B lymphopoiesis is upregulated after orchiectomy and is correlated with estradiol but not testosterone serum levels in aged male rats

Previous studies have shown that B lymphopoiesis is augmented after androge n withdrawal in male mice. As an analogy to the skeletal system, some effec ts of androgens on the proliferation of B cells may be mediated via aromati zation into estrogens in vivo. The aim of the present study was to assess t he effects of androgen withdrawal on B lymphopoiesis in bone marrow of aged male rats sequentially over a period of 9 months, and to correlate the flo w-cytometric findings with changes in systemic levels of sex steroids. We f irst showed that androgen withdrawal is associated with enhanced B lymphopo iesis in bone marrow of 4-month-old male orchiectomized (ORX) rats, and tha t the changes in the bone marrow B cell compartment in ORX animals can be r eversed by testosterone supplementation. In a subsequent, sequential experi ment, we found that orchiectomy induced a sustained rise in Thy 1.1(+)/CD45 R(+) bone marrow cells committed for the B cell lineage that lasted for sev eral months in 13-month-old aged rats. In a stepwise model of multiple regr ession analysis using estradiol, free and total testosterone as independent variables, estradiol was the strongest predictor of the percentage of B pr ecursor cells in bone marrow in aged SHAM and ORX rats. Free and total test osterone did not correlate with B lymphopoiesis in aged SHAM rats. The curr ent experiment has clearly shown that androgen withdrawal upregulates the n umber of B lineage cells over several months in rat bone marrow. Furthermor e, our results provide evidence that estradiol may play an important role a s a physiological suppressor of B lymphopoiesis in aged male rats.