10 Where To Stay inside the Cell: a Homesteader's Guide to Intracellular Parasitism

Diagrammatic representation of the three different routes of invasion of mammalian cells by intracellular pathogens. In each instance, the “active” cell or cells are labeled with a plus sign. In phagocytosis, the infecting pathogen is relatively passive in the process following ligation to host cell receptors capable of triggering internalization. This process requires little if any metabolic activity from the parasite. Examples include Leishmania, Mycobacterium, and Histoplasma. In induced endocytosis and phagocytosis, the pathogen induces a normally nonphagocytic cell to internalize the microbe. This is the least well understood route of entry and involves subversion of the host cell signaling pathways. Examples include Salmonella in nonprofessional phagocytes and Trypanosoma cruzi. In active invasion, the pathogen invades the host cell without the participation of the contractile apparatus of the host cell. In this process, the host cell is inert. Examples include all the apicomplexan parasites, Plasmodium, Toxoplasma, Eimeria, and microsporidia.

Electron micrograph of a critical-point dried, detergent-extracted, whole-cell mount of a sporozoite of Eimeria acervulina. The structure of this zoite is typical of that observed throughout the phyla that includes both Plasmodium and Toxoplasma. The cell adopts a spiral shape dictated by its subpellicular microtubule network. Motility and host cell invasion are achieved through an actin-myosin contractile system that caps plasmalemma constituents from the anterior to the posterior of the cell.

Intracellular niches. Pathogens have evolved to exploit a variety of intracellular locations, which fall readily into three different groups. The first group includes those that reside in acidic, hydrolytically competent lysosomes and appear undeterred by the hostile nature of their compartment. Examples include Leishmania, Coxiella, and possibly Salmonella (in macrophages at least). The second group includes those that remain vacuolar yet avoid the normal progression of their vacuole into a lysosomal compartment. This group of pathogens is the most diverse with respect to the nature of their intracellular vacuole. Examples include Plasmodium, Toxoplasma, Legionella, Chlamydia, and Mycobacterium. The third group includes those that avoid the consequence of remaining within a phagocytic vacuole by escaping into the cytoplasm. Examples include T. cruzi, Shigella, Rickettsia, and Listeria.

Hoffman modulation contrast micrograph of a monolayer of murine bone marrow-derived macrophages infected with Leishmania mexicana. This species of Leishmania tends to form large fluid-filled vacuoles that contain multiple parasites, which tend to line up along the periphery of the vacuoles (arrow). The vacuoles are acidic and contain active lysosomal hydrolases.

Salmonella induces an extreme response in mammalian cells during entry. In contrast to tight, zippering phagocytosis through which many particles are internalized, these bacteria induce a membrane “splash” or ruffle that captures the bacteria along with an appreciable volume of fluid. This phenomenon is illustrated in a series of time-lapse video frames. The point of initial contact of the bacterium is marked with an arrow in the 30-s and all subsequent time frames. The macropinosome forms (120 s), and several fluid-filled vesicles coalesce (135 and 170 s), until, finally, the phagocytosed bacilli are translocated toward the cell body (250 s). The mechanism appears analogous to the formation of macropinosomes. Courtesy of Hiroshi Morisaki, Michelle Rathman, and John Heuser.

(A) Electron micrograph of a platinum replica from an isolated Mycobacterium avium-containing phagosome. The view is of the cytoplasmic face of the phagosomal membrane and reveals the atypical smooth texture of the phagosome. (B) Region of an L. mexicana-containing phagosome viewed at comparable magnification. The stud-like structures represent proton-ATPase complexes, which are rare on mycobacterial vacuoles. Proton-ATPases are responsible for the normal acidification of phagosomes. Courtesy of David G. Russell and John Heuser.

Electron micrograph of a freeze-etch preparation of a HeLa cell infected with Chlamydia psittaci. The bacteria (black arrows) form an inclusion body or parasitophorous vacuole (PV) that lies within the host cell cytosol (host cell) and is excluded from the normal endocytic routes of that cell. The vacuole membrane is smooth over most of its surface; however, it is ruffled with processes (white arrows) that extend into the host cell cytoplasm in the region that subtends the host cell endoplasmic reticulum. Courtesy of David G. Russell and Ted Hackstadt.

An autoradio electron micrograph of an erythrocyte (RBC) infected with Plasmodium falciparum (Plasmodium). The micrograph illustrates the polymerization of hemozoin (arrowed) within the degradative food vacuole (V). The parasite culture was incubated with [3H]chloroquine prior to processing, and the antimalarial drug can be seen to localize to the hemozoin polymer. This is consistent with the proposed mode of action of the drug. Courtesy of David G. Russell and Daniel Goldberg.