Complement Factor 3 Mediates Particulate Matter–Induced Airway Hyperresponsiveness

Epidemiologic studies have suggested that exposure to airborne particulate matter (PM) can exacerbate allergic airway responses; however, the mechanism(s) are not well understood. We and others have recently shown that development of airway hyperresponsiveness (AHR) may be a complementmediated process. In the present study, we examined the role of complement factor 3 (C3) in the development of PMinduced AHR and airway inflammation by comparing responses between C3-deficient (C3 � / � ) and wild-type mice. Mice were exposed to 0.5 mg of ambient particulate collected in urban Baltimore. Forty-eight hours later, airway responsiveness to intravenous acetylcholine was assessed and bronchoalveolar lavage was conducted. PM exposure of wild-type mice resulted in significant increases in AHR, whereas it did not significantly increase airway reactivity in C3 � / � mice. Interestingly, PM induced similar inflammatory responses in both wildtype and C3 � / � mice. Immunohistochemical staining demonstrated marked C3 deposition in the airway epithelium and connective tissue of wild-type mice after PM exposure. These results suggest that exposure to PM may induce AHR through activation of complement factor 3 in the airways. The morbidity and mortality of asthma has increased dramatically over the last few decades. Although the factors that contribute to this increase are not well understood, epidemiologic studies have suggested that increased exposure to air pollutants such as ozone and particulate matter (PM) are clearly associated with increased hospitalizations, increased asthma medication usage, and decrements in pulmonary function (1, 2). Animal studies have supported a role for PM exposure in development of asthmalike parameters, in that exposure of animals to PM surrogates such as residual oil fly ash (ROFA) and diesel PM induce significant airway inflammation concomitant with increases in airway responsiveness (3‐6). To date, the exact mechanisms by which PM exposure can induce acute bronchoconstriction and airway hyperresponsiveness (AHR) are unknown. Upon activation of the complement system, cleavage of the third and fifth components of the complement system (C3 and C5, respectively) generates the peptides C3a and C5a, both of which are potent anaphylatoxins. C3a and C5a can trigger contraction of smooth muscle, increase the permeability of small blood vessels, and regulate vasodilation (reviewed in Ref. 7). In addition, complement factors are potent chemoattractants for a variety of inflammatory cells including neutrophils, eosinophils, and macrophage/ monocytes. Recent studies suggest that these molecules of the innate immune response are produced by airway epithelial cells and macrophages at the airway surface (8, 9). Receptors for both of these anaphylatoxins (C5aR and C3aR) have been shown to be constitutively expressed on bronchial and alveolar epithelial cells, as well as on vascular endothelial and smooth muscle cells (10). Recent reports suggest that allergen exposure upregulates C3aR expression (11) on bronchial smooth muscle cells. Furthermore, recent studies in C3aR- and C3-deficient mice support a role for C3 in antigen-induced AHR (11, 12). However, the role of complement components in non‐antibody-mediated inflammatory responses has not been explored. We have previously shown that PM collected in urban Baltimore induces AHR concomitant with significant elevations in bronchoalveolar lavage (BAL) cellularity (13). In the present study, we examined the role of C3 in AHR and inflammation induced by ambient Baltimore PM by comparing airway reactivity and inflammatory responses in wild-type and C3 � / � mice. We demonstrate that particulate-induced AHR is C3-dependent, whereas the accompanying inflammatory response is independent of C3 activation.