Paternal antigen and progesterone effects on conceptus size in laboratory mice

Reports in the scientific literature conflict regarding the possible effects of immunizing mothers with paternal antigens on placental and fetal size of the progeny. Several investigators noted an enhancement effect, while others were unable to demonstrate any effect or found an inhibitory influence. The principal differences among these previous experiments were in dosage and frequency of administration of antigen. A series of mice (C57BL/6J females immunized against A/J-strain male spleen cells and the reciprocal) was tested over a broad range of dosages, including repetitions of those given previously by other workers. Late-gestation conceptus weights and other measurements were negatively but highly correlated, with the loge of total paternal spleen cells used for preimmunization. However, the preimmunization effect could be nullified by treatment with medroxyprogesterone during pregnancy. Prior impregnation by males of the spleen-cell donor strain also countered the preimmunization inhibition of conceptus growth. Preimmunization dosage differences do not account for discrepancies observed by previous investigators.