Legionella in the lung: development of a human ex vivo tissue culture system to study host microbe interactions

Lung infections with Legionella pneumophila may lead to an aggressive pneumonia with a typical histological phenotype. The majority of current investigations focusses either on the late stages of infections in patients, is limited to cell culture experiments or uses rodent models. Therefore we developed a human ex vivo tissue culture model to study the early events of L. pneumophila. Human tissue explants were stimulated with wild-type or replication-deficient mutant strain dotA as well as Outer Membrane Vesicles (OMVs). After stimulation, tissue samples were HOPE-fixed and paraffin-embedded. Paraffin blocks were used for histological and genome wide gene expression analyses as well as localization of bacteria b immunohistochemistry. Bacterial replication was investigated with homogenized and plated tissue lysates.

Ex vivo infection with wild-type L. pneumophila resulted in a distinct phenotype of alveolar damage, while dotA mutant exhibited only little damage. OMVs caused a comparable phenotype to wild-type strain. The observed damage aggravated over time with a peak after 48h of incubation. Genome wide gene expression analysis by microarray revealed a specific list of gene ontology-terms associated with L. pneumophila tissue damage.

The application of a human ex vivo tissue culture model enables detailed investigations of the host response to L. pneumophila infections and permits a holistic approach on molecular and morphological level.