Rosette formation with unsensitized sheep erythrocytes is a characteristic of human thymus dependent lymphocytes. Release of glycopeptides from the sheep erythrocyte by trypsin reduces rosette formation. These tryptic glycopeptides inhibit rosette formation by untrypsinized sheep erythrocytes; this suggests that rosetting is mediated by erythrocyte surface glycopeptides. To investigate the molecular nature of this interaction, the authors examined the abilities of various model compounds to act as haptenic inhibitors of rosette formation. Inhibition is given by glycopeptides bearing oligosaccharide units rich in sialic acid, galactose, N acetylglucosamine, and mannose linked to asparagine residues through glycosylamine bonds. Among compounds tested, fetuin glycopeptide is most effective, but human transferrin glycopeptide and human erythrocyte glycopeptide I also inhibit rosette formation. Other compounds including human erythrocyte glycopeptide II, human IgG glycopeptide, lacto N neotetraose, 3' and 6' sialyllactose show no significant inhibition. Neither sialic acid, galactose mannose, nor N acetyl glucosamine alone inhibits rosette formation. Stepwise degradation of fetuin glycopeptide establishes the galactose residues as important determinants of inhibitory activity. Fetuin glycopeptide blocks rosette formation when added to a suspension of human lymphocytes and sheep erythrocytes, or when preincubated with human lymphocytes, but not when preincubated with sheep erythrocytes. Studies of the binding of [3H]fetuin glycopeptide to normal human lymphocytes demonstrate 7.5 x 106 saturable binding sites per cell. No saturable binding of this compound to sheep erythrocytes membranes is observed. Compared to normals, lymphocytes from patients with chronic lymphatic leukemia demonstrate decreased fetuin glycopeptide binding with a mean of 0.9 x 106 sites per cell. This decreased binding correlates with the impaired ability of these cells to form rosettes. The data suggest that fetuin glycopeptide inhibits rosette formation by binding to the thymus dependent cell where competition occurs with sheep erythrocytes for specific lymphocyte surface receptors.
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