Host Immune Response Module 4B PDF

Lecture 7: Host Immune Response – Intrinsic, Innate and Adaptive  The coordinated host response to infection These may seem These are cell-autonomous Induced by infection Tailored to primitive, but these responses (can be achieved...

Lecture 7: Host Immune Response – Intrinsic, Innate and Adaptive  The coordinated host response to infection These may seem These are cell-autonomous Induced by infection Tailored to primitive, but these responses (can be achieved (non-specific pathogen barriers block majority by a single cell in isolation) response) (specific of infections! response) 1 2 3 4 1  Integration of intrinsic defense with innate and adaptive immune systems If viruses bypass these physical + chemical barriers, a series of immune responses are engaged… 1 Quick Review: Physical Barriers 1  The coordinated host response to infection These may seem These are cell-autonomous Induced by infection Tailored to primitive, but these responses (can be achieved (non-specific pathogen barriers block majority by a single cell in isolation) response) (specific of infections! response) 1 2 3 4 0  Intrinsic Cell Response MAMPs: Macromolecules that are shared among groups of microorganisms & Receptor-mediated recognition of MAMPs recognized as foreign to the host. (microbe-associated molecular patterns) Examples of MAMPs include (but are not *not exclusively cell-surface receptors – can be Intracellular* limited to) dsRNA, peptidoglycan, LPS, flagellin, viral proteins. Cell signalling induced by MAMP recognition by PRRs (Pattern Recognition Receptors) PRRs: Host receptors that recognizes MAMPs, may be located on the host cell surface, endosomal membranes, cytoplasmic, or secreted. Examples of Cellular change that occurs due to MAMP- PRRs include the membrane-bound Toll- receptor engagement (i.e. gene expression) like receptors (TLRs) and cytoplasmic NOD-like receptors (NLRs), RIG-1-like receptors (RLRs), and protein kinase R MAMP-PRR engagement generally leads to signaling events that (PKR) ultimately activate transcription factors such as NFkB & the Interferon regulatory factors IRFs (i.e. IRF3, IRF7), promoting the expression of IFNs and inflammatory cytokines 2  homodimerize or heterodimerize Myd88 Cell surface PRRs Endosomal membrane PRRs Cytoplasmic PRRs MAMPs can be double stranded, single stranded RNA, siRNAs, and CpG motifs cytosolic region that acts as a binding site adaptor proteins will activate your Nf-kb (nuclear factor kappa b) and Irf, Interferon regulatory factors The major outcome here is the expression of cytokine genes such as the inflammatory cytokines & Type I Interferons (IFNs) 2  Intracellular PRR detectors of viral infection Toll-like receptors (TLRs) are generally expressed in certain cell types (i.e. sentinel cells like macrophages, dendritic cells). Other PRRs may be more ubiquitously expressed. 2  PRRs that are good to know… RIG-1 & MDA5: Cytoplasmic RNA helicases that function as RNA sensors RIG1 - Detects 5’ triphosphate RNA (without 5’ cap) Why RNA without 5’ Cap? Nucleus or Cytoplasm? MDA5 - Detects long dsRNA (and RNA without 5’ cap) Why dsRNA? Protein kinase R (PKR): Sensor for viral dsRNA, inhibits cap-dependent translation by eIF2α (eukaryotic Recall what is eIF2 Initiation Factor? cGAS: Cyclic GMP-AMP synthase (cGAS) binds to viral dsDNA in the cytoplasm Why in the cytoplasm? 2 confirmational change will expose a part that will allow the binding of phosphotases and Both RIG-1 & MDA5 dephosphorylate __ contain tandem N- domain and activate CARD terminal CARD domains Dephosphorylated CARD is In uninfected cells, CARD further activated through 3 1 domains are phosphorylated polyubiquitylation by Ubiquitin (inactive conformation) Ligases (TRIM25 and/or Riplet). Upon binding of viral RNA, PRRs undergo conformational 2 change were CARDs are exposed dephosphorylated by specific phosphatases Ubiquitilated MAVs and (active conformation) RIG (or MDA5) acts as a platform for recruitment 5 and activation of IKK and In their active conformations, TBK1 to activate NF-KB, RIG-1 and MDA5 are and IRF3 and IRF7 4 translocated to binds with the mitochondrial antiviral signalling protein (MAVS) Chan & Gack, Nature Reviews Microbiology, 2016 RIG-1 & MDA-5 2  What are some examples of how viruses evade RIG-1 & MDA5 responses? 1 2 1 Sequestration or modification of viral RNA ligands 3 Manipulation of post- 2 2 translational modifications on RIG-1, MDA5, MAVS 3 4 3 Cleavage of RIG-1, MDA5, MAVS 2 3 4 Sequestration or relocalization of RIG-1, MDA5 Chan & Gack, Nature Reviews Microbiology, 2016 RIG-1 & MDA-5 2  Recall: Cap-dependent translation initiation PKR McCormick & Khaperskyy, Nature Reviews Microbiology, 2017 2  Translation arrest by the dsRNA-activated protein kinase R When PKR binds to viral dsRNA, it undergoes dimerization & autophosphorylation which leads to its activation Activated PKR phosphorylates the α ? subunit of eIF-2 which causes it to remain in an inactive GDP-bound form Inactive GDP-bound eIF-2 is not able to recruit Met-tRNA; leads to an arrest of translation; translation arrest may lead to apoptosis PKR McCormick & Khaperskyy, Nature Reviews Microbiology, 2017 2  What are some examples of how viruses evade PKR responses? Some viruses may also use of eIF2- some viruses are inhibiting the activation IFNR independent translation mechanisms... other viruses will activate protein phosphatase to remove the Protein that was added by the PKR, some viruses Keeps eIF2α in a non- activate RAS phosphorylated state pathway to inhibit the PKR Viral proteins Viral protein some viruses activate viral may promote PKR antagonists host systems that inhibit activation of PKR phosphatase PKR Chiocca, Nature Reviews Cancer, 2002 2  Chan & Gack, Nature Reviews Microbiology, 2016 1 Cyclic GMP-AMP synthase (cGAS) binds to viral dsDNA in the cytoplasm Following DNA binding, cGAS 2 generates cyclic GMP-AMP (cGAMP) cGAMP binds and activates STING which 3 is located on the ER. STING dimerizes & is further activated by ubiquitylation Activated STING translocates to 4 perinuclear structures where it promotes expression of Type I IFNs & pro- inflammatory cytokines cGAS 2  What are some examples of how viruses evade cGAS responses? 3 1 1 Viral manipulation of STING post-translational modifications 2 2 Cleavage of STING 1 3 Prevent/limit cGAS sensing of nucleic acid ligand Chan & Gack, Nature Reviews Microbiology, 2016 cGAS 2 Cytokines Cytokines are small signalling proteins that are secreted by specific immune cells. Cytokines mediate cell-cell communication to regulate a range of immune responses. Interferons (IFNs) IFNs are a group of cytokines that are generated in response to several pathogens (‘interfere’ with viral infection). There are 3 types of IFNs: Type I (α/β), II (ϒ), III (λ1, 2, 3). Type I IFNs are induced following PRR signalling and bind to IFNAR receptors on target cells – help to establish antiviral response. What are IFNARs? -> Next Slide 2 Type I IFN synthesis, secretion, receptor binding & signal transduction Viruses (or viral components) bound by PRRs trigger downstream signalling events that lead to the production of Type I IFNs α/β). These cytokines are released from the cell and then bind to IFNAR receptors on the surface of adjacent cells to stimulate synthesis of IFN-responsive genes Then what? *Do not memorize this* 2  Type I IFNs promote upregulation of antiviral proteins PKR dsRNA sensor (already discussed this!) 2’,5’-oligo A synthetase Activated by dsRNA, promotes production of oligo A which, in turn, activates RNase L. RNase L then promotes degradation of viral RNA Mx GTPases GTPase with diverse roles in the inhibition of virion assembly IFN-sensitive response element (ISRE) 2 Type I IFNs promote upregulation of antiviral proteins PKR dsRNA sensor (already discussed this!) 2’,5’-oligo A synthetase Activated by dsRNA, promotes production of oligo A which, in turn, activates RNase L. RNase L then promotes degradation of viral RNA Mx GTPases GTPase with diverse roles in the inhibition of virion assembly 2 Effects of the inflammatory cytokines IL-1, IL-6, TNF-α in response to viral infection Effects on brain (hypothalamus) IL-1, IL-6, TNF-α all act on the brain (particularly the hypothalamus) to produce fever & fatigue in response to viral infection. Effects on bone marrow Colony-stimulating factor (CSF) have effects in the bone marrow to promote hematopoiesis and Effects on liver lymphocyte mobilization IL-1, IL-6, TNF-α act on the liver to release iron, zinc, and acute-phase proteins (mannose binding protein, fibrinogen, C-reactive protein – these acute-phase proteins have roles in the innate immune response) 2