Bactericidal activity of human eosinophilic granulocytes against Escherichia coli

Eosinophils participate in allergic inflammation and may have roles in the body's defense against helminthic infestation. Even under noninflammatory conditions, eosinophils are present in the mucosa of the large intestine, where large numbers of gram-negative bacteria reside. Therefore, roles for eosinophils in host defenses against bacterial invasion are possible. In a system for bacterial viable counts, the bactericidal activity of eosinophils and the contribution of different cellular antibacterial systems against Escherichia coli were investigated. Eosinophils showed a rapid and efficient killing of E. coli under aerobic conditions, whereas under anaerobic conditions bacterial killing decreased dramatically. In addition, diphenylene iodonium chloride (DPI), an inhibitor of the NADPH oxidase and thereby of superoxide production, also significantly inhibited bacterial killing. The inhibitor of nitric oxide (NO) production L-N 5 -(1-iminoethyl)-ornithine dihydrochloride did not affect the killing efficiency, suggesting that NO or derivatives thereof are of minor importance under the experimental conditions used. To investigate the involvement of superoxide and eosinophil peroxidase (EPO) in bacterial killing, EPO was blocked by azide. The rate of E. coli killing decreased significantly in the presence of azide, whereas addition of DPI did not further decrease the killing, suggesting that superoxide acts in conjunction with EPO. Bactericidal activity was seen in eosinophil extracts containing granule proteins, indicating that oxygen-independent killing may be of importance as well. The findings suggest that eosinophils can participate in host defense against gram-negative bacterial invasion and that oxygen-dependent killing, i.e., superoxide acting in conjunction with EPO, may be the most important bactericidal effector function of these cells. Eosinophilic granulocytes are believed to participate in the body's defense against infestation with parasites, especially helminths (such as nematodes and trematodes) in the larval stage (35). They are also typically present in the allergic in- flammatory response seen in conditions such as allergy and asthma, where they may contribute to tissue damage and con- nective tissue remodeling (35). However, the roles of eosino- phils in host defense functions are not settled. Even under normal, noninflammatory conditions, eosinophils reside in tis- sues close to mucous membranes that are in close contact with a potentially hostile environment, such as the content of gram- negative bacteria of the large intestine (23). Furthermore, eo- sinophils possess the adhesion molecule a4b7, which binds to MAdCAM-1, an adhesion molecule expressed by the endothe- lial cells of the vessels in the gastrointestinal tract, also sug- gesting roles for eosinophils after recruitment to these tissues (16). Their number increases dramatically during some condi- tions, such as ulcerative colitis, where the integrity of the mu- cosal barrier is disturbed and bacteria may invade the tissue (27).