Sialomucin CD34 Is the Major L-Selectin Ligand in Human Tonsil High Endothelial Venules

Peripheral node addressin (PNAd) is a complex mixture of glycoproteins with L-selectin ligand activity that functions in lymphocyte homing. We have investigated the contribution of the sialomucin CD34 relative to other components of PNAd in lymphocyte tethering and rolling in in vitro laminar flow assays. PNAd was isolated with MECA-79 mAb-Sepharose from tonsillar stroma, and the CD34 component (PNAd, CD34+) and CD34-negative component (PNAd, CD34-) separated on CD34 mAb-Sepharose. Lymphocytes on the PNAd, CD34- fraction tether less efficiently, roll faster and are less resistant to shear detachment than on PNAd. The PNAd, CD34+ fraction constitutes about half the total functional activity. These studies show that CD34 is a major functional component of PNAd. Ligand activity in both the PNAd, CD34+ and PNAd, CD34- fractions is expressed on mucin-like domains, as shown with O-sialoglycoprotease. The CD34 component of PNAd has about four times higher tethering efficiency than total tonsillar CD34. CD34 from spleen shows no lymphocyte tethering. Although less efficient than the PNAd, CD34+ fraction from tonsil, CD34 from the KG1a hematopoietic cell line is functionally active as an L-selectin ligand despite lack of reactivity with MECA-79 mAb, which binds to a sulfation-dependent epitope. All four forms of CD34 are active in binding to E-selectin. KG1a CD34 but not spleen CD34 are active as L-selectin ligands, yet both lack MECA-79 reactivity and possess E-selectin ligand activity. This suggests that L-selectin ligands and E-selectin ligands differ in more respects than presence of the MECA-79 epitope.