In vitro complement activation by ragweed allergen extract in the sera of ragweed allergic and non-allergic persons

Molecular basis for downregulation of C5a-mediated inflammation by IgG1 immune complexes in allergy and asthma

Allergy and asthma are triggered primarily by the binding of allergen-specific immunoglobulin E (IgE)-allergen complexes to their receptors, recognition of the allergens by antigen-presenting cells, and allergen presentation to the T cells. These events lead to mucus secretions, runny nose, itchy eyes, sneezing, airway hyperresponsiveness, and nasal congestion. Complement 5a (C5a) has emerged as a central molecule that mediates these allergic reactions. Many allergens and allergen-specific IgG immune complexes (IgG-ICs) cause complement activation and C5a generation. C5a interaction with its receptor (C5aR) leads to the infiltration and activation of several immunologic cell types and the secretion of pathogenic inflammatory and proinflammatory mediators. However, IgG1-IC binding to the IgG inhibitory Fc gamma receptor (FcγRIIB) suppresses C5aR-mediated inflammatory signaling and, hence, may reduce the inflammatory immune responses through this FcγRIIB-mediated pathway. Reviews of the IgG1-IC interactions with C5a-mediated inflammatory immune responses suggest that IgG1-IC-C5a inhibitory therapy may reduce inflammation in allergic diseases.

The role of complement in the diagnosis and management of allergic rhinitis and allergic asthma

Allergic rhinitis and asthma are common chronic inflammatory diseases of the nasal mucus membranes and the upper airways with a high prevalence in Western countries. In addition to maladaptive T-helper type 2 (Th2) immunity, Th17 cells can drive the inflammatory responses in both diseases. Several reports have shown that the complement system is activated locally and systemically in allergic rhinitis and/or allergic asthma patients. Importantly, recent findings in experimental models of allergic rhinitis and allergic asthma suggest that the complement cleavage products complement 3a and complement 5a and the activation of their corresponding receptors in antigen-presenting cells regulate the development of maladaptive Th2 and Th17 immunity. These findings in experimental asthma are corroborated by genome-wide searches and candidate gene studies in humans. We discuss recent findings in experimental and human allergic airway diseases suggesting that complement may serve as a new diagnostic and therapeutic target for both disorders.

Overexpression of the anaphylatoxin receptors, complement anaphylatoxin 3a receptor and complement anaphylatoxin 5a receptor, in the nasal mucosa of patients with mild and severe persistent allergic rhinitis

BACKGROUND

Although the complement anaphylatoxin peptides, complement anaphylatoxin 3a (C3a) and complement anaphylatoxin 5a (C5a), are implicated in the inflammatory process in allergic rhinitis, a direct interaction between allergic mucosa and complement receptors has not been demonstrated.

OBJECTIVE

We investigated the expressional levels and distributional patterns of the C3a receptor (C3aR) and C5a receptor (C5aR) in normal, mild, and severe persistent allergic nasal mucosa.

METHODS

Immunohistochemistry and Western blotting using C3aR and C5aR antibodies were applied to normal nasal, mild, and severe persistent allergic nasal mucosa.

RESULTS

Complement anaphylatoxin 5a receptor was detected in the inflammatory cells of normal and allergic nasal mucosa, and its expression level was significantly higher in allergic nasal mucosa than normal nasal mucosa. C3aR in normal and allergic nasal mucosa was commonly expressed in nonmyeloid cells such as epithelial cells, submucosal glands, and nerve fibers. In addition, C3aR was expressed in the endothelium of cavernous sinuses and the surrounding perivascular muscle layer in severe persistent allergic nasal mucosa, but not in normal and mild allergic nasal mucosa. Western blotting demonstrated that the expression level of C3aR was significantly increased in severe persistent allergic nasal mucosa compared with normal and mild nasal mucosa.

CONCLUSION

On the basis of the location of C3aR and C5aR, C5aR may play a role in activation of inflammatory cells, whereas C3aR may mediate mucus secretion and mucosal swelling in allergic nasal mucosa, especially severe persistent allergic nasal mucosa.

Complement C3a regulates Muc5ac expression by airway Clara cells independently of Th2 responses

RATIONALE

The factors that control the secretion of epithelial mucins are essential to understanding obstructive airway diseases such as asthma. Although the complement anaphylatoxin C3a and its receptor have been shown to promote many features of allergic lung inflammation, the contribution to mucin expression has not been elucidated.

OBJECTIVES

To determine if the C3a receptor with its ligand regulates airway epithelial mucin production.

METHODS

Mice deficient in the C3a receptor were examined in a model of allergic airway disease for the presence of goblet cells and the gel-forming secreted mucin Muc5ac.

MEASUREMENTS AND MAIN RESULTS

Lungs from antigen-challenged C3a receptor-deficient mice revealed a dramatic decrease in goblet cells and Muc5ac compared with challenged wild-type control animals. These differences were dependent on C3a binding to its receptor since intranasal challenge with C3a induced the formation of goblet cells only in wild-type but not C3a receptor-deficient mice. Increased numbers of goblet cells were also found in C3a-stimulated RAG-1-deficient mice demonstrating a mechanism independent of T lymphocytes and Th2 cytokines, mediators which have been shown to regulate mucin expression. A direct physiological role for C3a in these models was further demonstrated in cultures of airway epithelial Clara cells, which not only express the C3a receptor but also produce Muc5ac in response to C3a.

CONCLUSIONS

These studies identify a novel C3a receptor-dependent mechanism in the development of airway epithelial goblet cells and regulation of Muc5ac production and implicate C3a as a mediator of airway obstruction in asthma.

Circulating immune complexes and complement C3 and C4 levels in a selected group of patients with rhinitis in Lebanon

Background

A number of reports indicate that circulating immune complexes (CIC) and activation of the complement system contribute to the pathogenesis of Type I allergy. The aim of this study was to investigate the status of CIC in 113 patients with rhinitis in Lebanon and determine complement components C3 and C4 serum levels in the CIC-positive patients. Serum specific IgE antibodies were previously detected and reported in 74 of the 113 patients.

Methods

CIC were detected by polyethylene glycol precipitation and serum C3 and C4 levels quantified by radial immunodiffusion.

Results

CIC was positive in 20 of the specific IgE-positive and 13 of the specific IgE-negative patients. C3 and C4 levels were within the normal range in all the 33 CIC-positive patients.

Conclusions

The antibody class that constitutes the complexes does not seem to be IgG or IgM. Moreover, complement activation does not seem to be involved in the allergic reaction since both C3 and C4 levels were normal in all patients. The role of these complexes, if any, in the pathogenesis of rhinitis is yet to be determined.

Studies on the mechanisms of allergen-induced activation of the classical and lectin pathways of complement

Allergen extracts are efficient activators of the complement system trough the classical pathway. Involvement of the lectin pathway was not previously studied. To further examine the mechanism of complement activation by allergens, in vitro experiments, which covered early steps both of classical and lectin pathways, were performed. Two types of allergens used in these studies: parietaria (PA) and house dust (HD) mite extracts. These allergen extracts bound to the globular head of C1q and interacted with purified mannan-binding lectin (MBL) as measured by solid-phase ELISA. None of the allergen extracts was able to activate human C1 in vitro, as measured by the determination of the split products of C1s in a reconstituted precursor C1 preparation. Neither the HD nor the PA extracts induced C4d generation above background in the serum of three subjects with hypogammaglobulinaemia but normal complement haemolytic activity. After reconstitution to normal level with purified human IgG, allergen extracts induced C4d formation above control at a level comparable to that measured in normal serum incubated with the same amounts of the extracts. HD-induced C4d generation was about the same comparable in MBL-depleted serum and in normal sera. In contrast PA induced no C4d formation in the MBL-depleted serum, whereas reconstitution with purified MBL restored C4d generation. These in vitro findings indicate that although the allergen extracts can bind purified C1q and MBL, they require IgG for efficient complement activation. Depending on the allergens, this activation may be initiated through C1, MBL, or both.

Complement activation in the nasal mucosa following nasal ragweed-allergen challenge

The aim of this study was to explore complement activation in the nasal lavage following a nasal ragweed-allergen challenge. The study was carried out with 15 adolescents who were allergic to ragweed and with six non-allergic healthy volunteers. Following the baseline measurement after the symptoms were registered, subjects were given increasing doses of ragweed allergen. Lavage fluid was collected and tested for a complement-activation product (C3bBbP). The allergic patients responded to allergen provocation with an increase in C3bBbP formation compared to the initial lavage (p = 0.001). The C3bBbP level remained low in the lavage fluids of the non-allergic controls. We found a strong correlation between the threshold dose that induced symptoms and the dose where the maximum complement activation was detected (r = 0.78, p = 0.001). Our findings indicate that in allergic patients nasal challenge with ragweed allergen induces a rise in complement activation in the nasal lavage fluid. These results highlight the role of the complement system in the allergic inflammation on the nasal mucosal surface.

Ragweed allergy: correlation between skin reactivity and in vitro complement activation

Recently we have reported on several observations which indicate that allergen-induced complement activation contributes to the development of the symptoms of ragweed allergy. In the present paper a new finding that supports this assumption is summarized. In 48 ragweed-allergic patients individual skin reactivity to ragweed allergen extract (RWA) was assessed using dilution skin prick testing. Sera of these patients were incubated with 20, 100, and 400 U/ml RWA and generation of two complement activation products, alternative pathway C3-convertase (C3bBbP) and terminal pathway activation complex (C5b-9) was measured by ELISA methods. A strong positive correlation (Spearman correlation coefficient r = 0.495, P = 0.0004, and r = 0.454, P = 0.0012, respectively) was found between individual skin reactivity to RWA and C3bBbP generation induced by 20 and 100 A allergological units/ml (U/ml) RWA. This finding further supports the role of complement activation products in the aggravation of the basic IgE-mediated immunopathology of ragweed allergy.

The severity of clinical symptoms in ragweed-allergic patients is related to the extent of ragweed-induced complement activation in their sera

We have previously reported a correlation between the extent of ragweed allergen (RWA)-induced in vitro serum complement activation and the symptom scores registered daily during the ragweed (RW)-blooming season in RW-allergic patients. The present study was performed in 22 15-17-year-old RW-allergic adolescents. Serum samples were incubated with 100 micrograms/ml RWA, and the generation of different complement activation products was measured by ELISA or RIA. Symptom scores were registered for 4 weeks during the RW-blooming season. The patients were divided according to the extent (low or high) of the generation of complement activation products, and symptom scores registered in the two groups were compared by two-way ANOVA. Significantly higher symptom scores were obtained in the high than in the low complement activation group (P values: 0.049 for C1rC1sC1inh, 0.022 for C3bBbP, 0.015 for C5b-9, 0.0001 for C3a, and 0.0008 for C5a). Similar results were obtained at the measurement performed in the sera obtained from the same patients half a year before the season (P values: 0.022 for C3bBbP, and 0.005 for C5b-9). These findings indicate that complement activation induced by the allergen may enhance the clinical symptoms of RW allergy.

Complement inactivation by allergenic plant pollen extracts

Dialyzed aqueous extracts of plant pollen are widely used in the clinical practice of allergy for diagnostic and therapeutic purposes. The present investigation shows that such allergenic extracts are capable of consuming complement in every human serum, independent of the clinical condition. Complement is engaged by way of the first component C1, but without the participation of allergen-specific antibodies. The capacity of distinct pollen extracts to inactivate haemolytic complement was found to depend on the plant species, the most potent extracts being from the pollen of the weeds and trees. Analysis by UV-spectroscopy of the flavonoids remaining firmly bound to the proteins gives rise to the proposal that complement inactivation by allergenic and non-allergenic pollen extracts is due to polyphenolic (flavonoid) structures complexed with, or chemically conjugated to, the pollen proteins.

Complement peptides and mast cell triggering

Mucosal type mast cells have been earlier shown to be unresponsive to the so called 'peptidergic' stimulus provided by cationic agents, such as anaphylatoxins, neuropeptides or polyamines. We studied the relationship between mast cells' secretory response to stimulation via their type I Fc epsilon receptors (Fc epsilonRI) and that provided by C5a and C3a fragments of the complement system, in the rat mucosal-type mast cell line RBL-2H3. Our results shown here reveal a novel function of C3a, its inhibitory capacity on IgE-mediated triggering of mucosal mast cells. This activity of C3a is most probably mediated by its interaction with the beta-chain of Fc epsilonRI. While connective tissue type mast cells are known to be activated by micromolar concentrations of the complement peptides C3a and C5a, the amount of C3a necessary for the inhibition of antigen-induced degranulation of mucosal cells in our assays is in the nanomolar range. Interestingly, the other anaphylatoxic peptide C5a, which is known to be much more effective in several biological assays, did not show any activity in the same test-system.