Phagocytic interactions of sialated glycoprotein, sugar, and lectin coated beads with rat retinal pigment epithelium

Latex beads coated with mucin (a sialic acid containing glycoconjugate), N-acetyl glucosamine (GlcNAc), or with the lectins, succinylated wheat germ agglutinin (sWGA) (binds to GlcNAc) or Limax flayvus agglutinin (LFA) (binds to sialic acid residues) were used as phagocytic particles-Phagocytosis of these coated beads by retinal pigment epithelial (RPE) explants was determined by bead uptake in normal (Long Evans) and dystrophic (Royal College of Surgeons, RCS/p+) rat retinas. Electron microscopy showed that beads coated with mucin or LFA lectin were not phagocytlzed by either dystrophic or normal RPE. sWGA-coated beads were phagocytized by both dystrophic and normal RPE, while GlcNAc-coated beads were taken up by dystrophic RPE only. Specificity of uptake for sWGA and GlcNAc bead coatings was shown by the reduction in the number of beads phagocytized in the presence of the appropriate competing sugar or lectin. The lack of phagocytic uptake of beads coated with a sialated glycoprotein or a sialic acid binding lectin suggests that sialic acid residues are not recognized as particulate ligands In this phagocytic assay. The data which show the uptake of beads coated with sWGA (binds only GlcNAc) together with results which showed WGA (binds both GlcNAc and sialic acid)-coated beads were not taken up, further suggest that GlcNAc residues may be involved in bead phagocytosis. The most striking difference between normal and dystrophic RPE engulfment of coated beads is the uptake of GlcNAc-coated beads by dystrophic RPE only. These results suggest that the receptor molecules on dystrophic RPE eell surface membranes may be different from those on normal RPE membranes.