A COMBINATION OF STEM-CELL FACTOR AND GRANULOCYTE-COLONY-STIMULATING FACTOR ENHANCES THE GROWTH OF HUMAN PROGENITOR B-CELLS SUPPORTED BY MURINE STROMAL CELL-LINE MS-5

We have developed a long-term culture system using the murine bone mar row stromal cells MS-5 to support the growth of progenitor B cells wit h CD34(-), CD10(+), CD19(+), and cytoplasmic mu chain (C mu)-negative surface phenotype from human CD34(+) cells purified from umbilical cor d blood (CB). When 10(3) CB CD34(+) cells/well were seeded on MS-5 str omal cells at the beginning of culture in the absence of exogenously a dded cytokines, progenitor B cells first appeared after 14 days, and t he maximal cell production was achieved during the 6th week of culture . Intriguingly, the addition of recombinant human stem cell factor (rh SCF) and granulocyte colony-stimulating factor (rhG-CSF), but not rhIL -7, strikingly enhanced the growth of progenitor B cells from CB CD34( +) population cultured on MS-5 stromal cells. The culture of progenito r B cells could be maintained until the 6th week of culture when some cells were revealed to have a C mu(+) phenotype, and a small number of cells had immunoglobulin Ir chain on their cell surface in the presen ce of both rhSCF and rhG-CSF. When CD34(+) cells were cultured physica lly separated from the stromal layer by membrane, supportive effects o f MS-5 stromal cells for the growth of progenitor B cells were not obs erved. These results suggest that the present culture system could gen erate progenitor B cells to proliferate from CB CD34(+) cells, that so me of these progenitor B cells could differentiate into immature B cel ls in conjunction with rhSCF and rhG-CSF, and that a species-cross-rea ctive membrane-bound factor(s), which stimulates early human B lymphop oiesis, may exist in MS-5 stromal cells. Further studies are required to investigate the mechanism how rhG-CSF acts on progenitor B cells to allow their proliferation and differentiation.