CD40 ligand expression on the surface of colostral T cells.

The proportion of T lymphocytes, mainly CD4 positive, co-expressing the CD40 ligand (CD40-L) was significantly greater (P = 0.001) in the colostrum of 10 breast-feeding mothers than in either autologous or heterologous blood. This surface glycoprotein is a T cell molecule involved in B cell isotype switching and immunoglobulin production with its natural counter-receptor, CD40, expressed by both adult and infant B lymphocytes. As the T cells of newborn infants fail to express the CD40-L when stimulated in vitro, the in vivo upregulation on milk T lymphocytes may be one of the mechanisms through which the mother transfers immune protection to the suckling infant.