Molecular mechanism of biological activity expressed by hemolysins derived from Listeriae inside macrophages.

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On the molecular mechanisms of biological activity expressed by hemolysins derived from various Listeria spp., the following results have been obtained by using recombinant proteins and mutant strains. (1)All the hemolysin proteins, LLO, ILO and LSO, consist of 4 domains and the known hemolytic activity is highly dependent on domain 4, while the newly found cytokine-inducing activity is exclusively dependent on domain 1-3 at he N-terminus. (2)The very low cytokine-inducing activity observed in ILO appeared to be due to the absence of PEST-like sequence at the N-terminus portion. (3)Upon stimulation with LLO, macrophage first produce IL-12 and IL-18, then these IFN-γ-inducing cytokines stimulate NK cells to produce a massive amount of IFN-γ. (4)TLRs, especially TLR2 and TLR4 are both required for the cytokine production by macrophages in response to LLO. (5)The cytokine response in epithelial cells that do not express surface TLRs appeared to be due to the cytokine induction triggered by the Ca-influx caused by sublethal level of membrane damage by LLO. (6)The in-flame deletion mutant of hly (a gene encoding LLO) has been established. By using recombinant strains, it was confirmed that both escape of bacteria from phagosome into cytosolic space in macrophages and the presence of PEST-like sequence are required for cytokine response. (7)Upon infection of macrophages by Listeria monocytogenes, a significant level of upregulation was induced in hly and orfA gene expression. Oxygen radicals produced by infected macrophages appeared to be responsible for this upregulation of virulence gene expression.