CHARACTERIZATION OF THE HUMAN CD27 LIGAND, A NOVEL MEMBER OF THE TNF GENE FAMILY

CD27 is a lymphocyte-specific member of the TNF/NGF-R family that is h ighly induced on T cells after TCR stimulation, primarily on lymphocyt es belonging to the unprimed CD45RA(+) subset. However, after prolonge d activation both in vivo and in vitro, CD27 expression is gradually s witched off. In search of physiologic signals that influence CD27 expr ession, we found that B cell lines can mediate down-regulation of CD27 surface expression on activated T cells via direct cell-to-cell conta ct and that preincubation of activated T cells with CD27 mAbs prevente d this modulation. Based on these observations, we hypothesized that a ligand for CD27L is expressed on B cell lines and that ligand interac tion would result in down-modulation of the molecule from the T cell s urface. To identify this putative CD27L, mAbs were raised against the Burkitt cell line Ramos and screened for their ability to block down-m odulation of CD27 on activated T cells. Using this approach, we isolat ed a mAb, designated 2F2, that inhibits CD27 down-regulation in a dose -dependent manner. We demonstrate that this mAb recognizes CD27L as it 1) reacted to mouse fibroblasts expressing the recently cloned CD27L, a novel surface-expressed member of the TNF gene family, and 2) preve nted binding of a soluble CD27-Fc construct to a CD27L expressing cell line. Down-regulation of CD27 from the T cell surface by recombinant CD27L was shown to be at least partially caused by release of soluble CD27. The 2F2 mAb enabled us to begin to analyze the biochemical prope rties, tissue distribution, and function of the CD27L on human cells. From cell lysates of I-125 surface-labeled Burkitt cells a complex imm unoprecipitation pattern with dominant bands of 29, 55, and 122 to 127 kDa was obtained using the 2F2 mAb. Phenotypical analyses showed that freshly isolated lymphocytes lacked CD27L expression, but that expres sion of the molecule could be induced after in vitro stimulation of T and 8 cells. No expression was found on cells of the myeloid lineage o r on endothelial cells. Finally, blocking of naturally expressed CD27L by the 2F2 mAb exerted a potent inhibitory effect on the proliferatio n of T cells in response to allogeneic B cells and PHA. The data prese nted in this paper identify CD27 and its ligand as potentially importa nt structures involved in cellular interactions between T and B lympho cytes.