17-β estradiol directly influences B lymphocyte precursors in stromal cell free cultures

B lymphopoiesis is dramatically downregulated in vivo during pregnancy or after sustained exposure to 17-β estradiol (E2). We found previously that E2 can influence critical early events in B lymphopoiesis, including rag-1 and Ig transgene expression in pro-B cells. In addition, the hormone modifies life spans of B lineage precursors in normal and Rag gene targeted mice. We have now developed a novel in vitro system which mimics the in vivo effect of estrogen on B cell formation, revealing that early precursors represent one direct target of estrogen. Bone marrow cultures initiated with lineage negative (lin^-), c-kit⁺ hematopoietic precursors in a stromal cell free system gave rise to CD45R⁺,CD19^+/-,CD43⁺ B lineage precursors. Addition of E2 resulted in downregulation of rag-1 gene expression in a dose dependent fashion and a substantial expansion of CD45R⁺/ 0043^-/cμ^- B lineage precursors. These changes are similar to ones we observed in vivo after prolonged exposure to this hormone. Identification of this cellular target will facilitate further investigation of molecular mechanisms through which sex steroids regulate numbers of new B cells produced in bone marrow. In addition, our culture system can be used to identify key cellular and molecular events required for initiation of B lymphopoiesis.