Role of the thymus in generation of lymphocyte functions: II. Serum-mediated inhibition of development of mitogen-reactive lymphocytes in embryonic mouse thymus organ cultures

The thymuses of 14-day-old mouse embryos could be grown in serum-free organ cultures for at least 14 days with development of relatively large numbers of lymphocytes. These also acquired a strong reactivity to the mitogens concanavalin A (Con A) and leucoagglutinin (LA). Supplementing the organ culture medium with serum from calf (CS), fetal calf (FCS), mouse (MS), or fetal mouse (FMS) gave a serum concentration-dependent inhibition of development of mitogen reactivity, without clearly altering the quantitative lymphoid development in the organ cultures. Adult sera were more suppressive than fetal sera. All of nine tested FCS lots were inhibitory and the inhibiting activity was mainly found in the albumin fraction upon Sephadex G-200 gel filtration. Although FCS prevented development of mitogen-reactive cells in organ cultures of thymuses of 14-day-old embryos, it had much less effect on cultures of 15-day-old thymuses. FCS present during the entire organ culture period most efficiently inhibited generation of mitogen reactivity. If present only during the first or second half of the 14-day culture period, the inhibition was still marked but less complete.