Effect of silica treatment on resistance to Babesia rodhaini infection in immunized mice

The effect of silica treatment on the course of Babesia rodhaini infection was investigated in ICR mice pretreated with either a mixture of B. rodhaini parasitized red blood cell hemolysation-sonication (S) antigen and Propionibacterium acnes bacterin or P. acnes bacterin alone and not subjected to immunization. In the normal mice (non-immunized), the mean survival time was decreased by approximately 1 day when 5 mg per mouse of silica were intraperitoneally inoculated at 5 h before infection. In immunization with S antigen and P. acnes bacterin, all mice survived regardless of the dose of silica (15-25 mg per mouse) treatment. In contrast, in immunization with P. acnes bacterin alone, 3/10, 4/10 and 1/10 mice died of B. rodhaini, when inoculated with 15 mg per mouse, 20 mg per mouse and 25 mg per mouse of silica treatment 5 h before infection, respectively. These results suggest that peritoneal macrophages (M phi), especially when activated non specifically by P. acnes bacterin, play an important role in inhibiting multiplication of organisms at the early stage of infection and the mice immunized with S antigen were ready to quickly acquire an enhanced solid specific protection even if the function of M phi was damaged by a high dose of silica treatment.