Contributed by Craig Vollert, Ph.D. candidate
The TCR-CD3 complex plays an essential role in the adaptive immune response. Present on the cell surface, the T-cell receptor (TCR) initiates T-cell activation by recognizing antigens bound to the major histocompatibility complex (MHC) molecule1. However, due to the TCR’s extremely sort cytoplasmic tail it is dependent on the CD3 complex for signaling, effectively making the CD3 complex the gatekeeper for T-cell activation2. While the CD3 complex consists of multiple components, CD247/CD3Z is arguably the most important due to its role in signal transduction and its potential use as a biomarker for evaluating the status of immune system3.
Structurally, the TCR-CD3 complex consists of the TCR and the CD3 complex. The TCR is a heterodimer composed of two different protein chains. In 95% of T-cells the TCR consists of an alpha (α) and beta (β) chain, whereas 5% of T-cells have a gamma (γ) and delta (δ) chain instead. Only the α/β heterodimer interacts with the CD3 complex with both the α and β chain have a variable domain that is capable of binding to an MHC molecule1,4. The CD3 complex consists of four CD3 chains (CD3D. CD3G, CD3E and CD247/CD3Z) which form two heterodimers (CD3D-CD3E and CD3G-CD3E) flanking either side of the TCR and one homodimer of CD247/CD3Z sitting below the α/β heterodimer1,2. All four CD3 chains contain immunoreceptor tyrosine-based activation motifs (ITAMs) in their cytoplasmic domain that are capable of being phosphorylated.
The activation of the TCR by an MHC molecule sets off a signaling cascade that allows T-cells to differentiate, proliferate and secrete cytokines. First, depending on the type of T-cell either the TCR co-receptor CD4 or CD8 binds to the MHC molecule activating the tyrosine kinase Lck5,6. Next, Lck phosphorylates the intracellular ITAMs of the CD3 complex creating a docking site for protein kinase ZAP-70. CD247/CD3Z is particularly important because it’s the only CD3 component that has multiple ITAM sites which allows ZAP-70 to bind in a position that enables the phosphorylation of the transmembrane protein linker of activated T cells (LAT)7,8. Once phosphorylated, LAT acts as a docking site for SH2 domain-containing proteins recruiting multiple downstream signaling molecules. Ultimately, this culminates in the activation of important signaling pathways including the Ras-MEK-ERK pathway and transcription factors NF-kB, NFAT and AP-11.
Given the role of CD247/CD3Z in T-cell activation it’s not surprising that it may also be an important biomarker for evaluating the immune status of patients with diseases characterized by chronic inflammation9,10,11,12,13. During chronic inflammation T-cell activation is suppressed, which is associated with the downregulation of only CD247/CD3Z while the remaining components of the TCR-CD3 complex are unaffected14. Additionally, when inflammation is treated CD247/CD3Z levels normalize suggesting it’s a correlate for T-cell activation. Given this, CD247CD3Z is being investigated as a potential biomarker for determining the immune status and disease severity where traditional inflammation biomarkers like hs-CRP and TNF-α are not specific to T-cell activation14.
Taken together, CD247/CD3Z plays an incredibly important role in the activation of T-cells. As part of the TCR-CD3 complex, CD247/CD3Z is an important docking site for ZAP-70 which is critical for downstream signaling of the TCR-CD3 complex. Lastly, CD247/CD3Z is also potentially a valuable biomarker for checking the immune status in patients.
Detection of human CD247/CD3Z in FFPE tonsil IHC. Antibody: Rabbit anti-CD247/CD3Z recombinant monoclonal [BL-336-1B2] (A700-017). Secondary: HRP-conjugated goat anti-rabbit IgG (A120-501P). Substrate: DAB.