Hereditary angioedema
179
Complement is a system of plasma proteins that participates in a cascade of reac- tions, generating active components that allow pathogens and immune complexes to be destroyed and eliminated from the body. Complement is part of the innate immune defenses of the body and is also activated via the antibodies produced in an adaptive immune response. Complement activation is generally confined to the surface of pathogens or circulating complexes of antibody bound to antigen.
Complement is normally activated by one of three routes: the classical pathway, which is triggered by antigen:antibody complexes or antibody bound to the surface of a pathogen; the lectin pathway, which is activated by mannose-binding lectin (MBL) and the ficolins; and the alternative pathway, in which complement is acti- vated spontaneously on the surface of some bacteria. The early part of each pathway is a series of proteolytic cleavage events leading to the generation of a convertase, a serine protease that cleaves complement component C3 and thereby initiates the effector actions of complement. The C3 convertases generated by the three path- ways are different, but evolutionarily homologous, enzymes. Complement compo- nents and activation pathways, and the main effector actions of complement, are summarized in Fig. 31.1.
The principal effector molecule, and a focal point of activation for the system, is C3b, the large cleavage fragment of C3. If active C3b, or the homologous but less potent C4b, accidentally becomes bound to a host cell surface instead of a patho- gen, the cell can be destroyed. This is usually prevented by the rapid hydrolysis of
Case 31 Hereditary angioedema
Regulation of complement activation.
This case was prepared by Raif Geha, MD, in collaboration with Arturo Borzutzky, MD, and Andrew Maginnitie, MD.
topics bearing on tHis case:
Classical pathway of complement activation
Inhibition of C1 activation
Alternative pathway of complement activation
Inflammatory effects of complement activation
Regulation of C4b
case31.indd 179 23/12/2015 14:47
180 Case 31: Hereditary Angioedema
active C3b and C4b if they do not bind immediately to the surface where they were generated. Protection against inappropriate activation of complement is also pro- vided by regulatory proteins.
One of these, and the most potent inhibitor of the classical pathway, is the C1 inhibitor (C1INH). This belongs to a family of serine protease inhibitors (called serpins) that together constitute 20% of all plasma proteins. In addition to being the sole known inhibitor of C1, C1INH contributes to the regulation of serine pro- teases of the clotting system and of the kinin system, which is activated by injury to blood vessels and by some bacterial toxins. The main product of the kinin system is bradykinin, which causes vasodilation and increased capillary permeability.
C1INH intervenes in the first step of the complement pathway, when C1 binds to immunoglobulin molecules on the surface of a pathogen or antigen:antibody complex (Fig. 31.2). Binding of two or more of the six tulip-like heads of the C1q component of C1 is required to trigger the sequential activation of the two asso- ciated serine proteases, C1r and C1s. C1INH inhibits both of these proteases, by presenting them with a so-called bait-site, in the form of an arginine bond that they cleave. When C1r and C1s attack the bait-site they covalently bind C1INH and dis- sociate from C1q. By this mechanism, the C1 inhibitor limits the time during which antibody-bound C1 can cleave C4 and C2 to generate C4b2a, the classical pathway C3 convertase.
Activation of C1 also occurs spontaneously at low levels without binding to an antigen:antibody complex, and can be triggered further by plasmin, a protease of the clotting system, which is also normally inhibited by C1INH. In the absence of C1INH, active components of complement and bradykinin are produced. This is seen in hereditary angioedema (HAE), a disease caused by a genetic deficiency of C1INH.
Case Studies in Immunology | Raif Geha ISBN: 978-0-8153-4512-1 | 7th edition Hereditary Angioedema | CS-31.01 | Figure 31-1 © Garland Science design by blink studio limited
Removal of immune complexes
Binds to complement receptors on phagocytes
Opsonization of pathogens
Peptide mediators of in�ammation,
phagocyte recruitment
Membrane-attack complex,
lysis of certain pathogens and cells
Terminal complement components
C5b C6 C7 C8 C9
C3a, C5a C3b
C3 convertase
CLASSICAL PATHWAY ALTERNATIVE PATHWAY
C3 B D
C1q, C1r, C1s C4 C2
Antigen:antibody complexes (pathogen surfaces) Pathogen surfaces
LECTIN PATHWAY
Mannose-binding lectin or �colin binds carbohydrate
on pathogen surfaces
MBL/�colin, MASP-2 C4 C2
Fig. 31.1 Overview of the main components and effector actions of complement. The early events of all three pathways of complement activation involve a series of cleavage reactions that culminate in the formation of an enzymatic activity called a C3 convertase, which cleaves complement component C3 into C3b and C3a. The production of the C3 convertase is the point at which the three pathways converge and the main effector functions of complement are generated. C3b binds covalently to the bacterial cell membrane and opsonizes the bacteria, enabling phagocytes to internalize them. C3a is a peptide mediator of local inflammation. C5a and C5b are generated by the cleavage of C5b by a C5 convertase formed by C3b bound to the C3 convertase (not shown in this simplified diagram). C5a is also a powerful peptide mediator of inflammation. C5b triggers the late events in which the terminal components of complement assemble into a membrane- attack complex that can damage the membrane of certain pathogens. Although the classical complement activation pathway was first discovered as an antibody-triggered pathway, it is now known that C1q can activate this pathway by binding directly to pathogen surfaces, as well as paralleling the lectin activation pathway by binding to antibody that is itself bound to the pathogen surface. In the lectin pathway, MASP stands for mannose-binding lectin-associated serine protease.
case31.indd 180 23/12/2015 14:47
Case 31: Hereditary Angioedema 181
The case of Richard Crafton: a failure of communication as well as of complement regulation.
Richard Crafton was a 17-year-old high-school senior when he had an attack of severe abdominal pain at the end of a school day. The pain came as frequent sharp spasms and he began to vomit. After 3 hours, the pain became unbearable and he went to the emergency room at the local hospital.
At the hospital, the intern who examined him found no abnormalities other than dry mucous membranes of the mouth, and a tender abdomen. There was no point tenderness to indicate appendicitis. Richard continued to vomit every 5 minutes and said the pain was getting worse.
Case Studies in Immunology | Raif Geha ISBN: 978-0-8153-4512-1 | 7th edition Hereditary Angioedema | CS-31.02 | Figure 31-2 © Garland Science design by blink studio limited
C1INH dissociates C1r and C1s from the active C1 complex
Binding of C1q to Ig activates C1r, which cleaves and activates the serine protease C1s
C1q binds to at least two IgG molecules on bacterial surface
C1q binds to IgM on bacterial surface
C1s
C1r
C1INH C1s
C1r
C1INH
Fig. 31.2 activation of the classical pathway of complement and intervention by C1INH. In the left panel, one molecule of IgM, bent into the ‘staple’ conformation by binding several identical epitopes on a pathogen surface, allows binding by the globular heads of C1q to its Fc pieces on the surface of the pathogen. In the right panel, multiple molecules of IgG bound to the surface of the pathogen allow binding by C1q to two or more Fc pieces. In both cases, binding of C1q activates the associated C1r, which becomes an active enzyme that cleaves the proenzyme C1s, a serine protease that initiates the classical complement cascade. Active C1 is inactivated by C1INH, which binds covalently to C1r and C1s, causing them to dissociate from the complex. There are in fact two C1r and two C1s molecules bound to each C1q molecule, although for simplicity this is not shown here. It takes four molecules of C1INH to inactivate all the C1r and C1s molecules.
Richard, age 17, presents as an
acute abdominal emergency.
case31.indd 181 23/12/2015 14:47
182 Case 31: Hereditary Angioedema
A surgeon was summoned. He agreed with the intern that Richard had an acute abdominal condition but was uncertain of the diagnosis. Blood tests showed an elevated red blood cell count, indicating dehydration. An ultrasound examination of the abdomen was inconclusive. The surgeon decided to proceed with a laparoscopic appendectomy. Intestinal edema was noted, but no other cause for his pain was seen. The surgeon removed Richard’s appendix, which was normal, and Richard recovered and returned to school 5 days later.
What Richard had not mentioned to the intern or to the surgeon was that, although he had never had such severe pains as those he was experiencing when he went to the emergency room, he had had episodes of abdominal pain since he was 14 years old. No one in the emergency room asked him if he was taking any medication, or took a family history or a history of prior illness. If they had, they would have learned that Richard’s mother, his maternal grandmother, and a maternal uncle, also had recurrent episodes of severe abdominal pain, as did his only sibling, a 19-year-old sister.
As a newborn, Richard was prone to severe colic. When he was 4 years old, a bump on his head led to abnormal swelling. When he was 7, a blow with a baseball bat caused his entire left forearm to swell to twice its normal size. In both cases, the swelling was not painful, nor was it red or itchy, and it disappeared after 2 days. At age 14 years, he began to complain of abdominal pain every few months, sometimes accompanied by vomiting and, more rarely, by clear, watery diarrhea.
Richard’s mother had taken him at age 4 years to an immunologist, who listened to the family history and immediately suspected hereditary angioedema. The diagno- sis was confirmed on measuring key complement components. C1INH levels were 16% of the normal mean and C4 levels were markedly decreased, while C3 levels were normal.
When Richard returned for a routine visit to his immunologist a few weeks after his surgical misadventure, the immunologist, noticing Richard’s abdominal scar, asked what had happened. When Richard explained, he prescribed purified C1INH to be infused intravenously for acute attacks. Richard has utilized C1INH on several occa- sions and is considering changing to one of the other recently approved products for treatment of HAE attacks. He is also aware that C1INH can be given prophylactically to prevent attacks either before surgical and dental procedures or routinely.
Richard subsequently married and had two children. The C1INH level was found to be normal in both children.
Hereditary angioedema.
Individuals like Richard with a hereditary deficiency of C1INH are subject to recur- rent episodes of circumscribed swelling of the skin (Fig. 31.3), intestine, and airway. Attacks of subcutaneous or mucosal swelling most commonly affect the extremi- ties, but can also involve the face, trunk, genitals, lips, tongue, or larynx. Cutaneous attacks cause temporary disfigurement but are not dangerous. When the swell- ing occurs in the intestine it causes severe abdominal pain, and may obstruct the intestine causing vomiting. When the colon is affected, watery diarrhea may occur. Laryngeal swelling is most dangerous as it can lead to asphyxiation. HAE attacks do not usually involve itching or hives, which is useful to differentiate this disease from allergic angioedema. However, a serpiginous, or linear and wavy, rash (called erythema marginatum) is sometimes seen before the onset of swelling symptoms. HAE attacks may be triggered by trauma, menstrual periods, excessive exercise, exposure to extremes of temperature, mental stress, and some medications such as angiotensin-converting enzyme inhibitors and oral contraceptives.
Appendectomy performed.
Appendix appears normal.
Family history of colic.
Case Studies in Immunology | Raif Geha ISBN: 978-0-8153-4512-1 | 7th edition Hereditary Angioedema | CS-31.03 | Figure 31-3 © Garland Science design by blink studio limited
Fig. 31.3 Hereditary angioedema. Transient localized swelling that occurs in this condition often affects the face.
case31.indd 182 23/12/2015 14:47
Case 31: Hereditary Angioedema 183
HAE is not an allergic disease, and attacks are not mediated by histamine. Rather, generation of bradykinin by the enzyme kallikrein leads to HAE symptoms. As seen in Figure 31.4, activated Factor XII generates active kallikrein from its precursor. Kallikrein, which circulates bound to high-molecular-weight kininogen (HMWK) then cleaves bradykinin from HMWK. Bradykinin is the main mediator respon- sible for HAE attacks by causing vasodilation and increasing the permeability of the postcapillary venules by causing contraction of endothelial cells so as to create gaps in the blood vessel wall (Fig. 31.5). This is responsible for the edema; move- ment of fluid from the vascular space into another body compartment, such as the gut, causes the symptoms of dehydration as the vascular volume contracts.
Treatment of HAE can focus on preventing attacks or on resolving acute episodes. Purified C1INH can be used either routinely to prevent attack or before known trig- gers such as surgery or dental work. Attenuated androgens, such as danazol, are also indicated for prophylaxis. Purified or recombinant C1INH are effective ther- apy in terminating acute attacks. A kallikrein inhibitor and a bradykinin receptor antagonist have also been developed, which reverse HAE symptoms.
Questions.
1 Activation of the complement system results in the release of histamine and chemokines, which normally produce pain, heat, and itching. Why is the edema fluid in HAE free of cellular components, and why does the swelling not itch?
2 Richard has a markedly decreased amount of C4 in his blood. This is because it is being rapidly cleaved by activated C1. What other complement component would you expect to find decreased? Would you expect the alternative pathway components to be low, normal, or elevated? What about the terminal components?
3 Despite the complement deficiency in patients with HAE, they are not unduly susceptible to infection. Why not?
Fig. 31.5 Contraction of endothelial cells creates gaps in the blood vessel wall. A guinea pig was injected intravenously with India ink (a suspension of carbon particles). Immediately thereafter the guinea pig was injected intradermally with a small amount of activated C1s. An area of angioedema formed about the injected site, which was biopsied 10 minutes later. An electron micrograph reveals that the endothelial cells in post-capillary venules have contracted and formed gaps through which the India ink particles have leaked from the blood vessel. L is the lumen of the blood vessel; P is a polymorphonuclear leukocyte in the lumen; rbc is a red blood cell that has leaked out of the blood vessel. End is endothelial cell. Arrows point to India ink particles Scale bar: 10 μM. Micrograph courtesy of Kaethe Willms.
Case Studies in Immunology | Raif Geha ISBN: 978-0-8153-4512-1 | 7th edition Hereditary Angioedema | CS-31.04 | Figure 31-4 © Garland Science design by blink studio limited
Angiodema
kallikrein
+ bradykinin
factor XIIa
HMW kininogen
kallikrein
HMW kininogenHMW kininogen
prekallikrein
Endothelial cell
B2R
C1INH
Case Studies in Immunology | Raif Geha ISBN: 978-0-8153-4512-1 | 7th edition Hereditary Angioedema | CS-31.05 | Figure 31-5 © Garland Science design by blink studio limited
rbc
L
P
end
Fig. 31.4 Pathogenesis of hereditary angioedema. Activation of Factor XII leads to the activation of kallikrein, which cleaves kininogen to produce the vasoactive peptide bradykinin; bradykinin binds to its receptor on the surface of endothelial cells, triggering the edema characteristic of HAE.
case31.indd 183 23/12/2015 14:47
184 Case 31: Hereditary Angioedema
4 Emergency treatment for HAE cases is sometimes necessary because of airway obstruction. In most cases, however, a patient with obstruction of the upper airways is likely to be suffering from an anaphylactic reaction. The treatment in this case would be epinephrine. How might you decide whether to administer epinephrine or intravenous C1INH?
5 Fig. 31.6 shows Richard’s family tree. What is the mode of inheritance (dominant or recessive, sex-linked or not) of HAE? Can Richard’s two children pass the disease onto their offspring?
Case Studies in Immunology | Raif Geha ISBN: 978-0-8153-4512-1 | 7th edition Hereditary Angioedema | CS-31.06 | Figure 31-6 © Garland Science design by blink studio limited
affected female
affected male
normal female
normal male
propositus (Richard Crafton)
chromosome 11 bearing C1INH mutation
normal chromosome 11
Fig. 31.6 The inheritance of hereditary angioedema in Richard’s extended family.
case31.indd 184 23/12/2015 14:47