Complement activation is needed to restore tissue injury; however, inappropriate activation of complement, as seen in chronic wounds can cause cell death and enhance inflammation, thus contributing to further injury and impaired wound healing. Therefore, attenuation of complement activation by specific inhibitors is considered as an innovative wound care strategy. Currently, the effects of several complement inhibitors, for example, the C3 inhibitor compstatin and several C1 and C5 inhibitors, are under investigation in patients with complement-mediated diseases. Although (pre)clinical research into the effects of these complement inhibitors on wound healing is limited, available data indicate that reduction of complement activation can improve wound healing. Moreover, medicine may take advantage of safe and effective agents that are produced by various microorganisms, symbionts, for example, medicinal maggots, and plants to attenuate complement activation. To conclude, for the development of new wound care strategies, (pre)clinical studies into the roles of complement and the effects of application of complement inhibitors in wound healing are required.
Wound healing is often completed within two weeks after injury, although tissue remodeling may take several months up to two years. The process of wound healing consists of three, overlapping phases, that is, inflammation, tissue proliferation and tissue remodeling [
Chronic wounds occur in individuals having defects that either prevent the healing process or allow healing to continue without leading to a proper anatomical and functional result. Risk factors for the development of chronic wounds include vascular diseases, diabetes mellitus, pressure (necrosis), alcohol and nicotins abuse, and old age [
Tissue injury immediately initiates an array of physiological processes that lead to wound repair and regeneration. Although the exact underlying mechanisms of action are unclear, it is known that the immune systems play an essential role in the regulation of these processes [
Following blood capillary vessel injury, an immediate reflex promotes vasoconstriction, slowdown of blood flow, and the local formation of a platelet clot. In addition, injured tissue cells release factors that stimulate the formation of a fibrin clot (containing a.o. fibronectin and vitronectin), that traps blood cells including platelets and red blood cells. This provisional extracellular matrix allows tissue cells to migrate to the wound area. The activated kallikrein-kinin system provides vasoactive kinins that mediate vasodilation and increased vascular permeability. The complement system is activated by distinct carbohydrate and lipid residues on altered self-molecules and injured cells and the cellular inflammatory response is subsequently initiated. Neutrophils are the first inflammatory cells that migrate into wounds to debride necrotic and apoptotic cells and eliminate infectious agents from the wound bed [
The chemotactic mediators and growth factors produced by macrophages and healthy bystander cells stimulate angiogenesis and attract endothelial cells and fibroblasts that contribute to the proliferative phase of wound healing [
As described above, the first response to tissue injury is characterized by activation of the cellular and molecular effectors of the innate immune system, including the complement system. However, inappropriate complement activation, for example, in chronic wounds, will result in detrimental effects due to its ability to induce cell death and promote prolonged inflammation [
Based on these considerations, this paper focuses on (1) the current understanding of the dual roles of complement activation in wound healing and (2) the present and novel complement inhibitors to be considered for treatment of chronic wounds.
The activated complement system is a crucial effector mechanism of the innate immune response to tissue injury. In general, the complement system can be activated by a number of pathways: the classical pathway (by immune complexes), the lectin pathway (by mannose residues and ficolins), and the alternative pathway (by spontaneous activation and microbial structures) and by properdin and thrombin [
A simplified overview of the complement activation cascade after injury leading to wound healing. Three major pathways of complement activation, that is, the classical pathway (CP), the alternative pathway (LP), and the lectin pathway (LP), and two minor pathways initiated by properdin and thrombin are known. C is a complement component, MASP is mannan-binding serine peptidase, fB and D are factors B and D, SCIN is staphylococcal complement inhibitor, sCR1 is soluble complement receptor 1, fH is factor H, CR2 is complement receptor 2 and MAC is membrane attack complex. For simplicity, not all of the natural regulators of complement activation are shown in this diagram. The (pre)clinical complement inhibitors are denoted in bold and the complement factors that have been investigated in burn wound models in italic. C1 inhibitor affects C1r, C1s from the CP, and MASP 1 and MASP 2 from the LP. C4 knockout also affects both CP and LP.
There are a few studies that report beneficial effects of complement-activating components on wound healing. First, Strey et al. reported that complement C3a and C5a are absolutely required for liver repair in a mouse model of liver injury [
While enhanced levels of complement activating factors are found in chronic wounds, it is interesting to study the outcomes of wounds in which complement activation is attenuated. It has been shown that animals with a genetic complement deficiency or individuals treated with a complement inhibitor are protected from the symptoms resulting from chronic inflammatory processes [
Studies by Van de Goot et al. into the roles of complement in burn wounds showed enhanced levels of complement degradation factor C3d, indicative of complement activation, in the wound [
In clinical practice, only a few complement inhibitors are currently available (Table
An overview of (pre)clinical complement inhibitors.
Complement inhibitor | Medicine | Diseases | Study phase |
---|---|---|---|
Recombinant C1 inhibitor | Conestat alfa | HAE |
In clinical use, EU approved. |
(Ruconest in Europe/Rhucin in USA) | |||
| |||
Plasma-derived C1 inhibitors | Berinert P/cinryze | HAE | In clinical use, FDA approved. |
| |||
C3 inhibitors | Compstatin (POT-4) | AMD | Phase II |
Staphylococcal complement inhibitor (SCIN) | Preclinical | ||
| |||
Myristoylated peptidyl derived from soluble CR1 | Mirococept (APT070) | Delayed graft function of cadaveric kidney after transplantation. | Phase II |
| |||
Factor H | Plasma-derived factor H concentrate | HUS, AMD | Preclinical |
TT30/targeted alternative pathway inhibitor/factor H | PNH, AMD | Phase I | |
| |||
Factor D inhibitor | Anticomplement factor D | AMD | Phase II |
| |||
Factor B inhibitor | TA106/anti-complement factor B | AMD | Preclinical |
| |||
C5 inhibitors | Eculizumab | PNH |
In clinical use, FDA approved. |
Various other diseases, for example, kidney transplants, HUS, AMD. | Phase I | ||
Pexelizumab | Phase III study failed | ||
Mubodina | HUS | Preclinical | |
Ergidina | Ischemia/reperfusion injury | Preclinical | |
ARC 1905 | AMD | Phase I | |
| |||
C5a inhibitor | PMX 53 and several other compounds | AMD | Phase II study discontinued |
Osteoarthritis | Phase I | ||
| |||
Targeted complement inhibitors | Targeted |
Chronic glomerulonephritis | Phase I |
HAE: hereditary angioedema; AMD: acute macular degeneration; HUS: haemolytic uraemic syndrome; PNH: paroxysmal nocturnal haematuria.
Recently, the C5 inhibitor pexelizumab failed in a Phase III study as it did not reduce infarction and mortality in patients after coronary intervention [
Phase I studies are performed with targeted factor H (TT30), that is, factor H coupled to CR2, for AMD and PNH [
Larvae of medicinal maggots (
Recently, we found that maggot ES efficiently reduced complement activation in normal and immune-activated sera in a dose-dependent fashion with maximal inhibition of 99.9% (Figure
Dose-dependent effect of fresh collected maggot ES on activation of the classical pathway (white bars), the alternative pathway (grey bars), and the lectin pathway (black bars) in normal human sera. The complement activation in four different sera was determined with the enzyme immunoassays from Wieslab (EuroDiagnostica BV, Arnhem, The Netherlands) according to manufacturer’s instructions. The percentages inhibition was calculated using the values in the sera without maggot ES as 0%. The results are means and SD of four independent experiments.
As the complement system is a rapid and effective defense system, practically each successful microorganism has developed strategies and molecules to evade the actions of complement [
Although plants lack genes encoding complement molecules, complement inhibitors have been found in extracts from various species of plants and trees (Table
An overview of complement inhibitors in extracts from plant species.
Plant L. | Part of plant (extract) | Mode of action | Beneficial effects | References |
---|---|---|---|---|
|
Aerial parts | CP inhibition. |
Antispasmodic, antipyretic, antihelmintic, antibacterial, antiviral. Stimulant, emmenagogue, excitant. | [ |
| ||||
|
Leaves | AP activation, resulting in consumption of C3. | Antibacterial, antifungal, antiparasitic, antitumor, laxative. Used for seborrheic dermatitis, radiation dermatitis, psoriasis vulgaris, genital herpes, burn wounds, diabetes, HIV infection, ulcerative colitis, pressure ulcers, mucositis, aphthous stomatitis, acne vulgaris, lichen planus, frostbite, alopecia, systemic lupus erythematosus, arthritis, tic douloureux. | [ |
| ||||
|
Leaves | CP and AP inhibition. | Antispasmodic, mucilaginous, and pectoral properties. Used for rheumatism. | [ |
| ||||
|
Leaves | CP inhibition. |
Used for colic pain, vomiting, diarrhea, dysmenorrhea. | [ |
| ||||
|
Leaves | CP inhibition. Fucoidan binds C1q and prevents the formation of active C1. It forms a complex with C4 | Anti-inflammatory, antiangiogenic, anticoagulant, antiadhesive. | [ |
| ||||
|
Stem bark | CP and AP inhibition. |
Used for wound healing, bone healing, inflamed sores, gastric ulcers, uterine hemorrhages, metrorragias, cervicitis. | [ |
| ||||
|
Stem bark | CP inhibition. |
Antitumor, anti-inflammatory, antiviral. Used for skin diseases, wound healing, rheumatism, smallpox, ulcers, malaria. | [ |
| ||||
|
Aerial parts | CP inhibition. |
Used for wound healing, inflammation. | [ |
| ||||
|
Oleogum resin | CP inhibition, it inhibits C3 convertase | Antihelminthic, antiseptic, haemostatic, analgesic, cardiotonic, diuretic, aphrodisiac, laxative. Used for Crohn’s disease, ulcerative colitis, bronchial asthma, rheumatoid arthritis, osteoarthritis, wound cleaning, reducing fat, diarrhea, improving menstruation. | [ |
| ||||
|
Stem bark | CP and AP inhibition. Inhibition of C1 and terminal complex. | Used for rheumatism. | [ |
| ||||
|
Stem bark | CP and AP inhibition. |
Used for diabetes, hepatobiliary and cardiovascular diseases, hypertension, pain, kidney diseases, ulcers. | [ |
| ||||
|
Latex | CP and AP inhibition. |
Antibacterial, antitumor, antiviral. Used for wound healing, inflammation. | [ |
| ||||
|
Roots | CP inhibition. |
Hepatoprotective, haemostatic, antipyretic, antiseptic, diuretic, antigonococci, antisyphilitic, antiparasitic, abortifacient. Used for wound healing, malaria, respiratory diseases, psoriasis, rheumatism, cataract, dysentery. | [ |
| ||||
|
Stem bark | CP inhibition: IC50 (CP) = 12 |
Used for general debility, sore throat, wound healing, candidiasis, venereal diseases, tuberculosis, digestive tract disorders. | [ |
| ||||
|
Aerial parts | CP and AP inhibition. |
Antimicrobial, antiviral, antinociceptive. Used for gastric pain. | [ |
| ||||
|
Aerial parts | CP inhibition. |
Hepatoprotective, antiviral, antiseptic. | [ |
| ||||
|
Roots | CP and AP inhibition. |
Antioxidant. Used for muscular pain, sciatic pain, liver and kidney diseases, wound healing, skin ulcers, edema, inflammatory diseases. | [ |
| ||||
|
Seeds |
|
Antioxidant, anti-inflammatory, antitumor, antioestrogenic, antifungal, insulinotropic. Used for atherosclerosis, skin whitening, | [ |
| ||||
|
Roots and rhizomes | Glycyrrhizin binds to C3a and C3. It induces conformational changes in C3 and it inhibits CP at the level of C2. | Anti-inflammatory, antiviral, antimicrobial, antioxidative, antitumor, immunomodulatory, hepatoprotective, cardioprotective, diuretic, anabolic, laxative, contraceptive. Used for wound healing, cystitis, diabetes, cough, stomachache, tuberculosis, nefrolitiasis, lung ailment, Addison’s disease, gastric ulcers, improvement of voice, improvement of male sexual function. | [ |
| ||||
|
Roots and aerial parts | CP inhibition. Ca2+ and Mg2+ dependent complement inhibition. It inhibits C1 formation. | Rheumatism, neuralgia, silicosis, malaria. | [ |
| ||||
|
Latex | CP inhibition, mediated by Ca2+ depletion | Used for infected wounds. | [ |
| ||||
|
Leaves | CP inhibition. |
Anti-arthritic, haemostatic, diuretic, tonic. Used for respiratory diseases. It causes allergic contact dermatitis. | [ |
| ||||
|
Leaves | CP inhibition. |
Haemostatic. Used for wound healing. | [ |
| ||||
|
Stem bark | It inhibits C5a-induced chemotaxis and decreased the stimulated production of TNF- |
Asthma, rheumatic arthritis | [ |
| ||||
|
Leaves and stems | CP and AP inhibition. |
Hypoglycemic, diuretic, laxative, antiseptic, antinociceptic. |
[ |
| ||||
|
Leaves | CP and AP inhibition. It binds C3 and inhibits C5 convertase. C5a generation is decreased. IC50 (CP) = 2 |
Antispasmodic, choleretic, hepatoprotective, anti-inflammatory, antitumor, antioxidant. Used for renal colic pain, dysmenorrhea, respiratory disorder (bronchial asthma), stimulation of hair growth, relaxation of smooth muscles of trachea and intestine, peptic ulcers, atherosclerosis, ischaemic heart disease, cataract, improvement of sperm motility. | [ |
| ||||
|
Leaves | CP inhibition. |
Antipyretic, antiepileptic, antigonococci, antisyphilitic, anti-parasitic. Used for wound healing, dysmenorrhea, asthma, vomiting, hepatitis, improvement of fertility (women), gastric diseases, malaria, hypertension, rheumatism, lumbago. | [ |
| ||||
|
Stem bark | CP and AP inhibition. |
Antioxidant, parturifacient. Used for metrorragias, diarrhea, stomachache, intestinal worms, leishmaniasis, skin ulcers. | [ |
| ||||
|
Seeds | It attenuates MBL binding on human endothelial cells and inhibited C3 deposition. The dcreased LP activation resulted in less complement-dependent neutrophil chemotaxis. |
None. | [ |
| ||||
|
Stem bark | CP and AP inhibition. |
Anti-inflammatory, antiviral, immunostimulating, antimutagenic, antioxidant. Used for gastritis, dermic and urogenital inflammations, asthma, rheumatism, irregular menstruation, digestive, liver and kidney diseases, adjuvant therapy for breast cancer. | [ |
CP: classical pathway; AP: alternative pathway; LP: Lectin Pathway; IC50: concentration required for 50% complement inhibition. Most of these complement inhibition tests were performed using complement haemolytic activity assays. Compounds in these plant species inhibiting the complement system are; for example; flavonoids, glucosides, polysaccharides, terpenes, iridoids, polymers, peptides, alkaloids, and oils [
Complement serves as a rapid and efficient immune surveillance system to control infection and tissue injury. The complement system regulates the clearance of necrotic and apoptotic cells, inflammation, and tissue regeneration. However, elevated levels of C3, C3a, C3d, and MAC have been reported in chronic wounds and burn or traumatic wounds [
However, several challenges have to be overcome before complement inhibitors can be included in the therapeutic arsenal for wound care. For example, complement inhibitors should act locally at the site of inflammation or injury, thus avoiding the adverse effects of a systemic complement blockade, that is, infection and impaired wound healing [
One more issue pertains to the contribution of local production and functional activities of complement components and their regulators. Although the liver is the main source of complement components, the production of several complement components, for example, properdin, C1, C3, and C7, at sites of inflammation/injury should be studied in more detail. Furthermore, good affinity of the complement inhibitors for the target and selectivity are important factors to consider in anti-complement therapies. Moreover, the complement inhibitor must have a long half-life.
The choice of the complement inhibitor depends on the role the complement has in the disease. C5 inhibitors are preferred for the treatment of diseases in which C5a and MAC play a major role, for example, in HUS and in patients suffering from an infection with the EHEC bacterium [
Although there are a lot of challenges to overcome, there are some promising complement inhibitors. For example, the pathway-independent inhibitor compstatin is extensively tested in clinical studies in patients suffering from acute and chronic inflammatory conditions. The results up to date are successful [
Another important question that remains unanswered is how much the complement system can be attenuated without the risk of loss of protection. Based on our finding that a single maggot produces approximately 2
To conclude, well-designed (pre)clinical studies aimed at understanding the roles of complement in the pathology of chronic wounds, with the hope of innovative drugs and their clinical implementation to promote healing in patients with chronic wounds, are urgently needed.
The authors would like to thank Ilse Haisma for critical reading of this paper. G. Cazander was financially supported by the Bronovo Research Foundation from the Bronovo Hospital, The Hague, the Netherlands, and P. H. Nibbering by the Dutch Burns Foundation, Beverwijk, the Netherlands (Grant no 10.106). Sterile