Interleukin-6 in human milk

Interleukin-6 (IL-6) in human milk collected during the first 2 days of lactation was investigated by a competitive radioimmunoassay (RIA) and column chromatography. The concentrations of IL-6 in the aqueous phase of fresh human milk were 151 ± 89 pg/ml. The concentrations of IL-6 in milk increased after storage at 4°C and decreased after storage at -20°C (P < 0.01). Column chromatography revealed two molecular weight peaks of IL-6 in human milk, the first ≥100 kDa and the second between 25 and 30 kDa. The 25-30-kDa peak corresponded to known isoforms of human IL-6 and to the elution pattern for ¹²⁵I-labeled recombinant human IL-6, whereas the higher molecular weight peak may be in keeping with a bound or compartmentalized form of that cytokine. The precise molecular forms of this protein, the compartmentalization of or binding proteins for this cytokine and the in vivo effects of IL-6 in human milk upon the mammary gland or the recipient infant remain to be explored.