Mechanism of T lymphocyte activation by OKT3 antibodies. A general model for T cell induction.

The OKT3 monoclonal antibody has unique reactivity for human T cells and induces a mitogenic response equal to that of concanavalin A (Con A). However, significant differences were found in activation requirements for the two T cell mitogens. Although both OKT3 and Con A required living accessory cells for stimulation of T cells, the OKT3 antibody was optimally mitogenic at a molar concentration 10(3) times less than Con A. Under such conditions, the two mitogens showed similar kinetics as well as magnitude or responses. Furthermore normal IgG blocked the OKT3-induced proliferation selectively. The inhibition is most likely caused by the Fc portion of IgG competing for Fc receptors on accessory cells. The different requirements for activation are discussed in relation to the T cell receptor and its role in T cell activation.