An estimated 18 millions are infected with Onchocerca volvulus which will result in visual impairment in one million and blindness in more than 400,000. Although periodic administration of the drug ivermectin promise a drastic reduction in the densities of skin microfilariae (mf), the severe limitations of the control programs demand the availability of alternative strategies for control, such as vaccines that target the infective stage larvae (L3) and the developing fourth-stage larvae (LA). Previous studies have provided evidence that naturally acquired immunity against O. volvulus infection can occur in humans. This project represents an ambitious attempt to study immunity in the putatively immune (PI) individuals who are exposed to infection but do not become infected. Natural immunity against O. volvulus infection in the PI has been shown to be associated with TH1-type of cellular responses when tested against OvAg, adult worms extracts. However, anti-L3 TH2-type responses were found to be associated with protection in the O. volvulus mouse-diffusion chamber model. In addition, there is some evidence that resistance may be also mediated by antibodies, and that the PI sera can identify unique larval stage-specific proteins. We hypothesize that protective immunity in humans can be elicited by larval antigens, and that it operates independently of immune responses to adult antigens or mf. To test this hypothesis we will: 1. Analyze the cellular and humoral immune responses to crude larval antigens in the PI in comparison to the infected individuals, and thus provide the missing information on immune responses associated with protective immunity and which are directed specifically against the infective stages of the parasite. 2. Identify and clone larval proteins that are targeted by anti-L3 protective immune responses of the PI, and then analyze the immune responses in the PI to the corresponding recombinant proteins in order to determine the candidate molecules to be used in the mouse-diffusion chamber for immunization and the induction of protection against O. volvulus L3 challenge. The emphasis will be on antigens expressed during molting from L3 to L4, as many of the previous results point to those antigens as potential inducer of protection and the target of effector mechanisms. 3. Test the vaccine efficacy of the selected cloned recombinant antigens. One of the major goals of this project is to discover recombinant proteins which can elicit protective immunity in the mouse-diffusion chamber model, and thus advance the development of a vaccine for human use. These studies should, therefore, be instrumental in developing a vaccine against onchocerciasis with anticipated clinical application.