Molecular events in late stages of T-cell functional maturation

Gene expression in CD8 and CD4 T-cell populations following activation with the calcium ionophore A23187

We have hypothesized that functional maturation of T lymphocytes can be dissected into a series of discrete stages. For example, activation of T lymphocytes with the calcium ionophore A23187 drives CD8+ T cells to become dividing blasts, referred to as 'pre-effector' cells in that these blasts do not express cytolytic function but are driven by IL-2 to do so. Here we characterize via Northern blots the functional maturation of CD8+ and CD4+ T-lymphocyte populations which have been activated via A23187 followed by stimulation with IL-2. Previously we have reported that no detectable IL-2 was found in the supernatants of A23187-activated pre-effector blasts. However, these cells do express levels of IL-2 mRNA very similar to those of OKT3-activated blasts, from which IL-2 is easily detected in the supernatant. Translational control may account for these findings. A23187-activated CD8+ pre-effector blasts do not respond to stimulation with IFN-gamma nor do they express IFN-gamma mRNA following stimulation with IL-2. These observations suggest that IL-2 may be sufficient to stimulate maturation of these cells. Activation via A23187 results in lower expression of the proto-oncogene c-myb relative to that found in OKT3 activation. C-myb mRNA levels are higher in CD8+ than in CD4+ A23187-activated pre-effector blasts and the c-myb level in the CD8+ pre-effector blasts falls in response to IL-2. This decrease in c-myb mRNA coincides with an increase in proliferation, and the expression of cytolytic activity.

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.