Opening a Window on Thymic Positive Selection: Developmental Changes in the Influence of Cosignaling by Integrins and CD28 on Selection Events Induced by TCR Engagement

Abstract

How TCR and non-TCR signals are integrated by thymocytes to generate a decision to undergo either positive or negative selection remains incompletely understood. Recent evidence suggests that TCR signal transduction changes its quality during thymocyte maturation, but whether the contributions of various cosignaling or costimulatory pathways to thymocyte selection also are modified during development is unclear. Questions also remain about the possible selective roles of specific costimulatory pathways in induction of differentiation vs death among thymocytes at any given stage of maturity. To address these issues, a quantitative in vitro analysis of initiation of CD4+CD8+ thymocyte differentiation as measured by CD69 up-regulation/coreceptor down-modulation was conducted in parallel with an analysis of induction of death. Using transfected cells varying in their surface display of ICAM-1 or B7.1 along with antibody blocking experiments, we demonstrate here that ICAM-1 provides a selective boost to signaling for differentiation without substantially affecting induction of death among CD4+CD8+ cells, a property that is lost as thymocytes mature further. In contrast, B7 engagement enhances both cell activation and death in parallel. Based on these data, we propose that the high level of ICAM-1 on cortical epithelial cells plays a special role in opening a window between TCR signaling for differentiation vs death, permitting efficient initiation of positive selection on epithelial ligands. In contrast, late CD28-dependent cosignaling on hemopoietic cells in the medulla would help enforce negative selection by augmenting the effects of TCR engagement by low levels of high affinity ligands.

Quantitative and Qualitative Adjustment of Thymic T Cell Production by Clonal Expansion of Premigrant Thymocytes

“…To measure the proliferation rate of individual T cell clones, we compared BrdU incorporation by a given TCR Tg clone in different MHC contexts, and by SP thymocytes with different clonotypic Tg TCRs in a given MHC context. Several reports have suggested that the affinity of the AND TCR is higher for I-E k than for I-A b MHC molecules (27)(28)(29). Indeed, this is consistent with the ability of I-E k bound to self peptides to negatively select many Tg thymocytes.…”

“…Although no significant differences in the percentages of CD4SP Tg thymocytes were observed among the three haplotypes, the absolute number of CD4SP thymocytes was 5-fold lower in H-2 k/k mice than in H-2 b/b mice. The small thymus size and the reduction in DP Tg thymocyte numbers observed in the H-2 k/k haplotype are consistent with the induction of a clonal deletion (thymic negative selection) as a result of higher AND TCR affinity for I-E k than for I-A b MHC molecules (27)(28)(29). It has been shown that CD5 surface levels on CD4SP thymocytes from Tg mice increase with TCR affinity for the selecting ligands (30).…”

TCR Signaling for Initiation and Completion of Thymocyte Positive Selection Has Distinct Requirements for Ligand Quality and Presenting Cell Type

“…In dispersed cultures of AND TCR transgenic RAG-2 Ϫ/Ϫ thymocytes from mice lacking selecting MHC class II molecule expression (AND DP), CD69 up-regulation can be induced in a cohort of cells without concomitant cell death using transfected ICAM-1 ϩ L cells bearing agonist cytochrome c peptide/I-E k complexes (12,57). These studies did not determine whether these agonist-activated, viable thymocytes are capable of further differentiation and, if so, whether any resulting mature cells would be reactive to the selecting ligand.…”

Negative regulation of nuclear factor-kappaB activation and function by glucocorticoids

“…Optimal T cell activation requires, in addition, the provision of costimulatory signals imparted by CD4/CD8 coreceptors, CD28 (Lucas & Germain 2000), and cytokine receptors (Curtsinger et al 1999) among others (signal 2), which synergize with primary calcium-dependent TcR-CD3 ligation (signal 1), resulting in induction of the interleukin (IL)-2 autocrine pathway (Germaine 1994), and the temporal expression of cytokine genes and high affinity cytokine receptors (Curtsinger et al 1999, Slifka & Whitton 2000. Signaling through the antigen-specific TcR in conjunction with noncognate costimulatory signals results in the elevation of intracellular calcium and the induction of calmodulin-regulated enzymes, including the serinethreonine phosphatase, calcineurin (Clipstone & Crabtree 1992).…”