During the 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 T-cell receptor (TCR)/ CD3 antigen (CD3) complex, a second co-stimulatory signal from APCs is necessary for activation of naive T-cells. T-cell activation induces co-stimulatory molecules, including the Inducible CO-Stimulator (ICOS)/ Activation-Inducible Lymphocyte Immunomediatory Molecule (AILIM). ICOS mediated signaling contributes mainly to the regulation of activated T-cells and to effector T-cell functions. The potency of ICOS is enhanced following its ligation to ICOSL. ICOS mediated signaling mainly generates Th1 and Th2 responses.Antigen presenting B-cells with MHC class-II complex is recognized by TCR/CD3 and CD4 on T-helper cells. CD4 is an accessory protein essential for MHC class-II/TCR/CD3 interaction. Interaction of MHC class-II/TCR/CD3 and CD4 activates Lck. CD45 also activates Lck by converting the inactive form of Lck to its active form, a mechanism which is counterbalanced by CSK. Lck remains attached to the cytoplasmic domain of either CD4 or CD8. Activated Lck in turn stimulates activation of ZAP70. LAT is a palmitoylated integral membrane adaptor protein that resides in lipid membrane rafts and upon MHC class-II/TCR engagement is phosphorylated by ZAP70 and binds to the adaptor - GADS, the SH2 domain containing protein - SLP76, ITK, Vav1 and Tec. This interaction activates PLC-γ1 and RLK, thereby facilitating the recruitment of key signal transduction components to drive T-cell activation.
CD28 possesses an MYPPY motif that is essential for binding to CD80 and CD86. ICOS, CD28 and TRIM activation stimulates the regulatory PI3Ks (PIK3R1, PIK3R2, PIK3R3). These catalytic PI3Ks again activate PIK3C3 /p110-γ. This process is further aided by PI3K effectors like GRB2 activating GAB2 and SHC and G-proteins like GN-β and GN-γ. The interaction of PI3K effectors and G-proteins with PI3K is facilitated by IL-2 through IL-2R and GPCRs. PIK3Cγ phosphorylates cellular PIP2 to PIP3. PIP3 activates the kinases PDK-1 and Akt. 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. Vav1 activity is instrumental in Rac1 induced cytoskeletal regulation by RhoGTPases, leading to enhancement of phagocytosis and cell motility. However, SHIP counteracts PIP3 action through dephosphorylation and conversion of PIP3 to PIP2. A similar PIP3 inhibitory function is also executed by PTEN, where excess of PIP3 accumulation is controlled by dephosphorylation to PIP2. Further PLC-γ1 utilizes PIP2 as a substrate to stimulate the production of PIP3 and DAG.
The interaction of PIP3 with its receptor, IP3R leads to release of Ca2+/calcium ions whereas DAG activates PKC-θ. PKC-θ then regulates the IKK complex indirectly. NF-κB is translocated to the nucleus followed by degradation of I-κB. Calcium ions also promote NF-κB translocation indirectly through activation of Calm, which in turn facilitates CaMKII activation. Once active, Calm indirectly enhances NFAT translocation to the nucleus via Caln. 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.