Testosterone-unresponsiveness of existing immunity against Plasmodium chabaudi malaria.

Testosterone (Te) is known to suppress immunity and to increase host susceptibility to many parasites. This study investigates the action of Te on immunity acquired against blood-stages of the malaria parasite Plasmodium chabaudi in female mice of the inbred strain C57BL/10. Our data show: (i) About 90% of mice infected with 10(6) P. chabaudi-infected erythrocytes are able to develop protective immune mechanisms which become evident in self-healing the infection. The capability of self-healing is lost when mice are pretreated with Te for 3 weeks. (ii) Mice which have self-healed infections acquire immunity to homologous rechallenge. Concomitantly, mice become Te-unresponsive in that their acquired immunity is not suppressible by Te-treatment. (iii) Flow cytometry reveals that Te-pretreatment entails an increase of CD8+ cells and a decrease of Ig+ cells by about 4% in spleens of non-immune mice. In immune mice, however, there is a Te-unresponsiveness of the percental distribution of splenic cell populations. (iv) Adoptive transfer experiments indicate that immunity is conferred by spleen cells, presumably non-T-cells. These cells are Te-unresponsive, since they exert their effect in Te-pretreated mice in the presence of Te. (v) Te-unresponsive immunity can be also transferred by serum, especially the IgG-fraction, obtained from immune mice. Our data demonstrate that Te prevents the development of protective immunity against P. chabaudi infections. However, when once established, protective immunity becomes unresponsive to Te. Our data suggest that the effector mechanisms of protective immunity involve Te-unresponsive B cells secreting protective IgG-antibodies.