Antigen (Ag) recognition by the T cell receptor (TCR) induces activation ofT lymphocytes. However, TCR-mediated signals alone are insufficient forefficient T cell activation, and additional co-stimulatory signals are required. One of the most important surface molecules that delivers co-stimulatory signals for T cells is CD28. The human T lymphocyte Ag CD28 (Tp44) is a homodimeric 90kDa glycoprotein expressed on the surface of themajority of human peripheral T cells and lymphocytes. Stimulation of CD4+ Tcells in the absence of CD28 co-signalling causes impaired proliferation, reduced cytokine production and altered generation of helper T cell subsets.Co-stimulation via CD28 promotes T cell viability, clonal expansion,cytokine production and effector functions, while also regulating apoptosisof activated T cells, suggesting its importance in regulating long-term T cell survival [, , , ].Ligands for CD28 and the structurally related CTLA-4 (CD152) are themolecules B7.1 (CD80) and B7.2 (CD86). B7.1 and B7.2 are expressed onprofessional antigen presenting cells (APCs) and their expression is up-regulated during an immune response. Ligation of CD28 by its natural ligandsresults in tyrosine phosphorylation at a YMNM motif within its cytoplasmictail. The phosphorylated motif subsequently interacts with the Src homology2 domain in the p85 regulatory subunit of P13K, activating the p110 catalytic subunit. One of the P13K-dependent downstream targets, resulting from the antibody cross-linking of CD28, is the phoshporylation and activation of Akt (or PKB). Constitutively active Akt is able to substitutefor CD28 signals, and stimulates IL-2 production when introduced into matureCD28-deficient cells. Another molecule affected by CD28 stimulation is theproto-oncogene Vav, which acts as a guanine-nucleotide exchange factor forRac and CDC42, allowing these molecules to switch from the inactive GDP-bound state to the active GTP-bound state [, ].Another interesting feature of CD28, is its ability to induce expression ofPDE7, a cAMP phosphodiesterase, thus reducing cellular cAMP levels. cAMP hasbeen reported to affect nearly every pathway important for lymphocyteactivation, leading to inhibition of T cell proliferation. Specifically,increased intracellular cAMP has been implicated in the induction of T cellanergy, a non-responsive state that occurs after T cells are stimulatedthrough TCR/CD3 in the absence of co-stimulation. This can have therapeutic implications, in that blockage of CD28 co-stimulation can be profoundlyimmunosuppressive, preventing induction of pathogenic T cell responses inautoimmune disease models, and allowing for prolonged acceptance of allografts in models of organ transplantation []. Finally, CD28 co-stimulation directly controls T cell cycle progression by down-regulating the cdk inhibitor p27kip1, which actually integratesmitogenic MEK and P13K-dependent signals from both TCR and CD28 [].
ICOS is an inducible T-cell co-stimulator with homology to CD28 []. CD28 and ICOS function similarly during expansion, survival and differentiation of T cells. They are necessary for proper IgG responses. However, CD28 induces IL-2 production, while ICOS is more potent in the induction of IL-10 production [].
HSH2D (hematopoietic SH2 domain-containing protein, also known as ALX) is an adapter protein involved in tyrosine kinase and CD28 signaling []. It is expressed exclusively in hematopoietic cells. It contains an Src homology 2 (SH2) domain, four PXXP polyproline sequences, and two likely sites of tyrosine phosphorylation. The SH2 domain plays a critical role in ALX function downstream of CD28 [].
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.
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 [, ].
STKs (serine/threonine-protein kinases) catalyse the transfer of the gamma-phosphoryl group from ATP to serine/threonine residues on protein substrates. STK10/LOK, also called polo-like kinase kinase 1 in Xenopus (xPlkk1) [], is highly expressed in lymphocytes and is responsible in regulating leukocyte function associated antigen (LFA-1)-mediated lymphocyte adhesion []. It plays a role in regulating the CD28 responsive element in T cells [], and may also function as a regulator of polo-like kinase 1 (Plk1), a protein which is overexpressed in multiple tumour types [].
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[, , , , ].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 [].
The CD2 adhesion molecule is a cell surface protein expressed by T cells and natural killer cells. CD2's extracellular domain contains immunoglobulin-like domains that are glycosylated at two sites and can mediate homodimerisation []. Ligation of CD2 by CD58 in humans or CD48 in mice helps T cells adhere to antigen-presenting cells, and initiates signal transduction pathways that enhance signalling through the T cell receptor for antigen. CD2 knockout mice exhibit essentially normal immune function [], and it is thought that CD2 is somewhat functionally redundant with other T cell co-stimulatory receptors such as CD28 [].
The LFA-3 (CD2) adhesion molecule is a cell surface protein expressed by T cells and natural killer cells. LFA-3's extracellular region contains immunoglobulin-like domains that are glycosylated at two sites and can mediate homodimerisation []. Ligation of LFA3 by CD58 in humans or CD48 in mice helps T cells adhere to antigen-presenting cells, and initiates signal transduction pathways that enhance signalling through the T cell receptor for antigen. LFA-3 knockout mice exhibit essentially normal immune function [], and it is thought that LFA-3 is somewhat functionally redundant with other T cell co-stimulatory receptors such as CD28 [].
This entry represents the SH3 domain of PI3K (phosphoinositide-3-kinase) subunit beta (also known as subunit p85-beta). Class I PI3Ks convert PtdIns(4,5)P2 to the critical second messenger PtdIns(3,4,5)P3. Class IA PI3Ks consist of a p110 catalytic subunit bound to one of five regulatory subunits, generated by alternative splicing of three different genes: p85alpha, p85beta, and p55gamma [, ]. The p85 subunits contain the SH3, RhoGAP, and SH2 domains. They recruit the p110 subunit to the membrane, where p110 phosphorylates inositol lipids []. Vertebrates contain two p85 isoforms, alpha and beta. In addition to regulating the p110 subunit, p85beta binds CD28 and may be involved in the activation and differentiation of antigen-stimulated T cells [].
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 []. The CTLA-4 and B7-2 monomers are both two-layer β-sandwiches that display the chain topology characteristic of the immunoglobulin variable (V-type) domains present in antigen receptors. The front and back sheets of B7-2 are composed of AGFCC'C"and BED strands, respectively []. Members of the IgV family are components of immunoglobulin (Ig) and T cell receptors. The basic structure of Ig molecules is a tetramer of two light chains and two heavy chains linked by disulfide bonds. In Ig, each chain is composed of one variable domain (IgV) and one or more constant domains (IgC); these names reflect the fact that the variability in sequences is higher in the variable domain than in the constant domain []. Within the variable domain, there are regions of even more variability called the hypervariable or complementarity-determining regions (CDRs) which are responsible for antigen binding [].
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 [].
Tumor necrosis factor receptor superfamily member 9 (TNFRSF9), also known as CD137, ILA or 4-1BB, plays a role in the immunobiology of human cancer where it is preferentially expressed on tumor-reactive subset of tumor-infiltrating lymphocytes. It can be expressed by activated T cells, but to a larger extent on CD8 than on CD4 T cells []. In addition, CD137 expression is found on dendritic cells, follicular dendritic cells, natural killer cells, granulocytes and cells of blood vessel walls at sites of inflammation []. It transduces signals that lead to the activation of NF-kappaB, mediated by the TRAF adaptor proteins. CD137 contributes to the clonal expansion, survival, and development of T cells []. It can also induce proliferation in peripheral monocytes, enhance T cell apoptosis induced by TCR/CD3 triggered activation, and regulate CD28 co-stimulation to promote Th1 cell responses. CD137 is modulated by SAHA treatment in breast cancer cells, suggesting that the combination of SAHA with this receptor could be a new therapeutic approach for the treatment of tumors [].Mostly, CD137 in teleosts have not been characterized.This entry represents the N-terminal domain of TNFRSF9/CD137 from teleosts. TNF-receptors are modular proteins. The N-terminal extracellular part contains a cysteine-rich region responsible for ligand-binding. This region is composed of small modules of about 40 residues containing 6 conserved cysteines; the number and type of modules can vary in different members of the family [, , ].
Tumor necrosis factor receptor superfamily member 9 (TNFRSF9), also known as CD137, ILA or 4-1BB, plays a role in the immunobiology of human cancer where it is preferentially expressed on tumor-reactive subset of tumor-infiltrating lymphocytes. It can be expressed by activated T cells, but to a larger extent on CD8 than on CD4 T cells []. In addition, CD137 expression is found on dendritic cells, follicular dendritic cells, natural killer cells, granulocytes and cells of blood vessel walls at sites of inflammation []. It transduces signals that lead to the activation of NF-kappaB, mediated by the TRAF adaptor proteins. CD137 contributes to the clonal expansion, survival, and development of T cells []. It can also induce proliferation in peripheral monocytes, enhance T cell apoptosis induced by TCR/CD3 triggered activation, and regulate CD28 co-stimulation to promote Th1 cell responses. CD137 is modulated by SAHA treatment in breast cancer cells, suggesting that the combination of SAHA with this receptor could be a new therapeutic approach for the treatment of tumors [].This entry represents the N-terminal domain of TNFRSF9. TNF-receptors are modular proteins. The N-terminal extracellular part contains a cysteine-rich region responsible for ligand-binding. This region is composed of small modules of about 40 residues containing 6 conserved cysteines; the number and type of modules can vary in different members of the family [, , ].
ITK (also known as Tsk or Emt) is a member of the Tec family, which is a group of nonreceptor tyrosine kinases containing Src homology protein interaction domains (SH3, SH2) N-terminal to the catalytic tyr kinase domain. It also contains an N-terminal pleckstrin homology (PH) domain, which binds the products of PI3K and allows membrane recruitment and activation [], and the Tec homology (TH) domain, which contains proline-rich and zinc-binding regions. ITK is expressed in T-cells and mast cells, and is important in their development and differentiation [, ]. Of the three Tec kinases expressed in T-cells, ITK plays the predominant role in T-cell receptor (TCR) signaling. It is activated by phosphorylation upon TCR crosslinking and is involved in the pathway resulting in phospholipase C-gamma1 activation and actin polymerization []. It also plays a role in the downstream signaling of the T-cell costimulatory receptor CD28 [], the T-cell surface receptor CD2 [], and the chemokine receptor CXCR4 []. In addition, ITK is crucial for the development of T-helper(Th)2 effector responses []. This entry represents the SH3 domain of ITK.
