Entamoeba histolytic is an enteric protozoan that infects 10% of the world's population and is one of the following leading parasitic causes of death worldwide. Recently, a number of important amebic antigens have been isolated, characterized and cloned, including the 170kDa heavy subunit of the galactose-inhibitable adherence protein (GIAP), a 29kDa liver abscess-specific surface antigen, the major cysteine proteinase of E. histolytic, and a highly conserved 125kDa surface antigen. However, before work can proceed on development of an amebiasis vaccine we must gain a further understanding of natural immunity to this parasite. The overall objective of this proposal is to determine if natural immunity develops following invasive amebiasis and asymptomatic intestinal infection, and if so, to characterize the specific immune responses associated. They hypotheses to be tested are: 1) Cure of invasive amebiasis is followed by protective immunity against invasive amebiasis and asymptomatic intestinal infection with E. histolytic. 2) Elimination of asymptomatic pathogenic and nonpathogenic E. histolyatica intestinal infection is followed by immunity for months to years against recurrent infection; and 3) Immunity to invasive amebiasis is dependent on sustained antigen-specific cell mediated immune (CMI) responses, immunity to intestinal infection requires a similarly specific sIgA response. The specific aims of this proposal are to determine: 1) the incidence of invasive amebiasis and asymptomatic intestinal infection in seropositive individuals cured of amebic liver abscess and controls; 2) if there is an association between the amebic infection or disease; 3) the host immune response during asymptomatic E. histolytic infection and its relation to clearance of the infection; and 4) the incidence of recurrent asymptomatic infection, its correlation to host immune response, and whether immunity is strain specific. This will be accomplished by following 100 patients cured of amebic liver abscess and their 900 close associates for > 3 years at the Medical Research Council (Natal) and the King Edward VII Hospital in Durban, South Africa. Each of the 1000 subjects will have the follow-up every three months with determination of clinical status, occurrence of E. histolytic intestinal infection by culture and direct detection of fecal antigen and zymodeme analysis of infecting strain. In addition, assays of serum adherence protein antigenemia, serum IgG antibodies to each of the four major recombinant antigens previously mentioned, and salivary IgA to the GIAP and soluble amebic antigen will be performed. All subjects will have CMI studies consisting of lymphocyte blastogenesis and determination of gamma interferon production in response to mitogen, soluble amebic antigen and the 4 well characterized recombinant E. histolytic antigens. Preliminary studies support the high feasibility for enrolling and following this number of subjects. Considering known infection rates in the endemic area of Durban, the size of the study has been determined to be adequate to address the questions posed. This proposal will address an incredibly important issues in amebiasis research and have a major impact on vaccine development and clinical practice in the field.