On the mechanisms of inhibition of lymphocyte mitogenesis by high conA concentrations

A monoclonal antibody to human B lymphoblastoid cells activates human and murine T lymphocytes

A B lymphoblastoid cell line can provide a comitogenic, accessory signal for mitogen-treated T cells. In a study evaluating the antigenic determinant of such cells that mediate this effect, a monoclonal antibody (I57) was raised against the Daudi cell line. This antibody was found to interact with a 30-kDa protein on these cells and had agonistic properties. It enhanced the B lymphoblastoid accessory cell and interleukin 1 (IL-1)-dependent stimulation of PHA-treated murine thymocytes. The stimulatory effect of I57 on PHA-treated thymocytes was more pronounced at high, supraoptimal concentrations of the lectin. This was in contrast with the effect of IL-1 that failed to stimulate these cells treated with PHA at high concentrations. I57 also enhanced stimulation of thymocytes treated with IL-2 alone or with both PHA and IL-2. I57 exhibited by itself mitogenic activity for human T cells. These cells, treated with IL-2, were further stimulated by I57. I57 seems to be different from other agonistic antibodies that have been described so far.

Determination of mitogenic properties and lymphocyte target sites of Dolichos lablab lectin (DLA): comparative study with concanavalin A and galactose oxidase cell surface receptors

A mannoside-directed lectin has been isolated and purified from the seeds of Dolichos lablab L. by affinity chromatography. We have established that this glycoprotein, which displays high erythroagglutinating activity without blood group specificity, highly activates murine T lymphocytes, and we have described for the first time its mitogenic properties. Although its main properties are close to those of concanavalin A (Con A), the well-known mannoside-directed mitogen devoid of sugar moiety, several differences were found in some of the early events triggered by the two lectins during lymphocyte mitogenic stimulation: higher level of interleukin-2 (IL-2) synthesis, optimal dose for IL-2 synthesis at suboptimal mitogenic concentration, lack of ecto-5' nucleotidase inhibition, and lack of mitogenic inhibition at high lectin concentration. Because the two lectins did not act on the cell surface in exactly the same way, we have compared their receptors involved in mitogenesis on the plasma membrane of murine lymphocytes. We had previously established that the polyclonal activation of these cells probably occurred through high-molecular-weight receptors (200-230 kDa). Since the mitogenic stimulation of lymphocyte by galactose oxidase (GO), like that of Con A, was inhibited by DLA, we analyzed the cell surface receptors that were common to these three polyclonal mitogens. After labeling the neuraminidase/GO-treated cell surface glycoproteins with NaB3H4, we immunoprecipitated the Con A and DLA receptors which are the target of GO mitogenic action. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis of the precipitates demonstrated that there exist on the lymphocyte cell surface receptors common to the polyclonal mitogens DLA, Con A, and GO. Because Con A and DLA sterically inhibit GO mitogenic stimulation, the common glycoproteins which represent the necessary sites of oxidative mitogenic action are probably those which are involved in DLA and Con A-triggered mitogenesis, despite the different properties of the two lectins. These differences could be explained by the lower molecular weight receptors of the two lectins which are not identical.

Mechanism whereby ouabain inhibits human T lymphocyte activation: effect on the interleukin 2 pathway

Through the blockade of the Na-K-ATPase, ouabain inhibits several biochemical and biological events leading to the proliferation of activated lymphocytes. Since we already found that interleukin 1 production was not prevented by ouabain, we investigated by which mechanism this drug inhibits mitogen-induced human T lymphocyte activation, with respect to the interleukin 2 (IL 2) pathway. Our data revealed that at concentrations lower than 0.2 microM, IL 2 accumulation was not reduced in ouabain-treated cultures, even when cell proliferation was completely inhibited (0.1-0.2 microM ouabain). Moreover, in this concentration range, ouabain stimulated in a dose-dependent manner the accumulation of IL 2 in the supernatant of Con A-stimulated lymphocytes (optimum for 0.05 microM corresponding to half inhibition of lymphocyte proliferation). Such an effect, which appears correlated to the inhibition of Na-K-ATPase, suggests a failure of the cell to utilize IL 2. At concentrations higher than 0.3 microM, ouabain inhibited both lymphocyte proliferation and IL 2 production. These observations show that the glycosteroid interacts differently with the different cell populations involved in the cascade of reactions leading to cell proliferation, and suggest that the mitogenic inhibition resulting from the blockade of Na-K-ATPase is not related to the blockade of IL 2 production. On the other hand, we observed that: ouabain inhibited the expression of the receptors for IL 2, an obligatory step in lymphocyte proliferation; ouabain blocked the proliferation of an IL 2 sensitive human T cell line; in both cases the inhibition paralleled that of lymphocyte proliferation. Our data suggest that the essential steps of lymphocyte proliferation in which Na-K-ATPase-dependent K+ fluxes play a critical role are the expression of IL 2 receptors and the IL 2-dependent proliferative step.

Enhancement of cell-cell contact by a nonmitogenic lectin increases blastogenic response and IL-2 release by mitogen-stimulated mouse thymocytes

We have examined the influence of peanut agglutinin (PNA), a lectin which agglutinates but does not stimulate mouse thymocytes, on the responsiveness of these cells to concanavalin A (Con A) or galactose oxidase stimulation. Binding low amounts of PNA on unseparated mouse thymocytes pretreated with neuraminidase highly enhances the mitogenic response and the level of interleukin 2 release in the culture medium upon Con A stimulation. We have shown that PNA present on the cell surface acts as a crosslinking agent which favors intercellular binding between accessory cells (macrophages) and thymocytes, leading through this enhanced cooperation by cell-cell contact to an enhanced blastogenic response.

T cell triggering by lectins. I. Requirements for interleukin 2 production; lectin concentration determines the accessory cell dependency

The requirements for lectin-induced interleukin 2 (IL2) production by human T cells have been investigated. With two different types of T cells, the Jurkat T cell lymphoma and highly purified HLA class II- peripheral T cells, the amount of IL2 produced was strongly dependent on the lectin concentration used. Addition of accessory cells caused a shift in the dose-response curve, resulting in strongly enhanced IL2 production at low concentrations. Thus, the (absolute) accessory cell dependency for T cells to produce IL2 is defined by experimental conditions. Only at lectin concentrations that were found to be optimal in the presence of accessory cells, removal of these cells abrogates IL2 production. Furthermore, after depletion of monocytes IL 2 production by peripheral T cells became almost completely dependent on the presence of thiols in the culture medium. In contrast, the IL2 production by the Jurkat line was not influenced by addition of thiols. The Jurkat model was used to study the nature of accessory cell because this cell line does not show any reactivity to allogeneic cells. Various myeloid and B lymphoid cell lines were tested as accessory cells. The capacity to function as accessory cell was not related to the monocytic origin of the cell. B cell lines were far more effective than monocytes, as two HLA class II- monocytic cell lines were not active. Even after HLA class II determinants were induced on these cells by incubation with an interferon-gamma-containing conditioned medium, they failed to act as as accessory cells. These experiments question the importance of HLA class II molecules and monokines, such as IL1, for lectin-induced IL2 production.

T cell triggering by lectins. II. Stimuli for induction of interleukin 2 responsiveness and interleukin 2 production differ only in quantitative aspects

We have investigated the requirements for lectin-induced proliferation of highly purified human T cells. To study activation, independent of growth factor production, we cultured the cells in the presence of an excess of interleukin 2 (IL2), which was a product of cDNA cloned in E. coli. In the presence of IL2, the same cooperative effect of lectin and accessory cells was found that we have previously described for IL2 production. Thus, analogous to induction of IL2 production, the acquisition of responsiveness to IL2 can be completely monocyte dependent, but a 10-fold increase in lectin concentration completely abolishes the requirement for accessory cells. Furthermore, two stimuli (IL 1 and phorbol myristate acetate), which are able to replace monocytes at the level of IL2 production, also induce responsiveness to IL2 under accessory cell-dependent conditions. Thus, very similar conditions are required for proliferation and for the induction of IL2 production. There is only a quantitative difference: proliferation of cells in the presence of exogenous IL2 occurs already at low lectin concentrations, whereas IL2 production and consequently proliferation in the absence of exogenous IL2 requires higher lectin concentrations. At high lectin concentrations, when IL2 production has become the only limiting factor, purified T cells cannot be induced to proliferate in the absence of exogenous IL2 because the lectin concentration that induces IL2 production independent of accessory cells inhibits mitogenesis. However, after addition of thiols to the medium, which enhances the IL2 production, a very narrow range of lectin concentration can be found which is just below toxic values and still high enough to induce IL2 production in the absence of accessory cells. Under these conditions, accessory cells are no longer a prerequisite for lectin-induced T cell proliferation.