A monoclonal antibody inhibiting leucocyte adhesion blocks induction of IL-2 production but not IL-2 receptor expression.

Monoclonal antibody 60.3 defines the leucocyte antigen CD 18 and recognizes a cell surface glycoprotein with an apparent molecular weight (MW) of 90,000 expressed by most human peripheral blood and bone marrow cells. This antibody can, among other things, block phorbol ester-induced adhesion among human mononuclear leucocytes. We show in this study that phorbol esters alone can induce peripheral blood mononuclear cells (PBL) to secrete interleukin-2 (IL-2) and that the IL-2-dependent cell line CTLL can be used for measuring this lymphokine without influence of the phorbol esters themselves. These findings make it possible to analyse the capacity of antibody 60.3 to interfere with IL-2 production and receptor expression by phorbol ester or phytohaemagglutinin (PHA)-treated human PBL. A significant positive correlation between blockage of induced cell aggregation by antibody 60.3 and reduction in IL-2 release was observed. The addition of interleukin-1 (IL-1) restored IL-2 secretion in PHA-treated, but not in 4-beta-phorbol 12, 13-dibutyrate [P(Bu)2]-treated, cells in the presence of this antibody. In parallel, IL-2 receptor expression was determined by immunofluorescence using biotinylated anti-IL-2 receptor (Tac) antibodies. FACS analysis showed that IL-2 receptor expression was unaffected by antibody 60.3, whereas DNA synthesis of the same P(Bu)2-treated PBL was inhibited. However, addition of external recombinant IL-2 overcame this proliferation blockade. These results indicate that a cell-to-cell adhesion step is necessary for the production of IL-2, but not for the expression of its receptor on both PHA- and P(Bu)2-treated human PBL.