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. 2021 Dec 13;6:412. doi: 10.1038/s41392-021-00823-w

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© The Author(s) 2021

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.

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Fig. 1 — a TCRα/TCRβ and TCRγ/TCRδ heterodimers form complexes with the CD3 molecules. Heterodimers of CD3ε/CD3δ and CD3γ/CD3ε, and a homodimer of CD3ζ/CD3ζ form complexes with TCR dimers. TCR heterodimers contain intramolecular and intermolecular disulfide bonds. CD3 chains contain 10 ITAMs distributed in different CD3 molecules. The variable region (V) of TCR heterodimers recognize the antigen peptide-loaded on MHC (pMHC). In the absence of pMHC, the intracellular part of the CD3 molecules forms a close conformation in which ITAMs are inaccessible to the kinases for phosphorylation. b Coreceptor CD4 acts as a single molecule while CD8α and CD8β can form homodimers or heterodimers. c MCH-I consists of an α-chain containing three immunoglobulin domains (α₁, α₂, α₃) and β2-microglobulin (β2m). MCH-2 is the heterodimer of an α chain and a β-chain containing two immunoglobulin domains (α₁, α_2, and β₁, β₂) in each chain. d LCK-loaded CD4 molecules bind to the MHC-II bound TCR (TCRα/TCRβ) complex. This allows LCK to phosphorylate two distinct sites on ITAMs. Then ZAP-70 interacts with the phosphotyrosine sites and mediates more tyrosine phosphorylation. CD4 and MHC-II interaction is mediated through the membrane-proximal α₂ and β₂ domains of MHC-II and the membrane-distal D1 domain of CD4.