Induction of suppressor cells, interleukin-2 production and mitogenesis with monomeric concanavalin A: different actions of tetrameric and monomeric concanavalin A

A novel mechanism of murine hepatocyte death inducible by concanavalin A

BACKGROUND

Concanavalin A (Con A) is a plant lectin that polyclonally activates T-cells. When given intravenously to mice it induces a selective liver failure. Hepatotoxicity following Con A administration involves the systemic release of tumor necrosis factor.

METHODS

We used primary murine hepatocyte cultures to investigate mechanisms of hepatocytotoxicity related to this animal model of inflammatory liver failure.

RESULTS

Con A was directly toxic for cultured hepatocytes. This toxicity did not require additional cytokines or the presence of T cells. Cytotoxicity due to Con A involved specific binding of the lectin to mannosyl cell surface receptors, but no internalization. Other structurally similar lectins lacked such an in vitro hepatocytotoxicity. Con A induced initially reversible alterations of the morphology that were different from the ones caused by classical hepatotoxins. Con A-induced cell death was highly specific for murine hepatocytes. It was neither apoptotic by morphology nor did it involve DNA fragmentation. In addition, Con A caused a fall in cellular total glutathione content and an increase in transcriptional activity. Stabilization of microtubules by taxol completely protected cells from the lectin.

CONCLUSIONS

Stimulation of hepatocytes with Con A elicits a novel mechanism of cytotoxicity due to inappropriate excessive stimulation of membrane receptors and subsequent disturbance of the cytoskeleton.

Induction of interleukin-2 receptor alpha-chain gene expression by cytochalasin B and 12-O-tetradecanoylphorbol-13-acetate in T lymphocytes

The high affinity form of interleukin-2 receptor (IL-2R) is composed of two subunits; the alpha (p55) and beta (p75). The alpha chain, unlike the beta, is expressed only on activated T lymphocytes. Therefore, high affinity binding of interleukin-2 (IL-2) is controlled by the expression of the IL-2R alpha-chain. In this study, we examined the effect of cytochalasin B (CB) plus 12-O-tetradecanoylphorbol-13-acetate (TPA) on expression of IL-2 and IL-2R. Northern blot and flow cytometric analysis showed that the IL-2R alpha-chain was expressed both at mRNA and protein levels. However, IL-2 gene expression was not induced by this treatment. Unlike the cells treated individually with CB or TPA, cells treated with CB plus TPA accumulated IL-2R mRNA at all the times examined. In order to determine the percentage of cells that incorporated tritiated thymidine ([3H]dT) in the presence of IL-2 after treatment with CB plus TPA, autoradiography was carried out. We found that about 11% of the cells were labelled. Because the percentage of labelled cells and cells expressing IL-2R alpha-chain was relatively low (11% and 9% respectively), perhaps CB plus TPA caused IL-2R expression in only a subset of T cells.

Analysis of T cell-replacing factor-like activity: potent induction of T helper activity for human B cells by residual concanavalin A and interleukin 2

At least two factors with the capacity to induce IgM synthesis in human B cells were found to be present in the 15-20-kDa fraction of the supernatant of mononuclear cells activated with concanavalin A (Con A) and phorbol ester. Previously, it has been shown (Sauerwein, R. W. et al., Eur. J. Immunol. 1985. 15: 611) that interleukin 2 (IL2) in this material is able to induce T cell-dependent IgM secretion in normal B cells. Evidence was obtained for the presence of another factor distinct from IL2 that could replace T cells in the induction of B cell differentiation. We have analyzed this factor with the use of a neoplastic B cell population of prolymphocytic origin that was functionally nonresponsive to IL2. T cell-replacing factor (TRF)-like activity and IL2 could be separated by ion-exchange chromatography, although a small amount of IL2 was recovered in the TRF fractions. This small amount of IL2 was found to be crucial for the observed TRF activity. Moreover, a substantial amount of monomeric Con A was detected in the TRF preparation. Our studies show that Con A in the presence of IL2 can act as a potent inducer of helper function in lower numbers of T cells for normal and neoplastic B cells. Functional assays for T cell contamination in B cell suspensions are therefore of limited value because they are determined by the efficiency of the stimulating signal. Particularly in those B cell factor preparations, obtained from mitogen-activated T cells with an obligatory or unidentified role of IL2, the possible effect of a contaminating mitogen must be considered.