CD28 is a co-receptor for the TCR/CD3 complex and is responsible for providing the co-stimulatory signal required for T-cell activation. Priming of naive T-cells in lymphoid organs depends on the interaction between CD28 on T-cells and both CD80 and CD86 on antigen presenting cells (APC), and induces subsequent IL-2 production and clonal expansion of T-cells for an effective cell-mediated immune response. CD28 is a major positive co-stimulatory molecule required for T-cell activation and functional differentiation, and upon ligation with CD80 and CD86, CTLA4 provides a negative co-stimulatory signal for the termination of activation and cellular function of T-cells. One of the important receptors on T-cells is CD45, which occurs as a component of a complex of proteins associated with the antigen receptor. CD45 can regulate signal transduction by modulating the phosphorylation state of tyrosine kinases such as Lck. Lck and Fyn remain attached to the cytoplasmic domain of either CD4 or CD8. Concomitantly, activation of Lck and Fyn phosphorylates ZAP70, SYK and Vav1. Activated Lck in turn activates CD28 and induces activation of LAT. LAT binds to a number of proteins, including GADS, SLP76, ITK, Vav1 and Tec. These interaction lead to the activation of PLC-γ,RLK,CARMA1,BCL10, CDC42 and Rac, thereby facilitating the recruitment of key signal transduction components to drive T-cell activation.Further binding of CD28 to Class-I regulatory PI3K recruits PI3K to the membrane, resulting in generation of PIP3 and recruitment of proteins that contain a pleckstrin-homology domain to the plasma membrane, such as PIK3C3. PI3K is required for activation of Akt, which in turn regulates many downstream targets that to promote cell survival. In addition to NFAT, NF-κB has a crucial role in the regulation of transcription of the IL-2 promoter and anti-apoptotic factors. For this, PLC-γ utilizes PIP2 as a substrate to generate IP3 and DAG. IP3 elicits release of Ca2+ via IP3R, and DAG activates PKC-θ. Under the influence of RLK, PLC-γ, and Ca2+; PKC-θ regulates the phosphorylation state of IKK complex through direct as well as indirect interactions. Moreover, activation of CARMA1 phosphorylates BCL10 and dimerizes MALT1, an event that is sufficient for the activation of IKKs.
The two CD28-responsive elements in the IL-2 promoter have NF-κB binding sites. NF-κB dimers are normally retained in cytoplasm by binding to inhibitory I-κBs. Phosphorylation of I-κBs initiates its ubiquitination and degradation, thereby freeing NF-κB to translocate to the nucleus. Likewise, translocation of NFAT to the nucleus as a result of calmodulin-calcineurin interaction effectively promotes IL-2 expression. Activation of Vav1 by TCR-CD28-PI3K signaling connects CD28 with the activation of Rac and CDC42, and this enhances TCR-CD3-CD28 mediated cytoskeletal re-organization. Rac regulates actin polymerization to drive lamellipodial protrusion and membrane ruffling, whereas CDC42 generates polarity and induces formation of filopodia and microspikes. CDC42 and Rac GTPases function sequentially to activate downstream effectors like WASP and PAK1 to induce activation of ARPs resulting in cytoskeletal rearrangements. CD28 impinges on the Rac/PAK1-mediated IL-2 transcription through subsequent activation of MEKK1, MKKs and JNKs. JNKs phosphorylate and activate c-Jun and c-Fos, which is essential for transcription of IL-2. Signaling through CD28 promotes cytokine IL-2 mRNA production and entry into the cell cycle, T-cell survival, T-Helper cell differentiation and Immunoglobulin isotype switching.