FUNCTIONAL EFFECT OF IL-7-ENHANCED CD19 EXPRESSION ON HUMAN B-CELL PRECURSORS

We have previously demonstrated that IL-7 can sustain the growth of no rmal human B cell precursors (BCP) for several weeks on bone marrow-de rived stromal cells. Flow cytometric analysis of BCP recovered from IL -7 supplemented cultures revealed two- to threefold higher levels of c ell surface CD19, compared with BCP maintained without IL-7. Short ter m culture of BCP showed that IL-7 enhancement of CD19 was dose-depende nt, with increases detected by day 1 and plateauing by days 3 to 4. IL -7 increased cell-surface CD1 9 on small lymphoid cells, and to a grea ter degree on lymphoblasts, whereas cell-surface CD10 was unchanged. T he CD34+/CD19+ pro-B cell population showed a greater increase in cell -surface CD19 compared with pre-B and immature B cells. IL-1, IL-3, IL -4, IL-6, and stem-cell factor had no effect on CD19. The potential fu nctional significance of IL-7-enhanced cell-surface CD19 was examined using a F(ab')2 fragment of anti-CD19. This reagent had no effect on [ H-3]TdR incorporation in BCP cultured in the absence or presence of IL -7 for 5 days, but homotypic adhesion of BCP was induced at a concentr ation as low as 1.0 ng/ml F(ab')2 anti-CD19. IL-7 enhanced the F(ab')2 anti-CD19 induced homotypic adhesion of BCP in a dose-dependent manne r. Blocking antibody studies indicated that members of the beta1 or be ta2 integrin families did not mediate anti-CD19-induced homotypic adhe sion, even though the adhesion was completely ablated by 10 mM EDTA. T he pre-B and immature leukemic B cell lines NALM-6 and 1E8 expressed c omparable levels of cell-surface CD19, and exhibited comparable increa ses after IL-7 stimulation. However, their homotypic adhesion response s to anti-CD19 were different. NALM-6 cells exhibited a strong homotyp ic adhesion response to anti-CD19 that was EDTA-resistant, and beta1/b eta2 integrin independent. 1E8 cells only responded to anti-CD19 after IL-7 stimulation; this response was EDTA-sensitive and beta1/beta2 in tegrin independent. These collective results indicate that IL-7 not on ly acts as a growth factor for human BCP, but also regulates signal tr ansduction through cell-surface CD19.