Allocytotoxic T cell clones: both Leu 2+3- and Leu 2-3+ T cells recognize class I histocompatibility antigens

Indirect T cell recognition in allograft rejection

Allograft rejection has been attributed predominantly to alloreactive T cells which recognize intact allogeneic MHC on donor antigen presenting cells (APC). Following transplantation, T cells may also recognize donor derived MHC antigens which have been processed and presented as antigenic peptides in the context of self-MHC by recipient APC--so called indirect allorecognition. This article discusses the possible role of indirect T cell recognition in the graft rejection response with particular emphasis on recent experimental data obtained from in vivo transplant models. Although the relative contribution of indirect allorecognition to the rejection process remains unclear the available evidence suggests that the contribution of self-MHC restricted T cells to graft rejection may have been underestimated.

CD4+ human immunodeficiency virus type 1 (HIV-1) envelope-specific cytotoxic T lymphocytes derived from the peripheral blood cells of an HIV-1-infected individual

Virus-specific cytotoxic T lymphocytes (CTL) are frequently of the CD8+ surface phenotype, although CTL of the CD4+ surface phenotype have also been described. Published reports of CTL derived from peripheral blood mononuclear cells (PBMC) of individuals infected with human immunodeficiency virus type 1 (HIV-1) have described primarily cells of the CD8+ surface phenotype. However, CD4+ HIV-1 envelope-specific CTL have been reported after in vitro stimulation with HIV-1 envelope protein of peripheral blood cells obtained from HIV-1-seronegative donors, in peripheral blood cells after vaccination of HIV-1-seronegative persons with HIV-1 envelope proteins, and in cerebrospinal fluid cells of HIV-1-infected individuals. Recently, CD4+ HIV-1 gag-specific CTL were also reported. We now report a patient from whom we derived HIV-1 envelope-specific CTL cell lines of the CD4+ surface phenotype. Our cell culture technique did not employ exogenous viral antigenic stimulation, and may therefore yield cells that more closely reflect those in the underlying populations from which they were derived. These CTL did not appear to have the clear human leukocyte antigen (HLA) class II restriction pattern typically seen in CD4-expressing cells and were not functionally inhibited by anti-CD3 antibody. Further work will be required to define the role of CD4+ CTL in the pathogenesis of HIV-1 disease.

Cytotoxic and proliferative allospecific T-cell clones contain perforin and mediate anti-CD3-induced cytotoxicity

Some in vitro-generated allospecific T-cell clones can kill target cells bearing specific antigen, whereas others can only proliferate in response to that antigen. The mechanism of target lysis by clones that exhibit antigen-specific cytotoxicity is thought to involve the exocytosis of lytic granules, which contain the pore-forming protein perforin. Here, CD4+, CD8+, and CD4-8- T-cell clones, positive for CD3 and the alpha/beta T-cell receptor, were tested for their ability to lyse the mouse-anti-human CD3 hybridoma OKT3; this hybridoma has been shown to trigger the cytolytic mechanism in cytotoxic T cells regardless of their clonal specificity. We found that all in vitro-generated allospecific T-cell clones can efficiently lyse the OKT3 targets whether or not they can kill alloantigen-bearing lymphoblastoid B-cell line targets. Furthermore, all tested clones contained perforin. The OKT3 hybridoma was not lysed by perforin-negative, CD3+ leukemic T-cell lines or by CD3- NK clones. Thus, the presence of perforin in T-cell clones correlated with their ability to lyse OKT3 targets, but not with their ability to lyse alloantigen-bearing targets. These results demonstrate that T-cell clones that are nonlytic when activated by specific antigen nevertheless contain a complete lytic mechanism and also support the proposed central role in perforin in that mechanism.

MHC class-I-restricted auto-tumor-specific CD4+CD8- T-cell clones established from autologous mixed lymphocyte-tumor-cell culture (MLTC)

Autologous mixed lymphocyte-tumor cell cultures (MLTC) were initiated with cytokine (IFN gamma and TNF alpha)-treated ex-vivo tumor cells of lung, ovarian, breast and stomach carcinomas. The cytokine-treated tumors expressed class-I but not class-II molecules. Although the proportion of CD8+ lymphocytes increased in the bulk culture of MLTCs, in 5/7 experiments the majority of the established T-cell clones were CD4+. Among the CD8+ clones a high proportion (77%) was cytotoxic, while the proliferative response was more frequent among the CD4+ clones (70%). In 4/26 cytotoxic T-lymphocyte (CTL) clones (3/17 CD4+ and 1/9 CD8+), derived from a patient with class I+ class II- stomach carcinoma, lysis was restricted to the autologous tumor cells. These auto-tumor-specific clones did not lyse the autologous ConA blasts, the 5 allogeneic ex-vivo tumors, the NK-sensitive K562 or the relatively sensitive Daudi cells. The cytotoxicity of these clones was inhibited by pre-incubation of the tumor cells with W6/32 (alpha-class I) MAb, or by preincubation of the lymphocytes with OKT3 (alpha-CD3) MAb. The alpha-CD4 (OKT4) MAb had only a marginal effect on the CD4+ clones, while the lytic function of the CD8+ clone was inhibited by the alpha-CD8 (OKT8) MAb. The 3 CD4+ CTL clones also responded with proliferation to the autologous tumor cells. This proliferative response was inhibited by the presence of W6/32 MAb. Our results indicate that the auto-tumor lysis exerted by CD4+ CTL clones was restricted by the class-I antigens, and that the CD4 molecules of the clones were not essential for the lytic interaction.

Processed MHC class I alloantigen as the stimulus for CD4+ T-cell dependent antibody-mediated graft rejection

The traditional view of graft rejection is one of direct recognition of allogeneic MHC molecules by effector T cells, the phenotype of which may be predicted by the nature of the MHC disparity. In this article, Andrew Bradley and colleagues discuss recent evidence that suggests this view may be an oversimplification, and argue that additional effector mechanisms, such as alloantibody, need to be reconsidered.

T cell requirements for the rejection of renal allografts bearing an isolated class I MHC disparity

This study has examined the cellular and humoral responses underlying the rejection of rat renal allografts bearing an isolated RT1Aa class I MHC disparity. RT1Aa disparate kidneys were rejected promptly by high responder RT1u but not by low responder RT1c recipients (median survival time 10 d and greater than 100 d, respectively). The magnitude and phenotype of the cellular infiltrate were similar in rejecting and nonrejecting RT1Aa disparate kidneys. Paradoxically, graft infiltrating cells and spleen cells from RT1u recipients showed minimal ability to lyse donor strain lymphoblasts in vitro, whereas effector cells from RT1c recipients showed modest levels of cytotoxicity. Injection of RT1u rats with MRC OX8 mAb was highly effective at selectively depleting CD8+ cells from graft recipients but had no effect in prolonging the survival of RT1Aa disparate grafts despite the complete absence of CD8+ cells from the graft infiltrate, which included numerous CD4+ T cells and macrophages. RT1u, but not RT1c, recipients mounted a strong alloantibody response against RT1Aa disparate kidneys. Immune serum obtained from RT1u recipients that had rejected a RT1Aa disparate graft was able, when injected into cyclosporin-treated RT1u recipients, to restore their ability to reject a RT1Aa, but not a third-party RT1c, kidney. These results suggest that CD8+ cells in general and CD8+ cytotoxic effector cells in particular are unnecessary for the rapid rejection of RT1Aa class I disparate kidney grafts by high responder RT1u recipients. By implication, CD4+ T cells alone are sufficient to cause prompt rejection of such grafts and they may do so by providing T cell help for the generation of alloantibody.

Differential expression and regulation of the human CD8 alpha and CD8 beta chains

The CD8 glycoprotein is expressed by thymocytes, mature T cells and natural killer (NK) cells and has been implicated in the recognition of monomorphic determinants on major histocompatibility complex (MHC) Class I antigens, and in signal transduction during the course of T-cell activation. Both human and rodent CD8 antigens are comprised of two distinct polypeptide chains, alpha and beta. The majority of monoclonal antibodies (mAb) reactive with the human CD8 antigen bind the CD8 alpha chain, while a single mAb, T8/2T8-5H7, has been identified which binds to the CD8 alpha/beta heterodimer. While the two chains of CD8 have been presumed to be coordinately expressed in normal T cells, this is not always the case. Northern blot analysis of a panel of T-cell leukemias and normal cells demonstrate that CD8 alpha and CD8 beta are not invariably co-transcribed and phenotypic analysis of fresh and interleukin 2 (IL-2) expanded peripheral blood mononuclear cells (PBMC) confirm that the CD8 alpha and CD8 beta chains are differentially expressed at the cell surface. Four distinct subpopulations of CD8+ cells have been identified based on the expression of CD8 alpha/alpha or CD8 alpha/beta complexes: (1) T-cell receptor (TcR) alpha beta+ T cells which are CD8 alpha+/beta+; (2) TcR alpha beta+ T cells which are CD8 alpha+/beta-; (3) TcR gamma delta+ T cells which are CD8 alpha+/beta- and (4) natural killer (NK) cells which are CD8 alpha+/beta-. We also demonstrate the down-regulation of the CD8 alpha/beta heterodimers from the surface of a CD8+ T-cell clone following treatment with phorbol myristate acetate (PMA) while CD8 alpha/alpha homodimers remain on the cell surface. This observation demonstrates that a) a CD8+ T-cell clone can express both CD8 alpha/alpha homodimers and CD8 alpha/beta heterodimers and b) these two complexes do not have identical biological properties. Together, these data suggest that CD8 alpha/alpha and CD8 alpha/beta dimers may not subserve identical functions. The differential contribution of these two CD8 complexes should be considered in models of T-cell-mediated cytotoxicity and T-cell activation.

Recognition of self class II major histocompatibility complex antigens by CD8+ T cell clones derived from rheumatoid arthritis synovial membrane

T cells from rheumatoid synovium have been expanded in vitro as lines and clones using autologous Epstein-Barr virus-transformed stimulator cells. Both lines and clones recognized autologous class II MHC antigens in the absence of defined exogenous antigens i.e. the equivalent of the autologous mixed lymphocyte response. Surprisingly, despite their MHC specificity, several clones expressed CD8 rather than CD4, but were not cytotoxic. The function of CD8+ T cells within synovium has not previously been defined; in view of their unusual phenotype, they may exert an immuno-modulating role upon the inflammatory response within the joint, by responding to the high density of class II MHC antigens expressed in the rheumatoid synovium.

Lymphokine mRNA profile and functional analysis of a human CD4+ clone with unique antitumor specificity isolated from renal cell carcinoma ascitic fluid

We here describe the isolation, characterization, profile of lymphokine expression and T-cell-receptor gene rearrangement pattern of 444P.3, a CD3+ CD4+ CD8- 4B4+ interleukin-2 (IL-2)-dependent clone derived from the malignant ascites of a patient with renal cell cancer. The 444P.3 clone exhibited unique antitumor specificity between days 45 and 84 in culture and then lost its lytic, but not its proliferative, capacity. To our knowledge this is the first description of a specific antitumor reaction in a patient with renal cell cancer against autologous tumor. IL-2-expanded 444P.3 cells, tested on day 104 in culture, expressed mRNA for tumor necrosis factor (TNF), IL-2 and tumor growth factor beta (TGF-beta) but not for IL-1, lymphotoxin or granulocyte/macrophage-colony stimulating factor (GM-CSF). The parental noncloned population expressed mRNA for TNF, lymphotoxin, GM-CSF and TGF-beta but not for IL-1 beta or IL-2. Analysis of established human T cell clones should include profiles of lymphokine secretion in addition to growth and proliferation patterns, antitumor activity and surface phenotype. Such characterization of clones may provide a better understanding of the immunoregulatory role and functional potential of various T cell subsets involved in human antitumor reactivity.

Phosphorylation and down-regulation of CD4 and CD8 in human CTLs and mouse L cells

The CD4 and CD8 molecules are rapidly phosphorylated following exposure of CD4+ or CD8+ human cytotoxic T lymphocytes (CTL) clones to B-lymphoblastoid cell lines bearing the relevant target alloantigens. Treatment of CD4+ or CD8+ CTL clones with phorbol myristate acetate (PMA), phytohemagglutinin, or mitogenic combinations of CD2-specific antibodies also resulted in CD4 or CD8 phosphorylation. Down-regulation of the surface expression of these molecules could be demonstrated in both CD4+ and CD8+ clones following exposure to the relevant alloantigen or PMA. Parallel experiments were conducted using mouse L cells in which the human CD4 or CD8 antigens were stably expressed. Exposure of these transfectants to PMA induced rapid phosphorylation of the CD4 and CD8 molecules. As in CD4+ CTL clones, rapid modulation of the CD4 antigen could be demonstrated in L cells following PMA treatment. In contrast, there was no demonstrable down-regulation of the CD8 antigen in PMA-treated CD8+ L cell transfectants. These studies demonstrate a significant differential property of the CD4 and CD8 antigens and suggest that down-regulation of the CD8 antigen may require its expression in a T-cell environment and/or the association of CD8 with the T-cell receptor or other T cell-specific molecules.

Interplay between the TCR/CD3 complex and CD4 or CD8 in the activation of cytotoxic T lymphocytes

Interactions between CTL and target cells occur in the absence of specific antigen recognition and precede subsequent interaction of the TCR with its specific antigen. This antigen-independent adhesion progresses through two different pathways, one involving the interaction of CD2 with LFA-3 on the target cell, the second the interaction of LFA-1 with ICAM-1. Such antigen-independent adhesions are critical for the activation of T cells via the TCR. Also, CD4 and CD8 can serve as adhesion molecules by binding to monomorphic determinants expressed on class II and class I MHC antigens, respectively, on the target cells, but compared to LFA-1 and CD2 antigens their contribution to conjugate formation is minor. CD4 and CD8 are required for effective T-cell activation in situations where the intrinsic affinity of the TCR or antigen expression is low, suggesting that CD4 and CD8 enhance the avidity of T cells for target cells by binding to class II and class I antigen, respectively. However, CD4 and CD8 are also involved in post-binding events that lead to CTL activation and subsequent lysis of the target cells. On the other hand, blocking of anti-TCR/CD3 mAb-induced CTL reactivity by anti-CD4/CD8 mAbs does not necessarily involve an interference with the binding of CD4 and CD8 to their respective ligands and it has been proposed that the TCR and CD4 or CD8 form functional complexes that are required for optimal T-cell activation. It is still unclear whether blocking by anti-CD4/CD8 mAbs is based on the prevention of complex formation of the TCR with CD4 or CD8, since formation of such complexes has yet to be demonstrated. The alternative hypothesis, that anti-CD4/anti-CD8 mAbs can directly confer negative regulatory signals to the CTL is not supported by our studies with antibody-directed lysis mediated by a CD4+, CD8+ CTL clone. Anti-CD4/CD8 mAbs can also inhibit T-cell cytotoxicity induced by other T-cell surface activation antigens such as CD2 or Tp103. In these situations, the triggering may involve signals transferred via CD3 requiring functional CD3/CD8 or CD3/CD4 complexes. Although most studies investigating the sequence of events leading to T-cell activation are carried out with CTL, preliminary data indicate that the same mechanisms described here for CTL activation are probably also valid for the interactions of T-helper cells with APC or B cells.(ABSTRACT TRUNCATED AT 400 WORDS)

Regulatory role of the CD8 antigen in both CD3 and CD2 monoclonal antibody-induced nonspecific cytotoxicity of class I- and class II-allospecific cytotoxic T cell clones

We investigated the function of the CD8 moiety in antigen-specific and alternative activation of HLA class I- and HLA class II-allospecific CD8+ cytotoxic T lymphocyte (CTL) clones. Monoclonal antibodies (mAb) directed against the CD8 structure were only found to inhibit antigen-specific cytotoxicity of class I-allospecific CD8+ CTL clones and not of a class II-allospecific CD8+ CTL clone. However, cytotoxicity induced by CD3 mAb (used at suboptimal concentrations) or CD2 mAb in both types of CTL clone was blocked by CD8 mAb. The class II-allospecific CD8+ CTL clone was uniformly more difficult to inhibit than the class I-allospecific CD8+ CTL clones and, moreover, also easier to induce to exert nonspecific cytotoxicity by CD2 mAb and CD3 mAb. The absence of CD8 mAb blocking of antigen-specific cytotoxicity of the class II-specific CD8+ CTL clone is, therefore, assumed to result from too strong a triggering signal to be overcome by the down-regulatory signal of the CD8 antigen. These combined findings suggest a down-regulatory function of CD8 not only in T cell receptor (TcR)/CD3 activation, but also in TcR/CD3-controlled alternative activation routes such as the CD2 activation pathway.

Phenotypic heterogeneity of cerebrospinal fluid-derived HIV-specific and HLA-restricted cytotoxic T-cell clones

A variety of clinical syndromes, including AIDS and neurological disorders, may follow as a consequence of infection with the human immunodeficiency virus type 1 (HIV-1). It is not yet clear, however, to what extent the destruction of lymphocytes and neural cells associated with these conditions is caused by adverse immune responses to HIV-1 or how much is due to cytopathic effects of the virus itself. Here we document the existence of HLA-restricted, HIV-1-specific cytotoxic T lymphocytes in the cerebrospinal fluid of two AIDS patients manifesting neurologic disorders. These cytotoxic T lymphocytes showed dual specificity, recognizing target cells coated with purified HIV-1 envelope glycoprotein (gp 120) or inactivated HIV-1 in the context of HLA antigens. Cytotoxic T-cell clones derived from one of the AIDS patients revealed restriction specificities representing both HLA class I and HLA class II antigens. Considerable phenotypic heterogeneity was observed amongst these clones, some expressing conventional combinations of cytotoxic T-cell surface markers, and others displaying unusual phenotypes. The presence of HIV-specific cytotoxic T lymphocytes in AIDS patients, and in particular in their cerebrospinal fluid, suggests that these cytotoxic effectors may participate in the lymphoid cell and/or neurologic damage observed in such patients.

Mapping of a restriction fragment length polymorphism associated with defective DR beta 4 chain expression to the HLA-DRB1 gene

The HLA-DR beta 4 chain, encoded by the DRB4 gene, carries two DRw53 determinants normally expressed by DR4, DR7, and DR9 individuals. However, some DR7 individuals (DR7, Dw11) fail to express the DR beta 4 chain. At the genomic level, a HindIII restriction fragment length polymorphism can be detected in these individuals with a DR beta cDNA probe. The association of this altered HindIII fragment with defective beta 4 chain expression suggested the possibility that the polymorphic fragment was derived from the DRB4 gene and might, therefore, be related to the defect in expression. However, detailed Southern blot analysis has now mapped the polymorphic fragment to the 3' end of the DRB1 gene, approximately 100 kb away from the defective DRB4 gene. Although the alteration in the DRB1 gene might involve sequences important in regulating the expression of the DRB4 gene, it is more likely that the association results from strong positive linkage disequilibrium between these DR beta chain genes.

Human allospecific TLCs generated against HLA antigens associated with DR1 through DRw8. II. Population analyses and blocking studies with monoclonal antibodies

Serologic, cellular, and molecular evidence supports the concept of extreme complexity within the HLA-D region. To study the complexity and fine specificity of the HLA-D region at the level of T-cell recognition, a panel of T-cell clones was generated against alloantigens associated with HLA-DR1 through -DRw8. After initial screening of more than 800 clones, 89 representative lines were selected for extensive testing against 204 unrelated stimulator cells. Clone-by-clone correlation analyses were performed to test whether any clones recognized similar or identical epitopes. In addition, clonal reactivity patterns were correlated with known HLA specificities. Twelve clusters of clones were identified with similar reactivity patterns using clone-by-clone correlation analysis. Some groups were significantly correlated with specificities associated with various D-region haplotypes; others had no significant correlation with any defined D-region specificity. Five general types of clones obtained in our study can be categorized as follows: Those recognizing epitopes clearly demonstrating a primary association with the classically defined D-region molecules against which the clones were primed. Clones recognizing epitopes associated with one of the priming antigens and also with another unrelated D-region specificity. Clones detecting epitopes which showed significant correlation with D-region molecules totally different from those against which they were originally primed. Clones with limited reactivity in population studies and no correlation with defined D-region molecules. Clones recognizing class I-associated epitopes.

Activation of human T lymphocytes. III. Triggering of bystander cytotoxicity in cytotoxic T cell clones by antibodies against the T3 antigen or by a calcium ionophore

The role of T cell differentiation antigens in antigen-specific and nonspecific cytotoxicity by human cytotoxic T lymphocyte (CTL) clones was investigated. In contrast to other reports, several monoclonal antibodies (mAb) against the T3 antigen only marginally blocked antigen-specific cytotoxicity at high concentrations but induced cytotoxicity against third party cells at concentrations from 10 to 0.001 micrograms/ml. Susceptibility to anti-T3-induced lysis was variable but was found with all target cells. Incubation of CTL with anti-T3 mAb even led to self-destruction of the CTL. The effect was independent of the presence of Fc receptors on the target cell and could be obtained with F(ab')2 fragments of the antibody as well. Only activated but not resting T cells could be induced to lyse by anti-T3. Furthermore, this type of bystander killing of target cells could also be induced by the Ca2+ ionophore A23187. Antibodies against the T8 differentiation antigen inhibited antigen-specific, oxidation-induced and anti-T3-induced cytotoxicity by T8+ CTL clones, whereas triggering by the ionophore A23187 was not inhibited. These results show that undirected killing can be triggered in CTL by activating a transducing molecule directly without involving the antigen receptor. Since this triggering of the lethal hit can still be inhibited by mAb against the T8 molecule, the T8 molecule probably has a regulatory role in a late phase of CTL triggering.

Change in functional phenotype of cloned human alloreactive cytolytic T cells

Alloreactive T cell clones primed in vivo were tested for the expression of T cell differentiation antigens CD2, CD3, CD4, and CD8. Each of 29 different clones were found to express CD2 and CD3, but were variable in their expression of CD4 (7 positive clones) and CD8 (15 positive clones). Six clones were positive for both CD4 and CD8. One of the 29 clones expressed neither CD4 or CD8. Over a period of 12-18 weeks of culture, these clones began to lose their alloreactivity but acquired NK-like activity. By changing the concentration of TCGF, the "allo" and "NK-like" lytic activities could be modulated. After 18 weeks of culture, these clones lost their alloreactive specificity, but not their NK activity. The expression of surface markers was unchanged. CD2 and CD3 molecules were determined to play a role in both the alloreactive and NK activity of these clones.

Reactivity patterns with HLA-A2 variants indicate lack of identity between determinants defined by monoclonal antibodies and cytotoxic-T-cell clones

The anti-HLA-A2 monoclonal antibodies (MoAb) CR11-351 and 4B inhibit the binding of each other to HLA-A2 lymphoid cells and block the cytotoxicity of the anti-HLA-A2 cytotoxic-T-cell clone R32. The blocking does not reflect reactivity of the MoAb CR11-351 and 4B and of the cytotoxic-T-cell clone R32 with the same determinant, since they display differential reactivity with four HLA-A2 variants which carry amino acid substitutions at different positions. These results show for the first time in the human system that Class I HLA variants represent useful reagents to compare the fine specificities of monoclonal antibodies and T-cell clones. Furthermore our data suggest that T-cell recognition depends upon the tertiary structure of the antigen.

A cloned human T cell line cytotoxic for autologous and allogeneic B lymphoma cells

A human cytotoxic T cell clone (MWS-14) with auto-tumor reactivity was established in serum-free medium in a mixed tumor cell culture by repetitive stimulation with fresh autologous lymphoma cells. This clone and its subclones are of the T3+ T4+ T8- phenotype. They were strongly cytotoxic for the autologous lymphoma cells, whereas autologous PHA blasts were not killed. Analysis of the specificity of MWS-14, MWS-14-30, and MWS-14-34 indicated that these CTL clones were cytotoxic for 7/7 allogeneic lymphoma cells, whereas only 3/23 of normal and non-lymphoma cells were lysed. Blocking studies with monoclonal antibodies directed at MHC class I and class II antigens showed that this preferential, anti-lymphoma reactivity was not directed at HLA determinants. The anti-lymphoma activity is not due to an aspecific susceptibility of the lymphoma cells to lysis. In contrast to CTL clones specific for HLA antigens present on the lymphoma cells, T3 and T4 were not involved in the cytotoxic reaction of MWS-14 against the autologous lymphoma cells. The reactivity of this clone could be blocked by a monoclonal antibody directed at leukocyte function-associated antigen. It can be concluded from these results that these T4+ CTL clones recognize a determinant, which is preferentially expressed on autologous and allogeneic lymphoma cells.

HLA class-II-specific T-lymphocyte clones with dual alloreactive functions

The relationship between T lymphocytes that proliferate in response to HLA class II antigens and those that mediate the cytotoxic response toward HLA class II target antigens was investigated. Alloreactive T-cell clones were derived under conditions in which the likelihood of clonality was high. Three populations of HLA class-II-specific T cells were identified. Two of these populations exhibited only HLA class-II-directed cytotoxicity or HLA class-II-induced proliferation. The third population of T cells exhibited both of these responses.

HLA class I specific T lymphocyte clones with dual alloreactive functions

Four human T lymphocyte clones exhibiting proliferative responses to class I HLA antigens were isolated from an in vitro mixed lymphocyte culture (MLC). Three clones expressed the Leu-2+3- phenotype and demonstrated proliferation in response to HLA-B8, while the fourth clone expressed the Leu-2-3+ phenotype and proliferated in response to HLA-A2. These clones were also cytotoxic towards cells bearing the same target antigens. Blocking studies utilizing monoclonal antibodies demonstrated that proliferation was triggered by determinants on the class I molecule itself, and these determinants appear to be spatially close to those which determine serologic allospecificity. These findings support the concept that the class I molecules themselves are the weak MLC stimulating determinants previously mapped to the HLA-A and B regions of the major histocompatibility complex.