Mitogen-Stimulated Lymphoid Cells from Human Newborns Suppress the Proliferation of Maternal Lymphocytes across a Cell-Impermeable Membrane

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Abstract
The immunologic consequences of the interaction between lymphocytes from human newborns and their mothers were investigated in vitro. Maternal and male baby lymphocytes were cocultured in the presence or absence of PHA, and the resulting cellular proliferation was assessed by a) the fluorescent chromosome technique with the Y chromosome as a marker for dividing male baby cells, and by b) measuring the incorporation of 3H-thymidine in one- and two-way mixed lymphocyte cultures (MLC). In confirmation of previous reports, we found that approximately 90 to 100% of the dividing cells in two-way MLC were of baby origin, indicating that lymphocytes from human newborns effectively inhibit the proliferative capacity of maternal lymphocytes in 6-day MLC. The simultaneous measurement of 3H-thymidine uptake in the co-cultures revealed that the maternal cells with an inhibited proliferative capacity did cause a strong allogeneic stimulation in the newborn cell population. The suppressor activity associated with newborn lymphocytes was completely abrogated by irradiation with 6000 rads, showing a requirement for cell division in the in vitro development of the anti-proliferative effect on maternal cells. In contrast to the suppressive effect mediated by actively dividing newborn lymphocytes, irradiated cells from newborns exerted a strong stimulating effect on maternal lymphocyte proliferation in one-way MLC. Experiments using a double chamber culture system showed that mitogen-stimulated cells from newborns could effectively suppress the proliferation of lymphocytes from recently delivered mothers and nonpregnant adult females across a cell-impermeable membrane. PHA-stimulated lymphocytes from nonrelated adults did not, however, inhibit the division of maternal lymphocytes or lymphocytes from another nonpregnant woman when physically separated in double chamber cultures. Furthermore, mitogen-stimulated cells from one newborn failed to inhibit the proliferation of cells from another newborn. These findings suggest that proliferating newborn cells inhibit maternal lymphocyte proliferation by releasing a low m.w.-soluble suppressive factor(s). The characterization of the inhibitory substance released from mitogen- or alloantigen-activated human newborn suppressor cells is in progress.