Recombinant HcGAPDH Protein Expressed on Probiotic Bacillus subtilis Spores Protects Sheep from Haemonchus contortus Infection by Inducing both Humoral and Cell-Mediated Responses.

Probiotic Bacillales are effective in controlling pathogens. Live probiotic bacteria improve the composition of the gastrointestinal microbiota, leading to a reduction in pathogen colonization. However, it remains largely unknown how probiotics regulate the host's immunologic responses and protect the host from parasitic infection. In this study, we addressed whether Bacillales were effective against Haemonchus contortus, a parasitic nematode that infects small ruminants worldwide. Using 165 rRNA sequencing, we found that Bacillales were largely depleted in the abomasal microbiota of sheep infected with H. contortus. We constructed a recombinant Bacillus subtilis named rBS(CotB-HcG) that express the glyceraldehyde-3-phosphate dehydrogenase of H. contortus (HcGAPDH) on its spore surface using the Bacillus subtilis spore coat protein B (CotB) as a carrier. Mice receiving rBS(CotB-HcG) orally showed strong Th1-dominated immune responses. More importantly, sheep administered rBS(CotB-HcG) per os showed increasing proliferation of the peripheral blood mononucleates, elevated anti-HcGAPDH IgG in sera, and higher anti-HcGAPDH slgA in the intestinal mucus than the control sheep. The average weight gain of H. contortus-infected sheep treated with rBS(CotB-HcG) (Hc+rBS(CotB-HcG)) was 48.73% greater than that of unvaccinated sheep. Furthermore, these Hc+rBS(CotB-HcG) sheep had fewer eggs per gram of feces by 84.1% and adult worms by 71.5%. They also demonstrated greatly lessened abomasal damage by H. contortus with an abundance of probiotic species in the abomasal microbiota. Collectively, our data unequivocally demonstrate the protective roles of CotB-HcGAPDH-expressing B. subtilis spores in against H. contortus infection and showed great potential of using probiotic-based strategy in controlling parasitic nematodes of socioeconomic importance in general. IMPORTANCE Initial analyses of the abomasal microbiota of sheep using 165 rRNA sequencing suggested that probiotic bacteria played a protective role in against H. contortus infection. A recombinant Bacillus subtilis expressing a fusion protein CotB-HcGAPDH on its spore's surface induced strong Th1 immune response in a murine model. The same probiotic recombinant, upon only one oral application, protected sheep against H. contortus infection by reducing egg shedding and decreasing adult worm loads of the parasite and increasing body weight gain of infected sheep. Both Th1 and Th2 immune responses were evident in these immunized sheep.