THE SELECTIVE TRANSPORT OF MATERNAL PLASMA. PROTEINS ACROSS THE MAMMARY GLAND

Abstract

Colostrum and milk are relatively rich in antibodies, frequently in higher titers than are present in the maternal circulation. To study the selectivity and the mechanisms of protein transport across the mammary gland, purified proteins were labeled with ¹³¹I, and injected intravenously into non-lactating mice and into lactating mice after parturition. All animals including the sucklings were assayed at intervals for total body radioactivity. The transmammary transfer rate of a given protein was measured as the difference in the disappearance rates for that protein between lactating and non-lactating mice, and confirmed by the uptake of labeled protein via milk by the sucklings. Of the proteins studied, human IgA with and without attached secretory piece, human IgG and bovine IgG were most actively transported. Mouse IgG and human transferrin were also transferred into milk, but to a lesser degree than human IgG. There was little transmammary passage of human albumin or human IgG light chains. The data indicate that the selectivity of the mammary gland in its permeability to maternal plasma proteins is quite different from that exhibited by the placenta. In addition, although some investigators attribute transport properties to the “secretory piece” found attached to IgA in various secretions including milk, this is not supported by the present data, since IgA without secretory piece traversed the mammary gland as readily as did IgA with secretory piece.

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