Variations in the number of peptide-MHC class I complexes required to activate cytotoxic T cell responses

We determined equilibrium constants for the binding of 16 peptides (based on four T cell epitopes) to three MHC class I proteins (A2, Kb, and Ld) on intact cells and estimated the number of accessible peptide-binding sites on these cells. From these results, and the concentrations of peptides required to sensitize target cells for lysis by CD8+ CTL, we conclude that the critical number of peptide-MHC complexes required per target cell for the activation of CTL responses varies with different combinations of peptide-MHC complexes and CTL clones from several thousand complexes to fewer than ten per target cell.

Variations in the number of peptide-MHC class I complexes required to activate cytotoxic T cell responses

We determined equilibrium constants for the binding of 16 peptides (based on four T cell epitopes) to three MHC class I proteins (A2, Kb, and Ld) on intact cells and estimated the number of accessible peptide-binding sites on these cells. From these results, and the concentrations of peptides required to sensitize target cells for lysis by CD8+ CTL, we conclude that the critical number of peptide-MHC complexes required per target cell for the activation of CTL responses varies with different combinations of peptide-MHC complexes and CTL clones from several thousand complexes to fewer than ten per target cell.

High-affinity reactions between antigen-specific T-cell receptors and peptides associated with allogeneic and syngeneic major histocompatibility complex class I proteins

We report here that the intrinsic affinities of the antigen-specific T-cell receptors (TCR) of two unrelated CD8+ T-cell clones for their respective peptide-major histocompatibility complex (MHC) ligands are higher than the values generally thought to prevail for TCR. The TCR of one clone (2C) binds an allogeneic class I MHC protein (Ld) in association with an alpha-ketoglutarate dehydrogenase nonapeptide (QLSPFPFDL, termed QL9) with an intrinsic affinity (intrinsic equilibrium association constant) of 1-2 x 10(7) M-1. The TCR of the other clone (4G3) binds a syngeneic class I MHC protein (Kb) in association with an ovalbumin octapeptide (SIINFEKL, termed pOV8) with an intrinsic affinity of 1.5 x 10(6) M-1. A comparison of the two clones, combined with current views of T-cell repertoire selection in the thymus, leads us to propose that TCR affinities are generally likely to be higher for allogeneic MHC-peptide complexes than for syngeneic MHC-peptide complexes.

Naturally processed viral peptides recognized by cytotoxic T lymphocytes on cells chronically infected by human immunodeficiency virus type 1

We have established long-term cultures of several cell lines stably and uniformly expressing human immunodeficiency virus type 1 (HIV-1) in order to (a) identify naturally processed HIV-1 peptides recognized by cytotoxic T lymphocytes (CTL) from HIV-1-seropositive individuals and (b) consider the hypothesis that naturally occurring epitope densities on HIV-infected cells may limit their lysis by CTL. Each of two A2-restricted CD8+ CTL specific for HIV-1 gag or reverse transcriptase (RT) recognized a single naturally processed HIV-1 peptide in trifluoroacetic acid (TFA) extracts of infected cells: gag 77-85 (SLYNTVATL) or RT 476-484 (ILKEPVHGV). Both processed peptides match the synthetic peptides that are optimally active in cytotoxicity assays and have the consensus motif described for A2-associated peptides. Their abundances were approximately 400 and approximately 12 molecules per infected Jurkat-A2 cell, respectively. Other synthetic HIV-1 peptides active at subnanomolar concentrations were not present in infected cells. Except for the antigen processing mutant line T2, HIV-infected HLA-A2+ cell lines were specifically lysed by both A2-restricted CTL, although infected Jurkat-A2 cells were lysed more poorly by RT-specific CTL than by gag-specific CTL, suggesting that low cell surface density of a natural peptide may limit the effectiveness of some HIV-specific CTL despite their vigorous activity against synthetic peptide-treated target cells.

A cytotoxic T lymphocyte clone can recognize the same naturally occurring self peptide in association with a self and nonself class I MHC protein

The alloreactive CD8+ cytotoxic T lymphocyte (CTL) clone 2C was previously shown to recognize complexes made up of the class I MHC (MHC-I) molecule Ld and an octapeptide (LSPFPFDL, termed p2Ca) isolated from tissues of H-2d mice. Because peptide p2Ca has also been found in BALB.B (H-2b) mice, the strain from which clone 2C originated, the question arises as to whether these T cells can recognize peptide p2Ca in association with a self MHC protein of the H-2b haplotype. Here we show that 2C CTL do indeed recognize peptide p2Ca in association with Kb on the surface of H-2b cells or on transfected cells expressing Kb, but that an approximately 1000-fold higher concentration of this peptide is required to sensitize Kb+ than Ld+ target cells for lysis by 2C cells. However, the peptide's binding to Kb was not much weaker than to Ld, with only an approximately 10-fold difference in the respective equilibrium constants. These results predict that the T cell receptor (TcR) of clone 2C has a much lower intrinsic affinity for p2Ca-Kb complexes than for p2Ca-Ld complexes, and they provide some quantitative limits on the requirements for triggering T cell-mediated autoimmune reactivity.

Kinetics and affinity of reactions between an antigen-specific T cell receptor and peptide-MHC complexes

We show here that the net rate of accumulation of complexes formed by the antigen-specific receptor of T cells (TCR) of a T cell clone with its natural ligand, an octapeptide in association with Ld, a class I protein of the major histocompatibility complex (MHC), approaches the maximal value determined by the affinity of the TCR for this peptide-MHC ligand in 1-2 min, which is well within the lifetime of transient T cell-target cell conjugates. Consistent with this finding, we also found that the widely divergent affinity values (equilibrium constants) of this TCR for six related peptide-MHC complexes correlate well with the extent of specific lysis of target cells bearing various level of these complexes.

A ubiquitous protein is the source of naturally occurring peptides that are recognized by a CD8+ T-cell clone

We previously isolated from mouse spleen an octapeptide (LSPFPFDL) that in association with the class I major histocompatibility complex protein Ld is recognized by the antigen-specific receptor of an alloreactive CD8+ T-cell clone (2C). Guided by an assay dependent upon the same 2C T-cell receptor, we have now isolated from the same source another naturally occurring peptide. The second peptide (VAITRIEQLSPFPFDL) includes the entire octapeptide sequence and preliminary evidence suggests that it may be a natural precursor of the octapeptide. On finding extensive sequence homology between the 16-mer and part of human 2-oxoglutarate dehydrogenase, we determined the cDNA sequence of mouse 2-oxoglutarate dehydrogenase and found that the deduced amino acid sequence matches precisely the two naturally occurring peptides, indicating their origin by cellular processing of this ubiquitous self protein.

Cognate peptide-induced destruction of CD8+ cytotoxic T lymphocytes is due to fratricide

In the absence of other cells, cloned CTL in culture can undergo massive destruction upon the addition of a peptide that is recognized, in association with the CTL's class I MHC proteins, by the CTL's Ag-specific TCR. To determine whether the destruction is a result of the individual CTL's recognition via its own TCR of peptide-MHC-I complexes on its own surface ("suicide"), or to cytolytic attack by some CTL on others in the same culture ("fratricide"), we compared the rate of peptide-induced cell death in conventional cultures, where CTL are free to establish cell-cell contacts, with other cultures in which individual CTL were prevented from forming cell-cell contacts by encasing them individually in agarose gel microdrops. The differences were dramatic: in the presence of high concentrations of peptide (10 millionfold greater than is necessary to support 50% lysis of conventional target cells by these CTL) cell death was linear over 0 to 8 h in conventional cultures, at a rate of about 10% per hour, whereas in the presence of the same high concentration of peptide over the same time course, no death was detected among the cells encased in agarose gel microdroplets. The results demonstrate an absolute requirement for cell-cell contact in the destruction of cloned CTL in culture with their cognate peptides at high concentration. Using an increase of intracellular calcium ion concentration ([Ca2+]i) as a measure of T-cell activation, we also found that peptide-dependent activation of CTL likewise depends upon cell-cell contact.

Cognate peptide-induced destruction of CD8+ cytotoxic T lymphocytes is due to fratricide

In the absence of other cells, cloned CTL in culture can undergo massive destruction upon the addition of a peptide that is recognized, in association with the CTL's class I MHC proteins, by the CTL's Ag-specific TCR. To determine whether the destruction is a result of the individual CTL's recognition via its own TCR of peptide-MHC-I complexes on its own surface ("suicide"), or to cytolytic attack by some CTL on others in the same culture ("fratricide"), we compared the rate of peptide-induced cell death in conventional cultures, where CTL are free to establish cell-cell contacts, with other cultures in which individual CTL were prevented from forming cell-cell contacts by encasing them individually in agarose gel microdrops. The differences were dramatic: in the presence of high concentrations of peptide (10 millionfold greater than is necessary to support 50% lysis of conventional target cells by these CTL) cell death was linear over 0 to 8 h in conventional cultures, at a rate of about 10% per hour, whereas in the presence of the same high concentration of peptide over the same time course, no death was detected among the cells encased in agarose gel microdroplets. The results demonstrate an absolute requirement for cell-cell contact in the destruction of cloned CTL in culture with their cognate peptides at high concentration. Using an increase of intracellular calcium ion concentration ([Ca2+]i) as a measure of T-cell activation, we also found that peptide-dependent activation of CTL likewise depends upon cell-cell contact.

Stoichiometric labeling of peptides by iodination on tyrosyl or histidyl residues

Radioiodination with 125I or 131I is a favored technique for labeling biologically active peptides or proteins because of high specific radioactivities and convenience in counting gamma-emissions. Previous studies used trace labeling, in which fewer than 1% of the molecules are iodinated. We describe procedures for obtaining stoichiometrically iodinated and therefore chemically homogeneous peptides with specific activities exceeding 10(7) cpm/micrograms (approximately 10 Ci/mmol). By analyzing the pH dependence of iodination on tyrosyl and histidyl residues, we show that the method described can be applied to many short peptides and optimized for labeling on tyrosine and/or histidine. The power of reverse-phase HPLC is exploited to resolve multiple products substituted with different molar equivalents of iodine from each other and from unlabeled peptide. Specific radioactivity ratios can be used to identify the products, as confirmed by Edman sequence analysis under conditions that separate iodinated tyrosine and histidine derivatives from all other amino acids. We also show that the biological activities of iodinated and uniodinated peptides can differ by several orders of magnitude in a T cell assay and demonstrate the usefulness of stoichiometric labeling to overcome ambiguities inherent in studying biological activities with trace-labeled peptides.

A naturally occurring peptide recognized by alloreactive CD8+ cytotoxic T lymphocytes in association with a class I MHC protein

The antigenic structures that initiate T cell responses to foreign (allogeneic) cells have long attracted considerable interest. We have purified and sequenced a peptide from mouse spleen that is recognized in association with the class I MHC protein H-2Ld by 2C, an alloreactive CD8+ T cell clone. The peptide (LSP-FPFDL) greatly enhances the susceptibility of Ld+ cells to lysis by 2C, and this activity is completely blocked by a clonotypic antibody against the 2C T cell receptor. Thus, this study characterizes the naturally occurring peptide moiety of an MHC-I/peptide complex recognized by alloreactive CD8+ T cells. The peptide, which occurs in the thymus of MHC-disparate mice, can be used to study T cell development in mice expressing transgenes for the 2C T cell receptor.

Quantitation of reversible binding by particle counting: hapten-antibody interaction as a model system

With a view toward developing a general method for measuring intrinsic equilibrium constants for the reversible interactions between two ligands, we used an antibody-hapten model system [2,4-dinitrophenyl (DNP) hapten and anti-DNP antibody] to explore an approach based on particle counting of uniform polystyrene spheres to which the hapten is coupled covalently. This approach was made possible by an optical pulse particle size analyzer that accurately counts individual sphere clusters and quantitates with high precision specific aggregation of spheres crosslinked by antibody. The reduction in crosslinking that results from competition for antibody binding sites between a soluble DNP ligand and immobilized DNP groups on the spheres provides the basis for measuring the intrinsic equilibrium constant for the soluble ligand-antibody interaction. The binding constants measured in this way for several DNP ligands and an anti-DNP antibody (2A1) agreed with the values obtained by conventional methods. The range of intrinsic equilibrium constants that can be determined by particle counting is likely to be exceptionally wide and a value as low as 10(3) liters/mol has been measured. And since all soluble antigens, regardless of their mass, acquire the same ability to scatter light as a result of their immobilization on the much larger uniform spheres (0.36 microns), the approach described here should be applicable to virtually any molecularly dispersed antigen and its monoclonal antibody.

Effects of cognate peptides on cytolytic and proliferative activities of cloned cytotoxic T lymphocytes

When a cognate peptide is added to a culture of the corresponding clone of CD8+ cytotoxic T lymphocytes (CTLs) having the appropriate major histocompatibility complex (MHC) each cell can serve as both an antigen-presenting target cell and as a responding CTL. Under these circumstances many of the cells die. The extent of cell death is greatly diminished by Ca2+ chelation (by Mg2 EGTA) and by mAbs to CD8 and to LFA-1. Cell death is also blocked by cyclosporin A and FK-506, but not by inhibitors of protein synthesis and gene transcription (cyclohexamide and actinomycin D respectively). In contrast to the stimulatory effect on cytolytic activity, proliferation of recently stimulated CTLs is profoundly inhibited by the cognate peptide, as well as by conventional target cells that are specifically recognized and lysed by the CTLs. The inhibition of proliferation is transient and is followed by enhanced DNA synthesis of surviving CTLs. Cognate peptides also elicit fragmentation of CTL DNA, and this effect is likewise decreased by cyclosporin A and FK-506. Thus the addition of a target, either in the form of a synthetic cognate peptide that associates with MHC proteins on intact CTLs or other cells, or in the form of a conventional target cell, can simultaneously stimulate cytolytic activity and inhibit the proliferative activity of mature CTLs.

An optimal viral peptide recognized by CD8+ T cells binds very tightly to the restricting class I major histocompatibility complex protein on intact cells but not to the purified class I protein

CD8+ cytotoxic T lymphocytes recognize cell surface complexes formed by class I major histocompatibility complex (MHC-I) glycoproteins and antigenic peptides. We have identified a peptide nonamer (termed IV9) derived from the human immunodeficiency virus that is over a millionfold more active (at subpicomolar concentrations) than peptide analogues longer or shorter by one or two amino acid residues. Although IV9 does not detectably bind to isolated MHC-I molecules as measured by equilibrium dialysis, we quantitated its specific binding in unaltered form to MHC-I on intact cells. Less than 1% of cell surface MHC-I forms complexes with IV9, which suffices to trigger maximal cytotoxic T-lymphocyte activity. By contrast, a peptide dodecamer that includes the IV9 sequence and is active at micromolar concentrations does not bind to MHC-I on intact cells, raising the possibility that this longer peptide undergoes processing. Using stoichiometrically iodinated IV9 to obviate the ambiguities associated with trace labeling methods, we measured the dissociation kinetics of purified peptide/MHC-I complexes isolated by affinity chromatography and found these complexes to be exceedingly stable (t1/2 = 200-600 hr).

Cognate peptides induce self-destruction of CD8+ cytolytic T lymphocytes

Cytotoxic T lymphocytes (CTLs) have been shown to be relatively resistant to cytolytic attack by other CTLs. We show here, however, that cloned CTLs, in the absence of other cells, are destroyed by exposure to their cognate peptides (defined as those that in association with major histocompatibility complex proteins are recognized by the antigen-specific receptor of the T cell). Destruction is proportional to peptide concentration and can be prevented by a second peptide that competes with the cognate peptide for presentation by the class I major histocompatibility complex proteins of the CTLs. The speed and extent of peptide-induced changes in the appearance of CTLs suggest that the destruction may be due primarily to self-recognition and self-destruction of individual CTLs (suicide) rather than to the destruction of some CTLs by others of the same clone in the same culture (fratricide). This effect may also take place in vivo because the appropriately timed injection of a cognate peptide into ovalbumin-immunized mice appeared to deplete their spleens of primed anti-ovalbumin CTLs. The results point to a possible physiologic mechanism for postthymic elimination of cytolytic T cells that recognize their own peptides in association with their own major histocompatibility complex protein. The results also raise the possibility that cognate peptides might eventually prove therapeutically useful for eliminating CTL clones that cause pathological cell destruction, as in some autoimmune diseases and some viral infections.

Cytotoxic T lymphocytes recognize a reconstituted class I histocompatibility antigen (HLA-A2) as an allogeneic target molecule

Recent findings suggest that peptide fragments of newly synthesized proteins may associate intracellularly with nascent chains of class I histocompatibility antigens (termed MHC-I proteins because they are encoded by genes of the major histocompatibility complex) and that these peptide adducts may be required for the folding or stability and perhaps even the transport of these proteins to the cell surface. To determine whether these proteins can be reconstituted from their separated subunits into ostensibly native molecules in the absence of added peptides, we denatured a purified human MHC-I protein (HLA-A2) with 4 M NaSCN, separated its heavy (alpha) and light (beta 2-microglobulin) chains by gel filtration, and then mixed them in the presence of a 3-fold molar excess of beta 2-microglobulin and absence of added peptides. The reconstituted protein, recovered in 10% yield, was indistinguishable from native A2 in its reactivity with a monoclonal antibody (BB7.7) and its ability to specifically activate A2-specific CD8+ T cells. Inasmuch as the reconstituted A2 contained no detectable peptide adducts (we estimate less than 1 per 100 on a molar basis, assuming peptides of 2-5 kDa), the results suggest that peptide-free A2 can be recognized by CD8+ T cells and that peptide adducts are not essential for the MHC-I protein to maintain an ostensibly native structure.

Enzymatically activated microencapsulated liposomes can provide pulsatile drug release

A system for the delayed or pulsed release of biologically active substances was achieved by encapsulating liposomes containing the substance of interest inside microcapsules. The microcapsules retain the liposomes but allow controlled diffusion of the active substance when it is released from the liposomes. Furthermore, by coating the liposomes with phospholipase A2 (an enzyme that removes an acyl group from the 2 position of phospholipids) before placing them within the microcapsule, a pulsatile release pattern was achieved both in vitro and in vivo. The time of onset of the pulse as well as the release rate can be controlled by the amount of phospholipase A2, the molecular weight of the poly(L-lysine) that is used to coat the microencapsulated liposomes, and the composition of the phospholipid bilayer membrane. Even at 37 degrees C the system would protect a model enzyme (horseradish peroxidase). When not placed inside the microencapsulated liposomes, the enzyme lost its activity in solution at 37 degrees C in a few days, whereas it retained 40% of the initial activity after 30 days of incubation at 37 degrees C inside the microencapsulated liposomes.