TALL-1 is a novel member of the TNF family that is down-regulated by mitogens

Members of the tumor necrosis factor (TNF) family play important roles in modulation of immune responses. We describe the identification and cloning of a novel TNF family member that has been designated as TALL‐1. TALL‐1 is a 285‐amino acid type II transmembrane protein. Its carboxy terminus shares ∼35% sequence identity with the recently identified APRIL and ∼20–25% with TNF, FasL, TRAIL, and lymphotoxin‐α, suggesting that TALL‐1 and APRIL belong to a subfamily of the TNF family of ligands. Northern blot analysis suggests that TALL‐1 is expressed abundantly in peripheral blood leukocytes and weakly in spleen but is barely detectable in all other tissues examined. Reverse transcriptase‐polymerase chain reaction analysis indicates that TALL‐1 is specifically expressed in monocytes and macrophages but is undetectable in T and B lymphocytes. Furthermore, TALL‐1 expression is dramatically down‐regulated by phorbol myristate acetate/ionomycin. J. Leukoc. Biol. 65: 680–683; 1999.