Maturation Versus Death of Developing Double-Positive Thymocytes Reflects Competing Effects on Bcl-2 Expression and Can Be Regulated by the Intensity of CD28 Costimulation

Abstract

Immature double-positive (DP) thymocytes mature into CD4+CD8− cells in response to coengagement of TCR with any of a variety of cell surface “coinducer” receptors, including CD2. In contrast, DP thymocytes are signaled to undergo apoptosis by coengagement of TCR with CD28 costimulatory receptors, but the molecular basis for DP thymocyte apoptosis by TCR plus CD28 coengagement is not known. In the present study, we report that TCR plus CD28 coengagement does not invariably induce DP thymocyte apoptosis but, depending on the intensity of CD28 costimulation, can induce DP thymocyte maturation. We demonstrate that distinct but interacting signal transduction pathways mediate DP thymocyte maturation signals and DP thymocyte apoptotic signals. Specifically, DP maturation signals are transduced by the extracellular signal-related kinase (ERK)/mitogen-activated protein kinase (MAPK) pathway and up-regulate expression of the antiapoptotic protein Bcl-2. In contrast, the apoptotic response stimulated by CD28 costimulatory signals is mediated by ERK/MAPK-independent pathways. Importantly, when TCR-activated thymocytes are simultaneously coengaged by both CD28 and CD2 receptors, CD28 signals can inhibit ERK/MAPK-dependent Bcl-2 protein up-regulation. Thus, there is cross-talk between the signal transduction pathways that transduce apoptotic and maturation responses, enabling CD28-initiated signal transduction pathways to both stimulate DP thymocyte apoptosis and also negatively regulate maturation responses initiated by TCR plus CD2 coengagement.

CD28 Ligation Costimulates Cell Death but Not Maturation of Double-Positive Thymocytes due to Defective ERK MAPK Signaling

“…However, ligation of CD2 receptors by CD48 in TCR-activated thymocytes does costimulate thymocyte maturation. Consistent with previous studies (5)(6)(7)(8)(9)(10), ligation of CD28 receptors in TCR-activated DP thymocytes signals a death response. Mutation analysis of the cytoplasmic domain demonstrates that CD28-initiated costimulatory signals that enhance cell death responses can be regulated by multiple signaling motifs in the CD28 cytoplasmic domain.…”

“…Similarly, apoptotic responses of TCR-engaged DP thymocytes can be selectively initiated by signals initiated through CD28 receptors (5)(6)(7)(8)(9). The outcome (maturation or apoptosis) of TCR plus CD28 coengagement of DP thymocytes has been reported to be regulated by the intensity of CD28 engagement (10). The in vitro identification of a role for CD28 costimulation of thymic development has been supported by multiple in vivo studies (11)(12)(13)(14)(15).…”

“…In addition, CD28 costimulatory signals also have been shown to initiate the T regulatory (Treg) cell differentiation program in the thymus (16). Previous studies evaluating the role of costimulatory receptor coengagement in thymocyte development have largely relied on stimulating purified DP thymocytes with anti-receptor Abs bound to a solid phase matrix rather than using APCs expressing biologically relevant ligands (5)(6)(7)(8)(9)(10)16). This approach permitted selectivity in defining the specific receptor-initiated responses but may not reproduce biologically relevant interactions that occur between ligands on APCs and receptors on DP thymocytes.…”

Constitutively Active Protein Kinase B Enhances Lck and Erk Activities and Influences Thymocyte Selection and Activation

“…Maturation of DP cells is initiated by the ligation of the TCR and a number of coinducer/costimulatory receptors including CD28 (61) and the net effect of these interactions will decide whether DP cells mature or undergo apoptosis. Interestingly, in a recent report it was shown that CD28 coengagement of DP cells can either induce CD4 T cell maturation or negative selection, depending on the intensity of CD28 costimulation (62). Assuming that myr PKB reflects some aspects of CD28 signaling in thymocytes, it is more conceivable that subtle differences in myr PKB expression and modification of downstream target proteins in individual cells could have differential effects on selection.…”

No immunotoxic effect on T cells with di (2-ethylhexyl) phthalate in male C57BL/6 mice

“…of six mice. molecules, including CD3, CD4, CD8 and CD28 (16)(17)(18). In addition to the indispensability of these molecules in thymic differentiation, they play pivotal roles in recognizing antigens associated with the major histocompatibility complex expressed on antigen-presenting cells (19).…”