Role of the spleen and lymphocytes in regulation of the circulating platelet number in mice.

The role of the spleen in the regulation of the circulating platelet number was studied in mice. The increased 75Se-Met incorporation found after removal of the spleen indicates that postsplenectomy thrombocytosis is due to enhanced platelet production. The increased thrombopoietic activity observed in the plasma of splenectomized mice proves the presence of a humoral factor which mediates this process. Administration of spleen lymphocytes to control mice caused mild thrombocytopenia and prevented postsplenectomy thrombocytosis in splenectomized animals. Moreover, culture medium obtained from peripheral blood and spleen lymphocytes contained a substance which induces thrombocytopenia when injected into control mice. The mechanism by which the spleen and lymphocytes affect the amount of circulating platelets is discussed.