Natural cytotoxicity for Plasmodium berghei in vitro by spleen cells from susceptible and resistant rats.

The susceptibility of 30-day-old rats to Plasmodium berghei infection has traditionally been ascribed to the higher levels of circulating blood reticulocytes for which P. berghei has a predilection. However, spleen cells soon develop natural cytotoxicity for P. berghei which may account, in part, for the increased natural resistance of older rats. Spleen cells from normal 30- or 50-day-old rats were cultured overnight with erythrocytes parasitized by P. berghei and then injected into MF1 mice. Six days later, the percentage parasitaemia was determined and the extent of killing by the spleen cells in vitro determined. Spleen cells from 50-day-old resistant rats were found to be four times better at killing P. berghei in vitro than those from 30-day-old susceptible rats. Antibody-dependent cell cytotoxicity (ADCC) was, at best, only a minor component. About 12% of total cytotoxicity was destroyed by pretreatment of spleen cells with monoclonal anti-Thy-1.1 antibody and complement. The possibility that natural cytotoxicity in these experiments is mediated by natural killer cells is discussed.