Characterisation of Antigen B Protein Species Present in the Hydatid Cyst Fluid of Echinococcus canadensis G7 Genotype

Abstract

The larva of cestodes belonging to the Echinococcus granulosus sensu lato (s.l.) complex causes cystic echinococcosis (CE). It is a globally distributed zoonosis with significant economic and public health impact. The most immunogenic and specific Echinococcus-genus antigen for human CE diagnosis is antigen B (AgB), an abundant lipoprotein of the hydatid cyst fluid (HF). The AgB protein moiety (apolipoprotein) is encoded by five genes (AgB1-AgB5), which generate mature 8 kDa proteins (AgB8/1-AgB8/5). These genes seem to be differentially expressed among Echinococcus species. Since AgB immunogenicity lies on its protein moiety, differences in AgB expression within E. granulosus s.l. complex might have diagnostic and epidemiological relevance for discriminating the contribution of distinct species to human CE. Interestingly, AgB2 was proposed as a pseudogene in E. canadensis, which is the second most common cause of human CE, but proteomic studies for verifying it have not been performed yet. Herein, we analysed the protein and lipid composition of AgB obtained from fertile HF of swine origin (E. canadensis G7 genotype). AgB apolipoproteins were identified and quantified using mass spectrometry tools. Results showed that AgB8/1 was the major protein component, representing 71% of total AgB apolipoproteins, followed by AgB8/4 (15.5%), AgB8/3 (13.2%) and AgB8/5 (0.3%). AgB8/2 was not detected. As a methodological control, a parallel analysis detected all AgB apolipoproteins in bovine fertile HF (G1/3/5 genotypes). Overall, E. canadensis AgB comprised mostly AgB8/1 together with a heterogeneous mixture of lipids, and AgB8/2 was not detected despite using high sensitivity proteomic techniques. This endorses genomic data supporting that AgB2 behaves as a pseudogene in G7 genotype. Since recombinant AgB8/2 has been found to be diagnostically valuable for human CE, our findings indicate that its use as antigen in immunoassays could contribute to false negative results in areas where E. canadensis circulates. Furthermore, the presence of anti-AgB8/2 antibodies in serum may represent a useful parameter to rule out E. canadensis infection when human CE is diagnosed.

Antigen B from Echinococcus granulosus is a novel ligand for C‐reactive protein

“…On the other hand, as PLD treatment did not provoke a total reduction in hCRP‐binding activity, other EgAgB moieties would participate in CRP‐EgAgB interaction. EgAgB protein moieties seem not to be involved since rEgAgB8/1—the most abundant apolipoprotein in native EgAgB—as well as rEgAgB8/2 and rEgAgB8/3 were unable to compete with native EgAgB for CRP binding. Cholesterol, which is also present in native EgAgB, may play a role as it has been associated with CRP binding to LDL particles …”

“…Regarding EgAgB moieties involved in CRP recognition, PC is of foremost importance as the removal of the phosphocholine group of PC caused a significant reduction (60%) of hCRP binding to EgAgB. Given that PC is the most abundant phospholipid in EgAgB purified from different host species (bovine, porcine and human), CRP recognition of EgAgB might be a common event in natural CE infections. On the other hand, as PLD treatment did not provoke a total reduction in hCRP‐binding activity, other EgAgB moieties would participate in CRP‐EgAgB interaction.…”

The secreted protein signature of hydatid fluid from pulmonary cystic echinococcosis

“…However, the detection and the abundance of AgB8/2 to 5 in HF is rather variable [8,9,14,37]. Also, the AgB subunits representation in HF vary among different Echinococcus species [14,38]. multilocularis [13,37,39].…”

Antigen B from Echinococcus granulosus enters mammalian cells by endocytic pathways

“…A recent study with AgB particles obtained from fertile HF of swine origin, infected by Echinococcus canadensis , a closely related specie, also found AgB8/1 as the most represented subunit. Additionally, the authors found AgB8/4, AgB8/3 and AgB8/5, while AgB8/2 was not detected [ 13 ].…”

“…It has been demonstrated that delipidated AgB is able to bind hydrophobic compounds in vitro [ 14 ]. The lipid moiety associated with AgB represent 40–50% of the total particle mass and is constituted by different lipid classes, from hydrophobic lipids, like sterol esters and triacylglycerides, to a variety of phospholipids [ 9 , 13 ]. Moreover, delipidated recombinant AgB8/2 and AgB8/3 subunits were capable of transferring fatty acids analogues to artificial phospholipid membranes [ 15 ].…”