In this study, we present evidence that the decidua inhibits DC surveillance of the maternal/fetal interface and thus plays a pivotal role in preventing immune rejection of the fetus. This evidence is based on the identification of LN-homing uterine DCs as cells with CD11chi
surface phenotypes. A recent report also detected these populations in the E8.5 pregnant uterus but did not separately visualize them in the myometrium versus decidua nor assess their migratory potential (10
). Interestingly, another report that demonstrated a requirement for uterine CD11c+
cells in the decidual response itself found these cells, in aggregate, to be largely F4/80+
). While we cannot rule out the possibility that such cells have local DC-like function, our Transwell migration assay data and analysis of DC behavior in the nonpregnant uterus argue against MHCII+
cells having LN-homing potential. Strikingly, the decidua progressively loses DC tissue density over the first half of postimplantation development (E4.5–E12.5) and appears to possess only those DCs already present in the endometrium at the time of implantation. In contrast, DC tissue densities in the growing myometrium remain relatively constant. Provocatively, the distribution of DCs in the myometrium and decidua parallel that previously described for macrophages (27
) (see Supplemental Figures 1 and 3), suggesting that the tissue layer–specific densities of both cell types are differentially regulated by the same cellular and molecular pathways.
Although the contribution of decidual DCs to the anti-fetal/placental T cell response might be limited by their low tissue density, it is likely more critical that these cells are unable to exit the tissue and migrate to the uterine LNs. The entrapment of DCs within the decidua is supported by 4 independent lines of investigation. First, in contrast to the large number of CFSE+ DCs that appeared in the uterine LNs after we labeled the undecidualized uterus with CFSE, only few such cells appeared after CFSE labeling of the decidua. Second, LPS induced the disappearance of DCs from the myometrium but not the decidua, while similar experiments performed on nonpregnant mice showed equal loss of DCs from the myometrium and endometrium in a CCR7-dependent fashion. Third, DCs failed to emigrate from the decidua when explanted tissue was placed in CCL21-containing medium. Last, wild-type and Ccr7–/– female mice showed an equivalent early phase of fetal/placental antigen presentation in the uterine LNs, indicating that the regional T cell response to the fetus and placenta is not driven by migratory DCs. Bolstering this direct evidence is our data on Ccr7–/– mice suggesting that the steady-state flux of DCs through the decidua is quite low.
A failure of maternal DCs to monitor the maternal/fetal interface has major implications for afferent mechanisms of fetomaternal tolerance. By analogy to solid organ transplantation (30
), two distinct cellular pathways involving migratory DCs would be expected to prime anti-fetal/placental T cells in the uterine LNs. We previously showed that the “direct” pathway is inoperative over the entirety of gestation, most likely because the fetus simply does not contain a population of migratory DCs analogous to the donor DCs that emigrate from solid organ transplants (3
). Here, we show that the “indirect” pathway, which would be driven by maternal DCs originating from the maternal/fetal interface, is largely, if not entirely, attenuated over the first half of gestation (through about E12.5). The maternal T cell response to the implanted embryo thus appears akin to what might transpire if an organ depleted of “passenger leukocytes” were to be transplanted into an anatomical location rendered devoid of a draining lymphatic vasculature, two experimental manipulations that dramatically and independently prolong graft survival (31
). Indeed, DC entrapment within the decidua explains our inability to detect T cell responses to fetal/placental OVA early in gestation. Although we cannot rule out the possibility that DCs exit the decidua later in pregnancy, this would seem unlikely given our current set of data.
In addition to DCs, our CFSE tracking experiments suggested that no other cell type could migrate in appreciable numbers from the decidua to the spleen or LNs. Thus, the response of naive maternal T cells to natural fetal/placental antigens is likely mediated through the cell-free, passive transport of these antigens via the blood or regional lymphatics and their local uptake by spleen- and LN-resident DCs. This pathway is clearly apparent with our surrogate antigen, OVA, which is shed from the placenta in immunologically detectable quantities starting at mid-gestation. Indeed, shed OVA also accumulates systemically on FDCs, a nonmigratory cell type resident within secondary lymphoid organs that binds complement-fixed, cell-free material (3
). Although the specific mechanism has remained elusive, T cell priming is impaired when peripheral tissue antigens arrive in the LN solely by passive transport and are thus presented exclusively by LN-resident DCs (33
). Not only might this explain why anti-OVA CD8+
T cells are not primed to fetal/placental OVA despite their proliferative response (3
), but it also predicts that the presentation of natural fetal/placental antigens would be similarly tolerogenic.
Strikingly, decidual DCs retain maturation capacity in situ and the intrinsic ability to become CCL21-responsive upon stimulation. On the other hand, our Transwell migration assays suggest that they are not actively retained within the tissue, a finding that is consistent with our chemokine expression data indicating that only the myometrium produces DC-attracting chemokines. Thus, DC entrapment within the decidua is likely the result of downstream, DC-extrinsic factors that more passively preclude DC emigration. One possibility is that the specialized extracellular matrix produced by decidual stromal cells provides an insufficient scaffold for interstitial DC migration. Along with many unusual features (35
), this matrix contains low amounts of hyaluronan, a molecule known to be required for DC migration likely through interactions with its cell-surface receptor CD44 (37
). Second, it is possible that the periphery of the decidua creates some sort of physical barrier, although the relatively homogenous distribution of decidual DCs within LPS-treated mice (Figure D) would argue against this idea. Last, decidualization might disrupt the formation of chemokine gradients or simply increase the distance between the myometrium and decidual DCs to such an extent that they can no longer sense CCL21 produced by myometrial lymphatic vessels.
Interestingly, the human endometrium, like the mouse endometrium, is largely devoid of lymphatic vessels, whereas the human decidua shows robust lymphangiogenesis unlike that in the mouse (24
). Thus, an outstanding question is whether human decidual DCs are able to exit the uterus and, if so, whether their stimulation of anti-fetal/placental T cell responses in some circumstances might contribute to poor pregnancy outcomes. Along the same lines, a pathological decidual response in itself might promote inappropriate or overly robust T cell activation mediated by migratory DCs. Interestingly, our demonstration that uterine DC migration can be inhibited by decidualization, a stromal cell developmental process, suggests that analogous processes might also inhibit the sentinel function of other tissue-resident DCs. For example, tumor-associated stromal cells might inhibit the immune surveillance of tumor antigens because they produce an extracellular matrix that cannot support interstitial DC migration or because their proliferation sufficiently deranges tissue architecture so as to interfere with the function of lymphatic vessel-derived CCL21. Similar stromal cell–based processes might be of clinical use to prevent the rejection of transplanted tissues and cells.