Plasmodium berghei: long lasting immunity induced by a permanent attenuated mutant.

A long-lasting immunity against challenge with highly virulent Plasmodium berghei (NK65) was observed in Balb/c mice immunized with a permanently attenuated parasite (XAT), a derivative (XAT) of the NK65 strain. Mice infected with living XAT parasites showed an extremely low self-resolving type of parasitaemia followed by a strong immunity against a challenge with the lethal parent NK65 strain. This immunity lasted for nearly one year. Cross immunity was also observed in the immune mice after challenges with P. berghei ANKA, P. yoelii 17XL, P. vinckei, and P. chabaudi. In addition stage specific immunity was found after P. yoelii nigeriensis sporozoites were inoculated into immune mice. Late radical treatment of immunized mice had little effect on the immunity. Both IgG and IgM anti-Plasmodium titers did not correlate with the degree of immunity. Immune suppression was not observed in the test mice as far as responsiveness of spleen cells to several mitogens is concerned.