L11 Complement, MHC, Cytokines Lecture Notes PDF

11 The Complement system, major histocompatibility complex & Cytokines ILOs By the end of this lecture, students will be able to 1. Interpret the role of the complement as an effector mechanism of both innate and adaptive immunity. 2. Correlate the biological functions of com...

11 The Complement system, major histocompatibility complex & Cytokines ILOs By the end of this lecture, students will be able to 1. Interpret the role of the complement as an effector mechanism of both innate and adaptive immunity. 2. Correlate the biological functions of complement activation with its components. 3. Compare between MHC Class I and MHC class II antigens regarding its types and role in adaptive immunity. 4. Interpret the role of MHC polymorphism in paternity testing, disease association and organ transplantation. 5. Interpret the importance of various cytokines in the immune and inflammatory responses. 6. Explain the biologic effects of interferons to control viral infection. 7. Relate the clinical uses of cytokines as antimicrobial, immunomodulatory & antiproliferative factors. The complement system The complement consists of approximately 30 proteins that are present in normal human serum. The term complement refers to the ability of these proteins to complement or assist the effects of other components of the immune system. Activation of Complement: There are three pathways of complement activation: the classical the alternative and lectin pathways. (Figure 1) o The classical pathway (the pathway of acquired immunity) is activated by antigen-antibody complexes; thus, it is triggered after the generation of specific antibody to a particular Ag. Both IgG and IgM can activate the system by this pathway. o The alternative pathway can be activated by microbial cell surface substances. Bacterial polysaccharides and lipopolysaccharides of the cell envelop of gram-negative bacteria both serve as potent initiating stimuli. o The lectin binding or mannose binding pathway (MBP) is activated when mannose binding lectin binds to carbohydrates on the pathogen. TheThe lectin and the alternative alternative pathway andpathway are pathway the lectin considered arepart of the innate considered part ofimmune system innate immunity itysystem The activation of complement components involves sequential proteolytic cleavage of complement proteins, leading to the generation of effector molecules that participate in eliminating microbes in different ways. Activated complement proteins become covalently attached to the microbial cell surfaces where the activation occurs. Page 1 of 8  Many components are pro-enzymes which must be cleaved to form active enzymes. On cleavage of a complement component, the large fragment is given the suffix “b” e.g. C5b and the smaller fragment is given the suffix “a” e.g. C5a.. Complement pathways lead to the formation of a complex enzyme (C3 convertase) capable of binding and cleaving a key protein C3, common to all three pathways. Ø C3 activation is the common central event. After that, the three pathways proceed in the same fashion together through binding of the late-acting components to form a membrane-attack complex (MAC) which is C5b,6,7,8,9 which becomes inserted in the lipid bilayers of foreign membranes, ultimately causing cell lysis. Biologic Functions of the Complement System: 1. Cell lysis: Insertion of the membrane attack complex (C5b,6,7,8,9) into the cell membrane leads to lysis of many types of cells, including erythrocytes, bacteria and tumour cells. 2. Opsonization of pathogens: Microbes such as bacteria and viruses are phagocytized much more efficiently in the presence of C3b bound on their surfaces, because of the presence of C3b receptors on the surface of many phagocytes. 3. Inflammatory function: a. Chemotaxis: C5a attracts phagocytic cells, mainly polymorphs to the site of inflammation and increases their activity. b. Anaphylatoxins: C3a, C4a and C5a can produce degranulation of mast cells with release of mediators which cause increased capillary dilatation. 4. Enhancement of antibody production: The binding of C3b derivatives to its receptor on the surface of B lymphocytes enhances antibody production. Page 2 of 8  Figure 1: The classical, alternative & lectin pathways of the Complement system MAJOR HISTOCOMPATIBILITY COMPLEX (MHC) The MHC is a collection of highly polymorphic genes on the short arm of chromosome 6 in the human. There are 2 major classes of cell bound MHC gene products: I and II. MHC gene products are also called Human Leucocytic Antigens (HLA). Figure 2 Figure 2: The major histocompatibility gene complex and the corresponding MHC class I and II molecules or antigens on the cell surface o Class I MHC–antigens: are glycoproteins and include HLA-A, HLA-B, and HLA-C. They are expressed on all nucleated cells in the body. They are expressed in a codominant fashion, meaning that each cell expresses 2 A ,2B,2C products (one from each parent). Class I MHC enables cytotoxic T cells (CD8) to recognize foreign antigen on the surface of graft cells, tumour cells or virus infected cells and kill these cells. In other words, cytotoxic T cells are triggered only when they recognize both antigen and class I MHC molecules in close association on the surface of cells. This is known as MHC-restriction (Figure 3). Page 3 of 8  o Class II MHC-antigens: or HLA-D antigens are glycoproteins, and include HLA- DP, HLA-DQ and HLA-DR. They are expressed on antigen presenting cells of the body such as macrophages, dendritic cells and B lymphocytes. Helper T cells (CD4) will recognize foreign antigen on the surface of APC only if they are associated with class II MHC molecules (MHC restriction). (Figure 3) - Both classes could be typed by: Flow cytometry or molecular techniques such as PCR. Figure 3: Role of MHC in the immune response Ø Importance of MHC: Organ transplantation: the likelihood that a transplanted organ is accepted by the recipient’s immune system depends on the compatibility of the MHC genes of the donor and the recipient. MHC restricted antigen presentation: The ability of T cells to recognize antigen is dependent on the association of the antigen with either class I or class II proteins. o Tc cells recognize the Ag in association with MHC I. o Th cells recognize the Ag in association with MHC II. Disease association: It is found that the presence of certain HLA antigens is often associated with a particular disease: o HLA-B27 with ankylosing spondylitis. o B8 with myasthenia gravis. o DR2 with multiple sclerosis. o DR4 with rheumatoid arthritis. Paternity testing and forensic investigations (Each person have two sets of these genes -one on the paternal and the other on the maternal chromosome 6). Page 4 of 8 CYTOKINES Cytokines are glycoproteins of low molecular weight consisting of a large group of molecules involved in signalling between cells during the immune response. Mode of action: Cytokines are secreted by many cell types in response to specific stimuli. A cytokine acts on a target cell through a high-affinity cytokine receptor triggering subsequent signalling inside the cell. Cytokines mediate and regulate immune and inflammatory reactions. Most cytokines act in a paracrine or autocrine manner, whereas only a small part of the cytokines may function at the systemic level, by the endocrine way, like hormones (Figure 1) Figure 1: Mechanism of action of cytokines Classification of cytokines: Classifications based upon structural and biochemical properties of cytokines are complex. Classical nomenclature of cytokines includes: 1. Interleukins (ILs) 2. Colony-stimulating factors (CSFs) 3. Tumor necrosis factors (TNFs) 4. Transforming growth factors (TGFs) 5. Interferons (IFNs) 6. Chemokines and a variety of other proteins 1. Interleukins: This is a large group of cytokines which are produced mainly by T-cells, but also by mononuclear phagocytes or by tissue cells. Most are involved in directing other cells to divide and differentiate or their activation during an immune response. Each interleukin acts on a specific limited group of cells which express the correct receptors for that interleukin E.g.IL 2: The most powerful activator and growth factor for T-cells. 2. Colony stimulating factors (CSFs): are cytokines primarily involved in directing the division and differentiation of bone marrow stem cells. The CSFs direct immature bone marrow stem cells to develop into red blood cells (RBCs), platelets, and the various types of white blood cells. The balance of different CSFs is partially responsible for the proportion of different immune cell types produced. Some CSFs also promote further differentiation of cells outside the bone marrow, e.g., macrophage CSF (M-CSF) promotes the development of monocytes in blood and macrophages in tissues. Page 5 of 8 3. Tumour necrosis factors: TNFα acts synergistically with TNFβ, IL1, IL6, and IFNγ. It is a potent pro- inflammatory cytokine and even endogenous toxin, an endogenous pyrogen, and a regulator of adaptive immune responses. Historically, TNFα was discovered and named in such manner as could lyse particular tumor cell lines. TNFβ (“lymphotoxin”) is secreted by lymphocytes. Its effects are similar to TNFα’s effects, but TNFβ is more critical for the development of lymphoid tissue. 4. Transforming growth factors (TGFs): Transforming growth factors (TGFs) are a subset of a larger family of protein hormones that induce growth regardless of target cell anchorage and can play a role in embryological development, wound healing and tissue repair. TGF-β acts as a potent immunosuppressor, inhibiting proliferation in T and B cells and downregulating T-cell activity. 5. Interferons (IFN):-IFNα is produced by leukocytes and other cell types and takes part in the innate immunity as a potent antiviral agent to promote the cytostasis of target cells. IFNα is an anti- inflammatory cytokine and a stimulator of NK cell and macrophage activity. It belongs to type I of the IFN subfamily. IFNβ is secreted by fibroblasts and other cell types. It exerts the same effects as IFNα, in particular, in defense against viral infections. It also belongs to type I of the IFN subfamily. IFNγ is produced by lymphocytes upon their activation, belongs to the Th1 profile of cytokines, and exhibits wide immunoregulatory qualities in the immune processes. IFNγ is related to type II of the IFN subfamily. -Type I can be induced in response to viral infections by; double stranded RNA produced during virus replication. This explains why RNA viruses are stronger inducers than DNA viruses. -Type II interferons are induced by mitogens and not in response to most viral infections. Effects of Interferon (Figure 2) 1. Anti-viral Action Type I interferons (INF-α and INF-β) mediate the early innate immune response to viral infections. Interferon itself is not a direct antiviral agent but it initiates an antiviral state in cells surrounding the infected ones. Type I IFNs are produced by virus infected cells in response to intracellular signalling pathways in response to viral RNA. They bind to a specific cell surface receptor on neighbouring uninfected cells. They induce the synthesis of antiviral proteins. This subsequently blocks viral replication. Interferons are host species specific in function but are not specific for a given virus. 2. Immunomodulatory effect: -Interferons stimulate innate antiviral response: Type I IFN activates NK cells providing an early, natural defence against viral infection. Type II INF (INF-γ) is a potent inducer of macrophage activation. -Interferons also stimulate cell mediated immunity: Type I IFN enhances the expression of class I MHC antigens. INF-γ has an important role in antigen presentation. INF-γ enhances the expression of class I and II MHC antigens, increases the efficiency of presentation of antigens to both T-cytotoxic and T-helper cells and activation of cells. Page 6 of 8 -Antiproliferative effect: All types of interferon block cell proliferation at appropriate doses. Figure 2: Effects of Interferon 6. Chemokines: Chemokines are specialized cytokines, which drive the directed migration of immune system’s cells for homing and/or inflammation (chemotaxis). Chemokines are divided into homeostatic and inflammatory chemokines. Chemokines are produced by various cells in infected areas as endothelial cells and macrophages. e.g., IL-8 Proinflammatory cytokines (IL-1, IL-6, TNF-α): secreted mainly by macrophages. These cytokines have local and systemic inflammatory effects on the tissues including induction of fever, increased production of leukocytes by the bone marrow and production of acute phase proteins (C reactive protein (CRP), mannose binding lectin and complement components) by the liver. During fever, bacterial and viral replication is less efficient, whereas the adaptive immune response operates more efficiently. CRP acts as opsonin, facilitating removal of organisms by enhancing phagocytosis. However, at high concentrations, IL1, IL6 and TNF-α are an important mediator of endotoxin induced septic shock. They mediate septic shock by inducing fever and causing hypotension through vasodilatation and an increase in capillary permeability. Also, these cytokines activate the coagulation cascade causing disseminated intravascular coagulation resulting in anoxia of vital organs. Clinical uses of cytokines: 1. Antiviral: as in the treatment of viral hepatitis caused by HBV or HCV by IFN-α. Genetically engineered recombinant INF is used in management of severe or chronic viral infection. Pegylated interferon (Peg-INF) is a recombinant interferon supplemented with a molecule of polyethylene glycol that increased its plasma half-life to 10 times longer than the native interferon alpha. 2. Anti-tumour: as antiproliferative in treatment of certain leukaemia’s & lymphomas. 3. Anti-autoimmune (immunomodulatory): as in treatment of inflammatory diseases such as Crohn's disease and rheumatoid arthritis by TNF-α inhibitors or IL-1 receptor antagonist. Page 7 of 8  4. Antibacterial in multidrug resistant tuberculosis in conjunction with anti-tuberculous drugs. 5. Antifungal in treating resistant fungal disease. 6. Colony stimulating factors are used clinically to boost leukocyte numbers and prevent infections in patients who have received cancer chemotherapy. Side effects of cytokines: - Both naturally induced and manufactured INF can trigger systemic symptoms associated with many viral infections as malaise, myalgia, chills, and fever specially during viremia. That is why interferon therapy causes flu-like side effects. - During a cytokine storm, various inflammatory cytokines are produced at a much higher rate than normal. This overproduction of cytokines causes positive feedback on other immune cells to occur, which allows for more immune cells to be recruited to the site of injury that can lead to organ damage Page 8 of 8