TISSUE AND T-CELL DISTRIBUTION OF PRECURSOR AND MATURE IL-16

IL-16 is a novel cytokine, which is chemoattractant for CD4(+) T cells , macrophages, and eosinophils, Recently, it was reported that IL-16 i s synthesized as an approximately 80-kDa precursor molecule, pro-IL-16 , Since little is known about the processing and tissue distribution o f IL-16 and pro-IL-16, we investigated the distribution of IL-16 mRNA and protein in human lymphoid tissue. Northern blotting identified IL- 16 mRNA predominantly in normal lymphoid organs, including PBMC, splee n, and thymus, Immunohistochemistry of human lymph node localized IL-1 6 protein to lymphocyte cytoplasm within T cell zones and occasionally in lymphocytes in B cell zones. Flow cytometric detection of intracel lular IL-16 showed that >70% of CD4(+) and CD8(+) T cells constitutive ly expressed IL-16 protein. Western blot analysis of PBMC revealed nea rly all of this protein to be approximately 80-kDa pro-IL-16 in unstim ulated PBMC, and upon cell activation, the amino terminus of pro-IL-16 is processed into multiple fragments. These results show that pro-IL- 16 is widely and constitutively expressed and suggest that the amino t erminus of the protein can be processed upon cell activation.