Combinations of proinflammatory and procoagulant reactions are associated with a variety of
disorders affecting the cardiovascular system. Vascular leakage contributes to the pathology
of conditions such as, sepsis, allergy and anaphylactic reactions. Edema formation is the
result of extravasated proteins and fluid and the peptide hormone bradykinin is considered to
be one of the key mediators in the regulation of vascular leakage. Bradykinin is produced by
the kallikrein-kinin system, which consists of factor XII, plasma prekallikrein, high molecular
weight kininogen and C1 esterase inhibitor. Activated factor XII generates active prekallikrein
(kallikrein), which cleaves kininogen, leading to the liberation of bradykinin. Activation of
mast cells during allergic reactions mediates inflammatory responses, which cause
increased vascular permeability. We report a new mechanism by which mast cell-released
heparin increases vascular leakage. Upon allergen challenge mast cells release the
negatively charged polysaccharide heparin that efficiently activates factor XII and initiates
the kallikrein-kinin system. Heparin-driven kallikrein-kinin system activation culminates in
bradykinin formation causing excessive vascular leakage in mice that are deficient in C1
esterase inhibitor, the major endogenous inhibitor of factor XII and kallikrein. These findings
also have implications in anaphylactic and allergic diseases and we show that the factor XII–
driven kallikrein-kinin system critically contributes to the pathogenesis of anaphylaxis in both
murine models and human subjects. The data indicate that heparin-initiated bradykinin
formation plays a fundamental role in mast cell mediated diseases.
Hereditary angioedema (HAE) is a rare inherited disease that is characterized by acute
swelling that involves the skin, extremities and mucosa. HAE types I and II are caused by
deficiency in or dysfunctional C1 esterase inhibitor. In contrast, a third HAE variant exists in
patients that have normal C1 esterase inhibitor (HAE III). HAE III is associated with a single
point mutation at residue Thr309 in factor XII. However, the mechanism of HAE III was
unknown. This study characterizes the mechanism and therapy of HAE III. HAE III patientplasma
and recombinant Thr309 mutated factor XII result in a double band or in a band with
a lower molecular weight than wild-type factor XII in Western blotting. This is the
consequence of a loss of glycosylation. The mutation in factor XII causes excessive
activation of the kallikrein-kinin system resulting in enhanced production of bradykinin.
Addition of C1 esterase inhibitor dose-dependently blocked bradykinin production in HAE
types I and II, but not in HAE III. We generated a fully humanized antibody (3F7) that
specifically interferes with activated factor XII proteolytic activity. 3F7 inhibits activated factor
XII-driven cleavage of high molecular weight kininogen in a dose dependent manner and
interferes with aberrant kallikrein-kinin system-triggered bradykinin formation in HAE III
plasma. We reconstituted factor XII deficient mice with recombinant human mutated factor
XII and established an HAE III transgenic mouse that expresses human Thr309-mutated
factor XII in the liver using Tet-off transgenic technology. Intravital confocal scanning
microscopy and tracer extravasation-based methods show excessive bradykinin-mediated
vascular leakage in both F12-/- mice reconstituted with mutated factor XII and in HAE III
transgenic mice when challenged with factor XII-contact activator. Both a kallikrein inhibitor
and 3F7 reduce edema in HAE III associated leakage in mice. This study characterizes the
mechanism of HAE III and establishes factor XII inhibition as a novel therapeutic strategy to
interfere with excessive vascular leakage in HAE III and potentially, other causes of edema
