Negative selection of CD4+CD8+ thymocytes by T cell receptor-induced apoptosis requires a costimulatory signal that can be provided by CD28.

Abstract

SummaryCD4+CD8 + thymocytes expressing self-reactive T cell antigen receptors (TCR) are deleted in the thymus as a consequence of TCIL/sdf-antigen/major histocompatibility complex interactions. However, the signals that are necessary to initiate clonal deletion have not yet been clarified. Here we demonstrate that TCR engagement does not efftdently induce apoptosis of CD4+CD8 + thymocytes, although it generates signals that increase expression of CD5, a thymocyte differentiation marker. In fact, TCR signals fail to induce thymocyte apoptosis even when augmented by simultaneous engagement with CD4 or lymphocyte function 1-associated molecules. In marked contrast, signals generated by engagement of both TCR and the costimulatory molecule CD28 potently induce apoptosis of CD4 +CD8 + thymocytes. Thus, the present results define a requirement for both TCR and costimulatory signals for thymocyte apoptosis and identify CD28 as one molecule that is capable of providing the necessary costimulus. These results provide a molecular basis for differences among cell types in their ability to mediate negative selection of developing thymocytes.T olerance to self-proteins is maintained among T ceils through the elimination or inactivation of clones which express antigen receptors reactive to self-antigen/MHC complexes (1, 2). Elimination of stir-reactive immature T cells takes place in the thymus by donal deletion which occurs via apoptosis (3-5). CD4+CD8 + thymocytes, the major targets of donal deletion, can be induced to undergo apoptoffs both in vivo and in vitro through engagement of their antigen receptors by either intrathymic self-ligands or by antireceptor antibodies (6-13). However, it is not known whether induction of apoptosis requires signals in addition to those transduced by the TCR. In fact, donal deletion of thymocytes is usually assayed in the presence of dedicated APCs that are capable of providing ligands for costimulatory molecules present on thymocytes. Whether APCs bearing costimulatory ligands are uniquely capable of mediating TCRdriven apoptosis of CD4+CD8 + thymocytes, or whether any cell type capable of presenting self-antigen/MHC complexes can mediate negative selection (14) is not dear.In this report, we show that isolated TCR signals do not efficiently drive apoptosis of CD4+CD8 + thymocytes even when enhanced by coengagement with CD4 or LFA-1. However, TCR signals deliver a potent apoptotic stimulus when combined with signals provided by the costimulatory molecule, CD28. These results demonstrate that both TCR and costimulatory signals are necessary to induce thymocyte apoptosis and indicate that only ceUs expressing costimulatory ligands can mediate negative selection. Materials and MethodsIsolation of CD4 + CD8 + Thymoqtes. CD4+CD8 + thymocytes were purified from young adult C57BL/6 thymuses by panning on anti-CD8 coated phtes (15). More than 95% of the harvested cells were CD4+CD8 +.Culture Conditions and Antibodies. 24-wen tissue cuhure plates were coated with antibody incubating them overnight...

Distinct gene signature revealed in white blood cells, CD4+ and CD8+ T cells in (NZBx NZW) F1 lupus mice after tolerization with anti-DNA Ig peptide

“…There have been reports of CD28 having both pro-apoptotic 46,47 and anti-apoptotic effects. [48][49][50] Studies on the aging immune system have found that the number of CD8 þ CD28 À T cells increases with age and that these cells are resistant to apoptosis, having increased bcl-2 expression and decreased caspase 3 expression.…”

CD28 Signal Enhances Apoptosis of CD8 T Cells After Strong TCR Ligation

“…Negative selection of double-positive thymocytes was significantly reduced in response to either Ag or anti-TCR/CD3 Ab in CD28-deficient mice (8). CD28 is also involved in thymocyte negative selection mediated by superantigen both in vitro (9, 10) and in vivo (11).…”

Functional and Phenotypic Evidence for Presentation of Eα52–68 Structurally Related Self-Peptide(s) in I-Eα-Deficient Mice

“…The thymic medulla is rich in BM-derived cells expressing various costimulatory molecules and, therefore, is a very efficient site of negative selection. For instance, circulating Ag or endogenous superantigen (SAg), which are dominantly expressed by BM-derived cells (10), cause massive deletion in the medulla (11)(12)(13)(14). In addition, autoreactive CD4 ϩ T cells are detected in mice expressing MHC class II only in the thymic epithelium (the K14-A ␤ b Tg mice), presumably as a result of a lack of negative selection by BM-derived cells in the medulla (15).…”