We report, for the first time, the changes in global placental gene expression patterns in VUE. This study also uncovers another novel immunopathologic feature of VUE: the presence of a systemic CXC chemokine response both in the mother and fetus. VUE has been regarded as a placental lesion analogous to allograft rejection by the mother (host) against fetal Ags (graft) (10
). However, the evidence supporting a fetal systemic inflammatory response strongly suggests that VUE has a component similar to graft-vs-host disease (GVHD) by maternal lymphocytes (graft) to fetal placental tissue (host) (6
The transcriptome profiles of the placentas with VUE essentially reflect the presence of an inflammatory response involving T lymphocytes and APCs in the chorionic villi. The gene expression profile in this lesion is quite similar to that reported in other organs in the setting of either transplantation rejection or GVHD (20
). Gene Ontology analysis indicates that VUE, allograft rejection, and GVHD share enrichment of genes involved in Ag presentation, leukocyte migration, T cell activation, and induction by IFN-γ. Up-regulation of the expression of MHC class II molecules in the placentas with VUE is consistent with previous observations in which increased expression of MHC class II Ags in villous trophoblasts and macrophages in and around areas of VUE was demonstrated (26
). In a murine model of hepatic GVHD, mRNA for MHC class II molecules and for genes related to peptide processing for MHC molecules were up-regulated in the liver 1 wk after experimental allogeneic bone marrow transplantation (24
). In acute GVHD, HLA-DR protein was expressed by human keratinocytes. This is interesting because HLA-DR is not found in normal skin or after regression of GVHD (28
). A mouse renal allograft rejection model also demonstrated increased mRNA expression of MHC class II molecules in the kidney as a major feature of INF-γ-dependent, rejection-induced transcripts (29
). Indeed, the expression levels of MHC class II molecules in human organ allograft show a gradient in the order of rejection, nonrejection, and normal organ before transplantation, showing the critical nature of MHC class II molecules for successful transplantation (22
). Interestingly, HLA-DR expression in Hofbauer cells increases as a function of gestational age (31
). Enhanced potential for Ag presentation by Hofbauer cells with increased MHC class II expression at term may be an explanation of why VUE occurs mostly at term (1
The ligands for CXCR3 (CXCL9, CXCL10, CXCL11) and for CCR5 (CCL4, CCL5) are among the most common chemokines expressed during the course of transplantation rejection (33
) and GVHD (25
). Increased expression of these chemokines and their receptors in the placentas with VUE strongly suggests the presence of the signals required for leukocyte migration akin to those observed in transplantation rejection and GVHD. Hofbauer cells and endothelial cells in the villi of cases with VUE showed increased expression of selective chemokines, suggesting an interaction with CXCR3+
T cells, which infiltrate chorionic villi. Transplant rejection and GVHD also show increased expression of chemokines in the parenchymal tissue, including renal tubules and epidermis, in addition to leukocyte infiltration (33
). A homeostatic chemokine, CXCL13, which is not commonly found in either transplantation rejection or GVHD, except for a few examples of allograft rejection associated with B cell infiltration (39
), was overexpressed in VUE, although B cells were minimally present in VUE (26
). Similarly, we could not detect the expression of CXCR5, the primary CXCL13 receptor, in placentas with VUE. However, CXCL13 in VUE may be involved in the chemotaxis of activated T cells, since CXCL13 is also a potential ligand for CXCR3 in addition to IFN-γ-induced CXC chemokines without the ELR motif (42
). CXC chemokines are known to regulate angiogenesis by acting on their receptors on endothelial cells. Chemokine ligands for CXCR2 and CXCR4 (CXCL1, CXCL6, CXCL8) drive proangiogenic signals, while CXCL9, CXCL10, CXCL11, and CXCL13 are antiangiogenic (43
). This is interesting because VUE is commonly associated with obliterative changes of villous vessels (obliterative fetal vasculopathy) (47
). We propose that such a finding is associated with an increase in antiangiogenic chemokine expression in the placenta.
The most meaningful and novel observation in the present study is the demonstration of a systemic derangement in chemokine concentrations in both maternal and fetal circulation associated with VUE. The expression pattern of each chemokine mRNA indicates that circulating chemokines could be produced by the placenta or peripheral leukocytes. Chemokine up-regulation in systemic circulation could also be a result rather than a cause of VUE. A few studies have described changes in systemic chemokine concentrations either in allograft rejection or in GVHD. Circulating CXCL11 was elevated in patients with coronary artery disease, which developed in a transplanted heart (48
). The serum concentrations of CXCL10 and CCL5 have been reported to be increased in patients with cutaneous GVHD (38
). Interestingly, VUE-associated changes in the mother are distinct from those observed in acute chorioamnionitis in which systemic CXC chemokine concentrations were not changed (CXCL9, CXCL10, CXCL11) or even decreased (CXCL13), while those in the fetus were similar to those of acute chorioamnionitis. This stark difference in maternal plasma chemokine concentrations and the similarity in fetal plasma concentrations between VUE and acute chorioamnionitis strongly suggest that the immune responses mounted by the mother are more finely tuned and specific depending on the etiology of the inflammatory process. Another incidental but intriguing finding was the differences in maternal plasma concentrations of CXCL9 and CXCL11 between PTL and TIL patients. Whether those differences are related to gestational age or to the pathologic vs physiologic nature of labor needs further study. Nevertheless, this is the first evidence showing that the maternal systemic chemokine profile varies between preterm labor and spontaneous labor at term.
Pregnancy has been likened to a semiallograft. However, the fetus is not a simple allograft because it may also be a host, as demonstrated by the development of microchimerism (maternal cells in fetal circulation) (49
). VUE is a unique fetal response associated with maternal T cell infiltration in the placenta. The overall findings reported herein indicate that VUE is akin to placental GVHD in the fetal compartment and that increased circulating antiangiogenic chemokines, in addition to inflammatory destruction of the placenta, may have a causal link in the development of fetal growth restriction or fetal death. Systemic involvement of the skin or the gastrointestinal tract is a typical clinical presentation of GVHD (50
). From this perspective, VUE could be an atypical form of GVHD with its full-blown histologic lesion confined to the placenta. Tissue damage due to underlying diseases or previous treatment is a prerequisite for the development of GVHD (51
). This prerequisite is met during pregnancy because there is ongoing damage to the villous tree as the pregnancy progresses (53
). Restricted tissue damage in the placental villous tissue, exposure of villous tissue to maternal circulation isolated from fetal circulation, and the absence of lymphatics in the placenta may be plausible explanations for the confinement of VUE to the placenta. Secondary lymphoid organs are important for T cell activation, but they are not an absolute requirement for allorecognition (54
). Fetal placental macrophages (Hofbauer cells) seem to play a key role in establishing chemokine-chemokine receptor interaction.
In conclusion, we propose that VUE is a disorder characterized by distinct up-regulation of local and systemic CXC chemokines both in the mother and the fetus. Such a distinct pattern of chemokine up-regulation clearly differs from that observed in acute chorioamnionitis due to microbial invasion of the amniotic cavity. Therefore, we propose that placental VUE be considered a unique and genuine pathologic link between maternal allograft transplantation rejection and fetal GVHD. VUE is the only example in human biology where two hosts in intimate contact deploy a bidirectional inflammatory response resulting in a unique form of tissue destruction.