Interleukin-32 (IL-32) was initially identified as a transcript (NK4) that is selectively expressed in lymphocytes and NK cells and whose expression is increased following activation by IL-2. It was later re-isolated from an IL-18-treated lung carcinoma cell line and re-named IL-32. IL-32 is unusual in that it does not share sequence homology with known cytokine families and is highly expressed in immune tissues, existing in at least four differentially spliced isoforms. Because treatment of human monocytic and mouse macrophage cells with IL-32 induces several proinflammatory cytokines such as TNF-α, IL-8 and MIP-2, and because it is also induced in human peripheral lymphocyte cells after mitogen stimulation and in epithelial cells by IFNγ, it has been suggested that IL-32 may play a role in autoimmune and inflammatory diseases such as rheumatoid arthritis.