Studies on the monocyte dependence of T-cell proliferation induced by monoclonal antibodies directed against regions I and II of the CD2 antigen

We produced three murine monoclonal antibodies (mAb) directed against the human T-cell differentiation antigen CD2 (formerly T11) and analysed their epitope specificity by E-rosette inhibition, cross-blocking and proliferation-induction experiments. When added together, two mAb (both IgG1κ) were able to induce proliferation in peripheral blood T cells. This proliferation was found to be strictly dependent on the presence of monocytes. The polymorphism of Fc receptors (FcR) expressed on human monocytes for murine IgG1 antibodies, which is readily demonstrable in anti-CD3-induced T-cell activation, was not found in the anti-CD2-driven system. Moreover, mAb directed against the 40,000 MW FcRII were unable to inhibit proliferation induced by the mitogenic anti-CD2 combination. Still, F(ab')₂ fragments of the mitogenic anti-CD2 antibody combination could not initiate T-cell mitogenesis, despite their ability to induce a rise in the free intracellular [Ca²⁺]. The contributions of monocytes and of antibody Fc moieties to T-cell proliferation, induced by combinations of anti-CD2 mAb, will be discussed.