| Summary: | Onchocerciasis (river blindness), caused by the filarial nematode <i>Onchocerca volvulus</i>, is a neglected tropical disease mainly of sub-Saharan Africa. Worldwide, an estimated 20.9 million individuals live with infection and a further 205 million are at risk of disease. Current control methods rely on mass drug administration of ivermectin to kill microfilariae and inhibit female worm fecundity. The identification and development of efficacious vaccines as complementary preventive tools to support ongoing elimination efforts are therefore an important objective of onchocerciasis research. We evaluated the protective effects of co-administering leading <i>O. volvulus</i>-derived recombinant vaccine candidates (<i>Ov</i>-103 and <i>Ov</i>-RAL-2) with subsequent natural exposure to the closely related cattle parasite <i>Onchocerca ochengi</i>. Over a 24-month exposure period, vaccinated calves (<i>n</i> = 11) were shown to acquire infection and microfilaridermia at a significantly lower rate compared to unvaccinated control animals (<i>n</i> = 10). Furthermore, adult female worm burdens were negatively correlated with anti-<i>Ov</i>-103 and <i>Ov</i>-RAL-2 IgG1 and IgG2 responses. Peptide arrays identified several <i>Ov</i>-103 and <i>Ov</i>-RAL-2-specific epitopes homologous to those identified as human B-cell and helper T-cell epitope candidates and by naturally-infected human subjects in previous studies. Overall, this study demonstrates co-administration of <i>Ov</i>-103 and <i>Ov</i>-RAL-2 with Montanide™ ISA 206 VG is highly immunogenic in cattle, conferring partial protection against natural challenge with <i>O. ochengi</i>. The strong, antigen-specific IgG1 and IgG2 responses associated with vaccine-induced protection are highly suggestive of a mixed Th1/Th2 associated antibody responses. Collectively, this evidence suggests vaccine formulations for human onchocerciasis should aim to elicit similarly balanced Th1/Th2 immune responses.
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