Anaphylatoxin C3a enhances mucous glycoprotein release from human airways in vitro

Airway Gland Structure and Function

Submucosal glands contribute to airway surface liquid (ASL), a film that protects all airway surfaces. Glandular mucus comprises electrolytes, water, the gel-forming mucin MUC5B, and hundreds of different proteins with diverse protective functions. Gland volume per unit area of mucosal surface correlates positively with impaction rate of inhaled particles. In human main bronchi, the volume of the glands is ∼ 50 times that of surface goblet cells, but the glands diminish in size and frequency distally. ASL and its trapped particles are removed from the airways by mucociliary transport. Airway glands have a tubuloacinar structure, with a single terminal duct, a nonciliated collecting duct, then branching secretory tubules lined with mucous cells and ending in serous acini. They allow for a massive increase in numbers of mucus-producing cells without replacing surface ciliated cells. Active secretion of Cl(-) and HCO3 (-) by serous cells produces most of the fluid of gland secretions. Glands are densely innervated by tonically active, mutually excitatory airway intrinsic neurons. Most gland mucus is secreted constitutively in vivo, with large, transient increases produced by emergency reflex drive from the vagus. Elevations of [cAMP]i and [Ca(2+)]i coordinate electrolyte and macromolecular secretion and probably occur together for baseline activity in vivo, with cholinergic elevation of [Ca(2+)]i being mainly responsive for transient increases in secretion. Altered submucosal gland function contributes to the pathology of all obstructive diseases, but is an early stage of pathogenesis only in cystic fibrosis.

Complement components as potential therapeutic targets for asthma treatment

Asthma is the most common respiratory disorder, and is characterized by distal airway inflammation and hyperresponsiveness. This disease challenges human health because of its increasing prevalence, severity, morbidity, and the lack of a proper and complete cure. Asthma is characterized by T(H)2-skewed inflammation with elevated pulmonary levels of IL-4, IL-5, and IL-13 levels. Although there are early forays into targeting T(H)2 immunity, less-specific corticosteroid therapy remains the immunomodulator of choice. Innate immune injury mediated by complement components also act as potent mediators of the allergic inflammatory responses and offer a new and exciting possibility for asthma immunotherapy. The complement cascade consists of a number of plasma- and membrane-bound proteins, and the cleavage products of these proteins (C3 and C5) regulate the magnitude of adaptive immune responses. Complement protein are responsible for many pathophysiological features of asthma, including inflammatory cell infiltration, mucus secretion, increases in vascular permeability, and smooth muscle cell contraction. This review highlights the complement-mediated injury during asthma inflammation, and how blockade of active complement mediators may have therapeutic application.

Therapeutic potential of complement modulation

The complement system is an essential component of innate immunity that has been more recently recognized as an unexpected player in various pathological states. These include age-related macular degeneration, atypical haemolytic uraemic syndrome, allergy, foetal loss, and axonal and myelin degradation after trauma. Its importance has also been recognized in physiological processes including haematopoietic stem cell homing to the bone marrow, liver regeneration and modulation of adaptive immune responses. Although the complement system has long been known to be involved in autoimmune and inflammatory diseases, few agents that target the complement system are currently approved for clinical use. However, renewed interest in modulating this system in various pathological conditions has emerged, and several agents are now in development.

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.

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.

Complement C3 cleavage and cytokines interleukin-1beta and tumor necrosis factor-alpha in otitis media with effusion

OBJECTIVES

To analyze whether complement C3a anaphylatoxin, other C3 fragments, interleukin-1beta (IL-1beta), or tumor necrosis factor-alpha (TNF-alpha) contributes to inflammation in chronic otitis media with effusion (OME).

METHODS

The amount of C3a was measured by enzyme-linked immunoassay. Further breakdown of C3 was analyzed by Western blotting. IL-1beta and TNF-alpha concentrations were measured by radioimmunoassay. Bacteria were analyzed by culture and polymerase chain reaction.

RESULTS

Highly elevated levels of C3a and other C3 cleavage fragments were found in all middle ear effusion (MEE) samples. The mean values (+/- SEM, n = 26) for C3a, IL-1beta, and TNF-alpha were 5,973 +/- 1,124 ng/mL, 1,043 +/- 490 pg/mL, and 79 +/- 14.3 pg/mL, respectively. Comparison to an average C3 level of 555 (+/-108) microg/mL indicated that at least 40.5% +/- 6% of total C3 had become activated within the MEE. C3a concentrations were higher in the group in which the effusion had been present in the middle ear for a prolonged period (> or =4 mo) (P = .04). Children with multiple tube insertions had higher C3 (P = .006) and TNF-alpha (P = .04) concentrations in their MEE samples than those receiving their first tubes. C3 and C3a concentrations in MEE correlated to each other (correlation coefficient [r] = 0.513, P = .0056), as did concentrations of IL-1beta and TNF-alpha (r = 0.7016, P < .0001). No significant correlation was found between complement C3 or C3a levels and IL-1beta, TNF-alpha, or bacterial growth.

CONCLUSIONS

Highly elevated levels of C3a in MEE indicate ongoing complement activation, which is stronger than in almost any other disease demonstrated previously. Elevated C3a levels contribute to chemotactic and inflammatory potential in the MEE and correlate with the chronicity of the disease.

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

Ragweed allergen (RWA)-induced complement activation in sera of 40 RW allergic patients and of 40 non-allergic controls was investigated. After treatment of the sera with RWA, levels of C3a, C5a, C3bBbP, C1rC1sClinh, SC5b-9 and granulocyte-aggregating activity were determined. Concentration of RW-specific IgG was also measured. After RWA treatment dose-dependent complement activation was detected in sera of RW allergic and non-allergic persons. C3a generation was observed mostly in the sera of RW allergic individuals, while levels of C3bBbP and of RW-specific IgG were significantly higher in sera of allergics, and a strong correlation was found between these two parameters. In a prospective clinical study, a significant positive correlation was observed between the extent of RWA-induced alternative pathway (AP) activation and the severity of symptoms of allergic rhinoconjunctivitis that were developed in a 4-week period subsequent to blood sampling. These observations suggest that complement activation has a role in the development of the symptoms of RW allergy.

Interactions between respiratory epithelial cells and cytokines: relationships to lung inflammation

Epithelial cells lining respiratory airways can participate in inflammation in a number of ways. They can act as target cells, responding to exposure to a variety of inflammatory mediators and cytokines by altering one or several of their functions, such as mucin secretion, ion transport, or ciliary beating. Aberrations in any of these functions can affect local inflammatory responses and compromise pulmonary defense. For example, oxidant stress can increase secretion of mucin and depress ciliary beating efficiency, thereby affecting the ability of the mucociliary system to clear potentially pathogenic microbial agents. Recent studies have indicated that airway epithelial cells also can act as "effector" cells, synthesizing and releasing cytokines, lipid mediators, and reactive oxygen species in response to a number of pathologically relevant stimuli, thereby contributing to inflammation. Many of these epithelial-derived substances can act locally, affecting both neighboring cells and tissues, or, via autocrine or paracrine mechanisms, affect structure and function of the epithelial cells themselves. Studies in our laboratories utilized cell cultures of both human and guinea pig tracheobronchial and nasal epithelial cells, and isolated human nasal epithelial cells, to investigate activity of respiratory epithelial cells in vitro as sources of cytokines and inflammatory mediators. Primary cultures of guinea pig and human tracheobronchial and nasal epithelial cells synthesize and secrete low levels of IL-6 and IL-8 constitutively. Production and release of these cytokines increases substantially after exposure to specific inflammatory stimuli, such as TNF or IL-1, and after viral infection.

Interactions between airway epithelium and mediators of inflammation

Epithelial cells lining the respiratory airways classically are considered to be "target" cells, responding to exposure to a variety of inflammatory mediators by altering one or several of their functions, such as mucin secretion, ion transport, or ciliary beating. Specific responses of epithelial cells in vivo or in vitro to many of these inflammatory mediators are discussed. Recent studies have indicated that airway epithelial cells also can act as "effector" cells, responding to a variety of exogenous and/or endogenous stimuli by generating and releasing additional mediators of inflammation, such as eicosanoids, reactive oxygen species, and cytokines. Many of these epithelial-derived substances can diffuse away and affect neighboring cells and tissues, or can act, via autocrine or paracrine mechanisms, to affect structure and function of epithelial cells themselves. Studies dealing with airway epithelium as a source of inflammatory mediators and related compounds also are discussed.

ELISA of complement C3a in bronchoalveolar lavage fluid

Activated complement factors within the lung may induce several local biological effects. In order to investigate local complement activation we have developed non-competitive two-site ELISAs of C3a and total C3 in bronchoalveolar lavage fluid (BALF). For the assay of C3a, both C3 and C3(H2O) were removed from the samples by precipitation with polyethylene glycol. It was necessary to add carrier proteins to BALF to remove C3 and C3(H2O) fully. The ELISA of C3a has the lowest limit of detection reported thus far, namely 0.045 nM (= 0.405 ng/ml). In BALF from healthy persons (n = 9) the C3a concentration was 0.20 nM (0.12-0.31 nM) (median, range). C3a was higher in BALF from patients with asthma or with sarcoidosis; asthma (n = 10), 0.45 nM (0.20-5.79 nM); sarcoidosis (n = 19), 1.31 nM (0.095-5.65 nM) (Mann-Whitney U test, p less than 0.005). In BALF from patients with Pneumocystis carinii pneumonitis (n = 10) the C3a concentration was 0.18 nM (0.07-0.57 nM). C3a concentrations in BALF may reflect local complement activation in the lung and/or diffusion into the lumen. This was studied by normalizing C3a concentrations in BALF into values for epithelial lining fluid (ELF), and calculating serum-to-ELF quotients of C3a, and C3a/total C3 quotients.

Glycoprotein synthesis and secretion by cultured small intestinal mucosa in coeliac disease

Glycoprotein biosynthesis by jejunal mucosa was examined during culture in vitro in 26 patients with coeliac disease and 19 controls. The incorporation rates of tritiated glucosamine into tissue and secreted glycoproteins were determined using established techniques. The total glucosamine incorporation in untreated coeliac patients was significantly greater than that of histologically normal mucosa (p less than 0.001) and jejunal tissue from patients with treated coeliac disease (p less than 0.01). Enhanced secretion of in vitro labelled glycoproteins was observed in untreated coeliac patients. The total incorporation of tritiated glucosamine in intestinal tissues was correlated with goblet cell numbers. These results indicate that quantitative changes in glycoprotein synthesis and secretion occur in coeliac disease.

The role of C5a in hypersensitivity reactions in the lung

What role the complement system in general, and C5a in particular play in hypersensitivity reactions in the lung is unknown. A review of the available studies indicates that C5a can mimic hypersensitivity reactions in the lung by virtue of its ability to cause cell infiltration and edema in the lung, cause a short lived airway hyperreactivity and cause bronchoconstriction. Its ability to cause contraction of isolated airways and in vivo bronchoconstriction in the guinea-pig is due to the release of histamine and arachidonate metabolites, mediator systems similar to those evoked by antigen-antibody reactions in the guinea-pig lung. To begin to determine if C5a generation with complement system activation was an important contributor to antigen-induced bronchoconstriction, the response to antigen was assessed in animals depleted of complement by cobra venom factor and unresponsive to C5a. Unexpectedly, these animals responded to antigen with a much greater bronchoconstriction than did animals with an intact complement system. This suggested that an intact complement system was somehow important in limiting allergic bronchoconstriction or conversely that allergic bronchoconstriction might be heightened when preceded by complement system activation. Thus, the data implicates C5a and/or complement system activation as a modulator of antigen-induced bronchoconstriction rather than directly contributing to the bronchoconstrictor component of hypersensitivity reactions in the lung.

Gastrointestinal mucus, a medium for survival and for elimination of parasitic nematodes and protozoa

Mucus is a sticky visco-elastic material which coats all mucosal surfaces. Florey, in 1955, noted the following three functions for gastrointestinal mucus: protection of the underlying mucosa from chemical and physical injury, lubrication of the mucosal surface to facilitate passage of luminal contents, and removal of parasites by binding and entrapment. In the 31 years since Florey's review, detailed analyses of the composition of mucus and of the biochemistry of mucin glycoproteins, as well as measurements of the physical properties of mucus from different organs and sites have yielded information at the molecular level which provide additional support for his views on its function (Allen, 1981; Forstner, Wesley & Forstner, 1982).