Antisperm Antibody Binding to Human Sperm Inhibits Capacitation Induced Changes in the Levels of Plasma Membrane Sterols

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Abstract
Human spermatozoa express mannose ligand receptors (MLRs) over the entire head when incubated under conditions that promote loss of free cholesterol. Binding of IgA, IgG, and/or IgM head-directed anti-sperm antibodies (ASAs) to freshly isolated sperm blocks MLR expression in a dose dependent manner. Comparison of Western blots of immunoprecipitated glycoproteins from fresh sperm exposed to ASAs from women with those of sperm membrane proteins isolated from capacitated sperm probed with a goat anti-human macrophage MLR antiserum showed that ASAs do not react with human sperm MLRs of 48 and 61 kd apparent molecular weight. The direct effect of ASA binding was to increase membrane free cholesterol content which remained greater than 0.005 mumol/10(9) sperm after 18 h incubations, whereas the sterol content of controls decreased to less than 0.001 mumol/10(9) sperm. Cholesterol addition to sperm inhibited MLR expression in a manner analogous to ASA binding, while increasing temperatures up to the crystalline/liquid-crystalline phase transition for sperm membranes; less than 45 degrees C failed to promote the appearance of MLRs on the surface of fresh sperm. We conclude that ASA effects on membrane cholesterol content prevent the membrane fluidity changes needed for MLR expression.