BINDING OF GPI-PLD-TREATED DAF TO THE SURFACE OF SCHISTOSOMA-MANSONI SCHISTOSOMULA

Decay accelerating factor (DAF, CD55) is a 70-kDa glycosylphosphatidyl inositol (GPI)-anchored protein that protects human erythrocytes (HuE) from complement-mediated damage by regulation of the C3-convertase. P urified human DAF can be incorporated into sheep red blood cell (SRBC) membrane and confer complement resistance on these DAF-deficient cell s. Here, we demonstrate that normal HuE or their stroma (HuES) incubat ed at 37 degrees C for 24 h release soluble DAF in a biologically acti ve form into the culture medium. This soluble DAF neither inserts into SRBC plasma membranes nor presents the cross-reacting determinant (CR D) characteristic of the hydrolysis by phosphatidylinositol-specific p hospholipases C (PI-PLC) but binds to schistosomula of S. mansoni prot ecting them from antibody-mediated complement-dependent damage. To stu dy the binding of DAF to schistosomula in vitro, we have used purified human DAF labeled with I-125(I-125-DAF), intact or treated with eithe r PI-PLC or GPI-PLD (glycosylphosphatidylinositol-specific phospholipa se D). We have found that GPI-PLD-treated DAF binds to the surface of parasites more readily than intact or PI-PLC-treated DAF. Immunoprecip itation of the samples with a monoclonal anti-human DAF antibody (IA10 ) revealed that schistosomula incubated with GPI-PLD-treated I-125-DAF emit a stronger signal than their counterparts. This result indicates that the surface of schistosomula is capable of acquiring GPI-PLD-tre ated DAF more effectively than intact or PI-PLC-treated molecules.