B6CBAF1 females mated to Act-mOVA or B6 control males were adoptively transferred with CFSE-labeled B6CBAF1 OT-I T cells. (A) OT-I proliferation in secondary lymphoid organs. OT-I cells were visualized by flow cytometry at the end of their indicated 2-day window of exposure. The percentage of cells undergoing more than 1 cell division is noted; such cells were only rarely observed in B6-mated females at any stage of gestation. (B) Midgestational onset of OT-I proliferation in secondary lymphoid organs. Act-mOVA–mated female mice at different stages of gestation were transferred with OT-I cells and then sacrificed 2 days later. Using raw values for proliferation determined as in A, the percentage of OT-I cells in antigen-driven proliferation was calculated by subtracting mean B6-mated control values for each of the 3 secondary lymphoid organ sets in a given experiment from their respective values in individual Act-mOVA–mated mice. The graph shows all data for n = 21 Act-mOVA–mated females over 5 independent experiments. (C) Absence of OT-I reactivity at the maternal/fetal interface in early gestation. Transferred OT-I cells were visualized in the uterine LNs and in dissected decidua/placentas pooled from several implantation sites. Many of these cells lie along the x axis of the plot and so are difficult to see. We therefore note the total number of OT-I cells recovered from these sites. Percentages indicate the fraction of OT-I cells with a CFSE^undilutedCD69^lo naive phenotype (cells in the lower-right quadrant).