Role of Recruited Nonimmunospecific Defenses in Legionnaires' Disease

Abstract

Neutrophils are important in limiting L pneumophila growth following aerosol infection and are a major recruited nonimmunospecific defense mechanism in early pneumonia. Neutrophils may be recruited into the airspaces by several mechanisms during the course L pneumophila pneumonia. Chemotactins derived from AM that have ingested and are infected with L pneumophila are a likely means of neutrophil recruitment. Products of bacterial growth may also contribute to neutrophil chemotaxis into an area of infection. Activated complement components (C5a) are potent neutrophil chemotactins and might add to this chemotactic stimulus, but were not assessed in the present studies. Complement depletion did not affect the influx of neutrophils during infection, however.

Complement, with or without antibody, did not kill L pneumophila in the absence of phagocytes. The depletion of complement did not alter either the recruitment of neutrophils to the lung or the degree of bacterial growth during the course of L pneumophila pneumonia in rats. We conclude from these studies that complement is not an important recruited defense mechanism in combating L pneumophila infection in the lung.

Further studies are required to define the role of the neutrophil more fully and the means by which this cell assists in bacterial killing or the stabilization of bacterial growth during L pneumophila pneumonia. The chemotactic factors elaborated by infected macrophages and by growing bacteria deserve characterization. A more complete understanding of the host defense mechanisms that control the course of L pneumophila infection may provide a model for bacterial pneumonia and better means of treating this serious and prevalent disease.