Proliferation of bone marrow pro-B cells is dependent on stimulation by the pituitary/thyroid axis

The frequency and absolute number of pro-B, pre-B, and B cells in the bone marrow of the hypothyroid strain of mice are significantly reduced compared with those of their normal littermates. Tea investigate why this is the ca se, various B cell developmental processes were examined in the thyroid hor mone-deficient mice. These studies revealed that the frequency of pro-B cel ls in the S-G(2)/M phase of the cell cycle was significantly reduced in hyp othyroid mice. That thyroid hormone deficiency was responsible for this pro liferation defect was established by demonstrating that treatment of hypoth yroid mice with thyroxine resulted in a specific increase in the frequency and total number of cycling pro-B cells. The latter effect was paralleled b y increases in the frequency and number of bone marrow B lineage cells. Add itional in vitro experiments revealed that at least some thyroid hormone ef fects were directly mediated on the bone marrow, Taken together, these data demonstrate that thyroid hormones are required for normal B cell productio n in the bone marrow through regulation of pro-B cell proliferation and est ablish a role for the pituitary/thyroid axis in B cell development.