T cell response to Hu-D peptides in patients with anti-Hu syndrome.

The anti-Hu syndrome is the most common paraneoplastic neurologic syndrome but the exact mechanism of immune mediated neuronal injury remains unknown. Anti-Hu antibodies do not appear to play a pivotal role in the pathogenesis of the disease. To assess cell-mediated immunity, we selected 51 peptides from the Hu-D sequence and tested their ability to bind to six common HLA class I molecules. Stable complexes with purified HLA molecules were obtained with 19/51 (37%) selected peptides. Subsequently, the ability of the 19 HLA-binding peptides to stimulate T cells from 10 patients and 10 control subjects was evaluated by detecting IFN-gamma secretion. An anti-peptide T-cell response was observed in 7/10 Hu-positive patients but also in 3/10 control subjects. Overall, a significant T-cell activation occurred in response to 74% (14 out of 19) of the selected peptides in the Hu-positive patients vs. 16% (3 out of 19) in the control group (p < 0.001). In addition, T cells of patients tested within 3 months of the onset of anti-Hu syndrome responded to 82% (14 out of 17) of assessed Hu-D peptides vs. 37% (7 out of 19) in patients tested 1 year or more after developing the syndrome (p < 0.01). Thus, the present study suggests a role of cellular immunity during the course of anti-Hu syndrome.

Study of antigen-processing steps reveals preferences explaining differential biological outcomes of two HLA-A2-restricted immunodominant epitopes from human immunodeficiency virus type 1.

Cytotoxic T-lymphocyte (CTL) responses directed to different human immunodeficiency virus (HIV) epitopes vary in their protective efficacy. In particular, HIV-infected cells are much more sensitive to lysis by anti-Gag/p17(77-85)/HLA-A2 than to that by anti-polymerase/RT(476-484)/HLA-A2 CTL, because of a higher density of p17(77-85) complexes. This report describes multiple processing steps favoring the generation of p17(77-85) complexes: (i) the exact COOH-terminal cleavage of epitopes by cellular proteases occurred faster and more frequently for p17(77-85) than for RT(476-484), and (ii) the binding efficiency of the transporter associated with antigen processing was greater for p17(77-85) precursors than for the RT(476-484) epitope. Surprisingly, these peptides, which differed markedly in their antigenicity, displayed qualitatively and quantitatively similar immunogenicity, suggesting differences in the mechanisms governing these phenomena. Here, we discuss the mechanisms responsible for such differences.

Identification of p53 peptides recognized by CD8(+) T lymphocytes from patients with bladder cancer.

In many types of cancer, p53 frequently accumulates in tumor cells and anti-p53 antibodies can be detected. However, only four CD8(+) T-cell epitopes from p53 have been identified in humans so far. To further analyze the development of a T-cell response against p53, peptides having binding motifs specific for HLA-A1, -A2, -A3, -A24, -B7, -B35, -B44, and -B51 molecules have been defined. The HLA-binding capacity of those peptides was tested, and the stability of formed complexes was defined. Thirteen peptides that bound to HLA-A24 and -B44 molecules are presented. The positive peptides were then used to detect the anti-p53 response of CD8(+) T lymphocytes from patients with bladder cancer. Six peptides, presented by HLA-A2, -B51, or -A24, were able to stimulate T cells from two patients (among 16) with tumor cells that strongly accumulated p53. On the contrary, p53 peptides systematically failed to stimulate T cells from healthy donors or patients with low or undetectable levels of p53 in their tumor cells. These results have led to the identification of four new potential T CD8(+) epitopes from p53: 194-203 associating with HLA-B51 and 204-212, 211-218, and 235-243 associating with HLA-A24.

Characteristics of HIV-1 Nef regions containing multiple CD8+ T cell epitopes: wealth of HLA-binding motifs and sensitivity to proteasome degradation.

First and foremost among the many factors that influence epitope presentation are the degradation of Ag, which results in peptide liberation, and the presence of HLA class I molecules able to present the peptides to T lymphocytes. To define the regions of HIV-1 Nef that can provide multiple T cell epitopes, we analyzed the Nef sequence and determined that there are 73 peptides containing 81 HLA-binding motifs. We tested the binding of these peptides to six common HLA molecules (HLA-A2, -A3, -A24, -B7, -B8, and -B35), and we showed that most of them were efficient binders (54% of motifs), especially peptides associating with HLA-A3, -B7/35, and -B8 molecules. Nef peptides most frequently recognized by T cells of HIV-1-infected individuals were 90-97, 135-143, 71-81, 77-85, 90-100, 73-82, and 128-137. The frequency of T cell recognition was not directly related to the strength of peptide-HLA binding. The generation of Nef epitopes is crucial; therefore, we investigated the digestion by the 20S proteasome of a large peptide, Nef(66-100). This fragment was efficiently cleaved, and NH(2)-terminally extended precursors of epitope 71-81 were recognized by T cells of an HIV-1-infected individual. These results suggest that a high frequency of T cell recognition may depend on proteasome cleavage.

Synthesis and antigenic properties of reduced peptide bond analogues of an immunodominant epitope of the melanoma MART-1 protein.

Backbone modifications have been introduced into the melanoma derived peptide MART-1(27-35) to increase its binding to class I major histocompatibility complex HLA-A2 molecule, and ultimately to enhance its immunogenicity. Each analogue was obtained by replacing one peptide bond at a time in the natural epitope by the aminomethylene (CH2-NH) surrogate. All analogues displayed an increased resistance to proteolysis. Interestingly, the comparative results showed that five analogues bound more efficiently to HLA-A2 than the parent peptide. On the other hand, two pseudopeptide/HLA-A2 complexes were recognized by one melanoma-specific T cell clone. Close examination of the impact of such modifications at the molecular level provides useful supports for the rational design of stable compounds with applications in anti-tumour specific immunotherapy and in vaccine development.

Melanoma peptide MART-1(27-35) analogues with enhanced binding capacity to the human class I histocompatibility molecule HLA-A2 by introduction of a beta-amino acid residue: implications for recognition by tumor-infiltrating lymphocytes.

The design of heteroclytic antigens with high MHC binding capacity is of particular interest to overcome the weak immunogenicity of peptide epitopes derived from tissue antigens expressed by tumors. In the present study, double-substituted peptide analogues of the tumor-associated antigen MART-1(27-35) incorporating a substitution at a primary anchor residue and a beta-amino acid residue at different positions in the sequence were synthesized and evaluated for binding to the human histocompatibility class I molecule HLA-A2 and for recognition by tumor-infiltrating lymphocytes. Interestingly, by combining a Leu for Ala substitution at P2 (which alone is deleterious for antigenic activity) with a beta-amino acid substitution at a putative TCR contact residue, recognition by tumor-infiltrating lymphocytes was partially restored. The analogue [Leu(28),beta-HIle(30)]MART-1(27-35) displays both a higher affinity to HLA-A2 and a more prolonged complex stability compared to [Leu(28)]MART-1(27-35). Overall, these results suggest that double-substitution strategies and beta-amino acid replacements at putative TCR contact residues might prove useful for the design of epitope mimics with high MHC binding capacity.

Identification in humans of HPV-16 E6 and E7 protein epitopes recognized by cytolytic T lymphocytes in association with HLA-B18 and determination of the HLA-B18-specific binding motif.

Human papilloma virus type 16 (HPV-16) is the HPV most frequently associated with cervical carcinoma in humans. For the prevention or treatment of cervical carcinoma, the E6 and E7 oncoproteins appear to be good targets for vaccine-induced cytotoxic T lymphocytes (CTL). Lipopeptide vaccination is an efficient way of stimulating cellular responses. However, to synthesize effective lipopeptides, it is necessary to define which epitopes are immunogenic. In this study we first determined that peptide 80 - 88 of the E6 protein was recognized by CTL from a healthy donor in association with the HLA-B18 molecule. We then defined the HLA-B18 anchoring peptide motif by testing the binding of various short peptides with the HLA-B18 molecule and showed that it was related to the HLA-A1-specific peptide motif. Furthermore, in analyzing the potential E7 epitopes susceptible to associating with HLA-B18, we demonstrated that peptide E7 44 - 52 gave the strongest binding. It could also be recognized by CTL from peripheral blood mononuclear cells (PBMC) of the same healthy donor. Finally, with PBMC from a patient with a cervical intraepithelial neoplasia grade 3, we found CTL which recognized the E6 80 - 88 epitope. We have hence identified two peptides encoded by the E6 and E7 proteins which are presented by the HLA-B18 molecule and could be included in a vaccine against HPV-16.

Antileukemic HLA-restricted T-cell clones generated with naturally processed peptides eluted from acute myeloblastic leukemia blasts.

Recent studies have shown that transfusions of HLA-compatible donor lymphocytes may induce complete remission in marrow-grafted patients with relapses of acute myeloblastic leukemia (AML). We investigated the in vitro generation of antileukemia T-cell clones obtained from the peripheral blood mononuclear cells of a partially HLA-compatible donor (HLA-A2 and B7 molecules in common with the leukemic blasts) after stimulation with a pool of naturally processed peptides extracted from leukemic blast cells collected at diagnosis from a patient with hyperleucocytosis AML. We recovered a significant quantity of peptides that bound to the HLA-A2 or HLA-B7 molecules that were able to induce cytolytic T-lymphocyte (CTL) lines and clones specific for the eluted AML peptides and restricted to the HLA-A2 or B7 molecules. Such CTL line did not recognize the patient's nonleukemic cells, and one clone was able to interact with the leukemic blasts from which the naturally processed peptides had been eluted. Such T-cell clones might provide a rationale for the development of adoptive immunotherapy and could be used to improve the efficiency of HLA-compatible T-lymphocyte transfusions and the graft-versus-leukemia response in patients with AML.

Differential presentation of endogenously processed cytotoxic T lymphocyte epitopes by mouse hepatocarcinoma cell lines induced by SV40 large T antigen.

Tumor cells can have different morphologic or metabolic phenotypes and display genetic instability. Thus they could also vary in their ability to present epitopes to the immune system. We have analyzed the presentation of H-2 Kb- and Db-restricted cytotoxic T lymphocyte (CTL) epitopes of a tumor-associated antigen by three cell lines derived from hepatocarcinomas developed in vivo by mice transgenic for SV40 T targeted to the liver. SV40 T is the obvious tumor-specific antigen and epitopes derived from this antigen were therefore studied. The study included four already known epitopes that can be presented by SV40-transformed kidney cells and two new CTL epitopes that were identified in the present work. CTL lines specific for each epitope were obtained from C57BL/6 mice and were used to map the presentation of SV40 T peptides by the hepatocarcinoma cells. These tumor cells were derived from the same tissue, induced by the same agent and all naturally presented peptide p232-240 from p53. Despite these common features, they all had different patterns of spontaneous presentation of SV40 T CTL epitopes. The mechanisms underlying this disparity are discussed, together with the possible consequences for establishing immunotherapeutic strategies.

Generation of Melan-A/MART-1-specific CD8+ cytotoxic T lymphocytes from human naive precursors: helper effect requirement for efficient primary cytotoxic T lymphocyte induction in vitro.

This study investigates the generation of primary melanoma cell-specific cytotoxic T lymphocytes (CTLs) in vitro. Induction of peptide-specific CTLs from unfractionated naive peripheral blood mononuclear cells from HLA-A2 healthy donors was assessed using 2 recently described 9-mer epitopes from the melanoma tumor antigen Melan-A/MART-1. The need for help from CD4+ T lymphocytes for the long-lasting induction of CTLs and the capacity of the peptide-induced CTL lines to recognize many melanoma cells were evaluated. CTL lines were obtained reproducibly when CD4+ T-lymphocyte help was provided during the primary stimulation either in an autologous way, in the case of tetanus toxoid antigen (TT) responder donors, or with allogeneic TT-activated T-helper cells, separated by an insert well, in the case of tetanus toxoid non-responder donors. We also investigated helper T-cell-derived factors that are produced by TT-activated lymphocytes. Our results strongly suggest that a complex network of cytokines like interleukin-2 (IL-2), interferon-gamma, IL-6 and IL-1 exerts stimulatory effects for the initiation process of CTLs. In contrast, cytokine-like IL-4 might inhibit generation of cytolytic activity if provided by TT-activated T cells at early stages of induction. Our approach can be used to generate CTLs of a desired specificity for clinical use in adoptive immunotherapy protocols.

Peptides derived from the whole sequence of BCR-ABL bind to several class I molecules allowing specific induction of human cytotoxic T lymphocytes.

Chronic myeloid leukemia (CML) is characterized cytogenetically by a t(9;22) translocation which generates a hybrid bcr-abl gene, encoding a p210(bcr-abl) fusion protein. The induction in vitro of leukemia-specific T cells reactive with p210(bcr-abl) is a strategy developed for an immunological therapeutic approach in CML. Peptides from the junction region of this chimeric protein have been considered as potential targets for a cytotoxic response against leukemic cells. However, only a few peptides encompassing the two p210(bcr-abl) breakpoints have been shown to bind to the most common HLA class I molecules, which limits the number of patients who could benefit from this approach. We assume that the presence of chimeric BCR-ABL protein in leukemic cells may affect processing and delivery of peptides, possibly giving rise to new epitopes at the cell surface. We selected 162 peptides from the whole sequence of this protein, including 14 peptides of the b2a2 and b3a2 junctions, which had an anchor motif for a common HLA class I molecule. We tested their ability to bind to eight HLA class I molecules (HLA-A1, -A2, -A3, -A11, -B7, -B8, -B27, -B44). We identified 48 peptides from outside the junction region, with intermediate or strong binding capacities to these HLA class I molecules contrasting with only six junction peptides with a moderate binding capacity to HLA-A3/A11, -B8, or -B44 molecules. Moreover, cytotoxic T lymphocyte lines specific for various peptides outside the junction were generated from peripheral blood mononuclear cells of HLA-A2 or -B7 healthy donors and from one CML patient. These results contribute to evaluation of immunity to the BCR-ABL chimeric protein. Further studies are required to investigate whether such epitopes are correctly processed and presented by leukemic cells.

Differential binding to frequent HLA-A alleles of p21 RAS derived peptides bearing oncogenic substitutions at position 12 or 13.

RAS oncogenic proteins are frequently found mutated in human cancers, where they are known to be implicated in the tumoral process. Mutations occur preferentially at positions 12, 13 or 61. Identification of potential T cell epitopes is the first step to determine it RAS mutated proteins can generate tumor specific antigens which could be further used as targets for cancer immunotherapy protocols. We have investigated the capacity of synthetic wild-type and mutant RAS derived peptides encompassing positions 12 and 13 to bind to three frequent HLA-A alleles: HLA-A*0201, HLA-A*0301 and HLA-A*1101. Binding was evaluated by two methods using TAP-defective cell lines: a cytometric assay based on HLA molecules stabilization at the cell surface, and an assembly assay detecting interactions between solubilized HLA molecules and peptides. Positive HLA binding was observed for two sets of synthetic peptides, one specific for HLA-A*0201 allele (RAS 5-14), and the other one specific for HLA-A*0301 and HLA-A*1101 alleles (RAS 8-16). Interestingly, the different substitutions at positions 12 and 13 were not equivalent for HLA binding. These observations will be useful for the in vitro generation of restricted CD8+ T lymphocytes specific for mutated RAS proteins and recognizing tumoral cells expressing such RAS mutations.

Among all human T-cell leukemia virus type 1 proteins, tax, polymerase, and envelope proteins are predicted as preferential targets for the HLA-A2-restricted cytotoxic T-cell response.

The human T-cell leukemia virus type 1 (HTLV-1) is a human retrovirus associated with two diseases for which no successful treatment is yet available; the development of a vaccine is therefore an important issue. Since HTLV-1 is a persistent virus, an efficient vaccine will probably require a cytotoxic T-lymphocyte (CTL) response in addition to the production of antibodies. To identify potential CTL epitopes, we have selected, within all of the HTLV-1 proteins, nonapeptides containing anchor residues required for association with HLA-A2 molecules (residues at positions 2 and 9), which is the most frequently occurring A allele in all human populations. A set of 111 peptides was synthetized and tested in vitro in two assembly assays using processing-defective T2 cells. Anchor motifs selected were those containing two major anchor residues (L2/M2/12-V9/L9/I9) (one letter amino-acid code) and those including tolerated anchor residues (V2/A2/T2 and/or A9/M9/T9). The analysis of the binding capacity of the peptides confirms the high efficiency of the L2-V9 anchor motif and shows that a systematic research of potential binding peptides should exclude peptides containing known detrimental residues rather than select only peptides with known favored residues. We show that 39 peptides representative of all the HTLV-1 proteins are able to bind to HLA-A2 molecules. Strong binder peptides which are very likely good CTL epitopes were identified in three HTLV-1 proteins, Tax, envelope, and polymerase. Three of the strong binder peptides correspond to previously described HLA-A2-restricted CTL epitopes in the Tax protein, and two others are localized in a domain of the viral envelope recognized by natural neutralizing antibodies. This latter result has important implications for the development of an anti-HTLV-1 vaccine.

Partially modified retro-inverso pseudopeptides as non-natural ligands for the human class I histocompatibility molecule HLA-A2.

Syntheses of a series of partially modified retro-inverso analogues of the antigenic peptide M58-66 derived from the influenza virus matrix protein are reported. The retro-inverso modification phi(NH-CO) was obtained by replacement of two successive amino acid residues with a 2-substituted malonate derivative and gem-diaminoalkyl residue. The resulting compounds 1-8 were tested for their binding to the human histocompatibility class I molecule HLA-A2 in an assembly assay using lysates of peptide transporter-deficient cells T2. Specific peptide-dependent HLA-A2 assembly was revealed by an enzyme-linked immunosorbent assay. Significant HLA-A2 assembly was detected in the presence of analogues [gGly58-(S)mLeu59]-M58-66 (1a), [gGly61-(R,S)mPhe62]M58-66 (4), [gVal63-(R,S)mPhe64]M58-66 (6), and [gPhe64-(R,S)mAla65]M58-66 (7). The introduction of the retro-inverso modification between P2-P3, P3-P4, P5-P6, and P8-P9 (compounds 2, 3, 5, and 8, respectively) however led to a dramatic reduction in peptide binding to HLA-A2. Interestingly, compound 1a which contains modification between P1-P2 was found to be the most potent analogue, being able to retain the original HLA-A2 binding profile of the parent peptide M58-66. Taken together, these results and recent binding data obtained in the context of murine MHC class I molecule H-2Kd suggest that the incorporation of peptide bond surrogates in MHC class I-restricted epitopes is a useful approach to design molecules having both increased stability and high MHC-binding capacity. Depending on their agonist or antagonist effects at the T-cell receptor, such non-natural MHC ligands are likely to find many applications in the development of peptide-based vaccines or as potential therapeutic agents in the treatment of allergies and autoimmune diseases.

Direct detection of peptide-dependent HLA variability by surface plasmon resonance.

Cytotoxic T lymphocytes (CTL) recognize antigens as peptides associated with molecules of the major histocompatibility complex (MHC). The accurate characterization of antigenic peptides requires knowledge of how peptides bind to MHC molecules, and hence the conformational changes they can induce. Several reports have indicated that the conformation of the MHC class I molecule plays a role in T cell recognition. We therefore studied the interaction of a series of viral epitopes with HLA-A2, -A3, -B7 and -B8 molecules to determine how peptides could induce conformational changes in HLA molecules. This was done either directly with class I heavy chains in lysates of peptide-loading deficient T2 cells, or with purified material from B-EBV transformed cell lines. The peptide-induced HLA conformations were assessed using monoclonal anti-HLA antibodies (mAbs) and detected by surface plasmon resonance (SPR). Antigenic peptides specifically bound to the HLA molecule, even when assembly occurred in a mixed solution of HLA molecules. Distinct patterns of reactivity to a given peptide-bound class I molecule were obtained with monomorphic and allele-specific anti-HLA mAbs.

HLA-dependent variations in human immunodeficiency virus Nef protein alter peptide/HLA binding.

In human immunodeficiency virus (HIV) infection, sequence variations within defined cytotoxic T lymphocyte (CTL) epitopes may lead to escape from CTL recognition. In a previous report, we have shown that the variable central region of HIV Nef protein (amino acids 73-144) that contains potential CTL epitopes, can escape the CTL response. We suggested that this non recognition occurs through a variety of mechanisms. In particular, we provided evidence that HIV Nef sequences recovered from HLA-A11-expressing individuals have alterations in the major anchor residues essential for binding of the two Nef epitopes (amino acids 73-82 and 84-92) to the HLA-A11 molecule. Here, we investigate in more detail whether variations in autologous Nef sequences affect HLA binding, leading to CTL escape. Potential epitopes were sought by testing Nef peptides containing the HLA-A11-specific motif or related motifs. We confirmed that only the two previously described epitopes identified in cytolysis tests have optimal reactivity with the HLA-A11 molecule. We then sequenced several viral variants from donors that do not express the HLA-A11 molecule and compared the variability of these epitopes with those obtained from HLA-A11-expressing individuals. One substitution (Leu85) found in the sequences isolated from both populations increase the reactivity of the HLA-A11-restricted epitope 84-92, and might explain the difference in immunogenicity observed between the two HLA-A11-restricted epitopes from HLA-A11+ individuals. In addition, selective variations were only detected in virus isolated from HLA-A11-expressing individuals. Furthermore, examination of the association of variant peptides with the HLA-A11 molecule demonstrated that a single substitution within the minimal epitope could not always completely abrogate HLA binding, suggesting that multiple alterations within a particular epitope may accumulate during disease progression, allowing the virus to escape CTL recognition.

A simple assay for detection of peptides promoting the assembly of HLA class I molecules.

Synthetic peptides derived from influenza virus and human immunodeficiency virus were tested for their ability to promote the assembly of HLA-A2 and HLA-B51 molecules in T2 cell lysates. Specific assembly was detected by an enzyme-linked immunosorbent assay. The most significant HLA-A2 assembly was obtained in the presence of peptides known to be targets for HLA-A2-restricted cytotoxic T lymphocytes (influenza matrix M.58-66 and HIV Pol 476-484). Three of a batch of Nef peptides corresponding to epitopic regions for cytotoxic T lymphocytes, caused significant assembly of HLA-A2 (Nef 83-91, 137-145 and 144-153), but only at high concentrations (100 microM). As these peptides bound relatively weakly, it is unlikely that they are good candidates for HLA-A2-restricted CTL epitopes. Peptides matrix M.60-68, Nef 186-194, and Plasmodium falciparum sh.77-85 produced the most significant assembly of HLA-B51. These peptides have a dominant hydrophobic anchor residue (V, L. I) at position 9 that could occupy pocket "F". Our results also suggest that another hydrophobic residue (V, L) at position 3 or 4 may anchor to hydrophobic pocket "D" of HLA-B51. Proline at position 2 greatly increases HLA-B51 anchoring.

HLA-binding regions of HIV-1 proteins. I. Detection of seven HLA binding regions in the HIV-1 Nef protein.

The physical association of HLA class I or H-2 molecules with 36 HIV-1 Nef synthetic peptides was studied using a direct peptide binding assay (PBA) in solid phase. To assess the functional significance of the PBA results, the Nef peptides were also tested for their ability to inhibit the lytic activity of human or murine CTL. The PBA results showed that seven partly overlapping regions of the Nef protein contained MHC binding peptides (4-18, 46-67, 73-94, 100-128, 126-155, 182-198, and 192-206). Five of these seven regions included all the already described epitopes recognized by CD8+ human CTL. The two other regions, 4-18 and 46-67, are not yet described as antigenic for human CD8+ cells but they are located in the N-terminal part of Nef that was previously described as being stimulator for rat or chimpanzee CD4+ cells. Altogether, it can be concluded that 1) In virtually 100% of the cases, the PBA is capable to detect known antigenic peptides recognized by CTL. 2) The PBA and the functional inhibition assay provide similar results, supporting the functional significance of PBA results. 3) The PBA is easy to handle on a large scale, using multiple peptide and several MHC molecules, so that it can be used as a routine method for prevision of possibly epitopic sequences. 4) Systematic studies of peptides issued from the whole sequence of a given protein allow to map polyepitopic areas that are probably the most interesting parts of proteins for a vaccine purpose.

HLA-binding regions of HIV-1 proteins. II. A systematic study of viral proteins.

To detect HLA-binding peptides in 10 HIV-1 proteins (Rev, Tat, Vif, Vpr, Vpu, Gag p18, Gag p24, Gag p15, Env gp120 and Env gp41), the peptide binding assay (PBA) has been performed using three HLA class I molecules. Correlations have been searched between the PBA results and the peptide competitor activity in a functional test using HLA-A2-restricted CTL and target cells. A correlation between the data found in the PBA and well-defined CTL epitopes could be attempted only for the three Gag proteins. For these proteins, our results are in agreement with the known existence of epitopes reacting with human CD8+ CTL, with some exceptions. Together with the results reported with a panel of Nef peptides, these experiments showed that at least 18/20 of the already reported CTL epitopes from HIV-1 Gag, Nef, and Env proteins could be detected by the PBA, most (17/18) corresponding to strong reactivities. Perhaps more important, the regions of HIV-1 Gag p24 or Nef proteins that contain multiple associated CTL epitopes, with different HLA restrictions, were clearly identified by the reactivities in the PBA of several overlapping peptides and the major practical interest of the PBA might be the detection of such polyepitopic regions. Prediction are proposed in this report for 10 proteins, including several proteins for which CTL epitopes remain presently unknown.