Thymic microenvironment: selective activities of soluble and insoluble components.

Two components of the thymic microenvironment have been separated, and their biological properties have been studied. The first component, soluble thymic factor (STF), after injection into syngeneic recipients, provoked a temporary decrease in thymic weight on day 7, concomitant with an increase in the level of T cells in lymph nodes (but not in the spleen), followed by a general hypertrophy of lymph nodes peaking on day 21. In contrast, after the injection of the second component, insoluble thymic fraction (ITF), there was an increase in the number of prothymocytes in the thymic subcapsulary cortex, followed by general thymic hypertrophy on days 14 and 21. STF is believed to trigger a selective migration of lymph-node-seeking T cells from the thymus, whereas ITF seems to play a role in the differentiation step preceding that imparted by STF.