Host Protein-Degrading Cathepsin L-like Proteases of Ostertagia ostertagi

Ostertagia ostertagi is the most pathogenic gastrointestinal nematode of cattle and causes major productivity losses. With rising anthelmintic resistance and significant economic impact, alternative control measures, such as vaccination, are urgently needed. This study aimed to generate functionally active recombinant cysteine proteases from O. ostertagi, enzymes involved in nutrient acquisition and immune evasion, to evaluate their potential as vaccine candidates. Two phylogenetically predicted cathepsin B (catB)-like proteases, OoCBP1 and OoCBP2, were cloned into the pcDNA3.1+ vector and expressed in FreeStyle 293-F cells. Purified proteins were assessed using fluorogenic substrates and various host protein substrates, including fibrinogen, albumin, hemoglobin, IgG, mucin, and gelatin, at pH 5.0 and 7.4 with or without a reducing agent. OoCBP1 was detected in the parasite across the life stages, while OoCBP2 was only detectable in adult worms. OoCBP1 partially degraded host proteins at pH 5.0, with reduced activity at pH 7.4, whereas OoCBP2 efficiently degraded all tested proteins at both pH levels except IgG, which was only partially degraded. OoCBP1 showed catB/L activity only at acidic pH, while OoCBP2 was active at both pH conditions. Activities were inhibited by catB, catB/L, or other cysteine protease inhibitors. OoCBP1, but not OoCBP2, required low pH-mediated autoactivation for substrate degradation. OoCBP1 had no effect on coagulation under physiological conditions, while OoCBP2 accelerated clot formation with reduced clot density. With in vitro activation, both proteases delayed coagulation. These findings indicate that OoCBP1 and OoCBP2 may contribute to nutrient acquisition and immune evasion, supporting their potential as targets for vaccines or chemotherapeutics against ostertagiosis.