Host responses induced in mice by a radiation-attenuated Plasmodium berghei (NK65) malaria parasite

Abstract

AN ATTENUATED Plasmodium berghei (NK65) strain was obtained by irradiating infected blood with 40 krad X-ray dose and inoculating it into nude mice. The parasites isolated from the nude mice produced a self-limiting parasitaemia in immunocompetent mice, while the original strain always caused lethal infection. Maintenance of the attenuated parasite required passage through the thymus-deficient nude mice; after 50 passages, the attenuated features of the parasite were still preserved. The parasite did not grow in mice exposed to 800 rad irradiation, and showed a strong affinity for polychromatophilic erythrocytes. In addition, erythrocytes showing multiple infections with the parasite greatly exceeded those showing single infections, even at low levels of parasitaemia. These observations indicated that the attenuated parasite could grow preferentially in immature erythrocytes containing subcellular organelles which are deficient in mature erythrocytes. A single inoculation of the attenuated parasite into normal mice induced a potent immunity against challenge inoculation with original virulent strain. Immunopathological changes provoked by the attenuated and the qriginal virulent Plasmodium berghei were compared. Accumulation of theta and lymphocytes in the liver and loss of lymphocytes from the thymus were much more prominent in the infections with the virulent strain.‘However, the attenuated strain induced a more marked splenomegaly. Extensive efforts are now being made to produce a safe and effective vaccine against malaria. To date, three major approaches have been tried. Application of an irradiated sporozoite vaccine has been investigated notably by NBSENZWEIG and her colleagues.“’ Merozoites have also been found to be effective in inducing immunity against challenge inoculations of intra-erythrocytic parasites of the same species.(2’ Gamete vaccination is the third method, against showing a predominantly stage-specific immunity.‘3’ It is generally agreed that “premunition” is necessary for development of a potent acquired immunity to malaria in man. Production of an attenuated malaria parasite is an essential step in studies on this kind of immunity, aiming finally to produce an acceptable living vaccine which would establish artificially a state of premunition without causing harm to the host. Irradiation of blood stage parasites has been studied by WELLDE and co-workers.(5p7) Recently this group demonstrated that P. falciparum parasites irradiated from a 6oCo source were effective in the same strain. (*) Recovery and maintainance of radiationattenuated malaria parasites, however, have not been reported previously. The first section of the present paper reports the isolation of radiation-attenuated parasites and the features of the strain. The second section shows the effectiveness of the parasite in producing immunity in the host, while the third section will briefly review our studies on immunopathologic reactions in infected or immunized animals; this part is concerned particularly with “virulence” of the attenuated parasites. In Section 4, comparisons are made of immunopathological changes in mice infected with either the attenuated or the original virulent parasite.