Immunologic memory in the placenta: a lymphocyte recirculation hypothesis

The placenta is an immunologically unique organ where a balance between mat ernal immunity and fetoplacental well-being must be maintained for successf ul pregnancy to occur. The intervillous blood is important in this context, yet little is known about local immunologic processes, particularly how pl acenta-specific memory immune responses are maintained. Using malaria as an illustrative case, we describe an hypothetical model in which recirculatio n of memory T lymphocytes from the intervillous blood to local lymphoid tis sue facilitates maintenance of local memory immunity. This explains how mem ory cells might be retained when the placenta is expelled at parturition an d thus remain available for rapid recall from the local lymphoid tissue to the intervillous space when exposure to the same antigenic stimulus occurs in subsequent pregnancies. Study of cell-mediated immunity to infections li ke malaria in the intervillous blood and the use of animal models will be n ecessary to provide proof for this hypothesis. (C) 2000 Harcourt Publishers Ltd.