Mechanisms of immunity to leishmaniasis. I. Evidence for a changing basis of protection in self-limiting disease.

Methods of adoptive immunization have been employed to analyse the mechanisms of acquired immunity to leishmanial infection in the guinea pig. It was found that resistance developed 2 weeks after infection and that protection could be passively transferred to normal recipients with short-lived T lymphocytes. Although cells taken from animals at progressive stages of the disease were consistently able to adoptively immunize normal recipients their effectiveness waned from 8 weeks onward. Concomitant serum transfer performed at progressive times during the infection failed to augment the level of immunity generated in normal recipients, and at certain times appeared to have an inhibitory effect on this phenomenon. Serum from convalescent animals however did augment the level of adoptive immunity expressed in recipients. Further experiments revealed that serum taken from recovered animals, 24 hr after re-challenge, was alone capable of transferring immunity to normal recipients. Using immunofluorescent techniques to assay titres of anti-leishmanial antibody, a temporal relationship was found between high antibody titres and the ability of serum to contribute positively to the adoptive immunization of normal recipients. These observations were taken as evidence that the basis of the protective immune response to leishmanial infection may change the course of the disease from a purely cell-mediated mechanism to one involving protective antibody.