Magnitude of protein tyrosine phosphorylationlinked signals determines growth versus death of thymic T lymphocytes
Using concanavalin A (Con A) as a multireceptor-reactive agonist, we studied the relationship between the growth or death of thymic T lymphocytes and the agonist concentration-dependent magnitude of the intracellulary delivered signal. Both immature and mature thymic T lymphocytes were subjected to a high concentration of Con A-mediated signal for apoptotic cell death. In this model, a number of cellular proteins including mitogen activated protein kinases were phosphorylated at tyrosine depending on the concentration of Con A. This effect was followed by corresponding increase in serine 73 phosphorylation of cjun and transcription of c-fos. DNA fragmentation and cell membrane disruption developed concomitantly after stimulation with high concentrations of Con A. The addition of inhibitors of protein kinases which completely inhibited the growth of cells stimulated with low concentrations of Con A only partially prevented death, and even promoted DNA fragmentation of cells stimulated with high concentrations of Con A. The dissociated sensitivities of Con Amediated cell growth and cell death to the inhibitors were, however, shown to be due to the different efficiency of inhibition of high and low levels of intracellularly delivered signals. The results indicate that the magnitude of signaling could be the principal element that determines the growth versus death of thymic T lymp hocytes.
1 Introduction
Little is known about the intracellular signals involved in the negative selection of immature T lymphocytes in the thymus, even though the signal pathway governing early thymocyte maturation has been extensively studied [ 1-31. Many of the signal elements that are needed for thymic T lymphocyte maturation such as nonreceptor protein tyrosine kinases (PTK), protein kinase C (PKC), intracellular Ca2+ concentration ([Ca2+Ii), p38 mitogen-activated protein kinase (MAPK) and protein phosphatases may [4-121 or may not [13, 141 be included in the signal cascade of the TCR-mediated apoptotic cell death for negative selection. A co-stimulatory signal that can be provided by CD28 [15, 161, which is needed for positive selection [ 171, may also cooperate with the TCR-mediated signal for cell death. Recent studies reported that the avidity of cell-[I 163781