The depressed response of spleen cells from rats infected by Trypanosoma lewisi in producing a secondary response in vitro to sheep erythrocytes and the ability of soluble products of the trypanosome to induce this depression.

The non-pathogenic Trypanosoma lewisi depresses the ability of infected rat spleen cells to respond immunologically to sheep erythrocytes. A study was made to determine whether the parasite would produce this same phenomenon in vitro in Marbrook culture vessels with erythrocyte-primed spleen cells from normal rats; the antibody response of the spleen cells was measured by the plaque-forming cell assay. Soluble factors from 1 X 10(7) T. lewisi depress the ability of primed cells to form a secondary antibody response in vitro to sheep red cells. The intensity of the inhibitory activity depends on the stage of the life cycle of the parasite and can also be demonstrated using sonicated, frozen-thawed or boiled preparations of the organism. It is suggested that the immunodepressive substances may be a glycoprotein and that it (they) may arise both from the intracellular and from the extracellular compartments of the organism. It is likely that the host-parasite associations in trypanosomiasis are regulated by such soluble parasite factors.