Synthesis of a tetravalent sialyl Lewis x glycan, a high-affinity inhibitor of L-selectin-mediated lymphocyte binding to endothelium.

Kidney transplant rejection is an inflammatory process characterized by lymphocyte infiltration. Our earlier observations have shown that peritubular capillary endothelium (PTCE) is the site of lymphocyte entry into the rejecting renal allograft. During rejection, PTCE begins to express sialyl Lewis x de novo, and binds lymphocytes by a mechanism largely dependent on L-selectin. Hence, inhibiting the lymphocyte-endothelial interaction with oligosaccharide ligands of L-selectin offers an attractive possibility to prevent the inflammation and rejection. Here, we report enzyme-assisted synthesis of N-acetyllactosamine-based tetra-, deca-, and docosameric saccharides carrying one, two or four distally located sialyl Lewis x groups [Neu-NAc alpha 2-3Gal beta 1-4(Fuc alpha 1-3)GlcNAc] (sLex), respectively. When tested for their ability to inhibit lymphocyte-endothelial interaction during rat kidney transplant rejection, all sLex-saccharides were inhibitors in the Stamper-Woodruff binding assays; the analogues lacking fucose showed no inhibitory potency. The tetravalent sLex glycan proved to be a high-affinity adhesion inhibitor with an IC50 < 50 nM. While less powerful than the tetravalent glycan, also the divalent sLex saccharide was a much better inhibitor than the monovalent glycan. Hence, increasing multivalency and, possibly, increasing chain length of the polylactosamine backbone, enhances the inhibitory potency of sLex bearing glycans in the lymphocyte-endothelial adhesion assay. This suggests that L-selectin behaves as a "functional oligomer" on lymphocyte surfaces.