Development of a peptide vaccine against hookworm infection: Immunogenicity, efficacy, and immune correlates of protection

Background: Approximately 400 million individuals are infected with hookworms globally. Protective vaccines are needed to prevent reinfections, which often occur after drug treatment in endemic areas. Ideal vaccines are highly efficacious and well tolerated, and do not present risks to patient safety. Peptide vaccines can generate specific, highly protective responses because they focus on minimal antigenic target(s) with a specific immunoprotective mechanism. Necator americanus aspartyl protease 1 (Na-APR-1) is one of the most promising hookworm vaccine antigens. The neutralizing epitope p3 (TSLIAGPKAQVEAIQKYIGAEL), together with universal the TH epitope P25 (KLIPNASLIENCTKAEL), has been used previously to produce peptide vaccines and was found to protect BALB/c mice against rodent hookworm infections, resulting in worm burden reductions of up to 98%. However, because of extensive digestion in the gastrointestinal tract, large oral vaccination doses were necessary to achieve this level of efficacy. Objective: We sought to overcome the limitations of oral vaccine delivery and to investigate protective efficacy and immune correlates of protection. Herein, we examined 5 different peptide vaccines following intraperitoneal injection, to compare their efficacy with that of the clinical protein antigen APR-1. Methods: BALB/c mice were immunized with p3-P25-based antigen that was adjuvanted with (1) lipid core peptide, (2) polymethyl methacrylate, (3) linear polyleucine, and (4) branched polyleucine (BL10), or with (5) CpG/aluminum hydroxide adjuvant (alum)-adjuvanted control and protein-based (6) CpG/alum-adjuvanted Na-APR-1. The mice sera, saliva, and feces were sampled for immune response evaluation. The immunized mice were further challenged via hookworm larvae infection, and protection was evaluated by conducting intestinal hookworm counts. Results: BL10 and lipid core peptide generated the highest serum anti-Na-APR-1 IgG and fecal anti-APR-1 IgG titers, but only BL10 generated significant fecal anti-Na-APR-1 IgA titers. Upon challenge, immunization with CpG/alum-adjuvanted p3-P25, BL10, and lipid core peptide provided the highest worm burden reductions of 75%, 77%, and 59%, respectively, whereas the group immunized with Na-APR-1 had only modest worm reduction of 26%. The relationships between serum anti-Na-APR-1 IgG, fecal anti-Na-APR-1 IgA and IgG, and worm burden reduction were established with R-2 values greater than or equal to 0.9, and the crucial role of both anti-Na-APR-1 IgG and IgA responses was identified. Conclusions: We demonstrated for the first time that p3-based vaccine candidates are safer and can deliver higher protection against hookworm infection compared with the clinical vaccine candidate, Na-APR-1.