Several accessory molecules were shown to play important roles in T cell functions and be in close proximity to the T cell receptor (TcR/CD3). The L-selectin molecule (Leu8, LAM1-1, LECAM1) also plays an important role in lymphocyte homing and proliferation. We were interested in determining the proximity of this molecule to the TcR/CD3 complex on live peripheral human T cells. Using a fluorescence energy transfer method, designed to study individual cells, we could show that L-selectin is within 170 A of the TcR/CD3 complex. Monoclonal antibody directed against the LAM1-1 (Leu8) epitope of the L-selectin molecule suppressed the mitogenic activity of antibodies specific for various CD3 epitopes in vitro. Intracellular Ca2+ mobilization obtained with wt31 followed by cross-linking antibody or with anti-CD3 was not influenced by anti-leu8 antibody. Also antibody directed against the LAM1-1 epitope did not influence the binding of the mitogenic antibodies, as shown by fluorescence-based flow cytometry. Therefore, we suggest that binding of TcR/CD3 bound mitogenic antibodies to accessory cell Fc receptors may be hindered by antibodies bound to the close proximity L-selectin molecule
