Functional dissection of the hexamerin receptor and its ligand arylphorin in the blowfly Calliphora vicina.

The process of receptor-mediated uptake of hexamerin storage proteins from insect haemolymph by fat body cells is a unique feature of the class Insecta. We identified the binding domains of the hexamerin receptor and the hexamerin ligand arylphorin in the blowfly, by means of the yeast-two-hybrid-system. The receptor-binding domain of arylphorin was located within domain 3 of the arylphorin monomer. The ligand-binding domain of the hexamerin receptor was mapped to the extreme N-terminus of the receptor. The binding domains identified exhibit no similarity to any functional protein domains known to date. Additionally, we identified two previously unknown protein-interactors of the hexamerin receptor. The results of this study provide further insights regarding the mechanism of the receptor-mediated endocytosis of storage proteins in insects.

Age-dependent reduction of schistosome fecundity in Schistosoma haematobium but not Schistosoma mansoni infections in humans.

Understanding the dynamics of schistosome infections is problematic because direct measurements of worm burden are not possible. Hitherto, the relative intensity of infection has been estimated by the number of parasite eggs excreted. Egg excretion is assumed to have a consistent relationship with worm burden with duration of infection. We have tested this assumption in Schistosoma mansoni- and S. haematobium-infected populations by looking at the relationships between a circulating parasite antigen, egg excretion level, host age, and parasite density. The study was carried out in two populations because experimental models suggested that S. haematobium but not S. mansoni suffers immune-mediated reduction of fecundity. The results were consistent with this observation, showing that S. mansoni egg output remains stable irrespective of host age or infection intensity while S. haematobium has a substantially reduced egg production with host age. This information is fundamental to understanding the immunology and epidemiology of human schistosomiasis and thus practical approaches to disease control.

The relationship between worm burden and levels of a circulating antigen (CAA) of five species of Schistosoma in mice.

This study examines the ability of an assay which measures the amount of a schistosome specific antigen (CAA) in the host circulation to reliably reflect relative worm burden. Mice were infected with 5 species of schistosome with a range of infection dose. The levels of serum CAA increased during schistosome maturation. In all species tested CAA levels correlated well with adult worm burden once the parasites achieved sexual maturity and remained relatively stable during the establishment of egg production. The amount of CAA produced varied between species but within each species CAA levels were proportional to worm numbers: no density-dependent effects on CAA levels were observed even when mice carried worm burdens that were very large relative to host size. T-cell deprivation of the host had no effect on the CAA/worm burden relationship in either Schistosoma mansoni or S. haematobium infections and the CAA equilibrium was unaltered in intact mice when reduction of worm fecundity occurred. These data support the use of the CAA as an accurate and robust estimate of relative schistosome burden in man.