Complement J. Ochotná

Complement  system of about 30 serum and membrane proteins (humoral component of nonspecific immunity)  complement components in serum are present in inactive form  complement activation has cascade character

 complement proteins are synthesized in the liver, less by tissue macrophages and fibroblasts  the main complement components: C1-C9 (C3 is the central component)  other complement components: factor B, factor D, factor P  regulatory proteins: C1 - inhibitor, factor I, factor H, C4bp, DAF, MCP, CR1, factor S (vitronectin), CD59 (protektin), inactivator of anafylatoxin

Functions of complement  Opsonization (C3b, C4b)  Chemotaxis (C3a, C5a)  Osmotic lysis (MAC C5b-C9)  Anafylatoxins (C3a, C4a, C5a)

Complement activation  Alternative pathway  Classical pathway  Lektin pathway

Riedemann N.C.

Alternative pathway  C3 component of complement rarely spontaneously breaks into C3b and C3a  C3b can covalently bind on the surface of a particle (own cell, microorganism) or reacts with water and inactivate  to bound C3b joins a factor B, which is cleaved by factor D to Ba and Bb, resulting complex C3bBb is stabilized by factor P and functions as an alternative C3 convertase

 C3 convertase cleaves C3 to C3a (chemotactic for phagocytes) and C3b, which binds to the surface of the particles (opsonization), or gives rise to other C3 convertases  from some C3 convertases form C3bBbC3b that act as an alternative C5 convertase, which cleaves C5 to C5a (chemotaxis) and C5b (starts terminal lytic phase)

Alternative pathway

Classical pathway  can be initiated by antibodies IgG ( not by IgG4) and IgM or by pentraxins (CRP, SAP - acute phase proteins)  after binding of antibodies to the bacteria surface, change its conformation and than can bind C1 protein  C1 than change its conformation and get proteolytic activity, than cleaves C4 and C2 proteins

 fragments C4b and C2a bind to the surface of microorganism and create the classic C3 convertase (C4bC2a), which cleaves C3 to C3a and C3b  then creates a classic C5 convertase (C4bC2aC3b) that cleaves C5 to C5a and C5b

Zde dřívější nomenklatura – záměna 2a za 2b

Lectin pathway  is initiated by serum mannose binding lectin (MBL)  MBL binds to carbohydrate structures on the surface of some microbes, after the binding starts cleave C4 and C2  this way is similar to the classical pathway

Terminal (lytic) phase of the complement cascade  C5b fragments creates a complex with C6, C7 and C8, the complex dive into the lipid membrane of the cell and attached to it into a circle molecules of C9 (MAC – membrane attack complex), thus create in the membrane pores and cell can lysis (G-bacteria, protozoans, some viruses).  Most microorganisms are resistant to this lytic effect of complement (protection by cell wall).

Regulation of complement and protection of own cells  Activation of complement cascade is controlled by the plasma and membrane inhibitors.  C1 inhibitor ( C1-INH – inhibits C1, when the deficit → HAE )  DAF (decay-accelerating protein)-degradation of C3 and C5 convertase

 factor I, MCP (membrane cofactor protein), CR1, factor H – C3b cleavage  factor S (vitronectin) – inhibits complex C5bC6  CD 59 (protectin) - prevents the polymerization of C9  inactivator of anafylatoxins - inactivates anafylatoxins (C3a, C4a, C5a)

Complement receptors  Bind fragments of complement components  CR1 - on various cells - removing of immunecomplexes  CR2 - on B lymphocytes and FDC - activation of B cells  CR3, CR4 - on phagocytes - participation in opsonization, adhesion

4 basic complement functions  Opsonization (C3b, C4b)  Chemotaxis (C3a, C5a)  Osmotic lysis (MAC C5b-C9)  Anafylatoxins (C3a, C4a, C5a)

Antigens

Antigen (immunogen)  substance that the immune system recognizes and responds to it  usually proteins or polysaccharides (lipids and nucleic acids only in the combination with proteins or polysaccharides)  molecules <5 kDa can´t trigger an immune response, the optimal size of the antigen molecules to initiate immune response is about 40 kDa

Hapten  small molecules, that are able to induce specific immune response only after the establishment to the macromolecular carrier  separate haptens are not immunogenic  typically drugs (eg penicillin antibiotics, hydralazin)

Epitope  = a part of the antigen which is recognized by immune system (lymphocytes- BCR, TCR, Ig)  cross-reactive antigens - shares one or more identical or similar epitopes

Interaction antigen – antibody  Binding site of antibody (paratop) form non-covalent complexes with the corresponding part on antigen molecule (epitope)  participation: the hydrogen bonds, electrostatic and hydrophobic interactions, van der Waals forces  antigen-antibody complex is reversible

Antigen  endogenous antigens - autoantigens (self Ag)  exogenous antigens - foreign substances from the environment  allergen is exoantigen that in the susceptible individualscan cause pathological (allergic) immune response  allergen is exoantigen that in the susceptible individuals can cause pathological (allergic) immune response

Antigen features  immunogenicity proteins> carbohydrates> macromolecule complexes (glycoproteins, nucleoproteins, and glycolipids)> lipids  specificity

Factors affecting the immunogenicity  Physical: solubility - insoluble more immunogenic molecular weight - ideal 5-40 kDa  Chemical: structure - the number of determinants degradability - "ease" of uncovering the determinants in antigen presenting cells (APC cell)  Biological: biological heterogeneity genetic and physiological disposition of the body

Degree of foreignness  Autogeneic - antigens of the same individual  Syngeneic - antigens of genetically identical individuals (eg twins)  Allogeneic (alloantigens) - antigens genetically different individuals of the same species  Xenogeneic (heterologous) - antigens derived from individuals of different species (eg monkey kidney transplant man)

Types of antigens according to antigen presentation  thymus dependent antigens  thymus independent antigens

Thymus dependent antigens  more frequently, mostly protein Ag  for specific humoral immune response to antigen is necessary to cooperate with T H lymphocytes (or response isn´t enough effective)  assistance implemented in the form of cytokines produced by T H lymphocytes

Thymus independent antigens  in a small number of antigens can be induced antibodies production directly without the participation of T lymphocytes  mainly bacterial polysaccharides, lipopolysaccharides and polymer forms of proteins (e.g. Haemophilus, Str.pneumoniae)

Superantigens  proteins (microbial products) which have 2 binding sites, one interacts with the epitope presented on all MHCgpII, second interacts with other structures presented in many different TCR molecules (connection of T lymphocyte with APC)  stimulate polyclonaly and massively  massive activation of T lymphocytes can cause shock  e.g. bacterial toxins (Staph.aureus, Str.pyogenes, Pseud.aeruginosa)

Differcence between antigen and superantigen binding

Sequestered antigens  autoantigens that are normally hidden to immune system and therefore unknow (e.g. brain, the lens of the eye, testes)  if they are "uncovered" by demage, exposed to the immune system, are recognized as foreign (one of the theories of autoimmune processes)

Immunologically privileged sites  brain, eye, gonads  are protected from potentially damaging inflammatory immune responses  this tissues are far less rejected in allogeneic transplant (cornea)  this privileged position is not absolute

Immunologically privileged sites  Mechanisms of protection from the immune system:  isolation from the immune system (blood-brain barrier)  preferences of Th2 and suppression of Th1-response  active protection against effector T-lymphocytes  production of immunosuppressive cytokines (TGFβ)  FasL expression  increased expression of membrane complement inhibitors

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