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

Dendritic cells are the principal cells in mouse spleen bearing immunogenic fragments of foreign proteins

We monitored the APC function of cells taken from the spleen and peritoneal cavity of mice that had been given protein antigens via the intravenous or intraperitoneal routes. Using the mAb 33D1 and N418 to negatively and positively select dendritic cells, we obtained evidence that dendritic cells are the main cell type in spleen that carries the protein in a form that is immunogenic for antigen-specific T cells. In vivo pulsed macrophages were not immunogenic and did not appear capable of transferring peptide fragments to dendritic cells.

Identification of the region of T cell receptor beta chain that interacts with the self-superantigen MIs-1a

Superantigen-MHC complexes are known to stimulate T cells primarily via the V beta element of the T cell receptor. In this paper we identify a number of amino acid residues that define the region of a particular V beta element interacting with one of the self-superantigens, MIs-1a. These residues are predicted to lie on a beta-pleated sheet of the T cell receptor, away from the complementarity determining regions of the receptor, which are thought to interact with complexes of conventional peptide antigens and MHC. In support of this prediction, mutations affecting MIs-1a activity have no effect on the response to conventional antigen and MHC.

Generation and characterization of peptide-specific, MHC-restricted cytotoxic T lymphocyte (CTL) and helper T cell lines from unprimed T cells under microculture conditions

We describe a microculture system for the generation of CTL and T helper cells against peptides. Tryptic digest and cyanogen bromide fragments of chicken ovalbumin and synthetic peptides of ovalbumin (323-339) and influenza virus (NP 365-380) were used to generate CTL and T helper lines from unprimed T cells. These lines were both peptide-specific and MHC-restricted. The relative ease of generating peptide-specific, MHC-restricted CTL and helper T cell lines with as few as 10(6) unprimed lymphocytes can be an efficient method of detecting potential immunogenic determinants of an antigen.

Antigen presentation by brain microvessel smooth muscle and endothelium

It has been previously reported that cultured brain microvessel smooth muscle cells (SM) express major histocompatibility complex (MHC) class II antigen. Here we report that SM is able to present ovalbumin (OVA) antigen to an OVA-specific T cell hybridoma (A2.2E10) and also presents keyhole limpet hemocyanin (KLH) to a KLH-specific T cell clone (HDK-1). Both the class II expression and the antigen-presenting capacity of SM cells is increased by interferon-gamma stimulation. Antigen presentation by SM is also MHC restricted as it is blocked by anti-Ia monoclonal antibodies. In contrast to SM, brain endothelium (En) presents whole OVA, digested OVA and KLH poorly, to a much lesser degree than SM, to the same antigen-specific T cells.

Presentation of Salmonella antigens by peritoneal cells of normal and Salmonella-infected mice

A comparison of the ability of normal peritoneal cells (PC) and those harvested from mice 1-3 days after intraperitoneal immunization with live Salmonella enteritidis 11RX (11RX) to present antigen to 11RX-primed T cells was made using formalin-killed 11RX and a soluble 11RX antigen extract as antigens. Unfractionated PC and the adherent and non-adherent PC populations were analysed separately and the effects of the lysosomal function-impairing drug chloroquine and the fixative paraformaldehyde, used before or after antigen-pulsing, were also determined. The results presented indicate that immunization with live 11RX did not induce any detectable modulation of APC function which could account for the ability of live 11RX to induce cell-mediated immune responses involving Lyt 2+ T cells.

Selective killing of hepatitis B envelope antigen-specific B cells by class I-restricted, exogenous antigen-specific T lymphocytes

Specific B lymphocytes can act as very efficient antigen-presenting cells. They bind antigen with high affinity via their immunoglobulin receptors, process it through the class II major histocompatibility complex (MHC) pathway, and present its fragments to class II-restricted T lymphocytes. In general, exogenous antigens and noninfectious viral particles enter the class II pathway and are selectively associated with class II MHC molecules. The presentation of an exogenous antigen in association with class I molecules has been reported for only a few antigens, including the hepatitis B envelope antigen (HBenvAg). Here we demonstrate that antigen-specific B cells can efficiently deliver HBenvAg to the class I pathway, presenting its fragments to class I-restricted cytotoxic T lymphocytes (CTLs) which kill the specific B cells. This could represent a mechanism of suppression of neutralizing anti-hepatitis B virus (HBV) antibody response, a phenomenon that accompanies the development of the chronic HBV-carrier state.

The staphylococcal enterotoxins and their relatives

Staphylococcal enterotoxins and a group of related proteins made by Streptococci cause food poisoning and shock in man and animals. These proteins share an ability to bind to human and mouse major histocompatibility complex proteins. The complex ligand so formed has specificity for a particular part of T cell receptors, V beta, and by engaging V beta can stimulate many T cells. It is likely that some or all of the pathological effects of these toxins are caused by their ability to activate quickly so many T cells. It is also possible that encounters with such toxins have caused mice, at least, to evolve mechanisms for varying their T cell V beta repertoires, such that they are less susceptible to attack by the toxins.

Invariant chain influences the immunological recognition of MHC class II molecules

Recent experiments have implicated intracellular events in the formation of the MHC class II-peptide complexes recognized by CD4-positive T cells. These data raise the possibility that the intracellular association of class II with the non-polymorphic glycoprotein, invariant chain (Ii), may regulate the interaction between processed antigen and MHC class II molecules. To address this possibility, we have generated a series of transfected fibroblast cell lines that express class II with and without Ii. Although the presence of Ii does not seem to affect the ability of the cells to process and present intact antigen, Ii-negative cells express an altered form of class II at the cell surface. This modified conformation of class II in Ii-negative cells is detectable by an increase in the ability to present antigenic peptides to T cells and a decrease in the binding of several class II-specific monoclonal antibodies.

Immune function of the blood-brain barrier: incomplete presentation of protein (auto-)antigens by rat brain microvascular endothelium in vitro

The endothelial blood-brain barrier (BBB) has a critical role in controlling lymphocyte traffic into the central nervous system (CNS), both in physiological immunosurveillance, and in its pathological aberrations. The intercellular signals that possibly could induce lymphocytes to cross the BBB include immunogenic presentation of protein (auto-)antigens by BBB endothelia to circulating T lymphocytes. This concept has raised much, though controversial, attention. We approached this problem by analyzing in vitro immunospecific interactions between clonal rat T lymphocyte lines with syngeneic, stringently purified endothelial monolayer cultures from adult brain micro-vessels. The rat brain endothelia (RBE) were established from rat brain capillaries using double collagenase digestion, density gradient fractionation and selective cytolysis of contaminating pericytes by anti-Thy 1.1 antibodies and complement. Incubation with interferon-gamma in most of the brain-derived endothelial cells induced Ia-antigens in the cytoplasm and on the cell surface in some of the cells. Before the treatment, the cells were completely Ia-negative. Pericytes were unresponsive to IFN-gamma treatment. When confronted with syngeneic T cell lines specific for protein (auto-)antigens (e.g., ovalbumin and myelin basic protein, MBP), RBE were completely unable to induce antigen-specific proliferation of syngeneic T lymphocytes irrespective of pretreatment with IFN-gamma and of cell density. RBE were inert towards the T cells, and did not suppress T cell activation induced by other "professional" antigen presenting cells (APC) such as thymus-derived dendritic cells or macrophages. IFN-gamma-treated RBE were, however, susceptible to immunospecific T cell killing. They were lysed by MBP-specific T cells in the presence of the specific antigen or Con A. Antigen dependent lysis was restricted by the appropriate (MHC) class II product. We conclude that the interaction of brain endothelial cells with encephalitogenic T lymphocytes may involve recognition of antigen in the molecular context of relevant MHC products, but that this interaction per se is insufficient to initiate the full T cell activation program.

Characteristics of peptides which compete for presented antigen-binding sites on antigen-presenting cells

The T cell recognition of globular protein antigens requires the cell surface presentation of the protein by Ia-expressing antigen-presenting cells (APC). The mechanisms by which APC function remain to be elucidated. To gain a better understanding of association of antigen with APC surfaces, a large panel of peptides of diverse physicochemical properties was assayed for the ability to compete with presented antigen for binding sites on the APC surface. Competition was measured by the ability of a peptide to block the I-Ek-restricted T cell response to pigeon cytochrome c (Pc) as presented by APC. The panel assayed included overlapping peptides representing the entire length of sperm whale myoglobin and the alpha and beta chains of human adult hemoglobin as well as synthetic conformational peptides of lactate dehydrogenase C4 exhibiting stable secondary, alpha-helical structures. The results presented here show that several peptides of this group compete with the presented form of Pc for binding sites on the APC. However, there is no single biochemical property or amino acid sequence algorithm which predicts the blocking ability. The peptides which compete with presented Pc are not predicted to assume the amphipathic alpha-helical conformation hypothesized by De Lisi and Berzofsky (Proc. Natl. Acad. Sci. USA 1986. 82: 7048) for T cell antigenic peptides. However, peptides designed and synthesized to adopt a stable alpha-helical secondary structure show more potent blocking activity than the corresponding linear peptides, suggesting that the secondary structure may indeed be a contributing factor in the ability of presented antigenic peptides to be bound by the APC. The results with the myoglobin and hemoglobin peptides show no connection between any particular secondary structure of the peptide in the native proteins and the ability of the peptides to block presentation. Further, there is no correlation between the major histocompatibility complex restriction of the competing peptides and their ability to block the I-Ek-restricted Pc-specific T cell response. This suggests that antigen presented by the APC may be bound to APC structures other than Ia prior to association with Ia. Such additional binding sites for presented antigen may be necessary to facilitate association with Ia.

Induction of B cell responsiveness to growth factors by Epstein-Barr virus conversion: comparison of endogenous factors and interleukin-1

Immortalized B lymphocytes produce a factor(s) that stimulates growth of B cell lines carrying Epstein-Barr virus (EBV). Stimulatory supernatants derived from B cells also exhibit interleukin-1 (IL-1) activity in costimulator assays with the D10.G4.1 helper T cell line. Experiments with purified macrophage-derived IL-1 and recombinant IL-1 beta demonstrate that IL-1 stimulates proliferation of the cell lines that respond to the factors from B lymphocyte lines. One B cell line, Ramos, an EBV-Burkitt's lymphoma, contrasts with other B cell lines in that it is refractory to the growth enhancing effects of B cell conditioned medium and macrophage-derived IL-1. When EBV was introduced into Ramos cells, growth was enhanced by the factor(s) in B cell conditioned medium (six out of seven lines); growth of EBV-converted Ramos lines (six out of seven lines) also was enhanced by IL-1. These findings demonstrate that infection of a non-responsive transformed B lymphocyte by EBV induces cellular responsiveness to factor-mediated growth stimulation.

Epstein-Barr virus (EBV) antigens processed and presented by B cells, B blasts, and macrophages trigger T-cell-mediated inhibition of EBV-induced B-cell transformation

The ability of B cells, B blasts, and macrophages to present Epstein-Barr virion antigens to autologous T cells and trigger their capacity to inhibit Epstein-Barr virus-induced B-cell transformation was tested. Macrophages were as efficient as B cells and B blasts in presenting the virus to T lymphocytes. This function required antigen processing, because it was inhibited by chloroquine treatment and by fixation of the antigen-presenting cells immediately after viral exposure but not 18 h later. T cells exposed to the purified Epstein-Barr virus envelope antigen gp350 coupled to immunostimulating complexes also showed inhibitory function. These results suggest that recognition of processed virion antigens elicits the generation of T-cell-mediated inhibition of Epstein-Barr virus-induced B-cell transformation.

Anchor sequence-dependent endogenous processing of human immunodeficiency virus 1 envelope glycoprotein gp160 for CD4+ T cell recognition

Human CD4+ T cell clones and cell lines were shown to lyse recombinant vaccinia virus-infected cells that synthesize the HIV-1 envelope glycoprotein gp160. The processing of endogenously synthesized gp160 for recognition by CD4+ T cells required that the protein, after synthesis on the rough endoplasmic reticulum and during subsequent cellular transport, remain attached to the luminal/extracellular membrane face by a hydrophobic anchor sequence.

Structural requirements for the interaction between class II MHC molecules and peptide antigens

Previous work from our and other laboratories indicates that T cells recognize a complex between the MHC restriction element and peptide antigen fragments. This paper reviews the structural characteristics of the formation of such a complex. By analyzing in detail the interactions between purified IA(d) and IE(d) molecules and their peptide ligands, we found that some structural characteristics apply to both antigen-MHC interactions. In particular, we found: 1) each MHC molecule is capable of binding many unrelated peptides through the same peptide-binding site; 2) despite this permissiveness of binding, it is possible to define certain structural features of peptides that are associated with the capacity to bind to a particular MHC specificity (IA(d) or IE(d)); 3) IA(d) and IE(d) molecules recognize different and independent structures on the antigen molecule; 4) only about 10% of the single amino acid substitutions tested on two IA(d)- and IE(d)-binding peptides had significant effect on their MHC-binding capacities, while over 80% of these substitutions significantly impaired T cell recognition of the Ia-peptide complex; 5) based on the segregation between residues that are crucial for T cell activation and Ia binding, the easiest model for the antigen-Ia-T-cell-receptor complex pictures the antigen molecule sandwiched in a planar conformation between the MHC and the T cell.

On the peptide model of allorecognition: cytotoxic T lymphocytes recognize an alloantigen encoded by two HLA-linked genes

The peptide model of allorecognition hypothesizes that alloreactive cytotoxic T lymphocytes recognize peptides associated with major histocompatibility complex (MHC) molecules. This study characterizes an unusual alloantigen that is recognized by two human cytotoxic T-cell lines and may represent a complex formed by the association of a naturally selected peptide with an MHC molecule. Family studies demonstrate that both components of the alloantigen are products of MHC-linked genes. One component is a class I molecule most likely encoded by the HLA-B locus. The second component is encoded by an MHC-linked gene that shows a limited polymorphism; whether it represents the product of a second class I gene or of some other MHC gene remains to be determined. These data provide experimental evidence supporting the peptide model of allorecognition in human beings and indicate that some naturally selected peptides involved in allorecognition may be derived from MHC-encoded proteins.

Isolation of a functional antigen-Ia complex

The helper T-cell recognition of globular protein antigens requires that the antigen be processed and presented by an I-region associated (Ia)-expressing antigen-presenting cell (APC). Processing involves the uptake of antigen into an intracellular, proteolytic, acidic compartment; release of peptide fragments containing the T-cell antigenic determinant; association of these peptides with Ia; and presentation of these complexes on the cell surface for recognition by the specific T cells. The molecular mechanisms by which processed antigenic peptides associate with Ia within the APC are poorly understood. To date, functional antigen-Ia complexes have not been isolated from cells that have processed native antigens, although the resolution of the structure of a major histocompatibility complex (MHC) class I protein indicates that peptide is bound in a groove between two alpha-helical regions of the molecule and synthetic peptides have been demonstrated to bind purified MHC both in detergent solution and incorporated into planar membranes, where the MHC-peptide complexes function to activate specific T cells. Here we demonstrate that Ia purified from APCs that have processed the native globular protein antigen cytochrome c, when incorporated into lipid membranes, stimulates cytochrome c-specific T cells in the absence of exogenous antigenic peptide. The T-cell response to Ia purified from cytochrome c-pulsed APCs shows the same MHC restriction and antigen fine specificity as the response to antigen-pulsed APCs. Indeed, T-cell recognition of pigeon cytochrome c (Pc) shows a well documented high-affinity heteroclitic cross-reaction to insect cytochromes c-namely, those of Drosophila melanogaster (DMc) and tobacco hornworm moth (THMc). The same heteroclitic response is observed when purified Ia from unpulsed cells, incorporated into lipid membranes, is used to present antigenic peptides of Pc and of THMc. Significantly, Ia purified from APCs that have processed DMc is approximately 50-fold more active in stimulating specific T cells compared to Ia purified from APCs that have processed Pc. The peptide-Ia complex isolated here may provide the necessary material for analysis of the physiochemical properties of the processed form of the antigen that is produced by the APC and associates with Ia.

An intracellular pathway is required for ABA-tyrosine presentation to T cells

Although tyrosine-azobenzenearsonate (ABA-Tyr) is not degraded by proteolytic enzymes, its presentation by accessory cells is inhibited by lysosomotropic agents such as chloroquine. Presentation of ABA-poly-L-glutamic, alanine, tyrosine (ABA-GAT) is similarly inhibited by chloroquine, but in contrast to ABA-Tyr it is also inhibited by leupeptin. Finally formaldehyde fixation of accessory cells after pulsing with ABA-Tyr but not before permits successful stimulation of ABA-specific hybridoma cells. These results suggest that a lysosomal pathway but not digestion is necessary for the association of ABA-Tyr and la molecules for presentation.

Reversal of autoimmune encephalomyelitis by membranes presenting myelin basic protein-associated class II MHC molecule as an approach to immunotherapy of organ-specific autoimmune diseases

Experimental autoimmune encephalomyelitis (EAE), a well-accepted experimental model for multiple sclerosis in humans, is a paralytic disease mediated by CD4+ T cells specific for myelin basic protein (MBP). Several approaches to immune-specific therapy of EAE as a model for other organ-specific autoimmune diseases have previously been reported. We now show that macrophages (M phi) or B cells, as antigen-presenting cells, when pulsed with MBP and intraperitoneally (but not intravenously) inoculated after the encephalitogenic challenge, are highly effective in blocking the development of EAE. Moreover, M phi pulsed with an organ tissue homogenate, mouse spinal cord homogenate, can also present the relevant target antigen, MBP, and are as effective as MBP-pulsed M phi in blocking the development of EAE. This capacity of the M phi to identify and present the relevant target antigen indicates that this approach is also applicable to organ-specific autoimmune diseases other than EAE, regardless of how much is known about their etiological agent or specific target antigen. Nonviable glutaraldehyde-fixed MBP-pulsed M phi or membranes derived from MBP-pulsed M phi retain their capacity to block the development of EAE.

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

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

Defective processing and presentation of exogenous antigens in mutants with normal HLA class II genes

Presentation of an exogenous protein antigen to helper (CD4+)T-lymphocytes by antigen presenting cells (APC) generally requires that the APCs degrade the native protein antigen into an immunogenic peptide, a process termed 'antigen processing', and that this peptide bind to a major histocompatibility complex (MHC) class II molecule. The complex of peptide and MHC molecule on the APC surface provides the stimulatory ligand for the alpha beta T cell receptor. The intracellular pathways and molecular mechanisms involved in the generation of the peptide-MHC complex are not well understood. Here, we describe several mutant APCs which are altered in their ability to present native exogenous protein antigens but effectively present immunogenic peptides derived from these proteins. The lesions in these mutants are not in the class II structural genes, but they affect the conformation of mature class II dimers.

Antigen presentation by liposomes bearing class II MHC and membrane IL-1

Liposomes containing membrane IL-1, Iak, and the antigen conalbumin were evaluated as "synthetic antigen presenting cells." The role of these three molecules in macrophage-T cell interaction was studied by testing their ability to induce the proliferation of a T-cell clone specific to conalbumin (the D10 cell line) or immune spleen cells sensitized three times in vivo with conalbumin. In the latter case, splenic macrophages were eliminated by adherence and a lysomotropic agent. The antigen conalbumin was presented on the surface of the liposomes as native undigested protein. When the liposomes presented native conalbumin, Iak, and membrane IL-1, significant proliferation occurred, but if the liposomes lacked membrane IL-1, the proliferation of the T-cell clone and the spleen cells reached only about 60 percent of the previous signal. Native conalbumin and class II antigen alone were required for T-cell activation, while membrane IL-1 only amplified the response. When the liposomes were made with only Iak and membrane IL-1, lacking conalbumin, there was no proliferation of antigen-specific target cells. These results indicated that in this synthetic system, membrane IL-1 increases the magnitude of the response but is not essential for the proliferative response of antigen-specific T cells.

Selective expression of V delta 6 genes by B2A2- CD4- CD8- T cell receptor gamma/delta thymocytes

Most CD4-CD8- adult murine thymocytes characterized by absence of the B2A2 (J11d) antigen express T cell receptors (TcR) alpha/beta and utilize preferentially V beta 8.2 segments. To a much lesser extent, B2A2-CD4-CD8- thymocytes also express TcR gamma/delta, as evidenced by biochemical and Northern blot analysis. We have now been able to exclude the possibility that these cells might co-express both types of TcR: V beta 8+ B2A2- CD4-CD8- thymocytes expressed no TcR delta mRNA whereas the corresponding V beta 8- subset transcribed full-length TcR gamma as well as delta mRNA. Furthermore, the TcR gamma/delta expressing B2A2- thymocytes were found to use preferentially V delta 6 genes. Conversely, they did not express V delta 5 genes which are most frequently used by other TcR gamma/delta-bearing populations such as B2A2+ CD4-CD8- thymocytes or CD4-CD8- peripheral lymph node cells. RNase protection experiments showed that two particular V delta 6 transcripts predominate in these gamma/delta populations, the most prominent V delta 6 sequence being highly homologous if not identical to V alpha 7.1. Our observations extend previous information on overlapping V alpha and V delta gene repertoires, particularly in the cross-hybridizing V alpha 7/V delta 6 gene family. Moreover, our data suggest that B2A2-CD4-CD8- thymocytes represent a developmentally unique subset in which both V delta and V beta segments are nonrandomly expressed.

Antigen processing by isolated rat intestinal villus enterocytes

The processing of ovalbumin by isolated rat enterocytes and by splenic adherent cells (SAC) was compared. Paraformaldehyde fixation blocked presentation of ovalbumin to T cells by both cell types when given before, but not after, an antigen pulse. Presentation of ovalbumin by both cell types was blocked by treatment with chloroquine, ammonium chloride or monensin before the antigen pulse. Leupeptin treatment before the antigen pulse inhibited presentation by SAC, but not by enterocytes. Ovalbumin processed by enterocytes was not presented by fixed SAC. Radiolabelled ovalbumin was degraded to small molecular weight fragments by SAC, but not by enterocytes. It is suggested that, compared to the degradative processing seen in macrophages. B cells and dendritic cells, processing of ovalbumin by enterocytes comprises a more subtle, non-degradative mechanism.

The T-lymphocyte proliferative response to synthetic peptide antigens of defined secondary structure

Immunodominant sites in proteins recognized by T lymphocytes are segments consisting of at least 7-8 amino acids. It has previously been proposed that these sites in proteins are alpha-helical and amphipatic structures. We synthesized and investigated the immunogenicity of three synthetic peptides (MP7, MP8, and MP9), each consisting of the same 15 amino acids, but differing with respect to sequence. Based on information analysis and circular dichroism measurements, MP7 was shown to have an alpha-helical secondary structure and, based on previously assigned hydrophilicity indices, was also strongly longitudinally amphipatic. MP8 also was conformed as an alpha-helix, but was amphipatic in the sense that the N-terminal half of the molecule was hydrophilic and the C-terminal half hydrophobic. MP9 had neither an amphipatic nor alpha-helical structure. All three peptides were immunogenic in some strains of mice but none was immunogenic in all strains. This supports other studies concluding that amphipaticity per se is neither a necessary nor sufficient requirement for immunogenicity of a peptide. On the other hand, the present experimental data suggest that longitudinally amphipatic alpha-helical peptides may function better as T-cell determinants than the other peptides investigated.

A single amino acid mutation in a protein antigen abrogates presentation of certain T cell determinants

Nase-specific T cell recognize the 86-100 peptide in association with B10.A APC. Clone N40 recognizes the 86-100 peptide in association with B10.A (Ek alpha Ek beta) and B10.A (5R) (Ek alpha Eb beta) APCs. We demonstrate here that a single amino acid substitution in the staphylococcal nuclease protein alters the structure of the processed peptide such that the T cell epitope recognized by clone N40 was only available for recognition in conjunction with B10.A (5R) but not the B10.A APCs. Other Nase-specific T cells recognize the mutant nuclease, and a synthetic peptide corresponding to the immunodominant region of the mutant protein was stimulatory for all the Nase-specific T cells. These results suggest that the mutation either affects the processing of the protein into antigenic peptides or affects the conformation of the processed fragment differently from that of the peptide.

A peptide binding protein having a role in antigen presentation is a member of the HSP70 heat shock family

The T cell recognition of globular protein antigens requires the processing and presentation of the antigen by Ia-expressing APCs. Processing is believed to involve the uptake of antigen into an acidic compartment where proteolysis occurs. The resulting peptides containing the T cell antigenic determinant are associated with Ia and presented at the cell surface to the specific T cells. The mechanisms by which antigenic peptides become associated with Ia is not known. We previously described a peptide binding protein of 72/74 x 10(3) Mr (PBP72/74) that plays a role in antigen presentation as shown by the ability of an antiserum raised in rabbits to affinity-purified PBP72/74 to block presentation of cytochrome c to a cytochrome c-specific T cell hybrid. Here we show that PBP72/74 is recognized by mAbs specific for members of the HSP70 family of proteins. In Western blots PBP72/74 is bound by mAb 7.10, specific for an evolutionarily conserved epitope of HSP proteins and by mAb N27, specific for both the constitutively expressed and inducible 72/73 x 10(3) Mr HSP70 proteins. In addition, PBP72/74 shares a second common feature of the HSP proteins, that of binding to ATP. Indeed, ATP causes the release of PBP72/74 from binding to a peptide fragment of cytochrome c (Pc 81-104) and PBP72/74 can be eluted from ATP columns by Pc 81-104. Finally, a portion of PBP72/74 is shown to be present on B cell surfaces by immunofluorescence staining. Thus, it appears that characteristics of the heat shock proteins are shared by a protein playing a role in antigen presentation, suggesting some commonality in function.

Reactivity of V beta 17a+ CD8+ T cell hybrids. Analysis using a new CD8+ T cell fusion partner

Tolerance to IE molecules leads to deletion of V beta 17a-bearing T cells. Both, the CD4+ as well as the CD8+ T cell subsets are affected. A large percentage of CD4+ V beta 17a+ T cell hybrids recognize IE molecules. We now have investigated the reactivity for IE antigens of CD8+ V beta 17a+ T cell hybrids. Using a transfection approach, we have introduced the murine CD8 molecule into different V beta 17a+ T cell hybrids. Furthermore, the CD8 cDNA was transfected into the BW5147 alpha-beta- fusion partner. This allowed us to generate a large number of V beta 17a+ T cell hybrids by fusion with the appropriate T cells. Only 6% of T cell hybrids were stimulated to produce IL-2 upon incubation with IE+ cells. However, in those, the CD8 molecule seemed not to contribute to the IE reactivity of the hybrid, since mAbs against the CD8 molecule failed to inhibit their reactivity. This low percentage of V beta 17a+ CD8+ IE-reactive T cell hybrids contrasts with the strong reduction of CD8+ V beta 17a+ T cells in IE+ mice, strongly suggesting that elimination of such cells in the thymus occurs when they are coexpressing CD4 and CD8. This view was confirmed by the occasional expression of CD4 in some hybrids in which case IE reactivity was detected. Furthermore, we demonstrated the functional integrity of the introduced CD8 molecule by: (a) reconstitution of the IL-2 response in a class I-restricted TNP-specific T cell hybrid; and (b) by generation of alloreactive class I-restricted T cell hybrids using the new CD8+ fusion cell line. This CD8+ fusion partner, BWLyt2-4, should prove useful to study antigen processing and antigen presentation requirements of class I-restricted T cells.

An analysis of the physical properties of peptides that influence the pigeon cytochrome c specific T lymphocyte response

The ability to modify T cell responses was analyzed using synthetic peptide analogues of the T cell determinant for pigeon cytochrome c. Although the B10.A T cell proliferative response is directed to residues 95-104, residues to the amino-terminal side of this determinant influence antigen-specific T cell recognition. The proposed role of this non-determinant leader sequence has been to stabilize the core determinant in a helical conformation. Previous studies from our laboratory, however, using non-native leader sequences that were designed to examine the changes to T cell recognition invoked when the determinant was made more or less helical, amphipathic, or lipid binding in character than the native determinant. The structure of each analogue in aqueous, non-polar (TFE) and lipid environments was determined by circular dichroism. The ability of each antigen analogue to bind to phospholipid membranes and to stimulate two different pigeon cytochrome c T cell hybridomas, 2B4 and 22.D11, was also investigated. Our findings suggest that neither helicity or amphipathicity are necessary features of T cell recognition but that electrostatic interactions involving either the lipid membrane or the I-Ek molecule may influence T cell stimulation.

Presentation of IgG2a antigens to class II-restricted T cells by stably transfected B lymphoma cells

Here we describe a panel of BALB/c T cells specific for IgG2a of the b allotype in association with I-Ad. We used DNA-mediated gene transfer techniques to localize antigenic determinants recognized by responding T cells. Initially a truncated IgG2aa gene comprising a variable domain and the CH3 domain (not including the membrane exons) from the BALB/c IgG2aa heavy chain was introduced into myeloma cells. The V-CH3 protein was expressed at high levels under control of the Ig heavy chain enhancer. Secretion of the V-CH3 protein did not require assembly of H-H dimers or an association with light chains. To generate stably transfected B cell lines that would stimulate our class II-restricted T cells, we replaced most of the BALB/c IgG2aa CH3 exon with CH3 coding sequences from a C57BL/6 IgG2ab cDNA clone and introduced these constructs into Ia+ B lymphoma cells. The IgG2ab CH3-transfected B cells were recognized by BALB/c Igh-1b-specific T cell hybrids in the absence of exogenous antigen. Experiments using glutaraldehyde-fixed cells as stimulators indicate that presentation of the secreted form of V-IgG2ab CH3 requires processing. We found that a significant fraction of the endogenously synthesized V-IgG2ab CH3 protein was, however, present as already processed antigen.

Theileria parva: bovine helper T cell clones specific for both infected lymphocytes and schizont membrane antigens

Two Theileria parva-specific bovine helper T cell clones were used to identify T. parva-derived antigens expressed on the surface of schizont infected lymphoblastoid cells. Although the clones proliferated in response to both the immunizing (Muguga) and heterologous stocks of T. parva, the patterns of the responses differed, showing that the two clones recognized different antigenic epitopes. Both clones were stimulated by autologous infected cells, without an additional source of antigen-presenting cells, as well as by purified schizonts and by a subcellular membrane fraction prepared from infected lymphoblastoid cells, when antigen-presenting cells were present. The membrane fraction was shown to be enriched for schizont membranes as indicated by the presence of a schizont surface antigen detected by immunoblotting using a schizont-specific monoclonal antibody. Elimination of schizonts with the anti-theilerial drug, parvaquone, resulted in reduced antigenicity of the membrane fraction as detected by both the T cell clones and the schizont-specific monoclonal antibody. We conclude that the T. parva-infected cell surface antigens recognized by the T cell clones are of schizont membrane origin. Although the antigens have not yet been characterized biochemically, the monoclonal antibody-specific epitope appears to be distinguishable from the T cell epitopes.

Regulation of T-cell activation: differences among T-cell subsets

Four regulatory phenomena appear to regulate differentially the activation of TH1, TH2, and CTL clones. First, IFN-gamma selectively inhibits proliferation of TH2 but not TH1 cells; lymphokine production by TH2 cells is not affected by IFN-gamma. In addition, when fresh OVA-specific HTL clones are derived in the presence of rIL-2 TH2 cells are preferentially obtained, whereas TH1 cells predominate if cloning is performed in rIL-2 plus rIFN-gamma. These results suggest that the presence of IFN-gamma during the course of an immune response would result in the preferential expansion of HTL of the TH1 phenotype. Proliferation of CTL clones is not influenced by IFN-gamma. Second, different APC populations appear to differentially activate TH1 and TH2 clones. Purified splenic B cells stimulate optimal proliferation of TH2 but not TH1 cells, whereas macrophage/dendritic cells appear to stimulate optimal proliferation of TH1 but not TH2 cells. Since both APC types stimulate lymphokine production by each of the HTL subsets, these results suggest the existence of TH1- and TH2-specific cofactors for growth. Third, high doses of immobilized anti-CD3 mAb inhibit IL-2-dependent proliferation of TH1 but not TH2 clones. Since this effect appears to require calcium, this observation suggests that TCR-mediated signalling events might differ between the two HTL subsets. Indeed, little or no increase in [Ca++]i can be detected in TH2 clones stimulated with Con-A, while such an increase is easily discernible in TH1 cells. Although high concentrations of immobilized anti-CD3 mAb inhibit IL-2-dependent proliferation of CTL clones, proliferation of these cells in response to immobilized anti-CD3 alone reaches a plateau. Since activation with anti-CD3 is thought to mimic antigenic stimulation, these results suggest that antigen concentration may play a role in determining which predominant T-cell types proliferate in a particular immunological situation. Fourth, pretreatment of TH1 cells, but not TH2 cells or CTL, with IL-2 results in decreased lymphokine production and proliferation in response to subsequent stimulation via the TCR. This antigen-responsive state appears to involve a defect in calcium-dependent signalling, providing additional evidence for different signalling mechanisms in TH1 and TH2 clones.(ABSTRACT TRUNCATED AT 400 WORDS)

The selective antigen-presenting cell capacity of activated B lymphocytes in HLA-II-restricted responses of CD4+ T lymphocytes

We examined the role of antigen-presenting B lymphocytes using panels of antigen-specific CD4+8-T-lymphocyte clones (TLC). All 19 TLC showed a class II major histocompatibility complex-encoded (HLA-II) restricted proliferation to antigen presented by antigen-presenting cells (APC) from the monocyte fraction of peripheral blood. Only six TLC were effectively activated by antigen presented by autologous B lymphocytes activated by EBV transformation. This failure of B lymphocytes was not due to: (i) a high degree of cell surface sialic acid; (ii) a low expression of the cell surface proteins HLA-II, ICAM-1 or LFA-3 that restrict antigen presentation; (iii) lack of secretion of the cytokine IL-1 or other soluble factors that may be required as secondary signals; or (iv) induction of incomplete T-cell activation resulting in the production of growth factor interleukin-2 (IL-2) or the expression of receptors for IL-2 only. These data suggest the involvement of another cell surface interaction in antigen presentation acting besides the interactions known so far.

Immunoglobulin-specific T-B cell interaction. II. T cell clones recognize the processed form of B cells' own surface immunoglobulin in the context of the major histocompatibility complex class II molecule

In the preceding report (Eur. J. Immunol. 1989. 19: 1677) we have demonstrated that normal B cells, including small B cells, are capable of presenting Ig kappa-1b allotypic determinants of their endogeneously synthesized Ig+ to Ig kappa-1b-immune major histocompatibility complex (MHC) class II-restricted T cells. A panel of Ig kappa-1b allotype-specific T cell clones from August rats has been developed to study further the presentation of self surface Ig by B cells from Ig kappa-1-congeneic August.1b rats. All the clones studied were of the T helper/inducer phenotype (W3/25+,OX8-) and restricted by the RT-1Bc molecule. These clones responded both to the serum IgG(Ig kappa-1b) in the presence of irradiated spleen cells (SC) from August rats and to the Ig kappa-1b-bearing irradiated B cells from August.1b rats. SC presentation of secreted IgG was much less effective than B cell presentation of membrane Ig. Using CNBr cleavage of isolated C kappa (Ig kappa-1b) domain followed by high-performance liquid chromatography fractionation of the derived antigenic peptides, the kappa chain sequence between amino acids 176 and 214 has been identified as the T cell epitope recognized by all T cell clones in association with RT-1Bc. The fragment 176-214 of the Ig kappa-1b allotype differs from that of Ig kappa-1a allotype by three amino acid substitutions at positions 184, 185, 188. T cell recognition of pL kappa-1b(176-214) required no additional processing by the antigen-presenting cell: the efficient presentation of the peptide but not of intact IgG(Ig kappa-1b) by the paraformaldehyde-fixed SC was observed. These data provide clear-cut evidence for an absolute requirement of the processing of Ig molecules for T cell recognition to occur in our experimental system. Although the fixation of B cells from August.1b rats diminished their Ig kappa-1b-presenting ability, fixed Ig kappa-1b+ B cells were still able to induce Ig kappa-1b-specific T cell clone responses. Our results suggest that B cells can express the processed form of self-synthesized surface Ig in addition to intact surface Ig molecules. The former can be recognized by MHC-restricted T cell.

T cells specific for alpha-beta interface regions of hemoglobin recognize the isolated subunit but not the tetramer and indicate presentation without processing

Processing of a protein antigen into fragments is believed to be a prerequisite for its presentation by the antigen-presenting cell to the T cell. This model would predict that, in oligomeric proteins, T cells prepared with specificity for regions that are buried within subunit association surfaces should recognize the respective regions in vitro equally well on the isolated subunit or on the oligomer. Three hemoglobin (Hb) alpha-chain synthetic peptides, corresponding to areas that are situated either completely [alpha-(31-45)] or partially [alpha-(41-45) and alpha-(81-95)] within the interface between the alpha and beta subunits of Hb, and a fourth peptide representing a completely exposed area in tetrameric Hb were used as immunogens in SJL/J (H-2s) mice. Peptide-primed T cells were passaged in vitro with the respective peptide to obtain peptide-specific T-lymphocyte lines. T-cell clones were isolated from these lines by limiting dilution. T-cell lines and clones that were specific for buried regions in the subunit association surfaces recognized the free peptide and the isolated subunit but not the Hb tetramer. On the other hand, T cells with specificity against regions that are not involved in subunit interaction and are completely exposed in the tetramer recognized the peptide, the isolated subunit, and the oligomeric protein equally well. The responses of the T-cell lines and clones were major histocompatibility complex-restricted. Since the same x-irradiated antigen-presenting cells were employed, the results could not be attributed to differences or defects in Hb processing. The findings indicate that in vitro the native (unprocessed and undissociated) oligomeric protein was the trigger of major histocompatibility complex-restricted T-cell responses.

MHC class II invariant chains in antigen processing and presentation

Most protein antigens cannot elicit a T-cell response unless they are processed to peptides, which are then presented to T lymphocytes by surface MHC class II molecules. Recent evidence supports an essential role of the invariant chain associated with class II MHC polypeptides in antigen processing.

A novel and simple procedure for determining T cell epitopes in protein antigens

A human T cell epitope was mapped on tetanus toxin by the use of a simple method. Tetanus toxin peptide preparations were produced by various specific chemical and proteolytic cleavages. T cell proliferative response to these peptide preparations was assayed to determine which amino acid residues were present or absent in the T cell epitope. Cyanogen bromide-derived tetanus toxin peptides were separated by electrophoresis and transferred onto nitrocellulose. T cell proliferative response to nitrocellulose fragments was assayed to determine the molecular weight of the antigenic tetanus toxin peptide. The combination of the two procedures allowed us to define a 21-mer tetanus toxin peptide recognized by the T cell clone. This method represents an easily performed alternative for the mapping of T cell epitopes on protein antigen.

Presentation of myelin basic protein by normal guinea-pig brain endothelial cells and its relevance to experimental allergic encephalomyelitis

Previous studies have shown that endothelial cells in the central nervous system (CNS) of normal guinea-pigs constitutively express certain MHC class II determinants, whilst the expression of other determinants is apparent during the acute phase of chronic relapsing experimental allergic encephalomyelitis (CREAE). The expression of MHC class II determinants is retained by endothelial cells derived from normal guinea-pig brain tissue and maintained in culture. This present study demonstrates that the MHC class II molecules on these cells can be recognized by allogeneic lymphocytes, resulting in a proliferative response which is enhanced by the addition of exogenous IL-2. The endothelial cells were incapable of presenting either purified protein derivative or ovalbumin, but they could present autologous myelin basic protein (MBP), an encephalitogen implicated in the pathogenesis of EAE. The resulting lymphocyte proliferative response was of the same magnitude as that obtained when a control population of macrophages was used to present MBP. These results, therefore, suggest that cerebrovascular endothelia have the potential to play a role in the pathogenesis of EAE.

Carrier presentation: a possible strategy for selective in vivo amplification of specific T cells

A strategy for selective in vivo amplification of specific T cells by carrier presentation is outlined here. This hypothesis addresses the possible physiological significance of expression of MHC II molecules by T cells, and attempts to highlight the possibility of harnessing the phenomenon for therapeutic use in medicine.

Presentation of CMV immediate-early antigen to cytolytic T lymphocytes is selectively prevented by viral genes expressed in the early phase

The regulation of antigen processing and presentation to MHC class I-restricted cytolytic T lymphocytes was studied in cells infected with murine cytomegalovirus. Recognition by cytolytic T lymphocytes of the phosphoprotein pp89, the immunodominant viral antigen expressed in the immediate-early phase of infection, was selectively prevented during the subsequent expression of viral early genes. The surface expression of MHC class I glycoproteins and their capacity to present externally added pp89-derived antigenic peptides were not affected. Because recognition of several other antigens occurred during the early phase, a general failure in processing and presentation was excluded. Since neither rate of synthesis, amount, stability, nor nuclear transport of pp89 was modified, the failure in recognition indicates a selective interference with pp89 antigen processing and presentation.

Conformation of a T cell stimulating peptide in aqueous solution

Using two-dimensional NMR spectroscopy and circular dichroism spectroscopy it is demonstrated that a T cell stimulating peptide corresponding to residues 132-153 of sperm whale myoglobin populates helical conformations in aqueous solution. This finding is in accordance with proposals that immunodominant sites in T cell stimulating peptides have a high conformational propensity. The observation of secondary structure in aqueous solutions of this and other immunogenic peptides has important implications for initiation of protein folding.

Epitope-directed processing of specific antigen by B lymphocytes

Proteolytic processing of specific antigen was studied using Epstein Barr virus transformed B-lymphoblastoid cells expressing membrane IgG against tetanus toxin. As previously reported (Watts, C., and H.W. Davidson. 1988. EMBO (Eur. Mol. Biol. Organ.) J. 7:1937-1945), receptor-mediated endocytosis of monovalent antigen bound at 0 degrees C began immediately upon shifting the cells to 37 degrees C. In contrast, degradation of antigen, assessed either by the release of acid-soluble radiolabel into the incubation medium, or by SDS-PAGE analysis of total cell-associated antigen, proceeded after a lag of 10-20 min. Degradation was abolished by exposure of the cells to metabolic inhibitors, or by incubation at 20 degrees C, and inhibited in a dose-dependent fashion by chloroquine and by the lysosomal protease inhibitors leupeptin, E-64, and pepstatin A. Analysis of the cell-associated radiolabel by SDS-PAGE and autoradiography after incubations at 37 degrees C revealed the time-dependent generation of distinct antigen fragments. Virtually quantitative immunoprecipitation of these fragments was obtained using a monoclonal anti-human IgG antibody, indicating that the antigen/mIg complex is the initial substrate for processing. We show that the pattern of fragmentation observed varies from one B cell line to another (a) depending on the epitope through which the antigen is bound and endocytosed and (b) depending on whether additional epitopes in the antigen are complexed with anti-tetanus Fabs. The implications of these results for the presentation of major histocompatibility complex restricted antigen fragments, and for intracellular trafficking of ligand/receptor complexes are discussed.