Abstract
Background: Toll-like receptors (TLRs) are highly conserved innate sensing receptors that activate host defenses upon detection of microbial products. Plasmodium falciparum glycosylphosphatidylinositols (PfGPI) have been shown to stimulate macrophage cytokine production via TLR2. In addition to their role in inflammation, TLRs may also regulate phagocytosis. P. falciparum parasitized erythrocytes (PEs) can be non-opsonically internalized by macrophages in a process predominantly mediated by scavenger receptor CD36. Moreover, uninfected erythrocytes (UEs) are rendered susceptibleto macrophage clearance during malaria infection due to surface modifications, and this is believed to contribute to the pathogenesis of severe malarialanemia. We hypothesized that stimulation of macrophage TLR2 by PfGPI would enhance innate clearance of PEs as well as malaria-exposed UEs. Methods: Primaryhuman and murine macrophages were pre-stimulated with PfGPI or asynthetic TLR2 agonist (FSL-1) and phagocytosis assays were performed using P.falciparum PEs, EBABs (PE modelconsisting of anti-CD36 antibodies conjugated to human erythrocytes), IgG-opsonized PEs, and malaria-exposed UEs. Results: PfGPI and FSL-1 pre-stimulation significantly increased uptake of EBABs and P.falciparum PEs in a TLR2-dependent manner. TLR2 stimulation modestly increased Fc-mediated phagocytosis of IgG-opsonized PEs, and enhanced phagocytosis of UEs isolated from P. falciparum culture. Conclusion: TLR2-mediated macrophage activation enhanced in vitro clearance of both PEs and malaria-exposed UEs. These data underscore the complexity of TLR involvement in malaria infection: TLR-enhanced phagocytosis may benefit infected individuals by decreasing parasite burden, but in other contexts may predispose to anemia by enhancing UE destruction. Therapeutic targeting of TLR pathways in malaria requires careful consideration.
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