This report describes studies on: a) Production of granulocyte- macrophage colony stimulating factor (GM-CSF) by B cell lymphoma and hybridoma cell lines stimulated by LPS or by antibodies against IgM. GM-CSF activity was assayed on the basophil/mast cell line PT-18 (GM-CSF and IL3 dependent). Antibodies against murine recombinant GM-CSF were used to identify CSF activity present in supernatants of the stimulated B cell lines. No spontaneous release of GM-CSF was found in unstimulated cells. However, when stimulated with LPS, 2 of 5 lymphoma and 5 of 6 hybridoma lines produced GM-CSF. Two lines, the lymphoma M12.4.1 and the hybridoma TH2.2 were analyzed more extensively. In these two lines production of GM- CSF was dependent on the dose of LPS used and time of exposure. Antibodies against IgM stimulated TH2.2 (IgM+), but not the M12.4.1 (IgM-) cells to produce GM-CSF. Northern blot analysis of the M12.4.1 cells showed that mRNA of GM-CSF detected in LPS-stimulated but not in unstimulated cells. Our data show that B cells can be stimulated to produce GM-CSF and thus suggest that they may take part in granulopoiesis. b) Association of the proliferation-related oncogene c-fos with the differentiation of myeloid cells by CSF. Enhanced expression of c-fos is involved in the differentiation of monomyelocytic cells into mature macrophages, which accumulate in the G1 phase of the cell cycle. M1 murine myeloid leukemia cells, which can be synchronized in early and late G1 and can also differentiate into mature macrophages when stimulated with G-CSF, were used to investigate whether enhanced expression of c-fos in differentiated monomyelocytic cells is associated with the position of the cells in the cell cycle or with their state of differentiation. Our data show that the enhanced expression of the c-fos gene occurs only in fully differentiated cells and not in proliferating or G1-arrested cells. These data imply that the enhanced expression of c-fos in M1 cells is restricted to functionally differentiated cells and not related to the position of the differentiated cells in the cell cycle.
