Cytokine LD78 is a human counterpart of the mouse macrophage inflammatory protein la/hematopoietic stem cell inhibitor. Promoters of the LD78a and LD7813 genes showed similar inducible activities in two leukemic cell lines, K562 and Jurkat, but the induction mechanisms differed between the two cell lines. Further characterization of the LD78a promoter indicated that multiple positive and negative regulatory elements are present, some of which are differentially required for induction and repression of the promoter activity in different cells. One of the negative regulatory elements, ICK-1, functioned in both cell lines in the absence and presence of stimulation and was shown to be a recognition site for positive and negative transcriptional factors. This ICK-1 element contained a direct repeat, and similar repeats were also found in the negative regulatory elements of hematopoietic growth factor interleukin-3 (IL-3) and granulocyte-macrophage colony-stimulating factor (GM-CSF) gene promoters. Nuclear extracts from K562 and Jurkat cells formed several protein-DNA complexes with the LD78a ICK-1 element, one of which was also observed with the IL-3 and GM-CSF ICK-1 elements. Results from in vivo and in vitro analyses suggested that the protein forming this complex functions as a negative factor. The binding affinity of this protein, ICK-1A, to the LD78Oa ICK-1 element was low and was significantly affected by the incubation temperature and the salt concentration in the binding buffer. ICK-lB, another protein bound specifically by the LD78ai ICK-1 element, was shown to be a positive factor important for induction of the promoter. These results suggested that ICK-IA plays an important role in balanced expression of LD78, IL-3, and GM-CSF during hematopoietic cell growth and differentiation.Human LD78 (48), also known as AT464 (69) or GOS19S (2), is a member of a large superfamily of small secreted cytokines involved in cell growth and inflammation (18,49,54). The mouse counterpart of LD78 is macrophage inflammatory protein la (MIP-la) (8), also known as hematopoietic stem cell inhibitor (19) or L2G25B (30). LD78 shows 74% amino acid sequence identity with MIP-la. By using purified recombinant proteins, both LD78 and MIP-la have been shown to suppress proliferation of hematopoietic stem and immature progenitor cells in vitro (3, 5, 19, 58) and in vivo (11, 32). MIP-lt has also been shown to enhance proliferation of more differentiated progenitor cells in vitro (5). In addition, LD78 has been found to stimulate osteoclast differentiation in a rat bone marrow culture system (29). As well as modulating proliferation and differentiation of hematopoietic cells, MIP-lot affects macrophage function in an autocrine manner (12), and LD78 is chemotactic for T lymphocytes (63).Three distinct LD78 genes, clustered on chromosome 17 (21, 23), have been identified (2,23,43). The LD78ot and LD781 genes both consist of three exons and share 94%