The immune system has the remarkable ability to defend against a diversity of microbial pathogens, yet not respond to self. The discovery of the T-cell antigen receptor (TCR) provided an explanation for the specificity and diversity of T-cell responses. Yet, functional studies of Lafferty and colleagues (1) suggested that antigen alone was not sufficient to drive the activation of naive T cells and led to the two signal concept of T-cell activation. According to this model, productive T-cell activation requires a first signal provided by the interaction of antigenic peptide/major histocompatibility complex (MHC) with the TCR and a second, antigen-independent co-signal, the ‘costimulatory signal’. In addition, seminal studies of Jenkins and Schwartz (2, 3) demonstrated that TCR-mediated activation of T cells in the absence of costimulation resulted in antigen-specific unresponsiveness (termed T-cell anergy), rendering the T cells unable to respond to subsequent exposure to antigen. Thus, costimulation was postulated to have a pivotal role in determining whether the outcome of T-cell encounter with antigen would be activation or anergy.

The critical role of costimulation in regulating the immune response has given impetus to the study of costimulation and resulted in incredible growth of this field, since an understanding of costimulation is of fundamental and therapeutic interest. Early studies defined the function of the CD28 receptor as a stimulatory receptor for the activation of naive T cells (2), and identified the B7 family members, B7-1 (CD80) and B7-2 (CD86) as its ligands (5–7). The interaction between CD28 and B7-1/B7-2 fulfilled many of the requirements for the costimulatory signal postulated by Lafferty, Schwarz, and colleagues (1). The CD28 homolog cytotoxic T lymphocyte-associated antigen-4 (CTLA-4)(3) was found to be a higher affinity binding partner for B7-1 and B7-2, as compared to CD28 (4). It was presumed that CTLA-4 would also be a stimulatory receptor; however, the dramatic fatal inflammatory phenotype of CTLA-4 deficient (−/−) mice revealed the critical inhibitory function of CTLA-4 (10, 11). Furthermore, the phenotype of CTLA-4