Antigen-specific proliferative human T-lymphocyte clones with specificity for Chlamydia trachomatis

Chlamydial protease-like activity factor induces protective immunity against genital chlamydial infection in transgenic mice that express the human HLA-DR4 allele

There is no licensed vaccine available against Chlamydia trachomatis, the leading cause of bacterial sexually transmitted disease. We have found that intranasal immunization with recombinant chlamydial protease-like activity factor (CPAF) induces CD4(+) T-cell- and gamma interferon (IFN-gamma)-dependent protective immunity against murine genital chlamydial infection, thus making CPAF a viable vaccine candidate for further characterization. HLA-DR4 is the predominant allele involved in chlamydial antigen presentation to CD4(+) T cells in humans. We used engineered mice that lack endogenous major histocompatibility complex class II (MHC-II) alleles but express a human HLA allele (HLA-DR4 transgenic [tg] mice) to examine primary immune and CPAF-mediated responses against genital Chlamydia muridarum challenge. Upon primary bacterial exposure, HLA-DR4 tg mice developed Chlamydia-specific IFN-gamma and antibody production and resolved the infection within 30 days, similar to challenged conventional C57BL/6 animals. Moreover, C. muridarum-challenged HLA-DR4 tg mice exhibited CPAF-specific antibody and IFN-gamma production. Upon CPAF-plus-interleukin-12 (IL-12) vaccination, HLA-DR4 tg animals exhibited robust CPAF-specific IFN-gamma production and elevated titers of anti-CPAF total antibody and immunoglobulin G2a (IgG2a) and lower titers of IgG2b and IgG1 antibodies. HLA-DR4 tg and C57BL/6 mice vaccinated with CPAF plus IL-12 resolved the primary genital chlamydial infection significantly earlier than mock-immunized animals, whereas similarly vaccinated MHC class II-deficient mice displayed minimal antigen-specific immune responses and failed to resolve the infection even at 30 days postchallenge. Together, these results demonstrate the importance of human HLA-DR4 molecules in the recognition and presentation of CPAF epitopes, leading to the generation of protective antichlamydial immunity and making these mice a valuable model for mapping HLA-DR4-restricted chlamydial epitopes.

Direct detection and magnetic isolation of Chlamydia trachomatis major outer membrane protein-specific CD8+ CTLs with HLA class I tetramers

We recently identified HLA class I-presented epitopes in the major outer membrane protein (MOMP) of Chlamydia trachomatis that elicit CTL responses in human genital tract infections. T cells possessing cytolytic activities specific for these epitopes could be detected following in vitro stimulation of peripheral blood CD8(+) T cells with peptides. In the present study we used HLA-A2 tetramers for detailed characterization of MOMP-specific CTL responses. Ex vivo tetramer analysis detected MOMP-specific T cells in the peripheral blood of infected individuals at significant frequencies (0.01-0.20% of CD8(+) T cells). After in vitro stimulation with peptides, the frequencies of MOMP peptide-specific T cells increased up to 2.34% of CD8(+) T cells in bulk cultures. In contrast, HLA-A2/MOMP tetramer-binding T cells were virtually undetectable in the peripheral blood from uninfected individuals, either ex vivo or after 3 wk of in vitro peptide stimulation of their T cells. Magnetically sorted, tetramer-bound T cells specifically lysed peptide-pulsed targets as well as C. trachomatis-infected epithelial cells with nearly 50-fold greater per cell efficiency than that of unsorted populations. This study provides conclusive evidence of in vivo induction of HLA class I-restricted CD8(+) CTL responses to C. trachomatis MOMP. Direct detection of these cells with tetramers will allow their further characterization without prior manipulation and facilitate monitoring of CTL responses during infections and in immunization trials with MOMP-based vaccines.

Stimulatory effects of Helicobacter pylori on human peripheral blood mononuclear cells of H. pylori infected patients and healthy blood donors

The ability of 23 different strains of Helicobacter pylori to induce proliferative response of human peripheral blood mononuclear cells (PBMC) was investigated. All tested strains stimulated the DNA synthesis of PBMC from both healthy and H. pylori infected blood donors, but with lower stimulation of PBMC of infected donors. Using different bacterial antigen preparations, such as crude membranes, cytoplasmic proteins, and urease, a significantly lower induction of the proliferative response of PBMC from H. pylori infected than from healthy blood donors could also be demonstrated. In contrast to this result the reaction to phytohemagglutinin and purified protein derivative of tuberculin was similar in both groups. The stimulation pathway was interleukin 2 (IL-2) dependent as proved by inhibition of the proliferative response with an alpha-IL-2-receptor antibody. Using an antibody against HLA-DR the lymphoproliferation could also be blocked showing the importance of the major histocompatibility class II (MHCII) complex. Only coincubation of T cells with monocytes plus antigen or with antigen-preincubated monocytes led to a proliferative response showing the necessity of antigen-presenting cells. At least a part of the lymphoproliferative response is MHCII restricted as could be shown with H. pylori specific T-cell lines. These results and the kinetics of the proliferative response with a maximum at day 7 suggest that the proliferative response of human PBMC was mainly induced by antigens than by a mitogen.

Cell-mediated immunity to Chlamydia pneumoniae measured as lymphocyte blast transformation in vitro

The purpose of the present study was to analyze Chlamydia pneumoniae-induced, antigen-specific, cell-mediated immunity. Peripheral blood mononuclear cells of four persons infected with C. pneumoniae Kajaani 6 and 17 healthy volunteers were stimulated with antigen composed of whole elementary bodies of C. pneumoniae Kajaani 6 (CP-Ag). Definitive antigen-specific lymphoproliferation (LP) responses were developed after recent infection. The LP responses of healthy people to CP-Ag varied considerably. There was no clear correlation between LP responses to CP-Ag and those to an antigen prepared from Chlamydia trachomatis serotype L2 (r > or = 0.50, P < 0.1). A larger study is required to demonstrate whether the LP responses to CP-Ag can be used for the diagnosis of C. pneumoniae infection.

At least five antigenic epitopes on the streptokinase molecule are recognized by human CD4+ TCR alpha beta+ T cells

The streptokinase molecule (415 AA) was cleaved at methionine 237, 347 and 370 yielding four polypeptide fragments. Human HLA-class II restricted streptokinase-specific T cell clones and cell lines (CD2+, CD3+, CD4+, CD8-, TCR alpha beta+, TCR gamma delta-) recognized antigenic epitopes on all four fragments AA 1-236, AA 238-346, AA 348-369 and AA 371-415. T cell clones recognizing fragment AA 1-236 were restricted by at least two different HLA-class II elements, this indicating that more than one antigenic epitope can be recognized on this fragment. In addition, two streptokinase-specific T cell clones recognized only the intact molecule and none of the molecular fragments. These two clones probably recognized an antigenic epitope including one of the methionine residues used for molecular cleavage. We conclude that T cell proliferative responses to streptokinase are determined by recognition of at least five different antigenic epitopes distributed along the entire streptokinase polypeptide chain.

T-cell recognition of HLA class II molecules induced by gamma-interferon on a colonic adenocarcinoma cell line (HT29)

HLA class II molecules may be induced on non-lymphoid cells by gamma-interferon (IFN-gamma). We investigated if HLA class II molecules induced by IFN-gamma on the HT29 colonic carcinoma cell line are functional, i.e. if they may be recognized by allogeneic T cells. We found that IFN-gamma-treated HT29 (HT29IFN) cells could not induce primary proliferative responses of peripheral blood T lymphocytes, nor were they able to induce proliferation in T-lymphocyte clones (TLC) specific for HLA class II molecules found on HT29IFN. However, in the presence of exogenous interleukin 2 (IL-2), 1 of 5 DQw8-specific TLC proliferated when restimulated with HT29IFN, and 3 of these 5 TLC could very effectively inhibit the growth of HT29IFN, probably due to a cytotoxic effect. Both the proliferative response and the cytotoxicity were inhibited by anti-DQ MoAb. We conclude that T cells may recognize HLA-DQ molecules on non-lymphoid cells, which may be of relevance for autoimmune diseases, graft-versus-host disease, and possibly for the recognition of malignant cells.

Positive selection of Tac- (CD25) positive cells following T-cell activation. Use of immunomagnetic separation and implications for T-cell cloning

We have investigated if positive selection for cells expressing activation antigens, which appear on the cell surface during T-lymphocyte activation, could be used for cloning purposes. For this purpose, we used paramagnetic, monodisperse Dynabeads coated with anti-Tac monoclonal antibody, which recognizes CD25 (interleukin-2 receptor light chain). After the first 6-12 h of a primary response, depletion of Tac+ cells could largely abrogate the specific response. This indicated that the specifically responding cells were found among the Tac+ population. T-cell cloning was thus performed on Tac+ blasts positively selected after 18 h of a primary response, at day 6 of a primary response or during secondary stimulation, and gave a high percentage of specific clones. This method is thus a good alternative to established techniques.

T-lymphocyte clones responsive to Shigella flexneri

T lymphocytes from a patient with Shigella flexneri dysentery and postdysenteric reactive arthritis were cloned by limiting dilution with recombinant interleukin-2 and a strain of S. flexneri different from that which had infected her. Five of eight clones produced proliferated in response to the shigellae used to generate the clones. The response required irradiated syngeneic blood mononuclear cells as antigen-presenting cells. One such clone, MC12, proliferated in response to both the shigellae used to generate the clones and the infecting shigellae but not to other shigellae, Salmonella heidelberg, or control Escherichia coli. MC12 was CD3+, CD4+, CD8-, and human histocompatibility leukocyte antigen (HLA)-DR+. The proliferative response to the shigellae was blocked by antibody to HLA-DR but not by antibody to HLA-A,B,C. The response required antigen-presenting cells that shared HLA-DR antigens with the clone and appeared to be restricted by HLA-DR2. The epitope recognized by MC12 was associated with the bacterial membranes. Thus, T-lymphocyte clones that proliferate in response to some shigellae can be isolated from patients with shigellosis.

T lymphocyte clones recognizing an HLA-DQw3.2-associated epitope involving residue 57 on the DQ beta chain

The DQw3.2 specificity has previously been recognized using genomic RFLP analysis and certain combinations of monoclonal antibodies. Here we report three CD4+ T lymphocyte clones (TLCs) generated from a DR3,4; DQw2,w3.1 responder stimulated with cells from a DR3,4; DQw2,w3.2 donor, and using a modified cloning procedure involving enrichment of IL-2 receptor-positive T cell during priming. The resulting TLCs were strongly inhibited by some monoclonal anti-DQ, but not anti-DR or -DP antibodies. In panel studies using HLA homozygous stimulating cells, it was found that the TLCs recognize an HLA epitope encoded by a DQ gene carried only by DR4,DQw3.2 haplotypes. By comparison with published DQ chain amino acid sequences of some stimulating cells able or not to induce a response in these clones, evidence was obtained that Ala at position 57 on the DQ beta chain is most probably involved in the epitope. The epitope is present on cells from 12 out of 12 DR4,DQw3 insulin dependent diabetes mellitus (IDDM) patients, but on cells only from 6 out of 12 healthy DR4,DQw3 controls. Thus, a DQ-encoded epitope involving residue 57 on the DQ beta chain, and which is strongly associated to IDDM, may be recognized by T cells.

Mucosal T cells provide helper function but do not proliferate when stimulated by specific antigen in lymphogranuloma venereum proctitis in nonhuman primates

To study antigen-specific immune responses of gut-associated T lymphocytes after gastrointestinal infection, Cynomolgus monkeys were inoculated rectally with Chlamydia trachomatis of the L2 [lymphogranuloma venereum (LGV)] strain. Infected monkeys developed a chronic proctitis with the appearance of LGV-specific immunoglobulin G-antibodies in the serum. Lymphocytes isolated from the peripheral blood, the spleen, and draining lymph nodes had a vigorous antigen-specific proliferative response to LGV in vitro. Both T and B cells proliferated in response to stimulation with LGV, but B-cell proliferation was T-cell-dependent, as shown by cell separation techniques and cell-cycle analysis with dual-laser flow cytometry. Lymphocytes isolated from both involved and uninvolved lamina propria did not proliferate in response to LGV stimulation, whereas mitogen-induced proliferation was not different in lamina propria lymphocytes and the other lymphocyte populations. This lack of antigen-specific proliferation was not caused by a suppressor effect of mucosal T cells or monocytes or the absence of antigen-presenting cells. In contrast, lamina propria T lymphocytes from infected animals were able to provide antigen-specific help for polyclonal immunoglobulin synthesis by immune B lymphocytes after stimulation with LGV. Thus, in LGV proctitis in monkeys, mucosal antigen-reactive T cells differ from lymphocytes in other sites in that they can provide helper function, but are not able to proliferate in response to LGV antigens.

Major histocompatibility complex (MHC) restricted antigen recognition: high frequency of human T-cell clones recognizing novel MHC class II determinants

We have used antigen-specific human T-cell clones to study the relationship between MHC and antigen recognition specificities expressed by T cells. Tetanus toxoid (TT)-specific T-lymphocyte clones were derived from a immunized HLA-DR2,7 heterozygous donor by limiting dilution from peripheral blood mononuclear cells (PBM) restimulated with TT in vitro. Clones were screened for MHC-restricted antigen recognition against antigen-presenting cells (APC) from a panel of HLA-typed donors, using an in vitro T-cell proliferation assay. Several distinct patterns of antigen recognition were identified. In addition to T cells that recognized TT in association with donor class II MHC antigens, we found clones that simultaneously expressed self-restricted antigen recognition and alloreactivity, and clones with specificity for antigen in the context of MHC antigens not expressed by the T-cell donor. This was confirmed in inhibition studies using well-characterized monoclonal antibodies against class II MHC antigens to block specific proliferative responses. We propose a possible structure for the determinant recognized by two of the clones. These results suggest that the T-cell antigen receptor undergoes random or antigen-dependent changes in vitro, and that this may be a mechanism for somatic diversification of the T-cell repertoire.

Cytotoxic and immunoregulatory function of intestinal lymphocytes in Chlamydia trachomatis proctitis of nonhuman primates

To study the role of natural killer cells and immunoregulatory T cells in the pathogenesis of proctitis due to Chlamydia trachomatis (L2 serovar), lymphocytes were obtained from the rectal mucosa and other sites of nonhuman primates and studied by using phenotypic and functional assays. In animals with lymphogranuloma venereum (LGV) proctitis, the percentage of lymphocytes with the natural killer cell phenotype (Leu-11+) was not significantly higher at any site in LGV infection, and natural killer cell function of lymphocytes isolated from the rectum was lower during LGV infection. This was not due to the suppressive effect of factors in serum, rectal lymphocytes, or LGV elementary bodies. In studies of regulatory T cells, the Leu-3+/Leu-2+ ratio was lower in the peripheral blood and the spleen during LGV infection, but the ratio did not decrease in lamina propria T cells. Both peripheral blood and rectal lymphocytes had higher helper T-cell function for polyclonal immunoglobulin G (IgG) synthesis in pokeweed mitogen-stimulated cultures 2 weeks following LGV infection. Increased suppressor T-cell function for pokeweed mitogen-stimulated IgG synthesis was found only in the peripheral blood of animals 2 weeks after infection, but not in isolated rectal lymphocytes. These results indicate that in LGV proctitis natural killer cells are not an important component of the inflammatory infiltrate at the site of infection, and helper T-cell function increases in peripheral blood and rectal lymphocytes.

Polymorphisms of the HLA-DQ subregion

The HLA-D region (analogous to the I region of the mouse H-2 complex gene) is now known to contain three subregions, one of which is HLA-DQ. The products of the HLA-DQ subregion are important in T-cell recognition. Three DQ specificities have been recognized by the International Histocompatibility Workshop but additional polymorphisms of DQ gene products have also been described. In this article, Massimo Trucco and René Duquesnoy discuss the relationship between serologically, cellularly and molecularly defined polymorphisms of HLA-DQ.

Mycobacterium bovis BCG-induced human T-cell clones from BCG-vaccinated healthy subjects: antigen specificity and lymphokine production

A total of 121 human T-cell clones were raised from nine Mycobacterium bovis BCG-vaccinated healthy individuals. Three clones were autoreactive, 74 responded to BCG in the presence of antigen-presenting cells, and the others required in addition exogenous interleukin 2. Only one clone was CD8+ CD4-, and the rest were CD4+ CD8-. Testing with a panel of mycobacteria suggested that the clones were recognizing epitopes of varied specificity. Out of 44 clones tested, 15 were specific to BCG and Mycobacterium tuberculosis, 22 showed limited cross-reactivity, and 8 were broadly cross-reactive. None of the 22 BCG responder clones could differentiate between Danish, French, Prague, and Moreau strains of BCG. BCG and M. tuberculosis H37Rv also paralleled very closely; however, 6 of 18 BCG- and M. tuberculosis H37Rv-responding clones did not proliferate to Mycobacterium africanum. BCG- and M. tuberculosis H37Rv-specific as well as cross-reactive T-cell clones could be induced to produce interleukin 2, gamma interferon, and granulocyte-macrophage colony-stimulating factor activity.

Chlamydia trachomatis (L2 serovar) binds to distinct subpopulations of human peripheral blood leukocytes

We have previously shown that infants with pneumonitis caused by Chlamydia trachomatis, an obligate intracellular bacterium, possess increased percentages of B lymphocytes but not T lymphocytes in their peripheral blood. It was then demonstrated that chlamydiae induce proliferation in vitro of human peripheral blood B lymphocytes and, in the presence of T cells, differentiation of B cells to immunoglobulin-secreting cells. In this study, we show that C. trachomatis (L2 serovar) binds preferentially to 50% of human B lymphocytes from peripheral blood but only to a small percentage, if any, of T cells. Both monocytes and granulocytes bind and ingest chlamydiae. Despite chlamydial binding to B cells and ingestion by monocytes, no uptake by B cells and limited growth (fewer than 0.5% inclusion-containing cells) in monocytes occur. There is a dramatic decrease in the percentage of cells associated with the bacteria after culture. These results are the first demonstration of binding of C. trachomatis (L2 serovar) to lymphocytes and represent a direct step toward correlating physical interactions between bacteria and lymphocytes with specific immunostimulatory activities in vitro.

Human T-cell clones recognize a major M. leprae protein antigen expressed in E. coli

Leprosy is a chronic infectious disease caused by Mycobacterium leprae. As with other intracellular parasites, protective immunity is dependent on T cells and cell-mediated immunity. In animal models, immunization with killed armadillo-derived M. leprae elicits strong T-cell responses, delayed-type hypersensitivity and protection against viable challenge. We have recently shown that killed M. leprae can induce delayed-type hypersensitivity in healthy human volunteers. Identification of the M. leprae antigens that are recognized by T cells and may be involved in protection has been hampered by the inability to cultivate the organism in vitro and by difficulties in antigen purification from limited quantities of armadillo-derived bacillus. Because genes for the major protein antigens of M. leprae as seen by mouse monoclonal antibodies have been isolated, it has become possible to test whether these individual antigens are recognized by T cells. We screened crude lambda gtll phage lysates of Escherichia coli containing individual M. leprae antigens using M. leprae-specific T-cell clones isolated from M. leprae-vaccinated volunteers. Using this method, we find that nearly half of the M. leprae-specific T-cell clones are stimulated to proliferate by lysates containing an epitope of a M. leprae protein of relative molecular mass 18,000 (18K).

Identification, at the genomic level, of an HLA-DR restriction element for cloned antigen-specific T4 cells

Two T4 cell clones (TLC) specific for antigenic epitopes on Chlamydia trachomatis were studied. Using a panel of allogeneic antigen-presenting cells (APC), both TLC were found to be restricted by HLA class II elements closely associated with, but not identical to the DRw5S specificity, as determined by highly selected alloantisera, a monoclonal antibody (mAb), 109d6, and confirmed on the DNA level by determination of restriction fragment length polymorphisms (RFLP) with a DR beta probe. Furthermore, HLA-DR-specific mAb, including 109d6, but not other HLA class II- or class I-specific antibodies inhibited the two TLC, strongly suggesting that the restriction element is expressed by a DR molecule. Using digestion with Hind III restriction enzyme and a DR beta probe, we found a complete concordance between the appearance of a 9.3 kilobase band and the ability of allogeneic APC to restimulate the T cell clones. Thus, the restriction element for these T cell clones appear to be expressed by DR molecules, but can, at present, only be detected at the genomic level.

Characterization of Chlamydia trachomatis serotypes by human T-lymphocyte clones

T cells primed to Chlamydia trachomatis serotypes A, F, and K were cloned by limiting dilution. All T-lymphocyte clones obtained reacted only with C. trachomatis antigens. The proliferative capacity of 89 clones was studied with autologous non-T cells as antigen-presenting cells and the chlamydia serotypes A, B, D, F, K, and LGV-2 as antigens. Most of the clones reacted to several of the chlamydia strains, indicating common antigenic determinants. Other T-cell clones reacted with only a few serotypes. On the basis of the proliferation of the T-cell clones to the chlamydia strains and to interleukin-2, different reactivity patterns were obtained, which possibly can be used to differentiate among the chlamydia strains.

Antigen-specific T cell clones restricted by DR, DRw53 (MT), or DP (SB) Class II HLA molecules. Inhibition studies with monoclonal HLA-specific antibodies

T lymphocyte clones (TLCs) specific for Chlamydia trachomatis were obtained after limiting dilution of activated T cells from a single donor, BS (HLA-A3; B7,15; DR1, 4; DRw53; DPw4). Most of the proliferative TLCs obtained apparently used restriction elements on DR1 or DR4 molecules, expressed in the antigen-presenting cells (APC). The restriction pattern of two TLCs, however, closely followed the DRw53 specificity, while one TLC seemed to be restricted by elements on DPw4 molecules. A panel of murine monoclonal antibodies (Mabs) was employed to further dissect the restriction specificities. The proliferative capacity of some of the TLCs was studied with and without Mabs directed against different HLA molecules, using non-T cells, monocytes, and peripheral blood dendritic cells as APC. Two Mabs, 7.2 and D-54, which seem to detect monomorphic determinants on HLA-DR, inhibited both DR- and DRw53-restricted TLCs. One Mab, 109d6 which reacts with DRw53 determinants, selectively inhibited the DRw53-restricted TLCs. The DP-restricted TLC was not inhibited significantly by any tested Mabs, including two Mabs with putative specificity for monomorphic determinants on the DP molecule.

Antigen-specific proliferative human T cell clones with specificity for diphtheria toxoid: genetic and molecular restriction by class II antigens

Human T lymphocyte clones (TLC) specific for diphtheria toxoid (DT) were isolated from a DR6/7 individual by cloning in soft agar in vivo sensitized T lymphocytes. We report here the isolation and characterization of 3 of these clones by studying: (a) the kinetic of activation, (b) the surface phenotypes, (c) the fine specificity for one of the 2 DT chains and (d) the genetic restriction of the proliferative response by the haplotype DR7. Moreover, blocking studies of the proliferative response to DT by various immunochemically characterized anti-HLA-DR monoclonal antibodies indicate that, on the DR7 molecule, more than one Ia determinant may participate in the clonal DT proliferative response. By using human TLC of a defined specificity and well-characterized anti-DR monoclonal antibodies, such studies may help to define the functional repertoire of Ia molecules in man.

Lack of cell-mediated cytotoxicity towards Chlamydia trachomatis infected target cells in humans

Induction of cell-mediated cytotoxicity towards Chlamydia trachomatis infected target cells was studied in humans. PHA-derived lymphoblasts, infected with inactivated or virulent LGV-2 chlamydial particles, were used as target cells. Chlamydial primed T cell blasts or specific, cloned T cells were used as effector cells. A highly sensitive 18 hours lytic assay employing 111In-labelled target cells was used. Specific cytotoxicity towards C. trachomatis infected target cells in humans could not be detected. This was the case regardless of the origin of the effector cells or the various protocols tested. However, this assay was capable of yielding significant positive results with effector cells primed to alloantigens. We conclude that cell-mediated cytotoxicity towards bacterial infected target cells probably does not play a major role.

Class-II HLA restriction of antigen-specific human T-lymphocyte clones. Evidence of restriction elements on both DR and MT molecules

Blast-enriched suspensions of T cells primed for Chlamydia trachomatis antigen were cloned by a limiting dilution technique. The class-II HLA restriction of T-lymphocyte clones (TLC) was studied by using allogeneic antigen-presenting cells (APC) carrying foreign class-II HLA antigens. Most of the TLC were restricted by one or the other of the D/DR determinants of the T-cell donor; that is, they did not respond when antigen was presented by APC expressing foreign D/DR determinants. Furthermore, heterogeneity of the DR4-expressing molecule could be demonstrated by T-cell clones from one person; APC from family members expressing DR4 gave high proliferative responses, whereas no proliferation was observed with most APC from unrelated persons expressing DR4. This heterogeneity of DR4 was confirmed by mixed lymphocyte culture (MLC) experiments, indicating a close relationship between restriction epitopes and those that activate allogeneic T cells. Other clones seemed to be restricted by other class-II HLA determinants, most probably MT determinants of the T-cell donor. The restriction specificities were confirmed by subcloning experiments.