IL-16 is a proinflammatory cytokine synthesized as a 631-amino acid precursor protein (pro-IL-16). The pro-IL-16 is expressed at high level in T cells. After cell activation via the T cell rceptor, the C-terminal cytokine domain (121 amino acids) can be released from the N-terminal prodomain by caspase-3 and become biological active. Self-aggregation into homotetramer is required for its biological activities. IL-16 is a chemoattractant for CD4+ T cells, monocytes, and eosinophils. IL-16 also induces the expression of CD25 and activates CD4+ T cells synergically with IL-2 or IL-15. IL-16 can also induce IL-1, IL-6 and TNF-α production in monocytes. It has been shown tha IL-16 can inhibit HIV-1 replication in vitro by repressing the transcription of the HIV-1 long terminal repeat. The pro-IL-16 has been identified as a T lymphocyte cell cycle growth suppressor. In primary T cells, nuclear presence of pro-IL-16 results in a resting state. Pro-IL-16 can function as a scadffold protein which can interact with GA-binding protein β, HDAC3, and heat shock protein 70.