Antigen recognition by H-2-restricted T cells. I. Cell-free antigen processing

Identical peptides recognized by MHC class I- and II-restricted T cells

Previous data from many groups show that both class I and class II-restricted T cells recognize short synthetic peptides in the context of their respective MHC molecules (9-18), all of the peptides described to date are restricted to only a single class of MHC molecules; however, structural homology between the class I and II MHC molecules and the use of similar TCRs by class I and II-restricted T cells suggest that antigen recognition mechanisms are similar in both systems. To directly compare antigen recognition in the two systems, we analyzed peptides for the ability to function in both a class I and II-restricted system and found that seven of seven individual peptides tested stimulate both class I and II-restricted T cell responses. In addition, two of the peptides can function in different species stimulating both human class I and murine class II T cell responses. Thus, the process of T cell recognition of antigen in the context of MHC molecules was highly conserved in evolution not only between the class I and class II MHC systems, but also between the murine and human species.

Reconstruction of the immunogenic peptide RNase(43-56) by identification and transfer of the critical residues into an unrelated peptide backbone

The involvement of each of the amino acid residues of the I-Ak-restricted T cell determinant RNase(43-56) was examined in detail using a series of peptides containing single amino acid substitutions. Four positions were identified as being essential for the formation of the determinant, Phe-46, Val-47, His-48, and Leu-51. When these four residues were substituted into the backbone of the unrelated peptide HA(130-144), a nonstimulatory peptide was obtained. The inclusion of an additional residue, Val-54, resulted in a chimeric peptide, RN/HA2, which was nearly as active as the native molecule. The peptide RN/HA2 was able to prime in vivo for RNase reactivity, confirming that these five residues contained all of the specificity of the RNase(43-56) determinant. The role of three of these critical residues was examined using both a functional competition assay and an in vivo priming assay. It was ascertained that the Phe-46 was directly involved in contacting the TCR, while the His-48 and Leu-51 were either involved in binding to the I-Ak molecule or in determining the conformation of the peptide. Thus, by critically evaluating the contribution of each of the amino acid residues in a T cell determinant, we were able to generate a chimeric peptide only containing 5 of 15 residues from the RNase(43-56) sequence that was functionally identical to the native RNase(43-56) molecule both in vitro and in vivo.

Self recognition by T cells. I. Bystander killing of target cells bearing syngeneic MHC antigens

Activated CTL can kill any cell to which they bind or by which they are bound. This observation has been used to determine whether alloreactive CTL can recognize cells bearing self-MHC. When activated by their specific targets, 19 CTL clones of 4 different specificities and origins killed bystander targets bearing syngeneic but not third-party MHC antigens. Using target cells derived from MHC-recombinant animals, syngeneic bystander killing was shown to be restricted to a single self MHC-encoded molecule. These results provide the first clear demonstration that T cells, or more precisely CTL, are capable of self recognition in the absence of their specific antigen. Our findings support the model that T cell repertoire selection occurs as a result of positive selection during maturation in the thymus of precursor cells whose antigen receptors have low but real affinity for self-MHC.

Relative contribution of "determinant selection" and "holes in the T-cell repertoire" to T-cell responses

Using BALB/c and CBA/J mice, the I-region associated (Ia) binding capacity and T-cell immunogenicity of a panel of 14 overlapping peptides that span the entire sequence of the protein staphylococcal nuclease (Nase) was examined to evaluate major histocompatibility gene complex (MHC) control of T-cell responses. Ia binding and Ia-restricted T-cell immunogenicity could be determined for a total of 54 peptide-MHC combinations. Only 30% of the 54 instances examined involved detectable Ia binding, but they represented almost all (12 of 13) of the immune responses found. However, binding to Ia was not sufficient to ensure T-cell immunogenicity, since only 70% of the binding events were productive--i.e., were associated with an immune response. Thus, Ia molecules have the expected characteristics of a highly permissive capacity for antigen interaction that allows them to function as restriction elements for a large universe of antigens. On the other hand, since the Ia molecules cannot distinguish between self and non-self, not all antigen-Ia interactions would be permitted to elicit a T-cell response. It appears that both Ia binding ("determinant selection") and T-cell repertoire act in concert to define the immune response status of an individual toward any particular T-cell epitope.

Inhibition of T cell response with peptides is influenced by both peptide-binding specificity of major histocompatibility complex molecules and susceptibility of T cells to blocking

Synthetic peptides corresponding to sequences 46-62 and 51-62 of mouse lysozyme and 46-61 of hen egg-white lysozyme (HEL) were used as competitors in a variety of T cell responses. The competitors, according to their binding specificity for major histocompatibility complex (MHC) were expected to inhibit T cell responses restricted to I-Ak, but not those restricted to I-Ad, I-Ek molecules. In competition experiments with T cell hybridomas, the poor binder I-Ed molecule required 10- to 15-fold higher competitor concentrations than the good binder I-Ak molecule to achieve 50% inhibition of antigen presentation. Similarly, the nonresponder state of H-2d mice to HEL peptide 46-61 could be overcome by increasing the immunizing dose, and proliferative T cell responses to different antigens in association with a variety of class II MHC molecules could be blocked by the mouse lysozyme and HEL peptides. Thus, the capability of some and failure of other MHC molecules to bind certain peptides appeared quantitative, rather than of an all or none nature, in these experimental systems. The susceptibility of uncloned T cell lines to peptide competitors was found to decrease with time. Lines maintained by repeated restimulation with antigen and APC, but without exogenous interleukin 2, acquired resistance within weeks. In contrast, T cell clones retained their susceptibility to peptide competitors over a long period of time. The latter data raise the possibility that a competition between ubiquitous (self) peptides and foreign antigen may result in the selection of T cells that have high avidity for the activating antigen-MHC complex, and are thus relatively resistant to competition at the level of antigen presentation.

Binding of labelled influenza matrix peptide to HLA DR in living B lymphoid cells

T cells recognize protein antigens as fragments (peptides) held in a defined binding site of class I or class II major histocompatibility (MHC) molecules. The formation of complexes between various immunologically active peptides and different MHC molecules has been demonstrated directly in binding studies between the peptides and solubilized, purified molecules of class II MHC. Studies with intact cells, living or fixed, have not directly demonstrated the binding of the peptides to MHC molecules on antigen-presenting cells, but the formation of such complexes has been shown indirectly through the capacity of antigen-presenting cells to stimulate specific T cells. Here we report evidence that supports directly the binding of radiolabelled influenza matrix peptide 17-29 to products of the human class II MHC locus HLA-DR, on living homozygous B-cell lines, and we show that the kinetics of such binding is much faster with living cells than with fixed cells. Furthermore, whereas the peptide reacts with HLA-DR molecules of all alleles, it binds preferentially to DR1, the restricting element in antigen presentation.

Influence of the valency and hydrophobicity of an antigen on its efficiency of processing and presentation by antigen-specific B cells

We examined the influence of antigen structure on the efficiency of antigen-specific B-cell processing and presentation. The ability of untransfected and anti-TNP surface IgM (sIgM) bearing transfected TA3 B-hybridoma cells to process and present various conjugates of 2,4,6-trinitrophenyl-pork insulin (TNP-PI) to PI/I-Ad-specific T cells was investigated. Similar antigen concentrations were required for presentation of underivatized antigen by untransfected and transfected cells (antigen-non-specific presentation). Transfected cells present TNP-PI at about a 50-fold lower concentration and TNP-ovalbumin (TNP-OVA), at a 600-fold lower concentration than do untransfected cells (antigen-specific presentation). PI was derivatized at 3 possible residues, A1, B1 and B29, and the different TNP-PI conjugates obtained were separated by hydrophobic interaction high-pressure liquid chromatography. All TNP1-PI conjugates were presented equally by transfected and untransfected cells. Transfected TA3 cells presented a TNP2-PI conjugate derivatized at A1 and B29, TNP2(A1,B29)-PI, at about a 50-fold lower concentration than was required by untrasfected cells. Another TNP2-PI conjugate, derivatized at residues A1 and B1, TNP2(A1,B1)-PI, and a TNP3(A1,B1,B29)-PI conjugate were presented at about the same concentration by transfected vs untransfected B cells. Of the various TNP conjugates tested, only TNP2(A1,B29)-PI, which is more hydrophobic than any of the other TNP-PI conjugates, was processed more efficiently by transfected anti-TNP-specific TA3 cells vs untransfected cells. The efficiency of presentation of these TNP-PI conjugates was directly proportional to their rate of processing. Our data demonstrate that the valency and relative hydrophobicity of an antigen control its binding to sIgM and the efficiency of processing and presentation by B cells.

Presentation of antigen to T lymphocytes by non-immune B-cell hybridoma clones: evidence for specific and non-specific presentation

Non-immune SJL (H-2s) spleen cells were fused with non-secreting, non-antigen presenting (H-2d) Balb/c 653-myeloma cells and the hybridomas were cloned by two limiting dilutions. The resulting hybrid B-cell clones were tested for their antigen presentation capability to SJL T-cell lines that were specific for either lysozyme or myoglobin. In proliferative assays, 53% of the antigen presenting B-cell clones presented both myoglobin and lysozyme (general presenters) while the other 47% presented specifically either myoglobin or lysozyme (specific presenters). The ability to selectively present either myoglobin or lysozyme indicates that antigen presentation at the clonal level can be specific or non-specific depending on the particular B-cell clone.

Two genetically identical antigen-presenting cell clones display heterogeneity in antigen processing

Evidence from various antigen systems suggests that antigen processing can be one factor that determines the repertoire of immunogenic peptides. Thus, processing events may account for some of the disparity between the available and expressed helper T-cell repertoires. In this report, we demonstrate that the immunodominant T-cell determinant in ovalbumin [p323-339; ovalbumin-(323-339) heptadecapeptide] is processed differently by two genetically identical antigen-presenting cell lines, M12 and A20. The ovalbumin-specific T-cell-T-cell hybridomas, DO-11.10 and 3DO-54.8, were used to detect processed antigen. These T-T hybridomas have different fine specificities for the p323-339 determinant. A20 cells presented native ovalbumin well to both T-T hybridomas, whereas M12 cells presented native ovalbumin well to 3DO-54.8 but very inefficiently to DO-11.10. M12 and A20 cells effectively stimulated both T-T hybridomas with the same concentrations of the immunogenic synthetic peptide p323-339. Therefore, M12 cells and DO-11.10 can interact with each other, and both T-T hybridomas have similar sensitivities for the same immunogenic peptide. We conclude that genetically identical antigen-presenting cells can display heterogeneity in the fine processing of an immunodominant T-cell determinant, and synthetic model peptides that represent the minimal stimulatory sequence of a T-cell determinant are not necessarily identical to the structure of in vivo processed antigen. Heterogeneity in antigen processing by individual antigen-presenting cells would serve to increase the repertoire of immunogenic peptides that are presented to T cells.

Immunogenicity: role of dendritic cells

In the development of the immune response, the dendritic cell subset of leukocytes plays a key role in enhnacing immunogenicity. Dendritic cells can pick up antigens in the tissues and move to lymphoid organs, through which T cells continually recirculate. It is proposed that dendritic cells at these sites express functions which have been identified in tissue culture models. These involve efficient binding to antigen-specific T lymphocytes, as well as the induction of the lymphokines and growth factor receptors required for immunity. The dendritic cell system, apparently under the control of cytokines, is a sentinel designed to signal T cells that a significant antigen burden is present, and to generate the activated T lymphoblasts that interact with many other cell types to bring about an immune response.

Obligatory role of macrophages in dengue virus antigen presentation to B lymphocytes

The study was undertaken to investigate the role of dengue type 2 virus (DV)-infected mouse peritoneal macrophages (M phi) in presentation of the DV antigen to B lymphocytes as shown by counting virus-specific IgM antibody plaque-forming cells (PFC). It was observed that heat-killed or glutaraldehyde-fixed M phi did not present the antigen. Pretreatment of M phi with the lysosomotropic compounds ammonium chloride and chloroquine inhibited the antigen presentation. Depletion of M phi from the spleen cell cultures abrogated the immune response to DV. The tryptic-digested DV antigen could stimulate immune responses in B-lymphocyte enriched (depleted of M phi and T cells) spleen cell cultures, and the digested antigen could be presented by glutaraldehyde-fixed M phi. Pretreatment of M phi with a trypsin inhibitor abrogated antigen presentation. The findings thus show that even for presentation to B cells the DV antigen must be processed by M phi by a trypsin-like protease.

Antigen presentation in the sheep: generation of antigen-specific T-cell lines

Antigen-specific sheep T-cell lines have been generated in vitro from peripheral blood mononuclear cells (PBMC). PBMC prepared from antigen-primed animals were cultured in the presence of ovalbumin (OVA) or purified protein derivative (PPD). After 5 days of culture, activated antigen-specific cells were expanded by further culture in the presence of recombinant human interleukin-2 (IL-2). Cell lines generated after two cycles of antigen stimulation followed by expansion with IL-2 show a proliferative response to antigen only in the presence of autologous antigen-presenting cells (APC) and recognize only the antigen used in the original stimulation. An OVA-specific cell line was found to be capable of recognizing a synthetic peptide corresponding to amino acid residues 323-338 of OVA. The cell lines also responded by proliferation in an allogeneic mixed leucocyte reaction (MLR). Cell-surface phenotyping shows that the cell lines comprise both CD4- and CD8-positive cells.

Lewis rat lymphoid dendritic cells can efficiently present homologous myelin basic protein to encephalitogenic lymphocytes

Lymphocytes isolated from Lewis rats immunised with protein antigen in adjuvant were stimulated to proliferate in vitro by splenic dendritic cells (DC) which had been pulsed with purified homologous myelin basic protein (MBP). By contrast, in parallel experiments, lymphocytes did not respond to ovalbumin unless the protein was first processed by macrophages by a chloroquine-sensitive mechanism. DC, pulsed with rat MBP at concentrations as low as 6 micrograms/ml, activated lymphocytes for transfer of severe experimental autoimmune encephalomyelitis (EAE). MBP dissociating from myelin membranes in physiological medium, as well as MBP purified from highly acidic extracts of myelin, was effective for pulsing DC; preincubating myelin with macrophages led to a reduction rather than an enhancement in the severity of the EAE transferred. It is concluded that macrophage-mediated antigen processing is not required for immunogenic presentation of the determinants of MBP which cause EAE in Lewis rats. Furthermore, MBP-pulsed DC may prove useful in experiments requiring activation of encephalitogenic T cells.

Presentation of exogenous protein antigens by dendritic cells to T cell clones. Intact protein is presented best by immature, epidermal Langerhans cells

The capacity of dendritic cells to present protein antigens has been studied with two MHC class II-restricted, myoglobin-specific, T cell clones. Spleen dendritic cells and cultured epidermal Langerhans cells (LC) presented native myoglobin weakly and often not at all. These same populations were powerful stimulators of allogeneic T cells in the primary MLR. Freshly isolated LC were in contrast very active in presenting proteins to T cell clones but were weak stimulators of the MLR. Both fresh and cultured LC could present specific peptide fragments of myoglobin to the clones. These results suggest that dendritic cells in nonlymphoid tissues like skin can act as sentinels for presenting antigens in situ, their accessory function developing in two phases. First antigens are captured and appropriately presented. Further handling of antigen then is downregulated while the cells acquire strong sensitizing activity for the growth and function of resting T lymphocytes. The potent MLR stimulating activity of cultured epidermal LC and lymphoid dendritic cells probably reflects prior handling of antigens leading to the formation of allogeneic MHC-peptide complexes.

Inhibition by leupeptin and antipain of the intracellular proteolysis of Ii

Intracellular cleavage of Ii was evaluated in immunoprecipitates of radiolabeled Raji cells treated with protease inhibitors (leupeptin, antipain, chymostatin, and pepstatin) or blockers of endosomal function (chloroquine and monensin). Immunoprecipitates with anti-class II and anti-Ii(12-28) sera and VIC-Y1 MoAb revealed Ii cleavage products of 21,000 and 10,000 daltons (p21 and p10) only in leupeptin- and antipain-treated cells. Both p21 and p10 were judged to be N-terminal products because they were recognized with anti-Ii(12-28) and not with anti-Ii(183-193) or anti-Ii(192-211) sera. p10 might be derived from p21 because its intensity was increased in inverse proportion to p21 as a function of leupeptin or antipain concentration. p21, but not p10, was recognized by anti-class II antibody and thus might originate from class II-associated Ii. In pulse-chase studies, p21 and p10 appeared at 2 hr and later after Ii synthesis. p25, an Ii C-terminal fragment, was about 60% reduced by leupeptin or antipain. Intracellular proteolytic cleavage of class II-associated Ii appeared to follow two pathways leading either to N-terminal p21 and p10 or to C-terminal p25. Such cleavages might regulate or catalyze foreign antigen binding to class II.

A role of Ia-associated invariant chains in antigen processing and presentation

Most native antigens require processing in a cellular compartment for efficient presentation to T helper cells. The cellular elements that permit processing are not known. We investigated a possible role of the class II MHC-associated invariant chains in antigen processing. Fibroblast cells that were transfected with class II genes were compared with fibroblasts supertransfected with the invariant chain gene for their capacity to present the fifth component of complement (C5) to C5-specific class II restricted T cell clones or influenza virus protein to a virus-specific T cell clone. Only fibroblasts supertransfected with the invariant chain gene were able to present native antigen, even at very low antigen concentration, whereas both fibroblast types could present cyanogen bromide-fragmented C5 or the virus peptide. Presentation of intact antigen but not of fragmented antigen was totally abrogated by treatment of fibroblasts with chloroquine. The invariant chain gene encodes two polypeptides, li31 and li41. Expression of either li31 or li41 was sufficient to render class II-expressing fibroblasts capable of presenting intact antigen.

A pentapeptide as minimal antigenic determinant for MHC class I-restricted T lymphocytes

Peptides that are antigenic for T lymphocytes are ligands for two receptors, the class I or II glycoproteins that are encoded by genes in the major histocompatibility complex, and the idiotypic alpha/beta chain T-cell antigen receptor. That a peptide must bind to an MHC molecule to interact with a T-cell antigen receptor is the molecular basis of the MHC restriction of antigen-recognition by T lymphocytes. In such a trimolecular interaction the amino-acid sequence of the peptide must specify the contact with both receptors: agretope residues bind to the MHC receptor and epitope residues bind to the T-cell antigen receptor. From a compilation of known antigenic peptides, two algorithms have been proposed to predict antigenic sites in proteins. One algorithm uses linear motifs in the sequence, whereas the other considers peptide conformation and predicts antigenicity for amphipathic alpha-helices. We report here that a systematic delimitation of an antigenic site precisely identifies a predicted pentapeptide motif as the minimal antigenic determinant presented by a class I MHC molecule and recognized by a cytolytic T lymphocyte clone.

FBL-reactive CD8+ cytotoxic and CD4+ helper T lymphocytes recognize distinct Friend murine leukemia virus-encoded antigens

Immunization of C57BL/6 (B6) mice with FBL, a Friend murine leukemia virus (F-MuLV), induces both tumor-specific cytolytic CD8+ (CTL) and lymphokine-producing CD4+ Th that are effective in adoptive therapy of B6 mice bearing disseminated FBL leukemia. The current study evaluated the F-MuLV antigenic determinants expressed on FBL that are recognized by FBL-reactive CD8+ and CD4+ T cells. To identify the specificity of the FBL-reactive CD8+ CTL, Fisher rat embryo fibroblast (FRE) cells transfected with plasmids encoding F-MuLV gag or envelope (env) gene products plus the class I-restricting element Db were utilized. FBL-reactive CTL recognized FRE target cells transfected with the F-MuLV gag-encoded gene products, but failed to recognize targets expressing F-MuLV env. Attempts to generate env-specific CD8+ CTL by immunization with a recombinant vaccinia virus containing an inserted F-MuLV env gene were unsuccessful, despite the generation of a cytolytic response to vaccinia epitopes, implying that B6 mice fail to generate CD8+ CTL to env determinants. By contrast, CD4+ Th clones recognized FRE target cells transfected with env and not gag genes, and immunization with the recombinant vaccinia virus induced an env-specific CD4+ T cell response. These data show that in a Friend retrovirus-induced tumor model in which tumor rejection can be mediated by either CTL or Th, antigens derived from discrete retroviral proteins are predominantly responsible for activation of each T cell subset.

T cell-recognized antigenic peptides derived from the cellular genome are not protein degradation products but can be generated directly by transcription and translation of short subgenic regions. A hypothesis

It is now widely accepted that cytolytic T cells recognize their antigens in the form of small peptides bound to major histocompatibility complex molecules at the surface of the target cells. We present here the hypothesis that, when these antigenic peptides are derived from the cellular genome, they are not degradation products of cellular proteins but can be generated directly by the autonomous transcription and translation of short subgenic regions that we propose to name "peptons". We discuss some consequences of the notion that antigenic peptides can be produced in the absence of synthesis of messenger RNA and protein from the corresponding genes.

Antigen-induced inhibition of autoimmune response to rat male accessory glands. Role of macrophages in the induction of suppressor cells

The present paper describes a mechanism responsible for the induction of inducer-phase suppressor cells effective to suppress the autoimmune response to rat male accessory glands (RAG). In fact, we reported here that marked suppression of delayed type hypersensitivity (DTH) reaction and humoral response to chemically modified rat male accessory glands (MRAG) can be obtained when previously to be immunized with MRAG in complete Freund's adjuvant (CFA) syngeneic rats were pretreated with peritoneal cells (PC) coupled with a purified fraction of RAG (containing the autoantigen). The involvement of MRAG-specific inducer-phase suppressor cells was demonstrated by adoptive transfer experiments of spleen mononuclear cells from unresponsive donors to normal syngeneic rats 24 h prior to immunization of the recipients with MRAG-CFA. The PC used to treat the animals show a large proportion of non-specific-esterase positive, Ox-41 bearing macrophage-like cells. Moreover, the antigen-coupled PC able to trigger the suppressor cells showed the presence of the autoantigen of RAG on their surface. The role of the antigen presenting cells in the induction of MRAG-specific inducer-phase suppressor cells is discussed.

B-lymphoma cells process and present their endogenous immunoglobulin to major histocompatibility complex-restricted T cells

Antigen-presenting B-lymphoma cells were transfected with the gene encoding the immunoglobulin lambda 2 light chain of MOPC315 cells (lambda 2(315). The lambda 2 chain is expressed on the cell surface of the transfectants together with the endogenous heavy chain. The transfectants present an idiotope of the lambda 2(315) light chain to class II-restricted T-cell clones. Recognition by the T cells requires processing of the lambda 2(315) light chain. From these data we conclude that B-lymphoma cells constitutively process and present their immunoglobulins. Secretion and reuptake of the light chain was not necessary for the presentation. Thus, B cells bear two types of idiotypes on their membrane, a native form as surface immunoglobulin and a processed form in the context of products of the major histocompatibility complex.

Purification and characterization of an 80-kilodalton Trypanosoma cruzi urinary antigen

A Trypanosoma cruzi antigen eliminated in the urine of experimentally infected dogs was detected by enzyme-linked immunosorbent assay between 9 and 28 days after infection. The parasite urinary antigen (UAg) was purified by affinity chromatography with polyclonal antibodies to T. cruzi. The eluate of the antibody column was subjected to high-performance liquid chromatography and showed a single peak of A280. This antigen was the only parasite component found in the urine of infected dogs during the course of acute T. cruzi infection. Antigen characterization was performed by two-dimensional gel electrophoresis, lectin affinity chromatography, proteolytic digestion, and Western blotting (immunoblotting). The isolated UAg exhibited a relative molecular size of 80 kilodaltons (kDa), an isoelectric point of 6.2 to 6.8, binding to concanavalin A, and sensitivity to trypsin. The parasite antigen was electroeluted from polyacrylamide gels and subjected to acid hydrolysis and amino acid analysis by reverse-phase high-performance liquid chromatography. The 80-kDa glycoprotein was recognized by serum antibodies from a wide variety of T. cruzi-infected hosts. The UAg proved to be a highly antigenic component present in different strains of T. cruzi. This 80-kDa polypeptide resembles one of the parasite antigens previously found in the urine of patients with acute Chagas' disease.

Recognition of oligonucleotide-encoded T cell epitopes introduced into a gene unrelated to the original antigen

We have previously demonstrated that H-2Kd-restricted CTL specific for HLA-CW3 or HLA-A24 can recognize synthetic peptides corresponding to residues 170-182 of the HLA molecules. Synthetic oligonucleotides encoding region 170-182 of CW3 or A24 were inserted into the influenza nucleoprotein (NP) gene. We demonstrate herein that P815 (H-2d) cells transfected with the NP-oligo recombinant genes are specifically lysed by HLA-specific Kd-restricted CTL clones. Our results imply that there must be a high degree of flexibility for the expression of T cell epitopes in different molecular contexts.

An analysis of T-cell receptor variable region gene expression in major histocompatibility complex disparate mice

To define the impact of major histocompatibility complex (MHC)-encoded glycoproteins on the selection of the T-cell receptor repertoire, we have determined the frequency with which T-cell receptor variable region (V alpha and V beta) genes are expressed in T cells from MHC disparate mice. Approximately 500 T-cell hybridomas were generated from each of three strains of MHC congenic mice [B10 (H-2b), B10.BR (H-2k), and B10.Q (H-2q)] by fusing mitogen-stimulated lymph node T cells with the thymoma BW5147. RNA was prepared from 1629 individual hybridomas and analyzed for the expression of 10 V alpha and 16 V beta gene families. These experiments reveal significant differences in the relative contributions of 1 V alpha gene family (V alpha 3) and several V beta gene segments (V beta 5.1, -5.2, -11, and -12) to the T-cell receptor repertoire of MHC disparate mice.

Processing requirements for T cell activation by Mycoplasma arthritidis-derived mitogen

Mycoplasma arthritidis produces in culture a polyclonal mitogen which is active for murine and human T lymphocytes in the presence of accessory cells (AC). We studied the requirements for processing and presentation by AC of Mycoplasma arthritidis supernatant (MAS) mitogen to human T cells. As inhibitors of AC processing, several agents were used which inhibit lysosomal function: the weak bases chloroquine and NH4Cl, the cationic ionophore monensin and the competitive protease inhibitor leupeptin. When these agents were used to inhibit processing by presenting cells and washed out before T cells were added to culture, they inhibited lymphocyte activation and, therefore, we assume that they interfered with the presentation of the mitogen. Thus, if MAS requires a processing step, it appears to involve lysosomal proteolysis which can be blocked in vitro.

The role of intracellular acidification in antigen processing

The experiments described here were designed to investigate the role of intracellular acidic organelles in the processing of foreign antigens. Simultaneous addition of antigen and the inactive precursor of an acid protease, cathepsin L, greatly diminished the stimulation of a T-cell hybridoma specific for pigeon cytochrome c. The effect occurred during antigen processing as the presentation of a peptide fragment was unaffected by precursor-cathepsin L. The effect required uptake of the enzyme via mannose-6-PO4 receptors and was specific for antigenic determinants cleavable by the enzyme. The results confirm, without any pharmacologic manipulation, that foreign antigen moves into an acidic environment during some phase of its processing. We also explored the role in antigen processing of one acidic compartment, the early endosome, by examining CHO cells expressing a temperature-sensitive defect in endosomal acidification. These cells were transfected with MHC class II genes to convert them into APC for CD4+T cells. When these cells were incubated at the nonpermissive temperature, their ability to process antigen was impaired. There was no negative effect on presentation of antigenic peptides nor on the uptake and degradation of the antigen. Wildtype CHO cells similarly transfected and tested did not show any aberrancy in antigen processing. The inhibition of processing by the mutant cells was only partial, even under stringent conditions, and the residual processing was eliminated by chloroquine. Our findings provide the first evidence for a contribution by acidified early endosomes to the pathway of antigen processing and suggest that other acidified compartments are also involved.

Antigen processing for presentation to T lymphocytes: function, mechanisms, and implications for the T-cell repertoire

Antigen processing encompasses the metabolic events that a protein antigen must undergo in or on the antigen-presenting cell before it can be recognized by the T lymphocyte. It appears that a primary goal of these events is to unfold the protein to expose residues that are buried in the native conformation, which is designed to be soluble in water. The APC usually accomplishes this task by proteolytic cleavage of the protein, but we have found that artificial unfolding without proteolysis is sufficient. The purpose of unfolding may be to allow different faces of the antigenic site to bind simultaneously to the T-cell receptor and the MHC molecule on the APC, or to interact with other structures on the membrane of the APC. This requirement for unfolding appears to apply to everything from small peptides to large multimeric proteins. We have found that the way the antigen is processed and the structure of the fragments produced can greatly affect the availability of antigenic sites. For instance, some antigenic sites are not recognized when the native protein is used as immunogen, despite the fact that immunization with a small peptide corresponding to that site reveals both the ability of the site to bind to MHC molecules of the animal in question and the presence of a T-cell repertoire specific for that site. The antigenic site is not destroyed by processing, since it can be presented by the same F1 APC to T cells of another MHC type. Similarly, cross-reactivity between homologous epitopes of related proteins may occur at the peptide level even though the native proteins do not crossreact for the same T-cell clone. Since these events occur with monoclonal T cells, they cannot be due to suppressor cells specific for other sites on the native molecule. The best explanation is that the products of natural processing of the protein are larger than the peptides corresponding to the minimal antigenic sites, and contain hindering structures that interfere with binding to some MHC molecules and not others, or to some T-cell receptors and not others. Thus, antigen processing is a third factor that can lead to apparent Ir gene defects - in addition to MHC specificity and holes in the T-cell repertoire - and can significantly influence which antigenic sites are immunodominant.(ABSTRACT TRUNCATED AT 400 WORDS)

Immunomodulation of in vitro antigen presentation by cations

The ability of various cations to alter an antigen-specific (ovalbumin) T cell activation system in vitro has been assessed. This in vitro system includes analysis of antigen capture, processing, and presentation by antigen presenting cells (APC) (splenocytes, B cells or macrophages) in a major histocompatibility complex (MHC) restricted fashion and the evaluation of the activation of a T-cell hybridoma, DO-11.10, which is specific for "processed" ovalbumin in association with I-Ad. Activation is determined by production of IL2 which is quantitated in a bioassay with HT-2 cells. Numerous metals (10 and 100 microM) were screened in the coculture assay (APC and DO-11.10 plus ovalbumin). Metals with inhibitory effects were Cd greater than Cu greater than Pb greater than Zn. Co and Cr had no modulatory effect and Ni had an enhancing effect (increased IL2 production). The effects of the modulatory metals were further assessed for influences on the individual cellular components of this system. Cd was toxic to all cell types whereas Cu was toxic only to irradiated splenocytes and Do-11.10. Pb was the only metal which was not toxic to any cell type but still inhibited antigen presentation. To prevent influences of metals on DO-11.10 or carryover into the bioassay, APC were preincubated with metals and then assessed for presentation capability. After preincubation, only Cd, Pb, and Cu were inhibitory. The inhibition by Cd was due to toxicity. Cu inhibited only irradiated splenocyte presentation and this effect was due to toxicity. Pb inhibition was due to a mechanism other than toxicity and its biochemical influences on APC are discussed.

Facts on the fragmentation of antigens in presenting cells, on the association of antigen fragments with MHC molecules in cell-free systems, and speculation on the cell biology of antigen processing

The processing of a protein antigen is a multi-step event taking place in antigen-presenting cells. Processing is a prerequisite for the recognition of most antigens by T lymphocytes. The antigen is ingested by endocytosis, transported to an acid cellular compartment and subjected to proteolytic fragmentation. Some of the antigen fragments bind to MHC class II molecules and are transported to the surface of the antigen-presenting cell where the actual presentation to T lymphocytes occurs. The nature of the processed antigen, how and where it is derived and subsequently becomes associated with MHC molecules are the questions discussed in this review. To us, the entire concept of processing has appeal not only because it explains some hitherto well-established, but poorly understood, phenomena such as the fact that T lymphocytes focus their attention entirely upon antigens on other cells. It has appeal also because processing allows for a thorough scrutiny of all the proteins of a cell including both the proteins which have been taken up from the environment (mostly MHC class II-restricted) and the cell's own proteins (mostly MHC class I-restricted). Through the mechanism of processing fragments of all these proteins including "internal" fragments which are not present on the globular protein's surface are brought out on the cell surface in association with MHC molecules and displayed to the T-lymphocyte system. This allows for the identification and, if necessary, the destruction of cells which in their interior harbor abnormal proteins, be they derived from virus-encoded genes or other abnormal genes.

Processing requirements for the recognition of insulin fragments by murine T cells

In this study we investigated aspects of antigen processing using insulin and insulin A chain-derived fragments as model antigens in Ab alpha Ak beta-restricted T-cell stimulation. Similarly to other proteins, the immunodominant region of insulin recognized by these T cells is limited in size. It is located on the insulin A chain and encompasses a portion of the molecule that is represented faithfully by peptide A1-14(SSO3-)3. Efficient presentation of intact insulin and its entire A chain is dependent on uptake and processing by APC. Whereas peptides stemming from various globular proteins are known to be presented to T cells by APC without requiring processing, this is not the case with A-chain fragment A1-14 (SSO3-)3. This observation suggested that, in addition to proteolytic degradation, other mechanisms might play a role in the processing of these antigens. Three cys-residues are located in close proximity to those amino acid residues of the insulin A chain that are inferred to participate in the specific interaction with MHC class II molecules and the TcR. In A-chain derivatives that are stimulatory for the T cells or in intact insulin these cys residues are engaged in disulfide bonds or are S-sulfonated. Both linkages can be reversibly modified by reaction with thiols. Functional data indicate that from intact insulin and from structurally distinct A-chain derivatives a closely similar or identical peptide is formed and bound to class II molecules for recognition by the T cells. The question arises as to whether, in this processed peptide, the cys residues are present in reduced form, engaged in disulfide bonds, or are modified in some other way. Taken together, these findings suggest that modification of cys residues or isomerization of disulfide bonds may play a role in insulin processing. It can be expected that other proteins carrying cys residues in their immunodominant peptides may show similar processing requirements. The inhibition of N-glycosylation of proteins by tunicamycin in APC blocked the processing and presentation of insulin and OvA whereas, under the same conditions, the presentation of a processing-independent peptide was not affected. Furthermore, an autoreactive T-cell clone was capable of recognizing tunicamycin-treated APC. Since the expression of class II molecules was found to be unaltered as demonstrated by cytofluorometric analysis the deficient N-glycosylation appears to have little influence on class II antigen-mediated T-cell recognition but interferes with uptake of antigen and/or its processing by APC.

Autologous peptides constitutively occupy the antigen binding site on Ia

Low molecular weight material associated with affinity-purified class II major histocompatibility complex (MHC) molecules of mouse (Ia) had the expected properties of peptides bound to the antigen binding site of Ia. Thus, the low molecular weight material derived from the I-Ad isotype was efficient in inhibiting the binding of 125I-labeled I-Ad-specific peptide to I-Ad, but did not significantly inhibit the binding of an I-Ed-specific peptide to I-Ed; the reciprocal isotype-specific inhibition was demonstrated with low molecular weight material derived from I-Ed. The inhibitory material was predominantly peptide in nature, as shown by its susceptibility to protease digestion. It was heterogeneous as measured by gel filtration (mean molecular weight approximately 3000), and when characterized by high-performance liquid chromatography, it eluted over a wide concentration of solvent. Such self peptide-MHC complexes may have broad significance in the biology of T cell responses, including generation of the T cell repertoire, the specificity of mixed lymphocyte responses, and the immune surveillance of self and nonself antigens in peripheral lymphoid tissues.

Cooperative effect of interferon-gamma and tumor necrosis factor-alpha on the induction of the class II antigen-associated invariant chain expression

The regulation of the invariant chain (Ii) expression was studied in the human colon carcinoma cell line HT-29 that constitutively expressed neither Ii nor class II antigens. Upon stimulation of HT-29 cells with a combination of human recombinant tumor necrosis factor-alpha (TNF-alpha) and interferon-gamma (IFN-gamma), expression of mRNA and protein of the invariant chain were induced. In contrast, administration of TNF-alpha or IFN-gamma alone had no effect. A delayed induction of Ii mRNA, which was first detected 10-12 h after stimulation, was observed; this suggests an indirect regulatory mechanism. Stimulation with both IFN-gamma and TNF-alpha led to the co-expression of class II antigens with the invariant chain. In order to study the genetic basis for this stimulation the murine invariant chain gene (800 bp 5' flanking sequences and the structural gene) was transfected into HT-29 cells and transfected cells were tested for the ability to respond to IFN-gamma and TNF-alpha. Simultaneous application of both cytokines had a strong effect on the induction of the murine invariant chain. IFN-gamma alone had no effect and TNF-alpha only marginally stimulates murine invariant chain expression. The transfection experiment indicates that the murine invariant chain gene construct contains the structural elements which are responsible for regulation with IFN-gamma and TNF-alpha. We determined whether the cooperative effect of TNF-alpha and IFN-gamma is also found in vivo. Stimulations of mice were performed with TNF-alpha, IFN-gamma and a combination of both. The immunohistological analysis of kidney tissue sections revealed that TNF-alpha had no effect on Ii and Ia expression. Upon IFN-gamma treatment a minor subset of renal tubules showed staining for Ii, and less prominently also for Ia. However, simultaneous application of both cytokines led a strong induction of both Ii and Ia antigens in renal epithelial cells, thus suggesting that this synergistic effect potentially occurs under physiological conditions.

T cell hybridomas define the class II MHC-restricted response to vesicular stomatitis virus infection

T cell hybridomas with specificity for VSV (vesicular stomatitis virus)-infected cells were generated in an attempt to better define the la-restricted helper T cell response to VSV. The hybridomas were created by fusing BALB/c (H-2d) anti-VSV immune spleen cells to the murine thymoma BW 5147. These hybridomas produce IL-2 when stimulated with VSV-infected spleen cells. They were found to recognize viral antigens in association with I-Ad and, in addition, could also be stimulated by VSV-infected A20 cells (an Ia-positive B cell lymphoma of H-2d origin). The purified viral membrane glycoprotein, G protein, and Gs (secreted G protein that lacks the hydrophobic and intracytoplasmic domains) both stimulated IL-2 production when added to cultures of A20 and the hybridomas. These hybridomas therefore recognize a viral antigenic determinant on G protein. Since chemically-fixed antigen-presenting cells fail to stimulate the hybridomas after exogenous addition of purified G protein we can conclude that these T cell hybridomas recognize a processed form of the G protein. Stimulator cells created by expression in A20 of a transfected cDNA encoding G protein were also recognized. Recognition in this case was I-Ad-restricted, as anti-I-Ad monoclonal antibodies blocked stimulation, and an Ia-negative cell (P815) expressing a transfected G protein gene failed to stimulate the hybridomas. Even after paraformaldehyde fixation, G gene-transfected, Ia-positive cells could stimulate the hybridomas, suggesting that processing of this endogenously-synthesized antigen has occurred.

Molecular basis for cytolytic T-lymphocyte recognition of the murine cytomegalovirus immediate-early protein pp89

The murine cytomegalovirus protein pp89, which is encoded by gene ieI, is a nonstructural regulatory protein expressed in the immediate-early phase of the viral replication cycle and located mainly in the nucleus of infected cells. Protection of BALB/c (H-2d) mice against a lethal murine cytomegalovirus challenge infection is achieved by vaccination with a recombinant vaccinia virus, MCMV-ieI-VAC, expressing pp89 as the only murine cytomegalovirus gene product. The protection is entirely mediated by T lymphocytes of the CD8+ subset. In the present report, we analyzed the molecular basis of the recognition of pp89 by BALB/c CD8+ cytolytic T lymphocytes. A series of internal and terminal deletion mutants of gene ieI was constructed and cloned in vaccinia virus, and the antigenicity and immunogenicity of the fragments of pp89 expressed by the recombinants were studied. A region of only one-sixth of the protein, from amino acids 154 to 249 and encoded by the fourth exon of gene ieI, was sufficient for both the recognition in vitro of the protein by pp89-specific cytotoxic T lymphocytes and the induction in vivo of pp89-specific cytotoxic T lymphocytes. By using synthetic peptides, the sequence between residues 161 and 179, which is located within the defined domain, was identified as an epitope presented to BALB/C cytotoxic T lymphocytes by the class I major histocompatibility antigen Ld.

Immune responses to an adjuvant-free native syngeneic myeloma protein (M315)

Myeloma protein 315 (M315; isotype IgA, lambda 2) is used in this report as a model to explore the immunogenicity of a syngeneic Ig under nearly physiological conditions. We have previously shown that a synthetic peptide spanning the mutated HV3 loop of the L-315 chain, when emulsified in complete Freund's adjuvant, elicits T helper cells (Th) that respond to a boost with L-315 or M315, indicating that M315 is recognized as a processed protein antigen. We now show that the adjuvant-free 7S monomer of native or of mildly reduced and alkylated M315, given in divided doses totalling 300 or 800 micrograms to BALB/c mice, induced persistent anti-M315 antibodies (Ab), a large part of which was IgG1 directed mainly to idiotypes (Id) associated with M315's hapten-binding site. Polymers of M315 IgA (800 micrograms) failed to induce Ab, due probably to their rapid clearance into bile. Short-term treatment with anti-CD4 monoclonal Ab GK1.5 at the time of priming with 7S M315 inhibited the responses almost completely. The spleens of M315-immune mice contained Th that recognized the L-chain subunit of M315 as a carrier indicating that these Th did not require an assembled (VH-VL) pair of 315 V regions to be activated. We also observed low amounts of Ab specific for epitopes of the C alpha region. This evidence opens the possibility that a distinct autoimmune pathway exists for elicitation of rheumatoid factor (RF; autoAb to Fc gamma) that involves help to RF-producing B cells by Id-specific Th. We suggest that these Th recognize V-region peptides from IgG that have been captured, processed and presented by these B cells.

A single amino acid substitution in the human histocompatibility leukocyte antigen DR3 beta chain selectively alters antigen presentation

Activation of T lymphocytes by immunogenic peptides bound to HLA molecules is a central event in the generation of an immune response. To determine the sites on HLA molecules involved in this process, we isolated mutant EBV-transformed B cell clones that express altered HLA-DR3 molecules. One mutant has lost the ability to stimulate a T cell clone specific for a mycobacterial protein, but retains the ability to stimulate other antigen-specific T cells. The DNA sequence of the complete DR alpha and beta coding regions revealed a single nucleotide change resulting in a glutamic acid to lysine substitution at amino acid 9 in the first hypervariable region of the DR beta chain. These results are discussed in relation to a recently proposed model of class II molecule structure.

Antigen presentation by liposomes as model system for T-B cell interaction

Using the earlier established liposome system for antigen presentation, in which liposomes bearing major histocompatibility complex (MHC)-class II molecules inserted into and protein antigen covalently linked to the membrane were found to be sufficient for an antigen-specific and MHC-restricted activation of T cells, the minimal requirements for T-B cell interaction were investigated. Liposomes carrying MHC class II molecules and antigen-specific monoclonal antibodies were constructed and tested for their ability to present soluble antigen to T cells. With the antigens lactate dehydrogenase B and pigeon cytochrome c, a specific stimulation of T cell clones and hybridomas could be obtained. These results demonstrate the possibility of a direct involvement of the immunoglobulins on the B cell surface in their interaction with T cells.

A single antigen-specific B cell can conjugate to either a type 1 or a type 2 helper T cell

Keyhole-limpet-hemocyanin-specific I-Ad-restricted T helper cells type 1 (TH1 cells) and type 2 (TH2 cells) were studied for their ability to physically conjugate to trinitrophenyl-specific antigen-binding B cells (TNP-ABCs). The same TNP-ABCs (which had already processed TNP-keyhole limpet hemocyanin) formed conjugates with one or the other T helper (TH) cell type, and conjugation was antigen-specific and major histocompatibility complex-restricted. The conjugation of a fixed number of one type of TH cell to the TNP-ABCs was inhibited by co-incubation with increasing numbers of cells of the other TH cell type. Thus, the vast majority of TNP-ABCs can conjugate to either type of TH cell. A similar pattern of inhibition of the conjugation of one TH cell type to the TNP-ABCs was seen when cells were co-incubated with increasing numbers of cells from an alloreactive T-cell clone. In all cases, conjugates contained only one T cell bound to one B cell, suggesting that interaction of an antigen-presenting B cell with one TH cell prevents the simultaneous binding of another TH cell.

The heme moiety of cytochrome c is an autoreactive Ir gene-restricted T cell epitope

In these studies, we have shown that the heme moiety of cyt c is a dominant T cell epitope that induces a large proliferative response in lymph node T cells derived from SJL and B10.A mice when presented on either unfixed or fixed syngeneic APCs. Not only is this vigorous response observed for cyt c-primed T cell populations but also for populations obtained from naive SJL or B10.A mice. The reactivity to the heme moiety falls under strict MHC restriction, in that it is present only in murine strains bearing either the I-Ak or I-As molecule and can be blocked by antibodies specific for these class II molecules. Therefore, these findings require that the current models describing the nature of T cell epitopes be extended to include nonpeptide molecules. Furthermore, as the heme moiety is ubiquitous throughout the organism, although sequestered within proteins, the existence of heme-reactive T cell populations in unprimed animals provides another example of the existence of self-reactive T cell clones.

T-cell antigen receptor genes and T-cell recognition

The four distinct T-cell antigen receptor polypeptides (alpha, beta, gamma, delta) form two different heterodimers (alpha:beta and gamma:delta) that are very similar to immunoglobulins in primary sequence, gene organization and modes of rearrangement. Whereas antibodies have both soluble and membrane forms that can bind to antigens alone, T-cell receptors exist only on cell surfaces and recognize antigen fragments only when they are embedded in major histocompatibility complex (MHC) molecules. Patterns of diversity in T-cell receptor genes together with structural features of immunoglobulin and MHC molecules suggest a model for how this recognition might occur. This view of T-cell recognition has implications for how the receptors might be selected in the thymus and how they (and immunoglobulins) may have arisen during evolution.