Immunohistologic localization of "facteur thymique serique" (FTS) in human thymic epithelium.

Facteur thymique sérique (FTS) has been studied extensively because of its role as a T cell inducer, but its presence in human thymic structures, although likely, has not yet been demonstrated. Indirect immunofluorescence (IF) was performed to characterize this molecule in human thymic epithelium by using highly specific anti-FTS antibodies. This antiserum was prepared by the immunization of rabbits with chemically pure, synthetic FTS that was covalently coupled to a nonimmunogenic, insoluble microparticular carrier. To eliminate a low nonspecific staining of the background, the serum was absorbed on a series of human tissue powders. Anti-FTS activity was demonstrated by passive hemagglutination of FTS-coated, tanned sheep red blood cells and disappeared after treatment with immunosorbent made of FTS that was covalently linked to acrylamide agarose beads. Six thymuses were studied; 3 from normal young adults, 2 from myasthenic patients, and 1 from a 16-wk-old fetus. In every case, FTS could be visualized in 2 thymic structures: Hassall’s corpuscles and reticuloepithelial cells. The specificity of the labeling was assessed by: 1) the disappearance of IF staining when the serum was absorbed on an FTS immunosorbent or incubated with free FTS; and 2) the IF negativity of frozen cut sections of human gut, skin, thyroid, muscle, spleen, lymph nodes, tonsils, parathyroid, and brain. The presence of FTS in the epithelial reticular framework of the human thymus, in the direct vicinity of differentiating lymphocytes, suggests that this molecule could be an active component of the immunoregulatory microenvironment of this lymphoid organ as early as 16 wk of intrauterine life. The absence of FTS in a panel of other human organs that were studied brings strong evidence for a thymic origin of FTS.