C1 Inhibitor: Regulation of Vascular Permeability and Inhibition of Leukocyte Migration Across the Vascular Endothelium
Alvin E. Davis III, M.D.
Senior Investigator, CBR Institute for Biomedical Research
Professor of Pediatrics, Harvard Medical School
Boston, Massachusetts, USA
A serpin (serine proteinase inhibitor).
Serpins inactivate proteases by forming a stable complex with the protease.
Protease recognizes a substrate-like region of the inhibitor that is referred to as the reactive center loop.
C1 INHIBITOR FUNCTION
Protease
Serpin
Reactive center
The protease recognizes
the reactive center of the
inhibitor. Cleavage of the
reactive center loop is
followed by:
covalent bond formaton between the protease active site serine and the reactive center amino acid of the serpin.
rearrangement of the serpin with insertion of the reactive center loop (in yellow) into ? sheet A (in red), which results in movement of the protease from one pole of the molecule to the other. This results in structural distortion of the protease.
Proteases Inactivated by C1 Inhibitor
Complement System
Classical pathway: C1r, C1s
Lectin pathway: MASP 1 & 2
Intrinsic Coagulation/Contact Systems
Factor XIa, Plasma kallikrein, factor XIIa
C4b2a3b
C3bBb3b
C5
C5a
C5b + C6, C7, C8, C9
C5b-9
Membrane Attack Complex
CLASSICAL PATHWAY
Antigen-
antibody
complex
microbe
MBL + MASP 1,2
LECTIN PATHWAY
ALTERNATIVE PATHWAY
microbe
+ C3b, B, D, P
C3b,Bb,P
C1q, C1r, C1s
C4
C2
C4b2a
C3
C3a
C3b + C4b2a
C3b + C3bBbP
C1INH
C1INH
Factor I
Factor H
C1INH
Endothelial cell prolylcarboxypeptidase
Factor XII
Negatively
charged
surface
XIIa
C1INH
Prekallikrein
High molecular
weight kininogen
Kallikrein
High molecular
weight kininogen
Bradykinin
C1INH
C1INH
Contact System Activation
C1 Inhibitor is a Multifunctional
Anti-Inflammatory Protein
1. Regulation of vascular permeability
Inhibition of contact system activation.
C1 inhibitor knockout mouse.
2. Modulation of leukocyte transmigration.
Inhibition of complement activation.
Interaction with E- and P-selectins on
endothelial cells.
3. Protection from endotoxin shock.
Inhibition of both complement and contact
Systems.
Interaction with gram negative endotoxin
lipopolysaccharide (LPS).
Recurrent localized edema of skin or mucosa (larynx, gastrointestinal tract)
Dominant inheritance: heterozygous for deficiency of C1 inhibitor
Pathophysiology:
Decreased C1INH function
Spontaneous activation of the complement system (C1r, C1s) ? decreased C4, C2
Spontaneous activation of the contact system (plasma kallikrein, factor XIIa) ? cleaved high molecular weight kininogen
Mediator?
Hereditary Angioedema
Pathophysiology of Hereditary Angioedema
Because there is clearcut evidence for both complement and contact system activation, the question arises as to which is responsible for generation of the mediator of angioedema.
Until recently, some data suggested that the mediator was a product of complement system activation while other data suggested that it was derived from contact system activation.
Define mediator(s) of angioedema
Test new therapies and define their mechanism of action
Pathophysiology of vascular permeability
Analysis of variability of symptoms in HAE
C1 Inhibitor Knockout Mouse. Why?
C1 INHIBITOR KNOCKOUT MICE
C1INH protein levels:
-/- mice: C1INH not detectable.
+/- mice: ? 50% of normal.
Decreased C4 levels.
Normal in size, appearance and growth.
Reproduction, fetal growth and development are normal.
Two +/- and six -/- mice ? intestinal wall edema with obstruction at ~ 10-11 weeks.
IV Evan’s blue dye in +/- & -/- mice revealed enhanced vascular permeability.
Evans Blue Dye Injection
Wild Type
C1INH +/-
C1INH genotype +/+ +/+ -/- -/-
Evans blue dye - + + +
C1INH treatment - - - +
Does intravenous C1INH reverse the
increased vascular permeability in the
C1INH knockout mice?
C1INH genotype +/+ -/- -/-
Bk2R genotype +/+ +/+ -/-
Evans blue dye + + +
Is Angioedema Mediated by Bradykinin?
Bradykinin induces increased vascular permeability via the Bk type 2 receptor (Bk2R).
C1INH -/- mice were mated with Bk2R -/- mice.
Hypothesis: C1INH -/-, Bk2R -/- mice will be resistant to the development of increased vascular permeability.
C1INH genotype +/+ -/- -/- -/- -/-
Treatment 0 0 Captopril Bk1RA Hoe140
Mean .2577 .4087 .8860 .4477 .0252
Standard error .0284 .0806 .0773 .0667 .0252
Intestinal Vascular Permeability
Is Angioedema Mediated by
Bradykinin?
In addition, the increased vascular permeability in C1INH -/- mice was reversed by treatment with the following:
A recombinant variant Kunitz domain inhibitor, DX88 (Dyax), that is highly specific for plasma kallikrein.
A recombinant C1INH Ala443 ? Val, that inhibits the contact system but not the complement system.
A Bk2R antagonist, Hoe140 (Icatibant).
These data all support the hypothesis that the mediator of vascular permeability in C1INH deficiency is bradykinin.
Selectins: adhesion molecules responsible for leukocyte rolling on the endothelium.
E-selectin: endothelial cells; expression induced by TNF-?, IL-1, LPS.
P-selectin: platelets & endot