DF3 antigen, a human epithelial cell mucin, inhibits adhesion of eosinophils to antibody-coated targets.

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Abstract
Although mucins provide lubrication and physical protection for epithelial cell surfaces, other functional roles for these large glycoproteins are unknown. One human mucin, designated DF3 Ag, is detectable on apical surfaces of normal epithelial secretory cells and in normal milk, urine, and plasma. The present studies have examined the effects of DF3 Ag purified from both normal and malignant sources on the antibody-dependent cytotoxicity of Schistosoma mansoni by eosinophils (ADCC-E) and on the adherence of eosinophils to inert antibody-coated targets. DF3 Ag purified from tissue culture supernatant of a human breast carcinoma cell line or from human milk inhibited ADCC-E in a concentration-dependent manner, with half-maximal inhibitory activity at 3 to 10 micrograms/ml. Inhibition of ADCC-E was specific for the DF3 mucin, because no inhibition was observed with two other unrelated, circulating glycoproteins: carcinoembryonic Ag and alpha 1-acid glycoprotein. Inhibition was not a result of direct cytotoxicity of the DF3 Ag for eosinophils, as demonstrated by the lack of detectable effect of the mucin on cellular trypan blue exclusion or PMA-induced H2O2 release. The inhibitory effect was time dependent, requiring the presence of DF3 Ag in the ADCC-E culture for at least 4 h, beginning within the first 2 h of eosinophil-schistosomula interaction. Furthermore, inhibition was not a result of interaction between DF3 Ag and the activating lymphokine. These data suggest that inhibition of ADCC-E by DF3 Ag is a result of interference of adhesion of eosinophils to Ig-coated targets. In this regard, purified DF3 tumor Ag prevents eosinophil adherence to human Ig-conjugated Sepharose 4B beads. Preincubation of the inert Ig-coated targets with DF3 Ag did not inhibit subsequent eosinophil adherence, suggesting that DF3 Ag interacts with a moiety present on the eosinophil. Inhibition of adhesion occurred at 37 degrees C, but was also observed at 4 degrees C. These results suggest that DF3 Ag acts as an immunomodulating agent. Because activated eosinophils can damage surrounding normal tissues as well as infectious organisms, DF3 Ag may serve to protect secretory epithelium from the cytotoxic effects of activated inflammatory cells.

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