Mechanisms by which accessory cells contribute in growth of resting T lymphocytes initiated by OKT3 antibody

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Abstract
This study was undertaken to determine how accessory cells (AC) participate in growth of normal resting T cells initiated by anti-T3 monoclonal antibodies. Highly purified peripheral blood resting T cells were obtained by sequentially using three procedures (adherence to plastic surface, adherence to nylon wool columns and treatment with four monoclonal antibodies against antigens on AC and activated T cells plus complement). The assays for T cell growth were carried out at low cell density (104 cells/well) and with T cell populations where we could not detect cells bearing OKM1 and Ia antigens. Soluble OKT3 antibody, concanavalin A, recombinant interleukin 2 (IL2) or purified interleukin 1 (IL1) alone did not induce proliferation of purified resting T cells. Recombinant IL2 together with soluble OKT3 antibody stimulated significant growth whereas purified IL1 and two distinct preparations derived from AC containing IL1 activity did not. Nevertheless, purified IL1 amplified the proliferation of T cells induced by soluble OKT3 antibody in the presence of a small number of irradiated AC (3%). Phorbol myristate acetate (PMA) together with soluble OKT3 antibody activated purified resting T cells to proliferate, but PMA alone had little growth-promoting activity only. Soluble OKT3 antibody did not by itself induce a detectable number of resting T cells to express receptors for IL 2 as determined by direct immunofluorescence staining and FACS analysis with monoclonal anti-IL2 receptor antibody. Cloned IL2 or purified IL1 alone did not induce resting normal T cells to express receptors for IL2 either. In contrast, T cells exposed to both soluble OKT3 antibody and IL2 exhibited IL2 receptors. PMA alone stimulated some resting T cells to express IL2 receptors and this response was significantly increased when the drug was used together with soluble OKT3 antibody. Studies were performed with unfractionated mononuclear cells from a donor whose cells respond to OKT3 (IgG2) but not to Leu 4 (IgG1) anti-T3 antibodies. Recombinant IL2 but not purified IL 1 corrected the defective response to Leu 4 antibody. Finally, OKT3 antibody linked to beads, but not in soluble form, and purified IL1 replaced AC in growth of purified resting T cells. Based on these data I conclude the following: (a) AC participate in growth of resting normal T cells initiated by anti-T3 antibodies through their Fc receptors in two ways, namely, by providing a matrix to favour cross-linking of the T3 complex and simultaneously by secreting IL 1. (b) The contribution of IL 1 is that of amplifying rather than inducing the production of IL 2. This cytokine has no detectable effect on the expression of IL2 receptors by normal resting T cells, but it may improve the action of IL2, and (c) PMA bypasses the mechanisms by which AC contribute to T cell growth in this system. The efficacy of this drug is likely to rest on its capacity to both induce resting T cells to express IL2 receptors and increase the transcription of the gene encoding IL2. Both activities are most likely due to its property of binding to and activating protein kinase C. Finally, I propose that AC and PMA may ultimately act via the same mechanisms. Thus, both cross- linking of the T3 complex and IL 1 (the functions of AC) would stimulate the synthesis of diacylglycerol and a raise in T cell intracellular Ca2+. Diacylglycerol and Ca2+acting in concert efficiently activate protein kinase C and thereby, like PMA, trigger a sequence of events culminating in DNA synthesis.