IDENTIFICATION OF AN IL-7-ASSOCIATED PRE-PRO-B CELL GROWTH-STIMULATING FACTOR (PPBSF) - I - PRODUCTION OF THE NON-IL-7 COMPONENT BY BONE-MARROW STROMAL CELLS FROM IL-7 GENE-DELETED MICE

Mouse bone marrow (BM) stromal cell conditioned medium (CM) from our l ong-term lymphoid culture system selectively induces the in vitro prol iferation and presumptive differentiation of pre-pro-B cells (B220(+), HSA(-), TdT(-) or TdT(+), c mu(-)) from adult rat, mouse, and human B M. However, the responsible growth factor(s) has not yet been identifi ed, Inasmuch as IL-7 is one of the cytokines most closely associated w ith early B-lineage development, we utilized BM adherent cells and str omal cell lines from IL-7 gene-deleted (-/-) mice in combination with rIL-7 and anti-IL-7 mAb to investigate its possible regulatory role in our culture system, The results show that, although rIL-7 and IL-7 (- /-) CM each can maintain the viability of freshly harvested pre-pro-B cells in vitro, neither induces them to proliferate and/or differentia te, even in the presence of recombinant stem cell factor (rSCF) and/or recombinant insulin-like growth factor (rIGF), The results also show that anti-IL-7 mAb fails to neutralize the pre-pro-B cell growth-stimu lating activity in IL-7 (+/+) CM. Yet rIL-7 enables IL-7 (-/-) CM to i nduce proliferation of pre-pro-B cells, and to ''prime'' them to respo nd directly to monomeric IL-7, Furthermore, anti-IL-7 mAb adsorbs the pre-pro-B cell growth-stimulating activity from both IL-7 (+/+) CM and rIL-7-supplemented IL-7 (-/-) CM; but rIL-7 does not restore this act ivity. Lastly, both pre-pro-B cell growth-stimulatory activity and IL- 7 are quantitatively recovered by ultrafiltration in the 50 to 100 kDa , rather than the 10 to 50 kDa, apparent molecular mass fraction. Thes e results suggest that the pre-pro-B cell growth-stimulating activity in our culture system is the property of a self-associating complex of IL-7 and a second BM stromal cell-derived cofactor.