Inherited disorders of complement

The role of C5a-C5aR1 axis in bone pathophysiology: A mini-review

Bone remodeling is a physiological, dynamic process that mainly depends on the functions of 2 cell types: osteoblasts and osteoclasts. Emerging evidence suggests that complement system is crucially involved in the regulation of functions of these cells, especially during inflammatory states. In this context, complement component 5a (C5a), a powerful pro-inflammatory anaphylatoxin that binds the receptor C5aR1, is known to regulate osteoclast formation and osteoblast inflammatory responses, and has thus been proposed as potential therapeutic target for the treatment of inflammatory bone diseases. In this review, we will analyze the role of C5a-C5aR1 axis in bone physiology and pathophysiology, describing its involvement in the pathogenesis of some of the most frequent inflammatory bone diseases such as rheumatoid arthritis, and also in osteoporosis and bone cancer and metastasis. Moreover, we will examine C5aR1-based pharmacological approaches that are available and have been tested so far for the treatment of these conditions. Given the growing interest of the scientific community on osteoimmunology, and the scarcity of data regarding the role of C5a-C5aR1 axis in bone pathophysiology, we will highlight the importance of this axis in mediating the interactions between skeletal and immune systems and its potential use as a therapeutic target.

Leiner's disease (erythroderma desquamativum): A review and approach to therapy

Leiner's disease (LD) is a rare and serious syndrome of infantile erythroderma of severe and progressive generalized seborrheic-like dermatitis, recalcitrant diarrhea, malabsorption and wasting, and recurrent local and systemic infections. The purpose of this study is to provide an updated review on management with a summarized review of available peer-reviewed articles on LD. The mechanisms underlying this disease process remain unclear. The diagnosis includes demonstration of deficient opsonic activity along with the clinical tetrad of erythroderma, persistent gastrointestinal disturbance, superimposed bacterial or candidal infection, and marked wasting. An important correlation between LD and defective yeast and Staphylococcus aureus opsonization has been established. For the familial form of LD, an association of either complement three deficiency or complement five dysfunction has been made. LD should be distinguished from other types of infantile erythroderma, including Omenn syndrome. Treatment includes fluid and nutrition replacement, antibiotics to control infection, and fresh-frozen plasma therapy. The prognosis is unclear; it depends on treatment. LD is a life-threatening condition that requires prompt identification and hospitalization. Affected infants who receive vigorous treatment not only have the prospect of surviving, but also generally lead a normal life after infancy.

Complement modulates the cutaneous microbiome and inflammatory milieu

The skin is colonized by a plethora of microbes that include commensals and potential pathogens, but it is currently unknown how cutaneous host immune mechanisms influence the composition, diversity, and quantity of the skin microbiota. Here we reveal an interactive role for complement in cutaneous host-microbiome interactions. Inhibiting signaling of the complement component C5a receptor (C5aR) altered the composition and diversity of the skin microbiota as revealed by deep sequencing of the bacterial 16S rRNA gene. In parallel, we demonstrate that C5aR inhibition results in down-regulation of genes encoding cutaneous antimicrobial peptides, pattern recognition receptors, and proinflammatory mediators. Immunohistochemistry of inflammatory cell infiltrates in the skin showed reduced numbers of macrophages and lymphocytes with C5aR inhibition. Further, comparing cutaneous gene expression in germ-free mice vs. conventionally raised mice suggests that the commensal microbiota regulates expression of complement genes in the skin. These findings demonstrate a component of host immunity that impacts colonization of the skin by the commensal microbiota and vice versa, a critical step toward understanding host-microbe immune mutualism of the skin and its implications for health and disease. Additionally, we reveal a role for complement in homeostatic host-microbiome interactions of the skin.

A novel interaction between type IV pili of Neisseria gonorrhoeae and the human complement regulator C4B-binding protein

C4b-binding protein (C4BP) is an important plasma inhibitor of the classical pathway of complement activation. Several bacterial pathogens bind C4BP, which may contribute to their virulence. In the present report we demonstrate that isolated type IV pili from Neisseria gonorrhoeae bind human C4BP in a dose-dependent and saturable manner. C4BP consists of seven identical alpha-chains and one beta-chain linked together with disulfide bridges. We found that pili bind to the alpha-chain of C4BP, which is composed of eight homologous complement control protein (CCP) domains. From the results of an inhibition assay with C4b and a competition assay in which we tested mutants of C4BP lacking individual CCPs, we concluded that the binding area for pili is localized to CCP1 and CCP2 of the alpha-chain. The binding between pili and C4BP was abolished at 0.25 M NaCl, implying that it is based mostly on ionic interactions, similarly to what have been observed for C4b-C4BP binding. Furthermore, the N-terminal part of PilC, a structural component of pili, appeared to be responsible for binding of C4BP. Membrane cofactor protein, previously shown to be a receptor for pathogenic N. gonorrhoeae on the surface of epithelial cells, competed with C4BP for binding to pili only at high concentrations, suggesting that different parts of pili are involved in these two interactions. Accordingly, high concentrations of C4BP were required to inhibit binding of N. gonorrhoeae to Chang conjunctiva cells, and no inhibition of binding was observed with cervical epithelial cells.

Complement C6 deficiency protects against diet-induced atherosclerosis in rabbits

Low-density lipoprotein (LDL) can be transformed to an atherogenic moiety by nonoxidative, enzymatic degradation. Enzymatically degraded LDL induces macrophage foam cell formation, provokes release of cytokines, and also activates complement. To determine whether complement activation may contribute to atherogenesis, 6 pairs of homozygous C6-deficient rabbits and their non-C6-deficient heterozygous siblings were fed a cholesterol-rich diet for 14 weeks. Cholesterol levels and plasma lipoprotein profiles of the animals in the C6-competent and C6-deficient groups did not significantly differ, and the high density lipoprotein and LDL cholesterol ratios at the end of the experiment were 0.07+/-0.01 and 0.08+/-0.01 (SEM), respectively. However, differences in atherosclerotic plaque formation were discernible macroscopically, with extensive aortic lesions being visible in all C6-competent animals and absent in all C6-deficient animals. Aortas were sectioned from thorax to abdomen, and 10 sections were stained from each aorta. Quantification of atherosclerotic lesions and lumen stenosis with the use of computer-based morphometry documented a dramatic protective effect of C6 deficiency on the development of diet-induced atherosclerosis. We conclude that the terminal complement sequence is centrally involved in atherosclerotic lesion progression.

Lupus erythematosus profundus with partial C4 deficiency responding to thalidomide

A female patient with disfiguring lupus erythematosus profundus (LEP) from the age of 13 years was found to have an isolated partial C4 deficiency, with reduced levels of both allotypes, C4A and C4B. A genetic basis for the hypocomplementaemia was confirmed by a family study of complement and HLA types which revealed heterozygous null alleles for C4A and C4B in the proband. Marked improvement in her cutaneous lesions occurred with thalidomide.

Subacute cutaneous lupus erythematosus presenting as erythroderma

We report the first case of subacute cutaneous lupus erythematosus presenting with exfoliative erythroderma. Although our patient had uniquely extensive skin disease, she had the clinical, histologic, serologic, and immunogenetic features of subacute cutaneous lupus erythematosus. Complete remission was obtained with a short course of systemic corticosteroids and long-term hydroxychloroquine therapy. We briefly discuss the usual features of subacute cutaneous lupus erythematosus.

Selective C1q deficiency in a patient with systemic lupus erythematosus

We report a case of systemic lupus erythematosus with selective C1q deficiency. C1q deficiency had been presumed to be a non-hereditary disorder; however, the 10 reported cases in the world literature, and our case, suggest that C1q deficiency could also be an inherited disease.

Myelopathy in neonatal and infantile lupus erythematosus

Two children with lupus erythematosus and myelopathy were studied. The first child developed the neonatal lupus erythematosus syndrome associated with transplacentally acquired anti-Ro/SSA antibodies. The cutaneous manifestations of neonatal lupus erythematosus disappeared but a residual myelopathy was confirmed at 16 months of age. The second child developed cutaneous lupus erythematosus at 3 months of age associated with a total deficiency of the Clr component of complement. A myelopathy and mesangial glomerulonephritis developed at 2 years of age which required treatment with corticosteroids. These two children with CNS lupus erythematosus, one associated with transplacentally acquired antibodies and the other associated with a complement deficiency, may suggest an immune-mediated mechanism for the pathogenesis of myelopathy in childhood lupus erythematosus.

The functional inhibition of activated C1 inhibitor in normal human serum causes spontaneous consumption of the complement components C2, C3, C4, and factor B

The human complement components C1r, C1s, C4, C3, factor B, and/or activated C1INH were functionally blocked in normal human serum (NHS) and EGTA- or EDTA-treated NHS by polyclonal monospecific Fab'-fragments to the individual components. The results of inhibition experiments are compatible with the formation of a classical pathway fluid-phase C3 convertase (C4b2a) spontaneously generated by the inhibition of activated C1INH. This process in both NHS and EGTA-NHS was accompanied by the consumption of C2, C4, C3, and factor B but only by poor enhancement of C5 conversion. Blocking subcomponent C1r, completely inhibited spontaneous activation of the complement components, indicating that the control of C1r hydrolysis is the essential role of activated C1INH as a regulator of C1 activation in NHS. Non-complement serum proteases were inactive during the initiation of the activation process. The presence of blood cells during functional inhibition of activated C1INH in NHS slightly decreased the consumption of C3 but not of C2 and C4.

The hereditary and acquired deficiencies of complement

The identification of hereditary and acquired complement deficiencies in humans has led to a better understanding of the biologic importance of the complement system in immunity and autoimmune disease. Although the understanding of the relevance of complement in the pathogenesis of disease is incomplete, several characteristic clinical syndromes associated with complement deficiencies have been recognized and should be known to the practicing clinician. In allergic diseases, one need recognize the C1 inhibitor deficiency syndromes which can present as severe, recurrent angioedema in childhood or in the adult as recurrent angioedema in association with a lymphoid malignancy or autoimmune disease. Complement analyses allow one to readily diagnose C1 inhibitor deficiency in angioedema. Correct diagnosis is critical because safe effective therapy is available. Chronic urticaria is also uncommonly associated with complement deficiencies, particularly acquired C1q deficiency. Again, effective therapy for hypocomplementemic urticarial vasculitis and C1q deficiency is available and differs significantly from the usual management of chronic urticaria. Homozygous and acquired deficiencies of C3 are associated with severe immune deficiency and recurrent infections with gram-positive and gram-negative bacteria. Recurrent meningococcemia and gonococcemia are being identified frequently in patients with a deficient membrane attack mechanism relating to deficiency of C5, C6, C7, or C8. Nearly one third of the patients developing meningococcemia may have an associated complement deficiency indicating the importance of complement determinations in understanding the treatment and prognosis for these patients. Deficiency of almost every complement component has been reported in association with one or more rheumatic diseases, particularly systemic lupus erythematosus. Extensive studies of C2 deficiency and limited studies of C4 deficiency indicate that these components of the classical pathway of complement are important in preventing the development of SLE or are linked to other genes predisposing to SLE. The clinical presentations of SLE in association with C2 or C4 deficiency are relatively uniform. The patients exhibit typical skin manifestations suggestive of SLE and DLE and often exhibit antibodies to SSA (Ro). The association of complement deficiencies with clinical syndromes is important for today's physician. The syndromes and deficiencies described here are the beginning of an expanding knowledge relating to the pathobiology of complement in human disorders.