Wednesday, August 18, 2004

My Notes on Complement

(a.k.a. Why You Shouldn't Go to Medical School)

These actually sort of read like a Dr. Seuss book:

The complement system involves a cascade of proteolytic reactions that produce several different products which contribute to the elimination of invading microbes by:
1. direct killing of microbes by destruction of biological membranes
2. chemotactic attraction of inflammatory cells, and by
3. enhancing phagocytosis of microbes.
Complement components are synthesized by macrophages, monocytes, intestinal epithelial cells and the liver.
--They are released into the serum in an inactive form.
Complement may be activated via 3 different mechanisms, the Classical pathway, the Alternate pathway, and the Lectin Pathway.
--The Classical Pathway requires antigen-antibody complexes, but the Alternate and Lectin Pathways do not.

Classical Pathway

First component to become activated is C1.
C1 is a complex consisting of 3 proteins, C1q, C1r and C1s.
C1 =(C1q:C1r2:C1s2) held together by Ca+ ions.
C1q contains 6 globular heads.
It is activated when >2 of these heads bind to Fc's of either IgG or IgM.
--C1q receptors of Fc are exposed due to conformational changes of Ig molecule following antigen binding.

2 IgG molecules must be close to each other to activate C1q.
Activated C1q activates C1r which in turn activates C1s.
--C1s cleaves C4 into C4a and C4b.
C4b binds onto cell membrane adjacent to Ig molecules and also binds C2.
--C2 is cleaved by the action of C1s and C4b, but remains associated with C4 as an active C4b2a complex = C3 convertase.
--C4b2a cleaves C3 into C3a and C3b.

A single C4b2a complex can activate hundreds of C3 molecules so this is an important amplification step.
--C3b binds to the cell membrane in the immediate vicinity of its activation and binds onto C4b2a to form C4b2a3b which activates C5.
--C3a remains in the fluid phase and promotes inflammation.

The assembly and activation of components C5-C9 results in cytolytic activity.
--C5-C9 is called the membrane attack complex.
C4b2a3b = C5 convertase - cleaves C5 into C5a and C5b.
--C5a remains in fluid phase and promotes inflammation.
--C5b combines with C6 and C7 to form the C5b67 complex which inserts itself into the membrane's lipid bilayer and focuses the activity of C8 and C9.

C5b678 forms small pores and has some lytic activity, but polymerization of C9, a perforin like molecule, around the C5b678 complex results in transmembrane channels that contribute substantially to cell lysis.
--Ions pass through these channels resulting in osmotic disturbance.
--Water rushes into the cell resulting in swelling, membrane damage, loss of macromolecules and eventually, death.

Lectin Pathway
Activated by terminal mannose residue that are common to bacteria, but rare in mammalian cells.
Circulating Mannose Binding Lectin (MBL) forms a complex with 2 Mannose Associated Serine Proteases (MASP 1 and 2) on the surface of bacteria with terminal mannose residues.
MASP-1 becomes activated in the complex, cleaving C4 and C2 to form C4b,2a, the classic C3 convertase.
The remainder of the pathway is the same as the classic pathway.

Alternate Pathway
Can become activated whenever there is some free C3b around.
--Free C3b can be generated by classical pathway, or by proteases derived from coagulation cascade, bacteria, or inflammatory cells.
--C3b like molecules (C3*H2O) can also appear due to spontaneous conformational changes.
C3b exists in trace amounts in normal serum and can bind onto foreign substances (e.g. bacterial surfaces).

C3b can combine with serum factor B producing a C3bB complex which becomes activated when serum factor D cleaves B yielding C3bBb.
--C3bBb is a C3 convertase.

The C3bBb complex dissociates rapidly, but is stabilized by a serum protein called properdin (P).
The C3bBbP complex is a stable C3 convertase.
--The C3b product of this cleavage can bind more factor B and factor D, etc.
C3bBbPC3b acts as a C5 convertase, and the rest of the pathway is similar to the classical pathway.
Free C3b is normally inactivated rapidly in serum by factors H and I (C3b inactivator).
It is believed that factors setting off the alternate pathway somehow protect C3b from inactivation.

Controls
C1 esterase inhibitor (C1INH) forms a complex with C1r and C1s causing their dissociation from C1q, thus inhibiting classical pathway.
C1INH deficiency is inherited in an autosomal dominant manner, and results in herditary angioedema.
--Patients suffer from local edema in various organs throughout the body.

Several other proteins regulate C3 convertase (C4b2a) generated in classical pathway.
They bind C4b, displacing C2a.
--These include:
--C4b binding protein (C4BP)
--Decay Accelerating Factor (DAF)
--Membrane Cofactor Protein (MCP)
--Complement Receptor 1 (CR1)
--Once C2a is displaced, factor I can cleave C4b, (C4BP, MCP, or CR1 are required as cofactors).

Alternate pathway is also regulated.
Factor H competes with factor B for binding sites on C3b present on cell surfaces.
--Sialic acids of mammalian cells favor binding of Factor H, terminating cascade.
--Bacterial cells, which lack sialic acid, favor binding of Factor B, continuing cascade.
Factor H also displaces Bb from C3 convertase (C3bBb) by binding C3b.

CD59, a protein that is widely distributed in mammalian membranes, prevents lysis of bystander cells by binding C5b-8 in the membrane and preventing polymerization of C9.

Other Biological Effects
C3b and C4b are opsonic.
--PMN's & macrophages have receptors for these, and other C' components.
C3a, C5a, and to a lesser extent, C4a are anaphylatoxins = substances that induce degranulation of mast cells and/or basophils, which release several substances including histamine.
--These cells have specific receptors for these molecules.

Anaphylatoxins produce increased capillary permeability and smooth muscle contraction--local edema--delivery of more antibody and complement.
--C5a is also very chemotactic for neutrophils.
C' can enhance B cell response to antigens by means of C3d binding to CD21 of the B cell coreceptor complex (CD21:CD19:CD81).
--When C3d binds CD21, B cell is activated by lower concentrations of Ag.
Deposition of C3b on immune complexes aids their removal.

Complement Deficiency
Heterozygotes have about 1/2 normal levels of the component in question and have few clinical manifestations.
Patients who are homozygous for a defective component may have clinical manifestations.
--C3 deficiency - recurrent pyogenic infections.
--C1, C4, or C2 deficiency - not real significant, but may have difficulty clearing immune complexes (predisposed to SLE).
--Late component deficiencies - gonococcal and meningococcal infections.

1 Comments:

At 6:26 PM, Anonymous Anonymous said...

Thanks Lucas...Send this one to my brother in order to make his studying look better. :)
~Sarah

 

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