OKT3 induces suppressor cells for mixed lymphocyte and PHA mitogenic responses in human peripheral lymphocytes

Generation and function of human regulatory CD8+ T cells induced by a humanized OKT3 monoclonal antibody hOKT3gamma1 (Ala-Ala)

Recent studies with a humanized anti-CD3 antibody in clinical trials suggested that this drug can induce regulatory CD8+ T cells. In this review, we discuss how these regulatory human CD8+ cells can be generated and the possible molecular tools they may use to inhibit immune responses. We compare our data on CD8+ regulatory cells induced by anti-CD3 stimulation with those reported in other experimental systems to identify both common and unique features of these cells.

Differential responses of CD4+CD45RA+ and CD4+CD29+ subsets to activated CD8+ cells: enhanced stimulation of the CD4+CD45RA+ subset by cells from patients with multiple sclerosis

To examine whether functionally different CD4+ cells respond uniformly to the immunoregulatory influences of allogeneic activated CD8+ cells (*CD8+), we subfractionated the CD4+ population into two subsets, based on the high expression of either CD45RA or CD29. We confirmed that the CD45RA+ cells proliferated poorly in response to soluble anti-CD3 mAb, compared to the vigorous response obtained with the CD29+ subset; the CD45RA+ cells were more responsive to stimulation with Con A. Using normal healthy controls, we found that whereas *CD8+ had a significant suppressive effect on the proliferation of the CD29+ subset, they augmented the mitogen-induced proliferative response of the CD45RA+ cells. We further demonstrated that *CD8+ derived from MS patients augmented the response of the CD45RA+ subset to a significantly higher degree compared to healthy age- and sex-matched controls. There were no significant differences between the degree of suppression exerted by the *CD8+ of either the MS or the control group on the CD29+ cells. These results demonstrate that helper/memory CD4+CD29+ cells are more sensitive to the suppressive influences of *CD8+ compared to the CD4+CD45RA+ subset. In addition, in MS, *CD8+ may contribute to a more pronounced "on" signal for virgin CD4+CD45RA+ cells, which might serve as a means to perpetuate the autoimmune disease process.

Suppressor T cell induction by soluble OKT3 antibody does not require cross-linking of the T3 molecule

The monoclonal antibody OKT3 can induce T suppressor cells of the in vitro antibody response when used at concentrations as low as 10 ng/ml and in cultures containing as few as 4% OKT3-pretreated T cells. These suppressor cells are radioresistant, cyclosporin sensitive and belong to the T4 as well as the T8 subpopulations. In contrast with the mitogenic effect of OKT3 the suppressor activity can be induced and expressed in highly monocyte-depleted cultures. Moreover F(ab')2 fragments of OKT3, although not mitogenic, are able to induce suppressor cells. Thus cross-linking of the T3 molecule (such as that realized when intact OKT3 IgG and monocytes are present) is not required to induce suppressor cells among resting normal T cells.

The role of the T3 molecular complex on human T lymphocyte-mediated cytotoxicity

The above overview of the experimental data clearly indicates that the T3 molecular complex is intimately involved in T cell activation. The precise role of the T3 complex in the activation process, however, is not clearly understood. The surface association of the T3 complex with the antigen receptor, along with the ability of antibodies to these molecules to render T cells receptive to IL2, reveal a possible mechanism by which specific antigen initiates T cell activation and growth. However, it would be difficult to reconcile this specific effect of anti-T3 antibodies with their effect on CTL function. Since the T3 complex is not a specific marker of any particular effector T cell population, but it is found on all T lymphocytes, we favor the hypothesis that the complex is involved in a more fundamental step of T cell activation, and we believe that triggering of the 'lethal hit', expression of IL2 receptors, and secretion of IL2 are mere manifestations of this basic process. The natural ligand of the antigen receptor is obviously the specific antigen. However, the natural ligand of the T3 complex is unknown. Possibly, its natural ligand is the antigen receptor itself after it has interacted with antigen. A simple scenario, then of the early events of T cell activation would include antigen recognition and binding, followed by an interaction between the antigen receptor and the T3 complex which then activates or allows expression of specific pathways depending on the particular effector population involved. Thus, the inhibition of CTL function by anti-T3 antibodies could be explained by interference with the antigen receptor-T3 complex interaction following target cell recognition. This interaction may be the event that signals the initiation of the 'lethal hit' process.

Capping of the surface OKT3 binding molecule prevents the T-cell proliferative response to antigens: evidence that this molecule conveys the activation signal

The human T-lymphocyte receptor for antigen appears to have been localized to a cluster of associated surface glycoprotein molecules, among which is the OKT3 binding molecule. We have tested the hypothesis that selective removal of the OKT3 binding molecule eliminates the cellular immune response to antigens by clearing the surface of the molecule that conveys the activation signal. Enriched T cells were obtained from donors immune to purified protein derivative (PPD), streptokinase-streptodornase (SK-SD), or keyhole limpet hemocyanin (KLH). T-3 molecules were cleared from the cell surface by capping with OKT3 and F(ab')2 goat anti-mouse IgG. Regeneration of surface molecules was prevented by culturing the T-3 capped cells with OKT3 625 ng/ml. The capacity of capped T cells to proliferate in culture with antibody in response to antigens, alloantigens, and the mitogens, PHA and ionophore A23187, was compared to uncapped cells pretreated with media and to capped cells permitted to regenerate the OKT3 binding molecule. T-3 capped cells cultured in the presence of antibody failed to proliferate to antigens or alloantigens. However, T-3 capped cells cocultured with antibody also did not significantly proliferate to PHA, but did respond to A23187. In contrast, both media-treated T cells and cells which had regenerated the OKT3 binding molecule proliferated to mitogens, antigens, or alloantigens. The requirement for the OKT3 binding molecule was determined by utilizing T-1, T-4, and T-8 capped cells. T-1, T-4, or T-8 capped cells cultured in the presence of OKT1, OKT4, or OKT8 proliferated in response to antigens, alloantigens, and mitogens. These results demonstrate that in the absence of the OKT3 binding molecule, antigens, alloantigens, and PHA failed to induce a significant cellular proliferative response. In the absence of this molecule, PHA cannot bind to its carbohydrate moiety, and therefore cannot activate T cells to proliferate. These data support the concept that the molecule binding OKT3 conveys the transmembrane signal resulting in cellular activation.

A mutant human T-cell line producing immunosuppressive factor(s)

6T-CEM-20, a subclone of a 6-thioguanine-resistant mutant derived from the human-T-cell line CEM, secreted into the medium, a high titered immunosuppressive factor specific for T cells. The cell-free supernatant was very potent in suppressing, via a noncytotoxic mechanism, mitogen-activated T-cell proliferation, cytotoxic T-cell functions, and pokeweed mitogen (PWM)-stimulated plaque-forming cells (PFC). Lower dilutions of the supernatant (10(-1)-10(-2] abrogated T-cell functions within 24 hr whereas higher dilutions (10(-3)-10(-7] required a culture period of up to 4 days with lymphocytes to arrest T-cell activities. The suppressive activity was most pronounced when the factor was added in the early part of the culture period. The factor was sensitive to heat treatment and both low and high pH (most stable at physiological pH). Preliminary purification with column chromatography indicates that the active moiety was contained in the high-molecular-weight fraction (MW greater than 200,000). Data from coculture experiments suggested that T lymphocytes, which were exposed for 5-12 hr to the active supernatant or the partially purified material, suppressed allogeneic T-cell proliferation in a dose-related manner. This suppressor factor which we called suppressor-activating factor (SAF) might have activated a suppressor population or induced the production of a suppressor factor which in turn mediated the observed suppression. Both the molecular structure and the detailed mechanism of action are under investigation.

OKT3 monoclonal antibody inhibits cytotoxic T lymphocyte mediated cell lysis

Human alloreactive cytotoxic T lymphocytes (CTLs) were generated in culture by stimulating peripheral blood lymphocytes against lymphocytes of unrelated donors. OKT3 monoclonal antibody, which reacts with all human peripheral T cells, inhibited more than 50% of the lysis of the allogeneic cells by the CTLs in the four-hour cytotoxicity assay at a concentration of 1 microgram ml-1. OKT4, which reacts with inducer/helper T cells, did not demonstrate this effect. OKT5 and OKT8, both of which react with cytotoxic/suppressor T cells, had a small but distinct inhibitory effect (about 20-30% maximally). OKT3 antibody is also known to induce mitogenesis of peripheral T cells, but this mitogenic effect is probably of itself not the cause of the inhibitory effect of OKT3 on CTLs. Human plasma abolished OKT3-induced mitogenesis of T cells but not OKT3-mediated inhibition of CTL activity. In addition, there exist conditions that allow both full mitogenesis and intact CTL activity, suggesting CTLs can maintain cytolytic activity under mitogenic conditions. The generation of CTLs in the 6-day mixed lymphocyte culture was also inhibited by OKT3. Several possible mechanisms for this effect are discussed.