Effects of antiserum on the hydrolytic activity of activated rAc-cathB-1 and larval gut penetration. (A) The inhibition of activated rAc-cathB-1 by antiserum. One microgram of rAc-cathB-1 was activated and incubated with the positive serum (6.0 μg) or negative serum (6.0 μg) for 30 min, respectively, prior to assessment of the degradation of Z-RR-AMC. The hydrolytic reaction was performed at 37 °C for 30 min and the fluorescence of released AMC was measured. All data were presented as relative activities of activated rAc-cathB-1, where the activity of the control (without serum treatment) was taken as 100%. (B) Inhibition of larval penetration ability. Two hundred L3 larvae were pretreated with the undiluted positive serum, the undiluted negative serum, or PBS at 37 °C for 30 min, respectively. The three groups of larvae were separately injected into lumens of rat gut sacks and kept in sterilized Tyrode’s solution at 37 °C for 3 h. Each trial was conducted in triplicate and the numbers of larvae remaining in the gut lumen were counted. Numbers of L3 that penetrated the isolated gut were calculated and presented as indicated. Asterisk (*), P < 0.05; ns, not significant.