The human cytokine I-309 is a monocyte chemoattractant

The human cytokine I-309 is a small glycoprotein secreted by activated T lymphocytes and structurally related to a number of inflammatory cytokines. To investigate the biological activities of I-309 protein, we produced a stable Chinese hamster ovary cell transfectant, CDI.10, which constitutively secretes I-309 protein into culture supernatant. Affinity chromatography on a heparin-Sepharose matrix followed by reverse-phase HPLC was used to purify to homogeneity a glycoprotein doublet of 15-16 kDa from culture supernatant. Biochemical analysis showed the purified recombinant I-309 glycoprotein to be indistinguishable from the natural I-309 glycoprotein constitutively secreted by the T-cell line IDP2. Purified recombinant I-309 stimulated migration of human monocytes but not neutrophils when tested by in vitro chemotaxis assay. Furthermore, the purified protein transiently increased cytoplasmic free calcium concentration in human peripheral blood monocytes but did not do so in lymphocytes or neutrophils. These results demonstrate that the I-309 gene encodes an inflammatory mediator that specifically stimulates human monocytes.