CD28 Signaling in T-Helper Cell

 

CD28 (Antigen CD28) is characterized as a co-receptor for the TCR (T-Cell Receptor)/CD3 (CD3 Antigen) complex and is responsible for providing the co-stimulatory signal required for T-cell activation. CD28 also act as a receptor independent of the TCR and can initiate signaling events without concomitant TCR ligation. Priming of naive T-cells in lymphoid organs depends on the interaction between CD28, which is constitutively expressed in T-cells, and both CD80 (CD80 Antigen) and CD86 (CD86 Antigen) and induces subsequent IL-2 (Interleukin-2) production and clonal expansion for effective immune response. CD28 is a major positive co-stimulatory molecule required for T-cell activation and functional differentiation, and that CTLA4 (Cytotoxic T-Lymphocyte Antigen-4) upon ligation with CD80 and CD86 provides a negative co-stimulatory signal for the termination of activation and cellular function of T-cells (Ref.1&2). T-cells also help to control and eliminate microbial infections by detecting the causative agent and initiation of this process DCs (Dendritic Cells. The message transferred through the DCs is essentially composed of two signals. One signal is captured by the TCR and the other signal is relayed by CD28, which is a T-cell specific homodimeric receptor that serves to amplify the signal triggered by TCR ligation. The T-Helper cells recognize antigen when processed by antigen presenting cells/DCs containing MHC Class-II and CD4 (CD4 Antigen) (Ref.1&3). In naive T-cells, CD28 signaling is initiated by activation of two dominant signaling cascades, one that requires phosphorylation of a tyrosine residue within the membrane proximal YMNM motif and subsequent binding of the p85 subunit of PI3K whereas another pathway is initiated by the more distal proline-rich regions (Ref.4).

In the first signaling, CD28 binds to the adaptor protein GRB2 via its SH2 domain at the proximal YMNM motif or its SH3 domain at the distal PYAP motif. GRB2 through an SH3 interaction may subsequently bind VAV which has the potential to initiate two signaling complexes. The VAV-SOS complex results in CDC42/RAC1 activation an initiator of cytoskeletal rearrangement and downstream MAPK activation (JNK) that induces the formation of the AP1 transcriptional complex. VAV also binds the SLP76-LAT complex, which activates PLCγ1 a kinase that increases intracellular Ca2+, through IP3 production, and activates PKCδ via DAG. The increase in intracellular Ca2+ results in calcineurin activation, a phosphatase that acts on NFAT allowing for its nuclear translocation. PKCδ activation leads to the formation of a multi-protein complex with BCL10, MALT1 and CARMA-1 that induces NF-κB transcriptional activation. The distal PYAP motif also binds LCK a SRC kinase that phosphorylates PDK1 which phosphorylates and activates PKCδ and subsequently inactivates GSK3β via phosphorylation resulting in enhanced transcription of NFAT–dependent genes. Stabilization of cytokine mRNA is also dependent on signals originating at the PYAP through a mechanism that may involve LCK binding or an unknown protein whereas cytoskeletal rearrangement at the PYAP motif is FLNA (filamin-A) dependent (Ref.3, 4&5).

In another type of signaling, tyrosine phosphorylation of the YMNM motif via SRC family kinases initiates the binding of the p85 subunit of PI3K. PI3K activity leads to the production of D-3 lipids, which recruit proteins via their PH (pleckstrin homology) domain, including PDK1 and PKB/AKT. Once PKB is phosphorylated by PDK1, PKB phosphorylates its downstream targets including MTOR, IκB, GSK3β and Bad. Active MTOR and IκB result in increased NF-κB transcriptional activity whereas the phosphorylation of BAD and GSK3β results in increased survival and NFAT transcriptional regulation, respectively. The activation of NF-κB and NFAT induces the transcription of both BCL2L1, a prosurvival factor, and IL-2, an important T-cell cytokine required for proliferation (Ref.4).

CDC42 and RAC, function sequentially to activate downstream effectors like WASP (Wiskott - Aldrich syndrome Protein) and PAK1 (p21/CDC42/Rac1-Activated Kinase-1) to induce activation of ARPs (Actin-Related Proteins) resulting in Actin alterations in the cytoskeleton (Ref.3 & 5). CD28 impinges on the RAC/PAK1-mediated IL-2 gene transcription through subsequent activation of MEKK1 (MAP/ERK Kinase Kinase-1), MKKs (Mitogen-Activated Protein Kinase Kinases) and JNKs (c-Jun Kinases). JNKs phosphorylate and activate c-Jun (Jun Oncogene) and c-Fos (Cellular Oncogene Fos) -a part of the Activator Protein-1 complex essential for transcription of IL-2 (Ref.5&6).

The central positioning of the CD28/CD80/86/CTLA-4 receptor–ligand system in the initiation and control of effector and T cell responses makes it an attractive target for therapeutic modulation of the immune system. Blockade of the shared ligands CD80/86 is now firmly established in RA management and is also effective in boosting immune attack on cancer by blocking CTLA-4 ( Ref.3).