SummaryWe have used the cryosection immunogold technique to study the composition of the Mycobacterium tuberculosis phagosome. We have used quantitative immunogold staining to determine the distribution of several known markers of the endosomal-lysosomal pathway in human monocytes after ingestion of either M. tuberculosis, Legionella pneumophila, or polystyrene beads. Compared with the other phagocytic particles studied, the M. tuberculosis phagosome exhibits delayed clearance of major histocompatibility complex (MHC) class I molecules, relatively intense staining for MHC class II molecules and the endosomal marker transferrin receptor, and relatively weak staining for the lysosomal membrane glycoproteins, CD63, LAMP-l, and LAMP-2 and the lysosomal acid protease, cathepsin D. In contrast to M. tuberculosis, the L. pneumophila phagosome rapidly dears MHC class I molecules and excludes all endosomal-lysosomal markers studied. In contrast to both live M. tuberculosis and L. pneumophih phagosomes, phagosomes containing either polystyrene beads or heat-killed M. tuberculosis stain intensely for lysosomal membrane glycoproteins and cathepsin D. These findings suggest that (a) M. tuberculosis retards the maturation of its phagosome along the endosomal-lysosomal pathway and resides in a compartment with endosomal, as opposed to lysosomal, characteristics; and (b) the intraphagosomal pathway, i.e., the pathway followed by several intracellular parasites that inhibit phagosome-lysosome fusion, is heterogeneous.

Previous studies have shown that L. pneumophila multiplies intracellularly in human monocytes and alveolar macrophages within a membrane-bound cytoplasmic vacuole studded with ribosomes. In this paper, the formation of this novel vacuole is examined. After entry into monocytes, L. pneumophila resides in a membrane-bound vacuole. During the first hour after entry, vacuoles containing L. pneumophila are found surrounded by smooth vesicles fusing with or budding off from the vacuolar membrane and by mitochondria closely apposed to the vacuolar membrane. By 4 h, vacuoles are found less frequently surrounded by these cytoplasmic organelles, but now ribosomes and rough vesicles are found gathered about the vacuole. By 8 h, the ribosome-lined vacuole has formed. Erythromycin, at concentrations that completely inhibit the intracellular multiplication of L. pneumophila, has no effect on vacuole formation. Formalin-killed L. pneumophila also reside in a membrane-bound vacuole after entry into monocytes. In contrast to the situation with live L. pneumophila, cytoplasmic organelles are not found surrounding vacuoles containing formalin-killed L. pneumophila at any time after entry. Formalin-killed bacteria are rapidly digested, and by 4 h, few remain intact. The L. pneumophila-containing vacuole has certain features in common with other intracellular organisms that inhibit phagosome-lysosome fusion; these organisms may share a common mechanism for vacuole formation and inhibition of phagosome-lysosome fusion.

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.

A B S T R A C T We have studied the interaction between virulent egg yolk-grown Legionella pneumophila Philadelphia 1 and human blood monocytes in vitro. The leukocytes were cultured in antibiotic-free tissue culture medium supplemented with 15% autologous human serum.L. pneumophila multiplied several logs, as measured by colony-forming units, when incubated with monocytes or mononuclear cells; the mid-log phase doubling time was 2 h. The level to which L. pneumophila multiplied was proportional to the number of mononuclear cells in the culture. L. pneumophila multiplied only in the adherent fraction of the mononuclear cell population indicating that monocytes but not lymphocytes support growth of the bacteria. Peak growth ofL. pneumophila was correlated with destruction of the monocyte monolayer. By fluorescence microscopy using fluorescein conjugated rabbit anti-L. pneumophila antiserum, the number of monocytes containing L. pneumophila increased in parallel with bacterial growth in the culture. At the peak of infection, monocytes were packed full with organisms. By electron microscopy, L. pneumophila in such monocytes were found in membrane-bound cytoplasmic vacuoles studded with structures resembling host cell ribosomes.

Francisella tularensis, the agent of tularemia, is an intracellular pathogen, but little is known about the compartment in which it resides in human macrophages. We have examined the interaction of a recent virulent clinical isolate of F. tularensis subsp. tularensis and the live vaccine strain with human macrophages by immunoelectron and confocal immunofluorescence microscopy. We assessed the maturation of the F. tularensis phagosome by examining its acquisition of the lysosome-associated membrane glycoproteins (LAMPs) CD63 and LAMP1 and the acid hydrolase cathepsin D. Two to four hours after infection, vacuoles containing live F. tularensis cells acquired abundant staining for LAMPs but little or no staining for cathepsin D. However, after 4 h, the colocalization of LAMPs with live F. tularensis organisms declined dramatically. In contrast, vacuoles containing formalin-killed bacteria exhibited intense staining for all of these late endosomal/lysosomal markers at all time points examined (1 to 16 h). We examined the pH of the vacuoles 3 to 4 h after infection by quantitative immunogold staining and by fluorescence staining for lysosomotropic agents. Whereas phagosomes containing killed bacteria stained intensely for these agents, indicating a marked acidification of the phagosomes (pH 5.5), phagosomes containing live F. tularensis did not concentrate these markers and thus were not appreciably acidified (pH 6.7). An ultrastructural analysis of the F. tularensis compartment revealed that during the first 4 h after uptake, the majority of F. tularensis bacteria reside within phagosomes with identifiable membranes. The cytoplasmic side of the membranes of ϳ50% of these phagosomes was coated with densely staining fibrils of ϳ30 nm in length. In many cases, these coated phagosomal membranes appeared to bud, vesiculate, and fragment. By 8 h after infection, the majority of live F. tularensis bacteria lacked any ultrastructurally discernible membrane separating them from the host cell cytoplasm. These results indicate that F. tularensis initially enters a nonacidified phagosome with LAMPs but without cathepsin D and that the phagosomal membrane subsequently becomes morphologically disrupted, allowing the bacteria to gain direct access to the macrophagic cytoplasm. The capacity of F. tularensis to alter the maturation of its phagosome and to enter the cytoplasm is likely an important element of its capacity to parasitize macrophages and has major implications for vaccine development.