ENGAGEMENT OF THE LEWIS-X ANTIGEN (CD15) RESULTS IN MONOCYTE ACTIVATION

We previously reported that monocyte adhesion to tumor necrosis factor -alpha (TNF-alpha)-treated endothelial cells increased expression of t issue factor and CD36 on monocytes. Using immunological cross-linking to mimic receptor engagement by natural ligands, we now show that CD15 (Lewis X), a monocyte counter-receptor for endothelial selectins may participate in this response. We used cytokine production as a readout for monocyte activation and found that CD15 cross-linking induced TNF -alpha release from peripheral blood monocytes and cells from the mono cytic cell line MM6. Quantitative reverse transcriptase-polymerase cha in reaction (RT-PCR) showed an increase in steady-state TNF-alpha mRNA after 3 to 4 hours of cross-linking. CD15 cross-linking also concomit antly increased interleukin-1 beta (IL-1 beta) mRNA, while no apparent change was observed in the levels of beta-actin mRNA, indicating spec ificity. To examine transcriptional regulation of cytokine genes by CD 15 engagement, a CAT plasmid reporter construct containing IL-1 beta p romoter/enhancer sequences was introduced into MM6. Subsequent cross-l inking of CD15 increased CAT activity. CD15 engagement by monoclonal a ntibody also attenuated IL-1 beta transcript degradation, demonstratin g that signaling via CD15 also had posttranscriptional effects. Nuclea r extracts of anti-CD15 cross-linked cells demonstrated enhanced level s of the transcriptional factor activator protein-1, minimally changed nuclear factor-kappa B, and did not affect SV40 promoter specific pro tein-1. We conclude that engagement of CD15 on monocytes results in mo nocyte activation. In addition to its well-recognized adhesive role, C D15 may function as an important signaling molecule capable of initiat ing proinflammatory events in monocytes that come into contact with ac tivated endothelium. (C) 1997 by The American Society of Hematology.