Antigen processing and presentation to T cells

In this article Howard Grey and Robert Chesnut describe recent insights into the mechanism of antigen presentation and discuss the needfor antigen processing in the stimulation of T cells.

Measurement of MHC/peptide interactions by gel filtration or monoclonal antibody capture

This unit describes a technique for the direct and quantitative measurement of the capacity of peptide ligands to bind Class I and Class II MHC molecules. The binding of a peptide of interest to MHC is assessed based on its ability to inhibit the binding of a radiolabeled probe peptide to purified MHC molecules. This unit includes protocols for the purification of Class I and Class II MHC molecules by affinity chromatography, and for the radiolabeling of peptides using the chloramine T method. An alternate protocol describes alterations in the basic protocol that are necessary when performing direct binding assays, which are required for (1) selecting appropriate high-affinity, assay-specific, radiolabeled ligands, and (2) determining the amount of MHC necessary to yield assays with the highest sensitivity. After a predetermined incubation period, dependent upon the allele under examination, the bound and unbound radiolabeled species are separated, and their relative amounts are determined. Three methods for separation are described, two utilizing size-exclusion gel-filtration chromatography and a third using monoclonal antibody capture of MHC. Data analysis for each method is also explained.

Measurement of MHC/peptide interactions by gel filtration

This unit describes a technique for the direct and quantitative measurement of the capacity of peptide ligands to bind Class I and Class II MHC molecules. The binding of a peptide of interest to MHC is assessed based on its ability to inhibit the binding of a radiolabeled probe peptide to MHC molecules. The establishment of an MHC/peptide binding assay, and its subsequent use in determining the MHC binding capacities of peptide ligands, requires sufficient stocks of purified MHC and both labeled and unlabeled peptides. Accordingly, this unit includes protocols for the purification of Class I and Class II MHC molecules by affinity chromatography, and for the radiolabeling of peptides using the chloramine T method. A support protocol describes alterations in the basic protocol that are necessary when performing direct binding assays, which are required for (1) selecting appropriate high-affinity, assay-specific, radiolabeled ligands and (2) determining the amount of MHC necessary to yield assays with the highest sensitivity. After a 2-day incubation, the bound and unbound radiolabeled species are separated, and their relative amounts are determined. Two methods for separation by size-exclusion gel-filtration chromatography are described, as is data analysis.

HLA class I-restricted responses to vaccinia recognize a broad array of proteins mainly involved in virulence and viral gene regulation

We have analyzed by ex vivo ELISPOT the anti-vaccinia cytotoxic T lymphocyte responses of peripheral blood mononuclear cells from humans vaccinated with Dryvax vaccine. More than 6,000 peptides from 258 putative vaccinia ORFs predicted to bind the common molecules of the HLA A1, A2, A3, A24, B7, and B44 supertypes were screened with peripheral blood mononuclear cells of 31 vaccinees. A total of 48 epitopes derived from 35 different vaccinia antigens were identified, some of which (B8R, D1R, D5R, C10L, C19L, C7L, F12, and O1L) were recognized by multiple donors and contain multiple epitopes recognized in the context of different HLA types. The antigens recognized tend to be >100 residues in length and are expressed predominantly in the early phases of infection, although some late antigens were also recognized. Viral genome regulation and virulence factor were recognized most frequently, whereas few structural proteins were immunogenic. Finally, most epitopes were highly conserved among vaccinia virus Western Reserve, variola major and modified vaccinia Ankara, supporting their potential use in vaccine and diagnostic applications.

Activation of naïve CD4 T cells by anti-CD3 reveals an important role for Fyn in Lck-mediated signaling

Although there was no impairment in IL-2 secretion and proliferation of Fyn-deficient naïve CD4 cells after stimulation with antigen and antigen-presenting cells, stimulation of these cells with anti-CD3 and anti-CD28 revealed profound defects. Crosslinking of purified wild-type naïve CD4 cells with anti-CD3 activated Lck and initiated the signaling cascade downstream of Lck, including phosphorylation of ZAP-70, LAT, and PLC-gamma1; calcium flux; and dephosphorylation and nuclear translocation of the nuclear factor of activated T cells (NFAT)p. All of these signaling events were diminished severely in Fyn-deficient naïve cells activated by CD3 crosslinking. Coaggregation of CD3 and CD4 reconstituted this Lck-dependent signaling pathway in Fyn(-/-) T cells. These results suggest that when signaling of naïve T cells is restricted to the T cell antigen receptor, Fyn plays an essential role by positive regulation of Lck activity.

Effector T cells have a lower ligand affinity threshold for activation than naive T cells

It has been previously established that effector and memory T cells are more sensitive to antigen stimulation than naive T cells. In this study, we compared the effect of ligand affinity on the activation of naive and effector T cells derived from pigeon cytochrome c (PCC)-specific TCR transgenic mice by stimulating these cells with a variety of ligands with widely differing antigenicity. The data obtained indicated the following. (i) The differences in antigen dose requirements for activation of naive and effector cells widened as the affinity of the antigen decreased. Most dramatically, peptides that were TCR antagonists for naive T cells were recognized as agonists by effector T cells. (ii) While both naive and effector T cells were activated by the bacterial superantigen staphylococcal enterotoxin A, specific for the transgenic TCR V(beta)3 chain, effector, but not naive, T cells were stimulated to proliferate by toxic shock syndrome toxin-1, a superantigen not previously described to be stimulatory for V(beta)3 T cells. (iii) Effector T cells, but not naive cells, proliferated in response to endogenous self-peptides presented by antigen-presenting cells in a syngeneic mixed lymphocyte reaction. Taken together these data indicate that effector T cells have a lower affinity threshold for activation than naive T cells. Further studies demonstrated that the heightened reactivity of effector T cells to low-affinity ligands declined progressively with repeated stimulations by antigen such that after repeated stimulation effector T cells were no longer stimulated by low-affinity ligands but recognized them as TCR antagonists similar to naive T cells.

Study of the mechanism of TCR antagonism using dual-TCR-expressing T cells

The mechanism of action of TCR antagonists is incompletely understood. T cells expressing two distinct TCRs have been used to test competition for TCR occupancy as a potential mechanism. Previous studies with CD4 T cells showed that an antagonist for one TCR inhibited the response to the other TCR (cross-antagonism), whereas studies with CD8 cells failed to demonstrate cross-antagonism. To determine whether CD4 and CD8 cells were intrinsically different or whether the differences were the result of the use of different effector assays, we studied both CD4 and CD8 dual-TCR-expressing T cells. In the CD4 system, consistent with previous reports, cross-antagonism of proliferation was observed. In the CD8 system, cross-antagonism was observed using proliferation as readout but not when target cell cytolysis was used. These results suggest that different mechanisms may be involved in the inhibition of proliferation and inhibition of cytotoxic effector function, the latter only involving competition for TCR occupancy. Inhibition of proliferation appears to be more complex and other mechanisms such as sequestration of signaling molecules or negative signaling may be involved. The fact that 10- to 20-fold more antagonist was needed to achieve cross-antagonism compared with inhibition of the cognate TCR is consistent with the hypothesis that competition for TCR occupancy is also a major, albeit not sole, mechanism of antagonism of the proliferative responses of CD4 and CD8 cells.

CD28 plays a critical role in the segregation of PKC theta within the immunologic synapse

The signaling pathways that lead to the localization of cellular protein to the area of interaction between T cell and antigen-presenting cell and the mechanism by which these molecules are further sorted to the peripheral supramolecular activation cluster or central supramolecular activation cluster regions of the immunologic synapse are poorly understood. In this study, we investigated the functional involvement of CD28 costimulation in the T cell receptor (TCR)-mediated immunologic synapse formation with respect to protein kinase C (PKC)theta; localization. We showed that CD3 crosslinking alone was sufficient to induce PKC theta; capping in naive CD4(+) T cells. Studies with pharmacologic inhibitors and knockout mice showed that the TCR-derived signaling that drives PKC theta; membrane translocation requires the Src family kinase, Lck, but not Fyn. In addition, a time course study of the persistence of T cell molecules to the immunologic synapse indicated that PKC theta;, unlike TCR, persisted in the synapse for at least 4 h, a time that is sufficient for commitment of a T cell to cell division. Finally, by using TCR-transgenic T cells from either wild-type or CD28-deficient mice, we showed that CD28 expression was required for the formation of the mature immunologic synapse, because antigen stimulation of CD28(-) T cells led to a diffuse pattern of localization of PKC theta; and lymphocyte function-associated antigen-1 in the immunologic synapse, in contrast to the central supramolecular activation cluster localization of PKC theta; in CD28(+) T cells.

T-cell receptor antagonists induce Vav phosphorylation by selective activation of Fyn kinase

T cell receptor (TCR) antagonists inhibit antigen-induced T cell activation and by themselves fail to induce phenotypic changes associated with T cell activation. However, we have recently shown that TCR antagonists are inducers of antigen-presenting cell (APC)-T cell conjugates. The signaling pathway associated with this cytoskeleton-dependent event appears to involve tyrosine phosphorylation and activation of Vav. In this study, we investigated the role played by the protein tyrosine kinases Fyn, Lck, and ZAP-70 in antagonist-induced signaling pathway. Antagonist stimulation increased tyrosine phosphorylation and kinase activity of Fyn severalfold, whereas little or no increase in Lck and ZAP-70 activity was observed. Second, TCR stimulation of Lck(-), Fyn(hi) Jurkat cells induced strong tyrosine phosphorylation of Vav. In contrast, minimal increase in tyrosine phosphorylation of Vav was observed in Lck(hi), Fyn(lo) Jurkat cells. Finally, study of T cells from a Fyn-deficient TCR transgenic mouse also showed that Fyn was required for tyrosine phosphorylation and activation of Vav induced by both antagonist and agonist peptides. The deficiency in Vav phosphorylation in Fyn-deficient T cells was associated with a defect in the formation of APC-T cell conjugates when T cells were stimulated with either agonist or antagonist peptide. We conclude from these results that Vav is a selective substrate for Fyn, especially under conditions of low-affinity TCR-mediated signaling, and that this signaling pathway involving Fyn, Vav, and Rac-1 is required for the cytoskeletal reorganization that leads to T cell-APC conjugates and the formation of the immunologic synapse.

Epitope affinity for MHC class I determines helper requirement for CTL priming

We show here that priming and memory generation of antigen-specific CD8+ cytotoxic T lymphocytes (CTL) does not require help if the immunogen binds major histocompatibility complex (MHC) class I molecules with high affinity. This conclusion was based on the study of three chemically distinct optimal length CTL epitopes with high affinity for the restriction element Kb. In contrast, when two subdominant epitopes with intermediate MHC binding affinity were studied, either a class II MHC-restricted T helper cell epitope or administration of antibody to CD40 was required to obtain significant CTL priming. Depending on the epitope, one source of help was much more efficient than the other.

TCR antagonist peptides induce formation of APC-T cell conjugates and activate a Rac signaling pathway

T cell receptor antagonists inhibit T cell activation by antigen, and by themselves fail to induce phenotypic changes associated with T cell activation. However, they can induce limited tyrosine phosphorylation of TCRzeta chain. Here we show that TCR antagonists are potent inducers of APC-T cell conjugates, cytoskeletal reorganization, and capping of certain T cell proteins. These events are associated with a signaling pathway involving tyrosine phosphorylation of Vav and SLP-76, activation and capping of Rac-1, a protein previously linked with cytoskeletal reorganization, and activation of JNK. The finding that antagonist peptides stimulate this pathway, while failing to stimulate other TCR-mediated signaling pathways, indicates the presence in T cells of a hierarchy of signaling that is sensitive to the avidity of Ag / MHC-TCR interaction.

The stimulation of low-affinity, nontolerized clones by heteroclitic antigen analogues causes the breaking of tolerance established to an immunodominant T cell epitope

H-2K mice injected, intravenously in saline or intraperitoneally in incomplete Freund's adjuvant, with large quantities of the immunodominant I-E(k)-restricted epitope from moth cytochrome c (MCC) 88-103 fail to respond to subsequent immunization with this epitope when administered in complete Freund's adjuvant. This state of tolerance can be broken by immunization with certain MCC 88-103 analogues that are heteroclitic antigens as assessed on representative MCC 88-103 specific T cell clones. In this paper, the mechanism of breaking tolerance by heteroclitic antigens was investigated. The following observations were made: (a) T cell hybridomas derived from tolerance-broken animals required higher concentrations of MCC 88-103 to be stimulated than hybridomas derived from normal immune animals, suggesting that they have T cell receptors (TCRs) of lower affinity; (b) in contrast to normal immune animals whose MCC-specific TCRs are typically Vbeta3(+)/Valpha11(+), none of the hybridomas derived from tolerance-broken animals expressed Vbeta3, although they were all Valpha11(+). Also, the Vbeta complementarity determining region 3 (CDR3) regions from the tolerance-broken animals did not contain the canonical structure and length characteristics of the normal MCC 88-103 immune repertoire; and (c) adoptive transfer and tolerization of MCC-specific Vbeta3(+)/Valpha11(+) transgenic T cells followed by immunization with heteroclitic antigen failed to terminate the state of tolerance. Collectively, these data strongly suggest that the mechanism involved in breaking tolerance in this system is the stimulation of nontolerized, low-affinity clones, rather than reversal of anergy. Further support for this mechanism was the finding that after activation, T cells apparently have a lowered threshold with respect to the affinity of interaction with antigen required for stimulation.

Target cells for an immunosuppressive cytokine, glycosylation-inhibiting factor

Receptors for bioactive glycosylation-inhibiting factor (GIF) were demonstrated using a bioactive mutant of recombinant human (rh) GIF, which is comparable to the suppressor T (Ts) cell-derived bioactive GIF in its affinity for the receptors on helper T (Th) hybridoma cells. Both naive T and B cells in normal mouse spleen lacked GIF receptors. However, presentation of specific antigen to naive T cells resulted in the expression of the receptors on activated T cells. Furthermore, activation of small resting B cells with F(ab')2 fragments of anti-mouse IgM plus IL-4, lipopolysaccharide (LPS) plus IL-4 or LPS plus dextran sulfate induced the expression of the receptors within 48 h of B cell stimulation. It was also found that NK T cells freshly isolated from mouse spleen, but not conventional NK cells, expressed receptors for GIF. CD4(+) and CD4(-) subpopulations of NK T cells showed a similar binding capability. Mature dendritic cells derived from bone marrow did not bear the receptors. The dissociation constant (Kd) of the interaction between the bioactive rhGIF mutant and the high-affinity receptors was 10-100 pM, whereas inactive wild-type rhGIF failed to bind to the receptors. A bioactive derivative of rhGIF suppressed both IgG1 and IgE synthesis by purified B cells activated by LPS and IL-4, indicating that the binding of bioactive GIF to its receptors on activated B cells results in suppression of their differentiation.

Fine Specificity and MHC Restriction of Trinitrophenyl- Specific CTL

In this study, the fine specificity and MHC restriction of a CTL response specific to the trinitrophenyl (TNP) hapten was analyzed. Based on the structure of peptide/Kb complexes and ternary TCR/Ag/MHC complexes, four TNP peptides, two octamers, and two nonamers were chosen for eliciting anti-TNP CTL responses. Hapten was conjugated at position 4 in the octamers and at position 5 in the nonamers, positions which should allow engagement of the hapten by TCRs. Potent CTL activity for each of the TNP peptides was obtained that was highly hapten-specific; however, there were considerable differences in the extent of cross-reactivity with other TNP peptides, with the octamers generating more cross-reactive CTL than the nonamers. MHC restriction analysis suggested that anti-hapten responses were less dependent on MHC recognition than anti-peptide responses. This was evidenced by the relative ease of detecting cross-reactivity to haptenated peptides presented by allo-MHC and by the relative insensitivity of anti-hapten vs anti-peptide CTL to mutations in the Kb molecule at potential TCR interaction sites. One potential explanation for this insensitivity to MHC mutation was the finding that the anti-hapten response appeared to be of higher avidity, since a >100-fold difference in the amount of Ag required to sensitize target cells was found between these two types of Ags.

Fine specificity and MHC restriction of trinitrophenyl-specific CTL

In this study, the fine specificity and MHC restriction of a CTL response specific to the trinitrophenyl (TNP) hapten was analyzed. Based on the structure of peptide/Kb complexes and ternary TCR/Ag/MHC complexes, four TNP peptides, two octamers, and two nonamers were chosen for eliciting anti-TNP CTL responses. Hapten was conjugated at position 4 in the octamers and at position 5 in the nonamers, positions which should allow engagement of the hapten by TCRs. Potent CTL activity for each of the TNP peptides was obtained that was highly hapten-specific; however, there were considerable differences in the extent of cross-reactivity with other TNP peptides, with the octamers generating more cross-reactive CTL than the nonamers. MHC restriction analysis suggested that anti-hapten responses were less dependent on MHC recognition than anti-peptide responses. This was evidenced by the relative ease of detecting cross-reactivity to haptenated peptides presented by allo-MHC and by the relative insensitivity of anti-hapten vs anti-peptide CTL to mutations in the Kb molecule at potential TCR interaction sites. One potential explanation for this insensitivity to MHC mutation was the finding that the anti-hapten response appeared to be of higher avidity, since a > 100-fold difference in the amount of Ag required to sensitize target cells was found between these two types of Ags.

Termination of Peripheral Tolerance to a T Cell Epitope by Heteroclitic Antigen Analogues

Treating mice with an immunodominant T cell epitope from moth cytochrome c (MCC88–103) can induce T cell unresponsiveness under certain conditions of administration. In this report, we determined whether T cell tolerance to MCC88–103 in adult animals can be overcome by immunization with cross-reactive analogues of the tolerizing Ag. A panel of analogues of the tolerogen were tested for their capacity to terminate the tolerant state following in vivo immunization. As analyzed by their stimulatory capacity for a representative MCC88–103-specific T cell clone, this panel covered a wide range of cross-reactivity, including nonantigenic, antagonistic, weakly, and strongly antigenic peptides. Interestingly, only heteroclitic analogues, as measured in vitro by their enhanced antigenicity for the T cell clone that was specific for MCC88–103, were capable of breaking tolerance. Thus, an immune response to the cross-reactive, heteroclitic analogues of tolerized self Ags may represent a mechanism by which Ag molecular mimicry operates.

Termination of peripheral tolerance to a T cell epitope by heteroclitic antigen analogues

Treating mice with an immunodominant T cell epitope from moth cytochrome c (MCC(88-103)) can induce T cell unresponsiveness under certain conditions of administration. In this report, we determined whether T cell tolerance to MCC(88-103) in adult animals can be overcome by immunization with cross-reactive analogues of the tolerizing Ag. A panel of analogues of the tolerogen were tested for their capacity to terminate the tolerant state following in vivo immunization. As analyzed by their stimulatory capacity for a representative MCC(88-103)-specific T cell clone, this panel covered a wide range of cross-reactivity, including nonantigenic, antagonistic, weakly, and strongly antigenic peptides. Interestingly, only heteroclitic analogues, as measured in vitro by their enhanced antigenicity for the T cell clone that was specific for MCC(88-103), were capable of breaking tolerance. Thus, an immune response to the cross-reactive, heteroclitic analogues of tolerized self Ags may represent a mechanism by which Ag molecular mimicry operates.

Pools of lipidated HTL-CTL constructs prime for multiple HBV and HCV CTL epitope responses

Various peptide-based approaches to simultaneous induction of multiple cytotoxic T lymphocyte (CTL) responses were evaluated as part of ongoing efforts to develop immunotherapeutic vaccines for use in humans. To this end, HLA (human histocompatibility leukocyte antigen)-A2-restricted epitopes from several specific viral proteins were tested in an HLA-A2 transgenic mouse model system, which mimics human CTL responses to these viral proteins. Multiple CTL responses were elicited by immunization with either peptides emulsified in incomplete Freund's adjuvant (IFA), or lipidated peptides administered in phosphate buffered saline (PBS). In the case of lipidated peptides, induction of CTL responses was crucially dependent on the presence of helper T lymphocyte (HTL) epitopes, and most efficient in the case of lipidated covalently linked HTL-CTL epitope constructs. CTL could also be induced by immunization with lipidated HTL epitopes simply mixed with CTL epitopes and formulated in PBS. However, this approach was highly dependent on the particular lipidated HTL/CTL combination utilized, and was marginally effective for simultaneous priming of multiple CTL responses. By contrast, all HTL/CTL combinations were potent immunogens when delivered as lipidated, covalently linked molecules. This was the most effective of the approaches analysed in terms of multi-epitope priming, as demonstrated by the induction of simultaneous CTL responses to a pool of five different epitopes.

Modulation of Naive CD4 T Cell Activation with Altered Peptide Ligands: The Nature of the Peptide and Presentation in the Context of Costimulation Are Critical for a Sustained Response

Altered peptide ligands containing single amino acid substitutions have the potential to be used for modulating immune function. Using a panel of moth cytochrome c peptides, we demonstrate that different phases of naive CD4 T cell response are alternately modulated depending on altered peptide ligand dose and accessory molecule expression by APC. Weak agonists presented at high concentration, and with costimulation, efficiently induced early phase naive T cell activation as assessed by IL-2R/CD69 expression, but could only promote sufficient IL-2 for a short-lived proliferative response. In contrast, strong agonists and heteroclitic peptides induced early phase T cell activation even at low concentrations with costimulation, and allowed sustained IL-2 secretion and proliferation. In the absence of accessory molecule help, early and late phase activation was impaired with weak agonists, whereas strong agonists partially compensated for a lack of costimulation for early phase activation, and also promoted enhanced IL-2 with sustained proliferation. These studies support the hypothesis that the naive T cell response will be determined by the balance between provision of accessory molecule help and the affinity of peptide/MHC complexes for individual TCRs, and suggest that extended IL-2 production is the main facet of naive CD4 activation that is affected by altering the nature of the peptide.

Modulation of naive CD4 T cell activation with altered peptide ligands: the nature of the peptide and presentation in the context of costimulation are critical for a sustained response

Altered peptide ligands containing single amino acid substitutions have the potential to be used for modulating immune function. Using a panel of moth cytochrome c peptides, we demonstrate that different phases of naive CD4 T cell response are alternately modulated depending on altered peptide ligand dose and accessory molecule expression by APC. Weak agonists presented at high concentration, and with costimulation, efficiently induced early phase naive T cell activation as assessed by IL-2R/CD69 expression, but could only promote sufficient IL-2 for a short-lived proliferative response. In contrast, strong agonists and heteroclitic peptides induced early phase T cell activation even at low concentrations with costimulation, and allowed sustained IL-2 secretion and proliferation. In the absence of accessory molecule help, early and late phase activation was impaired with weak agonists, whereas strong agonists partially compensated for a lack of costimulation for early phase activation, and also promoted enhanced IL-2 with sustained proliferation. These studies support the hypothesis that the naive T cell response will be determined by the balance between provision of accessory molecule help and the affinity of peptide/MHC complexes for individual TCRs, and suggest that extended IL-2 production is the main facet of naive CD4 activation that is affected by altering the nature of the peptide.

Several Common HLA-DR Types Share Largely Overlapping Peptide Binding Repertoires

The peptide binding specificities of HLA-DRB1*0401, DRB1*0101, and DRB1*0701 have been analyzed by the use of large collections of synthetic peptides corresponding to naturally occurring sequences. The results demonstrated that nearly all peptides binding to these DR molecules bear a motif characterized by a large aromatic or hydrophobic residue in position 1 (Y, F, W, L, I, V, M) and a small, noncharged residue in position 6 (S, T, C, A, P, V, I, L, M). In addition, allele-specific secondary effects and secondary anchors were defined, and these parameters were utilized to derive allele-specific motifs and algorithms. By the combined use of such algorithms, peptides capable of degenerate DRB1*0101, DRB1*0401, and DRB1*0701 binding were identified. Additional experiments utilizing a panel of quantitative assays specific for nine additional common DR molecules identified a large set of DR molecules, which includes at least the DRB1*0101, DRB1*0401, DRB1*0701, DRB5*0101, DRB1*1501, DRB1*0901, and DRB1*1302 allelic products, characterized by overlapping peptide-binding repertoires. These results have implications for understanding the molecular interactions involved in peptide-DR binding, as well as the genetic and structural basis of MHC polymorphism. These results also have potential practical implications for the development of epitope-based prophylactic and therapeutic vaccines.

Several common HLA-DR types share largely overlapping peptide binding repertoires

The peptide binding specificities of HLA-DRB1*0401, DRB1*0101, and DRB1*0701 have been analyzed by the use of large collections of synthetic peptides corresponding to naturally occurring sequences. The results demonstrated that nearly all peptides binding to these DR molecules bear a motif characterized by a large aromatic or hydrophobic residue in position 1 (Y, F, W, L, I, V, M) and a small, noncharged residue in position 6 (S, T, C, A, P, V, I, L, M). In addition, allele-specific secondary effects and secondary anchors were defined, and these parameters were utilized to derive allele-specific motifs and algorithms. By the combined use of such algorithms, peptides capable of degenerate DRB1*0101, DRB1*0401, and DRB1*0701 binding were identified. Additional experiments utilizing a panel of quantitative assays specific for nine additional common DR molecules identified a large set of DR molecules, which includes at least the DRB1*0101, DRB1*0401, DRB1*0701, DRB5*0101, DRB1*1501, DRB1*0901, and DRB1*1302 allelic products, characterized by overlapping peptide-binding repertoires. These results have implications for understanding the molecular interactions involved in peptide-DR binding, as well as the genetic and structural basis of MHC polymorphism. These results also have potential practical implications for the development of epitope-based prophylactic and therapeutic vaccines.

The role of peptides in thymic positive selection of class II major histocompatibility complex-restricted T cells

A thymic epithelial cell line transfected with I-Ek was used in reaggregate cultures to study the role of peptides in positive selection of T cell receptor transgenic thymocytes. In this system, positive selection of CD4 SP cells occurred only after the addition of exogenous peptide. Analysis of antigen analogs indicated an inverse relationship between the antigenicity for peripheral T cells and the concentration of peptide required for positive selection. These data are most consistent with an avidity (rather than an affinity) model of positive selection, in which ligand density and the affinity of T cell receptor act in concert to determine the fate of developing thymocytes.

The minimal number of antigen-major histocompatibility complex class II complexes required for activation of naive and primed T cells

Although previous studies have shown that 50-200 antigen-major histocompatibility complex complexes (Ag-MHC) are sufficient to stimulate significant secretion of interleukin (IL)-2 from MHC class II-restricted T cell hybridomas, there have been no studies of this nature on more physiologically relevant T cell populations. In this study we have analyzed the ligand requirements for stimulation of responses from naive and previously primed T cells derived from T cell receptor (TCR)-transgenic animals whose TCR is specific for the pigeon cytochrome c (PCC) 88-104 peptide presented by I-Ek. Primed T cells were as sensitive as the previously reported T cell hybridomas, requiring about 100 Ag-MHC complexes to synthesize readily detectable quantities of IL-2, whereas naive T cells required 15 times more ligand to produce equivalent quantities of IL-2. Similarly, primed T cells required about 40 Ag-MHC complexes to produce a significant proliferative response, whereas naive T cells required about 400 complexes. In contrast to these results, naive and primed T cells showed similar ligand requirements when early events in the T cell activation pathway were analyzed; i.e. TCR down-modulation, CD69 and CD25 expression, and blast transformation. A further analysis of IL-2 and IL-2R expression indicated: 1) The first synthesis of IL-2 was detected at the same ligand concentration in both primed and naive T cells, but primed T cells made much more IL-2 as the ligand concentrations increased; 2) primed T cells expressed about fivefold more IL-2 receptor (R) than naive T cells, despite the fact that the antigen dose-response curves with respect to the percentage of cells expressing IL-2R were identical. These results suggest that naive and primed T cells have the same threshold with respect to the number of Ag-MHC complexes required to initiate T cell activation, but that due to the inefficient expression of IL-2 and IL-2R, engagement of more complexes is needed to enable naive T cells to synthesize the necessary amounts of these two molecules to allow T cells to go through a complete cycle of replication.

Derivation of HLA-A11/Kb transgenic mice: functional CTL repertoire and recognition of human A11-restricted CTL epitopes

Transgenic mice expressing chimeric human (alpha1 and alpha2 HLA-A11 domains) and murine (alpha3, transmembrane, and cytoplasmic H-2Kb domains) class I molecules were derived. These mice were used as a model system to study the immunogenicity of human CTL epitopes and also to examine the aspects of Ag processing differences of mice vs man. Immunization of these mice with seven known HLA-A11-restricted CTL epitopes emulsified in IFA resulted in vigorous specific CTL responses. A larger panel of 45 A11-binding peptides was used to examine the relationship between immunogenicity in the HLA-A11/Kb transgenic mice and HLA-A11 binding capacity. Twenty-one of 28 (75%) peptides with high binding affinities (50% inhibitory concentration (IC50), 2-50 nM) and 7 of 13 (54%) intermediate binding peptides (IC50, 50-500 nM range) were immunogenic. In parallel, 19 of these peptides were used for in vitro primary immunizations of PBMC derived from HLA-A11 healthy human donors. It was found that 8 of 8 peptides that were able to elicit CTL in primary human in vitro cultures were also immunogenic in HLA-A11/Kb mice. Finally, HLA-A11/Kb transgenic mice were found to generate an A11/Kb restricted CTL response following immunization with influenza virus A/PR/8/34, suggesting that, at least to some extent, A11 epitopes are generated by transgenic mice as a result of natural in vivo processing and presentation.