During immune response, T-cells are optimally activated in secondary lymphoid tissues in order to properly migrate into areas of inflamed tissue. Upon antigen recognition via the TCR (T-cell receptor)/CD3 (CD3 antigen) complex, a second co-stimulatory signal from APCs or antigen-presenting cells is necessary for activation of naive T-cells. According to the “Two-Signal Model” for T-cell activation, although the engagement of TCR/CD3 by antigen/MHC (major histocompatibility complex) products is essential for the initial stages of T-cell activation, a second signal, termed a co-stimulatory signal, is required for clonal expansion and functional differentiation of antigen-specific T-cells. T-cell activation induces co-stimulatory molecules, including the ICos (inducible co-stimulator)/AILIM (activation-inducible lymphocyte immunomediatory molecule), which is the third member of the CD28 (antigen CD28) family and is only expressed at very low levels on naive T-cells. ICos-mediated signal contributes mainly to the regulation of activated T-cells and to effecter T-cell functions . The potency of ICos is enhanced following ICos ligation by ICosL (inducible T-cell co-stimulator ligand). The FDPPPF motif on ICos facilitates this ligation during T-cell proliferation and significantly increases the production of cytokines. ICos-mediated signaling mainly generates Th1 (T-Helper-1) and Th2 responses. Even though ICos belongs to the CD28 family of receptors and CD28 is a major positive co-stimulatory molecule for T-cell activation and functional differentiation, there are several unique features of the ICos system that differ from CD28 signaling. For example, while CD28-mediated co-stimulatory signaling facilitates the production of IL-2 (interleukin-2), the ICos ligation results in a negligible enhancement of IL-2 production. Furthermore, CD28 is constitutively expressed by naive T-cells, whereas ICos expression is inducible by activation .
ICos promotes T-cell/B-cell collaboration through the CD40 (B-Cell-Associated Molecule CD40)/CD40L (CD40 Antigen Ligand) pathway. The outcome of such interaction is required for both lymphocyte activation and development of adaptive immunity and has a key role in promoting Ig (immunoglobulin) isotype switching [3,4]. B-cells often act as APCs and T-Helper cells only recognize antigens when processed by APCs having MHC Class II as a ligand. In general, T-Helper cells recognize antigens in context with the MHC Class II complex. Antigen-presenting B-cells with MHC Class II complex is recognized by TCR/CD3 and CD4 (CD4 antigen) on the T-Helper cells. CD4 is an accessory protein essential for MHC Class II/TCR/CD3 interaction and plays a more general role in mediating cell recognition events than merely those of cellular immune response. Interaction of MHC Class II/TCR/CD3 and CD4 activates Lck (lymphocyte-specific protein–tyrosine kinase). CD45 (CD45 antigen) also activates Lck by converting inactive form of Lck to its active form, a mechanism which is counterbalanced by CSK (c-Src tyrosine kinase) [1,5]. Lck remains attached to the cytoplasmic domain of either CD4 or CD8 (CD8 antigen). Activated Lck in turn stimulates activation of ZAP70 (zeta-chain-associated protein kinase). ZAP70 remains associated with the TCR-Zeta chain and induce activation of LAT (linker for activation of T-cells) and TRIM (T-cell receptor interacting molecule). LAT is critical for coupling the pre-TCR surface complexes to intracellular signaling pathways in the developmental response. LAT is a palmitoylated integral membrane adaptor protein that resides in lipid membrane rafts and upon MHC Class II/TCR engagement. It is phosphorylated by ZAP70 and binds to the adaptor GADS (growth factor receptor-bound protein-2-related adaptor protein-2), SLP76 (SH2 domain-containing leukocyte protein-76), ITK (IL-2 inducible T-cell Kinase), Vav1 (oncogene Vav1), and Tec (Tec protein tyrosine kinase); this interaction activates PLC-Gamma1 (phospholipase-C-gamma1) and RLK (resting lymphocyte kinase), thereby facilitating the recruitment of key signal transduction components to drive T-cell activation [1,6].
ICos binds specifically to its counter-receptor ICosL, but not to CD80 (CD80 antigen) or CD86 (CD86 antigen). In a similar manner, CD28 does not bind to ICosL; thus the CD28 and ICos pathways do not cross-interact on the cell surface. CD28 possesses a MYPPY motif that is essential for binding to the CD80 and CD86 [5,6]. Although less is known about the mechanism of ICos signaling, the co-receptor shares with CD28 an ability to bind PI3K (phosphatidylinositde-3 kinase). ICos, CD28, and TRIM contain consensus PI3K-binding motifs (YXXM) which contribute to and complement TCR-dependent PI3K signaling. PI3Ks regulate numerous biological processes, including cell growth, differentiation, survival, proliferation, migration, and metabolism. In the immune system, impaired PI3K signaling leads to immunodeficiency, whereas unrestrained PI3K signaling contributes to autoimmunity and leukemia. ICos, CD28, and TRIM activation stimulates the regulatory PI3Ks—PIK3R1 (phosphoinositide-3-kinase-regulatory subunit polypeptide-1), PIK3R2 and PIK3R3 (phosphoinositide-3-kinase-regulatory subunit polypeptide-3)—that in turn activate the catalytic PI3Ks: PIK3CAlpha (phosphoinositide-3-kinase-catalytic-alpha polypeptide), PIK3CBeta (phosphoinositide-3-kinase-catalytic-beta polypeptide), and PIK3CDelta (phosphoinositide-3-kinase-catalytic-delta polypeptide). These catalytic PI3Ks again activate PIK3C3 (phosphoinositide-3-kinase-class-3)/p110-gamma. This process is further aided by PI3K effectors like GRB2 (growth-factor receptor-bound protein-2), activating GAB2 (GRB2-associated binding protein-2) and SHC (Src homology-2 domain containing transforming protein) and G-Proteins like GN-Beta (guanine nucleotide-binding protein-beta) and GN-Gamma (guanine nucleotide-binding protein-gamma). The interaction of PI3K effectors and G-Proteins with PI3K is facilitated by IL-2 through IL-2R (interleukin-2 Receptor) and GPCRs (G-protein-coupled receptors). PIK3CGamma phosphorylates the cellular PIP2 (phosphatidylinositol-4,5-bisphosphate) to PIP3 (phosphatidylinositol-3,4,5-trisphosphate) [1,7]. PIP3 activates the kinases PDK-1 (3-phosphoinositide-dependent protein kinase-1) and Akt (v-Akt murine thymoma viral oncogene homolog). Akt activation by PDK-1 and PIP3 up regulates Akt Signaling and BAD Phosphorylation that promotes events such as protein translation, prevention of cell death, and the up regulation of cellular metabolism. PIP3 along with GRB2, CD28, and c-Cbl (Cas-Br-M murine ecotropic retroviral transforming sequence homolog) indirectly activate PLC-Gamma1 by accelerating the function of LAT, GADS, SLP76, ITK, Vav1, and Tec, whereas active RLK facilitates direct activation of PLC-Gamma1. Vav1 activity is instrumental in Rac1 (Ras-related C3 botulinum toxin substrate) induced cytoskeletal regulation by RhoGTPases, leading to enhancement of phagocytosis and cell motility. However, SHIP (SH2-containing inositol phosphatase) counteracts PIP3 action through dephosphorylation and conversion of PIP3 to PtdIns(3,4)P2 (D-myo-Phosphatidylinositol 3,4-bisphosphate). This conversion terminates Akt signaling and BAD phosphorylation and the PI3K signal may then be “taken over” by the recently discovered TAPP1 (tandem PH-domain-containing protein-1) and TAPP2 adaptor proteins, which have binding specificity for PtdIns(3,4)P2. Whether these proteins propagate negative signaling pathways or have other functions remains to be determined. Although the downstream targets of TAPP proteins remain unidentified, they may influence the process of cytoskeleton rearrangement. A similar PIP3 inhibitory function is also executed by PTEN (phosphatase and tensin homolog), where excess of PIP3 accumulation is controlled by dephosphorylation to PIP2. Further PLC-Gamma1 utilize PIP2 as a substrate to stimulate the production of IP3 (Inositol 1,4,5-trisphosphate) and DAG (Diacylglycerol) [8,9].
The interaction of IP3 with its receptor, IP3R (IP3 receptor) leads to release of Ca2+/Calcium ions whereas DAG activates PKC-Theta (Protein Kinase-C-Theta). PKC-Theta then regulate the IKK (inhibitor of kappa light polypeptide gene enhancer in B-cells kinase) complex indirectly though CaMKII (calcium/calmodulin-dependent protein kinase-II) and it directly activate the complex but this activation ultimately results in translocation of NF-KappaB (nuclear factor-kappaB) to the nucleus, followed by degradation of I-KappaBs (inhibitor of kappa light chain gene enhancer in B-cells). Calcium ions also promote NF-KappaB translocation indirectly through activation of Calm (calmodulin), which in turn facilitates CaMKII activation. Once active, Calm indirectly enhances NFAT (nuclear factor of activated T-cells) translocation to the nucleus via Caln (calcineurin). Functionally, the human ICos, is one of the few “two-signal molecules”, which co-induces a variety of cytokines including IL-4, IL-5, IL-6, and IFN-Gamma (Interferon-Gamma), but not IL-2, and super induces IL-10 by promoting NFAT and NF-KappaB translocation to the nucleus [7,9]. Furthermore, ICos co-stimulation prevents the apoptosis of pre-activated T-cells. ICos thus has a novel and distinct functional role in T-cell migration and polarization compared to other co-stimulatory molecules. ICos has important physiological roles in the regulation of Th1 cells in endothelium and in the control of the selective entry of Th1 cells into inflamed peripheral tissue . Defective ICos-mediated co-stimulation leads to autoimmune diseases, encephalomyelitis, acute GVHD (graft-versus-host disease), and lung and mucosal inflammation, to name a few. Manipulation of the ICos-ICosL pathway also has important therapeutic implications beyond the Th1 and Th2 field and these may add to the growing arsenal of selective immunotherapeutics [11,12].
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