An essential role for interleukin-5 and eosinophils in helminth-induced airway hyperresponsiveness

Abstract

Infection with the parasitic helminth Brugia malayi can result in development of a severe asthmatic response termed tropical pulmonary eosinophilia. This disease, thought to result from a host inflammatory response to blood parasites which become trapped in the lung microvasculature, is characterized by a profound eosinophilic infiltration into the lungs. Recruitment of eosinophils also correlates with the development of airway hyperresponsiveness (AHR) to cholinergic agonists and severe asthmatic symptoms. Our studies examined the role of interleukin-5 (IL-5) in helminth-induced pulmonary eosinophilia and AHR. C57BL/6 mice immunized with killed B. malayi microfilariae and challenged intravenously with live microfilariae exhibit many of the characteristics of human disease, including peripheral and pulmonary eosinophilia. Cells recovered by bronchoalveolar lavage of sensitized mice consisted of 3.8% eosinophils on day 1 postchallenge and 84% on day 10. Extracellular major basic protein was present on the surface of airway epithelial cells as early as day 1 and continued to be evident after 8 days, indicating sustained activation and degranulation of eosinophils in the lung. These histologic changes correlated with the development of AHR to carbachol. In contrast to immunocompetent mice, immunization and challenge with B. malayi in IL-5(-/-) mice did not induce peripheral or pulmonary eosinophilia, and these mice failed to show AHR in response to cholinergic agonists. Taken together, these data indicate that IL-5 and eosinophils are required for the induction of AHR by filarial helminths.

FIG. 1

Eosinophils and MBP in lungs after i.v. inoculation with B. malayi larvae. C57BL/6 and IL-5^−/− mice were immunized s.c. with 100,000 killed B. malayi larvae (microfilariae) and injected i.v. with live microfilariae. On day 1 or day 8 after challenge, mice were sacrificed, and lungs were fixed in formalin. All sections were immunostained with antisera to eosinophil MBP and visualized with VectorRed. (A) Microfilariae in lung vasculature of a C57BL/6 mouse day 1 after i.v. challenge; (B) microfilariae surrounded by inflammatory cells, including eosinophils on day 1 (arrowheads indicate visible areas of the worm); (C) perivascular inflammation in a C57BL/6 mouse on day 8 after challenge (note the presence of numerous eosinophils and mononuclear cells); (D) blood vessel from an IL-5^−/− mouse sacrificed on day 8; (E) terminal bronchiole from a C57BL/6 mouse on day 8 postchallenge, with deposition of MBP on bronchial epithelial cells; (F) terminal bronchiole from an IL-5^−/− mouse on day 8 after i.v. challenge; (G) bronchiolar epithelium from a C57BL/6 mouse on day 8 postchallenge; (H) bronchiolar epithelium from an IL-5^−/− mouse. Photomicrographs are representative of five mice per time point in three repeat experiments. Original magnifications: A and B, ×600; C and D, ×400; E and F, ×200; G and H, ×400.

FIG. 2

Kinetics of inflammatory response in the airway. C57BL/6 mice were immunized three times with 100,000 killed microfilariae and challenged i.v. with live microfilariae. (A) At the time points indicated, BAL fluid was cytocentrifuged and cells were examined after staining with modified Wright-Giemsa stain. The data are the means of five animals/time point and are representative of two repeat experiments. (B) Cytospin preparation of BAL fluid harvested day 10 postchallenge from a C57BL/6 mouse and stained with modified Wright-Giemsa stain (magnification, ×600). (C) Cytospin of BAL fluid from an IL-5^−/− mouse (magnification, ×600).

FIG. 3

Contractile responses of tracheal smooth muscle after i.v. injection of B. malayi microfilariae. C57BL/6 and IL-5^−/− mice were immunized and injected i.v. with microfilariae as described in Materials and Methods. On day 8 postchallenge, tracheas were dissected and tracheal cylinders were submerged in an organ bath and exposed to cumulative doses of carbachol. (A) “Naive” data points represent contractile responses of unimmunized, unchallenged C57BL/6 mice. The data points represent force generated in grams (mean ± SE of eight animals per group). Data were analyzed by nonlinear regression using PRISM. Statistical significance was determined by an unpaired t test. Maximal contractile force in tracheas from sensitized animals (1.56 ± 0.07 g) increased significantly compared to naive controls (1.15 ± 0.08 g; P = 0.002). Enhanced tracheal reactivity was also observed at day 1 postchallenge. (B) Tracheal smooth muscle responses of naive IL-5^−/− and sensitized/challenged IL-5^−/− mice (mean ± SE of eight animals per group). Maximal contractile force in tracheas from sensitized animals (1.25 ± 0.16 g) did not differ significantly from the value for naive controls (1.23 ± 0.22 g; P = 0.96).