Sex steroids negatively regulate B lymphopoiesis in adult mice. Paradoxically, lymphocytes arise during fetal life, when estrogen levels are high and maternal lymphopoiesis is suppressed. Here we demonstrate that embryonic B lymphopoiesis was unaffected by estrogen, but sensitive to glucocorticoids. Both fetal and adult precursors contained glucocorticoid receptor transcripts, but only adult precursors expressed estrogen receptor alpha and beta together with the androgen receptor. Fetal hematopoietic cells did not efficiently acquire functional estrogen receptors after transplantation to irradiated adult mice. Sex steroid receptors were also expressed in a stage- and developmental age-dependent fashion in human precursors. A developmental switch in responsiveness of hematopoietic cells to sex steroids may be essential for formation of the immune system.
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