Complement pathway and B CELL FUNCTION - Copy.pptx

IT IS A COMPREHENISVE PPPT ABOUT COMPLEMENT PATHWAY AND ITS FUNCTIONS

1. Complement pathway and B cell activation

2. Contents Complement system: Introduction Nomenclature Functions Types Opsonisation Inflammatory Response Regulators Deficiency B cells: 1. Introduction 2. Function 3. B cell receptors 4. B cell activation 5. Affinity maturation 6. Isotype switching

3. Complement System: Introduction: It is a major component of innate immunity and a complement to antibody –triggered responses. It consists of nearly 60 plasma and membrane proteins. These proteins are synthesized mainly by liver hepatocytes, although significant amounts are also produced by monocytes, macrophages, and epithelial cells of the GI tract and genitourinary tracts. Most complements circulate in the serum in inactive forms as proenzymes, or zymogens , which are inactive until proteolytic cleavage.

4. Nomenclature : Complement components were named in the order as they were identified. They are designated by the letter “C” combined with numbers (C1–C9 ). Alternate pathway components are designated as factors e.g ., factor B and D After proteolytic activation, the smaller fragment resulting from cleavage of a component is designated “a” and the larger fragment designated “b ” ( e.g : C3a, C3b, exception is C2 whereC2a is larger fragment). The larger fragments bind to the target near the site of activation . The smaller fragments diffuse from the site and can initiate localized inflammatory responses. The complement fragments interact with one another to form functional complexes (eg:C4b2a , C3bBb)

5. Functions of Complement System include : 1.Lysis of cells, bacteria, and viruses via the membrane attack complex. 2. Opsonization , which promotes phagocytosis of particulate antigens 3. Promotion of inflammatory and immune responses by release of anaphylotoxins to induce mast cell degranulation and cellular chemotaxis. These proinflammatory processes require tight control to avoid damage to host tissue.

6. Stages of Complement Activation: The three main stage in the process of activation of complement by any process include : 1. formation of C3 convertase (C4b2a, C3bBb) 2. Formation of C5 convertase (C4b2a3b, C3bBb3b) 3. Formation of membrane attack complex.(C56789) The formation of C3 convertase occurs via different process in C lassical, Lectin and Alternate pathway , then they form C5 convertase by same route and lead to formation of MAC.

7. Classical pathway: IgM and certain classes of IgG( IgG1,IgG2 or IgG3) can activate complement . The Antigen-Antibody complex formation induces conformational changes in the Fc portion of the Ig molecule that expose a binding site for the C1 component of the complement system. Initial stage involve activation of C1,C2 ,C3 ,C4 which are present in plasma in functionally inactive forms. The C3 convertase cleave C3 and forms C3b. Most C3b molecules acts as opsonins although some C3b generate the C5 convertase (C4b2b3b) that cleaves C5 to C5a and C5b. C5a acts as a potent A naphylatoxin . C5b binds to C6,C7,C8,C9 and forms MAC.

8. Lectin Pathway: Lectins are proteins is s pecialised for prompt recogination of repetitive carbohydrate patterns(mannose) on the surface of microbial pathogens. Similar to classical pathway except that initiating antibodies are replaced by Mannose binding lectins and sugars on pathogens are analogous to antigens . Like C1q , Lectin pathway is activated by pattern recoginising molecules (PRM)of two major protein families ficolin , collectin . Also associated with large PRM ( such as MBL) are serine proteases( ie , MBL-associated serine protease[MASPs]). Like C1s in the classical pathway , the MASPs activate C4 and C2 by proteolytic cleavage.

9. Alternative pathway: It does not require the presence of antibodies or lectins to become activated. A small amount of autoactivated C3 is always present at low level , presence of a microbe , a damaged host cell, or a lack or complement regulatory protein amplifies its effect. The alternative pathway is engaged when this activated C3 binds factor B. Bound factor B undergo proteolytic cleavage mediated by another serine protease factor D, to produce fragments Bb and Ba. C3convertase C3bBb is formed . Another C3b binds to C3 convertase to form C3bBb3b, the C5 convertase. The assembly of the MAC is then same as for the classical pathway.

10. Membrane Attack Complex The terminal sequence of complement activation involves C5b, C6, C7, C8, and C9, which interact sequentially to form a macromolecular structure called the membrane-attack complex (MAC). MAC forms transmembrane channels or pores that displaces lipid molecules and other constituents , resulting in membrane perturbation and osmotic lysis.

11. Opsonisation : C3b is formed in complement activation process. It acts as an opsonin . Complement system destroys microbes and biological debris by covalent deposition of C3b on a target . Once a target is coated with C3b, then it can be recognized by host cells bearing complement receptors like neutrophils and macrophages and followed by ingestion by phagocytic cells.

12. C omplement system leads to the release of anaphylatoxins (C3a,C5a ) They are potent mediators of inflammatory and immune responses. Functions: 1.chemotaxis of granulocytes. 2.Release of mediators ,such as histamine from mast cells . 3. Activations of many cell types ,including epithelial and endothelial cells 4 . Contraction of smooth muscles 5.Dilation of blood vessels with exudation of plasma and cells Promotion of Inflammation:

13. Regulation of Complement activation: Unchecked complement activity can cause serious damage to tissues. several inbuilt control mechanisms regulate the complement cascade at different steps. There are mainly two kinds : Inhibitors and Inactivators Inhibtors : 1. C1 esterase Inhibitor: It inhibits C1 esterase and other esterases like plasmin , kininogen and the Hageman factor .This does not prevent the normal progress of the complement cascade but checks its autocatalytic prolongation. 2.The S protein : It is present in normal serum and binds to C67 and modulates the cytolytic action of the MAC. 3. DAF, aka CD55 : prevent complement mediated lysis of RBC

14. Inactivators : 1. Factor I :A serum beta globulin ,It provides homeostatic control of C3 activation ,particularly by alternative pathway. 2.Factor H : It acts in coordination with Factor I ,modulating C3 activation. 3.Anaphylatoxin inactivator is an alpha globulin that enzymatically degrades C3a,C4a,C5a which are anaphylatoxins released during complement cascade. 4.C4 binding protein controls activity of cell bound C4b .

15. Deficiency of Complement system Deficiency Syndrome 1. C1 esterase Inhibitor Hereditary Angioneurotic edema 2. Early components of classical pathway C1,C2,C4 SLE and other collagen vascular diseases 3.C3 and its regulatory protein Severe recurrent pyogenic infections 4. C5 –C9 Bacteremia, mainly with Gram negative diplococci 5.Anchors of complement inhibitors ( CD55, CD59) Paroxysmal nocturnal hemoglobinuria

16. Introduction to B cell

17. Surface proteins in B cell

18. B cell receptor (BCR) The receptor complex consists of a transmembrane form of Ig molecule and two heterodimers comprised of Ig α (CD 79a) and Ig β (CD 79b ). Recognize Ag via Ig heavy- and light- chain variable regions Ig αβ heterodimer send signal to the nucleus of B cell for activation to terminally differentiate to make Ag- specific Ab

19. Activation of B Lymphocytes : A mature B cell leaves the bone marrow expressing membrane-bound immunoglobulin (IgM and IgD ) . These B cells are educated to respond to specific antigens, but Have not encountered the appropriate antigen – are sometimes referred to as naïve B cell proliferation and differentiation occurs in secondary lymphoid organs like spleen and lymphnodes . If antigen entering these secondary lymphoid organs binds to and cross-links the idiotypes of the immunoglobulin, this provides the first signal for the activation of the B lymphocytes.

20. Activation of B Lymphocytes : The antigens that B lymphocytes encounter are divided into 2 categories: 1. Thymus- independent(TI) antigens 2. Thymus -dependent antigens(TD) antigens

21. T cell Independent B cell activation: Certain mature naïve B cells are capable of interacting with macromolecules such as lipids ,polysaccharides and lipopolysaccharides without having to interact with Helper T cells. These antigens are called TI antigens , and they directly stimulate B cells to proliferate and differentiate into plasma cells. The response is generally weaker ,resulting primarily in the secretion of IgM antibodies and absence of immunologic memory.

22. T Cell dependent B cell activation: Most of the antigens are proteins. They do not have multiple repeating units which make clustering of B Cell Receptors difficult. It requires direct contact of B cells with helper T cells and are influenced by cytokines . Exposure of ’naïve’ B cell to an antigen. BCR–mediated endocytosis . Exogenous antigen is presented on MHC II and recognized by TCR on Th cell. CD40 receptor on B cell binds CD40 ligand CD4+ T cell (CD40L) on Th cell. B cells are activated and undergo clonal expansion . The B cells respond by proliferating and differentiating into memory B cells or plasma cells.

23. During clonal expansion the clones undergo: 1.Somatic hyper mutation and affinity maturation ( variable regions) 2. C lass switching ( constant region)

24. References: 1.Harrison 21 st edition 2. Kuby immunology 6 th edition 3. Robins basic of pathology 10 th edtion

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