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

Abstract

The differentiation of double-positive (DP) CD4+CD8+ thymocytes to single-positive CD4+ or CD8+ T cells is regulated by signals that are initiated by coengagement of the Ag (TCR) and costimulatory receptors. CD28 costimulatory receptors, which augment differentiation and antiapoptotic responses in mature T lymphocytes, have been reported to stimulate both differentiation and apoptotic responses in TCR-activated DP thymocytes. We have used artificial APCs that express ligands for TCR and CD28 to show that CD28 signals increase expression of CD69, Bim, and cell death in TCR-activated DP thymocytes but do not costimulate DP thymocytes to initiate the differentiation program. The lack of a differentiation response is not due to defects in CD28-initiated TCR proximal signaling events but by a selective defect in the activation of ERK MAPK. To characterize signals needed to initiate the death response, a mutational analysis was performed on the CD28 cytoplasmic domain. Although mutation of all of CD28 cytoplasmic domain signaling motifs blocks cell death, the presence of any single motif is able to signal a death response. Thus, there is functional redundancy in the CD28 cytoplasmic domain signaling motifs that initiate the thymocyte death response. In contrast, immobilized Abs can initiate differentiation responses and cell death in DP thymocytes. However, because Ab-mediated differentiation occurs through CD28 receptors with no cytoplasmic domain, the response may be mediated by increased adhesion to immobilized anti-TCR Abs.

Low Ligand Requirement for Deletion and Lack of Synapses in Positive Selection Enforce the Gauntlet of Thymic T Cell Maturation

“…Importantly, this is fewer peptides than are required to elicit an effector response from mature T cells of the same antigen specificity (Irvine et al, 2002; Peterson et al, 1999) and also agrees very closely with earlier estimates of the range of peptides required (Peterson et al, 1999). Although CH27 B cells possess the costimulatory and adhesion molecules thought to be important for negative selection (Graham et al, 2006), thymic APCs may of course be different in terms of the number of peptides they require to delete self-reactive DP T cells, although it is hard to imagine that the threshold could be placed much lower. Negative selection can also occur at the semimature SP stage (Kishimoto and Sprent, 1997), although we and others (Villunger et al, 2003) find that 5c.c7 DP and semimature SP thymocytes are similarly sensitive to agonist stimuli.…”

Soluble recombinant CD69 receptors optimized to have an exceptional physical and chemical stability display prolonged circulation and remain intact in the blood of mice

“…Graham et al. [20] recently reported that hyperstimulation of human T lymphocytes with ligands for CD28 or integrins results in their activation, as demonstrated by high surface expression of CD69, and massive cell death. North et al.…”

Intracellular Signals of T Cell Costimulation

“…The molecular basis of how CD28 regulates the expression of bcl-xL has been defined as upon interacting with its ligand, B7-1 or B7-2 and CD28 gets phosphorylated on tyrosine residues. The CD28 cytoplasmic domain that binds with a cytosolic protein called translationally controlled tumor protein (TCTP), a novel multifunctional antiapoptotic bcl-xL-interacting protein to regulate cell survival (72,73). In particular, one tyrosine (Y170 in mouse CD28, Y173 in human CD28) that is important in PI3K activation permits CD28 to recruit SH2-containing signaling molecules, including PI3K, Grb2, and Gads that control the regulation of bcl-xL (21,74).…”