SOLUBLE RECEPTORS AND CYTOKINE ANTAGONISTS IN HUMAN-MILK

To determine whether human milk contained soluble receptors and cytoki ne antagonists that might contribute to its anti-inflammatory properti es, ELISA and enzyme-amplified sensitivity immunoassay methods were us ed to quantitate soluble intercellular and vascular cell adhesion mole cules, soluble E-selectin, soluble IL-6 receptor, IL-1 receptor antago nist, and soluble tumor necrosis factor-alpha (TNF-alpha) receptors I and II in human milk and colostrum. Soluble adhesion receptors (solubl e intercellular and vascular cell adhesion molecules and soluble E-sel ectin) were present in colostrum at levels approximately equal to seru m, whereas milk levels were significantly lower. Both colostrum and mi lk contained soluble IL-6 receptor, but the levels present were signif icantly lower than that reported for serum. The colostrum contents of IL-1 receptor antagonist (672 +/- 202 pg/mL), TNF-alpha receptor I (>3 703 +/- 305 pg/mL), and TNF-alpha receptor II (>4507 +/- 770 pg/mL) we re significantly elevated over serum/plasma levels. Milk levels of IL- 1 receptor antagonist and TNF-alpha receptor I were also greater than serum/plasma levels, but lower than colostrum levels. Examination of s equential milli specimens collected from seven women over st period of 2-6 mo showed that IL-1 receptor antagonist and TNF-alpha receptors I and II persisted throughout lactation. Column chromatographic fractio nation of colostrum and milk demonstrated that soluble TNF-alpha recep tors I and II had molecular sizes up to 60 kD, suggesting that they mi ght be associated with other molecules. Antigen assays for TNF-alpha i n colostrum and milk, as well as chromatographic fractionation experim ents, showed that, although present, most TNF-alpha was not ''free'' i n colostrum or milk, consistent with the observed content of soluble T NF-alpha receptors I and II. These studies demonstrate that human milk and colostrum contain soluble receptors and cytokine antagonists, mat erials which could contribute to their anti-inflammatory properties.