Plasmodium bergheiXAT, an attenuated variant of lethalP. berghei, causes a resolving infection in Balb/c mice from which they recover in about 3 weeks. The parasitaemia displays an early peak at about 5 days, followed by a steep drop in parasite number associated with the appearance of degenerating forms inside mature erythrocytes; the parasites remaining are inside reticulocytes. By contrast, no degenerating parasites were seen in infections caused by the virulent parent, which was mainly confined to mature erythrocytes. However,P. bergheiXAT was no more sensitive to reactive O2metabolites, generated by alloxan, or to tumour necrosis serum, than its virulent parent. Furthermore, its early drop in parasitaemia was unaffected by silica. The drop still occurred in the absence of T cells, although the infection was then ultimately lethal, and it was not mediated by NK cells since it occurred in nude mice treated with anti-asialo GM1 serum to abolish NK cell activity. However, it was absent in splenectomized mice, in whichP. bergheiXAT infection was lethal. Thus, the attenuation ofP. bergheiXAT infection is not due to increased susceptibility to some of the agents thought to cause parasite destruction, but to some other mechanism in which the spleen is involved.
The Role of NK Cell Activity in Age-Dependent Resistance of Mice to Murine Cytomegalovirus Infection