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To investigate the role of systemic suppression during allopregnancy, CBA/J female mice were immunised against H-2d prior to mating. Cytotoxic T lymphocyte (CTL) activity was tested at days 14-16 of pregnancy. A reduction of CTL activity was observed only in multiparous animals. Although a nonspecific suppression was detected in isopregnancy, suppression was more marked in allopregnancy. Conversely, the CTL activity observed in the spleen during the first pregnancy (iso or allo) was always significant in mice presensitized with 2 or 3 alloimmunizations prior to pregnancy. Such animals have in vivo effector cells, since allografts of Sarcoma Sa1 were rejected in secondary fashion in alloimmunized mice, while the fetus remained unharmed. These observations demonstrate that allospecific anti-MHC CTLs are specifically impaired in multiple allopregnancy, but also tend to rule out theories postulating that systemic suppression of CTL generation and function is required for successful allopregnancy.

The transfer of cells from allopregnant animals to syngeneic receivers allografted with paternal strain tumor leads to mild but significant enhancement. The effect can be defined as T cell mediated. Cells from allopregnant animals can suppress a mixed lymphocyte reaction (MLR) of maternal responders against paternal stimulators. The effect relies upon a THY 1+, Ly 2+, Ia+ cell. Cell-mediated lympholysis (CML) assay could also be suppressed by cells from allopregnant animals. Placental products are capable of interfering with allograft rejection in vivo. They can block MLR in vitro, and seem to act in part via the induction of suppressor cells. The respective roles of these depressive components, together with enhancing antibodies, is discussed.

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