Toward a Complete Immunity-Inducing Vaccine for Schistosomiasis

Cercariae of Schistosoma mansoni and S. haematobium invade the definitive mammalian host by penetrating intact skin. The cercariae adhere to the skin surface with the oral sucker and then penetrate the epidermis with the aid of muscular movement and lytic secretions discharged from the acetabular glands, losing their tail in the process and transforming into schistosomula. The schistosomula need 48–96 hr to negotiate the different layers of the stratified epidermis (Wilson, 1987; Sturrock, 1993, 2001). In this context, one wonders about the in vivo relevance of immunological data obtained with the in vitro–favored model of 3- and 24-hr schistosomula because in vivo parasites of this age are still in the epidermis, well protected from the effect of blood components and cells. In the blood and lymphatic capillary-free, immunologically privileged environment of the epidermis, schistosomula shed the glycocalyx and undergo metabolic changes...