Membrane Proteins of S. mansoni as Vaccine Candidates

Harn, Donald

Abstract

Schistosomes are extracellular blood worms that infect over 250 million people globally and cause about 500,000 deaths annually. The adult parasites can survive for many years with the host vascular system . Since the worms must interact closely with their host, performing functions such as nutrient uptake and attachment that are normally performed by integral surface membrane proteins, the parasites must have developed mechanisms for avoiding immune attack directed at these proteins. Our primary long-term goal is to test the hypothesis that these functional host-interactive membrane proteins (HIMPs) can be effective targets of a schistosomiasis vaccine. Adult schistosomes transport glucose across their outer body covering, or tegument, from the host's bloodstream. We have obtained a full-length S. mansoni cDNA clone encoding a glucose transporter protein (SGTP4) that was definitely demonstrated by immunolocalization and surface biotinylation to be exposed at the host/parasite interface in the worm tegument. SGTP4 is uniquely localized to the apical membranes of the tegument and is rapidly synthesized and deposited to the tegumental surface upon invasion of the vertebrate host. We have now produced abundant evidence to demonstrate that SGTP4, as a model HTMP, can be used as a protective immunogen. We have achieved 28-50% protection against cercarial challenge in C57Bl/6 mice following intramuscular DNA inmunization despite poor inmunogenicity, while intradermal DNA immunization has not provided protection. Immunization of mice with gel purified, full-length SGTP4 from recombinant insect cells is also poorly immunogenic, yet yelded highly sigmficant protection in C57Bl/6 mice but not in BALB/c mice. Recently, we have induced a strong anti-SGTP4 antibody response in both C57Bl/6 and BALB/c mice using thioredoxin fusion proteins expressing the major extrafacial loop of SGTP4. Moreover, these antibodies recognize the surface of lulled, non-fixed parasites cultured for > 22 hours in vitro. C57Bl/6 mice were protected up to 75% from cercarial challenge, equivalent to the levels of protection achieved with irradiated cercariae. In contrast, BALB/c mice were entirely unprotected. Antibody isotyping indicated that C57Bl/6 mice mounted a Th1- llke immune response whle BALB/c mice mounted a Th2 response. Our data support the hypothesis that the SGTP4 HlMP can be an effective vaccine in mice if the immune responses are primarily of the Th1 type. In this grant application, we propose to substantially expand on these findings by testing this hypothesis as we seek to determine whch aspects of Th1 immunity contribute to host protection. Specifically, we will examine the roles played by humoral and cellular immunity, and manipulate these with cytokines and genetic knockouts to begin deciphering the critical effector mechanisms. Data provided by these experiments will provide information about the mechanism of parasite killing and suggest whch formulations of SGTP4 will boost protective immunity.