SIGNALING IN LYMPHOCYTES-T - A POSSIBLE ACCESSORY PATHWAY INITIATED AT THE CD69 RECEPTOR

T cells interact with antigen presenting cells (APC) not only with their TCR/CD3 and CD4 or CD8 supercomplex, but also with a number of accessory receptors, many of which are able of independent signal transduction upon ligand crosslinking. Simultaneous signaling by accessory molecules is therefore likely to play a crucial role in the modulation and channeling of the response initiated at the TCR/CD3. Enhancement or interference mediated by second messengers on key intracellular relais may account for as different end responses as anergy and cell division, or for the activation of different sets of lymphokine genes. Defining the biochemical pathways of intracellular signaling from different receptors will provide the basic information to start to draw a temptative picture of how multiple signaling integrates in lymphocytes. Here we summarize the data about signals generated at the TCR/CD3, the main signal transducing apparatus in T cells, and CD69, a receptor expressed early during lymphocyte activation. While TCR/CD3 appears to utilize essentially a PI-PLC, CD69 preferentially activates a PLA2. CD69-mediated activation may therefore represent a model for alternative signaling in lymphocytes and prove useful in understanding potential interactions among signal transduction pathways generated by different receptors.