We have explored the thymus dependency of T cell precursors in mouse neonatal spleen. Neonatal spleen was grafted under the kidney capsule of adult syngeneic recipients [normal, adult thymectomized (ATx) or thymectomized, irradiated, and bone marrow reconstituted]. Grafts were tested for the presence of Ig+ and Thy 1+ cells and the response to phytomitogens and allogeneic cells. In normal and ATx recipients the grafts were partly populated with Thy 1+ cells of graft origin, which developed in situ from T cell precursors having migrated to the spleen before birth. The absence of the thymus (ATx recipients) did not prevent the appearance of Thy 1+ cells but impaired their functional maturation as assessed by Con A responses and mixed lymphocyte reaction. Such functions appeared to be sensitive to the humoral function of the thymus since they reappeared after grafting of a thymus gland in a cell-impermeable Millipore chamber. No T cell development was observed when neonatal spleens were grafted into T cell-deprived mice, suggesting that T cell precursors that preexisted in the spleen before grafting disappear in T cell-deprived mice whereas they persist in normal or ATx mice. One may thus hypothesize that the persistence and/or proliferation of T cell precursors in the neonatal spleen require the presence of a T cell environment, whereas its functional maturation is under the control of the humoral function of the thymus.

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This work was supported by Grants I.N.S.E.R.M. 56-78-88 and D.G.R.S.T. 35-94.

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