Villitis in normal term human placentae: frequency of the lesion determined by monoclonal antibody to HLA-DR antigen

Villitis of Unknown Etiology in the Placenta of a Pregnancy Complicated by COVID-19

Villitis of unknown etiology (VUE) is noninfectious chronic villitis thought to be associated with fetal growth restriction and stillbirth. COVID-19 and the pandemic SARS-CoV-2 infection can cause an increased risk in pregnant women for potential maternal and fetal complications from an immunological mechanism. We report a 39-week-gestational-age infant delivered to a 37-year-old mother diagnosed with SARS-CoV-2 infection at 37 weeks gestation. The placental examination showed the morphological features of VUE. We showed immunohistochemically that macrophages and CD4-positive T cells predominated in the villous tissue, although elevated numbers of CD8-positive cells were also present. We hypothesize that VUE may represent a maternal anti-viral immune response, in this case to SARS-CoV-2.

Characterizing Villitis of Unknown Etiology and Inflammation in Stillbirth

Villitis of unknown etiology (VUE) is an enigmatic inflammatory condition of the placenta associated with fetal growth restriction and stillbirth. Greater understanding of this condition is essential to understand its contribution to adverse outcomes. Our aim was to identify and quantify the cells in VUE in cases of stillbirth and to characterize immune responses specific to this condition. Immunohistochemistry was performed on placentas from stillborn infants whose cause of death was recorded as VUE to identify CD45(+) leukocytes, CD163(+) macrophages, CD4(+) and CD8(+) T cells, neutrophils, and proinflammatory and anti-inflammatory cytokines. Images were quantified with HistoQuest software. CD45(+) leukocytes comprised 25% of cells in VUE lesions: macrophages (12%) and CD4 T cells (11%) being predominant cell types; CD8 T cells were observed in all lesions. Leukocytes and macrophages were increased throughout the placenta in stillbirths; pan-placental CD4(+) and CD8(+) T cells outside VUE lesions were increased in stillbirth with VUE. There was increased IL-2 and IL-12 and reduced IL-4 immunostaining in VUE lesions. Our results suggest VUE in stillbirth has a similar immune cell profile to live birth. Pan-placental macrophages, CD4 and CD8 T cells indicate a wider inflammatory response unrestricted to VUE lesions. The cytokine profile observed suggests a skew towards inappropriate Th1 immune responses. Full characterisation VUE lesion phenotype confirms its immunological origins and provides foundations to develop novel investigations.

Chronic villitis of unknown etiology and massive chronic intervillositis have similar immune cell composition

INTRODUCTION

Chronic villitis of unknown etiology (CVUE) and massive chronic intervillositis (MCI) are placental lesions associated with infiltration of mononuclear cells in the chorionic villi and the intervillous spaces, respectively. It is not well known whether immune cells in CVUE and MCI have similar phenotypic characteristics.

METHODS

A cross-sectional study of third trimester placentas was conducted to identify immune cell subpopulations in CVUE and MCI (n = 17/group). CVUE was diagnosed with H&E staining and antibody to CD3 in serial sections; and MCI, by the presence of massive infiltration of mononuclear cells in the intervillous spaces. Immune cells, ICAM-1 expression and nuclear factor κB (NF-κB) activation were determined immunohistochemically.

RESULTS

CVUE and MCI showed similar infiltrates, mainly CD68+ and CD3+ cells. Most cells (>80%) were CD45RB+, and one third were CD45RO+ in both lesions. There were slightly more CD8+ than CD4+ cells in both CVUE and MCI. More than 90% of cells in CVUE and MCI were ICAM-1+ with NFκB nuclear localization. Syncytiotrophoblast ICAM-1 expression was significantly (p < 0.001) higher in MCI (mean of 81.0; range of 71.6-86.0) than in CVUE (52.4; 36.4-59.4) or normal placentas (0.2; 0.0-0.6). Both, failure of physiologic transformation of spiral arteries and placental atherosclerosis-like lesions of atherosis were significantly more frequent in MCI than in CVUE or normal placentas (p = 0.044 and p = 0.007, respectively).

DISCUSSION

These finding suggest that MCI and CVUE have very similar infiltration of immune cells although MCI has more severe placental lesions.

The fetus, not the mother, elicits maternal immunologic rejection: lessons from discordant dizygotic twin placentas

AIMS

Our objective was to elucidate the pathogenesis of twin discordance in four dizygotic pregnancies where only one of the twins had IUGR due to chronic villitis.

METHODS

We identified four cases of dizygotic twin placentas over a period of four years with evidence of chronic villitis. There was no clinical or pathologic evidence of TORCH, bacterial infection, preeclampsia or autoimmune disorders. Placentas were weighed, processed for histologic examination and stained with CD45RO (clone UCHL1) mouse monoclonal antibody, which identifies T-cells.

RESULTS

All placentas were dichorionic, with two being fused. Birth weight differences were 29%, 41%, 17% and 10%. Villitis was more marked in the placenta of the twin that weighed less and correlated with the degree of weight discordance. On examining the junction between the fused dichorionic placentas, the chorionic villi from the smaller twin contained numerous T-cells, whereas the villi associated with the less affected twin, showed little to no T-cells.

CONCLUSION

We describe a series of dizygotic twin placentas where the more severe the chronic villitis, the more affected the placenta and fetus. Since the maternal environment was constant for each of these twins, differences in villitis severity appears to be attributable to differences in the ability of each placenta to induce a maternal immune response.

Chronic villitis of unknown etiology

The diagnosis of chronic villitis of unknown etiology (CVUE), characterized by focal areas of inflammation with mononuclear cells and areas of fibrinoid necrosis in chorionic villi, can only be set-up after exclusion of a latent maternal-fetal transmission of infectious agents by sophisticated techniques such as polymerase chain reaction. Significant associations of CVUE with maternal body mass index, multigravidity and ethnicity were reported. While a fetal origin of the inflammatory cells has been evoked, there are many more arguments drawn from histopathology and immunohistology for a maternal immune response against the foreign fetal allograft. CVUE is detected in 7-33% of placentas, mainly after idiopathic intrauterine growth retardation, unexplained prematurity, preeclampsia, perinatal asphyxia and intrauterine fetal death. CVUE is also more frequent in pregnancies affected by autoimmune or alloimmune diseases. Considering the high rate of recurrences after an index case of CVUE, we would suggest to associate aspirine and corticosteroids in further pregnancies, a regimen that was successful in our experience but must be confirmed by other studies. The same is true for the alleviated inflammatory immunologic response recently obtained by a weekly use of maternal intravenous immunoglobulins.

Association des corticoïdes à l’aspirine pour la prévention des récidives de villite ou d’intervillite chroniques d’étiologie indéterminée

We report the cases of two patients who had a favorable outcome with aspirin and corticosteroid therapy during pregnancy for chronic villitis of unknown etiology complicated by labor asphyxia and further intrauterine fetal demise in one gravida 3 patient and for chronic intervillositis of unknown etiology diagnosed after three perinatal deaths in another patient (gravida 4). Chronic villitis of unknown etiology (CVUE) is detected in 7 to 33% of placentas, mainly after intrauterine growth retardation (IUGR), unexplained prematurity, preeclampsia, perinatal asphyxia and intrauterine fetal death (IUFD). The less frequent chronic intervillositis of unknown etiology (CIUE) (0.6 to 0.9/1.000) has been implicated in recurrent severe pregnancy complications, such as spontaneous abortions, IUGR and IUFD. Histopathology and immunohistology are in favor of an immune response against the foreign fetal allograft. The favorable results obtained with corticosteroids and aspirin remain to be confirmed by larger series.

Unique epitopes of lactoferrin expressed in human cytotrophoblasts involved in immunologic reactions

OBJECTIVE

Lactoferrin is an iron-binding protein that has been implicated in protection against infections and allogeneic recognition reactions and in the control of cell growth. We studied the biochemical characteristics and expression of the unique lactoferrin epitopes (LF(1)) in human placentas.

STUDY DESIGN

Immunohistologic studies of normal human term placentas were done by using monoclonal antibodies to LF(1). Double-antibody experiments were done by using monoclonal antibodies to markers of inflammation (macrophages, human leukocyte antigen [HLA-DR]). LF(1) was studied immunochemically by using lactoferrin fragments produced by the reaction of lactoferrin with trypsin or N-glycanase.

RESULTS

Anti-LF(1) monoclonal antibodies reacted with most interstitial cytotrophoblasts in the basal plate and with villous cytotrophoblasts of some but not all chorionic villi. Cytotrophoblasts expressing LF(1) were associated with large numbers of HLA-DR-reactive macrophages. Anti-LF(1) monoclonal antibodies reacted with 2 distinct tryptic fragments of lactoferrin, and these reactivities were not affected by treatment with N-glycanase.

CONCLUSION

Placental cytotrophoblasts express unique epitopes of lactoferrin (LF(1)). Such expression is increased in the presence of activated macrophages. This expression could be an extraembryonic response to inflammation and maternal allogeneic recognition as an effort to protect trophoblastic cells. The LF(1) epitopes might represent conserved polypeptide epitopes on 2 homologous lobes of lactoferrin.

Expression of MHC class II antigens by placental villi: no relationship with villitis of unknown origin

The aim of the study was to determine whether immunoreactivity to major histo-compatibility complex (MHC) class II antigens studied by immunohisto-chemistry could be used reliably to define villitis lesions in placenta. Eighteen placental sections with villitis and 32 without, as determined in a careful observer reproducibility study, were immunolabelled with a monoclonal antibody to monomorphic determinants of MHC class II antigens (CR3/43), using a standard avidin-biotin peroxidase technique. Placentas with villitis were found to express MHC class II antigens. However, some showed no immunoreactivity. Occasional villi unaffected by villitis, including those near placental infarcts, also expressed MHC class II antigens. The study therefore showed that immunohistochemistry cannot be used to define villitis of unknown aetiology. It provides further evidence of the difficulties that can arise when immunohistochemistry conflicts with previous light microscopy findings.

Maternal cells in chorionic villi from placentae of normal and abnormal human pregnancies

PROBLEM

We asked if activated macrophages and CD4 positive T lymphocytes in placental chorionic villi with villitis were of maternal or fetal origin.

METHOD

We employed a double antibody immunocytochemical technique on placental sections from three normal and four abnormal pregnancies with small-for-gestational-age infants. All studied placentae were mismatched for the maternal-fetal HLA-DRw 52 antigen. Areas of immunopathology were identified by using a monoclonal antibody to a monomorphic determination on HLA-DR, and the origin of immunological cells in areas of immunopathology was identified by using a monoclonal antibody to a polymorphic determinant on HLA-DRw 52.

RESULTS

We used a double antibody technique that employed monoclonal antibodies to HLA-DR and HLA-DRw 52 antigens and placentae that were mismatched for the maternal-fetal HLA-DRw 52 antigen. We found that the vast majority of immunological cells within villi with inflammation were of maternal origin. Quantitative studies showed that between 75 and 100% of the cells in normal as well as in abnormal pregnancies were of maternal origin, and that abnormal pregnancies had a significantly higher percentage of villi with maternal cellular infiltrates.

CONCLUSION

Our data show unequivocally that cells in areas of placental immunopathology are predominantly of maternal origin, and that abnormal pregnancies are associated with significantly more villi containing immunological cells of maternal origin.

Observer reliability in assessing villitis of unknown aetiology

AIMS

To evaluate observer variation in diagnosis of villitis of unknown aetiology.

METHODS

Fifty haematoxylin and eosin stained sections were circulated to three pathologists who were asked to assess if villitis was present. These slides, with an additional 20, were recirculated and reassessed by the same pathologists.

RESULTS

Intra-observer agreement was 84.7% (range 74--92%) and interobserver agreement was 81%. A conjoint review by the three pathologists revealed that sources of differences included the overlooking of isolated single or small numbers of affected villi, the difficulty in assessing stromal cellularity close to infarcted parenchyma, and apparent stromal hypercellularity in immature villi.

CONCLUSIONS

Experienced pathologists can show a significant interobserver variation in assessing villitis of unknown aetiology. Future studies on villitis of unknown aetiology should address the problem of observer reproducibility of diagnosis.

Immunological studies of lactoferrin in human placentae

Lactoferrin (LF) and transferrin (Trf) are glycoproteins with strong affinities for ferric ions. Human syncytiotrophoblastic membranes analyzed by enzyme linked immunosorbent assay (ELISA) and immunoblotting were negative with monoclonal and polyclonal antibodies to LF. Immunohistological studies of 35 normal placentae showed that LF was absent from the trophoblast basement membranes, stroma and fetal stem vessel endothelium, but positive cells were occasionally noted in intervillous spaces and fetal stem vessels. In contrast, many LF-positive cells were identified within areas of immunopathology identified by the presence of T cells, HLA-DR-positive macrophages and platelets. Double-antibody experiments showed that the LF-positive cells in these areas reacted with CD15 and CD16 monoclonal antibodies (mAbs), indicating that the cells were polymorphonuclear neutrophils (PMN). PMN from peripheral blood analyzed by flow cytometry and immunocytology also showed reactivities with anti-LF, CD15 and CD16 and we consistently found that circulating PMN reacted better than placental PMN with antibodies to MHC class I antigens and gp 100, (CD67), which is a neutrophil activation marker. PMN adherent within placentae had no detectable MHC class I or CD67 antigens. These findings suggest PMN adherent to placental tissues down-regulate or alter plasma membrane markers. LF appears to play a role in placental inflammation, for LF-positive cells were significantly enriched in areas of immunopathology.

Microvascular perturbations in human allografts: analogies in preeclamptic placentae

The thromboresistance of endothelium is maintained as long as natural anticoagulant pathways are functionally present on endothelial plasma membranes. The principal anticoagulant pathways in human hearts and kidneys are thrombomodulin (TM) and heparan sulfate proteoglycan-antithrombin III (HSPG-ATIII). The downregulation of TM or the loss of ATIII is associated with fibrin deposition. This sequence of events occurs when stable allografts of hearts or kidneys become unstable or rejected. Human placentae do not contain the HSPG-ATIII natural anticoagulant pathway, but the TM system is uniformly represented on endothelium of normal chorionic villi. However, many villi in placentae from preeclamptic pregnancies contain thrombomodulin-negative endothelium, and these vessels contain fibrin thrombi. These thrombi compromise blood flow through the placental microcirculation and are associated with ischemic changes either with or without the presence of cellular infiltrates.

Immunohistochemical study of the inflammatory infiltrate in villitis of unknown etiology. A qualitative and quantitative analysis

Villitis is characterized by an inflammatory infiltrate within the substance of the chorionic villi. Quantitative and qualitative analyses of the mononuclear infiltrate in areas of villitis were performed in placentas with villitis of unknown etiology (VUE). We used a panel of monoclonal antibodies and immunoperoxidase technique in paraffin sections from 17 placentas with VUE and 8 without VUE. Macrophages followed by T lymphocytes were the predominant inflammatory cells in areas of villitis in virtually all cases. B lymphocytes were not observed and monocytes were present usually in small number in 58 per cent of the cases. Mononuclear cells which expressed HLA-DR antigens were found in 75 per cent of the cases. In areas of villitis with trophoblastic necrosis, we found monocytes and some T lymphocytes adhered to them. These cells apparently had migrated from the maternal circulation. We suggest that in areas of villitis with destruction of the trophoblast and its basal membrane the inflammatory infiltrate might have a mixture of fetal and maternal cells. The maternal monocytes and T lymphocytes might be attracted to these sites of trophoblastic necrosis and activated due to exposure to fetal MHC antigens of the villous stroma.

Fetal stem vessel endothelial changes in placentae from normal and abnormal pregnancies

Areas of immunocytochemically defined immunopathology are common in placentae from secondary recurrent spontaneous aborters. Endothelial cells of affected villi were found to be negative for the thrombomodulin natural anticoagulant pathway, and these cells were reactive with monoclonal antibodies to tissue factor and fibrin. Endothelial cells of normal villi adjacent to affected villi also were negative for thrombomodulin and positive for tissue factor and fibrin, but endothelium of normal villi distant to affected villi were thrombomodulin-positive, tissue factor-negative, and did not contain deposits of fibrin. These findings indicate that a loss of endothelial anticoagulant activity precedes the onset of immunopathology. This observation, coupled with the findings that fetal stem vessels normally lack the heparan sulfate proteoglycan-antithrombin III natural anticoagulant pathway and that normal cord blood contains a heparin-like anticoagulant, suggest that a failure of endothelial anticoagulation may be a primary event in the development of placental immunopathology.

Current topic: trophoblastic pathology

Selected topics in trophoblastic pathology which are arousing current interest are briefly reviewed. These include the concept of villitis as a non-specific immunological lesion, changing views on the distinction between partial and complete hydatidiform mole, a reconsideration of the role of inadequate placentation in pregnancy hypertension and in spontaneous abortion, the significance of confined placental mosaicism and the importance of infection and defective collagen synthesis in premature rupture of the membranes.

Tissue factor in chronic villitis of unestablished etiology

We have previously reported the presence in normal human placentae of coagulation, macrophages and helper T lymphocytes in inflammatory foci known as villitis of unestablished etiology. In order to investigate the link between coagulation and immunity, we have studied fetal stem vessel endothelium for tissue factor, which is made available by cytokines and activates coagulation via the extrinsic pathway. We found that fetal stem vessel endothelial cells of normal chorionic villi did not react with antibody to tissue factor. Normal placentae contain small numbers of villitis areas and endothelium in these areas was reactive with antibody to tissue factor. Endothelial tissue factor reactivity was more prominent in placentae from secondary recurrent spontaneous aborters and these placentae have greatly increased numbers of villitis areas. The tissue factor availability on fetal stem vessel endothelium may result from immunologically mediated cytokine release. The net effect of these reactions is the presence of lymphocytes, macrophages, coagulation, necrosis and vasculitis in villitis.

HIV proteins in normal human placentae

Cryostat sections of human normal term placentae were studied for evidence of immunopathology by using antibodies to lymphocytes, macrophages, platelets, and coagulation factors. Areas of so-called chronic villitis of unestablished etiology were identified in all placentae. The same tissues were examined for HIV protein antigens gp120, p17, p24, and gp41. No evidence for gp41 was found. Antigens gp120 and p17 were identified in normal chorionic villi in vimentin-positive fibroblast-like cells and in endothelium, respectively. Antigen p24 was localized to HLA-DR positive cells that morphologically resembled macrophages in areas of villitis. The distribution of gp120 and p17 was similar to that observed for tissue factor. These findings prompted speculation that retroviral proto-oncogenes that are known to encode for certain placental receptors could be involved in the presentation of tissue factor, and that gp120 may be a hitherto unrecognized immunobiological mechanism for the blockade of CD4 on maternal lymphocytes if and when such cells gain entrance to chorionic villi.

Cytoskeletal proteins in chronic villitis of unestablished etiology

Cytoskeletal proteins (i.e., cytokeratins, vimentin, actin, and desmin) are normally present in the placental chorionic villi and are related to the maintenance of the villous shape, and to the ability of the villi to contract and permit a normal blood flow. In areas of villitis of unestablished etiology, the normal reactivity of these proteins in the villous core around fetal stem vessels disappears, and an increased number of cytokeratin positive cells is identified. The vascular damage in stem villi with villitis could develop ischemia in the villous tree promoting cytotrophoblast proliferation. Increased numbers of these areas could be related with the appearance of abnormal pregnancies.

Anchoring villi in human placental basal plate: lymphocytes, macrophages and coagulation

We have studied basal plates in 25 normal placentae, ten placentae from primary aborters and six placentae from secondary aborters for leukocyte markers and coagulation components. Anchoring villi contained class II MHC antigen-reactive (HLA-DR, DP and DQ) macrophages and T-lymphocytes of the helper (CD4) phenotype. Antibodies to coagulation components revealed the presence of tissue factor, factor IX and fibrin in and around anchoring villi. Immune cells and coagulation components in anchoring villi were more frequently identified in secondary aborters. These findings provide morphological support for an idea that there is a linkage between the presence of immune cells and coagulation in anchoring villi of the basal plate. Helper T-lymphocytes can activate macrophages which release cytokines that activate coagulation through the extrinsic pathway. We envision this as an expression of maternal allogeneic recognition of extraembryonic tissues in the basal plate.

Concordant expression of tissue factor and class II MHC antigens in human placental endothelium

Villitis of unestablished etiology is a placental lesion frequently associated with high risk pregnancies: it is also found in placentae from normal term pregnancies. The etiology of the lesion is unknown. Vasculitis and thrombosis have been described in villitis areas of placentae from normal and high risk pregnancies. We asked if fetal stem vessel endothelium in villitis lesions expresses MHC class II antigens, and if this is associated with a thrombogenic activity of these vessels. We found that endothelium of fetal stem vessels in villitis areas was usually MHC class II (HLA-DR, DP and DQ) reactive. Reactivity of fetal stem vessel endothelium for MHC class II antigens was associated with the presence of tissue factor reactivity and the absence of thrombomodulin reactivity. These changes on endothelial plasma membranes can promote intravascular coagulation, ischemic necrosis, vasculitis and other histological changes characteristic of villitis.

Immunohistologic evidence that villitis in human normal term placentas is an immunologic lesion

Villitis of unestablished origin is a lesion in placentas from normal and high-risk pregnancies. We have studied villitis areas in 25 normal term placentas for immune cells, coagulation components, and endothelial markers. Villitis areas were filled with activated (HLA-DR, HLA-DP, and HLA-DQ reactive) macrophages. B lymphocytes were not identified, and T lymphocytes were of the helper (CD4) phenotype. Antibodies to coagulation components revealed perivascular and trophoblastic basement membrane deposits of factor IX, increased numbers of platelets, and fetal stem vessels that did not react with endothelial markers. These findings suggest helper T lymphocytes activate macrophages that mediate coagulation activation and alter endothelium. This combination of immunologic events results in tissue changes that are histologically diagnosed as villitis. It is not known what triggers these immunologic events, but the finding of villitis in normal placentas suggests the causative factor(s) is present in all pregnancies.

Human placentae: view from an immunological bias

Human placentae form a major part of the interface between maternal and embryonic tissues. Placentae are morphologically complex organs that are lined with a trophoblastic tissue that provides anatomical constraints over which flows maternal blood. Maternal antibodies and immune cells thus contact the lining syncytiotrophoblast, and this paper considers some of the antigens that maternal immune components might encounter. The antigens have been grouped into those of unknown function, major histocompatibility antigens, and antigens of suspected function. The paper also details certain antigens of cytotrophoblasts. We discuss consequences of maternal immune responses to extraembryonic antigens and present evidence for amplification of these responses through the activation of hemostasis and fibrinolysis within the placenta and placental bed.