Identification of human complement Factor H as a ligand for L-selectin

The selectin family of adhesion molecules (E-, P- and L-selectins) is invol ved in leukocyte recruitment to sites of inflammation and tissue damage. Re cently it has been shown that L-selectin is involved not only in leukocyte tethering and rolling, but also plays an important role in leukocyte activa tion. For example, glycosylation-dependent cell-adhesion molecule 1 (GlyCAM -1), a known ligand for L-selectin, has been shown to enhance beta 2-integr in function. GlyCAM-1 is a secreted protein and is present in mouse serum a t a concentration of approx, 1.5 mu g/ml. There is no obvious GlyCAM-1 homo logue in man and, to date, L-selectin ligand(s) from human ser um have not been characterized. Therefore we have used L-selectin affinity chromatograp hy. follow-ed by ion-exchange chromatography, to isolate specific ligand(s) for L-selectin. Using this procedure, we have isolated three major glycopr oteins of apparent molecular masses 170 kDa, 70 kDa and 50 kDa, The 170 kDa protein band was digested with trypsin and peptides were analysed by delay ed er;traction matrix-assisted laser desorption ionization MS and protein d atabase searching. The 170 kDa protein was identified as the human compleme nt protein Factor H, Human Factor H, isolated by a different method, was sh own to bind specifically to L-selectin in the presence of CaCl2, and bindin g was inhibited by anti-L-selectin antibodies, fucoidan and lipopolysacchar ide. Only a part of the purified Factor H preparation bound to immobilized L-selectin, The interaction of Factor H with leukocyte L-selectin was shown to induce the secretion of tumour necrosis factor-alpha (TNF-alpha), Pretr eatment of Factor H with sialidase reduced both the binding of L-selectin t o Factor H and the Factor H-induced L-selectin-mediated TNF-alpha secretion by leukocytes. Taken together, these results demonstrate that a post-trans lationally modified form of human plasma Factor H is a potential physio-log ical ligand for L-selectin.