The effects of soluble cell product released by live thymocytes on lymphocyte movements in vivo

Thymocytes have been found to release a glycoprotein when cultured for 1-2 h in serum-free media. This substance has been purified and has been called thymocyte interaction modulation factor (IMF). Thymocyte IMF is assayed in vitro by its ability to reduce the mutual adhesiveness between B lymphocytes. When thymocyte IMF is injected intravenously into mice syngeneic with the source of IMF, a rapid but short-lived leucocytosis in the blood is seen with a concomitant increase in the percentage of surface immunoglobulin (sIg) positive lymphocytes in the blood and decrease in the splenic sIg-positive lymphocyte percentage. The IMF probably causes a mobilisation of cells from the red pulp of the spleen. When radiolabelled lymphocytes are pretreated with IMF and the injected into syngeneic recipients, the localisation of cells in the liver, lymph nodes and small intestine is lower than that found with untreated cells. The mechanism of action of IMF is compared to other substances which alter lymphocyte circulation.