Hormonal induction of the secretory immune system in the mammary gland.

The secretory immune system of the mammary gland is undeveloped in virgin mice but becomes active at term and during lactation. This change appears to depend on migration to the mammary gland of precursors of IgA-secreting cells derived from the gut-associated lymphoid tissue, an origin which explains the specificity of milk IgA antibodies for enteric organisms. Because development of the epithelial components of the mammary gland is clearly under hormonal control, we examined the effect of mammotropic hormones on differentiation of the immune elements. Under a combined regimen of progesterone, estrogen, and prolactin, development of the glandular epithelium occurs with concomitant increments in the number of IgA-secreting plasma cells and amount of intraepithelial IgA. These increases appear to be due to enhanced capacity of the gland to attract or retain precursors of IgA plasma cells derived from gut-associated lymphoid tissue. Testosterone, which antagonizes lactation, also antagonizes development of the secretory immune system and decreases cellular trapping in the lactating gland. The ability of the gland to trap IgA immunoblasts is probably contingent upon a hormone-induced increase in receptors.