T-cell receptor signaling (Homo sapiens)

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The T-cell antigen receptor (TCR) complex is composed of a ligand-binding subunit, the α and β chains, and a signaling subunit, namely the CD3ε, γ and δ chains and the TCRζ chain. This complex participates in T-cell activation upon the presentation of the antigen peptide (derived from the foreign antigen) bound to the MHC (Class I and Class II) residing on antigen-presenting cells (APCs), including dendritic cells, macrophages and B cells. Co-stimulatory receptors, such as CD2, CD28, CD4, CD8, and integrin molecules, contribute to signal transduction by modulating the response threshold. All the above components along with accessory proteins essential for MHC are a part of the immunological synapse that initiates T-cell activation.

Protein tyrosine phosphorylation mediated by the Src family kinases Lck and Fyn, in turn regulated by CD45, is the initial event in TCR signaling. Lck is activated by the interaction of MHC and CD4 or CD8. It then induces the phosphorylation and activation of TCR-CD3 complex and eventually mediates Fyn activation. The activated Src kinases phosphorylate ITAMs (Immune receptor tyrosine-based activation motifs) present on CD3γ, δ, ε and ζ chain. The 70-kd ζ chain–associated protein kinase (ZAP70) is then recruited to the ITAMs followed by its phosphorylation and subsequent activation which results in further amplification of the response. These activated PTKs induce tyrosine phosphorylation of several polypeptides, including the transmembrane adaptor LAT (Linker Activator for T-Cells). Protein tyrosine phosphorylation subsequently leads to the activation of multiple pathways, including ERK (Extracellular Signal Regulated Kinase), JNK (c-Jun N-terminal Kinase), NF-κB (Nuclear factor of kappa light polypeptide gene enhancer in B-cells 1) and NFAT(Nuclear Factor of Activated T-Cells) pathways, which ultimately induce effector functions.

CD28 ligation by B7-1 or B7-2 helps in bringing the T-Cell and Antigen Presenting Cell membranes into close proximity. This event also provides a co-stimulatory signal leading to the production of IL-2 and T- cell activation. Besides CD28, many other transmembrane receptors also modulate specific elements of TCR signaling. CD28 and CD45 activate Lck which in turn induces the phosphorylation and activation of the TCR-CD3 complex and consequently, the tyrosine kinases Fyn and ZAP70. CD45 plays a role in antagonizing the effect of inhibitory proteins on T-cell activation. ZAP70 induces activation of LAT (Linker for Activation of T-Cells), an integral membrane adaptor protein which further binds to GADS (Growth Factor Receptor-Bound Protein-2-Related Adaptor Protein-2), SLP76 (SH2 Domain-Containing Leukocyte Protein-76), and ITK (IL-2 inducible T-cell kinase). This complex facilitates the activation of PLCG1 (Phospholipase-C-Gamma1) that is responsible for the production of the second messengers DAG (Diacylglycerol) and IP3 (Inositol Triphosphate). DAG activates proteins including PKCθ (Protein Kinase-C-Theta) and Ras, whereas IP3 leads to Ca2+ release from ER and also facilitates influx of extracellular calcium. Increased intracellular Ca2+ activates the phosphatase Calcineurin that dephosphorylates NFAT which then enters the nucleus and promotes transcription.

Activated LAT also binds multiple adaptor proteins including GRB2, GRAP (GRB2- Related Adaptor Protein) and GADS to facilitate activation of serine/threonine kinases such as Raf1, MEK (MAPK/ERK Kinase) and dual-specificity kinases ERK1/2 that are involved in activation of MAPKs (Mitogen activated protein kinases) resulting in the induction of transcription factor Elk1.

ZAP70 also activates IKKs via the CARD11 (Caspase recruitment domain family, member 11) -BCL10 (B-Cell CLL/lymphoma-10)-MALT1 (Mucosa Associated Lymphoid Tissue Lymphoma Translocation Gene-1) complex and MAP3K (activated by PKCθ) which in turn relieve NF-κB of IκB (NF-kappa-B inhibitor beta) and allow its nuclear translocation and transcriptional activation. ZAP70 also activates p38 via Vav, Rac and MKK3/6 (Mitogen-Activated Protein Kinase Kinase-3 / Mitogen-Activated Protein Kinase Kinase-6). p38 then activates the transcription factor ATF2 (Activating Transcription Factor-2). Activated Rac also activates JNK via MEKK1 and MKK4/7 (MKK4 (Mitogen-Activated Protein Kinase Kinase-4/ Mitogen-Activated Protein Kinase Kinase-7) which eventually leads to activation of c-Jun mediated transcription.

SIT (SHP2-Interacting Transmembrane Adaptor Protein) and CTLA4 (Cytotoxic T-Lymphocyte Antigen-4) are transmembrane adaptor proteins that interact with the SHP2 (SH2-containing Protein tyrosine Phosphatase-2) and negatively regulate T-cell activation by inhibiting the phosphorylation of Fyn and CD28 respectively. CTLA4 has a greater affinity for its B7-1/B7-2ligands in comparison toCD28 and thus competes for them. During T-cell activation CTLA4 is endocytosed to quicken the response. ZAP70 facilitates translocation of CTLA4 to the membrane when the response needs to be controlled. PAG (Phosphoprotein Associated with Glycosphingolipid Microdomains), a transmembrane adaptor molecule is associated with CSK (c-Src Tyrosine Kinase), an inhibitor of Src-related protein tyrosine kinases. Overexpression of PAG inhibits TCR-mediated responses. Dual specificity phosphatase 3 (DUSP3) or VHR is also induced by ZAP70 which consequently down regulates ERK activation and thus controls T-cell response.

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