IL-7 responsiveness and IL-7Ra receptor levels are decreased on B-lineage cells exposed to the thymic microenvironment.

(A) CD45R⁺sIgM⁻ cells were cultured in FTOC, and 7 days later their proliferative response to increasing concentrations of IL-7 was compared to that of CD45R⁺sIgM⁻ cells harvested from the BM. The frequency of CD45R⁺CD43⁺ cells in each population was determined by FACS, and based on this information, cultures were initiated using the same total number of CD45R⁺CD43⁺ cells from each source. (B) Seven days after seeding thymic lobes with 5 × 10⁴CD45R⁺sIgM⁻ BM cells, CD127 expression on CD45R⁺CD43⁺ pro–B cells was examined by FACS. Freshly isolated CD45R⁺CD43⁺ pro–B cells from the BM were examined in parallel. Studies were performed using cells from BALB/c (top), Ifnar1⁻/⁻ (middle), and 129 strain mice. The frequency of high- and low-expressing CD127 cells is indicated in each plot. (C) Frequency of B-lineage cells in the thymus of BALB/c and Ifnar1⁻/⁻mice.