Phagocytic Receptors Dictate Phagosomal Escape and Intracellular Proliferation of Francisella tularensis

Abstract

Francisella tularensis, the causative agent of tularemia, survives and proliferates within macrophages of the infected host as part of its pathogenic strategy, through an intracellular life cycle that includes phagosomal escape and extensive proliferation within the macrophage cytosol. Various in vitro models of Francisellamacrophage interactions have been developed, using either opsonic or nonopsonic phagocytosis, and have generated discrepant results on the timing and extent of Francisella phagosomal escape. Here we have investigated whether either complement or antibody opsonization of the virulent prototypical type A strain Francisella tularensis subsp. tularensis Schu S4 affects its intracellular cycle within primary murine bone marrow-derived macrophages. Opsonization of Schu S4 with either human serum or purified IgG enhanced phagocytosis but restricted phagosomal escape and intracellular proliferation. Opsonization of Schu S4 with either fresh serum or purified antibodies redirected bacteria from the mannose receptor (MR) to the complement receptor CR3, the scavenger receptor A (SRA), and the Fc␥ receptor (Fc␥R), respectively. CR3-mediated uptake delayed maturation of the early Francisella-containing phagosome (FCP) and restricted phagosomal escape, while Fc␥R-dependent phagocytosis was associated with superoxide production in the early FCP and restricted phagosomal escape and intracellular growth in an NADPH oxidase-dependent manner. Taken together, these results demonstrate that opsonophagocytic receptors alter the intracellular fate of Francisella by delivering bacteria through phagocytic pathways that restrict phagosomal escape and intracellular proliferation.

Subversion of Host Recognition and Defense Systems by Francisella spp

“…The efficiency of Francisella uptake by macrophages depends in large part on whether the bacteria are opsonized, since serum-or antibody-opsonized bacteria are taken up by macrophages at 10-fold-higher levels than unopsonized bacteria (90). While this increased efficiency of uptake helps the bacteria evade extracellular defenses, it comes at a cost since the intracellular fates of opsonized and unopsonized bacteria are different (Fig.…”

“…5). Opsonized bacteria escape the phagosome with delayed kinetics and replicate modestly in the host cell cytosol (28,90,176,202). In contrast, unopsonized Francisella escapes the phagosome rapidly and replicate robustly in the cytosol (17,202).…”

“…Fc␥R Ϫ/Ϫ macrophages exhibit a 90% reduction in uptake of IgG-opsonized Francisella, although they have no defect in the uptake of unopsonized or serum-opsonized bacteria (90). Fc␥R-mediated uptake leads to increased activation of the NADPH oxidase, a phagosomal enzyme complex that produces toxic reactive oxygen species (ROS) and is discussed in greater detail below.…”

Antioxidant Defenses of Francisella tularensis Modulate Macrophage Function and Production of Proinflammatory Cytokines

“…Macrophages are considered to be the primary targets and are a widely studied cell type in Francisella research. F. tularensis has a unique intramacrophage cycle that involves entry (10), inhibition of phagosome-lysosome fusion (11)(12)(13), phagosomal escape, and cytosolic replication (14,15).…”

Putting the Jigsaw Together - A Brief Insight Into the Tularemia

“…In the non-opsonic uptake of F. tularensis by macrophages, a significant role is played by the mannose receptor [28] and possibly other cell surface receptors that have not yet been defined. The mode of entry may influence the fate of Francisella inside the host cell by triggering different signaling pathways that control the expression of intracellular defense mechanisms and, in parallel, influence the survival of intracellularly localized bacteria [31].…”