Glycoproteins B7-1 (CD80) and B7-2 (CD86) are expressed on antigen-presenting cells and deliver the co-stimulatory signal through CD28 and CTLA-4 (CD152) on T cells. Signalling through CD28 augments the T-cell response, whereas CTLA-4 signalling attenuates it []. CD80 contains two Ig-like domains, an amino-terminal immunoglobulin variable (IgV)-like domain characteristic of adhesion molecules and a membrane proximal immunoglobulin constant (IgC)-like domain similar to the constant domains of antigen receptors []. Members of the Ig family are components of immunoglobulin, T-cell receptors, CD1 cell surface glycoproteins, secretory glycoproteins A/C, and Major Histocompatibility Complex (MHC) class I/II molecules. In immunoglobulins, each chain is composed of one variable domain (IgV) and one or more IgC domains. These names reflect the fact that the variability in sequences is higher in the variable domain than in the constant domain. The IgV domain is responsible for antigen binding, and the IgC domain is involved in oligomerization and molecular interactions [, ].
Glycoproteins B7-1 (CD80) and B7-2 (CD86) are expressed on antigen-presenting cells and deliver the co-stimulatory signal through CD28 and CTLA-4 (CD152) on T cells. Signalling through CD28 augments the T-cell response, whereas CTLA-4 signalling attenuates it []. CD80 contains two Ig-like domains, an amino-terminal immunoglobulin variable (IgV)-like domain characteristic of adhesion molecules and a membrane proximal immunoglobulin constant (IgC)-like domain similar to the constant domains of antigen receptors []. Members of the Ig family are components of immunoglobulin, T-cell receptors, CD1 cell surface glycoproteins, secretory glycoproteins A/C, and Major Histocompatibility Complex (MHC) class I/II molecules. In immunoglobulins, each chain is composed of one variable domain (IgV) and one or more IgC domains. These names reflect the fact that the variability in sequences is higher in the variable domain than in the constant domain. The IgV domain is responsible for antigen binding, and the IgC domain is involved in oligomerization and molecular interactions [, ].
The basic structure of immunoglobulin (Ig) molecules is a tetramer of two light chains and two heavy chains linked by disulphide bonds. There are two types of light chains: kappa and lambda, each composed of a constant domain (CL) and a variable domain (VL). There are five types of heavy chains: alpha, delta, epsilon, gamma and mu, all consisting of a variable domain (VH) and three (in alpha, delta and gamma) or four (in epsilon and mu) constant domains (CH1 to CH4). Ig molecules are highly modular proteins, in which the variable and constant domains have clear, conserved sequence patterns. The domains in Ig and Ig-like molecules are grouped into four types: V-set (variable; ), C1-set (constant-1; ), C2-set (constant-2; ) and I-set (intermediate; ) []. Structural studies have shown that these domains share a common core Greek-key β-sandwich structure, with the types differing in the number of strands in the β-sheets as well as in their sequence patterns [, ].Immunoglobulin-like domains that are related in both sequence and structure can be found in several diverse protein families. Ig-like domains are involved in a variety of functions, including cell-cell recognition, cell-surface receptors, muscle structure and the immune system []. This entry represents the C2-set type domains found in the T-cell antigen CD80, as well as in related proteins. CD80 (B7-1) is a glycoprotein expressed on antigen-presenting cells []. The shared ligands on CD80 and CD86 (B7-2) deliver the co-stimulatory signal through CD28 and CTLA-4 on T-cells, where CD28 augments the T-cell response and CTLA-4 attenuates it [].
T cell-dependent immune processes require cell-surface interactions thatmediate the initiation, modulation and the ultimate course of the response.The specificity of T cell recognition is determined by the engagement of theT cell receptor (TCR) on T cells with cognate peptide-MHC complexes presented by antigen presenting cells (APCs). Additional signals arerequired to sustain and enhance T cell activity, the most important of whichis provided by the engagement of CD28 on T cells with its ligands B7-1(CD80) and B7-2 (CD86). By contrast, the interaction of B7 isoformswith cytotoxic T lymphocyte-associated molecule-4 CTLA-4, a CD28 homologue receptor on T cells (31% identity), provides inhibitory signals requiredfor down-regulation of the response, while it may also prevent T cell activation by weak TCR signals[, , , , ].Unlike CD28, which is not expressed on resting T cells, CTLA-4 is not detected on the cell surface until 24 hours after activation. In fact, Tcell activation leads to both increased CTLA4 gene expression andtrafficking of CTLA4 protein to the cell surface. In addition, CTLA-4exhibits an affinity for the B7 isoforms that is 10 to 100 times that forCD28. Covalent dimerisation of CTLA4 is required for its high bindingavidity, but each monomeric subunit also contains a binding site for CD80and CD86. It is likely that CTLA-4 directly competes with CD28 for bindingB7 and also directs the assembly of inhibitory signalling complexes thatlead to quiescence or anergy. Thus the balance between the opposing signals elicited by CD28 and CTLA-4 is central to the regulation of T cellresponsiveness and homeostasis. One mechanism by which CTLA-4 may performthis function is by regulating cell-cycle progression; by contrast with CD28, which down-regulates the cell-cycle inhibitor p27kip1, CTLA-4 prevents this degradation[, , ].Sequence comparison between human CTLA-4 and CD28 proteins suggests they arehomologous, with the highest of degree of similarity being in the juxta-membrane and cytoplasmic regions. In addition, the cytoplasmic domainsof human and murine CTLA-4 are identical, suggesting that this region hasimportant functional properties [].Typically, activation of T cells by TCR-engaging peptide-MHC is dramatically enhanced by interaction of the CD28 co-stimulatory receptor with its ligands CD80 (B7-1) and CD86 (B7-2) on the APC surface. Interestingly, CTLA-4 is transported from intracellular stores toward the region of the cell surface receiving activation signals. This suggests that binding of CD28 to its ligand may occur primarily at the centre of the mature immunological synapse, and that CTLA-4 may be transported to this site under certain circumstancesto block or reverse this effect.
