In vitro analysis of LTRCs and ITRCs purified from WT and TAC4^−/− bone marrow. (A) The kinetics of the first cell division of purified LTRCs and ITRCs in WT and TAC4^−/− bone marrow was determined by single-cell division assays. The time point where 50% of cells from single-cell cultures (1 cell/well) have divided was calculated (t₅₀). The figure shows 1 representative experiment performed 2 times with similar results (n = 25-50 wells). In this experiment, the values for t₅₀ were: LTRCs WT (53 hours), LTRCs TAC4^−/− (49 hours), ITRCs WT (36.7 hours), and ITRCs TAC4^−/− (40.6 hours). (B) Bar graph showing total numbers of pro-B cells (B220⁺IgM⁻CD43⁺) derived from WT and TAC4^−/− LTRC and ITRC cultures harvested after 20 to 22 days. TAC4^−/− LTRC-derived cultures contained significantly elevated numbers of pro-B cells (LTRC WT vs TAC4^−/−, P = .01; ITRC WT vs TAC4^−/−, P = .3). Furthermore, the effect of HK-1 (10nM and 100nM) on B-cell development in LTRC- and ITRC-derived cultures was assessed. HK-1 significantly decreased the total number of pro-B cells in all groups. This figure shows 1 representative experiment. Data are mean ± SD (n = 6). P values and levels of significance are indicated. Experiments were performed at least 3 times with similar results.