cytotoxic lymphocytes (CTL). Oral vaccination protected 55-64% of the immunized animals from tumor take (p < 0.01) and strongly reduced the average size of the tumor in the other 34-45% (p < 0.01). Vaccinated mice developed a long-lasting response, which resulted in 100% protection from a subsequent tumor challenge. Substitution of the whole TAA by its CTL-defined immunodominant epitope resulted in 43% protection, suggesting a contribution of the humoral response to the observed antitumor effect. No statistically significant differences were observed in the antitumor response when mice were immunized with strains expressing the immunodominant TAA epitope in the context of carrier proteins which were either exported or restricted to the bacterial cytoplasm. This suggests that the topology of the recombinant antigen does not play a major role in the outcome of the protective response.

TAP-dependent major histocompatibility complex class I presentation of soluble proteins using listeriolysin

Immunization of mice with mixtures of listeriolysin, a pore-forming hemolysin secreted by the pathogenic bacterium Listeria monocytogenes, together with soluble ovalbumin, nucleoprotein of influenza virus, or beta-galactosidase of Escherichia coli, resulted in strong cytotoxic CD8 T cell responses to each of the respective passenger proteins in vivo. Also, the concomitant addition of either protein with listeriolysin to target cells elicited efficient sensitization of these cells which could be attributed to the pore-forming activity of listeriolysin. This response was dependent upon a functional TAP transporter and was inhibitable by brefeldin A, indicating the transfer of the soluble proteins into the cytosol and the classical major histocompatibility (MHC) class I presentation pathway. The treatment of target cells with listeriolysin under our experimental conditions did not affect cell viability and the pores generated by listeriolysin treatment were repaired within 60 min. Introduction of soluble proteins into the MHC class I presentation pathway by listeriolysin provides a powerful system to study the cytotoxic response towards intracellular pathogens and would allow for rapid screening of potential antigens in vaccine formulations.

Antigen expression by dendritic cells correlates with the therapeutic effectiveness of a model recombinant poxvirus tumor vaccine

Recombinant poxviruses encoding tumor-associated antigens (TAA) are attractive as candidate cancer vaccines. Their effectiveness, however, will depend upon expression of the TAA in appropriate antigen-presenting cells. We have used a murine model in which the TAA is beta-galactosidase (beta-gal) and a panel of recombinant vaccinia viruses (rVV) in which beta-gal was expressed under early or late promoters at levels that varied over 500-fold during productive infections in tissue culture cells. Remarkably, only those rVV employing early promoters were capable of prolonging the survival of mice bearing established tumors expressing the model TAA. Late promoters were ineffective regardless of their determined promoter strength. The best results were obtained when beta-gal was regulated by a strong early promoter coupled to a strong late promoter. When a variety of cell types were infected with the panel of viruses in vitro, dendritic cells were found to express beta-gal only under the control of the early promoters even though late promoters were intrinsically more active in other cell types. Furthermore, in a functional assay, dendritic cells infected in vitro with rVV encoding beta-gal regulated by an early promoter activated beta-gal-specific cytotoxic T lymphocytes, whereas similar rVV with a late promoter-regulated gene did not. These data indicate that promoter strength per se is not the most critical quality of a recombinant poxvirus-based tumor vaccine and that the use of promoters capable of driving the production of TAA in "professional" antigen presenting cells may be crucial.

Expression levels of stress protein gp96 are not limiting for major histocompatibility complex class I-restricted antigen presentation

Immunization of mice with gp96 induces CTL with specificity for proteins that are expressed in the cells from which gp96 was isolated (Arnold et al., J. Exp. Med. 1995. 182: 885, Udono et al., Proc. Natl. Acad. Sci. USA 1994. 91: 3077). Recently, it has been shown that gp96 from cells transfected with vesicular stomatitis virus (VSV) nucleocapsid protein as well as gp96 loaded in vitro with peptides containing an epitope of this protein are taken up by phagocytic cells which obtain thereby the capacity for stimulating VSV-specific cytotoxic T lymphocytes (Suto and Srivastava, Science 1995. 269: 1585). The immunization experiments together with the peptide transfer from gp96/peptide complexes to major histocompatibility complex (MHC) class I molecules of phagocytic cells are consistent with the hypothesis that the endoplasmic reticulum-resident protein gp96 plays a crucial role in the antigen presentation of a cell (Srivastava et al., Immunogenetics 1994. 29: 93). To examine the involvement of gp96 in class I-restricted antigen presentation, we reduced gp96 RNA and protein levels by transfecting P13.1 cells with a vector containing part of gp96 cDNA in antisense orientation to the promotor. We found that antisense clones expressing strongly reduced levels of gp96 mRNA and gp96 protein show normal levels of MHC class I molecules on the cell surface and are recognized by T cells to the same extent as wild-type cells. Thus, our results show that normal levels of gp96 expression in a cell are not limiting for class I-restricted antigen presentation.

Murine dendritic cells loaded in vitro with soluble protein prime cytotoxic T lymphocytes against tumor antigen in vivo

The priming of an immune response against a major histocompatibility complex class I-restricted antigen expressed by nonhematopoietic cells involves the transfer of that antigen to a host bone marrow-derived antigen presenting cell (APC) for presentation to CD8+ T lymphocytes. Dendritic cells (DC), as bone marrow-derived APC, are first candidates for presentation of tumor-associated antigens (TAA). The aim of this study was to see whether DC are able to prime in vivo antigen-specific cytotoxic T lymphocytes after exposure to a soluble protein antigen in vitro. Lacking a well-defined murine TAA, we took advantage of beta-galactosidase (beta-gal)-transduced tumor cell lines as a model in which beta-gal operationally functions as TAA. For in vivo priming both a DC line, transduced or not transduced with the gene coding for murine GM-CSF, and fresh bone marrow-derived DC (bm-DC), loaded in vitro with soluble beta-gal, were used. Priming with either granulocyte macrophage colony-stimulating factor-transduced DC line or fresh bm-DC but not with untransduced DC line generated CTL able to lyse beta-gal-transfected target cells. Furthermore, GM-CSF was necessary for the DC line to efficiently present soluble beta-gal as an H-2Ld-restricted peptide to a beta-gal-specific CTL clone. Data also show that a long-lasting immunity against tumor challenge can be induced using beta-gal-pulsed bm-DC as vaccine. These results indicate that effector cells can be recruited and activated in vivo by antigen-pulsed DC, providing an efficient immune reaction against tumors.

Cross-priming of minor histocompatibility antigen-specific cytotoxic T cells upon immunization with the heat shock protein gp96

Vaccination of mice with heat shock proteins isolated from tumor cells induces immunity to subsequent challenge with those tumor cells the heat shock protein was isolated from but not with other tumor cells (Udono, H., and P.K. Srivastava. 1994. J. Immunol. 152:5398-5403). The specificity of this immune response is caused by tumor-derived peptides bound to the heat shock proteins (Udono., H., and P.K. Srivastava. 1993. J. Exp. Med. 178:1391-1396). Our experiments show that a single immunization with the heat shock protein gp96 isolated from beta-galactosidase (beta-gal) expressing P815 cells (of DBA/2 origin) induces cytotoxic T lymphocytes (CTLs) specific for beta-gal, in addition to minor H antigens expressed by these cells. CTLs can be induced in mice that are major histocompatibility complex (MHC) identical to the gp96 donor cells (H-2d) as well as in mice with a different MHC (H-2b). Thus gp96 is able to induce "cross priming" (Matzinger, P., and M.J. Bevan. 1977. Cell. Immunol. 33:92-100), indicating that gp96-associated peptides are not limited to the MHC class I ligands of the gp96 donor cell. Our data confirm the notion that samples of all cellular antigens presentable by MHC class I molecules are represented by peptides associated with gp96 molecules of that cell, even if the fitting MHC molecule is not expressed. In addition, we extend previous reports on the in vivo immunogenicity of peptides associated gp96 molecules to two new groups of antigens, minor H antigens, and proteins expressed in the cytosol.

Immunodominant minor histocompatibility antigens expressed by mouse leukemic cells can serve as effective targets for T cell immunotherapy

Numerous minor histocompatibility antigens (MiHAs) show tissue-specific expression and can induce vigorous T cell responses. They therefore represent attractive targets for leukemia immunotherapy mediated by adoptive transfer of T cells. The main objective of this work was to determine whether MiHAs expressed by normal hematopoietic cells were present on leukemic cells and whether they could trigger lysis by cytotoxic T lymphocytes (CTLs). CTL assays showed that mouse leukemic cells of both lymphoid and myeloid lineages were sensitive to CTLs targeted toward some but not all MiHAs. In four out of four strain combinations in which we primed CTLs against immunodominant MiHAs, effectors killed leukemic blasts, whereas no cytotoxicity was observed when CTLs were targeted toward four immunorecessive MiHAs. Testing of HPLC fractions obtained from normal and leukemic cells provided molecular evidence that leukemic blasts expressed only some of the MiHAs found on normal mouse hematopoietic cells. Decreased density of H-2 class I molecules at the surface of leukemic cells suggests that down-regulation of genes encoding either class I molecules or proteins involved in antigen processing played a role in the aberrant expression of MiHAs. In vivo resistance to the leukemic cells by various strains of mice correlated with in vitro CTL activity. These results show that leukemic cells express only some (immunodominant) MiHAs and suggest that this subset of MiHAs represent prime targets for adoptive immunotherapy.

Scattered micrometastases visualized at the single-cell level: detection and re-isolation of lacZ-labeled metastasized lymphoma cells

To study metastasis at the single-cell level we transduced highly metastatic ESb lymphoma cells with a retroviral expression vector containing the lacZ (bacterial beta-galactosidase) gene. This allowed single ESb-lacZ tumor cells to be detected in infiltrated target organs by means of X-Gal staining. Despite expression of the lacZ gene, the tumor cells were still tumorigenic, highly metastatic, unchanged in phenotype and therefore comparable to parental ESb cells. After spontaneous metastasis, whole-organ staining revealed metastatic foci at the surface of the liver. In histological liver sections, metastatic clusters and single dispersed tumor cells could be detected. In contrast to whole-organ staining, histological examination revealed scattered distribution of tumor cells throughout the organ, which was not evident with parental ESb cells. In addition, clusters with diffuse or dense (focal) appearance were found, in correlation with the whole-organ staining. Expression of the foreign lacZ gene allowed the metastatic spread of tumor cells to liver and spleen to be quantified approximately by FACS analysis. Furthermore, it was shown that the newly expressed beta-gal was expressed not only intercellularly but also at the cell surface. There it could be recognized by MAbs and cytotoxic T-cells (CTL). beta-gal did not affect CTL recognition of the ESb tumor-associated antigen. In conclusion, lacZ could be used as a genetic marker for a highly metastatic lymphoma, to define scattered metastatic spread in the liver at the single-cell level and to quantify the tumor load by FACS analysis.

Presentation of mouse tum- P91A antigen from chimeric proteins with different subcellular localizations by class I molecules of the major histocompatibility complex

Like many antigens presented by class I molecules the mouse Ld-binding tum- antigen P91A derives from a cytosolic protein. To decide how stringent this localization is for presentation to cytotoxic lymphocytes (CTL) the P91A template has been inserted in the cDNA of rat esterase ES-10, a protein located in the endoplasmic reticulum (ER), and in the cDNA of mouse interleukin-9, a secretory product of lymphocytes. The esterase construct was also engineered to replace the C-terminal leucine by arginine, which causes secretion of the protein, or to delete the N-terminal presequence, which prevents transfer of the nascent chain to the ER. After cell-free transcription-translation, or transfection in COS cells, the products of the chimeric cDNA had the expected size and localization; however, the truncated form of esterase remained undetected in COS cells. The various chimeric templates were transfected in P1.HTR cells (H-2d); upon challenge with Ld-restricted anti-P91A CTL the cells were lyzed almost as efficiently as cells transfected with the full-length P91A cDNA. We conclude that peptide fragments that bind to class I molecules of the major histocompatibility complex can be generated in the ER.

The tum- antigens P91A and P198 derive from proteins located in the cytosolic compartment of cells

To characterize the proteins P91Ap and P198p, of which mutants generate the tum- antigens P91A and P198, respectively, rabbit antisera were raised with ovalbumin-coupled synthetic peptides that correspond to their respective C terminus. In immunoadsorption tests using immobilized protein A the antisera recognized the translation products synthesized by rabbit reticulocyte lysates programmed with the SP6 polymerase transcripts of the P91A and P198 cDNA. The presence of the two proteins was demonstrated by SDS-PAGE and immunoblotting in all the mouse cells and organs examined. P91Ap is a constituent of the cytosol; despite a remarkable homology to the Drosophila diphenol oxidase DOX-A2, it separates from murine catechol oxidase activity in rate zonal sedimentation analysis. P198p is a ribosomal constituent, or a factor firmly linked to both the free and membrane-bound ribosomes. These subcellular localizations strengthen other evidence that the antigens presented to T lymphocytes by class I products of the major histocompatibility complex derive from proteins of the cytosol, or in direct contact with it.

Leishmania mexicana promastigotes induce cytotoxic T lymphocytes in vivo that do not recognize infected macrophages

The question is addressed whether antigens of Leishmania, a parasite residing in the endosomal compartment of macrophages, can be presented in the context of major histocompatibility complex class I molecules. We used E. coli beta-galactosidase as a model antigen which can be expressed in high levels in L. mexicana promastigotes (L. mexicana-gal). Infection of BALB/c mice with L. mexicana-gal induces beta-galactosidase-specific cytotoxic T cells (CTL), which can be isolated using a beta-galactosidase-expressing mastocytoma line as an antigen-presenting cell. These CTL recognize epitopes of beta-galactosidase in the context of H-2Kd; however, they do not recognize L. mexicana-gal-infected macrophages even after killing of the intracellular amastigotes by drug treatment or macrophage activation by lymphokines, although class I-peptide interaction and the presentation of endogenously produced antigens is normal. It is concluded that parasite antigens can induce a CTL response in vivo but that these CTL cannot recognize infected macrophages because the relevant epitopes cannot gain access to class I molecules. The effect of priming in vivo may be explained by the well-known but ill-understood phenomenon of cross-priming.

Can one predict antigenic peptides for MHC class I-restricted cytotoxic T lymphocytes useful for vaccination?

The cytotoxic T-lymphocyte (CTL) response can be crucial for efficient immunological control of intracellular pathogens and the MHC class I-restricted CTL have a major role to play in this process. They recognize complexes associating antigen-derived peptides with MHC class I molecules expressed on infected target cells. The characterization of these antigenic peptides is thus a key issue for developing vaccines efficient in inducing specific CTL. Recently, by sequencing the whole set of self-peptides eluted from a given MHC class I molecule, Falk and colleagues have found that they have a homogeneous 8-10 residue length and contain allele-specific peptidic motifs with two conservative dominant anchor residues. The existence of consensus motifs opens the way for a strategy to predict the MHC class I-restricted T-cell epitopes and here we discuss such an approach using hen egg lysozyme (HEL) as an antigenic model. Two HEL peptides corresponding to allele-specific motifs were found, HEL(49-56) and HEL(70-78) peptides, which can associate with MHC class I H-2Kb and H-2Db molecules, respectively. The HEL peptide HEL(70-78) was found to be able to induce HEL-specific CTL in H-2b mice also. Moreover, using an empiricial approach, we have also characterized the N-terminal HEL(1-17) peptide as an immunodominant antigenic peptide in the H-2k haplotype. This peptide presented by H-2Kk molecules neither contained the corresponding allele-specific binding motif nor fitted the expected 8-10 residue length.(ABSTRACT TRUNCATED AT 250 WORDS)

Cytomegalovirus prevents antigen presentation by blocking the transport of peptide-loaded major histocompatibility complex class I molecules into the medial-Golgi compartment

Selective expression of murine cytomegalovirus (MCMV) immediate-early (IE) genes leads to the presentation by the major histocompatibility complex (MHC) class I molecule Ld of a peptide derived from MCMV IE protein pp89 (Reddehase, M.J., J. B. Rothbard, and U.H. Koszinowski. 1989. Nature (Lond.). 337:651). Characterization of endogenous antigenic peptides identified the pp89 peptide as the nonapeptide 168YPHFMPTNL176 (del Val, M., H.-J. Schlicht, T. Ruppert, M.J. Reddehase, and U.H. Koszinowski. 1991. Cell. 66:1145). Subsequent expression of MCMV early genes prevents presentation of pp89 (del Val, M., K. Münch, M.J. Reddehase, and U.H. Koszinowski. 1989. Cell. 58:305). We report on the mechanism by which MCMV early genes interfere with antigen presentation. Expression of the IE promoter-driven bacterial gene lacZ by recombinant MCMV subjected antigen presentation of beta-galactosidase to the same control and excluded antigen specificity. The Ld-dependent presence of naturally processed antigenic peptides also in nonpresenting cells located the inhibitory function subsequent to the step of antigen processing. The finding that during the E phase of MCMV gene expression the MHC class I heavy chain glycosylation remained in an Endo H-sensitive form suggested a block within the endoplasmic reticulum/cis-Golgi compartment. The failure to present antigenic peptides was explained by a general retention of nascent assembled trimolecular MHC class I complexes. Accordingly, at later stages of infection a significant decrease of surface MHC class I expression was seen, whereas other membrane glycoproteins remained unaffected. Thus, MCMV E genes endow this virus with an effective immune evasion potential. These results also indicate that the formation of the trimolecular complex of MHC class I heavy chain, beta 2-microglobulin, and the finally trimmed peptide is completed before entering the medial-Golgi compartment.

CD8+ T cells specific for a single nonamer epitope of Listeria monocytogenes are protective in vivo

Class I major histocompatibility complex (MHC)-restricted CD8+ T cells have been demonstrated to be effective mediators of both acquired and adoptive immunity to the intracellular bacterium Listeria monocytogenes. We have recently determined that L. monocytogenes-infected H-2d mice recognize a nonamer peptide, residues 91-99, of the secreted protein listeriolysin O (LLO), in a H-2Kd-restricted fashion. In this report we have generated CD8+ T cell lines with specificity for LLO 91-99 in the context of H-2Kd by in vitro stimulation with P815 (H-2d) cells transfected with LLO. These CD8+ lines have been generated from immune donors after sublethal infection with L. monocytogenes, or after in vivo immunization with syngeneic spleen cells coated with synthetic LLO 91-99 peptide. LLO-specific CD8+ T cells derived from either protocol were capable of significant protection against L. monocytogenes infection. The in vivo protection by these CD8+ T cell lines has been shown to be solely due to recognition of LLO 91-99 in the context of H-2Kd. These studies demonstrate that CD8+ T cell immunity to a single, naturally produced peptide epitope has the potential for significant protection in a bacterial infection. Thus, the allele-specific motif approach to epitope prediction has identified a naturally produced bacterial epitope with biological relevance.

Specificity of antigen processing for MHC class I restricted presentation is conserved between mouse and man

Alloreactive mouse cytotoxic T lymphocytes (CTL) specific for particular peptides presented by H-2Kb molecules on mouse cells were found to recognize human cells transfected with Kb. The CTL-recognized peptides (probably derived from conserved proteins) were extracted from Kb-expressing human or mouse cells, respectively, and compared biochemically by high resolution high performance liquid chromatography. The results strongly suggest identity of peptides processed by cells from both species and thus indicate that the specificity of the processing machinery used in the major histocompatibility complex (MHC) class I-restricted pathway, probably including enzymes, transport mechanisms, and chaperons, is highly conserved across species. The results are consistent with the notion that MHC molecules themselves have an instructive role in processing.

Naturally-occurring peptide antigens derived from the MHC class-I-restricted processing pathway

The extraction of naturally-processed peptides from MHC class I glycoproteins has paved the way for a major advance in the understanding of the antigen processing pathway that ultimately induces cytotoxic T-cell responses. Here, Olaf Rötzschke and Kirsten Falk review these new developments and discuss their findings in terms of a novel hypothesis of MHC class-I-restricted processing.

On the nature of peptides involved in T cell alloreactivity

The strong reaction of T cells against foreign major histocompatibility complex (MHC) antigens, commonly termed "alloreactivity", is not only a nuisance for clinical organ transplantation; it also remains a puzzling question for immunologists. By making use of recent technical developments, alloreactive T cells nominally directed against a mutation in a single MHC class I molecule were found to fall into several major categories. One is recognizing peptides whose occurrence is dependent on one particular MHC allele, another is recognizing peptides supported by several MHC alleles, and a third is recognizing peptides occurring independently of MHC alleles. In a fourth category, the binding to MHC of any of a broad range of peptides appears sufficient. In addition, there are T cells for which no peptide involvement could be detected at all. Even within these categories, the heterogeneity of T cells is considerable: among 16 Kb-reactive T cells analyzed, 15 different modes of reactions were found.

Identification of naturally processed viral nonapeptides allows their quantification in infected cells and suggests an allele-specific T cell epitope forecast

Virus-specific cytotoxic T lymphocytes (CTL) recognize virus-derived peptides presented by major histocompatibility complex (MHC) class I molecules on virus-infected cells. Such peptides have been isolated from infected cells and were compared to synthetic peptides. We found previously the Kd- or Db-restricted natural influenza nucleoprotein peptides to coelute on reversed phase high performance liquid chromatography columns with certain peptidic by-products present in synthetic peptide preparations. Here we show by extensive biochemical and immunological comparison that the natural peptides in all respects behave as the surmised synthetic nonapeptides, and thus, must be identical to them. The absolute amounts of these natural peptides contained in infected cells could be determined to be between 220 and 540 copies by comparing with defined amounts of pure synthetic nonapeptides. The comparison of the natural Kd-restricted peptide with published synthetic peptides known to contain other Kd-restricted CTL epitopes suggested a new MHC allele-specific T cell epitope forecast method, based on the defined length of nine amino acid residues and on critical amino acid residues at the second and the last position.

New use of BCG for recombinant vaccines

BCG, a live attenuated tubercle bacillus, is the most widely used vaccine in the world and is also a useful vaccine vehicle for delivering protective antigens of multiple pathogens. Extrachromosomal and integrative expression vectors carrying the regulatory sequences for major BCG heat-shock proteins have been developed to allow expression of foreign antigens in BCG. These recombinant BCG strains can elicit long-lasting humoral and cellular immune responses to foreign antigens in mice.

Human T cell responses to beta-galactosidase

The peripheral blood of most normal individuals has been shown to contain T cells that respond to beta-galactosidase (beta-Gal), presumably as a result of natural priming. Three T cell clones (clones 1,2,4) specific for beta-Gal were isolated from peripheral blood mononuclear cells (PBMC) after pretreatment with leucine methyl ester (LeuOMe); a fourth clone from the same individual was isolated from untreated cells. All four clones were CD4+ CD8- alpha beta TcR+ and clone 1 was additionally shown to be cytotoxic. Epstein-Barr virus (EBV) transformed B cell lines were derived from LeuOMe-treated or untreated PBMC and used to study the efficiency of presentation of beta-Gal to one of the clones. The results indicated that B cells transformed after LeuOMe treatment presented beta-Gal at lower concentrations than untreated controls. beta-Gal would therefore appear to be a highly suitable model antigen for studies of immunoregulation in humans.

Cytosolic targeting of hen egg lysozyme gives rise to a short-lived protein presented by class I but not class II major histocompatibility complex molecules

A way to study the role of intracellular trafficking of an antigen in its presentation to T cells is to target the antigen to various cell compartments of the antigen-presenting cells (APC) and compare the nature of the complexes associating major histocompatibility complex (MHC) molecules and antigenic peptides, expressed on the cell surface. MHC class I+ and MHC class II+ mouse L fibroblasts secreting hen egg lysozyme (HELs cells) or expressing HEL in their cytosol (HELc cells) were obtained after transfection with HEL cDNA and signal sequence-deleted HEL cDNA, respectively. HEL was evidenced in both HELs- and HELc-transfected cells and the former type of transfectant secreted a large amount of HEL. However, HEL produced in the cytosol exhibited a short half-life of less than 5 min. HEL-derived peptides could not be shown biochemically either in HELc- nor in HELs-transfected cells. We then studied the capacity of these cells to present HEL to HEL-specific class I- and class II-restricted T cells. Both cell types could be recognized by the HEL-specific MHC class I-restricted CTL clones. In contrast, MHC class II-HEL peptide complexes, recognized by HEL-specific helper T cell hybridomas, could be detected on MHC class II+ HELs- but not HELc-transfected cells. In vivo experiments showed, however, that HELc-transfected cells could provide host APC with HELc-derived peptides able to associate with MHC class II molecules. This was inferred from the capacity of MHC class II-HELc-transfected cells, unable by themselves to elicit any anti-HEL antibody response, to prime syngeneic and allogeneic mice against HEL. The priming was revealed by the induction of an antibody response after a boost with an amount of HEL unable itself to elicit an antibody response.

Cellular peptide composition governed by major histocompatibility complex class I molecules

Major histocompatibility complex (MHC) class I molecules present peptides derived from cellular proteins to cytotoxic T lymphocytes (CTLs), which check these peptides for abnormal features. How such peptides arise in the cell is not known. Here we show that the MHC molecules themselves are substantially involved in determining which peptides occur intracellularly: normal mouse spleen cells identical at all genes but MHC class I express different patterns of peptides derived from cellular non-MHC proteins. We suggest several models to explain this influence of MHC class I molecules on cellular peptide composition.

Generation of hen egg lysozyme-specific and major histocompatibility complex class I-restricted cytolytic T lymphocytes: recognition of cytosolic and secreted antigen expressed by transfected cells

Syngeneic cells exogenously supplied with hen egg lysozyme (HEL) or endogenously synthesizing HEL were used as antigen-presenting cells to induce major histocompatibility complex class I-restricted cytotoxic T lymphocytes (CTL). Immunization of C57BL/6 mice followed by repeated stimulation of their splenocytes in vitro with trypsinized HEL peptides led to the generation of CTL lines specific for trypsinized HEL peptides and restricted by H-2K. Immunization of C3H mice with a mixture of soluble native HEL and irradiated syngeneic spleen cells followed by in vitro stimulation of immune spleen cells with soluble HEL could in a few cases result in HEL-specific CTL able to kill syngeneic transfectant L cells secreting HEL (HELs) or expressing cytosol-targeted HEL (HELc). The use of HELs or HELc transfectant L cells as in vivo and in vitro immunogens was a potent way for eliciting HEL-specific polyclonal CTL. These CTL and two CD8+ clones were found to be H-2K restricted and specific for the 1-17 N-terminal HEL peptide. In addition, the anti-HEL CTL could also exhibit a significant cross-reactivity against unsensitized and HEL-untransfected targets expressing the K restriction element. This cross-reactivity was likely due to recognition of unidentified HEL mimicking peptides (self-derived?) presented by the MHC class I (H-2K or H-2K) molecule used as the restriction element for the specific recognition of HEL. The CTL raised after immunization with HELs or HELc transfectant cells were found to recognize both the HELs and HELc transfectant cells even though HEL was not detected in the latter after a 2- or 5-min radiolabeling pulse. Recognition of both HELs and HELc transfectant cells by a given CTL clone suggests that HEL subjected to two separate processing pathways, each depending on the initial subcellular localization, can ensure the generation of similar MHC class I peptide complexes.

Characterization of naturally occurring minor histocompatibility peptides including H-4 and H-Y

Minor histocompatibility (H) antigens can be peptides derived from cellular proteins that are presented on the cell surface by major histocompatibility complex (MHC) class I molecules. This is similar to viral antigens, because in both cases cytotoxic T lymphocytes (CTLs) recognize artificially produced peptides loaded on target cells. Naturally processed minor H peptides were found to be similar to those artificial CTL-epitopes, as far as size and hydrophobicity is concerned. The peptides studied were isolated from a transfectant that expressed a model CTL-defined antigen, beta-galactosidase, from male cells that express H-Y, which has been known operationally since 1955, and from cells that express H-4, known since 1961.

Partial tolerance in beta-galactosidase-transgenic mice

A transgenic mouse line was produced which allowed the expression of E. coli beta-galactosidase (beta-Gal) under the regulatory elements of the immunoglobulin heavy chain locus. Expression of the transgene is found in spleen and bone marrow. Upon immunization of the transgenic mice with beta-Gal, a reduced but clearly detectable antibody response was obtained. Affinity purification with sera from immunized transgenic mice suggests that they contain lower affinity antibodies as compared to normal littermates. Transgenic and nontransgenic mice immunized with bovine serum albumin (BSA) alone or as a mixture with beta-Gal gave comparable anti-BSA responses. Immunization with a chemically cross-linked (Gal-BSA)-protein, however, showed a 10- to 30-fold difference in the anti-BSA response. Partial unresponsiveness to beta-Gal in the transgenic mice is best explained by a dominant, peripheral suppression mechanism linked to the antigen-presenting potential of B cells.

Limit of T cell tolerance to self proteins by peptide presentation

Cytotoxic T lymphocytes (CTLs) recognize foreign peptides bound to major histocompatibility complex (MHC) class I molecules. MHC molecules can also bind endogenous self peptides, to which T cells are tolerant. Normal mice contained CTLs specific for self peptides that were from proteins of ubiquitous or tissue-restricted expression. In vivo, these endogenous self peptides are not naturally presented in sufficient density by somatic cells expressing MHC class I molecules. They can, however, be presented if added exogenously. Thus, our data imply that CTLs are only tolerant of those endogenous self peptide sequences that are presented by MHC class I-positive cells in a physiological manner.

Class I-restricted processing and presentation of exogenous cell-associated antigen in vivo

MHC class I-restricted T lymphocyte responses are usually directed to cellular antigenic components resulting from endogenous gene expression. Exogenous, non-replicating antigens, such as soluble proteins, usually fail to enter the class I pathway of antigen processing and presentation. Consistent with this notion, we have recently shown that soluble, exogenous proteins can be efficiently processed for class I presentation in vitro only if they are introduced directly into the target cell cytoplasm. In this report we extend this work to the in vivo situation by introducing soluble protein into the cytoplasm of mouse splenocytes via the osmotic lysis of pinosomes and then using these cells for in vivo immunization. Our results show that cytoplasmic loading of OVA and beta-GAL into H-2b and H-2d splenocytes respectively, resulted in effective in vivo immunogens for class I-restricted CTL. To our surprise, control spleen cell preparations simply incubated with the exogenous, native protein for 10 min at 37 degrees C in isotonic medium and then washed could also induce a comparable class I-restricted CTL response following intravenous injection. Experiments using (H-2b X H-2d)F1 mice showed that protein pulsed splenocytes from one parental strain could effectively "cross prime" T cells restricted to the MHC of the other parental strain. In all cases, target cell recognition by the effector CTL generated by immunization with spleen cell-associated antigen required the antigen to be present in the cell cytoplasm. Thus the CTL do not recognize target cells exposed to soluble, exogenous antigen. These results, reminiscent of analogous experiments with cross priming by minor histocompatibility antigens, argue that class I-restricted processing and presentation of exogenous antigen can occur in vivo following immunization with cell-associated antigen.

Identification of classical minor histocompatibility antigen as cell-derived peptide

Histocompatibility antigens expressed on tissue grafted between individuals are recognized by host T cells, which reject the graft. The major histocompatibility complex (MHC) antigens have been identified on the molecular level, whereas the molecules representing the remaining ones, the minor histocompatibility antigens, are unknown, apart from some exceptions. The cytotoxic T lymphocyte (CTL) response against minor histocompatibility antigens shares many aspects with that against virus-infected cells. Virus-specific CTL recognize peptides derived from viral proteins produced in the infected cell. These peptides are presented by MHC class I molecules, as indicated by functional and crystallographic data. By analogy, minor histocompatibility antigens have been postulated to be peptides derived from normal cellular proteins presented by MHC class I molecules. Here we report that peptides derived from normal cellular proteins can indeed be recognized by CTL raised in the classical minor histoincompatible mouse strain combination, C57BL/6 against BALB.B. Thus, we have proven the above postulate, and isolated one of the minor histocompatibility molecules elusive for several decades.