Inhibition of lymphocyte proliferation by monoclonal antibody directed against the T3 antigen on human T cells

Cell markers and determinants in fish immunology

Despite the impressive increase in the cloning and expression of genes encoding fish immunoregulatory molecules, the knowledge on "in vivo" and "in vitro" functional immunology of the corresponding peptide products is still at an initial stage. This is partly due to the lacking of specific markers for immunoregulatory peptides, that represent an indispensible tool to dissect immune reactions and to trace the fate of cellular events downstream of the activation. In this review we summarise the available information on functional immune activities of some teleost species and discuss the obtained data in an evolutionary and applied context.

Down-regulation of T-cell proliferation in response to soluble anti-CD3 antibodies through development of redirected cytolytic activity eliminating costimulatory cells

CD4+ T-depleted spleen cells (CD8+ T cells) activated by anti-CD3 antibodies (aCD3) suppressed proliferation of CD8+ T-depleted spleen cells (CD4+ T cells) and fresh normal T cells in response to aCD3. Antigen-nonspecific cytolytic activity was induced in splenic CD8+ T cells by stimulation with aCD3 and showed the peak level on day 3, whereas cytolytic activity induced in CD4+ T cells was weak. Intact Ig but not F(ab')2 of aCD3 induced and mediated cytolytic activity. Correspondingly the cytolytic activity induced by aCD3 was directed against target cells bearing Ig-binding Fc-receptor activity and cytolysis was inhibited by the addition of free Ig into the assay system. We showed that aCD3-activated T cells carried a high level of aCD3 on their surface at the time after the peak proliferation when they attained high cytolytic activity. This raised the possibility that the anti-CD3-induced aCD3-redirected cytolytic activity eliminated Fc-receptor-bearing costimulatory cells in the culture for down-regulation of the T-cell proliferation. This view was supported by partial restoration of anti-CD3-induced low responsiveness of CD8+ T cells by the addition of fresh costimulatory cells. These results suggested a new pathway of down-regulation of T-cell proliferation by aCD3-activated cytolytic CD8+ T cells.

Modulatory effects of plasma and serum on T lymphocyte activation: distinctive patterns for different mitogens

Published reports have shown that fresh plasma, but not cryoprecipitate-depleted plasma or fresh serum, inhibits T cell activation by phytohemagglutinin (PHA). We sought to determine if this pattern of inhibition also characterized T cell responses to mitogens differing from PHA with regard to the cell surface molecules utilized for signal transduction. The activation system included colchicine, which limits the cells to one round of division, masking stimulatory factors that enhance proliferation by boosting the number of divisions per culture period. A distinctive modulatory pattern characterized T cell responses to each of 4 mitogens tested (PHA, anti-CD3 monoclonal antibody, desialyzed oxidized erythrocytes (DOE), pokeweed mitogen). Enhanced proliferative responses to anti-CD3 and DOE were observed in the presence of serum, and reflected an increased percentage of T cells expressing CD25. These findings suggest that concerns regarding a negative impact of plasma components on T cell responsiveness, when based on results from PHA-induced activations systems, may be unwarranted.

Monoclonal antibodies in renal transplantation: a review

OKT3 is the first anti-CD3 monoclonal antibody available for treatment in humans. Over the last few years it has proven to be a very powerful immunosuppressive agent in renal transplantation. Clinical studies have shown that OKT3 is superior to high-dose steroids as first-line treatment for acute renal allograft rejection. Furthermore, it is comparable to antithymocyte globulin (ATG) in treating steroid-resistant rejection and is also effective as rescue treatment in ATG- and antilymphocyte globulin-(ALG-) resistant rejection. Despite its excellent rejection-reversal rate, OKT3 treatment is followed by a substantial percentage of re-rejections, most of which respond well to steroids. In the early post-transplantation period, a prophylactic course of OKT3 is very effective in preventing acute rejections, and in this respect it is probably equivalent to ATG. Indirect evidence exists that a prophylactic course of OKT3 may be beneficial in immunologically high-risk patients and in patients with delayed graft function. However, more clinical studies are required to answer the question whether OKT3 should be given as induction treatment, as first-line treatment, or as rescue treatment. To answer this question, the side effects of OKT3 should also be taken into account. First-dose-related side effects, although frequent and disturbing, are usually transient and seldom life-threatening, provided overhydration has been corrected and steroids have been given before the first administration. These side effects are attributed to the release of cytokines as a result of T-cell activation or lysis. After exposure of patients to OKT3 an increased incidence of infections and malignancies has been reported. However, it is not yet clear whether this is due to OKT3 as such, or whether it merely reflects the total burden of immunosuppression. Xeno-sensitization represents an important limitation to OKT3 treatment, although a second or third course can still be effective in patients with low antibody titers. The precise immunosuppressive mechanism of anti-CD3 monoclonal antibodies is yet unknown. Monitoring of patients treated with OKT3 revealed CD3 and/or T-cell antigen receptor depletion and immunological incompetence of remaining T cells. More clinical data are required to establish the correct dose and duration of OKT3 treatment. In conclusion, OKT3 is a powerful immunosuppressive agent but its real value in renal transplantation remains to be determined. A practical approach may be to reserve it for the treatment of steroid-resistant rejections.(ABSTRACT TRUNCATED AT 400 WORDS)

CD3-T cell receptor modulation is selectively induced in CD8 but not CD4 lymphocytes cultured in agar

The CD3-T cell receptor (TcR) complex is central to the immune response. Upon binding by specific ligands, internalized CD3-TcR molecules increase, and either T cell response or unresponsiveness may ensue depending on the triggering conditions. Using semi-solid agar culture, we have shown previously that quiescent CD4 but not CD8 lymphocytes generate clonal colonies under phytohaemagglutinin stimulation. Here we have demonstrated that the agar induces selective CD3-TcR modulation in the CD8 and not in the CD4 subset. CD8 lymphocytes preactivated in liquid culture and recultured in agar with exogenous recombinant interleukin-2 generate colonies with a modulated CD3-TcR surface expression. The peptides composing the CD3-TcR complex are synthesized in CD8 colonies as well as in CD4; however, the CD3 gamma chain is phosphorylated at a higher level in CD8 colonies. A component of the agar polymer, absent in agarose, appears to be the ligand that induces differential CD3-TcR modulation in the CD8 subset. In contrast to agar culture, CD8 colonies can be derived from quiescent CD8 lymphocytes in agarose. These CD8 colonies express unmodulated CD-TcR. CD3-TcR modulation with anti-CD3 monoclonal antibody prior to culturing in agarose inhibits the colony formation. We conclude that given triggering conditions can result in both CD3-TcR modulation and inhibition of the proliferative response selectively in the CD8 lymphocyte subset and not in the CD4.

HLA-B8,DR3 phenotype and lymphocyte responses to phytohaemagglutinin

Several reports have shown that HLA-B8,DR3 positive subjects may display some changes in immune parameters when compared with HLA-B8,DR3 negative ones and are prone to develop several immunological diseases. In the present study we have analysed the proliferative response to phytohaemagglutin (PHA) in HLA-typed healthy subjects. A twin method was also employed to assess the role of genetic and environmental factors in the regulation of the response to the mitogen. It was not possible to demonstrate any difference in proliferative response to optimal doses of PHA between groups of subjects carrying or not carrying the HLA-B8,DR3 phenotype. When suboptimal responses were studied, however, the results showed that lymphocyte responses were significantly decreased in HLA-B8,DR3 positive subjects compared with the negative ones. Moreover, the experiments performed with twins demonstrated that environmental factors were more important than genetic factors in the proliferative response to mitogen. The fact that the HLA-B8,DR3 phenotype affects the suboptimal response to PHA although environmental factors are more important than genetic factors in the response to the mitogen seems of some interest. However, these results could be consistent with the high incidence of autoimmune disorders among HLA-B8,DR3 positive individuals.

Peripheral T-lymphocyte activation by human T-cell leukemia virus type I interferes with the CD2 but not with the CD3/TCR pathway

Human T-cell leukemia virus type I (HTLV-I) is etiologically associated with adult T-cell leukemia, an aggressive lymphoproliferative disorder, and with chronic neurological diseases. In vitro it can infect several types of cells but transforms only human T lymphocytes. We have previously shown that HTLV-I viral particles, even when noninfectious, were able to activate human resting T lymphocytes, suggesting that this activation step may be important in the initiation of the lymphoproliferative process. In the present study, we first demonstrate that in contrast to other mitogenic stimuli, HTLV-I has the unique property to activate human resting T cells in the absence of accessory cells. We then investigate the relationship between HTLV-I-induced T-cell activation and the classical well-known pathways of activation, namely, the CD3/TCR and CD2 pathways. Competitive blocking experiments were performed in which the effects of monoclonal antibodies (MAb) to the CD3/TCR complex or to the CD2 molecule were evaluated on the HTLV-I activation of T cells and compared with that obtained on phytohemagglutinin (PHA)-stimulated cells. It was found that anti-CD3 or -TCR MAb strongly suppress the proliferative response of T cells to PHA, but are significantly less efficient in inhibiting the activation initiated by HTLV-I. By contrast, MAb recognizing specific epitopes of the CD2 molecule inhibit the proliferative response of T cells to PHA or to HTLV-I to the same extent. The results provide evidence that HTLV-I virions interfere mainly with activation via CD2 but not via the CD3/TCR complex. Considering the earlier expression of the CD2 molecule on human T-cell precursors, these observations might be relevant to the characterization of the differentiation stage at which viral infection could interfere with the development and the maturation of T lymphocytes.

Requirement for both the CD3/T cell receptor complex and the CD2/lymphocyte function-associated antigen-3 adhesion system in monocyte-independent T cell activation by oxidized erythrocytes

Recent reports have shown that human T lymphocytes can be activated in vitro by desialyzed oxidized erythrocytes (DOE) in a monocyte-independent fashion. These findings suggest that, in addition to providing the activating stimulus, DOE may also fulfill accessory cell roles normally supplied by monocytes in T cell activation. DOE may thus be a useful substitute for antigen-presenting accessory cells in studies of T cell activation. This report describes experiments conducted to determine if T cell activation by DOE involves cell surface structures known to play a role in antigen-induced T cell activation. T cell stimulation by DOE was blocked by anti-CD3 monoclonal antibody, suggesting that perturbation of the CD3/T cell antigen receptor complex on T cells is required for activation. Activation was also blocked by anti-CD2 monoclonal antibody, indicating involvement of the CD2/LFA-3 adhesion system. Since DOE and monocytes both express LFA-3, the role of LFA-3 on DOE in T cell activation was assessed. DOE pretreated with anti-LFA-3 monoclonal antibody induced markedly lower levels of T cell proliferation. As further evidence for LFA-3 involvement, DOE prepared from 5 patients with paroxysmal nocturnal hemoglobinuria (PNH), known to exhibit deficient LFA-3 expression, were also deficient in T-cell stimulatory activity. These findings illustrate that T cell activation by DOE requires many of the surface structures involved in activation by antigen-bearing accessory cells, and may thus serve as a useful model for studies of T cell activation.

A functionally unique anti-CD3 monoclonal antibody cross-reactive with basal keratinocytes

Monoclonal antibodies (mAb) 147, 446, and 454 each recognize different epitopes of CD3. The CD3 epitope recognized by mAb 446 is functionally unique for the T cell. In contrast to mAb 147 and 454, mAb 446 induces modulation of surface CD3 with delayed kinetics and, hence, is impaired in inducing a refractory state in the T cell to subsequent anti-CD3-induced helper function. MAb 446 (but not other anti-CD3 mAb, including mAb 147, 454, OKT3, and anti-Leu4) recognizes a cytoplasmic determinant within basal keratinocytes. Extraction of keratinocytes with nonionic detergent and 2 M NaCl abolished subsequent staining with mAb 446 but enhanced subsequent staining with anti-keratin mAb, suggesting that this cross-reactive determinant is not keratin. Immunoprecipitation of internally labeled keratinocytes with the anti-CD3 mAb 147 and 446 failed to reveal specific bands, whereas these same mAb immunoprecipitated specific bands from internally labeled CD3+ Jurkat cells corresponding to previously identified CD3 subunits, suggesting that the cross-reactive determinant in keratinocytes is also not CD3. The cross-reactivity is not species specific, in that mAb 446 stained a mouse keratinocyte line, nor is it absolutely keratinocyte specific, in that mAb 446 stained one of the two nonkeratinocyte human epithelial cell lines tested. This study raises the possibility that perturbation of unique CD3 epitopes may have unique effects on T cell surface events and subsequent activation and function.

The anti-T cell monoclonal antibody 9.3 (anti-CD28) provides a helper signal and bypasses the need for accessory cells in T cell activation with immobilized anti-CD3 and mitogens

CD28 is an antigen of 44 kDa which is expressed on the membrane of the majority of human T cells. The present study examines the functional effects of an anti-CD28 monoclonal antibody (mAb 9.3) on T cell activation induced with immobilized anti-CD3 mAb OKT3 or with mitogens, in the absence of accessory cells. To this end, we used blood resting T cells that were completely depleted of accessory cells (monocytes, B cells, and natural killer cells), and consequently did not respond to recombinant interleukin-2 (rIL-2), to immobilized OKT3, to PHA, or to Con A. Addition of mAb 9.3 to the cultures enhanced IL-2 receptor expression (Tac antigen) on PHA- or immobilized OKT3-stimulated T cells and induced IL-2 receptors on Con A-stimulated T cells. Moreover, addition of mAb 9.3 to cultures of T cells stimulated with PHA, Con A, or immobilized OKT3 resulted in IL-2 production. Soluble mAb 9.3 was a sufficient helper signal for T cell proliferation in response to PHA or immobilized OKT3. Crosslinking of mAb 9.3 by culture on anti-mouse IgG-coated plates enhanced the helper effect and was an essential requirement for the induction of T cell proliferation in response to Con A. No other anti-T cell mAb (anti-CD2, -CD4, -CD5, -CD7, -CD8) was found to provide a complete accessory signal for PHA or Con A stimulation of purified T cells. T cell proliferation induced by the combination of PHA and mAb 9.3 was strongly inhibited by the anti-IL-2 receptor mAb anti-Tac. In conclusion, mAb 9.3 can provide a signal bypassing monocyte requirement in T cell activation with immobilized OKT3, PHA, and Con A, resulting in an autocrine IL-2-dependent pathway of proliferation.

Differential immunomodulation by anti-CD2 monoclonal antibodies of anti-CD3-induced T cell activation: dependence upon the individual anti-CD3 monoclonal antibody used for activation

Two monoclonal antibodies (mAb) recognizing different CD2 epitopes each inhibited anti-CD3-induced proliferation and anti-CD3-induced increase in surface CD2 expression. The magnitude of inhibition by either anti-CD2 mAb was dependent upon which anti-CD3 mAb was used as the stimulus, being more pronounced when the anti-CD3 mAb 454 was used as the stimulus than when either anti-CD3 mAb 147 or 446 was the stimulus. The effects of neuraminidase-treated sheep erythrocytes (which bind to CD2) were also more pronounced on mAb 454-induced proliferation than on mAb 147- or 446-induced proliferation. Furthermore, the effects of preincubation with anti-CD2 mAb depended upon the responder status of the donor to IgG1 anti-CD3 mAb. Preincubation of high-responder cells with anti-CD2 mAb had little effect on subsequent IgG1 anti-CD3-induced proliferation. In contrast, preincubation of low-responder cells with anti-CD2 mAb usually augmented the otherwise small proliferative response to IgG1 anti-CD3 mAb. Taken together, these observations suggest that interaction of surface CD2 with ligand alters the response of T cells to anti-CD3 mAb, but these effects depend upon the individual anti-CD3 mAb used for stimulation. These studies raise the possibility that perturbation of different parts of the CD3-T cell antigen receptor complex may lead to different sequelae, and, as a result, the T cell may respond to a given immunomodulator in different ways.

Activation of human T cells by neuraminidase-galactose oxidase-treated erythrocytes involving CD2 (T11) and its complementary structure

Human T lymphocyte proliferation induced by neuraminidase-galactose oxidase (NAGO)-treated autologous erythrocytes (HENAGO) plus polyethylene glycol (PEG) has previously been shown to be independent of accessory cells. Here, we show that the response to HENAGO + PEG was accompanied by interleukin 2 (IL-2) release and was inhibited by anti-IL-2 and anti-IL-2 receptor antibodies. HENAGO alone initiated DNA synthesis together with phorbol ester (12-O-tetradecanoyl-phorbol-13-acetate; TPA). To elucidate the nature of the stimulatory signals NAGO-treated sheep erythrocytes (SENAGO) were used in additional experiments. In parallel to the superior rosetting capacity of SE compared to HE. SENAGO were by themselves stimulatory, and the response was further enhanced by PEG or TPA. Antibody L180/1, specific for the T11 (CD2) target structure (T11TS) on SE, homologous to the human CD2 ligand LFA-3, abolished the response to SENAGO alone or when combined with PEG or TPA. The results suggest that ENAGO induce T-cell response through CD2-LFA-3-T11TS interaction, and via other surface antigens bound by the oxidatively induced aldehyde groups on ENAGO.

Activation of human T lymphocytes through CD3 and CD2 (T11) with anti-CD3-coupled sheep erythrocytes

Proliferative activation of T lymphocytes depends on cell-cell cooperation, i.e. interactions between specific cell surface receptors and their ligands. My co-workers and I have recently shown that an activating signal is mediated by interaction between the T cell surface antigen CD2 (T11) and its ligand on sheep erythrocytes (SE), the T11 target structure (T11TS), which is the homologue to the human LFA-3 antigen. Here I demonstrate that the anti-CD3 monoclonal antibody (MoAb) UCHT1 coupled to SE (SE-UCHT1) most efficiently induces accessory cell (AC)-independent T cell proliferation, and that SE can substitute for AC in stiumulation with phytohaemagglutinin (PHA). Inhibition studies with MoAb suggest that (1) concurrent stimulation of CD3 and CD2 is essential for interleukin 2 production and proliferation, since SE-UCHT1 treated with the anti-T11TS MoAb L180/1 are not mitogenic; (2) proximity of CD3 and CD2 is required during the stimulation, since a mixture of SE exposing either anti-CD3 or T11TS is not mitogenic; and (3) that T cell-AC interactions involving LFA-1 can be replaced by LFA-3-CD2 interactions, since the anti-LFA-1 MoAb 60.3 does not inhibit the SE-UCHT1 response, and only partly the SE + PHA response. These results demonstrate a functional linkage between the CD3 and CD2 structures, making accessory LFA-1 signals superfluous in proliferative activation of human resting peripheral T cell.

Immunoregulatory functions of paf-acether. II. Decrease of CD2 and CD3 antigen expression

Paf-acether (platelet-activating factor) is a phospholipid initially described as a potent platelet-aggregating compound. It is produced by numerous cell types and is now considered as an important mediator of cell-cell interactions. The effect of paf-acether on the expression of CD2 and CD3, two human T cell surface glycoproteins, was investigated by indirect immunofluorescence and flow cytometry. Paf-acether partially down-regulated, in a time- and dose-dependent manner, CD2 and CD3 but not HLA class I antigen expression on peripheral human T cells and Jurkat cells. Lysophosphatidylcholine, a phospholipid closely related to paf-acether, had no detectable modulatory effect on CD2 and CD3 expression. In addition to CD2/CD3 modulation, paf-acether markedly inhibited T cell proliferative response not only to phytohemagglutinin or concanavalin A but also to anti-CD3 or a stimulatory combination of anti-CD2 monoclonal antibodies. These data demonstrate for the first time that lipid mediators such as paf-acether might be involved in the regulation of the expression of cell surface glycoproteins that are essential in the execution of T cell function.

Anti-CD3 activation of T cells with immunoglobulin coupled red cells

We have studied the ability of anti-mouse IgG antibody coupled to Burro red blood cells (BRBC) to modulate the anti-CD3 stimulation of human T cells. These coupled BRBC restored anti-Leu-4 (anti-CD3) induced proliferative responses to purified T cells, in the presence of interleukin-1 (IL-1). Uncoupled BRBC were incapable of supporting anti-Leu-4-induced stimulation. T cells from individuals whose peripheral blood mononuclear cells were unresponsive to anti-Leu-4 did proliferative in the presence of anti-Leu-4, IL-1, and coupled BRBC. Ig-coupled red cells provide an efficient method for the study of murine antibody-induced modulation of T cell surface antigens.

Differences of T-cell activation by the anti-CD3 antibodies Leu4 and BMA030

Two anti-CD3 antibodies and their Fab/F(ab')2 fragments were compared with regard to their requirement for secondary signals and generations of intracellular messengers. The anti-CD3 antibody BMA030 was found to require monocyte contact to elicit T-cell mitogenesis. Cross-linking by plastic-bound goat anti-mouse antibodies (panning) failed to activate T cells, even in the presence of recombinant IL-1 or IL-2. In contrast, crosslinking of the anti-CD3 antibody Leu4 or Leu4 fragments was mitogenic in monocyte-free cultures. Measurements of intracellular Ca2+ ([Ca2+]i) and generation of inositol phosphates revealed that binding (+/- panning) of BMA030, Leu4, and their F(ab')2 fragments generated similar amounts of intracellular messengers and thus failed to explain the different responsiveness to passive crosslinking. Since the generation of these messengers was not necessarily followed by proliferation but was always observed when mitogenesis occurred, we conclude that the elevation of [Ca2+]i and the production of inositol phosphates are required but not sufficient to trigger mitogenesis.

Activation of T cells by cross-linking an anti-CD3 antibody with a second anti-T cell antibody: mechanism and subset-specific activation

Binding of anti-CD3 antibody BMA030-F(ab')2 to T cells is not able to induce T cell proliferation even after additional cross-linking by plastic-bound goat anti-mouse Ig antibodies (panning) and addition of either interleukin (IL) 1 or IL2. In search for agents able to complement the signals provided by BMA030-F(ab')2, several antibodies directed against CD2, CD4, CD5, CD6 and CD8 were used. Neither of these caused T cell proliferation either by themselves or when simply added together with BMA030-F(ab')2. However, if BMA030-F(ab')2 and any other of these anti-CD2, CD4, CD5, CD6 or CD8 antibodies were cross-linked, then proliferation of T cells occurred. The mitogenic effect of cross-linking BMA030-F(ab')2 and a second anti-T cell antibody is dependent on the presence of monocytes or exogenously added IL2. The enhancing effect of monocytes could not be replaced by addition of IL1, suggesting that other soluble factors or monocyte contact might be involved in induction of activation signals. The mechanism leading to this mitogenic effect is dependent on combining cross-linking of BMA030 with the second anti-T cell antibody, whereby the second antibody appears to act as an "anchor" for the CD3 antibody, controlling and/or changing the signal transduction via the CD3 structure. This "anchor" hypothesis could be substantiated by the following observations: the best stimulatory signals were obtained at a BMA030/anti-T cell antibody ratio of 1:1; the effect is independent of the antibody isotype and the epitope recognized by the second antibody; the binding of BMA030 and separate cross-linking of the anti-T cell antibody was not sufficient to trigger T cell mitogenesis; immobilization by direct binding of the CD3 antibody alone or of the CD3 and second membrane structure/antibody complex separately was not sufficient to stimulate T cell proliferation. A further interesting aspect of this finding is the fact that a selective T cell stimulation is possible, since cross-linking BMA030-F(ab')2 plus anti-CD4 antibodies induced a selective stimulation of T4 cells, while cross-linking BMA030-F(ab')2 plus anti-CD8 activated solely T8 cells. It is thus possible to selectively activate T cell subsets in vitro.

CD3 modulation inhibits pokeweed mitogen-induced T-cell help for immunoglobulin production

The CD3 molecule is considered to be a signal transducer in the process of T-cell activation. Modulation of the CD3 molecule of peripheral blood T cells can be accomplished by incubation at 37 degrees C with UCHT-1, a mouse IgG1 anti-CD3 monoclonal antibody, under experimental conditions avoiding T-cell activation. We have examined the effect of CD3 modulation on T-cell-dependent polyclonal immunoglobulin (Ig) production induced by pokeweed mitogen (PWM) in cultures of peripheral blood lymphocytes. CD3 modulation strongly inhibited (greater than 80%) IgG and IgM production. This was due to inhibition of the production of soluble helper factors by the T cells, and not to induction of suppressor cells. These data support the concept that the CD3 molecule is an essential signal transducer in the process of PWM-induced helper T-cell activity, and that CD3 can function as a receptor transmitting negative signals to helper T cells.

Impaired lymphocyte responsiveness to phytohaemagglutinin associated with the possession of HLA-B8/DR3

An examination was made of blastogenic response to phytohaemagglutinin (PHA) in HLA-B8 and/or DR3 positive subjects and B8/DR3 negative individuals. Both B8 and DR3 antigens were associated with a depression of the response at all three doses of PHA used. The possession of both these antigens did not lead to a further depression of the response.

Interleukin-2 production in response to phytohemagglutinin is not necessarily dependent upon the T3-mediated pathway of T-cell activation

The present study was designed to investigate whether heterogeneity exists in the requirement for T3 molecules in the mechanism of T-cell activation and interleukin-2 (IL-2) release by IL-2-producing T-cell clones. Clones were derived from peripheral blood by a culture system which allows clonal expansion of essentially all T lymphocytes or from mixed lymphocyte culture (MLC) activated T-cell population. IL-2-producing clones were selected and cell aliquots treated with anti-T3 monoclonal antibodies (mAb) to induce modulation of T3 surface molecules. Although stimulation of modulated clones with different anti-T3 mAb did not lead to IL-2 production, 4/39 of these clones produced IL-2 after stimulation with phytohemagglutinin (PHA). The ability of these clones to be activated by PHA could not be explained by incomplete T3 modulation. In addition, two alloreactive clones were isolated from MLC population in which modulation of T3 antigens abrogated the IL-2 production induced by either anti-T2 mAb or allogeneic stimulation but had no effect on PHA-induced IL-2 release. These data further support the concept that PHA may trigger some T cells via surface molecules that are independent from the T3-Ti cell receptor molecular complex.

Differentiation antigens on rhesus monkey lymphocytes. II. Characterization of RhT3, a CD3-like antigen on T cells

A monoclonal antibody FN18 is described, which is specific for mature rhesus monkey T lymphocytes. It defines a cell surface antigen, composed of two polypeptide chains with a molecular mass of 22 and 27 kDa. In view of these and other similarities with the human T3 or CD3 antigen, it was designated as RhT3. Expression, distribution and certain functions of RhT3 were compared with those of its human counterpart. Analogous to anti-CD3 antibodies in man, FN18 is able to modulate its target antigen, has mitogenic properties and is able to block the pokeweed mitogen and concanavalin A-driven cell proliferation, but not that caused by phytohemagglutinin. In spite of such minor dissimilarities the available data strongly suggest that the RhT3 antigen is the rhesus monkey's homologue of the human CD3 antigen.

Different T cell subset stimulation by IgG1 or IgG2a anti-T3 antibodies

In so-called responder cell cultures--able to be stimulated by mouse IgG1 anti-T3 antibodies (IgG1-CD3)--the proliferation, IL 2 secretion and IL 2 receptor expression were lower in IgG1-CD3 than in IgG2a-CD3-stimulated cultures. Using a double-fluorescent technique, we found that IgG1-CD3 antibodies stimulate fewer T4 cells, but the same amount of T8 cells, than IgG2a-CD3 antibodies. The suboptimal T4 cells stimulation could be enhanced by adding exogenous IL 2 or by crosslinking the IgG1-CD3 antibodies by plastic-bound goat anti mouse IgG antibodies. Following antibody-mediated crosslinking, high IL 2 levels could also be found in IgG1-CD3-stimulated cultures. We conclude that CD3 antibodies of the mouse IgG1 isotype stimulate fewer T4 cells, due to insufficient crosslinking by monocytes and that T3 structure crosslinking is essential for optimal IL 2 production and secretion.

Analysis of CD3-antibody-mediated inhibition of T-cell activation

In this study the influence of a non-mitogenic anti-CD3 antibody on accessory cell-dependent antigen and mitogen-induced T-lymphocyte proliferation has been investigated. The antibody was found to completely inhibit PHA, Con A, PWM, and tetanus toxoid stimulation, with no effect on the proliferation induced by the calcium ionophore A23187. VIT3 completely abrogated the production of IL-2 by lectin-stimulated T cells. It had no effect, however, on the IL-2-dependent proliferation of preactivated T-cell blasts. In addition, the antibody was able to elevate free cytoplasmic Ca2+ levels within minutes after the addition to T cells. Detailed time kinetic analyses revealed that the time interval critical for inhibition was significantly dependent on the interaction between T cells and accessory cells. Under standard conditions, in the presence of 10% non-T cells as accessory cells 50% inhibition was still achieved when VIT3 was added to PHA-stimulated T cells as late as 8 hr after the onset of culture. Delayed addition or a decrease in the number of added accessory cells significantly prolonged this time period. Lectin-stimulated T cells can thus obviously be inhibited via CD3 as long as they have not received all signals including those delivered by accessory cells. Although the underlying mechanisms are not clear so far, the observation that VIT3 at the same time triggers an early cytoplasmic Ca2+ response might indicate that it thereby actively interferes with antigen and lectin-initiated activation processes.

Phytohemagglutinin binds to the 20-kDa molecule of the T3 complex

Current findings have suggested that the T3 molecular complex is an essential antigenic signal transducer during T cell activation. Lectins, such as phytohemagglutinin (PHA), activate T cells nonspecifically. Conceivably, lectins may mediate their stimulatory action by affecting the T3 complex. In the present investigation we have studied the involvement of the T3 molecular complex in the PHA-mediated activation of T cells. We selectively modulated the surface expression of T3 molecules by anti-T3 antibody and subsequently tested the ability of the modulated cells to respond to PHA. Reduction of T3 expression by 70% resulted in 80% inhibition of the PHA response. This effect was specific for T3 since modulation of other T cell surface molecules (T4, T8) did not affect the PHA-induced mitogenesis. To determine if PHA could interact directly with the T3 complex, immunoblotting (Western blot) analyses of anti-T3 immunoprecipitates were performed. A 20-kDa member of the T3 complex reacted not only with the anti-T3 antibody, but also with PHA itself. These results provide the first evidence for direct binding of PHA to one of the molecules of the T3 complex. The combined data suggest that a major pathway of PHA-induced T cell activation involves the T3 complex. Possible activation mechanisms are discussed.

Human T cell activation. I. Monocyte-independent activation and proliferation induced by anti-T3 monoclonal antibodies in the presence of tumor promoter 12-o-tetradecanoyl phorbol-13 acetate

Three monoclonal antibodies (mAb), of IgG1, IgG2a, and IgM isotypes, raised against the T3 complex, were used to probe the activation of human T cells. The IgM antibody 235 was not mitogenic for peripheral blood mononuclear cells (PMC). It efficiently blocked the proliferation of PMC induced by T cell mitogens, alloantigens, and soluble antigens. The other two antibodies were mitogenic, and behaved similarly to Leu 4 and OKT3, respectively. In T cell preparations with less than 0.1% monocytes (as assayed by nonspecific esterase staining), all three mAb were not mitogenic. They failed to induce either interleukin 2 (IL-2) receptor expression or IL-2 secretion. Addition of IL-1 failed to collaborate with anti-T3 mAb to induce these T cells to proliferate, but IL-2 enhanced T cell proliferation slightly. Monocyte-depleted T cells, however, proliferated in response to all three anti-T3 mAb, when TPA was added, in a dose-dependent manner. TPA induced a low level of IL-2 receptor expression in monocyte-depleted T cells, without inducing IL-2 secretion. Anti-T3 plus TPA induced a marked enhancement in both quantity and intensity of IL-2 receptor expression. IL-2 secretion was also detected. These results indicate that anti-T3 IgM can deliver an inductive signal despite its blockage of T cell proliferation, and that two signals are necessary and perhaps sufficient to induce human T cell activation and proliferation.