Abstract. Immune tolerance expands to accommodate foreign paternal antigens expressed by the developing fetus during pregnancy. This vital process requires sustained expansion of a dedicated immune suppressive maternal CD4+ T cell subset, called regulatory T cells (Tregs), whereas blunted maternal Treg accumulation occurs in spontaneous abortion, preeclampsia and other pregnancy complications linked with fractured fetal tolerance. This necessity for maternal Tregs is reinforced in animals where even partial transient depletion from expanded pregnancy levels disrupts fetal tolerance and triggers fetal wastage. One important, but relatively uncharacterized feature of Tregs lies in their antigen specificity. Like other T cells, individual Tregs have defined specificity, and T cell receptor stimulation is required for activating their suppressive properties. By combining tetramer enrichment for tracking rare CD4+ T cells based on defined specificity and the use of transgenic mice that ubiquitously express defined model antigens to sire pregnancy in non-transgenic females that transforms model antigens into surrogate fetal antigens, our initial studies show pregnancy primes robust expansion of maternal Tregs with fetal specificity. By extending this analysis after parturition, maintenance of maternal Tregs with pre-existing fetal specificity, and their more robust re-accumulation with fetal-antigen re- stimulation in subsequent pregnancies were also revealed. In turn, sharply increased resiliency against fetal wastage during secondary compared with primary pregnancy coincides with this expanded pool of fetal- specific memory Tregs. These results highlighting potential protective memory features for maternal Tregs are in agreement, and may provide a scientific basis to explain human partner-specific protective benefits of prior successful pregnancy against complications in subsequent pregnancies. Nonetheless, despite this association, what remains unknown is whether fetal-specific memory Tregs confer protection against fetal wastage. Addressing these critical gaps in knowledge require new strategies for in vivo manipulation of maternal Tregs with defined fetal specificity. Our overall hypothesis is that maintenance of maternal memory Tregs requires ongoing stimulation by fetal cells that establish microchimerism in mothers after pregnancy. In turn, depletion of microchimeric fetal cells allows the necessity of memory Tregs in protection against fetal wastage during later pregnancies to be investigated. Furthermore, if postpartum maintenance of regulatory tolerance is restricted to antigens expressed by microchimeric fetal cells, the transient pregnancy-induced remission and swift postpartum recurrence of organ-specific autoimmune disorders (e.g. multiple sclerosis) may reflect inadequate expression of tissue restricted self antigens by fetal microchimeric cells. Therefore, accomplishing these aims are of exceptionally high significance for unraveling t...