Erythrocyte agglutination by wheat germ agglutinin: ionic strength dependence of the contact seam topology.

The topology of the cell-cell contact seam formed when normal or pronase pre-treated (PPT) erythrocytes are exposed to wheat germ agglutinin (WGA) in isotonic media of different ionic strengths was examined here. Lectin uptake and cell agglutination were also quantified. Agglutination of normal cells was gradually and significantly inhibited as ionic strength (IS) was reduced from 0.15(buffered 145 mM NaCl) to 0.105. Agglutination was less inhibited in PPT cells, even when IS was reduced to 0.09. Cell contact seams formed during agglutination showed patterns of localized contacts. The scale of the patterns, i.e, the average lateral separation distance of contact regions, was 0.62 mum for normal cells and was significantly shorter, at 0.44 mum, for PPT cells at an IS of 0.15. The scale increased significantly for both cell types when the IS was reduced to 0.09. Flow cytometry measurements showed that WGA uptake by normal cells increased slightly, whilst that for PPT cells was unchanged, as IS was decreased from 0.15 to 0.09. The results imply that, whilst ionic strength change does not exert a strong influence on intermolecular WGA-ligand binding, physico-chemical modification of the interaction between cells modulates not only the extent and progression of the biospecific lectin-induced cell-cell agglutination but also the topology of the contact seam. The IS dependence of contact separation in WGA-agglutinated cells is contrasted here with that reported for cells adhering in dextran solutions. The influence of IS change and pronase pre-treatment on contact pattern are consistent with predictions, from interfacial instability theory, of punctuate thinning of the aqueous layer separating bilayer membranes in close apposition.