Interaction between Schistosoma mansoni and the complement system: role of IgG Fc peptides in the activation of the classical pathway by schistosomula.

It was previously reported that Schistosoma mansoni schistosomula in the presence of IgG activate complement through the classical pathway (CCP). In the present work we have demonstrated that schistosomula incubated with IgG peptides resulting from IgG hydrolysis by schistosomula proteases are able to initiate complement activation through CCP, since a marked consumption of C1, C4, and C2 was observed. Our results eliminate an eventual direct action of the substances released by schistosomula on the activation of CCP. The first step of CCP activation required the preliminary binding of IgG peptides on the schistosomula surface. This interaction induced an increase in the C1q uptake by schistosomula. Since the involvement of the Fc portion of IgG molecules has been clearly evidenced, in this case the peripheral globular subunits of C1q recognize the CH2 region on IgG peptides or intact IgG molecules. By this mechanism, the local consumption of complement around schistosomula could contribute to its survival in the host.