Antigen presentation by supported planar membranes containing affinity-purified I-Ad

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.

Structural requirements and biological significance of interactions between peptides and the major histocompatibility complex

Previous studies indicate that T cells recognize a complex between the major histocompatibility complex (MHC) restriction-element and peptide-antigen fragments. Two aspects of this complex formation are considered in this paper: (1) what is the nature of the specificity of the interactions that allows a few MHC molecules to serve as restriction elements for a large universe of antigens; and (2) what is the relative contribution of determinant selection (i.e. antigen-MHC complex formation) and T-cell repertoire in determining the capacity of an individual to respond to an antigen? By analysing single amino acid substitution analogues of a peptide antigen (Ova 325-335) as well as by analysing the structural similarities between unrelated peptides capable of binding to the same MHC molecule, we have been able to document the very permissive nature of the antigen-MHC interaction. 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. With respect to the question of the relative role of 'determinant selection' and 'holes in the T-cell repertoire' in determining immune responsiveness, we present data that suggest both mechanisms operate in concert with one another. Thus only about 30% of a collection of peptides that in sum represent the sequence of a protein molecule were found to bind to Ia. Although immunogenicity was restricted to those peptides that were capable of binding to Ia (i.e. determinant selection was operative), we found that about 40% of Ia-binding peptides were not immunogenic (i.e. there were also 'holes in the T-cell repertoire').

Gamma interferon induces the appearance of a CD4-, major histocompatibility complex class II+ macrophage subpopulation in the rat peritoneal cavity

Peritoneal cells from gamma interferon (IFN-gamma)-treated rats were phenotypically characterized by flow cytometry. Intraperitoneal inoculation of 3 x 10(4) units of IFN-gamma induced, within 24 h, the appearance of a CD4-, major histocompatibility complex (MHC) class II+ macrophage subpopulation not present in rats treated with phosphate-buffered saline. IFN-gamma induced an increased expression of MHC class II I-A molecules on both CD4- and CD4+ macrophages. Both cell types adhered to plastic and expressed high levels of the macrophage membrane molecules CD11b and OX41. Histological examination of sorted CD4- and CD4+ macrophages confirmed the macrophage morphology of both populations with less granula in the former. We conclude that the appearance of CD4- macrophages in the peritoneal cavity after inoculation with IFN-gamma most probably reflects a selective recruitment of these cells from blood or surrounding tissues. The function of these cells is still unknown, although strong expression of MHC class II I-A indicates competence as antigen-presenting cells.

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.

A kinetic intermediate in the reaction of an antigenic peptide and I-Ek

Helper T cells are triggered by molecular complexes of antigenic peptides and cell surface glycoproteins of the MHC (gene products of the major histocompatibility complex) on antigen-presenting cells. There is now a lot of evidence that the complexes between isolated class II MHC molecules and selected peptides have long half-lives of approximately one day. The reported equilibrium binding constants between antigenic peptides and class II MHC molecules however, are only micromolar, suggesting that the association rate constants are very low. The only reported association rate constant is for a chicken ovalbumin peptide (OVA323-339) binding to I-Ad, and is indeed remarkably low, about 1 litre per mole per second. Prompted by these unusual data, we have used the pigeon cytochrome-c peptide pCytc(88-104) and I-E reconstituted in planar lipid bilayers on glass slides to investigate further the kinetics of peptide-MHC reactions. We report the formation of two IEk-pCytc peptide complexes. One complex has slow apparent association and dissociation kinetics, very similar to those reported previously for the chicken ovalbumin peptide and I-Ad. The second complex forms and dissociates about a hundred times more rapidly. The short-lived complex shows peptide-MHC specificity and is a kinetic intermediate in the formation of the long-lived complex; the long-lived complex is recognized by specific T-helper cells.

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)

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.

The T-cell accessory molecule CD4 recognizes a monomorphic determinant on isolated Ia

The membrane protein CD4 is commonly found on mature T cells specific for antigen in association with class II major histocompatibility complex (MHC; Ia) proteins. This correlation has led to the suggestion that CD4 binds to a monomorphic region of the Ia molecule on the antigen-presenting cell (APC) and functions either by enhancing interaction between the T cell and the APC, or conversely, by transducing negative signals to the T cell. To address this hypothesis, we have made use of sublines from an unusual T hybrid that is class I MHC restricted but also CD4+. By incorporating purified MHC proteins into a planar membrane system, we show that different Ia molecules can greatly enhance the ability of a CD4+ but not a CD4- variant of this class I-restricted T hybrid to respond to isolated class I molecules. T-cell responses can be strongly augmented by the concurrent expression of CD4 on the T cell and any of four different Ia proteins on planar membranes, thus supporting the idea that CD4 binds to a monomorphic region of the Ia molecule and increases the avidity with which the T cell can interact with its target.

Tumor necrosis factor and immune interferon act in concert to slow the lateral diffusion of proteins and lipids in human endothelial cell membranes

Vascular endothelial surface-related activities may depend on the lateral mobility of specific cell surface macromolecules. Previous studies have shown that cytokines induce changes in the morphology and surface antigen composition of vascular endothelial cells in vitro and at sites of immune and inflammatory reactions in vivo. The effects of cytokines on membrane dynamic properties have not been examined. In the present study, we have used fluorescence photobleaching recovery (FPR) to quantify the effects of the cytokines tumor necrosis factor (TNF) and immune interferon (IFN-gamma) on the lateral mobilities of class I major histocompatibility complex protein, of an abundant 96,000 Mr mesenchymal cell surface glycoprotein (gp96), and of a phospholipid probe in cultured human endothelial cell (HEC) membranes. Class I protein and gp96 were directly labeled with fluorescein-conjugated monoclonal antibodies; plasma membrane lipid mobility was examined with the phospholipid analogue fluorescein phosphatidylethanolamine (Fl-PE). In untreated, confluent HEC monolayers, diffusion coefficients were 30 x 10(-10) cm2 s-1 for class I protein, 14 x 10(-10) cm2 s-1 for gp96, and 80 x 10(-10) cm2 s-1 for Fl-PE. Fractional mobilities were greater than 80% for each probe. Cultures treated at visual confluence for 3-4 d with either 100 U/ml TNF or 200 U/ml IFN-gamma did not exhibit significant changes in protein or lipid mobilities despite significant changes in cell morphology and membrane antigen composition. In HEC cultures treated concomitantly with TNF and IFN-gamma, however, diffusion coefficients decreased by 71-79% for class I protein, 29-55% for gp96, and 23-38% for Fl-PE. Fractional mobilities were unchanged. By immunoperoxidase transmission electron microscopy, plasma membranes of untreated and cytokine-treated HEC were flat and stained uniformly for class I antigen. "Line" FPR measurements on doubly treated HEC demonstrated isotropic diffusion of class I protein, gp96, and Fl-PE. Finally, although TNF and IFN-gamma retarded the growth of HEC cultures and disrupted the organization of cell monolayers, the slow diffusion rates of gp96 and Fl-PE in confluent doubly treated monolayers were not reproduced in sparse or subconfluent untreated monolayers. We conclude that the slowing of protein and lipid diffusion induced by the combination of TNF and IFN-gamma is not due to plasma membrane corrugations, to anisotropic diffusion barriers, or to decreased numbers of cell-cell contacts.(ABSTRACT TRUNCATED AT 400 WORDS)

Non-H-2 involvement in antigen-specific T cell proliferative responses. I. Different blocking effect of anti-class II monoclonal antibodies between B6 and BALB.B mice on antigen-specific T cell activation

Two monoclonal antibodies (IAb1 and D2A2 mAbs) to I-Ab molecule were shown to have different inhibitory effects between B6 and BALB.B mice in antigen-specific T cell activation. Antigen-specific T cell proliferative responses of B6 were blocked by IAb1 mAb but not by D2A2 mAb, whereas those of BALB.B were blocked by both IAb1 and D2A2 mAbs. The different blocking effect of D2A2 mAb between B6 and BALB.B is attributed to the APC level but not responding T cell repertoires, because antigen-specific proliferative responses of (B6XBALB.B)F1 T cells stimulated by BALB.B APC but not by B6 APC were blocked by D2A2 mAb. These data indicate that not only H-2 but also non-H-2 genes participate in antigen-specific activation of T cells by APC.

Contribution of antigen processing to the recognition of a synthetic peptide antigen by specific T cell hybridomas

Most antigens recognized by T cells require unfolding or partial degradation (processing) followed by association with Major Histocompatibility Complex (MHC) molecules. We examined the processing requirements for the presentation of antigen to two T cell hybridomas which recognize the alpha-helical synthetic polypeptide antigen Poly 18, Poly [EYK(EYA)5], in association with I-Ad. Hybridoma A.1.1 responds to EYK(EYA)4 as the minimum antigenic sequence while hybridoma B.1.1 recognizes (EYA)5 sequence. It was found that these hybridomas responded to Poly 18 and to minimum peptide sequences presented by glutaraldehyde and chloroquine treated antigen presenting cells (APC), suggesting that antigen processing is not a requirement for the activation of these cells. The reactivity pattern of hybridoma B.1.1 in the presence of glutaraldehyde fixed APC revealed that antigens containing lysine were presented with much less efficiency than antigens without lysine, suggesting an interaction of these residues with the antigen presenting cell surface. We discuss the possibility that alanine residues in the alpha-helical Poly 18 form a hydrophobic ridge which may be required for appropriate interaction between antigen, the T cell receptor, and MHC molecules.

Differences between primed allogeneic T-cell responses and the primary mixed leucocyte reaction. Primed T cells become independent of the blocking effects of monoclonal antibodies against IL-1 beta and the CD5, CD11a (LFA-1), and CD11c (p 150,95) molecules

Recent investigations have demonstrated that the primary mixed lymphocyte reaction (MLR) is dependent on certain accessory molecules, e.g. CD4 and LFA-1. We have compared the requirements of the primary MLR and the responses of alloreactive, primed lymphocytes (PL) by inhibition studies using monoclonal antibodies (MoAb) directed against (i) adhesion molecules belonging to the CD11 cluster of leucocyte antigens (CD11a, LFA-1; CD11b, MAC1 = CR3; and CD11c, p 150,95); (ii) various T cell-related antigens (CD2, CD4, CD5 and CD8); and (iii) recombinant IL-1 beta. The CD5-, CD11a- and CD11c-reactive MoAb significantly inhibited the primary MLR (inhibition = 25%, P less than or equal to 0.01; 48%, P less than or equal to 0.01 and 13%, P less than or equal to 0.05, respectively) but these MoAb did not inhibit the primed lymphocyte reaction (PLR). The CD11b-reactive MoAb had no significant influence on either of the responses. CD2- and CD4- reactive MoAb significantly inhibited both primary MLR (greater than 80%, P less than or equal to 0.01) and to a lesser extent the PLR (40-65%, P less than or equal to 0.01). A MoAb reactive with IL-1 beta inhibited the primary MLR (38%, P less than 0.01) and the purified protein derivative (PPD) induced lymphocyte transformation response (42%, P less than or equal to 0.01) of peripheral blood mononuclear cells (PBMC), whereas primed allogeneic responses to PBMC and Epstein-Barr virus (EBV) cell lines were unaffected by this MoAb. In addition, preliminary data indicated that PL seemed neither to bind exogenous IL-1 (as opposed to CD4+ PBMC) nor to possess membrane-bound IL-1. The differences between 'virgin' and primed, allogeneic T-cell responses indicate that profound changes in the functional capability of the responding T-cell population take place during the bulk expansion. The results indicate that during repeated priming with alloantigen and bulk expansion, the proliferative response of T lymphocytes becomes independent of (i) the interaction with the CD11 adhesion molecule(s), (ii) the CD5 molecule, and (iii) the cytokine IL-1 beta.

Selective stimulation of human T lymphocyte subsets by heteroconjugates of antibodies to the T cell receptor and to subset-specific differentiation antigens

Ligand binding under conditions that generate microaggregates of the T cell receptor complex (Ti/CD3) with the membrane molecules CD4 or CD8 can induce activation of small resting T lymphocytes. We demonstrate this by using soluble dimeric heteroconjugates consisting of monoclonal antibodies to CD4/CD8 and of a novel anti-CD3 monoclonal antibody (BMA030) which even in aggregated form is not stimulatory on its own under limiting conditions in the absence of accessory cells. Using combinations of BMA030 with either anti-CD4 or anti-CD8, the corresponding T cell subpopulation could be selectively expanded in vitro. Selective expansion of T cell subpopulations in vitro or in vivo might be helpful for certain therapeutical manipulations. In addition, this finding may contribute to a better understanding of major histocompatibility complex-restricted T cell activation.

Insulin-dependent diabetes mellitus induced in transgenic mice by ectopic expression of class II MHC and interferon-gamma

We have produced transgenic mouse strains harboring class II major histocompatibility complex or interferon-gamma genes linked to the human insulin promoter. These experiments were designed to investigate the consequences of the expression of immunological effector molecules by nonimmunological cells. In both of these studies we observed the disappearance from the pancreas of the insulin-producing beta cells coinciding with the development of insulin-dependent diabetes mellitus. Transgenic mice expressing both chains of the I-A gene showed progressive atrophy of the islets of Langerhans, whereas mice expressing interferon-gamma suffered an inflammatory destruction of the islets.

The T cell receptor

The primary structure of T cell receptor proteins and genes is well understood. Immunologists are now trying to understand the properties of these interesting molecules. Evidence suggests that T cell alpha beta receptors recognize a complex of an antigen-derived peptide bound to one of the cell-surface products of the major histocompatibility complex (MHC) genes. It is likely that alpha beta receptors and MHC proteins have coevolved to have some affinity for each other. During T cell development in the thymus, cells bearing self-reactive receptors are deleted by the mechanisms of tolerance, and cells are preferentially allowed to mature if they bear receptors that will be able to recognize antigen plus self-MHC after they have become full-fledged T cells. Some explanations for these phenomena have been tested, but no satisfactory theory can yet be proposed to account for them.

Cross-linking of the T cell receptor complex with the subset-specific differentiation antigen stimulates interleukin 2 receptor expression in human CD4 and CD8 T cells

Nonpolymorphic interactions between the T cell differentiation antigens CD4 or CD8 and major histocompatibility complex (MHC)-encoded molecules have been postulated to participate in antigen recognition of MHC-restricted T cells. This would imply simultaneous binding of CD4/8 and of the T cell receptor complex (Ti/CD3) to MHC molecules on the stimulator or target cell. In this report experimental evidence is provided that simultaneous binding by antibodies of Ti/CD3 and of CD4 or CD8 leads to the expression of interleukin 2 (IL 2) receptors in resting human T cells and to their subsequent proliferation in the presence of recombinant IL 2 (rIL 2). This could be shown by using a novel anti-CD3 monoclonal antibody (BMA 030) which alone only marginally stimulates highly purified human T cells even when applied in cross-linked form. However, human T cell subpopulations could be stimulated to grow in the presence of rIL 2 when BMA 030 was fixed to a solid support in combination with antibodies to either CD4 or CD8. In limiting dilution experiments, the frequencies of CD4 and CD8 T cells activated by the antibody combinations were similar to those activated by phytohemagglutinin in the presence of irradiated adherent cells. No stimulation was achieved if both or one antibody was applied in soluble form. In contrast, soluble antibodies inhibited activation by solid-phase antibodies. Taken together, cross-linking of Ti/CD3 with CD4/8 seems to be essential for T cell activation in cases of ligands that bind but do not activate T cells on their own--a situation that may reflect the interaction of T cell receptors with MHC-encoded molecules in association with antigen.

A monoclonal antibody inhibiting leucocyte adhesion blocks induction of IL-2 production but not IL-2 receptor expression

Monoclonal antibody 60.3 defines the leucocyte antigen CD 18 and recognizes a cell surface glycoprotein with an apparent molecular weight (MW) of 90,000 expressed by most human peripheral blood and bone marrow cells. This antibody can, among other things, block phorbol ester-induced adhesion among human mononuclear leucocytes. We show in this study that phorbol esters alone can induce peripheral blood mononuclear cells (PBL) to secrete interleukin-2 (IL-2) and that the IL-2-dependent cell line CTLL can be used for measuring this lymphokine without influence of the phorbol esters themselves. These findings make it possible to analyse the capacity of antibody 60.3 to interfere with IL-2 production and receptor expression by phorbol ester or phytohaemagglutinin (PHA)-treated human PBL. A significant positive correlation between blockage of induced cell aggregation by antibody 60.3 and reduction in IL-2 release was observed. The addition of interleukin-1 (IL-1) restored IL-2 secretion in PHA-treated, but not in 4-beta-phorbol 12, 13-dibutyrate [P(Bu)2]-treated, cells in the presence of this antibody. In parallel, IL-2 receptor expression was determined by immunofluorescence using biotinylated anti-IL-2 receptor (Tac) antibodies. FACS analysis showed that IL-2 receptor expression was unaffected by antibody 60.3, whereas DNA synthesis of the same P(Bu)2-treated PBL was inhibited. However, addition of external recombinant IL-2 overcame this proliferation blockade. These results indicate that a cell-to-cell adhesion step is necessary for the production of IL-2, but not for the expression of its receptor on both PHA- and P(Bu)2-treated human PBL.

The relation between major histocompatibility complex (MHC) restriction and the capacity of Ia to bind immunogenic peptides

The capacity of purified I-Ad, I-Ed, I-Ak, and I-Ek to bind to protein derived peptides that have been previously reported to be T cell immunogens has been examined. For each of the 12 peptides studied strong binding to the relevant Ia restriction element was observed. All the peptides bound more than one Ia molecule; however, for 11 of 12 peptides, the dominant binding was to the restriction element, whereas in one instance the dominant binding was to a nonrestriction element. When the peptides were used to inhibit the presentation of antigen by prefixed accessory cells to T cells, an excellent correlation was found between the capacity of a peptide to inhibit the binding of an antigen to purified Ia and the capacity of the peptide to inhibit accessory cell presentation of the antigen. Thus, the binding of peptide to purified Ia is immunologically relevant, and Ia seems to be the only saturable molecule on the surface of the accessory cell involved in antigen presentation. Inhibition analysis also indicated that all peptides restricted to a particular Ia molecule competitively inhibited one another, suggesting that each Ia restriction element has a single binding site for antigen. Cross-linking of labeled peptides to Ia followed by electrophoretic analysis and autoradiography suggested that this single binding site is made up of portions of both alpha and beta chains of Ia.

High-affinity fluorescent peptide binding to I-Ad in lipid membranes

Membranes isolated from antigen-pulsed, antigen-presenting cells were extensively dialyzed and shown to retain the ability to present antigen to an I-Ad-restricted, antigen-specific T cell, 3DO-54.8. This ability to retain antigen on the membrane was duplicated in lysates of antigen-presenting cells as well as with pure I-A molecules in phospholipid vesicles. Measurement of the concentration of surface-associated fluorescent peptide on planar membranes prepared from antigen-pulsed phospholipid vesicles showed that about 1 peptide molecule was retained per 100 I-A molecules. We calculate that about 1000 I-A-associated peptide molecules are sufficient to stimulate the response of a single 3DO-54.8 cell. The association of fluorescent peptide with the surface depended on the presence of I-A and was blocked by unlabeled ovalbumin peptide or by a digest of hen egg lysozyme, added at the time of the pulse, but not after pulsing.

Cell-sized, supported artificial membranes (pseudocytes): response of precursor cytotoxic T lymphocytes to class I MHC proteins

Novel cell-sized, supported artificial membranes bearing class I antigens have been prepared by a simple dialysis procedure and then used to study the requirements for antigen recognition by precursor cytotoxic T lymphocytes (CTL). The membranes were made by mixing lipid, H-2 antigen, and C18 alkylated 5 microns silica beads in deoxycholate, and dialyzing to remove the detergent. The H-2 antigen-bearing, cell-sized beads, termed pseudocytes (artificial cells), were able to simulate generation of secondary CTL responses with the same specificity as alloantigen-bearing spleen cells. Comparative analyses demonstrated that the size of an antigen-bearing structure, and thus its potential for multivalent interaction, was a critical determinant of effectiveness of antigen recognition, and showed that H-2 antigen was recognized as effectively on cell-sized beads as on allogeneic spleen cells. Generation of a response to antigen on the cell-sized beads was completely dependent on addition of lymphokines to the cultures. Thus, unlike liposomes, H-2 antigen on beads was not available to accessory cells for stimulation of Ia-dependent production of lymphokines by T helper cells. These results, as well as direct observations by microscopy, strongly indicate that antigen is recognized on the surface of the beads. Despite effective stimulation of secondary CTL responses, antigen on beads was completely inactive in stimulating a primary CTL response by naive spleen cells. The results of mixing experiments by using beads and alloantigen-bearing cells or plasma membrane vesicles indicate that the lack of a primary response may result from a requirement for a soluble factor(s) that is not needed for generation of secondary responses. The unique advantages of cell-size supported membranes for studying antigen recognition by T cells are discussed. The beads can be handled and used like antigen-bearing cells in functional assays, while possessing well-defined, readily varied, and easily quantitated composition.

Total internal reflection fluorescence. Measurement of spatial and orientational distributions of fluorophores near planar dielectric interfaces

The fluorescence collected from a fluorophore which is near a planar interface and is excited by a laser beam that is totally internally reflected at the interface depends on the direction of the absorption and emission transition dipole moments of the fluorophore with respect to the interface, on the distance from the fluorophore to the interface, on the angle of incidence and polarization direction of the exciting beam, and on properties of the collection optics. Expressions are derived for the excitation and subsequent emission and collection of fluorescence from a population of fluorophores near a planar interface. Presented is a general model-independent method of obtaining characteristic parameters of the spatial and orientational distribution of the population of fluorophores, from a measure of the fluorescence collected as a function of the polarization and the incidence angle of the totally internally reflected laser beam. The method is illustrated with several simulation calculations.

Immunologically relevant peptide antigen exists on the presenting cell in a manner accessible to macromolecules in solution

Although studies of the association of antigen with APC have been complicated by antigen-processing requirements, recent studies have suggested that immunologically relevant antigen should be present on the APC surface. Nevertheless, blocking of antigen presentation with antibody to the antigen has not been demonstrable in most systems. To study this problem we developed a system using avidin to block presentation of amino-terminal biotinylated synthetic peptide 132-146 of sperm whale myoglobin (B132) to a murine T cell clone specific for this site in association with I-Ed. greater than 95% specific inhibition was observed with doses of B132 equipotent to unmodified peptide. Specific blocking could be observed: (a) after pulsing APC with antigen, washing, and incubating for a chase period of 8-16 h before addition of avidin and T cells to assure adequate time for intracellular trafficking and maximal display of antigen on the cell surface, or (b) when monensin is present during the antigen pulse to inhibit such traffic. Therefore, the inhibition appeared to be occurring at the cell surface unless dissociation and reassociation were constantly occurring. To distinguish these, B10.GD APC (I-Ed-negative) were pulsed with antigen and cocultured with B10.D2 APC (I-Ed-positive). No detectable antigen presentation resulted. Thus, minimal dissociation and reassociation between antigen and APC occurs and, consequently, blocking by extracellular solution-phase binding of avidin to antigen is unlikely. Taken together, these data suggest that the blocking is occurring at the cell surface. Thus, under physiologic conditions, immunologically relevant antigen necessary for T cell activation appears to be present on the APC surface and is freely accessible to macromolecules the size of avidin. These findings hold specific implications for models of antigen presentation for T cell recognition.

Antigen-Ia interaction and the proteolytic processing of antigen: the structure of the antigen determines its restriction to the A or E molecule of the major histocompatibility complex

The effect of a protease inhibitor, leupeptin, on the presentation of hen egg lysozyme (HEL) to cloned T cells was investigated. We found that leupeptin-sensitive thiol proteases are apparently less involved when HEL is presented by the I-Ad molecule, than when it is presented by the I-Ed molecule. This selectivity was more of a function of the antigen than that of the Ia molecule because presentation of denatured or fragmented HEL was not sensitive to leupeptin whereas antigen presentation to a number of I-A-restricted T cell clones specific to other antigens was sensitive to leupeptin. These data demonstrate that the particular combination of major histocompatibility complex/nominal antigen recognized by a certain T cell clone may require processing of the antigen molecule through a certain group of proteases and that other combinations are independent of that particular processing pathway. Furthermore, there is a preference for a certain type of processing depending on the Ia molecule involved.

The response of T cells to soluble protein antigens and fixed antigen-presenting cells

A polyclonal population of antigen-specific T cells has been used to study the presentation of a soluble protein antigen (ovalbumin) using a B-cell line (A.20) as antigen-presenting cells. We find that glutaraldehyde-fixed antigen-presenting cells will present undegraded antigen to this polyclonal population of T cells. T-cell proliferation is observed only if an exogenous source of T-cell growth factor is added to the cultures. These findings are compared to the results obtained using antigen-specific T-cell clones or T-cell hybridomas and provide further evidence to suggest that degradation of antigen is not essential for the presentation of antigen to T cells.

Antigenic competition at the level of peptide-Ia binding

We examined the direct binding of a hen egg white lysozyme peptide, HEL(46-61), to membrane I-Ak (protein encoded in the A locus of the I region) molecules in the presence of detergent. A number of synthetic peptide derivatives, which did not stimulate our T-cell reactive hybridomas, competed for the binding of HEL(46-61) to I-Ak and also inhibited the functional presentation of HEL(46-61). Inhibitors included a peptide lacking a tyrosine at position 53 and a peptide corresponding to the autologous lysozyme peptide. Presentation was examined with cells or with supported planar phospholipid membranes bearing only I-Ak and HEL(46-61). Other peptides that did not compete for the binding did not inhibit functional presentation. We concluded that the binding of an immunogenic peptide to I-A is critical for presentation, that the I-A molecule does not discriminate between autologous and foreign related determinants but does recognize structurally different peptides. Our evidence suggests that our immunogenic peptide bears noncontiguous amino acids critical for contact I-A binding interspersed with amino acids critical for interaction with T cells.

Cotransfer of the Ed alpha and Ad beta genes into L cells results in the surface expression of a functional mixed-isotype Ia molecule

Ia molecules play a key role in antigen recognition by T lymphocytes. To analyze the structural features of the individual alpha and beta chains relevant to the assembly of intact Ia molecules, mouse fibroblasts were cotransfected with various combinations of haplotype- and isotype-mismatched Ia alpha/beta gene pairs. Two important points emerged. First, the level of surface expression of a given haplotype-mismatched A alpha A beta pair appears to depend upon the alpha and beta chain alleles involved. Second, transfection with some isotype-mismatched combinations such as Ed alpha Ad beta results in a significant level of surface expression of a stable mixed-isotype dimer, which also appears to be normally expressed at a low level by an Iad-positive B lymphoma. Moreover, a T-cell hybridoma specific for human gamma globulin and restricted by the Ed molecule was found to be efficiently stimulated by the Ed alpha Ad beta-positive transfectant in the presence of antigen. The stimulation was specifically inhibited by monoclonal antibodies directed to either the Ia or the L3T4 molecule. These findings suggest that the estimates of the potential number of Ia molecules available in an animal for restricting T-lymphocyte recognition of antigens must be revised.

Antigen presentation by liposomes: inhibition with antibodies

The involvement of both antigen and class II major histocompatibility complex (MHC) molecules in T cell activation by liposome-bound antigens was investigated. We used a pigeon cytochrome c (PCC)-specific Ek-restricted T cell hybridoma that can be activated to produce interleukin 2 by liposomes carrying either PCC and Ek molecules, or a high concentration of PCC alone. We demonstrated that the MHC-restricted response of this hybridoma to liposomes is specifically blocked by both anti-MHC and anti-PCC monoclonal antibodies, whereas unrestricted activation is only inhibited by PCC-specific antibody.

T-cell-mediated association of peptide antigen and major histocompatibility complex protein detected by energy transfer in an evanescent wave-field

Helper T cells recognize foreign antigen displayed on antigenpresenting cells which also express self-molecules of the major histocompatibility complex (MHC). A single T-cell receptor mediates recognition of both MHC and foreign antigen. A proposed ternary complex between T-cell receptor, foreign antigen and MHC antigen has not yet been demonstrated (see ref. 1 for review). Here, we show that a fluorescein-labelled synthetic peptide, together with Texas red-labelled class II MHC antigen, I-Ad, stimulates the production of interleukin-2 by a peptide-specific I-Ad-restricted T-cell hybridoma when reconstituted in a lipid membrane on a glass substrate. Under the same conditions, resonance-energy transfer from donor peptide to acceptor I-A can be stimulated in an evanescent wave-field only in the presence of the specific T-hybrid. Our results show that the T cell stabilizes an association between peptide antigen and class II MHC protein to within a distance of about 40 A.

Lateral diffusion of specific antibodies bound to lipid monolayers on alkylated substrates

We have measured the lateral mobility of fluoresceinated monoclonal IgG antibodies bound specifically to a spin label lipid hapten in phospholipid monolayers supported on alkylated silicon oxide surfaces. Dimyristoyl phosphatidylcholine and dipalmitoyl phosphatidylcholine monolayers containing 5 mol% of the lipid hapten were transferred by conventional Langmuir-Blodgett techniques onto substrates alkylated with hydrocarbon chains containing 10, 16, and 18 carbon atoms. We show that the diffusion of the bound antibodies depends on their lateral density, the composition of the lipid monolayer, and the nature of lipid coupling to hydrocarbon chains on the alkylated substrate. Antibody diffusion coefficients at low antibody densities are within a factor of 2 of those displayed by the lipid hapten in the absence of the bound antibody. High antibody densities result in reduced antibody mobility, but the lateral diffusion of unbound lipids is unaffected.

High antigen concentration inhibits T cell proliferation but not interleukin 2 production: examination of limiting dilution microcultures and T cell clones

The effect of high antigen dose on the activation of cytochrome c peptide-primed lymph node cells was determined in several strains of mice by a limiting dilution analysis. It was found that proliferation of cytochrome c peptide-specific T cells was completely inhibited at high antigen concentration in C57BL/6 but only partially in DBA mice and had no effect in SJL mice. Clones derived from DBA mice showed a differential capacity to be inhibited by high antigen dose. On the other hand, interleukin 2 production by these clones was not impaired regardless of the antigen concentrations used.

Interactions between T helper cells and dendritic cells during the rat mixed lymphocyte reaction

This paper describes the interactions between dendritic cells (DC) and T helper (Th) cells in the rat mixed lymphocyte reaction (MLR). Th blasts that are actively proliferating were generated in a 5 d primary MLR; resting Th memory cells were derived from a 10-12 d MLR. The DC were purified from thoracic duct lymph derived from rats whose mesenteric lymph nodes had been removed. The results show that DC are the major stimulators in the primary MLR and also for the restimulation of Th blasts and Th memory cells. Th blasts rapidly formed large clusters when cultured with DC but not with Ia+ macrophages or B cells. This interaction was not dependent on a major histocompatibility complex (MHC) difference between the T blasts and the DC. Con A-activated T and B blasts also formed clusters when cultured with DC. Th memory cells formed small clusters with DC, but, in a different assay in which clusters are dispersed, we detected an antigen-specific interaction between Th memory cells and DC. The monoclonal antibodies W3/25 (anti-rat CD4) and MRC OX-6 (anti-MHC class II) blocked proliferation in the primary MLR and also inhibited the restimulation of Th memory cells. However, the restimulation of Th blasts in a secondary MLR was only blocked by MRC OX-6. These results suggest that there are different requirements for the restimulation of T blasts than for the activation of primary or memory Th cells. W3/25 and MRC OX-6 did not affect the clustering of T blasts with DC but they both inhibited the antigen-specific binding of Th memory cells to DC. The data suggest that the CD4 (W3/25) antigen is involved in antigen-specific interactions between Th cells and DC.

Analysis of antigen presentation by metabolically inactive accessory cells and their isolated membranes

Several amino acid copolymers are potent immunogens under the control of major histocompatibility complex (MHC)-encoded Ir genes. We have further characterized their accessory-cell-dependent, MHC-restricted presentation to T lymphocytes. We initially characterized their processing requirements by investigating the ability of paraformaldehyde-fixed antigen-presenting cells (APC) to present these copolymers. Fixed APC can present poly(Glu56Lys35Phe9) and poly(Glu60Ala30Tyr10) provided that they have been incubated with antigen prior to fixation. The inability of these same fixed preparations to present soluble antigen indicates a fixation-sensitive antigen-processing step. In contrast, the antigens poly(Glu55Lys35Leu10) and poly(Glu55Lys35Tyr10) can be presented by APC fixed before antigen exposure. This differential requirement for antigen processing was exploited to analyze the events of antigen presentation in two related systems. First, the ability of isolated APC membranes to process and present antigen was assessed. APC membranes can present the antigens poly(GluLysLeu) and poly(GluLysTyr) in a specific and MHC-restricted manner. However, the isolated membranes fail to present either poly(GluLysPhe) or poly(GluAlaTyr), suggesting that such preparations can present but not process antigen. Second, the distinct properties of the various copolymers were used with fixed APC to test the effects of antigen processing on the phenomenon of antigen competition. APC that had processed poly(GluLysPhe) or poly(GluAlaTyr) were subsequently fixed and used to present antigen in the presence or absence of various antagonists. Under these conditions, poly(GluLysLeu) and poly(Glu50Tyr50) could effect specific inhibition, clearly indicating that antigen competition occurs distal to and does not require antigen processing. In contrast, native antigen with an absolute processing requirement is not capable of competing with preprocessed antigen on fixed APC. Taken together, these results suggest that processing is important for the molecular interactions between the copolymer antigens and the APC cell surface that are relevant to both antigen presentation and competitive inhibition.