Z39Ig is a transmembrane protein containing two Ig homology domains with unknown functions. Immunohistochemical analyses of human carotid atherosclerotic plaques detected Z39Ig staining in areas rich in foamy macrophages. Z39Ig staining was also observed in macrophages in the lining layers and sublining areas of rheumatoid arthritis synovium. Z39Ig staining in the osteoarthritis synovium was restricted to macrophages in the lining layers. To identify the role(s) of Z39Ig in the function of macrophages, we used human monocytic cell lines TF-1A (Z39Ig-negative) and THP-1 (Z39Ig-positive). The expression of Z39Ig was induced in TF-1A cells ,when they were differentiated into macrophages by treatment with PMA. The stimulation of PMA-treated TF-1A or THP-1 cells with immobilized anti-Z39Ig mAb induced the secretion of IL-8 and matrix metalloproteinase (MMP)-9, which was dependent on NF-kappaB activation. These data indicate that the macrophage Z39Ig is involved in the pathogenesis of inflammatory diseases through chemokine induction, which will promote the migration of inflammatory cells into the lesion area, and MMP-9 induction, which will contribute to cartilage destruction or extracellular matrix degradation.