Induction and protective role of antibodies in Legionella pneumophila infection.

Legionella pneumophila (Lpn) is a ubiquitous Gram-negative bacterium found in aquatic environments and is the causative agent of Legionnaires' disease, a severe form of pneumonia. We have used Lpn-permissive A/J mice as a model to analyze the B cell response upon intravenous (i.v.) and intranasal (i.n.) infection with Lpn. A strong antibody (Ab) response was observed upon i.v. infection with wild-type (WT) Lpn and an icmT mutant strain, which is unable to replicate within permissive host cells. In contrast to i.v. infection, only WT but not icmT mutant Lpn was able to induce specific Ab responses upon i.n. infection. After primary i.n. infection with WT Lpn, a strict compartmentalization of Lpn-specific Ab isotypes was observed, as IgG was found exclusively systemically, while IgA was detectable only locally in the lung. Regardless of the infection route, isotype switching to IgG and to IgA was strictly dependent on CD4+ T cells, whereas IgM production was completely Th-independent. Finally, we analyzed the protective capacity of the Lpn-specific Ab response. Actively or passively immunized mice or mice that were infected with opsonized Lpn had 50-100-fold reduced bacterial titers compared to naive animals, clearly demonstrating the capacity of Ab to protect against infection with Lpn.