Type II Secretion Substrates ofLegionella pneumophilaTranslocate Out of the Pathogen-Occupied Vacuole via a Semipermeable Membrane

Abstract

Legionella pneumophila replicates in macrophages in a host-derived phagosome, termed the Legionella-containing vacuole (LCV). While the translocation of type IV secretion (T4S) effectors into the macrophage cytosol is well established, the location of type II secretion (T2S) substrates in the infected host cell is unknown. Here, we show that the T2S substrate ProA, a metalloprotease, translocates into the cytosol of human macrophages, where it associates with the LCV membrane (LCVM). Translocation is detected as early as 10 h postinoculation (p.i.), which is approximately the midpoint of the intracellular life cycle. However, it is detected as early as 6 h p.i. if ProA is hyperexpressed, indicating that translocation depends on the timing of ProA expression and that any other factors necessary for translocation are in place by that time point. Translocation occurs with all L. pneumophila strains tested and in amoebae, natural hosts for L. pneumophila. It was absent in murine bone marrow-derived macrophages and murine macrophage cell lines. The ChiA chitinase also associated with the cytoplasmic face of the LCVM at 6 h p.i. and in a T2S-dependent manner. Galectin-3 and galectin-8, eukaryotic proteins whose localization is influenced by damage to host membranes, appeared within the LCV of infected human but not murine macrophages beginning at 6 h p.i. Thus, we hypothesize that ProA and ChiA are first secreted into the vacuolar lumen by the activity of the T2S and subsequently traffic into the macrophage cytosol via a novel mechanism that involves a semipermeable LCVM.

A MicroRNA Network Controls Legionella pneumophila Replication in Human Macrophages via LGALS8 and MX1

“…Thus, the absence of galectin-8 upon knockdown could explain the enhanced replication of Legionella (27). An association of galectin-8 with the LCVs of infected human macrophages at 6 h postinfection was previously validated, strengthening the putative role as a danger receptor (99).…”

Stenotrophomonas maltophilia Encodes a VirB/VirD4 Type IV Secretion System That Modulates Apoptosis in Human Cells and Promotes Competition against Heterologous Bacteria, Including Pseudomonas aeruginosa

“…maltophilia VirB/D4 T4SS triggers apoptosis of human macrophages in a contact-dependent manner. When we extended our analysis to include infection of U937 cells, a human macrophage line that is widely used for infection studies (74,(94)(95)(96), the virB10 mutant attached to the host cells at a wild-type level ( Fig. S3B) but resulted in less cell death (i.e., more viable host cells) than did the wild-type strain (Fig.…”

The manipulation of host transcription by the ANKH effector of legionella.

“…It has also been shown that T2SS mutants trigger immune response pathways, like MyD88 and Toll Like receptor 2, resulting in an increase in inflammatory cytokine levels and suggesting T2SS effector proteins dampen cytosolic innate immunity sensors [118,120]. A recent study by Truchan et al elucidated the cellular localization of 2 T2SS effectors, ProA (a metalloproteinase) and ChiA (chitinase), which escape the LCV and then form a ring-like pattern around the LCV membrane in the host cell cytosol [121].…”

“…These studies help to shift views from the predominant paradigm in the L. pneumophila field, which is that only type 4 secretion system (T4SS) effectors are able gain access to the host cell cytoplasm during infection [121].…”