The Legionnaires' disease bacterium (Legionella pneumophila) inhibits phagosome-lysosome fusion in human monocytes.

Abstract

The interactions between the L. pneumophila phagosome and monocyte lysosomes were investigated by prelabeling the lysosomes with thorium dioxide, an electron-opaque colloidal marker, and by acid phosphatase cytochemistry. Phagosomes containing live L. pneumophila did not fuse with secondary lysosomes at 1 h after entry into monocytes or at 4 or 8 h after entry by which time the ribosome-lined L. pneumophila replicative vacuole had formed. In contrast, the majority of phagosomes containing formalin-killed L. pneumophila, live Streptococcus pneumoniae, and live Escherichia coli had fused with secondary lysosomes by 1 h after entry into monocytes. Erythromycin, a potent inhibitor of bacterial protein synthesis, at a concentration that completely inhibits L. pneumophila intracellular multiplication, had no influence on fusion of L. pneumophila phagosomes with secondary lysosomes. However, coating live L. pneumophila with antibody or with antibody and complement partially overcame the inhibition of fusion. Also activating the monocytes promoted fusion of a small proportion of phagosomes containing live L. pneumophila with secondary lysosomes. Acid phosphatase cytochemistry revealed that phagosomes containing live L. pneumophila did not fuse with either primary or secondary lysosomes. In contrast to phagosomes containing live bacteria, the majority of phagosomes containing formalin-killed L. pneumophila were fused with lysosomes by acid phosphatase cytochemistry. The capacity of L. pneumophila to inhibit phagosome-lysosome fusion may be a critical mechanism by which the bacterium resists monocyte microbicidal effects.

Legionella pneumophila inhibits acidification of its phagosome in human monocytes.

“…In contrast, the phagocytic vacuole containing formalin-killed L. pneumophila does not interact with these cellular organelles. Second, the phagocytic vacuole containing live L. pneumophila does not fuse with lysosomes, whereas the vacuole containing formalin-killed L. pneumophila does fuse (3). Third, as shown in this paper, phagocytic vacuoles containing live L. pneumophila maintain a significantly higher pH than phagocytic vacuoles containing formalin-killed L. pneumophila.…”

“…With L. pneumophila, in contrast to yeast and E. coli, live organisms enter a different intracellular compartment than do dead ones. Live L. pneumophila enter a phagosome, whereas dead L. pneumophila and both live and dead yeast and E. coli enter a phagolysosome (3,10). The pH difference between vacuoles containing live and dead L. pneumophila, and the absence of such a difference between vacuoles initially containing live and dead yeast or E. coli, may reflect the different intracellular fates of these organisms.…”

“…Unlike L. pneumophila, live E. coli are rapidly killed by monocytes and reside in phagolysosomes after phagocytosis (3,12). To determine the pH of the E. coli phagosome and the influence on phagosomal pH of killing the E. coli with formalin before phagocytosis, we measured the pH of E. coli phagosomes in monocytes incubated with live or formalinkilled E. coll.…”

The Response Regulator CpxR Directly Regulates Expression of SeveralLegionella pneumophila icm/dotComponents as Well as New Translocated Substrates

“…The S. enterica PmrAB system was shown to be activated by low pH, as well as in response to high levels of Fe(III) (45,60). Since L. pneumophila resides in a phagosome that does not fuse with lysosomes early during infection (23,59) but it was shown to acquire lysosomal markers late during infection (54), it is tempting to believe that one or both of these sensor histidine kinases respond to changes in pH. However, since we found three ceg genes which were shown here to be regulated by both CpxR and PmrA and since for two of these genes CpxR was found to act as a repressor and PmrA was found to act as an activator, it is unlikely that both CpxA and PmrB sense pH and that they control the levels of expression of the same target genes in opposite ways.…”

Intrazelluläres Überleben und Expression der Virulenzdeterminanten von Legionella pneumophila

“…As analyzed by Borwitz and co-workers, live Legionella strains enter eukaryotic cells by a common mechanism, termed coiled phagocytosis; furthermore they induce the formation of a novel ribosome-lined phagosome, inhibit phagosome-lysosome fusion and inhibit phagosome acidification [4,5). The bacterial factors responsible for intracellular survival of legionellae are still unknown.…”