A 15 amino acid fragment of influenza nucleoprotein synthesized in the cytoplasm is presented to class I-restricted cytotoxic T lymphocytes

Reduction of vector gene expression increases foreign antigen-specific CD8+ T-cell priming

Viral vectors have been shown to induce protective CD8(+) T-cell populations in animal models, but significant obstacles remain to their widespread use for human vaccination. One such obstacle is immunodominance, where the CD8(+) T-cell response to a vector can suppress the desired CD8(+) T-cell response to a recombinantly encoded antigen. To overcome this hurdle, we broadly reduced vector-specific gene expression. We treated a recombinant vaccinia virus, encoding antigen as a minimal peptide determinant (8-10 aa), with psoralen and short-wave UV light. The resulting virus induced 66 % fewer vector-specific immunodominant CD8(+) T cells, allowing the in vivo induction of an increased number of CD8(+) T cells specific for the recombinant antigen.

A lung-specific neo-antigen elicits specific CD8+ T cell tolerance with preserved CD4+ T cell reactivity. Implications for immune-mediated lung disease

The A/Japan/57 influenza hemagglutinin (HA) was expressed in BALB/c mice under the transcriptional control of the surfactant protein C (SP-C) promoter, resulting in expression of HA in type II alveolar epithelial cells, as well as low level variable expression in other tissues, including the thymus in some of the founder lines. Transgenic animals were able to recover from infection with A/Japan/57 influenza, and they were able to mount antibody responses to A/Japan/57 HA in titers similar to wild type. We therefore tested their CD4+ T lymphocyte responses to HA and found them to be similar to wild type responses. However, CD8+ T cells from A/Japan/57-infected transgenic animals were unable to express cytolytic activity against target cells expressing the A/Japan/57 HA. The CD8+ T cell tolerance was also extremely specific, since transgenics immunized with an influenza strain containing a single amino acid substitution in a dominant HA epitope were able to mount full cytolytic responses to that epitope, but not the wild-type epitope. Adoptive transfer of CD8+ T cell clones into transgenic animals resulted extensive interstitial pneumonitis that was antigen-specific and associated with significant morbidity and mortality. We conclude that a lung-specific transgene may lead to specific CD8+ T cell tolerance, with CD4+ T cell and B cell reactivity to the antigen, and that CD4+ T cell reactivity may remain intact to an antigen expressed in the thymus, even when CD8+ T cell tolerance exists. This observation may have profound implications concerning immune-mediated lung diseases, particularly those mediated by CD4+ T cells.

LMP2+ proteasomes are required for the presentation of specific antigens to cytotoxic T lymphocytes

BACKGROUND

Major histocompatibility complex (MHC) class I molecules present short peptides generated by intracellular protein degradation to cytotoxic T lymphocytes (CTL). The multisubunit, non-lysosomal proteinases known as proteasomes have been implicated in the generation of these peptides. Two interferon-gamma (IFN-gamma)-inducible proteasome subunits, LMP2 and LMP7, are encoded within the MHC gene cluster in a region associated with antigen presentation. The incorporation of these LMP subunits into proteasomes may alter their activity so as to favour the generation of peptides able to bind to MHC class I molecules. It has been difficult, however, to demonstrate a specific requirement for LMP2 or LMP7 in the presentation of peptide epitopes to CTL.

RESULTS

We describe a T-cell lymphoma, termed SP3, that displays a novel selective defect in MHC class I-restricted presentation of influenza virus antigens. Of the MHC-encoded genes implicated in the class I pathway, only LMP2 is underexpressed in SP3 cells. Expression of IFN-gamma in transfected SP3 cells simultaneously restores LMP2 expression and antigen presentation to CTL. Expression of antisense-LMP2 mRNA in these IFN-gamma-transfected cells selectively represses antigen recognition and the induction of surface class I MHC expression. Moreover, the expression of this antisense-LMP2 mRNA in L929 fibroblast cells, which constitutively express LMP2 and have no presentation defect, blocks the presentation of the same influenza virus antigens that SP3 cells are defective in presenting.

CONCLUSIONS

Our results show that the LMP2 proteasome subunit can directly influence both MHC class I-restricted antigen presentation and class I surface expression.

Functional analysis of amino acid residues encompassing and surrounding two neighboring H-2Db-restricted cytotoxic T-lymphocyte epitopes in simian virus 40 tumor antigen

Simian virus 40 tumor (T) antigen contains three H-2Db-and one H-2Kb-restricted cytotoxic T lymphocyte (CTL) epitopes (sites). Two of the H-2Db-restricted CTL epitopes, I and II/III, are separated by 7 amino acids in the amino-terminal one third of T antigen. In this study, we determine if the amino acids separating these two H-2Db-restricted CTL epitopes are dispensable for efficient processing and presentation. In addition, the importance of amino acid residues lying within and flanking the H-2Db-restricted epitopes I and II/III for efficient processing, presentation, and recognition by site-specific CTL clones was determined by using T-antigen mutants containing single-amino-acid substitutions between residues 200 and 239. Using synthetic peptides in CTL lysis and major histocompatibility complex class I stabilization assays, CTL recognition site I has been redefined to include residues 206 to 215. Substitutions in amino acids flanking either site I or site II/III did not affect recognition by any of the T-antigen-specific CTL clones. Additionally, the removal of the 7 residues separating site I and site II/III did not affect CTL recognition, thus demonstrating that these two epitopes when arranged in tandem in the native T antigen can be efficiently processed and presented to CTL clones. Differences in fine specificities of two CTL clones which recognize the same epitope (Y-1 and K-11 for site I and Y-2 and Y-3 for site II/III) have been used in conjunction with synthetic peptide variants to assign roles for residues within epitopes I and II/III with respect to TCR recognition and/or peptide-major histocompatibility complex association.

Processing of major histocompatibility class I-restricted antigens in the endoplasmic reticulum

We have introduced long precursor peptides directly into the endoplasmic reticulum (ER) of a mutant cell line (T2-Db) that lacks the ability to transport peptides from the cytosol to the ER in a transporter associated with antigen processing (TAP) dependent way. This was done by expressing various influenza A-derived peptides containing the naturally processed epitope ASNENMDAM (366-374) preceded by the influenza hemagglutinin ER translocation sequence. Peptides derived from these minigenes that became associated with Db were isolated and identified by combined reversed phase liquid chromatography and detection by cytotoxic T lymphocytes. Our results establish that NH2-terminal extensions of at least 40 residues can be trimmed from peptides entering the ER, but that proteolysis of larger proteins may be limited.

Simian virus 40 T antigen as a carrier for the expression of cytotoxic T-lymphocyte recognition epitopes

Simian virus 40 (SV40) large T antigen can immortalize a wide variety of mammalian cells in culture. We have taken advantage of this property of T antigen to use it as a carrier for the expression of cytotoxic T-lymphocyte (CTL) recognition epitopes. DNA sequences corresponding to an H-2Db-restricted SV40 T-antigen site I (amino acids 205 to 215) were translocated into SV40 T-antigen DNA at codon positions 350 and 650 containing EcoRI linkers. An H-2Kb-restricted herpes simplex virus glycoprotein B epitope (amino acids 498 to 505) was also expressed in SV40 T antigen at positions 350 and 650. Primary C57BL/6 mouse kidney cells were immortalized by transfection with the recombinant and wild-type T-antigen DNA. Clonal isolates of cells expressing chimeric T antigens were shown to be specifically susceptible to lysis by CTL clones directed to SV40 T-antigen site I and herpes simplex virus glycoprotein B epitopes, indicating that CTL epitopes restricted by two different elements can be processed, presented, and recognized by the epitope-specific CTL clones. Our results suggest that SV40 T antigen can be used as a carrier protein to express a wide variety of CTL epitopes.

Immunization of mice with the N-terminal (1-272) fragment of simian virus 40 large T antigen (without adjuvants) specifically primes cytotoxic T lymphocytes

Immunization of C57BL/6 (B6) mice (H-2b) with the "large tumor antigen" (T-Ag) of simian virus 40 (SV40) in its soluble form without adjuvants primed CD8+ cytotoxic T lymphocytes (CTL) in vivo. CD8+ CTL primed in vivo by this non-structural 708-amino acid (aa) viral protein, and specifically restimulated in vitro, lysed H-2b target cells, either transfected with an SV40 T-Ag-encoding vector, or transformed by SV40 infection. H-2b RMA-S transfectants expressing the complete 708 aa T-Ag (which fail to transport peptides through the endoplasmic reticulum membranes) were not lysed. CTL were also efficiently primed in vivo by injection of the N-terminal 272 aa fragment of the T-Ag. Hence, this fragment contains the structure(s) required for a soluble protein to enter the "endogenous" class I-restricted antigen processing and presentation pathway for CD8+ CTL activation. In soluble form, the complete T-Ag or the N-terminal T-Ag fragment sensitized in vitro RBL5 cells for lysis by T-Ag-specific CTL lines and clones. This in vitro sensitization was blocked by brefeldin A. In contrast, specific recognition of RBL5 cells pulsed in vitro with synthetic, immunogenic nonapeptides (derived from N-terminal T-Ag epitopes) by CTL lines was insensitive to brefeldin A. Hence, T-Ag and its 272-aa N-terminal fragment can enter the "endogenous" processing pathway and prime CD8+ CTL in vivo and in vitro.

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

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

A "string-of-beads" vaccine, comprising linked minigenes, confers protection from lethal-dose virus challenge

We have previously demonstrated that induction of antiviral cytotoxic T lymphocytes (CTL), in the absence of antiviral antibodies, can confer protection against a lethal-dose virus challenge. Here we extend those findings as follows. First, three discrete viral CTL epitopes expressed from minigenes encoding peptides as short as 12 amino acids can be recognized when expressed from recombinant vaccinia virus; second, concentrating on two of the three epitopes, we show that these vaccinia virus recombinants can confer protection in a major histocompatibility complex (MHC)-restricted manner; third, the minigenes can be fused to generate a "string of beads," and the close proximity of the two epitopes within one oligopeptide does not disrupt recognition of either epitope; fourth, this string-of-beads vaccine, in contrast to the single epitope vaccines, can protect on both MHC backgrounds; and, fifth, CTL to different epitopes may act synergistically, as protection is improved when the vaccine contains more than one CTL epitope for a given MHC background.

Expression of a membrane protease enhances presentation of endogenous antigens to MHC class I-restricted T lymphocytes

We find that expression of the membrane dipeptidyl carboxypeptidase angiotensin-converting enzyme (ACE) enhances presentation of certain endogenously synthesized peptides to major histocompatibility complex (MHC) class I-restricted cytotoxic T lymphocytes. ACE appears to function only in an intracellular secretory compartment of antigen-presenting cells. ACE-enhanced antigen presentation requires the expression of the putative antigenic peptide transporters, TAP1 and TAP2. These findings demonstrate that a protease can influence the processing of endogenously synthesized antigens and strongly suggest that longer peptides can be transported from the cytosol to a secretory compartment where trimming of antigenic peptides to the lengths preferred by MHC class I molecules can occur if the appropriate protease is present.

A transient transfection system for identifying biosynthesized proteins processed and presented to class I MHC restricted T lymphocytes

CD8+ cytotoxic T lymphocytes (CTL) constitute a major portion of immune responses to foreign and self antigens. CTL recognize class I major histocompatibility complex molecules complexed to peptides of 8-10 residues derived from cytosolic proteins. To understand CTL responses to these antigens and to manipulate CTL responses optimally, it is necessary to identify the specific peptides recognized by CTL. The methods currently used for this purpose have significant drawbacks. We describe a plasmid transfection method that results in significant lysis of histocompatible target cells. Influenza virus-specific CTLs specifically lysed target cells that were transfected with plasmids bearing cDNAs encoding full length gene products, fragments containing the region that encodes the CTL epitope, or even a ten residue peptide. This significantly lessens the time and effort required to define genes, and gene segments that contain CTL epitopes.

Restoration of the LFA-3 adhesion pathway in Burkitt's lymphoma cells using an LFA-3 recombinant vaccinia virus: consequences for T cell recognition

Conjugate formation between cytotoxic T lymphocytes (CTL) and target B cells, as observed in vitro, is mediated by interactions between adhesion molecules on the two cell surfaces rather than involving immune recognition through the T cell receptor. It is still not clear to what extent such adhesive contacts facilitate the process of immune recognition and target cell lysis. However, work on the Epstein-Barr virus (EBV)-associated malignancy Burkitt's lymphoma (BL) has suggested that down-regulation of one particular adhesion molecule, the lymphocyte function-associated antigen LFA-3, on the tumor cell surface is a key factor in allowing these target cells to escape EBV-specific T cell surveillance. To examine this directly, we used a cDNA for the full-length transmembrane form of LFA-3 to construct a recombinant vaccinia virus (Vacc-LFA 3), which is capable of restoring surface LFA-3 in adhesion molecule-negative BL cell lines to levels as high as seen in EBV-transformed lymphoblastoid cell lines (LCL); biochemical studies confirmed expression of the authentic N-glycosylated protein. The recombinant vaccinia-encoded LFA-3 was functional as an adhesion molecule since BL cells acutely infected with Vacc-LFA-3 then acquired the ability to form conjugates with activated T cells in vitro. However, there was no clear dependence upon LFA-3 when such BL cell lines were tested as targets for cytotoxic T lymphocytes (CTL). Firstly, LFA-3- BL cells could be killed by allospecific CTL recognizing HLA class I alloantigens, in some cases as efficiently as the corresponding LCL. In other cases where lysis was slightly below that of the LCL, Vacc-LFA-3 infection of the BL cells increased lysis up to, but never beyond, LCL values. Secondly, we studied the sensitivity of BL to EBV-specific HLA class I-restricted CTL using a BL target line which was LFA-3- but which expressed the same spectrum of EBV target proteins as an LCL. This line was not recognized by appropriately HLA-matched effectors, even after restoration of LFA-3 expression. We conclude that the LFA-3 status of BL cells influences their conjugate forming ability in in vitro assays but not necessarily their sensitivity to immune T cell-mediated cytolysis.

Processing pathways for presentation of cytosolic antigen to MHC class II-restricted T cells

Antigens presented to CD4+ T cells derive primarily from exogenous proteins that are processed into peptides capable of binding to class II major histocompatibility complex (MHC) molecules in an endocytic compartment. In contrast, antigens presented to CD8+ T cells derive mostly from proteins processed in the cytosol, and peptide loading onto class I MHC molecules in an early exocytic compartment is dependent on a transporter for antigen presentation encoded in the class II MHC region. Endogenous cytosolic antigen can also be presented by class II molecules. Here we show that, unlike class I-restricted recognition of antigen, HLA-DR1-restricted recognition of cytosolic antigen occurs in mutant cells without a transporter for antigen presentation. In contrast, DR1-restricted recognition of a short cytosolic peptide is dependent on such a transporter. Thus helper T-cell epitopes can be generated from cytosolic antigens by several mechanisms, one of which is distinct from the classical class I pathway.

A nonimmunogenic sarcoma transduced with the cDNA for interferon gamma elicits CD8+ T cells against the wild-type tumor: correlation with antigen presentation capability

To be recognized by CD8+ T lymphocytes, target cells must process and present peptide antigens in the context of major histocompatibility complex (MHC) class I molecules. The nonimmunogenic, low class I-expressing, methylcholanthrene (MCA)-induced murine sarcoma cell line, MCA 101, is a poor presenter of endogenously generated viral antigens to specific CD8+ T lymphocytes and cannot be used to generate tumor infiltrating lymphocytes (TIL). Since interferon gamma (IFN-gamma) has been shown to upregulate three sets of molecules important for antigen processing and presentation, we retrovirally transduced wild-type MCA 101 (101.WT) tumor with the mIFN-gamma cDNA to create the 101.NAT cell line. Unlike 101.WT, some clones of retrovirally transduced 101.NAT tumor expressed high levels of class I, and could be used to generate CD8+ TIL. More importantly, these TIL were therapeutic in vivo against established pulmonary metastases from the wild-type tumor. Although not uniformly cytotoxic amongst several separate cultures, these TIL did specifically release cytokines (IFN-gamma and tumor necrosis factor-alpha) in response to 101.WT targets. 101.WT's antigen presentation deficit was also reversed by gene modification with mIFN-gamma cDNA. 101.NAT had a greatly improved capacity to present viral antigens to CD8+ cytotoxic T lymphocytes. These findings show that a nonimmunogenic tumor, incapable of generating a CD8+ T cell immune response, could be gene-modified to generate a therapeutically useful immune response against the wild-type tumor. This strategy may be useful in developing treatments for tumor histologies not thought to be susceptible to T cell-based immunotherapy.

Immunization with soluble simian virus 40 large T antigen induces a specific response of CD3+ CD4- CD8+ cytotoxic T lymphocytes in mice

C57BL/6 (B6) mice (H-2b) were immunized with the large tumor antigen (T Ag) of simian virus 40 (SV40). Intraperitoneal or subcutaneous sensitization with soluble T Ag specifically primed cytotoxic lymphocyte precursors (CTLp). T Ag-specific cytotoxic T lymphocytes (CTL) were detected in a cytotoxicity assay after specific in vitro restimulation of effector cell populations from mice immunized with 2-10 micrograms purified, soluble T Ag and boosted with an injection of 2 micrograms T Ag 2-4 weeks after priming. Cells used for in vitro restimulation and as targets in cytotoxicity assays were syngeneic (B6-derived) RBL5 lymphoma cells expressing SV40 T Ag after transfection with a T Ag-encoding expression vector. Effector cells of this response were H-2 class I-restricted CD3+ CD4-CD8+ CTL. The magnitude of the anti-T Ag CTL response of B6 mice stimulated by soluble virus protein was comparable to the anti-T Ag CTL response of SV40-infected B6 mice. Injections of denatured or native T Ag protein primed CTLp equally well, but immunization with an equal dose of antigen emulsified in incomplete Freund's adjuvants inefficiently stimulated CTLp.

A defect in the presentation of intracellular viral antigens is restored by interferon-gamma in cell lines with impaired major histocompatibility complex class I assembly

Surface expression of the majority of class I major histocompatibility complex (MHC) heavy chains is known to require assembly with beta 2 microglobulin (beta 2m). To define other factors involved in class I MHC assembly, we have studied two tumor cell lines that are deficient in cell surface class I (H-2) expression. The BC2 fibrosarcoma and the CMT lung carcinoma express only intracellular unassociated heavy chains despite the presence of beta 2m. As described previously, when these cell lines are treated with interferon-gamma (IFN-gamma), they are capable of assembling and transporting class I molecules to the cell surface. In this study, we have shown that in the absence of IFN-gamma these mutant cells are unable to present intracellular viral antigens, although they can be lysed by specific cytotoxic T lymphocyte (CTL) after pre-incubation with the corresponding synthetic peptide. Flow cytometric analysis demonstrated that extracellular peptide was capable of increasing twofold the surface expression of beta 2m-heavy chain complexes. Furthermore, immunoprecipitation experiments confirmed that peptide stabilizes chain association in the BC2 cell lysates. However, infecting these mutants with vectors expressing either pre-processed antigen or rapidly degraded antigen, failed to overcome their defect in the presentation of endogenous peptide to specific CTL or to mediate surface expression of class I MHC. Preincubation with IFN-gamma completely reversed the endogenous peptide presentation defect, even in mutant cells transfected with a vector encoding a cDNA for the H-2 molecule restricting CTL recognition. This last result suggests that IFN-gamma corrects the defect by a mechanism separate from simple enhancement of the number of class I molecules produced by the cell. Because there is growing evidence that endogenous peptides can participate in class I MHC assembly, the defect in these mutants could be ascribed to the lack of access to class I molecules by the endogenous peptide. This would prevent stable association of the heavy and light chains and their subsequent transport. Our data suggests that IFN-gamma reestablishes class I MHC surface expression by restoring access of endogenously synthesized peptide to class I molecules.

Flanking sequences influence the presentation of an endogenously synthesized peptide to cytotoxic T lymphocytes

Cytotoxic T lymphocytes (CTL) recognize class I major histocompatibility complex molecules complexed to peptides of eight to nine residues generated from cytosolic proteins. We find that CTL recognize, in vitro and in vivo, cells synthesizing a 10-residue peptide consisting of an initiating methionine followed by nine residues corresponding to a naturally processed determinant from influenza virus nucleoprotein (NP) (residues 147-155). Addition of two COOH-terminal residues corresponding to NP residues 157 and 158 severely reduced presentation of the endogenously produced peptide to CTL in vitro and in vivo. Extension of NH2 and COOH terminal flanking residues to include residues corresponding to NP residues 137-146 and 159-168 failed to increase the antigenicity of this peptide. Its presentation was greatly enhanced, however, by further extending the NH2 and COOH termini to include all of the additional residues of NP. These findings indicate first, that a naturally processed viral ligand (with an NH2-terminal Met) of a class I molecule contains sufficient information to access intracellular class I molecules, and second, that flanking residues can influence the processing and presentation of antigens to CTL.

Characterization of two distinct major histocompatibility complex class I Kk-restricted T-cell epitopes within the influenza A/PR/8/34 virus hemagglutinin

Cytotoxic T-lymphocyte (CTL) clones specific for the influenza A/PR/8/34 virus hemagglutinin (HA) were isolated by priming CBA mice with a recombinant vaccinia virus expressing the HA molecule. The epitopes recognized by two of these clones, which were CD8+, Kk restricted, and HA subtype specific, were defined by using a combination of recombinant vaccinia viruses expressing HA fragments and synthetic peptides. One epitope is in the HA1 subunit at residues 259 to 266 (numbering from the initiator methionine), amino acid sequence FEANGNLI, and the other epitope is in the HA2 subunit at residues 10 to 18 (numbering from the amino terminus of the HA2 subunit), sequence IEGGWTGMI. These two peptides are good candidates for naturally processed HA epitopes presented during influenza infection, as they are the same length (eight and nine residues) as other naturally processed viral peptides presented to CTL. A comparison of the sequences of these two new epitopes with those of the three previously published Kk-restricted T-cell epitopes showed some homology among all of the epitopes, suggesting a binding motif. In particular, an isoleucine residue at the carboxy-terminal end is present in all of the epitopes. On the basis of this homology, we predicted that the Kk-restricted epitope in influenza virus nucleoprotein, previously defined as residues 50 to 63, was contained within residues 50 to 57, sequence SDYEGRLI. This shorter peptide was found to sensitize target cells at a 200-fold lower concentration than did nucleoprotein residues 50 to 63 when tested with a CTL clone, confirming the alignment of Kk-restricted epitopes.

Defective presentation of endogenous antigens by a murine sarcoma. Implications for the failure of an anti-tumor immune response

MHC class I-restricted CTL play a central role in the immune response against methylcholanthrene (MCA)-induced sarcomas in mice. We, therefore, hypothesized that MCA-induced tumors may evade immune recognition by failing to present Ag to CD8+ CTL. Of a number of previously described MCA-induced sarcomas, one, MCA 101, fails to induce CTL, is nonimmunogenic, and grows rapidly and lethally in nonimmunosuppressed recipients. To better understand the nonimmunogenicity of MCA 101 we examined its ability to present foreign Ag to CTL. Unlike immunogenic sarcomas, MCA 101 failed to present endogenously synthesized influenza virus Ag to influenza virus-specific CTL. The deficiency in presentation of endogenous Ag by MCA 101 was attributed to a markedly reduced rate of synthesis of class I molecules because up-regulation of class I synthesis by IFN-gamma greatly increased the presentation of influenza A virus Ag. Despite low levels of cell surface class I expression, MCA 101 presented exogenous peptide Ag to anti-influenza CTL with efficiency similar to immunogenic MCA sarcoma cell lines. These findings could not be attributed to deficiencies in class I assembly or transport, as has been suggested by others who have studied mutant cells with defective Ag presentation. Furthermore, our studies suggest that some tumor cells can escape recognition by CTL and subsequent immune eradication by suppressing presentation of endogenous Ag.

Protection against lethal cytomegalovirus infection by a recombinant vaccine containing a single nonameric T-cell epitope

The regulatory immediate-early (IE) protein pp89 of murine cytomegalovirus induces CD8+ T lymphocytes that protect against lethal murine cytomegalovirus infection. The IE1 epitope is the only epitope of pp89 that is recognized by BALB/c cytolytic T lymphocytes (CTL). Using synthetic peptides, the optimal and minimal antigenic sequences of the IE1 epitope have been defined. To evaluate the predictive value of data obtained with synthetic peptides, recombinant vaccines encoding this single T-cell epitope were constructed using as a vector the hepatitis B virus core antigen encoded in recombinant vaccinia virus. In infected cells expressing the chimeric proteins, only IE1 epitope sequences that were recognized as synthetic peptides at concentrations lower than 10(-6) M were presented to CTL. Vaccination of mice with the recombinant vaccinia virus that encoded a chimeric protein carrying the optimal 9-amino-acid IE1 epitope sequence elicited CD8+ T lymphocytes with antiviral activity and, furthermore, protected against lethal disease. The results thus show for the first time that recombinant vaccines containing a single foreign nonameric CTL epitope can induce T-lymphocyte-mediated protective immunity.

Vaccinia virus: a tool for research and vaccine development

Vaccinia virus is no longer needed for smallpox immunization, but now serves as a useful vector for expressing genes within the cytoplasm of eukaryotic cells. As a research tool, recombinant vaccinia viruses are used to synthesize biologically active proteins and analyze structure-function relations, determine the targets of humoral- and cell-mediated immunity, and investigate the immune responses needed for protection against specific infectious diseases. When more data on safety and efficacy are available, recombinant vaccinia and related poxviruses may be candidates for live vaccines and for cancer immunotherapy.

Signals arising from antigen-presenting cells

It has been customary to consider that antigen-presenting cells provide, in addition to the presented antigen, a second or co-stimulatory signal that leads to T-cell growth and effector function. The recent literature indicates that this two-signal notion oversimplifies the function of antigen-presenting cells. Instead it is useful to consider four groups of events: the formation of peptide-MHC complexes, the role of soluble cytokines, the action of antigen-presenting cell-T cell molecular couples distinct from the receptor for peptide MHC, and the function of antigen-presenting cells in situ.

Strategies for exploiting the immune system in the design of vaccines

There are three types of vaccines in medical use today--live, attenuated agents, mainly viruses; inactivated whole organisms; and subunit preparations. Though there are examples in each category of successful preparations, live attenuated viruses have almost invariably given long-lasting immunity after one or two administrations. Contributing reasons for this are gleaned, not from human vaccine studies, but from model systems and it is concluded that a vaccine needs to achieve four goals: activation of antigen-presenting cells; overcoming genetic variability in T cell responses; generation of high levels of T and B memory cells; and persistence of antigen for recruitment of B memory cells. Of the newer approaches to vaccine development, synthetic peptides have substantial limitations but should be successful in some cases. Subunit preparations may also be limited as a general method. Some live viral or bacterial vaccines, used as vectors of nucleic acid coding for other antigens, hold considerable promise as is illustrated by recent examples.

Pathways of antigen processing

Separate pathways exist for the processing of antigens to be presented by MHC class I and class II molecules. We are beginning to determine the subcellular location of certain events in both pathways.

Sequences encoded in the class II region of the MHC related to the 'ABC' superfamily of transporters

Class I molecules of the major histocompatibility complex (MHC) bind and present peptides derived from the degradation of intracellular, often cytoplasmic, proteins, whereas class II molecules usually present proteins from the extracellular environment. It is not known how peptides derived from cytoplasmic proteins cross a membrane before presentation at the cell surface. But certain mutations in the MHC can prevent presentation of antigens with class I molecules. In addition, mutations possibly in the MHC can affect presentation by class II molecules. Here we report the finding of a new gene in the MHC that might have a role in antigen presentation and which is related to the ABC (ATP-binding cassette) superfamily of transporters. This superfamily includes the human multidrug-resistance protein, and a series of transporters from bacteria and eukaryotic cells capable of transporting a range of substrates, including peptides.

Isolation and analysis of naturally processed viral peptides as recognized by cytotoxic T cells

Virus-infected cells can be eliminated by cytotoxic T lymphocytes (CTL), which recognize virus-derived peptides bound to major histocompatibility complex (MHC) class I molecules on the cell surface. Until now, this notion has relied on overwhelming but indirect evidence, as the existence of naturally processed viral peptides has not been previously reported. Here we show that such peptides can be extracted from virus-infected cells by acid elution. Both the naturally processed H-2-Db-restricted and H-2-Kd-restricted peptides from influenza nucleoprotein are smaller than the corresponding synthetic peptides, which have first been used to determine the respective CTL epitopes. As with minor histocompatibility antigens, occurrence of viral peptides seems to be heavily dependent on MHC class I molecules, because infected H-2d cells do not contain the H-2-Db-restricted peptide, and infected H-2b cells do not contain the H-2-Kd-restricted peptide. Our data provide direct experimental proof for the above notion on MHC-associated viral peptides on virus-infected cells.

Assessment of the specificity of cytotoxic T lymphocytes for the nucleoprotein of Pichinde virus using recombinant vaccinia viruses

Pichinde virus (PV) infection of mice results in induction of a strong H-2 restricted, virus-specific cytotoxic T lymphocyte (CTL) response and rapid clearance of the virus. To define the specificities of CTL induced by PV infection, we constructed vaccinia virus recombinants containing cloned cDNAs corresponding to full-length (VVNP) and a truncated form (VVNP 51-561) of the nucleoprotein (NP) gene of PV. Radioimmunoprecipitation analysis of infected cell lysates indicated that VVNP expressed a PV-specific product identical in size to that of authentic NP, while vaccinia virus recombinants containing truncated NP produced a polypeptide consistent with the synthesis of amino acids 51-561 of Pichinde virus NP. Interestingly, cells infected with VVNP synthesized easily detectable, but much lower levels of nucleoprotein relative to both PV and VVNP51-561. Primary virus-specific CTL induced in three different strains of inbred mice following intravenous infection with PV were able to lyse syngeneic target cells infected with PV but did not markedly lyse syngeneic targets expressing full-length or truncated NP following recombinant vaccinia virus infection. Similarly, secondary anti-PV specific CTL generated following in vitro restimulation by PV or selectively restimulated with vaccinia recombinants did not significantly lyse target cells expressing NP. Further, infection of mice with VVNP and VVNP51-561 did not induce CTLs specific for PV and did not prime mice for the generation of memory anti-PV CTL in vivo. These results suggest that PV gene products other than NP, such as the GPC or L protein, contain the major target epitope(s) recognized by PV-specific CTL.

An endogenous processing pathway in vaccinia virus-infected cells for presentation of cytoplasmic antigens to class II-restricted T cells

The recognition of virus-infected cells by class I MHC-restricted cytotoxic T cells requires endogenous processing of antigen for presentation. It is still unclear whether endogenous processing of antigen can be utilized by class II MHC molecules for presentation. To test this possibility, a human B cell line expressing HLA-A2 and HLA-DR1 was infected with a recombinant vaccinia virus expressing the Influenza A virus M1 matrix protein (VAC-M1) and was assayed for lysis by different M1-specific cytolytic T cell lines, restricted by either HLA-A2 or by HLA-DR1. Class II-restricted lysis of VAC-M1-infected cells did occur. This lysis required de novo M1 synthesis and was not due to exogenous antigen. Several properties of the endogenous processing pathway for class II-restricted presentation were different from those of the pathway utilized by class I molecules. First, class II-mediated recognition of VAC-M1 infected cells was less efficient, requiring higher doses of virus and longer infection times, than the class I-mediated recognition. Second, chloroquine completely blocked presentation of endogenous M1 to class II-restricted T cells but had no effect on the class I-restricted presentation. Third, the class II-restricted presentation of M1 was only mildly affected by Brefeldin A, a drug that prevents transport from the endoplasmic reticulum to the Golgi, whereas the class I-restricted presentation of M1 was completely abrogated by this drug. These data demonstrate the existence of an endogenous processing pathway for the presentation of cytosolic antigen by class II molecules and show that this pathway is distinct from the one used for presentation by class I molecules.