Regulation of immune responses is critical for functional protection of an organism. Controlling the activation of self-reactive cells is one of the most critical aspects of regulation. The immune system is faced with the problem of distinguishing harmful from innocuous stimuli and one way to regulate the immune response is by fine-tuned control of T cells and APCs. This can be achieved through developmental checkpoints, structural barriers, cytokines, localized antigen deposition, homing receptors, and activation control through molecules expressed by T cells and APCs.
A fundamental concept in T cell activation is the requirement of two signals for optimal T cell activation. Bretscher and Cohn first proposed a “two-signal” hypothesis in their theory of self-nonself discrimination. Lafferty and Woolnough further developed this hypothesis to explain why transplants of foreign tissue are not always rejected. Their theory that full activation of naive T cells requires signals through both antigen receptors (signal one) and another receptor (signal two) is now widely accepted. At the time when the two-signal model was first proposed, TCR and costimulatory receptors were not yet identified. CD28 molecule was the first receptor to be identified as being able to provide the second signal. The discovery that mice deficient in CD28 can still mount a response in some models suggested the presence of other receptor-ligand pairs able to contribute the second signal. In addition, it has been shown that signals through TCR are not uniform, as some altered peptides and superantigens can induce T cell responses that do not follow the dogmatic self-MHC/foreign peptide scheme. With the discovery of more accessory receptors came the discovery of negative regulators. It is now known that T cell activation and subsequent proliferation and survival are complex processes that are regulated by a variety of accessory molecules.
Full activation of T cells requires signals through TCR/CD3 complex with its coreceptors CD4 or CD8 and through a costimulatory molecule, such as CD28. However, in vitro, T cells stimulated with a high concentration of anti-CD3 antibody can induce cell division and upregulation of activation markers in the absence of costimulation. In vivo, the level of TCR stimulation is normally much lower, and such interactions are better represented in vitro by suboptimal concentrations of anti-CD3 antibody. Thus, costimulatory molecules are required to lower the threshold of TCR signal required for T cell activation. In addition, they also induce proliferation and survival and help shape the responses.
Submitogenic exposure to peptide antigen in the absence of a costimulatory signal induces a state of unresponsiveness known as clonal anergy. The anergic state is characterized by the acquired inability of a lymphocyte to respond to appropriate MHC/peptide complexes by proliferation, cytokine secretion, and differentiation. The hallmark of anergic T cells is the absence of interleukin (IL)-2 production upon exposure to antigen, even upon subsequent restimulation in the presence of costimulatory molecules.
At present, two major families of costimulatory molecules have been identified. The CD28/B7 family encompasses CD28 and CTLA-4 and their ligands B7.1 (CD80) and B7.2 (CD86), ICOS and ICOS ligand (ICOSL), and the coinhibitory receptor Programmed Death receptor 1 (PD-1) and its ligands PD-L1 and PD-L2. In addition, a CD28 homolog member B and T lymphocyte attenuator (BTLA) and two B7 homologs B7-H3 and B7-H4 (B7x, B7S1) have recently been identified. Both CD28 and CTLA-4 exist as disulfide-linked homodimeric glycoproteins. They both contain the MYPPPY ligand binding domain that allows binding of B7.1 and B7.2. ICOS contains a slightly altered motif FDPPPD and thus does not bind the same ligands. All three molecules have PI3K binding motifs in their cytoplasmic domains. The receptor/ligand pairs in this family mostly act directly on the T cells, but can also influence other cell types.
The TNFR/TNF family consists of a number of receptor/ligand pairs. A subset of these molecules is involved in survival signaling in T cells subsequent to the initial effects of CD28/B7 interaction as well as in cell death. Members of this family that induce survival contain cysteine-rich extracellular domains and interact with various TRAF adaptor proteins to activate NF-κB.
CD28 Family. Members of the CD28 family have a single extracellular immunoglobulin (Ig) variable-like domain followed by a short cytoplasmic tail. CD28, ICOS, and CTLA-4 genes are clustered in close proximity and have an unpaired cysteine that allows them to homodimerize on the T cell surface. The ligand binding sites on these three receptors surround a PPP motif and their cytoplasmic tails contain a central Src homology 2 (SH2) binding motif YXXM. In contrast, PD-1 and BTLA are positioned in distinct locations in the genome and are more similar to each other than to the other members of the family.
TNFR Family. TNF receptor (TNFR) superfamily currently consists of 29 members that are characterized by the presence of a cysteine-rich domain in the extracellular portion. Their ligands usually belong to the TNF family, although binding of the TNFR family member herpes virus entry mediator (HVEM) to a CD28 family member BTLA has been recently documented. The TNF superfamily members have a 20% to 30% amino acid identity in their TNF homology domains, which are responsible for binding to the receptors. Whereas the TNFR members are expressed on a wide variety of cells, almost all TNF ligands are expressed by the cells of the immune system such as T cells, B cells, NK cells, monocytes, and DCs.
Most of the TNFRs are characterized as type I transmembrane proteins (extracellular N-terminus and intracellular C-terminus). Also, most of them contain transmembrane domains, but soluble forms of several receptors have been identified, including the soluble form of 4-1BB generated by alternative splicing. Receptors of the TNFR superfamily are broadly divided into two groups: death receptors, consisting of death-domain-containing receptors and decoy receptors, and the TNF receptor-associated factor (TRAF) binding receptors. The majority of the TNFR superfamily members signal by binding to one or more TRAFs. Members of the TNF superfamily activate NF-κB through ubiquitin-mediated degradation of its inhibitor, IκBα. This process is initiated by phosphorylation of IκBα by the IκBα kinase (IKK) complex, and is dependent on IKKβ and IKKγ (NEMO) catalytic subunits. This results in proteasomal degradation of IκBα and liberation of NF-κB dimers. BAFF and CD40 signal through the concomitant NF-κB activation pathway, causing an IKKβ- and IKKγ-independent phosphorylation of p100 by IKK-α (NIK) and the proteasomal degradation of p100.