Modulation of the immune response by progesterone-induced lymphocyte factors

Rat spleen and peripheral blood lymphocytes express progesterone receptors whose concentration is increased greatly during the early phase of pregnancy. After stimulation of progesterone the expression of receptors was augmented 2-3 times. When cells were cultured in the presence of progesterone they released a soluble factor that inhibited cellular immunoreactions (MLR, CRC) and cellular proliferation as measured by thymidine incorporation by spleen-cell culture. This factor also inhibited the synthesis of anti-DNP antibodies by a mouse hybridoma and diminished the proportion of cells in phase S. However, the percentage of asymmetric molecules produced by the hybridoma remained unaltered. These results support the hypothesis that soluble factors released by rat lymphocytes modulate the immune response of the mother and participate in the mechanism that protects the fetus against antipaternal antibodies.