BINDING OF CLOSTRIDIUM-DIFFICILE TOXIN-A TO HUMAN-MILK SECRETORY COMPONENT

Toxigenic Clostridium difficile is isolated from a majority of healthy human infants. The exact mechanism of asymptomatic colonisation is un clear; however, previous studies in this laboratory have shown that co mponents of both the immunoglobulin and nonimmunoglobulin fractions of human milk bind to toxin A and prevent its interaction with hamster i ntestinal brush border membranes (BBMs). Secretory IgA (sIgA) is the p rimary immunoglobulin found in human milk. As sIgA resists digestion i n the infant stomach and passes at high levels into the colon, its abi lity to bind toxin A was the subject of this investigation. Purified s IgA in concentrations at and below those found in human milk inhibited the binding of toxin A to purified BBM receptors, Heating sIgA to 100 degrees C for 5 min did not affect its inhibitory activity. IgM, IgG and serum IgA did not appreciably inhibit the binding of toxin A to BB M receptors, SDS-PAGE separated sIgA into three major bands: secretory component, heavy chains and light chains. Autoradiography with radiol abelled toxin A revealed that toxin A bound to the secretory component (SC) of sIgA, When the three purified subunits of sIgA were coated on to microtitration wells, SC bound significantly more toxin A than the heavy or light chains of sIgA, Purified SC also inhibited toxin bindi ng to receptors in a dose-dependent fashion similar to sIgA, The heavy and light chains of sIgA did not inhibit toxin A receptor binding. Re moving carbohydrates from sIgA and SC by enzymic digestion showed that toxin A binds much less to deglycosylated SC than to glycosylated SC. These data suggest that SC in human milk binds to toxin A and may fun ction as a receptor analogue, protecting human infants against C. diff icile-associated disease.