THE DEVELOPMENT OF LEGIONELLA PNEUMOPHILA REACHES DIFFERENT END POINTS IN AMOEBAE, MACROPHAGES AND CILIATES

Abstract

The intracellular pathogen Legionella pneumophila thrives in both natural and man-made water habitats where it replicates inside freshwater amoebae. L. pneumophila follows a developmental cycle as it grows in amoebae. The actively-multiplying intracellular replicative forms (RFs) differentiate into highly virulent mature infectious forms (MIFs) late in the amoeba infection, and are then released extracellularly. L. pneumophila accidentally infects susceptible humans causing the non-communicable Legionnaires’ disease (LD). MIFs play a central role in the life cycle of L. pneumophila and are thought to be responsible for the transmission of LD. Early reports demonstrated that MIFs were poorly produced inside human macrophages, suggesting that the L. pneumophila progeny from human macrophages has fitness and infectivity disadvantages. Direct comparisons of the L. pneumophila progenies from amoebae and human macrophages have demonstrated that the progeny from amoebae is more morphologically differentiated, resistant to antibiotic challenges, and able to adhere to and initiate infections in host cells than the progeny from macrophages. Analysis of the transcriptomic and proteomic profiles of L. pneumophila inside different hosts has revealed a specific set of genes that are upregulated during differentiation of L. pneumophila into MIFs inside freshwater protozoa but not inside human macrophages, suggesting that these genes may be required for the full differentiation of L. pneumophila and, therefore, for the transmission of LD to susceptible humans. Since the expression of the gene lpg1669, which encodes a putative α-amylase, was upregulated in amoebae (highest level of upregulation among the tested genes) and inside Tetrahymena ciliates, but not inside human macrophages, the role of lpg1669 in the differentiation of L. pneumophila into MIFs was investigated. An isogenic lpg1669 deletion mutant did not display defects in morphological differentiation, in vitro (BYE broth) or in vivo (A. castellanii or U937 human macrophages) growth when compared to its parent strain, suggesting that the gene lpg1669 is not essential for the intracellular differentiation of L. pneumophila. Collectively, these findings demonstrate that L. pneumophila can reach different developmental end points in different hosts and could also provide a clue for the lack of transmission of LD among humans.