Different appearance of parasitized erythrocytes in blood between normal and toxoplasma-infected rats after infection of Plasmodium berghei.

When normal rats were infected with Plasmodium berghei (Pb), both IgG and IgM immunofluorescent antibody titers were found to rise in the 1st week with increase of parasitaemia. After reinoculation of P. berghei into Pb-immune rats, IgG titer was further increased remarkably, although no parasitaemia was observed. No elevation of IgM titer was found. In rats infected with Toxoplasma gondii (Tg), IgG and IgM antibody activities were demonstrated in the 1st to the 3rd week postinfection but only IgG titer was maintained to the 16th week. The challenge with T. gondii to Tg-immune rats stimulated the further increase in IgG titer but not in IgM. When Tg-immune rats were infected with P. berghei, little or no parasitaemia appeared. In infection of P. berghei in Tg-immune rats which were treated with anti-rat thymocyte serum (ATS) beforehand, highly increased parasitaemia was usually found in the rats as compared with that in ATS non-treated rats. Pb-immune macrophages were more effective in phagocytosis of Pb-parasitized erythrocytes in vitro than normal or Tg-immune macrophages. When Pb-parasitized erythrocytes were preincubated with fresh Pb-immune serum, the phagocytosis rate of macrophages was clearly heightened. It was observed that the phagocytic activity of normal macrophages to Pb-parasitized erythrocytes was stimulated by addition of the supernatant (lymphokines) taken from the incubation of Tg-immune lymphocytes with Tg-antigen or Pb-antigen.