L5_Immune Receptors and Signal Transduction_2024.pdf

Immune Receptors and Signal Transduction Dr Allison Imrie Learning Objectives You should understand, and be able to describe: The major signalling receptors of the immune system The structure and function of T cell and B cell receptor complexes The co-stimulatory molecules that support T cell activa...

Immune Receptors and Signal Transduction Dr Allison Imrie Learning Objectives You should understand, and be able to describe: The major signalling receptors of the immune system The structure and function of T cell and B cell receptor complexes The co-stimulatory molecules that support T cell activation The tyrosine kinase phosphorylation pathways of T cell activation, and the immune synapse The role of microRNA in regulation of T cell activation The role of complement in B cell activation The JAK-STAT pathway, as an example of cytokine activation Cytokines: low weight regulatory molecules (proteins or glycoproteins) Secreted by immune cells and various other cells in response to stimuli a: Expression of cytokines and their receptors is highly regulated b: Cytokines can act in Autocrine (same cell), Paracrine (close proximity) and Endocrine (long distance) ways Major categories of signalling receptors in the immune system a receptor that uses a non-receptor tyrosine kinase a receptor tyrosine kinase a nuclear receptor that binds its ligand and can then influence transcription a seven-transmembrane G protein– coupled receptor (GPCR) Notch, which recognizes a ligand on a distinct cell and is cleaved, yielding an intracellular fragment (IC Notch) that can enter the nucleus and influence transcription of specific target genes. ATP, Adenosine triphosphate; cAMP , cyclic AMP Cytokine signaling is mediated by non-receptor tyrosine kinase (NRTK) receptors Signalling from the cell surface involves cytosolic and nuclear phases A generic receptor that activates a non-receptor tyrosine kinase (NRTK) after it binds ligand is shown In this simplified example, the cytosolic phase has only a single enzymatic event; but many signal transduction pathways involve multiple steps Non-receptor tyrosine kinase-based receptors Non-receptor tyrosine kinases regulate cell growth, proliferation, differentiation, adhesion, migration and apoptosis NRTK receptors mediate cytokine responses Critical components in the regulation of the immune system In the cytosolic signalling phase, the NRTK phosphorylates a key tyrosine residue on the cytoplasmic tail of the receptor The phosphotyrosine-containing receptor tail is able to recruit a downstream enzyme that is activated once it is recruited Modifies a specific transcription factor that is located in the cytoplasm In the nuclear phase, this modified transcription factor enters the nucleus and binds to a specific site in the promoter or in some other regulatory region of target genes and thus facilitates their expression Cytokines are produced by receptor-expressing cell Signaling molecules are often composed of distinct modules, each with a specific binding or catalytic function SRC family kinases an immunologically important family of nonreceptor tyrosine kinases c-SRC is the cellular homolog of the transforming protein of the Rous sarcoma virus c-SRC contains several distinct domains, two of which, called SRC homology 2 (SH2) and SRC homology 3 (SH3) domains, mediate binding to other signaling proteins c-SRC also contains a catalytic tyrosine kinase domain and an Nterminal lipid addition domain that facilitates the covalent addition of a myristic acid molecule to the protein. The myristate helps target SRC family kinases to the plasma membrane. Selected members of the immune receptor family. FcγRIIB is an inhibitory receptor found on B cells and myeloid cells PD-1, an inhibitory receptor found on T cells, also has an immunoreceptor tyrosine-based switch motif (ITSM) in its cytoplasmic domain Typically, immune receptors that activate immune cells have separate polypeptide chains for recognition, and associated polypeptide chains that contain cytosolic ITAMs B cell receptor (BCR), the T cell receptor (TCR), and the highaffinity receptor for IgE (FcεRI) Inhibitory receptors in the immune system typically have ITIMs on the cytosolic portion of the same chain that uses its extracellular domain for ligand recognition Ig, Immunoglobulin; ITAM, immunoreceptor tyrosinebased activation motif; ITIM, immunoreceptor tyrosinebased inhibition motif Structure of the T cell receptor. The αβ T cell receptor (TCR) with domains of a typical TCR specific for a peptide–major histocompatibility complex (MHC) complex. The antigen-binding portion of the TCR is formed by the Vβ and Vα domains. The hypervariable segment loops that form the peptide-MHC binding site are at the top Ig, Immunoglobulin. Binding of a T cell receptor to a peptide–major histocompatibility complex V domains of a TCR interacting with a human class I MHC molecule, HLA-A2, presenting a viral peptide (yellow) The figure represents a front view of the x-ray crystal structure of the trimolecular MHC-peptide–TCR complex β2m, Beta-2 microglobulin; HLA, human leukocyte antigen; MHC, major histocompatibility complex; TCR, T cell receptor. The CD3 and ζ proteins are noncovalently associated with the TCR αβ heterodimer to form the TCR complex, and when the TCR recognizes antigen, these associated proteins transduce the signals that lead to T cell activation The CD3 proteins and the ζ chain are identical in all T cells regardless of specificity, which is consistent with their role in signalling and not in antigen recognition The CD3 and ζ proteins are also required for surface expression of the complete receptor complex on T cells Components of the T cell receptor complex. In addition to the TCR complex, T cells express several other proteins that recognize ligands on APCs and play important roles in T cell responses Early tyrosine kinase phosphorylation events in T cell activation On antigen recognition, there is clustering of T cell receptor (TCR) complexes with coreceptors (CD4, in this case) CD4-associated LCK becomes active and phosphorylates tyrosines in the ITAMs of CD3 and ζ chains Early tyrosine kinase phosphorylation events in T cell activation, cont. ZAP70 binds to the phosphotyrosines of the ζ chains and is itself phosphorylated and activated. Active ZAP70 then phosphorylates tyrosines on various adaptor molecules, such as LAT (LAT is a member of the SRC family of kinases) The adaptors become docking sites for cellular enzymes These enzymes activate various cellular responses: eg. RAS, MAPK pathways Shown here: one ZAP70 molecule binding to two phosphotyrosines of one ITAM in the ζ chain Likely that initiation of a T cell response requires assembly of multiple ZAP70 molecules on ITAMs of the two ζ chain as well as on CD3 chains The Immune Synapse When the TCR complex recognizes MHCassociated peptides on an APC, several T cell surface proteins and intracellular signalling molecules are rapidly mobilized to the site of T cell–APC contact: the immune synapse supramolecular activation cluster (SMAC) PKC is a protein kinase involved in signal transduction Integrins – LFA-1 and talin - function to stabilise binding of the T cell to antigen presenting cell, APC T cell activation is regulated by microRNAs (miRNAs): posttranscriptional inhibition of gene expression miRNAs are small noncoding RNAs that are transcribed from DNA but are not translated into proteins One strand of the miRNA can pair with a complementary sequence in a number of cellular messenger mRNAs mRNA may be targeted for degradation, or translation of the mRNA is inhibited In the cytosol, pre-miRNAs are processed by endoribonuclease Dicer into short double-stranded miRNAs, 21 to 22 base pairs in length, which associate with several proteins, including Argonaute, to form complexes known as RNA-induced silencing complexes (RISC) miRNAs participate in regulatory networks that control T cell activation, the expansion of the T cell population and effector T cell differentiation B cell antigen receptor complex Membrane immunoglobulin (IgM) (and IgD ) on the surface of mature B cells is associated with the invariant Igβ and Igα molecules, which contain ITAMs in their cytoplasmic tails that mediate signalling functions Cytoplasmic tail of membrane IgG contains a tyrosine containing motif called the Ig tail tyrosine (ITT) motif that helps amplify B cell receptor signalling in memory B cells. Signal initiation by antigens occurs by cross-linking of the BCR. Antigen-induced cross-linking of membrane immunoglobulin (Ig) on B cells Clustering and activation of SRC family tyrosine kinases and tyrosine phosphorylation of the ITAMs in the cytoplasmic tails of the Igα and Igβ molecules This leads to docking of SYK and subsequent tyrosine phosphorylation events Signaling cascades follow these events leading to the activation of transcription factors Signal transduction pathways are similar to those described in T cells AP1, Activator protein 1; GDP, guanosine diphosphate; GTP, guanosine triphosphate; ITAM, immunoreceptor tyrosine-based activation motif; NFAT, nuclear factor of activated T cell; NF-κB, nuclear factor κB; PLCγ1, phospholipase C γ1; PIP3, phosphatidylinositol trisphosphate; PKC, protein kinase C. Role of complement in B cell activation B cells express a complex of the CR2 complement receptor; CD19; and CD81 Microbial antigens that have bound the complement fragment C3d can simultaneously engage both the CR2 molecule and the membrane immunoglobulin (Ig) on the surface of a B cell This leads to the initiation of signalling cascades from both the B cell receptor complex and the CR2 complex: The response to C3d-antigen complexes is greatly enhanced compared with the response to antigen alone. Role of the ubiquitin ligase CBL-b in terminating T cell responses. Cytosolic and nuclear proteins are degraded via covalent attachment of ubiquitin residues to these proteins CBL-b is recruited to the T cell receptor (TCR) complex, where it facilitates the ubiquitination of CD3, ZAP70, and other proteins of the TCR complex These proteins are targeted for proteolytic degradation in lysosomes and other organelles Cytokines, the secreted messenger molecules of the immune system All cytokine receptors consist of one or more transmembrane proteins whose extracellular portions are responsible for cytokine binding and whose cytoplasmic portions are responsible for initiation of intracellular signaling pathways Most cytokine receptors signaling pathways are activated by ligand-induced receptor clustering Brings together the cytoplasmic portions of two or more receptor molecules This clustering induces the activity of non-receptor tyrosine kinases (NRTK) JAK-STAT signalling induced by cytokines Ligation of receptors for type I and type II cytokines results in: activation of an associated JAK tyrosine kinase phosphorylation of the receptor tail recruitment of an SH2 domain–containing activator of transcription (STAT) to the receptor The recruited STAT is activated by JAK phosphorylation, dimerizes, enters the nucleus, and turns on the expression of cytokine target genes. JAK , Janus kinases; STATs , signal transducers and activators of transcription The JAK-STAT pathway as a target for treatment of inflammatory disease Eg. rheumatoid arthritis; systemic lupus erythematosus; allergy; sepsis https://www.nature.com/articles/nrrheum.2017.23 https://www.nature.com/articles/s41584-021-00726-8 Summary Signaling receptors, typically located on the cell surface, generally initiate signaling in the cytosol, followed by a nuclear phase during which gene expression is altered Many different types of signaling receptors contribute to innate and adaptive immunity, the most prominent category being immune receptors that belong to a receptor family in which non-receptor tyrosine kinases phosphorylate tyrosine-containing immunoreceptor tyrosine-based activation motif (ITAM) on the cytoplasmic tails of proteins in the receptor complex Signaling from antigen receptors can be attenuated by inhibitory receptors such as CD22 and PD-1, which contain cytosolic immunoreceptor tyrosine-based inhibition motifs (ITIMs) and sometimes immunoreceptor tyrosine-based switch motifs (ITSMs) The T cell receptor (TCR) complex is made up of the TCR α and β chains that contribute to antigen recognition and the ITAM-containing signaling chains CD3 γ, δ, and ε and the ζ homodimer. The CD3 chains each contain one ITAM, whereas each ζ chain contains three ITAMs CR ligation results in tyrosine phosphorylation of CD3 and ζ ITAMs by SRC family kinases and the recruitment of ZAP70 to the phospho-ITAMs, with each SH2 domain of ZAP70 binding to one phosphorylated tyrosine of the ITAM. Activated ZAP70 phosphorylates tyrosine residues on adaptors, and downstream enzymes are recruited to the signalosome Summary Costimulatory receptors initiate signaling separately from antigen receptors, and signaling outputs from antigen receptors and costimulatory receptors synergize in the nucleus The BCR is made up of membrane-bound Ig and an associated disulfide-linked Igα and Igβ heterodimer. Both Igα and Igβ contain ITAMs in their cytoplasmic tails. Signaling pathways linked to the BCR are broadly similar to signaling pathways downstream of the TCR Many cytokine receptors use non-receptor tyrosine kinases called JAKs to phosphorylate transcription factors called STATs, which dimerize after phosphorylation, translocate to the nucleus, and induce transcription of target genes