Role of cross-linking in stepwise activation of T cells

Activation of T cells is a complex process which we have hypothesized involves a series of functional intermediates possessing some, but not all, of the characteristics of fully functional effector cells. We have identified two such functional intermediates, the poised T cell (poTc) and the pre-effector T cell (peTc). poTc do not proliferate or mediate cytolytic activity but are responsive to help in the form of interleukin 2 (IL-2); peTc are proliferating cells which are not cytolytic. Here the role of T-cell receptor/CD3 complex cross-linking in generating these functional intermediates is examined and it is shown that cells can be driven to different stages of functional maturation depending upon the sequence of antibody binding to CD3 and cross-linking. Highly purified T cells can be activated to poTc stage if they are first labelled with the anti-CD3 monoclonal antibody OKT3 and then cross-linked with goat anti-mouse IgG-coated beads. If the OKT3 antibody is first bound to the IgG-coated beads and then added to highly purified T cells, peTc are generated. In both cases the intermediates can be driven to become fully functional effector cells by the addition of IL-2. Finally, by removing the OKT3-bound beads during the activation process, were are able to show that poTc and peTc are sequential intermediates along the same pathway of activation.