The Dot/Icm Type IV Secretion System of Legionella pneumophila Is Essential for the Induction of Apoptosis in Human Macrophages

Abstract

We have previously shown that Legionella pneumophila induces caspase 3-dependent apoptosis in mammalian cells during early stages of infection. In this report, we show that nine L. pneumophila strains with mutations in the dotA, dotDCB, icmT, icmGCD, and icmJB loci are completely defective in the induction of apoptosis, in addition to their severe defects in intracellular replication and pore formation-mediated cytotoxicity. Importantly, all nine dot/icm mutants were complemented for all their defective phenotypes with the respective wild-type loci. We show that the role of the Dot/Icm type IV secretion system in the induction of apoptosis is independent of the RtxA toxin, the dot/icm-regulated pore-forming toxin, and the type II secretion system. However, the pore-forming toxin, which is triggered upon entry into the postexponential growth phase, enhances the ability of L. pneumophila to induce apoptosis. Our data provide the first example of the role of a type IV secretion system of a bacterial pathogen in the induction of apoptosis in the host cell.

Regulation of Bacterial Virulence by Csr (Rsm) Systems

“…When humans inhale aerosolized L. pneumophila, the bacterium either immediately triggers contact-dependent apoptosis or rapidly multiplies within the engulfing alveolar macrophage by establishing the Legionellacontaining vacuole (LCV) (204). Either fate depends on effectors that are secreted by the Dot/Icm type IV secretion system (T4SS), which is important for transporting L. pneumophila virulence components into the host.…”

“…The Dot/Icm complex includes approximately 27 protein components that are assembled into one of three subcomplexes: the inner membrane substrate receptor, the transmembrane bridge connecting the inner and outer core subcomplexes, and the outer membrane core containing the secretory components for host cell penetration (206). This T4SS secretes approximately 300 effectors (207)(208)(209)(210) and is important for intracellular replication, organelle trafficking, and effector-dependent egress from the host cell (204,(211)(212)(213). CsrA regulates the expression of at least 26 of 44 tested Dot/Icm effector genes and at least three genes encoding regulatory proteins that influence Dot/Icm expression (78,214).…”

Type V Protein Secretion Pathway: the Autotransporter Story

“…The prototypical TFSS is that of the A. tumefaciens nucleoprotein T-DNA transfer system. However, several other systems, which are required for the full virulence of pathogenic bacteria, have been described, including the B. pertussis Ptl (pertussis toxin) system (130), the Dot/Icm system of Legionella pneumophila (546), and those of Brucella suis (41), Bartonella henselae (443), and Helicobacter pylori (18). Representatives of the conjugal DNA transfer systems include the IncF, IncP, and IncW plasmid transfer systems (Tra) (277).…”

Coupling Factors in Macromolecular Type-IV Secretion Machineries

“…Legionella pneumophila, the cause of Legionnaire's disease, a form of lobar pneumonia [54,55], contains four icm/dot genes with significant sequence similarity to A. tumefaciens Vir proteins and those of various conjugal transfer systems [56]. This system is essential for apoptosis induction in human macrophages [57] and for secretion of protein DotA [58]. As reported for the VirB transporter, the icm/dot system transfers E. coli RSF1010 plasmid between bacteria, probably as a nucleoprotein complex [33,59,60].…”