Complement system

Complement System: Brain teasing topic

1. The
COMPLEMENT
SYSTEM
Dr.Riyaz Sheriff M.D

2. What is complement system?
“Complement”isasystemofsomenon
specificproteins(factors).
Presentinthenormalhumanandanimalserum
Activatedbyantigen-antibodyreactions
Hastheabilitytolyseordamagecells.
Mediateanumberofbiologicalactivities

3. Complement as we know it
Buchner(1889)
Bactericidal,bacteriolyticandhaemolyticactionsareinhibited
bypreheatingtheantiserumat56
0
cfor30minutes(heat
labile)
Pfeifferphenomenon(1894)
Vibriocholeraewerelysedwheninjectedintraperitoneally
intospecificallyimmunizedguineapigs
Bordet(1895)
Concludedthatbacteriolysisorhemolysisrequires
»Heatstableantibody
»Heatlabilefactor
Thisheatlabilefactorwas“ALEXINE”

4. ComplEmEnt as wE know it(Cont’D)
PaulEhrlich(1899)
RenamedALEXINEas“COMPLEMENT”.
Actionofantibodyiscomplementedbythis
factorandhencethename–COMPLEMENT
Bordet&Gengou(1901)
DescribedComplementFixationTestusing
hemolysisasasensitiveserologicalreaction
wassermanncomplementfixationtest–syphilis

5. What is special about complement??
•Belongs to group of Triggered enzyme
capsules
•Cascade system
•Enzyme in cascade has the ability to activate
molecules required for the next step
•Provides amplification of response
•Has its own control mechanism
•Can be regulated precisely

6. General properties
•Present in sera of all mammals, animals, birds,
amphibians and fish
•Constitutes 5% of normal serum protein
•Does not increase with immunization
•Heat labile
•Destroyed by heating @56°C for 30mins
•Such a serum is called “Inactivated serum”

7. GEnEral propErtiEs (Cont’D)
•Does not bind to free antigen or antibody
•Binds only with antibody which has already
combined with an antigen
•Binds to Fc fragment on the immunoglobulin
•Binding results in activation of classical complement
pathway
•Complement does not bind with all classes of
Immunoglobulin
•Binds only with IgM, IgG3, IgG1 & IgG2

8. Components
•20 serum proteins
•Distinct chemically and immunologically.
•Comprises of
–Complement components
–Properdin system
–Control proteins
•Complement has fractions C1-C9
•C1 occurs in serum as an calcium dependent complex
•On chelation with EDTA it gives rise to C1q, C1r &
C1s

9. How complement system is
explained ?
•Lets consider lysis of RBC
•E= RBC
•A= Antibody
•C= Complement
EA
EAC

10. How complement system is
explained ?
•EAC will be followed by numbers.
•Numbers represent the components which have reacted
EgEAC 123
•Active components are indicated with a bar on top
EAC 123
•Cleaved components will be mentioned in small letters
C1a
•Inactive form of complement is indicated with a prefix “I”
iC3b

11. How complement is activated ?
•Activated by binding of antigen & antibody
•Followed by series of reactions ( Cascade)
•Initial components acts as enzymes for the following steps
•These enzymes cleave the next member of cascade into
–Larger fragment : join the cascade
–Smaller fragment : Biological effects
•First to be identified Classical pathway Specific active
immunity
•Most common Alternate pathway Non specific innate
immunity
↑in inflammation, ↑Vascular
permeability, Smooth muscle
contraction, Chemotaxis, Detoxification

12. Classical pathway
142-356789
E A+
EA
C1
Ca
EAC1
C4
C4a
C4b
C14b
C2

13. Classical pathway
142-356789
C2
C2a
C2b
C14b2a
C3
C3 convertase
C3b
C3a
C14b2a3b
C5 convertase
C5
C5b
C5a
Membrane
attack phase
Anaphylotoxin
↑Vascular
permeability
Chemotaxis
Anaphylaxis
Mg

14. Classical pathway
142-356789
C5b
C14b2a3b567
Binds to cell membrane
Prepares for lysis
C6
Amplifies
lysis
By
Binding
To new cells
C14b2a3b56789
C e l l L Y S I S

15. Revision !!

16. Alternate complement pathway
•Classical pathway : Activation of C3 happens with the help of
C4b2a or C3 convertase
•Activation of C3 without the help of C3 convertase constitutes
“Alternate Pathway”
•First observed was with “Properdin System”
•Serum proteins provide defense against microbes without
involving an Antibody.
•Intital study was based on ZYMOSAN Polysacchride in
yeast cell wall
•Other activators : Bacterial endotoxins, IgA, IgD, Cobra
venom & Nephritic factor

17. Alternate complement pathway
C3b in circulation
C3b + Activator
Bound C3b
C3bB
C3bBb
C3
C3a
C3b
CASCADE
Free C3b inactivated by serum protein factors H & I
Protected from inactivation by factor B
(C3 Proactivator)
With factor B –Mg Dependent
C3bB cleaves in presence of Factor
D (C3Proactivator Convertase)
Ba Bb
Released
into system
•C3 Convertase
•Liable
•Stabilized by Factor P Properdin
Anaphylaxis
Chemotaxis

18. Lectin complement pathway
•Lectins?
•Proteins which recognize and bind to specific
carbohydrate targets
•Protein bind to mannose residues in the complement
•Hence known as Mannan-binding lectinpathway or
MB lectin(MBL)
•Does not depend on antibody for activation
•Mannose residues are seen in Salmonella, Listeria,
Neisseria,Cryptococcus&Candida

19. Lectin complement pathway
•MBL is an acute phase protein
•Acts like C1q
•After binding Proteases like MASP1 & MASP2 act
and for C1 like active complex
•This process cleaves C4 followed by C2 leading to
formation of C5 convertase (without participation of
antibody!!)
•This pathway is similar to classical pathway except
that it does not use C1 proteins …

21. Ok finally what do these pathways do???
•Converge to form membrane attack complex
(MAC)
•MAC forms large channel through the membrane
of pathogen
•Because of the open channel ions move freely in
& out!
•Cell cannot maintain osmotic stability
•Killed by
–Influx of water
–Loss of electrolytes
C14b2a3b56789

22. Regulation of complement
system
•Uncontrolled activity will lead to exhaustion
and extensive damage to host tissue
•System is self regulatory
•Comprises of
•Inhibitors: Bind to components of cascade and
halt further function
•Inactivators: Enzymes destroy complement
proteins

23. Inhibitors
•Inhibitor of C1 esterase C1sINH
•Heat labile
•Inhibits other esteraseslike
–Plasmin
–Kininogen
–Hageman factor
•Prevents Autocatalytic prolongation
•S protein
–Present in normal serum
–Binds to active C67
–Modulates lysisof membrane attack complex (MAC)

24. inactivators
•Factor I
–Controls C3 activation
–Important in alternate pathway
•Factor H
–Joins with factor I and controls C3 activation
•Anaphylotoxininactivator
–Enzymaticallydegrades C3a, C4a and C5a
(anaphylotoxins)
•C4 binding protein
–Controls activity of cell bound C4b

25. Biological effects of complement system
•Immunological membrane damage
•Amplifies inflammatory response :
Hypersensitivity
•Anti-Viral activity
•Promotes phagocytosis
•Promotes immune adherence

26. phagocytosis
•Complement helps phagocytic cells identify and destroy
pathogens
•This happens because of presence of receptors for complement
in the phagocytic cells (CR)
•Opsonisation
•CR CR 1,2,3,4 and C1q
•Opsonized immune complexes and bound to receptors of
phagocytic cells leading to destruction of such immune
complexes
•CR2 receptor on B-Cells also act as receptor for Epstein-Barr
virus

27. Inflammatory response
•C fragments released during cascade amplify inflammatory response
•C2 ↑ capillary permeability
•C3a, C5a Anaphylotoxins, Chemotactic
•C567 Chemotactic& lysis
Hypersensitivity & autoimmunity
•Type II hypersensitivity Incompatible transfusion, Thrombocytopenia
•Type III hypersensitivity Serum sickness , Arthusreactions
•SLE, Rheumatoid arthritis, Autoimmune hemolytic anaemia

28. Endotoxicshock
•Endotoxinsactivate alternative complement pathway
•Leads to large scale platelet lysisRealeaseof
platelet factors DIC and Thrombocytopenia
Immune adherence
•Antigen+ Antibody + Complement attaches to RBC
or Platelets : Immune adherence
•Suchcomplexes are phagocytosedrapidly
•C3& C4 are essential for immune adherence

29. Can we measure complement?
•Measured by estimating highest dilution of
serum lysing sheep RBC which is sensitized
by anti-erythrocyte antibody
•Individual components of complement cascade
can also be measured using hemolytic activity.
•Radial immunodiffusion
–Disadvantage : Cannot differentiate active and
inactive forms

30. Where is it synthesized?
C1: Intestinal epithelium
C2: Macrophages
C3: Liver
C4: Macrophages
C5: Spleen
C6: Liver
C8: Spleen
C9: Liver
C4, C5, C6 & C3 Acute phase reactants

31. Deficiencies in complement system
•Deficiency of
•C1 inhibitor : Heriditaryangioneuroticedema
•Episodic angioedemaof Subcutaneous tissue,
Respiratory or Alimentary tracts
•May be fatal
•Uncontrolled breakdown of C4 & C2
•Treated with infusion of FFP (source of C1 inhibitor)
•Prophylaxis : Epsilon aminocaproicacid