Active immunization against the malaria parasite Plasmodium berghei in mice. The immunizing inoculum

In the immunization procedure of Swiss and C3H/StZ mice against P. berghei the inoculum plays an important role. Only viable parasites are able to induce immunity when multiple inoculations (10(5) P.E. per mouse) or a single inoculation (1-4 X 10(7) P.E. per mouse) are administered. The inoculated parasitized erythrocytes should enter the vascular system. The subcutaneous route is inappropriate, since subsequent immune reactions are notably absent. In combination with a given suppressive regimen successful immunization depends on an optimum number of viable P.E. in the inoculum. All conditions that affect the proportion of viable parasites in the inoculum (route, storage, medium, temperature, donor) should be recognized and controlled. The actual immunizing capacity of the inoculum also depends on the magnitude and time of initiation of sulfathiazole treatment after inoculation. Suppressive treatment (300 mg/L) starting 2 days after inoculation was found optimal in order to render the procedure less sensitive to small differences in the number of P.E. inoculated. Conditions which lead to antimalarial immunity are apparently strain-specific.