Uptake of serum-opsonized Francisella tularensis by macrophages can be mediated by class A scavenger receptors

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Abstract
The bacterium Francisella tularensis is highly infective, and this is one of the chief attributes that has led to its development as a bioweapon. Establishment of infection requires efficient uptake of F. tularensis by host macrophages, which provide a safe in vivo environment for F. tularensis replication. Little is known, however, about the cellular entry mechanisms employed by this organism. This report shows that efficient uptake of F. tularensis live vaccine strain (LVS) by macrophages is dependent on a heat‐sensitive serum component and is mediated in part by types I and II class A scavenger receptors (SRA), demonstrating for the first time that SRA can act as a receptor for opsonized pathogens. Specifically, uptake of serum‐opsonized LVS was partially blocked by general scavenger receptor inhibitors [fucoidan and poly(I)] and was largely inhibited by a specific function‐blocking antibody against SRA. A role for SRA in LVS binding was confirmed by showing that ectopic expression of SRA in human embryonic kidney cells conferred the capacity for robust serum‐dependent LVS binding. Finally, SRA–/– macrophages ingested significantly fewer LVS than did macrophages from wild‐type mice. These findings support a novel role for SRA in innate immunity and suggest a potential therapeutic approach for modulating F. tularensis infection, namely, blocking SRA as a means of hindering F. tularensis access to its intracellular niche.