Soluble Fas and soluble Fas ligand proteins in human milk: Possible significance in the development of immunological tolerance

Human milk contains a complex uncharacterized, immune system able to exert actions both locally and systemically. This study reports the results of an ELISA-based quantitation of soluble Pas and soluble Fas ligand in human milk which may modulate the Fas/FasL system that is critical for the expression of immune tolerance and apoptosis. Production of Fas/FasL mRNAs by milk cells was also examined using RT-PCR. Pas is ubiquitously expressed in various cells and when bound by its ligand Fast, present predominantly on activated T- and NK cells, Fas-expressing cells are killed. A large amount of soluble Fas (1746-4320 pg/ml) is detected in colostrum, transitional milli and the mature milk of mothers delivering prematurely or at full-term, whereas Fast is present only in the range 123-310 pg/ml. Milk cells are positive for Fas mRNA, but negative for Fast mRNA. An excess of soluble Fas in human milk may bind to Fast preventing apoptosis and preserving epithelial barriers, and may represent an additional new mechanism whereby human milk favours immune tolerance and normal gastrointestinal development.