Immunological studies in rodent malaria. I: Protective immunity induced in mice by mild strains of Plasmodium berghei yoelii against a virulent and fatal line of this plasmodium.

Mild and virulent lines of Plasmodium berghei yoelii are readily distinguished both by their course of infection and by the preference of the virulent line for mature red blood cells. Mice given either mild line were fully protected against the virulent P.b. yoelii one week after they had become negative. Mice given the mild line of P.b. yoelii 17X were fully protected against a challenge by the virulent line on the third day of infection (D+3). Mice given the mild and virulent lines of P.b. yoelii 17X concomitantly showed only slight protection, all eventually succumbing. Mice given the virulent P.b. yoelii 17X and challenged with P.b. yoelii 17X mild on D+1 showed 2/6 with virulent-like infections and 4/6 infections which resembled the mild controls. Five out of eleven animals challenged with P.b. berghei after immunization with mild P.b. yoelii had infections which resembled the P.b. berghei control group and were dead by D+23. The other six mice became negative, but all of them relapsed and three of them subsequently died. In mice challenged four weeks after their mild P.b. yoelii 17X infections had become negative, 5/6 succumbed to a P.b. berghei infection. Mice immunized with P.b. yoelii 17X mild and challenged with P.v. vinckei one week after becoming negative had a survival rate of 2/10.