Pregnancy-induced maternal regulatory T cells, bona fide memory or maintenance by antigenic reminder from fetal cell microchimerism?

Long-term maintenance of immune components with defined specificity, without antigen is the hallmark feature of immunological memory. However, there are fundamental differences in how memory CD8(+) compared with CD4(+) T cells are maintained. After complete antigen elimination, CD8(+) T cells can persist as a self-renewing numerically stable cell population, and therefore satisfy the most stringent definition of "memory." Comparatively, CD4(+) T cell maintenance is considerably less stable, often requiring low-level antigen persistence or antigenic reminders. Recent studies show these basic memory features, classically ascribed to effector CD8(+) and CD4(+) T cells, extend to immune suppressive Foxp3(+) regulatory CD4(+) T cells (Tregs). In particular, gestational expansion and postpartum retention of maternal Tregs with fetal specificity may explain the protective benefits of primary pregnancy on complications in subsequent pregnancy. Herein, the possibility of ongoing antigenic reminders from fetal cell microchimerism in postpartum maintenance of maternal Tregs with fetal specificity is considered.