Protective Immunity Produced by the Injection of X-Irradiated Sporozoites of Plasmodium berghei. II. Effects of Radiation on Sporozoites

Abstract

The effects of X-radiation on the infectivity and development of sporozoites of Plasmodium berghei were determined. When susceptible mice were injected with X-irradiated sporozoites, results showed that the higher the dose of radiation, the smaller the percentage of mice that developed patent blood infections. Sporozoites irradiated with more than 10,000 rads failed to produce any blood infections. Small, but apparently mature, exoerythrocytic forms were found in the livers of rats that had been injected with X-irradiated sporozoites. Such forms were more numerous after a low dose of radiation, i.e., 2,000 rads, and were found to persist in the liver for at least 16 days after sporozoite injection. There may be a relationship between the presence of these forms and the protective immunity found in mice inoculated with irradiated sporozoites. Studies on the effects of ionizing radiations on parasitic protozoa have repeatedly shown that while an extremely high radiation dose is necessary to cause immediate death of the parasite, much lower doses can interfere with its infectivity (review by Kimball, 1955). Thus, doses of more than 100,000 roentgens (r) of X-rays were necessary to produce changes in motility in Trypanosoma gambiense in vitro, whereas a dose of 12,000 r abolished the infectivity of the trypanosome to mice (Halberstaedter, 1938). Although the immunogenicity of irradiated parasites, ranging from viruses (review by Luria, 1955) to helminths (I.A.E.A., 1964), has been investigated only a few such studies have been done with protozoa, e.g., Eimeria (Hein, in Urquhart, 1964), and Trypanosoma lewisi (Sanders and Wallace, 1966). These studies have shown that parasites irradiated with a dose which abolishes their reproductive potential and ability to produce patent infections may still retain their capacity to produce an immune response in a susceptible host. Much of the work on the effects of X-radiation on the malaria parasite has related primarily to the doses required to abolish the Received for publication 25 June 1968. * This work, contribution no. 430 from the Army Research Program on Malaria, was sponsored by the Commission on Malaria, Armed Forces Epidemiological Board, and was supported by the United States Army Medical Research and Development Command. t Department of Preventive Medicine. $ Department of Radiology. infectivity of the erythrocytic stages (Bennison and Coatney, 1945; Rigdon and Rudison, 1945; Ward, Bell, and Schneider, 1960; and Targett and Fulton, 1965), as well as the sporozoite stage (Bennison and Coatney, 1945). Ceithaml and Evans (1946) showed that erythrocytes parasitized by P. gallinaceum were capable of apparently normal oxygen uptake and glucose consumption after an X-ray dose sufficient to destroy reproductive capacity of the parasite (10,000 r). Only when the dose was increased to 30,000 r were these metabolic activities of the parasitized erythrocytes affected. The immunizing capability of X-irradiated erythrocytic stages of malaria parasites has been shown with P. gallinaceum (Ceithaml and Evans, 1946), and with P. berghei (Corradetti, Verolini, and Bucci, 1966; and Wellde and Sadun, 1967). P. gallinaceum sporozoites inactivated either with ultraviolet light or by drying were shown by Russell, Mulligan, and Mohan (1942) to be effective immunizing agents when injected into fowls. That such sporozoites might be more active agents of immunization than are inactivated erythrocytic parasites was suggested by the work of Richards (1966). Our own preliminary results (Nussenzweig et al., 1967) indicated the considerable protective effect induced by the injection of X-irradiated sporozoites of P. berghei into mice. The normal course of sporozoite-induced parasitemia in susceptible rodents, as well as quantitative aspects of the development of sporozoites into exoerythrocytic (E.E.) forms has been previously described (Vanderberg,