Onchocerca volvulus: Immunization of chimpanzees with X-irradiated third-stage (L3) larvae

To provide a theoretical basis for the potential development of vaccines against Onchocerca volvulus (Ov) a trial has been conducted to assess the protective efficacy of immunization of chimpanzees with X-irradiated L3 larvae. Approximately 1000 larvae were injected at 0, 1, and 7 months. The immunized animals, and unimmunized controls, were then challenged with 250 live L3. In order to provide possibly protective exposure to the immunologically distinct L4 epicuticle, a radiation dose (45 krad) was chosen which preserved about 50% of the molting ability of unirradiated larvae. Despite the presence of a strong immune response to crude adult worm extracts, and to cloned Ov antigens, at the time of challenge little or no significant protection against patent infection was observed: three of four immunized animals developed patent infection as compared to four of four controls. One immunized animal failed to become patent or to manifest the late antibody response to adult worm antigens seen in both subpatent and patent infections in this model, and may have been protected from infection. The implications of these studies for future attempts to immunize against O. volvulus are discussed.