Regulatory T cells in pregnancy. III. Comparison of early acting and late acting suppressor T cells in MLR: evidence for involvement of differential T-cell subsets.

The properties of mitomycin-treated and untreated spleen cells from allopregnant mice (bearing allogeneic foetuses) have been compared for suppressive activity of mixed lymphocyte reaction (MLR) of maternal strain spleen cells responding to paternal (or genetically related) strain cells. Mitomycin-treated cells in 1% normal mouse serum suppress an MLR if added at its onset while they cannot alter an already set up (day 2) MLR. Conversely untreated spleen cells are unable to affect an MLR if added at its onset while they suppress it when added on day 2 of an ongoing MLR. The two systems also differ in their in vitro retriggering requirements at the target cell (stimulator) level. Whereas the untreated, late acting population, in a k/d or k/a system, requires the contact with molecules coded by 'S-G' (or IC+S G) subregion, also present in the father's strain, the mitomycin-treated, early acting population, tested on similar panel does first appear non-specific. Suggestive evidence, however, is given that this apparent non-specificity may in fact veil clones of cells with specific recognition of 'private' Ia determinants. It is suggested that allopregnant mice elaborate two T suppressor cells, acting at different steps of the MLR. The usefulness of the first set (mitomycin-resistant suppression) for exploring the Ia chart is pointed out.