Role of CTLA-4 in the Activation of Single- and Double-Positive Thymocytes

Abstract

CTLA-4, a homologue of CD28, is a negative regulator of T cell activation in the periphery and is transiently expressed on the cell surface after T cell activation. However, the role of CTLA-4 in T cell activation in the thymus is not clear. This investigation was initiated to determine the role of CTLA-4 in the activation of CD4+CD8+ double-positive (DP) and CD4+CD8− and CD4−CD8+ single-positive (SP) thymocytes using fetal thymic organ cultures (FTOC) of MHC class II-restricted, OVA323–339-restricted TCR transgenic mice (DO11.10). We found that treatment of the FTOC with anti-CTLA-4-blocking Ab during activation with OVA323–339 increased the proportion and number of DP thymocytes, but decreased the proportion and number of SP thymocytes compared with OVA323–339-stimulated FTOC without anti-CTLA-4 Ab treatment. In addition, anti-CTLA-4 Ab treatment inhibited OVA323–339-induced expression of the early activation marker, CD69, in DP thymocytes, but increased CD69 in SP thymocytes. Similarly, CTLA-4 blockage decreased phosphorylation of ERK in DP thymocytes by Ag-specific TCR engagement, but increased phosphorylation of ERK in SP thymocytes. CTLA-4 blockage inhibited deletion of DP thymocytes treated with a high dose of OVA323–339, whereas CTLA-4 blockage did not inhibit deletion of DP thymocytes treated with a low dose of OVA323–339. We conclude that CTLA-4 positively regulates the activation of DP thymocytes, resulting in their deletion, whereas blocking CTLA-4 suppresses the activation of DP thymocytes, leading to inhibition of DP thymocyte deletion. In contrast, CTLA-4 negatively regulates the activation of SP thymocytes.

CTLA-4 blockade in murine bone marrow chimeras induces a host-derived antileukemic effect without graft-versus-host disease

“…Tolerance induction for organ-specific peptides, however, may depend on their appropriate expression by thymic epithelial cells, which could be sensitive to radiation and CTLA-4 blockade. Although the role of CTLA-4 in thymic clonal deletion is controversial, 13,14,[54][55][56][57] delayed CTLA-4 blockade in our model was not associated with altered clonal deletion of alloreactive T cells.…”

Enhanced selection of FoxP3 + T-regulatory cells protects CTLA-4-deficient mice from CNS autoimmune disease

“…Early reports suggested that thymic selection in CTLA-4KO mice was normal and that any findings of an altered thymic phenotype were the result of inclusion of the parathymic lymph nodes or the infiltration of the thymus by activated peripheral T cells (1,27). More recently, a role for CTLA-4 in the negative selection of DP thymocytes has been suggested (19,20). In addition to TCR signals, the deletion of autoreactive DP T cells in the thymus requires a costimulatory signal that can be provided by CD28 (28).…”

“…In addition to TCR signals, the deletion of autoreactive DP T cells in the thymus requires a costimulatory signal that can be provided by CD28 (28). Activation of DP thymocytes induces the expression of CTLA-4 (29), which can counteract the CD28-driven apoptosis of autoreactive cells following the encounter of high-dose, but not low-dose, specific antigen (20).…”

“…1). Based on the available literature, this could be the result of unresponsiveness of CD4 ϩ CD25 Ϫ T cells because of increased deletion of autoreactive T cells in the thymus (19,20). Alternatively, the lack of disease could result from augmented activity of Treg cells.…”

Molecular Determinants of Inverse Agonist Activity of Biologicals Targeting CTLA-4

“…For CD28, this has been shown with a subset of Abs known as superagonist Abs (2)(3)(4). For CTLA-4, in contrast, historical data had claimed the ability of some Abs to have inverse agonist properties (5)(6)(7)(8), but this has only recently been demonstrated using a bispecific, in-tandem single-chain Fv (ScFv) 3 fragment against CTLA-4 known as 24:26 (9).…”