P. brasiliensis, strain Pb-18, were cultured in BHI at 35⁰C. Cultures were irradiated with 6.5 kGy, in the presence of oxygen and at room temperature. The irradiation was performed in a uniform source of ⁶⁰Co gamma rays, at a dose rate of 950 Gy/h. Groups of eight BALB/c mice were immunized by intravenous injection of 10⁵ radioattenuated yeast cells. Forty-five days after immunisation, 8 BALB/c mice per group were inoculated with 10⁵ viable yeast forms of virulent P. brasiliensis. Organ colony-forming units (CFUs) were determinated after 1 month of infection in the lungs, spleen and liver. Antibody response was determined weekly by ELISA, using microtiter plates coated with P. brasiliensis extract.

A significant decrease in the organs CFUs of immunised mice was observed. In the lung and spleen the reduction was of 92 % and 86 % respectively. No CFU was recovered from the liver. The overall protection obtained was 89%. The immunization with the radioattenuated yeast did not significantly stimulate the antibody production. We concluded that the immunization with the radioattenuated yeast elicited a protective effect and the humoral response evoked was related with this picture, since the severe forms of the disease, leading to death, were associated with high titres of specific antibodies and the mild forms, leading to healing, were parallel to low levels of specific antibodies. Now, we are evaluating the cellular immune response elicited by the radioattenuated yeast.