Protective immunity in mice elicited by living infective third-stage hookworm larvae (Shanghai strain of Ancylostoma caninum).

OBJECTIVE

To elucidate the mechanisms of protective immunity in mice elicited by living hookworm (Ancylostoma caninum third-stage infective larvae (L3).

METHODS

The number of migrating infective larvae recovered from the lungs was used as an endpoint for vaccine immunity. The timing of maximal L3 lung entry was determined by counting the number of lung larvae at several time points after infection with 500 or 1000 L3. Mice were immunized either orally or subcutaneously with 500 L3, followed by two boosts of L3 once every two weeks. The immunized mice were challenged orally with 500 L3 one week after the final boost. To evaluate the protective immunity, the number of L3 recovered from the lungs of the immunized mice during the time of maximal larval entry was compared with that of controls. Host immunity was also evaluated by comparing circulating anti-L3 antibodies between immunized and controlled mice, using both enzyme immunoassays and immunoblotting techniques, and by lung histopathology.

RESULTS

The peak time of larval entry into the lungs occurred 48 hours after infection. Mice immunized either orally or subcutaneously with L3 exhibited a marked reduction (90.2% and 86.2% respectively) in the number of recovered lung larvae in comparison to controls (P < 0.01). The protection might be associated with circulating anti-L3 antibodies, including antibodies directed against 132-200 kDa L3 antigens, as well as three major antigens ranging from 28 to 51 kDa. Larvae migrating through the lungs of vaccinated mice showed cuticular damages accompanied with host-inflammatory cell invasion.

CONCLUSIONS

Immunization with living L3 protects mice against lung invasion after challenged with hookworm infection. Vaccine immunity is associated with circulating antibodies against L3 antigens and lung inflammatory responses. The mouse model is potentially useful for developing a hookworm vaccine.