CD28, CTLA‐4 and their ligands: who does what and to whom?

Abstract

T-cell activation is a critical event in the organization of effective cellular and humoral immune responses. Activated T cells are essential for provision of T-cell help, promoting the development of high-affinity antibody production and the generation of cytotoxic T-cell responses. Accordingly, defects in proteins required for T-cell activation give rise to significant infectious pathology and malignancies. However, the decision to allow T-cell activation also has potentially dangerous consequences for the host and must therefore also be tightly controlled. Defects in proteins involved in regulating activated T-cell behaviour therefore tend to lead to autoimmunity. Thus, the major challenge faced in regulating T-cell responses is how to maintain a sufficiently large immune repertoire capable of recognizing all possible foreign antigens, whilst at the same time maintaining T cells in an unresponsive state towards self-antigens. In recent years significant progress has been made in our understanding of the mechanisms of self-tolerance. In the thymus it is clear that large numbers of potentially 'self-reactive' T cells are eliminated during negative selection in a process termed central tolerance. However, paradoxically, the process of positive selection that permits the expansion of T cells with low avidity for self–major histocompatibility complex (MHC) interactions must also lead to a degree of self-reactivity which is presumably tolerable in peripheral T cells. The question is how such T cells (albeit weakly self-reactive) can be ensured to remain non-reactive amongst a different array of self-antigens in the periphery. In the last few years a number of proteins have been identified that may serve the function of 'quality controlling' peripheral T-cell activation. This review focuses on two proteins, CD28 and cytotoxic T lymphocyte antigen-4 (CTLA-4), and explores how their interactions with their natural ligands may regulate the outcome of T-cell receptor engagement amongst peripheral T cells. CD28 and CTLA-4 (CD152) are transmembrane protein members of the immunoglobulin gene superfamily containing a single extracellular 'V-like' domain. 1–3 Both proteins are predominantly expressed by T cells and whilst CD28 is found in substantial amounts on the cell surface of the majority of resting T cells, in contrast CTLA-4 surface expression is much more limited. 4 The levels of CTLA-4 expression in most resting T cells are extremely low (or probably absent), and CTLA-4 predominantly appears following T-cell activation. However, despite maximal expression being reported at 48–72 hr post-activation, remarkably little stable surface CTLA-4 is found, although mRNA is equivalent to that of CD28.5,6 This lower level of cell-surface expression results from a motif in the cytoplasmic domain of CTLA-4 that facilitates its interaction with the clathrin pit adaptor complex (AP-50) causing its rapid internalization from the cell surface. 7–9 Consequently the majority of CTLA-4 is found in intracellular vesicles that may be then targeted to the cell surface at the site of T-cell receptor (TCR) contact. 10 It has been suggested that phosphorylation of the CTLA-4 cytoplasmic domain results in disengagement from the AP-50 internalization system and therefore stabilizes cell-surface expression. 8 The complexity of the CD28/CTLA-4 receptor interactions stems from the fact that there are two natural ligands CD80 (B7-1) and CD86 (B7-2) for these receptors. 11–17 Whilst these ligands can both interact with either receptor, they are only approximately 25% identical in sequence and it has therefore been attractive to speculate that they may serve different functions. Predictably, for co-stimulatory ligands, CD80 and CD86 are found on professional antigen-presenting cells such as dendritic cells, monocytes and activated B cells, although they can be induced on other cell types including T cells. 13,17–21 In general CD86 is the more abundant in terms of expression, and is increased more rapidly upon activation. In contrast CD80 is not generally found on resting antigen-presenting cells (APCs) and is induced more slowly upon cellular activation. A large variety of stimuli have been investigated in the control of CD80 and CD86 expression. Most of these, such as CD40, interferon-γ (IFN-γ), interferon-α (IFN-α), granulocyte–macrophage colony-stimulating factor (GM-CSF) and lipopolysaccharide (LPS) appear to result in increased expression 18,22–27 whereas others such as interlekin-10 (IL-10) and interleukin-4 (IL-4) may inhibit expression. 28–30 These expression studies, together with findings in CD80 and CD86 KO mice, 31–33 tend to indicate that CD86 is probably the major initial ligand for CD28 during T-cell activation, based mainly on its more rapid and abundant expression on APCs. However, functional data indicate that CD80 is probably the more potent CD28 ligand in terms of activation,34,35 which is consistent with affinity measuremen