One parasite lipoprotein, two functions: antigen B uptakes cholesterol and acts as an efficient LPS-scavenger

Abstract

The larvae (hydatid) of Echinococcus granulosus s.l. grows within the host´s viscera causing a chronic infection. This highlights an excellent parasite adaptation to its hosts, involving a tight modulation of the immune response with several mechanisms likely involved. An E. granulosus lipoprotein, called antigen B (EgAgB), was postulated as an immunomodulator because of its capacity to interfere with innate cell activation in vitro and in vivo. EgAgB belongs to a cestode-specific family of hydrophobic ligands binding proteins, having putative participation in acquiring lipids not synthesized by Echinococcus (cholesterol and fatty acids). EgAgB physicochemical characterization (size, lipid/protein ratio, apolipoprotein secondary structure) revealed similarities to HDL, described as a plasma lipophilic PAMP scavenger and anti-inflammatory lipoprotein due to its ability to remove cholesterol from innate cells. To address EgAgB mechanisms involved in innate cell modulation, we compare in vitro EgAgB and HDL effects on dendritic cells (BMDC) activation. EgAgB was significantly more efficient in inhibiting LPS-induced IL6/IL12 secretion on BMDC than HDL. Unlike HDL, EgAgB did not alter LTA-induced cytokine secretion, revealing a specificity for LPS interference. Of note, EgAgB diminished LPS-induced TLR4 dimerization, an early step of TLR4 activation pathway, and bound equally to TLR4KO and wild-type BMDC suggesting it controls activation in a receptor independent manner, previous to LPS-TLR4 interaction. Additionally, EgAgB inhibited LPS binding to BMDC, possibly neutralizing LPS in the milieu as HDL3 does. A direct interaction between EgAgB and LPS was observed by an ELISA-like assay, supporting LPS neutralization might contribute to EgAgB´s modulatory effects on innate cells. Whether EgAgB neutralizes/carries other immune-relevant molecules deserves analysis. Notably, EgAgB removed cholesterol from macrophages and hepatocytes, by an SR-B1 and ABCA-1 independent mechanism (unaltered by specific inhibitors), suggesting an efficient passive diffusion mechanism. Further studies are needed to elucidate if EgAgB's ability to uptake cellular cholesterol impact innate cell activation.