Soluble lytic transglycosylase SLT of Francisella novicida is involved in intracellular growth and immune suppression

Francisella tularensis , a category-A bioterrorism agent causes tularemia. F . tularensis suppresses the immune response of host cells and intracellularly proliferates. However, the detailed mechanisms of immune suppression and intracellular growth are largely unknown. Here we developed a transposon mutant library to identify novel pathogenic factors of F . tularensis . Among 750 transposon mutants of F . tularensis subsp. novicida ( F . novicida ), 11 were isolated as less cytotoxic strains, and the genes responsible for cytotoxicity were identified. Among them, the function of slt , which encodes soluble lytic transglycosylase (SLT) was investigated in detail. An slt deletion mutant (Δ slt ) was less toxic to the human monocyte cell line THP-1 vs the wild-type strain. Although the wild-type strain proliferated in THP-1 cells, the number of intracellular Δ slt mutant decreased in comparison. The Δ slt mutant escaped from phagosomes during the early stages of infection, but the mutant was detected within the autophagosome, followed by degradation in lysosomes. Moreover, the Δ slt mutant induced host cells to produce high levels of cytokines such as tumor necrosis factor-α, interleukin (IL)-6, and IL-1β, compared with the wild-type strain. These results suggest that the SLT of F . novicida is required for immune suppression and escape from autophagy to allow its survival in host cells.