Acquirement of protective immunity in mice through infection with an attenuated isolate and its failure in parent virulent Plasmodium berghei.

In virulent Plasmodium berghei infection, mice showed suppressive responses to sheep red blood cells SRBC (PFC) as well as the parasite antigen (DTH) and developed autoantibodies against homologous lymphocytes. On the other hand, mice infected with an attenuated variant derived from P. berghei did not show these responses but developed solid protective immunity against parent parasite infection accompanied by high antibody titre. When such an immune serum was transferred into mice, attenuated parasite infection was completely eliminated. These results show that an attenuated variant stimulates antibody production, which contributes to protection against the parasites. In contrast, in virulent P. berghei infections harmful immunopathological responses against the host are more prominent than protective immune responses against the parasites.