Triggering of the lethal hit in human cytotoxic T lymphocytes: a functional role for a 103-kDa T cell-specific activation antigen

'Nonspecific' MHC-unrestricted killer cells and their receptors

The receptors involved in apparently nonspecific, MHC- unrestricted effector cell-target cell interaction and lysis continue to raise controversy. They bind to distinct ligands on their target cells and activate diverse cellular functions such as gene expression, lymphokine production, proliferation and/or cytolytic activity by the effector cells. Several distinct receptors may mediate MHC-unrestricted cytotoxicity. Here, Peter Hersey and Reinder Bolhuis review evidence that the four main receptors involved in triggering this form of lytic activity are the CD2 molecule (the sheep red cell receptor), CD3-associated αβ chain T-cell receptor (TCR), the γδ chain TCR-CD3 complex and the CD16 molecule (the IgG0Fc receptor). The apparent non-specificity specificity of killing is a reflection of the widespread expression of natural ligands for these receptors on target cells.

Inhibition of human leukocyte function, alanyl aminopeptidase (APN, CD13) and dipeptidylpeptidase IV (DP IV, CD26) enzymatic activities by aqueous extracts of Cistus incanus L. ssp. incanus

Short-boiled aqueous extract from leaves of Cistus incanus L. ssp. incanus (CIT) dose-dependently inhibit the enzymatic activities of both alanyl aminopeptidase (APN, CD13, EC 3.4.11.2) and dipeptidylpeptidase IV (DP IV, CD26, EC 3.4.14.5). This inhibition is not reversible and very likely results from a covalent binding of reactive compounds to the enzymes. Furthermore, we show that aqueous CIT extracts decrease the DNA-synthesis of human T cells and mononuclear cells and inhibit the proliferation rate of the human T cell line KARPAS-299 in a dose-dependent manner. Data are presented suggesting that the antiproliferative effects of CIT extracts are due to their strong cytotoxic activity.

Molecular analysis of CD26-mediated signal transduction in T cells

CD26 or dipeptidylpeptidase IV (DPP IV) is a cell surface protease involved in T cell activation. It is a type II transmembrane glycoprotein consisting of a large extracellular part, a single transmembrane region and a short cytoplasmic tail without any common signalling motifs. To eluciate the mechanisms involved in CD26-mediated signalling we have constructed C-terminal deletion mutants of the human CD26 molecule and transfected them into murine T cell hybridomas. Stimulation experiments show that most of the extracellular part of CD26 can be deleted without affecting its costimulatory activity. The membrane proximal glycosylation rich region of CD26 is sufficient to transduce costimulatory signals. Activation of T cells via CD26, however, is not mediated by the important T cell receptor associated adaptor proteins LAT and TRIM as shown in colocalization assays.

The adenosine deaminase-binding region is distinct from major anti-CD26 mAb epitopes on the human dipeptidyl peptidase IV(CD26) molecule

CD26 or dipeptidyl peptidase IV (DPP-IV) is a cell surface protease involved in T cell activation. Monoclonal antibodies (mAbs) directed against the CD26 molecule are able to stimulate CD26-expressing T cells. Although many different CD26-specific mAbs exist which are able to provide a triggering signal in T cells, little is known about their specific epitopes on the CD26 molecule. Whereas some mAbs were shown to compete with each other and to inhibit the association of adenosine deaminase (ADA) and human immunodeficiency virus 1 (HIV-1)-derived Tat protein with CD26, other CD26-specific mAbs obviously bind to distinct regions on DPP-IV. In the present study we have generated truncated versions of the human CD26 molecule and expressed them in COS-1 cells to study the binding pattern of a panel of 14 CD26-specific mAbs in confocal microscopy and, thus, correlated the CD26-specific mAbs epitopes with the binding region of ADA. We show that the majority of anti-CD26 mAbs is directed against the glycosylation-rich region of the molecule whereas the ADA-binding site could be located in the cysteine-rich region of DPP-IV. In contrast to binding experiments with purified ADA, which revealed a specific association with CD26 on CD26-positive Jurkat cells, HIV-derived Tat protein did not interact specifically with CD26 on transfected Jurkat cells, nor could Tat binding be competed by anti-CD26-specific mAbs.

Dipeptidyl-peptidase IV/CD26 on T cells: analysis of an alternative T-cell activation pathway

CD26 is a proteolytic enzyme (dipeptidyl-peptidase IV) with a wide tissue distribution and a unique specificity that was already described 27 years ago. CD26 is expressed on a fraction of resting T cells at low density but is strongly upregulated following T-cell activation. Recent results indicate that CD26 is a multifunctional molecule that may have important functions on T cells and in the immune system. It is associated with molecules of immunological importance such as the protein tyrosine phosphatase CD45 and adenosine deaminase (ADA) on the cell surface. Synthetic inhibitors of the enzymatic activity of CD26 have been shown to suppress certain immune reactions in vitro and in vivo. An interesting feature of CD26 is its ability to transmit a transmembrane signal to trigger functional programs in T cells. This triggering requires crosslinking of CD26 on a cell membrane. The enzymatic activity of CD26 is not obligatory for the activation of T cells via CD26. Since CD26 is a type II membrane protein with only six intracellular amino acids, it must deliver its signal via a signal-transducing molecule. Signaling is dependent on the expression of the T-cell receptor (TCR) complex with a special need for a functional zeta-chain. In this context the zeta-chain of the TCR complex is required for CD26-mediated signaling but, in contrast to other co-stimulatory molecules such as the CD2 molecule, is not sufficient for triggering the T cell.

Expression of CD26/dipeptidyl peptidase IV in adult T cell leukemia/lymphoma (ATLL)

The association of CD26/dipeptidyl peptidase IV (DPPIV) and human T lymphotropic virus type I (HTLV-I) was studied by two approaches. First, we examined the expression of CD26 in peripheral blood mononuclear cells (PBMC) from the patients with adult T cell leukemia/lymphoma (ATLL), an HTLV-I-related malignancy. The expression of CD26 on the surface of PBMC was decreased in all 20 patients with ATLL compared with those from normal individuals (P < 0.01) and the expression of the CD26 gene transcript was not detectable in seven out of eight patients with ATLL. Then we compared the quantity of viral DNA in CD26-negative (CD26-) and CD26-positive (CD26+) cells obtained from 17 HTLV-I healthy carries by using a polymerase chain reaction method. The CD26-cells had a higher copy number of viral DNA than CD26+ cells. These findings indicate that HTLV-I has in vivo tropism to CD26- cells, suggesting that some phenotypes of ATLL cells reflect the in vivo cellular tropism of HTLV-I.

CD26: a surface protease involved in T-cell activation

CD26 is a proteolytic enzyme (dipeptidylpeptidase IV) with a wide tissue distribution and a unique specificity. Recent developments indicate that CD26 is a multifunctional molecule that may have important functions in the immune system. Here, Bernhard Fleischer reviews the current knowledge of CD26 and discusses the possible functions of this molecule in T lymphocytes.

Enzymatic activity of CD26 (dipeptidylpeptidase IV) is not required for its signalling function in T cells

CD26 is a proteolytic enzyme (dipeptidylpeptidase IV) expressed on the T cell surface that defines an alternative activation signal for human T lymphocytes. Crosslinking of CD26 via monoclonal antibodies triggers proliferation and cytotoxicity in preactivated T cells. In this study, we used highly specific competitive and irreversible inhibitors of dipeptidylpeptidase IV to study the role of the enzymatic activity in activation of CD26-transfected T cells as well as of CD26-expressing normal human T cell clones. These inhibitors at concentrations that blocked up to 95% of the enzymatic activity, did not specifically inhibit T cell activation neither via TCR/CD3 nor via CD26 itself. This demonstrates that the enzymatic activity of CD26 is not required for its T cell activating properties.

Anti-CD26 monoclonal antibodies can reversibly arrest human T lymphocytes in the late G1 phase of the cell cycle

Three different anti-CD26 monoclonal antibodies (mAbs) are described, which specifically inhibited proliferation of human T lymphocytes after stimulation with PHA, tetanus toxoid or soluble anti-CD3 mAb. Anti-CD26 mAbs induced in T cells a dose-dependent shift of the maximum of DNA synthesis, which was due to a transitory arrest of cells in the cell cycle. This cell cycle arrest was found to occur in the late phase of G1 since the expression of the T cell activation markers CD25-, CD71-, and HLA-DR-positive cells was the same in anti-CD26 mAb-containing and control cultures. Propidium iodide staining further confirmed the assumption that the arrest occurs in the first round of the cell cycle before S phase cells were detectable. Because the cells were arrested before consuming IL-2 in the S phase, we observed in accumulation of IL-2 in anti-CD26 mAb-containing cultures, whereas IFN-gamma production by PHA-activated T lymphocytes was reduced. These data indicate that CD26 is involved in the processes of T cell activation and proliferation.

Expression of CD26 (dipeptidyl peptidase IV) on memory and naive T lymphocytes

It has been suggested that human CD26-positive T lymphocytes represent the memory pool of the cellular immune system. For proof of this suggestion we analysed the responsiveness of CD26-positive and CD26-negative T lymphocytes after antigenic stimulation in limiting dilution experiments. After stimulation with tetanus toxoid (TT) the number of proliferating cells within the CD26-positive subset was two- to sixfold higher than that within the CD26-negative subset. These differences in responsiveness were also detectable between CD4+/CD26+ and CD4+/CD26- T cells. To further investigate the memory character of the cells, human peripheral blood mononuclear cells were stimulated with TT for 7 or 14 days. Thereafter, CD26+ and CD26- T cells were isolated and restimulated with TT in limiting dilution experiments. Responding cells were found not only within the CD26-positive subset but also within the CD26-negative subset, and their number increased with time. Surface marker analysis of freshly isolated human T lymphocytes or T-cell clones indicated that CD26 is not a stable cell surface marker. Furthermore, CD26 is both absent and present on CD29-positive or CD45RA-positive cells, indicating no association of CD26 with surface markers for memory or naive T cells, respectively. These results strongly argue against the hypothesis that CD26-positive T cells represent the memory pool. We conclude that CD26 is an activation marker of T lymphocytes, which is associated with reactivity on naive and memory cells.

Expression of CD26 (dipeptidyl peptidase IV) on resting and activated human T-lymphocytes

CD26 is an activation antigen which is expressed on the surface of human T-lymphocytes. It has been characterized to be the dipeptidyl peptidase IV (DPP IV). Considerable amounts of CD26 are already present on resting T-lymphocytes. The expression of CD26 is enhanced by T-cell mitogens or antigens. A correlation of CD26 expression and of enhanced enzymatic activity was observed after T-cell activation. Our data indicate that not only the immunoreactivity, but also the enzymatic activity of CD26 are detectable on the cell surface. In addition, de novo expression of CD26 was demonstrated on CD26-negative T-cells after mitogenic or antigenic stimulation. CD26 expression is initiated during the G1 phase of the cell cycle. The expression occurs nearly simultaneously with HLA-DR, but later than CD25. Similar to CD25 and HLA-DR, CD26 is not a permanent marker on the surface of T-lymphocytes, but is down-regulated after 7 days of culture. When testing the influence of interleukin 1, interleukin 2, tumour necrosis factor, and interferon-gamma on the expression of CD26, no effect was found on unstimulated or on mitogen-stimulated T-lymphocytes. The binding of two different monoclonal antibodies against CD26 (anti-DPP IV and anti-Tal) to resting and activated T-lymphocytes revealed a different pattern of immunoreactivity. Resting T-lymphocytes reacted stronger with anti-DPP IV than with anti-Tal. However, binding of the two monoclonal antibodies to T-cell blasts did not show significant differences. These data indicate that CD26 may be expressed in differently modulated configurations on the surface of T-cells, which may be associated with a distinct status of activation and/or function.

Dipeptidyl peptidase IV in the immune system. Cytofluorometric evidence for induction of the enzyme on activated T lymphocytes

Dipeptidyl peptidase IV (DP IV) is a membrane peptidase with essential functional significance in thymus derived lymphocytes. This conclusion is drawn from 1) the induction of this enzyme after stimulation of T lymphocytes in vitro and 2) the impairment of T cell functions in presence of active site-specific inhibitors of the enzyme. The first item will be addressed in this paper, whereas the second one will be treated in a forthcoming article. Using flow cytofluorometry we investigated the expression of dipeptidyl peptidase IV on activated lymphocytes and the phenotype of lymphocytes expressing this enzyme. After stimulation by mitogenic lectins the number of epitopes on the cell surface binding polyclonal antibodies against DP IV increases 4 to 6 times. By means of double fluorescence staining the enzyme has been shown to be restricted nearly exclusively to T lymphocytes even after mitogenic stimulation. The highest density of DP IV epitopes has been found in cells coexpressing activation markers like receptors for interleukin 2 or transferrin in a high density.

FcR-mediated crosslinking of Ta1 (CDw26) induces human T lymphocyte activation

Ta1 (CDw26) is a 105-kDa glycoprotein of unknown function whose expression on human T lymphocytes is strongly correlated with activation and proliferation. The subset of peripheral blood T cells expressing Ta1 includes the principal responsive population to proliferative stimulation by recall antigens as well as monoclonal antibodies directed to the CD3/T cell receptor complex and the CD2 (T11) molecule. We now show that the Ta1 molecule is itself an alternate mediator of human T lymphocyte activation. T cell clones were induced to proliferate and exert their cytolytic effector function by anti-Ta1 monoclonal antibodies in the presence of Fc-receptor-positive accessory or target cells. Resting T cells from peripheral blood were also activated to proliferate by anti-Ta1, but only if both Fc-receptor-positive accessory cells and exogenous IL-2 were present. Anti-Ta1 antibodies induced increased expression of IL-2 receptors on purified T cells under these conditions. Activation via Ta1 was shown to be functionally interconnected to CD3/T cell receptor activation mechanisms, because modulation of the CD3/T cell receptor complex inhibited anti-Ta1-mediated cytolysis without affecting Ta1 surface expression. While demonstrating that the CDw26 antigen-mediated pathway of activation is not dependent on one unique epitope, our results suggest that the Ta1 glycoprotein can mediate T cell activation directly, suggesting that it may be associated with an important cellular component of the human T cell regulatory network.

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)

Agonistic effects of anti-CD2 and anti-CD16 antibodies on human natural killer killing

Two monoclonal antibodies (MoAb), 9-1 (anti-CD2) and 3G8 (anti-CD16), were previously shown to enhance the cytotoxic activity of human natural killer (NK) cells. The present study examined the effect of 9-1 and 3G8 with different effector and target cells to determine whether they activate NK cells through a common mechanism. Analysis of purified lymphocyte subpopulations demonstrated that the CD3+CD16+CD3- NK effector cell population is enhanced by both antibodies, while purified CD2+CD16-CD3+ T cells are not activated by either antibody. Although both antibodies enhance killing of K-562 and Daudi, killing of T-cell lines is enhanced by 9-1 and inhibited by 3G8. In contrast, killing of the promyelocytic cell line, U-937 is inhibited by 9-1 and enhanced by 3G8. On NK-susceptible cells the pattern of enhancement with 3G8, an IgG1 MoAb, is consistent with the pattern of target cell expression of an Fc receptor, FcR II, known to bind IgG1 antibodies. This suggests that 3G8 may cross-link effector and target cells through CD16 on the effectors and FcR II on these targets. This could activate NK killing by a mechanism similar to antibody-dependent cellular cytotoxicity reactions (ADCC) with the MoAb in the reverse orientation. The failure of 3G8 F(ab')2 fragments to enhance NK killing, further supports the reverse ADCC mechanism of enhancement by 3G8. The pattern of enhancement mediated by 9-1, an IgG3 MoAb, is not correlated with any target cell Fc-receptor known to bind IgG3 MoAb. The effect of 9-1 may result, instead, from its binding to the unique 9-1 epitope on the CD2 molecule involved in CD2-mediated T-cell activation, as previously described. Alternative mechanisms, including activation of NK killing by 9-1 mediated cross-linking of CD2 and CD16 on the effector cells, have also been discussed.

Very late activation antigen on synovial fluid T cells from patients with rheumatoid arthritis and other rheumatic diseases

We used flow cytometry and immunoprecipitation techniques to study the expression of the activation molecules transferrin receptor, interleukin-2 receptor, HLA-DR, and very late activation antigen 1 (VLA-1) on purified T lymphocytes from peripheral blood and synovial fluid of 9 patients with rheumatoid arthritis and 7 patients with other rheumatic diseases. We found a T cell subset bearing VLA-1 in synovial fluid from 8 rheumatoid arthritis patients and 4 patients with other rheumatic diseases. VLA-1 was not found in peripheral blood T lymphocytes from either group.

In vivo localization of a bispecific antibody which targets human T lymphocytes to lyse human colon cancer cells

A bispecific MAb was derived from the fusion of a hybridoma producing MAb CD3 with a hybridoma producing MAb L-DI (which is directed against a 41-kDa glycoprotein expressed in most gastro-intestinal and pancreatic carcinomas). Bispecific antibody molecules were isolated from parental antibody molecules by the use of hydroxylapatite-HPLC and shown to target human cytolytic T lymphocytes, irrespective of their original specificity, to specifically lyse human colon carcinoma cells. Localization studies in vivo using nude mice bearing human colon carcinoma xenografts showed significant accumulation of the HPLC-purified 125I-labelled bispecific antibodies into the tumor compared to 131I-labelled control CD3 antibody.

Contingent genetic regulatory events in T lymphocyte activation

Interaction of antigen in the proper histocompatibility context with the T lymphocyte antigen receptor leads to an orderly series of events resulting in morphologic change, proliferation, and the acquisition of immunologic function. In most T lymphocytes two signals are required to initiate this process, one supplied by the antigen receptor and the other by accessory cells or agents that activate protein kinase C. Recently, DNA sequences have been identified that act as response elements for one or the other of the two signals, but do not respond to both signals. The fact that these sequences lie within the control regions of the same genes suggests that signals originating from separate cell membrane receptors are integrated at the level of the responsive gene. The view is put forth that these signals initiate a contingent series of gene activations that bring about proliferation and impart immunologic function.

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.

Triggering of T cell proliferation through AIM, an activation inducer molecule expressed on activated human lymphocytes

In this report, we describe a novel activation antigen that appears very early after T cell activation and is absent in resting lymphocytes, through which agonistic proliferative signals can be triggered by mAb binding. It has been designated as activation inducer molecule (AIM) and is a disulphide-linked heterodimeric structure containing two polypeptide chains of Mr 33,000 and 27,000. The expression of AIM can be induced by different activation stimuli such as PMA, PHA, or anti-CD3 mAb, but not by the Ca2+ ionophore A23187, and it precedes the expression of other activation molecules such as 4F2 or the IL-2-R. Once AIM antigens are expressed on lymphocytes after stimulation with submitogenic doses of PMA, the binding of anti-AIM mAbs triggers a strong proliferative response. Furthermore, a comitogenic effect of the anti-AIM mAbs is exerted in the presence of either PHA or anti-CD3 mAb. The activation of lymphocytes through AIM antigens induces both IL-2 and IL-2-R receptor synthesis and is inhibited by anti-IL-2-R mAbs.

Lysis of tumor cells by the retargeting of murine cytolytic T lymphocytes with bispecific antibodies

The specificity and efficiency of tumor-cell lysis by the retargeting of murine cytolytic lymphocytes (CTL) with bispecific antibodies was examined. Bispecific antibodies (also known as heteroaggregated or hybrid antibodies) were produced by the chemical coupling of monoclonal antibodies (MAbs) against H-2 antigens and the murine T-cell receptor (TCR). Murine tumor cell lines which expressed on their plasma membrane an antigen reactive with one component of the bispecific antibody were efficiently lysed in the presence of polyclonal murine CTL. CTL capable of lysis were generated by stimulating spleen or lymph-node cells with ConA and IL-2, while unstimulated cells or cells incubated only with IL-2 showed no lysis of target cells with bispecific antibodies. Furthermore, the lysis of target cells by bispecific antibodies and CTL did not cause the lysis of bystander cells (cells not expressing an antigen recognized by the antibody which are mixed with the target cells). The efficient CTL-mediated lysis observed with these antibodies makes this a promising approach for the immunotherapy of human cancer.

Transfection of the CD8 gene enhances T-cell recognition

Antibodies against CD8 or CD4 antigens can prevent T-cell functions induced by T-cell targets. As CD8 or CD4 antibodies can also initiate negative signals in T cells in the absence of appropriate targets it is not clear whether CD8 and CD4 molecules are directly involved in the interaction of T cells with their targets. In previous experiments we have introduced the T-cell receptor alpha- and beta-chain genes from a CD8-positive cytolytic T cell specific for the antigen fluorescein (FL) and the H-2D molecule of the major histocompatibility complex (MHC) into a CD8-negative recipient cell. The CD8-positive donor cell lysed both FL-conjugated fibroblasts and lymphoblasts, which express relatively high and low amounts of H-2D molecules, respectively. In contrast the CD8-negative transfectant lysed FL-conjugated fibroblasts only. Here we show that recognition of FL-conjugated lymphoblasts by the transfectant is enhanced by supertransfecting it with the CD8 gene.

The use of hybrid hybridomas to target human cytotoxic T lymphocytes

This study describes a general strategy to produce hybrid monoclonal antibodies that are capable of targeting human cytotoxic T lymphocytes (CTL) against any cell carrying the appropriate target antigen. This is done by fusing a HAT-sensitive, G418-resistant anti-T3 hybridoma with immune spleen cells (or with other hybridomas) that produce antibodies against the desired target antigen. In the hybrid hybridomas the reassortment of Ig heavy and light chains results in the production of bifunctional antibody molecules. Because of their double specificity, these antibodies are able to bridge human CTL to target cells and trigger cytotoxic function. We have isolated several stable hybrid hybridomas in which one specificity is against T3 and one either against HLA antigens (class II, DC-1, A3), human Ig (IgM, IgE, kappa), Toxoplasma gondii or an ovary carcinoma-associated antigen. In all of these cases we show that culture supernatants can efficiently and specifically target any CTL clone against any target cell that possesses the relevant surface antigen recognized by the antibody. Furthermore, the killing requires as little as 0.1 ng/ml of antibody, occurs at effector to target ratios comparable to those used in conventional cytotoxic assays and does not affect bystander cells. Nonspecific killing of Fc receptor-positive cells can be avoided using F(ab')2 fragments. As an example, we show that it is possible to change the major histocompatibility complex class and allospecificity of a CTL clone and target CTL against non-major histocompatibility complex antigens such as Ig, parasites and tumor-associated antigens.

Lysis of bystander target cells after triggering of human cytotoxic T lymphocytes

Human cloned cytotoxic T lymphocytes (CTL) specific for class I HLA antigens were used to investigate whether triggering of CTL leads to killing of innocent bystander target cells. After triggering of CTL by their specific target cells, lysis of bystander targets was detected in a 7-h cytotoxicity assay. Considerable differences were found in the susceptibility of various target cells to this type of lysis. Targets susceptible to this bystander lysis were also susceptible to lysis by CTL triggered by F(ab')2 fragments of an anti-T3 monoclonal antibody, whereas other targets were resistant to both types of cytotoxicity. Triggering of CTL by oxidized target cells or via a T3-independent activation pathway led to bystander lysis detectable already after 4 h. Bystander lysis was considerably enhanced under conditions that facilitated a non-specific cell contact between CTL and bystander target. We conclude that a function besides antigen recognition of the T cell receptor on CTL is to direct killing to the target cell. This directing, however, is incomplete and destruction of innocent bystanders can be detected under appropriate conditions.