Susceptibility of mice treated with cyclophosphamide to lethal infection with Leptospira interrogans Serovar pomona.

Mice not normally susceptible to infection with Leptospira interrogans serovar pomona were rendered susceptible to lethal infections by treatment with a single dose of 300 mg of cyclophosphamide (Cy) per kg administered optimally from 4 days before to 1 day after infection. Cy-treated mice with either passively or actively acquired antibody were protected from death. Blood levels of leptospires in infected untreated and in Cy-treated mice remained similar until 2 days after infection, when untreated mice cleared the leptospires. Soon afterwards, opsonizing and agglutinating antibody appeared. Cy-treated mice given spleen cells from other normal or specifically immune mice were protected from infection. An important factor in the natural resistance of mice to leptospiral infection appears to be their capacity to produce circulating antibody within 48 to 72 h. Applications are suggested for this animal model in vaccination and protection studies.