Characterization of the Receptor-Ligand Pathways Important for Entry and Survival ofFrancisella tularensisin Human Macrophages

Abstract

Inhalational pneumonic tularemia, caused by Francisella tularensis, is lethal in humans. F. tularensis is phagocytosed by macrophages followed by escape from phagosomes into the cytoplasm. Little is known of the phagocytic mechanisms for Francisella, particularly as they relate to the lung and alveolar macrophages. Here we examined receptors on primary human monocytes and macrophages which mediate the phagocytosis and intracellular survival of F. novicida. F. novicida association with monocyte-derived macrophages (MDM) was greater than with monocytes. Bacteria were readily ingested, as shown by electron microscopy. Bacterial association was significantly increased in fresh serum and only partially decreased in heat-inactivated serum. A role for both complement receptor 3 (CR3) and Fc␥ receptors in uptake was supported by studies using a CR3-expressing cell line and by down-modulation of Fc␥ receptors on MDM, respectively. Consistent with Fc␥ receptor involvement, antibody in nonimmune human serum was detected on the surface of Francisella. In the absence of serum opsonins, competitive inhibition of mannose receptor (MR) activity on MDM with mannan decreased the association of F. novicida and opsonization of F. novicida with lung collectin surfactant protein A (SP-A) increased bacterial association and intracellular survival. This study demonstrates that human macrophages phagocytose more Francisella than monocytes with contributions from CR3, Fc␥ receptors, the MR, and SP-A present in lung alveoli.

Francisella Targets Cholesterol-Rich Host Cell Membrane Domains for Entry into Macrophages

“…Protease treatment of bacteria does not affect this entry pathway, whereas peroxidation and crosslinking of bacterial surface carbohydrates and/or lipopolysaccharides lead to conventional phagocytosis of Francisella. Using F. novicida and specifically looking at primary human monocytes, contributions from complement receptor 3, Fc␥ receptors, mannose receptors, and surfactant protein A have been described (7)(8)(9). Interestingly, these authors also described a 60% reduction of association of Francisella LVS with primary human monocytes in comparison to F. novicida.…”

Phosphatidylinositol 3-Kinase Activation Attenuates the TLR2-Mediated Macrophage Proinflammatory Cytokine Response toFrancisella tularensisLive Vaccine Strain

“…Indeed, one recent study demonstrated that 60% fewer LVS bacteria associate with human monocyte-derived macrophages compared with F. tularensis subsp. novicida (83), and another demonstrated that LVS has a 100-fold greater growth rate in rat macrophages (84). Moreover, we have observed that proinflammatory cytokine expression by macrophages is 10-fold greater from macrophages infected with F. tularensis subsp.…”

Inhalation of Francisella novicida ΔmglA causes replicative infection that elicits innate and adaptive responses but is not protective against invasive pneumonic tularemia

“…However, production of the Th2-type cytokine IL-4 by sensitized splenocytes was absent. Alternatively, the low level of IgG2 antibodies induced by ΔmglA infection may indicate insufficient opsonizing antibodies for Fc receptor mediated bacterial uptake, which others have shown promotes phagocytosis of F. novicida U112 [29]. IgG molecules are recognized by different Fcγ receptors, which trigger activating and inhibitory signaling pathways.…”