Legionella pneumophila is an accidental human pathogen that causes the severe pneumonia known as Legionnaire’s Disease. L. pneumophila evades predation and replicates within amoebae, which has equipped the bacteria with the ability to replicate in human alveolar macrophages. During infection, L. pneumophila establishes a replicative vacuole termed the Legionella-containing vacuole (LCV) that sustains intracellular replication in macrophages and amoebae. Establishment of the LCV requires the Dot/Icm type IV secretion system (T4SS), that injects more than 300 effector proteins into the infected host cell. Despite their central role in LCV biogenesis, to date most effector proteins remain uncharacterized. Therefore, to aid in the study of effector-associated phenotypes, in this study, we generated nine genomic deletions in L. pneumophila, which resulted in the deletion of 68 effector genes and 138 non-effector genes collectively.
Despite the loss of multiple effector genes, none of the mutants showed any replication defect in human macrophages compared to wild-type L. pneumophila, including one mutant (ΔFGHI) that was missing 42 effector genes. This supports the argument for large scale functional redundancy among Dot/Icm effector activity in mammalian cells. Interestingly, several mutants, such as ΔB and ΔFGHI showed a 10-fold increase in lung colonisation compared to wild-type L. pneumophila at 24 hours after infection in A-strain mice. This finding is especially surprising as, unlike wild-type L. pneumophila, both ΔB and ΔFGHI were unable to replicate in Acanthamoeba castellanii. In order to unravel this unusual observation, we will identify which L. pneumophila genes contribute to higher bacterial numbers in the lung using trans-complementation to restore the phenotype, and examine the immune response to infection. Together, this will shed light on to the pathogenesis of L. pneumophila in mammalian cells, as well as the pathogen interaction with host immune response.