The unique Legionella longbeachae capsule favors intracellular replication and immune evasion

Legionella longbeachae and Legionella pneumophila cause Legionnaires disease despite species-specific differences in environmental niches, disease epidemiology, and genomic content. Here, we characterized a new L. longbeachae virulence factor, a capsule that is expressed in post-exponential growth phase as shown by electron microscopy. Analysis of the capsule composition via HLPC revealed the presence of a highly anionic polysaccharide absent in a capsule mutant. The capsule is crucial for replication and virulence in vivo in a mouse model of infection and in the natural host Acanthamoeba castellanii. It has anti-phagocytic function when encountering innate immune cells, it is involved in a low cytokine responses in mice and in human monocyte derived macrophages and helps to dampen the innate immune response. The L. longbeachae capsule is a novel virulence factor, unique among known Legionella species, that aids L. longbeachae to survive in specific niches and partly confers L. longbeachae its unique infection characteristics.