Interleukin-16 is a proinflammatory cytokine synthesized as a 68 kD precursor molecule (pro-IL-16). After cleavage of pro-IL-16 by caspase-3, IL-16 is released as a 12 kD protein, but its biologic activities are exerted by a tetramer. IL-16 is a chemoattractant for CD4+ T cells, monocytes, and eosinophils. On T cells, IL-16 upregulates CD25 and class II expression, and also inhibits HIV-1 replication in vitro by repressing the transcription of the HIV-1 long terminal repeat. IL-16 is expressed by lymphocytes, macrophages, eosinophils, mast cells, epithelial cells, and fibroblasts. CD4 and CD9 are the receptors of IL-16.