Regulatory substances produced by lymphocytes

Human suppressor factors constitutively produced by T-T cell hybridomas: functional and biochemical characterization

We have recently developed a new method (Hybridoma 6:589, 1987) for the generation of human T-T cell hybrids. This method is based on a new selection procedure that involves cloning the hybrids in soft agar, screening by HLA-typing or appropriate functional tests and recloning by limiting dilution. T-T cell hybrids were separated from the parent line on the basis of their ability to form colonies in soft agar, whereas the parent lymphoblastoid T cell lines did not. HAT medium was not used in our selection procedure. Using this method, we have succeeded in developing human T-T cell hybrids (as determined by HLA-typing) constitutively producing B cell growth factor (BCGF) (Hybridoma 6:589, 1987) or suppressor factors. These hybrids were obtained by fusing MLC or Con A T cell blasts with cells from the Molt 4 or Jurkat lymphoblastoid T cell lines. T-T cell hybridomas, derived by fusing Con A-stimulated lymphocytes with cells from the Jurkat T cell line, produced suppressor factors inhibiting: (1) proliferative response in vitro of human peripheral blood mononuclear leukocytes to mitogens and to allogeneic cells in mixed lymphocyte culture; and (2) immunoglobulin synthesis and secretion by mononuclear leukocytes in the PWM-induced differentiation system in vitro. A suppressor factor with these inhibitory properties was also identified in supernatants of the Jurkat T cell line. These suppressor factors were ammonium sulphate precipitable, pH 2 labile, non-dialyzable and they were inactivated by treatment at 56 degrees C for 30 minutes. They exhibited a molecular weight in the range of 50,000-70,000, as determined by gel filtration, and were not gamma or alpha interferon or lymphotoxin/TNF. They did not lyse human lymphoblastoid tumor cell lines nor did they affect the viability and cell numbers of human mononuclear cells even after prolonged incubation (88 hr). They appeared to be cytostatic rather than cytotoxic molecules. The Jurkat suppressor factor is different from those produced by the hybrids on the basis of: (a) different isoelectric points; and (b) the ability of the Jurkat factor to arrest proliferation to PHA of human mononuclear cells in the S phase, whereas the 160 and 169 factors arrest proliferation at the G1 phase of the cell cycle. Certain of these suppressor factors (produced by the hybrids 153, 160, 170, and the Jurkat T cell line) also inhibited proliferative responses of mouse lymphocytes in vitro. In contrast, suppressor factors produced by the 169 and 77 hybrids did not inhibit any murine responses.

Hybridoma-derived human suppressor factors: differential effects on mouse lymphocytes

We have recently identified a family of suppressor factors produced by certain human T-T cell hybridomas that we developed (references 1 and 2) and by the Jurkat T cell line. These suppressor factors significantly inhibited proliferative responses to mitogens and allogeneic cells in mixed lymphocyte culture and antibody production by human peripheral blood mononuclear cells. We investigated and report here the effect of these suppressor factors on certain in vitro murine immune responses. Suppressor factors produced by certain of these hybrids, such as 153, 160, 170 and by the Jurkat T-cell line were able to inhibit: (1) proliferative responses to mitogens of mouse thymocytes and splenocytes; (2) proliferative responses of mouse splenocytes to allogeneic cells in mixed lymphocyte cultures; (3) primary in vitro antibody responses of mouse spleen lymphocytes to sheep erythrocytes; (4) primary in vitro antibody responses of mouse spleen lymphocytes to a T-cell independent antigen (TNP-Ficoll). Inhibition of murine immune responses in vitro by these suppressor factors was regular and reproducible and it was observed in a large number of experiments. In contrast, suppressor factors produced by the 169 and by the 77(38F3) hybrids did not suppress the murine immune responses. The basis for these differences are not known at the present. The ability of human suppressor factors to inhibit effectively mouse immune responses provides an additional opportunity for the characterization of the properties of these factors in vivo using mouse models of human disease.

Synthesis of sterols and 5-lipoxygenase products are required for the G1-S phase transition of interleukin-2-dependent lymphocyte proliferation

A murine killer T cell line, G-CTLL 1, whose proliferation depends on the presence of interleukin 2 (IL-2), was used to analyze the mechanism of IL-2 action with respect to sterol synthesis and arachidonate metabolism. De novo sterol synthesis was substantially enhanced much earlier than DNA synthesis, and the rate reached a maximum at 13 hr after the addition of IL-2. Compactin, which is a potent competitive inhibitor of 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMG CoA reductase, the enzyme in the rate-limiting step of the sterol synthesis), inhibited the IL-2-induced DNA synthesis. The addition of mevalonate, the product of HMG CoA reductase, prevented the inhibition of DNA synthesis by compactin, suggesting that the supply of a sufficient amount of sterol is an essential prerequisite for IL-2 action. The IL-2-induced DNA synthesis was also inhibited by AA861, a specific inhibitor of arachidonate 5-lipoxygenase, and by other lipoxygenase inhibitors such as nordihydroguaiaretic acid and esculetin. In contrast, indomethacin, an inhibitor of arachidonate cyclooxygenase, had no effect. These findings suggest that synthesis of 5-lipoxygenase products is also a prerequisite. The inhibition of DNA synthesis was effectively inhibited only when compactin or lipoxygenase inhibitors were added early enough to block the synthesis of sterols or 5-lipoxygenase products; addition of the reagents after 3 hr decreased the inhibition with time. Therefore, about 3 hr after the addition of IL-2, several drastic intracellular changes are assumed to begin and to lead to DNA synthesis.

Modulatory effect of zinc on the proliferative response of murine spleen cells to polyclonal T cell mitogens

To study the effect of zinc on the proliferative response to polyclonal T cell mitogens, spleen cells from C57BL/6 mice were cultured with or without ZnCl2 and stimulated with graded doses of concanavalin A or phytohemagglutinin. Addition of 10(-4) M ZnCl2 inhibited proliferation whereas 10(-5) to 10(-6) M ZnCl2 did not modify the response to suboptimal doses of mitogen but increased DNA synthesis in cultures stimulated with high doses of mitogen (10 or 20 micrograms/ml of concanavalin A and 10 or 25 microliters/ml of phytohemagglutinin) which are supraoptimal for C57BL/6 mice, and inhibited proliferation in cultures of spleen cells from animals of this strain, low responder to T cell mitogens. In contrast, supplementation with ZnCl2 did not enhance the response to mitogen of spleen cells from high responder BALB/c mice. The enhancing effects of ZnCl2 on the proliferative response of C57BL/6 cells were not observed following depletion of adherent cells or in cultures supplemented with 5 X 10(-5) M 2-mercaptoethanol, both conditions capable of abrogating the inhibitory effect of high mitogen doses on the response of C57BL/6 cells.

Murine splenocytes release an inhibitory peptide of lymphocyte DNA synthesis into serum-free culture medium

Murine splenocytes release a novel repressing activity of Concanavalin A-induced lymphocyte DNA synthesis. This activity was purified by gel filtration, ion exchange and thin layer chromatography. It was characterized to be a heat-stable peptide whose molecular weight was estimated to be about 1 000 daltons. The strong inhibition by this peptide was observed when added at early periods of the cell culture. This factor does not affect early RNA synthesis in lymphocytes, but nucleotide (UTP) incorporation at early periods was considerably decreased. In addition, although the factor inhibits drastically lymphocyte DNA synthesis, the cell viability is not changed significantly. These results suggest that the peptide suppresses the cell membrane function at early periods and specifically blocks the concomitant DNA synthesis.

Pharmacologic modulation in vitro of human histamine-induced suppressor cell activity

Various metabolic inhibitors and agents which increase intracellular cyclic nucleotide levels were investigated for their effects on the generation of suppressor cells in a co-culture system. Human blood mononuclear cells (MNC) were stimulated with histamine (10(-3) - 10(-5)M) for 24 h in the absence or presence of other agents, washed, co-cultured with lectin-stimulated autologous MNC and and suppression of 3H-thymidine uptake was measured. The generation of histamine-induced suppressor cells was shown to require active cellular metabolism since inhibitors of transcription (1 microgram ml-1 actinomycin D), translation (5 micrograms ml-1 puromycin and 1 microgram ml-1 cycloheximide), oxidative phosphorylation (10(-2)M sodium azide) and glycolysis (10(-2)M 2-deoxyglucose) significantly reduced their activity. Inhibitors of cytoskeletal function such as cytochalasin B (5 micrograms ml-1) and colchicine (3 X 10(-5)M) also markedly reduced suppressor activity. There was no apparent requirement for DNA synthesis in the generation of suppressor cells since treatment of MNC with mitomycin C (50 micrograms ml-1) had no effect on suppression. Incubation of MNC with cAMP-elevating agents (10(-3)M dibuteryl cAMP and 3 X 10(-5)M papaverine) for 2 h resulted in augmented suppressor activity while longer incubations (24 h) resulted in reduced function. Incubation (2 - 24 h) of MNC with cGMP-elevating agents (10(-3)M dibuteryl cGMP and 10(-5)M imidazole) had no effect on suppressor cell activity. These agents may interfere with critical lymphocyte - macrophage interactions (lymphokine/monokine synthesis and/or secretion) that are required for intact suppressor responses.

The role of regulatory components from resident T lymphocytes in polyclonal B cell activation

Resident T lymphocytes have been found to exert helper and suppressor regulatory influences with regard to polyclonal activation of murine splenic B lymphocytes elicited by lipopolysaccharide. In the normal adult spleen, only T cell helper influences are exercised over polyclonal B cell activation. This activity is a property of Lyt 1+2- T cells and does not appear to be subject to MHC restriction. Suppressive influence evidently is either latent or it exists at such a low level that its effects are difficult to detect. No regulatory activity can be recovered from the supernatants of T cells, cultured either with or without LPS. However, suppressor T cell function may be evoked by activating splenic T cells with Concanavalin A or by sonicating unstimulated splenic T cells in order to liberate a suppressive potential which is not expressed by these unstimulated cells when intact. The soluble fraction of resident splenic T cell sonicates exerts both helper and suppressor regulatory influences. The soluble helper activity is derived from Lyt l+2- T cells, whereas suppressor activity is generated from Lyt 1-2+ T cells. The suppressive activity of T cell sonicates is not restricted by the MHC gene complex. Helper and suppressor activities contained in splenic T cell sonicates were separated by gel chromatography; the suppressive activity was found to elute with a molecular weight between 68,000 and 84,000 daltons, and the helper activity eluted with a molecular weight between 15,000 and 23,000 daltons. The data indicate that helper and suppressor activities are distinct molecular entities derived from distinct splenic T lymphocyte subpopulations. The possibility that these molecules are precursors to or components of antigen-specific or nonspecific helper and suppressor factors described in the literature is discussed.

Regulatory factors of lymphocyte-lymphocyte interaction. I. Con A-induced mitogenic factor acts on the late G1 stage of T-cell proliferation

DNA synthesis in murine lymphocytes was augmented by a soluble factor in the supernatant of serum-free cultures of syngeneic spleen cells activated with concanavalin A (Con A). This so-called mitogenic factor (MF), which is probably identical with interleukin II, partially purified by DEAE-cellulose and Sephadex G-75 chromatography, is a fairly homogeneous molecule of 17-25 X 10(3) daltons. By using partially purified MF, the role of MF in lymphocyte proliferation was investigated. Pretreatment of lymphocytes with Con A for the first 3 hr of culture, which does not in itself induce cell proliferation, markedly augmented the effect of MF. The presence of MF, however, is necessary only in a restricted stage(s) of lymphocyte proliferation. The addition and removal of MF at various times during culture showed that MF exerts its effect on a process which occurs 3-6 hr before the beginning of DNA synthesis. These results strongly suggest that MF regulates the proliferation of mitogen-stimulated lymphocytes by acting on a restricted stage(s) of the cell cycle.

Immunosuppressive lymphocyte factors: characterization of the IDS-producing cell in the experimental model of antigenic competition

Inhibitor of DNA synthesis, is a soluble, protein lymphocyte factor which nonspecifically suppresses in vitro lymphocyte responses to antigens or mitogens. It is secreted in large amounts in vivo in some experimentally induced immunological paralysis. Here, we have defined the cell secreting IDS in one experimental model of non-specific immune-suppression, ie., that of antigenic competition. Lymphocytes of rats injected with a large dose of ovalbumin intravenously, show no immunologic response to the same or other antigens or mitogens 24 hr later. At this time, spleen cells of these rats secrete large amounts of the inhibitor into culture supernatants. However spleen cell supernatants of T-depleted rats do not contain the activity. Further, maximal inhibitor concentrations are obtained in the first 2 days of culture when more than 65% of cultured cells are large blasts actively synthesizing protein. As the number of actively metabolizing blast cells decrease in subsequent days of culture inhibitor concentration falls. Finally thymocytes of rats pretreated with hydrocortisone acetate, to deplete thymus cortex cells are unable to secrete inhibitor in culture. These findings reveal that the cells producing inhibitory DNA synthesis in an animal made tolerant with a supra-optimal dose of antigen is an active blast transformed T cell (present in the spleen and thymus). In the thymus the cell making inhibitor appears to reside in the thymus cortex. Previous experiments have confirmed that an identical cell causes nonspecific immune-suppression in vitro. We suggest that this cell produces in vivo tolerance in antigenic competition through the release of inhibitor to DNA synthesis.

Microcytotoxicity assay for cell-mediated immunity enumerating residual target number by 86Rb

Lymphocyte-mediated lysis of target cells grown as monolayers in microtiter wells is readily quantitated by an assay measuring the 86Rb incorporated after attaining isotopic equilibrium. Lysis of fibroblasts by allogeneic lymphocytes sensitized by skin grafts and of tumor cells by syngeneic spleen cells sensitized by intraperitoneal tumor inoculation were readily detected. Weakly cytolytic lymphocyte populations can be assayed by increasing incubation times to 48 h or longer. A potential problem, 86Rb incorporation by lymphocytes sticking to residual target cells, was controlled by comparing 86Rb incorporation by targets incubated with non-immune lymphocytes. Results by 86Rb incorporation were identical to those determined by microscopic counting or 51Cr release. 86Rb incorporation assays should be considered as an alternate to 51Cr release techniques, especially in those experimental systems where the cytolytic potential of a lymphocyte population is so low that lysis can be detected only after long incubation times and/or when the spontaneous release of 51Cr is prohibitively high.

Cytokine inhibition of DNA synthesis: effect on cyclic adenosine monophosphate in lymphocytes

A greater than twofold increase in intracellular adenosine 3',5'-monophosphate (cyclic AMP) inhibited DNA synthesis, in stimulated rat lymphocytes. A two- to fourfold rise of intracellular cyclic AMP, starting at 16 hours, was produced by purified inhibitor od DNA synthesis added to such cells either at 0 or 16 hours, in close association with the initiation of DNA synthesis.