Identification of a soluble Fc gamma-binding molecule (annexin II) in human serum using a competitive ELISA

Effect of Annexin A2 on prognosis and sensitivity to immune checkpoint plus tyrosine kinase inhibition in metastatic renal cell carcinoma

BACKGROUND

Immunotherapy (IO) plus tyrosine kinase inhibitor (TKI) therapy is the first-line recommendation for advanced renal cell carcinoma (RCC), but no biomarker has been approved for it. Annexin A2 (ANXA2) can induce immune escape in tumors.

METHODS

Two independent cohorts of advanced RCC treated by IO + TKI were utilized for survival analysis (ZS-MRCC, n = 45; Javelin-101, n = 726). ANXA2 expression was determined by RNA-sequencing. The impact of ANXA2 on the tumor microenvironment was assessed by RNA-sequencing, flow cytometry and immunohistochemistry in two localized RCC datasets (ZS-HRRCC, n = 40; TCGA-KIRC, n = 530).

RESULTS

ANXA2 was upregulated in non-responders of IO + TKI therapy (p = 0.027). High-ANXA2 group showed poor progression-free survival (PFS) in both the ZS-MRCC cohort (HR, 2.348; 95% CI 1.084-5.085; P = 0.025) and the Javelin-101 cohort (HR, 1.472; 95% CI 1.043-2.077; P = 0.027). Multivariate Cox regression determined ANXA2 as an independent prognostic factor (HR, 2.619; 95% CI 1.194-5.746; P = 0.016). High-ANXA2 was correlated with decreased proportion of granzyme B+ CD8+ T cells (Spearman's ρ = - 0.40, P = 0.01), and increased TIM-3+ (Spearman's ρ = 0.43, P < 0.001) and CTLA4+ (Spearman's ρ = 0.49, P < 0.001) tumor-infiltrating lymphocytes. A random forest (RF) score was further build by integrating ANXA2 and immune genes, which stratified patients who would benefit from IO + TKI therapy (low-RF score, IO + TKI vs TKI, HR = 0.453, 95% CI 0.328-0.626; high-RF score, IO + TKI vs TKI, HR = 0.877, 95% CI 0.661-1.165; interaction P = 0.003).

CONCLUSIONS

Upregulated ANXA2 was associated with poor PFS and therapeutic resistance in RCC treated by IO + TKI therapy, and related with T cell exhaustion. The integrated RF score could stratify patients who would benefit from IO + TKI therapy.

Diagnostic Value of Plasma Annexin A2 in Early-Stage High-Grade Serous Ovarian Cancer

Ovarian cancer (OC) is commonly diagnosed at advanced stage when prognosis is poor. Consequently, there is an urgent clinical need to identify novel biomarkers for early detection to improve survival. We examined the diagnostic value of the calcium phospholipid binding protein annexin A2 (ANXA2), which plays an important role in OC metastasis. Annexin A2 plasma levels in patients with high grade serous OC (n = 105), benign ovarian lesions (n = 55) and healthy controls (n = 143) were measured by ELISA. Annexin A2 levels were found to be significantly increased in patients with stage I (p < 0.0001) and stage IA (p = 0.0027) OC when compared to healthy controls. In the logistic regression models followed by receiver operating characteristics (ROC) curve analyses, plasma annexin A2 showed 46.7% sensitivity at 99.6% specificity in distinguishing stage IA OC patients from healthy controls and 75% sensitivity at 65.5% specificity in the diagnosis of stage IA versus benign ovarian tumors. In the diagnosis of stage IA OC versus normal controls, the combination of plasma annexin A2 and CA125 showed 80% sensitivity at 99.6% specificity (AUC = 0.970) which was significantly higher than for CA125 (53.3% sensitivity at 99.6% specificity; AUC = 0.891) alone. The diagnostic accuracy in distinguishing stage IA OC from benign ovarian disease when combining annexin A2 and CA125 (71.4% accuracy at 100% sensitivity) was almost twice as high compared to CA125 (37.1% accuracy at 100% sensitivity) alone. In conclusion, annexin A2 in combination with CA125 has potential as a biomarker for the early detection of OC and to predict malignancy in patients with ovarian lesions, warranting further investigations.

Annexin A2 promotion of hepatocellular carcinoma tumorigenesis via the immune microenvironment

Hepatocellular carcinoma (HCC) is the most common primary liver cancer with a dismal prognosis, especially when diagnosed at advanced stages. Annexin A2 (ANXA2), is found to promote cancer progression and therapeutic resistance. However, the underlining mechanisms of ANXA2 in immune escape of HCC remain poorly understood up to now. Herein, we summarized the molecular function of ANXA2 in HCC and its relationship with prognosis. Furthermore, we tentatively elucidated the underlying mechanism of ANXA2 immune escape of HCC by upregulating the proportion of regulatory T cells and the expression of several inhibitory molecules, and by downregulating the proportion of natural killer cells and dendritic cells and the expression of several inhibitory molecules or effector molecules. We expect a lot of in-depth studies to further reveal the underlying mechanism of ANXA2 in immune escape of HCC in the future.

Annexin A2 (ANX A2): An emerging biomarker and potential therapeutic target for aggressive cancers

ANX A2 is an important member of annexin family of proteins expressed on surface of endothelial cells (ECs), macrophages, mononuclear cells and various types of cancer cells. It exhibits high affinity binding for calcium (Ca⁺⁺ ) and phospholipids. ANX A2 plays an important role in many biological processes such as endocytosis, exocytosis, autophagy, cell-cell communications and biochemical activation of plasminogen. On the cell surface ANX A2 organizes the assembly of plasminogen (PLG) and tissue plasminogen activator (tPA) for efficient conversion of PLG to plasmin, a serine protease. Proteolytic activity of plasmin is required for activation of inactive pro-metalloproteases (pro-MMPs) and latent growth factors for their biological actions. These activation steps are critical for degradation of extracellular matrix (ECM) and basement proteins (BM) for cancer cell invasion and metastasis. Increased expression of ANX A2 protein/gene has been correlated with invasion and metastasis in a variety of human cancers. Moreover, clinical studies have positively correlated ANX A2 protein expression with aggressive cancers and with resistance to anticancer drugs, shorter disease-free survival (DFS), and worse overall survival (OS). The mechanism(s) by which ANX A2 regulates cancer invasion and metastasis are beginning to emerge. Investigators used various technologies to target ANX A2 in preclinical model of human cancers and demonstrated exciting results. In this review article, we analyzed existing literature concurrent with our own findings and provided a critical overview of ANX A2-dependent mechanism(s) of cancer invasion and metastasis.

Annexin A2 autoantibodies in thrombosis and autoimmune diseases

Antiphospholipid syndrome (APS) is an autoimmune disease characterized by arterial, venous or small-vessel thrombotic events, and recurrent miscarriages or fetal loss. APS diagnosis is based on the repeated detection of anti-phospholipid (PL) antibodies (Ab), typically associated with anti-β2 glycoprotein I (β2GPI)-Ab. Recent studies suggest that anti-β2GPI Ab activity involves a protein complex including β2GPI and annexin A2 (ANXA2). Anti-ANXA2 Ab recognizes this complex, and these Ab can effectively promote thrombosis by inhibiting plasmin generation, and by activating endothelial cells. Therefore, anti-ANXA2 Ab represent a new biomarker, which can be detected in up to 25% of APS patients. Moreover, anti-ANXA2 Ab have been detected, in thrombotic associated diseases including pre-eclampsia, in other autoimmune diseases, and in cancer.

Effects of novel vaccine/adjuvant complexes on the protective immunity against Eimeria acervulina and transcriptome profiles

SUMMARY. This study investigated the ability of two novel adjuvant formulations, QCDC (Quil A/cholesterol/DDA/ Carbopol) and QCDCR (QCDC/Bay R1005), in combination with a recombinant profilin vaccine, to modulate host protective immunity and to alter gene expression during experimental avian coccidiosis. Vaccination with profilin plus QCDCR significantly reduced the severity of intestinal lesions and increased mitogen-induced lymphocyte proliferation in infected chickens compared with immunization with profilin alone or profilin plus QCDC. Immunization with profilin plus QCDC or profilin plus QCDCR increased body weight gain but had no effect on fecal oocyst shedding of chickens infected with Eimeria acervulina compared with birds vaccinated with profilin alone. The results of global gene expression analysis revealed that, compared with PBS controls, (a) chickens vaccinated with profilin alone had 71 up-regulated and 56 down-regulated mRNA transcripts, (b) chickens immunized with profilin plus QCDC had 198 up-regulated and 247 down-regulated mRNAs, and (c) birds immunized with profilin plus QCDCR had 210 up-regulated and 267 down-regulated mRNAs. Compared with birds vaccinated with profilin alone, (a) chickens given profilin plus QCDC had 60 up-regulated and 104 down-regulated transcripts and (b) chickens immunized with profilin plus QCDCR had 103 up-regulated and 130 down-regulated mRNAs. Finally, chickens vaccinated with profilin plus QCDCR had 193 up-regulated and 204 down-regulated transcripts compared with birds given profilin plus QCDC. Biological function and network analysis revealed that the majority of altered transcripts were encoded by immune-related genes.

Annexin A2 in amniotic fluid: correlation with histological chorioamnionitis, preterm premature rupture of membranes, and subsequent preterm delivery

AIM

The aim of this study was to determine whether amniotic fluid levels of annexin A2, a phospholipid-binding protein that is abundant in amnion and regulates fibrin homeostasis, are associated with histological chorioamnionitis, preterm premature rupture of the membranes, and subsequent preterm delivery.

MATERIALS AND METHODS

Amniotic fluid was obtained from 55 pregnant women with preterm labor and/or preterm premature rupture of the membranes before 32weeks of gestation, and amniotic fluid levels of annexin A2 were measured with a sandwich enzyme-linked immunosorbent assay.

RESULTS

Amniotic fluid levels of annexin A2 in patients with histological chorioamnionitis was higher than that in the remainder (P=0.053), whereas amniotic fluid levels of annexin A2 in patients with preterm premature rupture of the membranes was significantly higher than that in the remainder (P=0.002). Amniotic levels of annexin A2 was a fair test (area under receiver-operator characteristic curve=0.679), and amniotic fluid levels of annexin A2>878.2ng/mL had a sensitivity of 68.8%, a specificity of 65.2%, a positive predictive value of 73.3%, and a negative predictive value of 60.0% for predicting delivery within 2weeks after amniotic fluid sampling. Furthermore, the combined use of amniotic fluid cut-off levels of 878.2ng/mL for annexin A2 and 13.3ng/mL for interleukin-8 improved the specificity (91.3%) and the positive predictive value (89.5%).

CONCLUSIONS

We identified amniotic fluid levels of annexin A2, especially in combination with amniotic fluid levels of interleukin-8, as a novel predictive marker for preterm delivery.

Analysis of the urine proteome in patients with pancreatic ductal adenocarcinoma

Pancreatic ductal adenocarcinoma (PDAC) accounts for over 213 000 deaths worldwide each year, largely due to late diagnosis. One of the risk factors for the development of PDAC is chronic pancreatitis (CP); the intense desmoplastic reaction makes differentiation between the two conditions extremely difficult. In order to identify biomarkers for noninvasive diagnosis, we performed 2-D DIGE analysis of urine samples from healthy individuals and patients with PDAC and CP. Despite considerable intersample heterogeneity, a total of 127 statistically valid (p<0.05), differentially expressed protein spots were detected, 101 of which were identified using MALDI-TOF MS. A number of these, including annexin A2, gelsolin and CD59 have already been associated with PDAC, however, their validation using immunoblotting proved challenging. This is probably due to extensive PTMs and processing thus indicating the need for raising specific antibodies for urinary proteins. Despite this, our study clearly demonstrates that urine is a valid source of noninvasive biomarkers in patients with pancreatic diseases.

Annexin A2 is a soluble mediator of macrophage activation

On the surface of the macrophage, annexin A2 tetramer (A2t) serves as a docking protein or recognition element for bacterial and viral pathogens. Plasma levels of free A2t have been reported to increase following infection, although the mechanistic significance of this observation is unclear. Although annexin A2 had generally been thought to play an anti-inflammatory role, soluble A2t stimulates MAP kinase activity in bone marrow stromal cells downstream of a recently cloned receptor. This raises the question of whether A2t activates human macrophages via MAP kinases and whether it might be capable of acting as an inflammatory mediator. To this end, human monocyte-derived macrophages were treated with soluble A2t and MAP kinase phosphorylation, p65 NF-kappaB activation, and inflammatory mRNA and protein levels were measured. It was found that A2t caused rapid phosphorylation of several MAP kinases, as well as translocation of p65 NF-kappaB to the nucleus. A2t stimulated the production of TNF-alpha, IL-1beta, and IL-6, as well as several members of the chemokine family within 24 h, which are capable of recruitment and/or activation of a broad range of leukocyte classes. Furthermore, A2t-activated macrophages demonstrated enhanced phagocytic ability for the ingestion of GFP-expressing Escherichia coli. These data are the first to suggest the participation of an annexin in microbial clearance, as well as the establishment of inflammation and the immune response, including the recruitment and activation of immune cells to the site of infection.

Annexins I and IV inhibit Staphylococcus aureus attachment to human macrophages

Annexins are a family of proteins that bind to phospholipids and carbohydrates in a calcium-dependent manner. They are present in a variety of body fluids. Previous studies have shown that annexins have anti-inflammatory activities for lipid A of Gram-negative bacteria. The present study investigated the effect of annexins on interaction between Gram-positive bacteria and immune cells such as macrophages. Annexins I and IV bound to lipoteichoic acids which are surface molecules on Gram-positive bacteria. Binding of annexins I and IV to whole Staphylococcus aureus (S. aureus) were observed and these bindings were inhibited by lipoteichoic acid from S. aureus. Moreover, annexins I and IV suppressed the attachment of S. aureus to phorbol 12-myristate 13-acetate-treated THP-1 cells (human macrophages). These results suggest that annexins I and IV have ligand specificities toward foreign substances, and that the annexins might have some anti-inflammatory property for Gram-positive bacteria.

Annexin expression in inflammatory myopathies

The pathogenesis of the inflammatory myopathies is still unclear, making their treatment largely empirical. Improved understanding of the molecular mechanisms of inflammatory muscle injury may, however, lead to the development of more specific immunotherapies. To elucidate a possible pathogenic contribution of calcium-binding proteins such as the annexins, we immunohistochemically investigated muscle biopsy specimens from patients with dermatomyositis (10 cases), polymyositis (9 cases), and inclusion-body myositis (4 cases), compared to control cases comprising sarcoid myopathy (3 cases), Duchenne muscular dystrophy (DMD; 4 cases), and normal muscle (3 cases). We found expression of annexins A1, A2, A4, and A6 in the vascular endothelium of all cases. Myofibers expressed annexins A5, A6, and A7 diffusely and weakly in the cytosol, whereas annexins A5 and A7 were also particularly localized to the sarcolemma. In the inflammatory myopathies, in areas of myonecrosis in DMD, and in granulomatous lesions of sarcoid myopathy, reactivity of annexins A1, A2, A4, A5, and A6 was observed in macrophages and T-lymphocytes. Whereas the latter annexins appear to be nonspecific indicators of activation, annexin A1 upregulation may represent endogenous anti-inflammatory mechanisms that merit further investigation.

No immunosuppressive effect of transfected annexin II

Annexin II (AXII) was reported to suppress lymphoproliferation and immunoglobulin production. To investigate this further, the AXII-negative human B-lymphoma cell line Raji was transfected with AXII followed by transfection with p11. Stable transfectants were generated. In vitro immunological effects of Raji, Raji-AXII and Raji-AXII-p11 were compared by using irradiated cells as stimulators for PBM in mixed lymphocyte reactions or by adding the supernatants to lymphoproliferation or ELISPOT cultures. None of the assays provided evidence of significant immunosuppression by AXII. Thus, expression of AXII or AXII plus p11 does not by itself give a cell immunosuppressive ability.

Fc receptors are critical for autoimmune inflammatory damage to the central nervous system in experimental autoimmune encephalomyelitis

Multiple sclerosis (MS) is simulated by various forms of experimental autoimmune encephalomyelitis, in which T cells, antibodies, cytokines and complementary factors interact with the central nervous system (CNS) myelin proteins and lead to inflammatory damage. We investigated the role of Fc receptors (FcRs), which link the cellular and humoral branches of the immune system, in myelin oligodendrocyte glycoprotein (MOG)-induced experimental autoimmune encephalomyelitis (EAE), using two different FcRgamma knockout DBA/1 mice. The first knockout were the FcRgamma chain-deficient mice, which lack FcgammaRI, FcgammaRIII and Fc(epsilon)RI, while the second knockout mice lack only FcgammaRII. The lack of FcgammaRII enhanced the disease susceptibility with associated increased CNS demyelination. While FcRgamma+/+ DBA/1 mice also developed pronounced CNS infiltration and myelin destruction, FcRgamma-/- littermates were protected despite initial peripheral autoimmune responses to MOG. In vitro analyses revealed equivalent potentials of fluid phase phagocytosis of myelin and MOG in bone-marrow macrophages derived from both FcRgamma+/+ and FcRgamma-/- mice, while MOG-immunoglobulin (Ig)G immune complexes were only internalized by FcRgamma+/+ macrophages. This was associated with cellular activation in FcRgamma+/+ but not FcRgamma-/- macrophages, as assessed by the activation of intracellular mitogen activated protein (MAP)-kinase signalling elements. We propose that protection from EAE in FcRgamma-deficient mice is due to the inefficient antigen processing/presentation of myelin proteins during the induction of secondary immune responses locally in the CNS, which leads to demyelination. This demonstrates the importance of FcR in the promotion of autoimmune inflammation of the CNS and highlights the therapeutic possibility of treatment of MS with FcR-directed modalities.

Annexins I and II bind to lipid A: a possible role in the inhibition of endotoxins

Annexins are Ca2+-dependent phospholipid-binding proteins with anti-inflammatory properties that are present on the surfaces of, and released from, certain cell types, such as leukocytes and secretory epithelia. The present study investigated the possibility that annexins may bind directly to bacterial endotoxin, inhibiting its interactions with cellular receptors or accessory binding proteins. An enzyme-linked immunoassay demonstrated calcium-dependent binding of low nanomolar concentrations of annexin-I and annexin-II p36/p11 heterotetramer to lipid A. In contrast, little or no annexin binding to lipopolysaccharide (LPS) was detected under similar conditions. LPS-binding protein binding to lipid A was blocked by annexin-I, and the annexins inhibited nitrite generation in RAW 264.7 cells induced by lipid A but not that induced by LPS. The data suggest that direct binding of annexins to lipid A may represent a mechanism for suppressing cellular and systemic responses to endotoxin.

Suppressive effect on lymphoproliferation in vitro by soluble annexin II released from isolated placental membranes

PROBLEM

Syncytiotrophoblast microvillous plasma membranes (StMPM) are potent suppressors of lymphoproliferation in vitro. We have previously shown that soluble annexin II (AII) is present at higher levels in retroplacental serum (RPS) than in peripheral serum, and that soluble AII has an immunosuppressive effect. The aims of this study were to determine whether AII can be released from StMPM and whether soluble AII from StMPM exerts any immunosuppressive effect.

METHOD OF STUDY

Isolated StMPM were incubated in growth medium for 18 hr and supernatants were prepared by ultracentrifugation. Soluble AII was detected by immunoblotting. StMPM, StMPM supernatant, and affinity-purified AII were analysed in a lymphoproliferation assay for immunomodulating activity.

RESULTS

AII heavy chain and its p11 light chain were detected both in StMPM supernatant and in RPS after removal of StMPM particles by ultracentrifugation. StMPM, StMPM supernatant, and purified AII suppressed lymphoproliferation in a dose-dependent manner. Absorption of AII from StMPM supernatant reduced the suppressive activity. The suppressive effect of StMPM supernatant and purified AII was completely reversed by heating at 100 degrees C for 30 min or by adding recombinant interleukin-2 at 100 units/ml. Although StMPM and affinity-purified AII suppressed the proliferation of lymphocytes from all donors tested, StMPM supernatant suppressed the proliferation of lymphocytes from 12 of 23 donors. Six of eight female non-suppressed donors were multiparae, whereas five of five female suppressed donors were nulliparae.

CONCLUSIONS

Annexin II is released by isolated placental membranes in vitro and is present in RPS, indicating in vivo release of AII at the fetomaternal interface, probably as AII heterotetramer. AII has immunosuppressive activity and may be important in fetal allograft survival.

Inhibition of phytohaemagglutinin-induced lymphoproliferation by soluble annexin II in sera from patients with renal cell carcinoma

Annexin II (AII) is a member of a family of glycoproteins which bind negatively charged phospholipids in a calcium-dependent manner. Annexins are membrane-associated proteins, expressed both in normal and malignant cells, but have also been detected as soluble molecules in serum and other body fluids. Because of their adhesive properties, it has been suggested that annexins play a role in the metastatic process. An ELISA was established for quantification of soluble AII. Within-run variation was 5.2-10.4% and run-to-run variation 12.4-15.6%. Soluble AII was detected in all sera studies. A strongly positive serum was arbitrarily given the value 100 AII units and used as reference serum. The mean level in sera from 20 normal blood donors was 49 (SE 5.6) AII units. Sera from peripheral blood of five patients with renal cell carcinoma and sera from blood obtained from the renal vein of the same patients contained 47 (SE 20) and 83 (SE 28) AII units, respectively. In two patients, AII levels were increased in renal vein serum as compared with peripheral blood serum. Interestingly, in both cases, and in none of the three remaining cases, phytohaemagglutinin-stimulated lymphoproliferation was suppressed by renal vein serum as compared with peripheral blood serum. Affinity absorption of AII from the renal vein sera with increased AII levels strongly reduced their immunosuppressive activity. Addition of affinity-purified AII to cell cultures suppressed lymphoproliferation. These data show that the level of AII is markedly increased in renal vein sera from some patients with renal cell carcinoma, suggesting that AII may be locally released in vivo. The study also demonstrates an immunosuppressive effect of soluble AII in vitro. We speculate that soluble AII released by the tumour has immunosuppressive properties. This study identifies soluble AII as a novel immunosuppressive factor in sera from patients with renal cell carcinoma. A further study including a larger number of patients is currently in progress, in order to investigate the pathological significance of this finding.

Human placental Fc gamma-binding proteins in the maternofetal transfer of IgG

Annexin II, a member of the annexin family of Ca2+ and phospholipid binding proteins, is present in human placenta. Placental annexin II has low affinity FcR activity, and is present as a heterotetramere on syncytiotrophoblast apical cell membrane extracellular surface. In addition to annexin II, transmembraneous leukocyte FcRIII is present on syncytiotrophoblast apical membrane. Either one, or both molecules may mediate the binding of IgG and thereby facilitate its transport through the syncytiotrophoblast layer. However, the presence of other maternal plasma proteins in syncytiotrophoblasts that are not transported to the human fetus is suggestive of nonspecific fluid phase endocytosis. The MHC class I like FcR, similar to the receptor found in neonatal rodent intestine, FcRn, is present intracellularly in human syncytiotrophoblasts, as is its light chain beta 2-microglobulin. The hFcRn is not detected on the apical plasma membrane. The placental hFcRn co-localizes with IgG in syncytiotrophoblast granules. It is likely that hFcRn binds and transcytoses IgG through the syncytiotrophoblast. Protected transfer of IgG may occur within syncytiotrophoblast endocytotic vesicles prior to release in the villous stroma and subsequent translocation into the lumen of fetal stem vessels by uptake and transport in endothelial caveolae.

Surface annexin II on placental membranes of the fetomaternal interface

PROBLEM

The phospholipidbinding membrane protein annexin II has been demonstrated to possess FcR activity for IgG and has been localized to the outer part of the syncytiotrophoblast cell layer. The question has arisen whether annexin II is exposed on the surface of syncytiotrophoblast cells thus enabling it to take part in the transport of IgG across the maternal barrier.

METHOD

Syncytiotrophoblast microvillous plasma membranes were analyzed by flow cytometry for annexin II as well as established surface molecules. Fresh, fixed placental tissue was preincubated with antibodies to annexin II or known trophoblast surface molecules, and analyzed by confocal laser scanning microscopy.

RESULTS

Annexin II and its subunit p11 were expressed on the surface of the syncytiotrophoblast microvillous plasma membranes as were other established surface proteins (CD46, CD59, placental alkaline phosphatase), using both flow cytometry and confocal microscopy. Annexin was not detected on the surface of viable cultured trophoblast cells.

CONCLUSION

Annexin II is exposed on the surface of syncytiotrophoblast cells as a heterotetramer together with its light chain p11. It is exposed to maternal blood and may be instrumental in IgG transport across the placental barrier by binding.

Co-localization of the neonatal Fc gamma receptor and IgG in human placental term syncytiotrophoblasts

Transfer of maternal IgG through the human placenta furnishes the newborn with passive immunity to a number of infectious agents. The exact mechanism of this transfer is still unknown, but it is agreed that it involves active receptor-mediated transport. The neonatal Fc receptor is a major histocompatibility complex class I-like receptor originally identified in the intestines of newborn rodents. A similar receptor has recently been detected in human placental syncytiotrophoblasts. Using multilabeling fluorescence immunohistochemistry and confocal laser scanning microscopy, we found that the neonatal Fc receptor co-localizes with IgG and beta 2-microglobulin in granules of human placental syncytiotrophoblast. The Fc receptor is not detected on syncytiotrophoblast apical plasma membrane. Localization to the outermost cellular barrier between the fetal and maternal blood further strengthens the role of the Fc receptor in transplacental transport of IgG.