Loss of sperm surface sialic acid induces phagocytosis: an assay with a monoclonal antibody T21, which recognizes a 54K sialoglycoprotein.

Using a monoclonal antibody T21, we reported that a mouse sperm maturation-associated antigen sialoglycoprotein of 54000 daltons (54K sialoglycoprotein) was secreted at the distal caput to proximal corpus epididymidis and that the 54K sialoglycoprotein had a hidden determinant (cryptodeterminant), which could be eliminated by sialidase treatment (Toshimori et al. (1988): Histochemistry 90:195-200; (1990a): Biol Reprod 42:151-160; (1990b): Arch Histol Cytol 53:339-349). This study evaluated the mouse sperm susceptibility to phagocytosis by macrophage in vitro. Comparisons were made between sperm from the caput epididymidis (caput sperm) incubated in modified Krebs Ringer's solution (MKR) and caput sperm incubated in MKR containing cauda fluid, and between sialylated (sialidase-untreated) sperm from the corpus and cauda epididymidis (corpus/cauda sperm) and desialylated (sialidase-treated) corpus/cauda sperm. The results showed that macrophages were least actively engaged in phagocytosis for caput sperm incubated in MKR containing cauda fluid, and most active for desialylated corpus/cauda sperm. Incubation of caput sperm in MKR containing cauda fluid revealed that the 54K sialoglycoprotein in cauda fluid could be bound to the flagellar surface of caput sperm. These results together with previous findings strongly suggest that the 54K sialoglycoprotein bound to immature sperm during maturation in the epididymis is implicated in the protection of sperm from phagocytosis with the aid of sialic acid residues.