VE-cadherin endocytosis controls vascular integrity and patterning during development.

Tissue morphogenesis requires dynamic intercellular contacts that are subsequently stabilized as tissues mature. The mechanisms governing these competing adhesive properties are not fully understood. Using gain- and loss-of-function approaches, we tested the role of p120-catenin (p120) and VE-cadherin (VE-cad) endocytosis in vascular development using mouse mutants that exhibit increased (VE-cadGGG/GGG) or decreased (VE-cadDEE/DEE) internalization. VE-cadGGG/GGG mutant mice exhibited reduced VE-cad-p120 binding, reduced VE-cad levels, microvascular hemorrhaging, and decreased survival. By contrast, VE-cadDEE/DEE mutants exhibited normal vascular permeability but displayed microvascular patterning defects. Interestingly, VE-cadDEE/DEE mutant mice did not require endothelial p120, demonstrating that p120 is dispensable in the context of a stabilized cadherin. In vitro, VE-cadDEE mutant cells displayed defects in polarization and cell migration that were rescued by uncoupling VE-cadDEE from actin. These results indicate that cadherin endocytosis coordinates cell polarity and migration cues through actin remodeling. Collectively, our results indicate that regulated cadherin endocytosis is essential for both dynamic cell movements and establishment of stable tissue architecture.

p120-catenin regulates VE-cadherin endocytosis and degradation induced by the Kaposi sarcoma-associated ubiquitin ligase K5.

Vascular endothelial (VE)-cadherin undergoes constitutive internalization driven by a unique endocytic motif that also serves as a p120-catenin (p120) binding site. p120 binding masks the motif, stabilizing the cadherin at cell junctions. This mechanism allows constitutive VE-cadherin endocytosis and recycling to contribute to adherens junction dynamics without resulting in junction disassembly. Here we identify an additional motif that drives VE-cadherin endocytosis and pathological junction disassembly associated with the endothelial-derived tumor Kaposi sarcoma. Human herpesvirus 8, which causes Kaposi sarcoma, expresses the MARCH family ubiquitin ligase K5. We report that K5 targets two membrane-proximal VE-cadherin lysine residues for ubiquitination, driving endocytosis and down-regulation of the cadherin. K5-induced VE-cadherin endocytosis does not require the constitutive endocytic motif. However, K5-induced VE-cadherin endocytosis is associated with displacement of p120 from the cadherin, and p120 protects VE-cadherin from K5. Thus multiple context-dependent signals drive VE-cadherin endocytosis, but p120 binding to the cadherin juxtamembrane domain acts as a master regulator guarding cadherin stability.

Cadherin tales: Regulation of cadherin function by endocytic membrane trafficking.

Cadherins are the primary adhesion molecules in adherens junctions and desmosomes and play essential roles in embryonic development. Although significant progress has been made in understanding cadherin structure and function, we lack a clear vision of how cells confer plasticity upon adhesive junctions to allow for cellular rearrangements during development, wound healing and metastasis. Endocytic membrane trafficking has emerged as a fundamental mechanism by which cells confer a dynamic state to adhesive junctions. Recent studies indicate that the juxtamembrane domain of classical cadherins contains multiple endocytic motifs, or "switches," that can be used by cellular membrane trafficking machinery to regulate adhesion. The cadherin-binding protein p120-catenin (p120) appears to be the master regulator of access to these switches, thereby controlling cadherin endocytosis and turnover. This review focuses on p120 and other cadherin-binding proteins, ubiquitin ligases, and growth factors as key modulators of cadherin membrane trafficking.

Ankyrin-G Inhibits Endocytosis of Cadherin Dimers.

Dynamic regulation of endothelial cell adhesion is central to vascular development and maintenance. Furthermore, altered endothelial adhesion is implicated in numerous diseases. Therefore, normal vascular patterning and maintenance require tight regulation of endothelial cell adhesion dynamics. However, the mechanisms that control junctional plasticity are not fully understood. Vascular endothelial cadherin (VE-cadherin) is an adhesive protein found in adherens junctions of endothelial cells. VE-cadherin mediates adhesion through trans interactions formed by its extracellular domain. Trans binding is followed by cis interactions that laterally cluster the cadherin in junctions. VE-cadherin is linked to the actin cytoskeleton through cytoplasmic interactions with ß- and α-catenin, which serve to increase adhesive strength. Furthermore, p120-catenin binds to the cytoplasmic tail of cadherin and stabilizes it at the plasma membrane. Here we report that induced cis dimerization of VE-cadherin inhibits endocytosis independent of both p120 binding and trans interactions. However, we find that ankyrin-G, a protein that links membrane proteins to the spectrin-actin cytoskeleton, associates with VE-cadherin and inhibits its endocytosis. Ankyrin-G inhibits VE-cadherin endocytosis independent of p120 binding. We propose a model in which ankyrin-G associates with and inhibits the endocytosis of VE-cadherin cis dimers. Our findings support a novel mechanism for regulation of VE-cadherin endocytosis through ankyrin association with cadherin engaged in lateral interactions.

Resistance of a subset of red blood cells to clearance by antibodies in a mouse model of incompatible transfusion.

BACKGROUND: Alloimmunization to antigens on transfused red blood cells (RBCs) represents a major barrier to chronic transfusion. In extreme cases of multiple alloimmunization, clinicians may be faced with the decision of transfusing incompatible RBCs or risking death from lack of transfusion. The disastrous results of hemolytic transfusion reactions are well understood, and major pathways of clearance have been described. However, well described but poorly understood is the survival of a subset of incompatible donor RBCs during hemolysis, despite antibody binding. STUDY DESIGN AND METHODS: We utilize a tractable murine model of incompatible transfusion in which RBCs from transgenic donor mice expressing human glycophorin A (hGPA) are transfused into recipients passively immunized with anti-hGPA. RESULTS: As in humans, the majority of RBCs are cleared but a subset of incompatible donor RBCs persist in circulation, despite being bound by antibodies. Data contained herein reject the hypothesis that lack of clearance is due to insufficient antibody or overwhelming of phagocytic machinery; rather, we establish that surviving RBCs represent a distinct population resistant to clearance. CONCLUSIONS: These studies demonstrate that surviving RBCs during incompatible transfusion can represent a population that is resistant to clearance.

Partial tolerance of autoreactive B and T cells to erythrocyte-specific self-antigens in mice.

BACKGROUND: Breakdown of humoral tolerance to RBC antigens may lead to autoimmune hemolytic anemia, a severe and sometimes fatal disease. The underlying mechanisms behind the breakdown of humoral tolerance to RBC antigens are poorly understood. DESIGN AND METHODS: In order to study the pathogenesis of autoimmune hemolytic anemia, we developed a murine model with RBC-specific expression of a model antigen carrying epitopes from hen egg lysozyme and ovalbumin. RESULTS: Humoral tolerance was observed; this was not broken even by strong immunogenic stimulation (lysozyme or ovalbumin with adjuvant). Autoreactive CD4(+) T cells were detected by tetramer enrichment assays, but failed to activate or expand despite repeat stimulation, indicating a nonresponsive population rather than deletion. Adoptive transfer of autoreactive CD4(+) T cells (OT-II mice) led to autoantibody (anti-lysozyme) production by B cells in multiple anatomic compartments, including the bone marrow. CONCLUSIONS: These data demonstrate that B cells autoreactive to RBC antigens survive in healthy mice with normal immune systems. Furthermore, autoreactive B cells are not centrally tolerized and are receptive to T-cell help. As the autoreactive T cells are present but non-responsive, these data indicate that factors that reverse T-cell non-responsiveness may be central to the pathogenesis of autoimmune hemolytic anemia.

Biphasic clearance of incompatible red blood cells through a novel mechanism requiring neither complement nor Fcγ receptors in a murine model.

BACKGROUND: Antibody binding to red blood cells (RBCs) can induce potentially fatal outcomes, including hemolytic transfusion reactions (HTRs), hemolytic disease of the fetus and newborn, and autoimmune hemolytic anemia. The mechanism(s) of RBC destruction following antibody binding is typically thought to require complement activation and/or the involvement of Fcγ receptors (FcγRs). In the current report, we analyzed mechanisms of HTRs during incompatible transfusions of murine RBCs expressing human glycophorin A (hGPA) into mice with anti-hGPA. STUDY DESIGN AND METHODS: C3 and Fcγ receptor knockout, splenectomized, Fcγ receptor blocking antibody-treated, and clodronate-treated mice were passively immunized with anti-hGPA (10F7 or 6A7) and transfused with RBCs expressing the hGPA antigen. Posttransfusion blood and serum were collected and analyzed via flow cytometry and confocal microscopy. RESULTS: This HTR model results in both rapid clearance and cytokine storm. Neither complement nor FcγRs were required for RBC clearance; in contrast, FcγRs were required for cytokine storm. Circulating aggregates of hGPA RBCs were visible during the HTR. Splenectomy and phagocyte depletion by clodronate had no effect on acute RBC clearance; however, incompatible RBCs reentered over 24 hours in clodronate-treated mice. CONCLUSION: These data demonstrate a biphasic HTR, the first phase involving sequestration of incompatible hGPA RBCs and the second phase involving phagocytosis of sequestered RBCs. However, the mechanism(s) of phagocytosis in the second phase required neither C3 nor FcγRs. These findings demonstrate novel mechanistic biology of HTRs.

Generation of transgenic mice with antithetical KEL1 and KEL2 human blood group antigens on red blood cells.

BACKGROUND: KEL1, also known as "K", is one of the most immunogenic red blood cell (RBC) antigens. KEL2, also known as "k," differs from KEL1 by a single amino acid. Anti-Kell system antibodies can lead to significant adverse clinical outcomes in humans, including hemolytic complications in alloimmunized transfusion recipients or in infants of alloimmunized mothers. To provide a platform for in-depth immunologic studies of alloimmunization and subsequent sequelae, we generated transgenic mice expressing the human KEL1 or KEL2 antigens. STUDY DESIGN AND METHODS: Vectors were created in which cDNAs encoding either KEL1 or KEL2 were regulated by an erythroid specific ß-globin promoter and enhancer. Pronuclear microinjections were carried out into a C57BL6 background, and founder pups were identified by polymerase chain reaction and screened for expression by flow cytometry. RBC life span and antigen stability were assessed by dye labeling RBCs, transfusing into agammaglobulinemic (µMT) recipients, and tracking by flow cytometry. RESULTS: The expression of either KEL1 or KEL2 is RBC specific and first occurs on early RBC precursors. Both KEL1 and KEL2 RBCs have a normal circulatory life span and stable antigen expression. Expression of KEL1 or KEL2 does not result in altered levels of murine Kell, and resulting RBCs have normal hematologic variables. CONCLUSION: The KEL1 and KEL2 mice represent the first murine system of RBC immunity with antithetical antigens, allowing a more precise modeling of human RBC immunology in general and also a platform for development of novel therapeutics to prevent or minimize the dangers of RBC alloimmunization to the KEL1 and KEL2 antigens in particular.

Rapid clearance of transfused murine red blood cells is associated with recipient cytokine storm and enhanced alloimmunogenicity.

BACKGROUND: Fourteen-day stored red blood cells (RBCs) containing an RBC-specific transgenic antigen (HOD) induce a recipient proinflammatory cytokine storm and are significantly more immunogenic compared to fresh RBCs. Given that recipient mice clear transfused stored RBCs more rapidly than fresh RBCs, we hypothesized that rapid RBC clearance was associated with adverse transfusion outcomes. STUDY DESIGN AND METHODS: HOD RBCs were treated by two distinct methods known to lead to rapid posttransfusion RBC clearance: phenylhydrazine or heat. HOD antigen expression was analyzed on the treated cells before transfusion, and RBC recovery, recipient cytokine response, and recipient anti-HOD alloimmunization response were measured after transfusion. RESULTS: Phenylhydrazine and heat treatment each led to near complete RBC clearance in recipients by 24 hours posttransfusion, without significantly altering HOD antigen expression on the transfused RBCs. Recipients of phenylhydrazine- or heat-treated RBCs had elevated circulating levels of keratinocyte-derived chemokine/CXCL-1, monocyte chemoattractant protein-1, and interleukin-6 after transfusion. Furthermore, phenylhydrazine- or heat-treated RBCs were significantly more immunogenic than control RBCs, with a mean 25.1- and 10.3-fold enhancement, respectively, of anti-HOD alloimmunization magnitude by flow cytometric crossmatch. CONCLUSIONS: Three separate insults to RBCs (storage, phenylhydrazine, or heat treatment) result in rapid posttransfusion clearance, with a recipient proinflammatory cytokine storm and enhanced alloimmunogenicity. These data are consistent with the hypothesis that rapid clearance of RBCs is causally involved in these outcomes and suggest that human donor RBCs with favorable posttransfusion clearance profiles may be less immunogenic.

Alloimmunization against RBC or PLT antigens is independent of TRIM21 expression in a murine model.

Generation of alloantibodies to transfused RBCs can be a serious medical problem for patients who require chronic RBC transfusion therapy. Patients with sickle cell disease have a substantially increased rate of alloimmunization compared to other chronically transfused populations. A recent study has forwarded the hypothesis that a polymorphism in an immunoregulatory gene in close proximity to beta-globin (TRIM21 rs660) plays a role in the increased rates of RBC alloimmunization in sickle cell patients. In particular, it was hypothesized that rs660C/T decreases expression of TRIM21, resulting in loss of a negative feedback pathway in immune responses and increased RBC alloimmunization. To test the effects of TRIM21 expression on alloimmunization, we analyzed antibody responses to alloantigens on RBCs and platelets transfused into wild-type and TRIM21 KO mice. No significant increases were seen in the frequency or magnitude of humoral immunization to alloantigens on transfused RBCs or platelets in adult or juvenile TRIM21 KO recipients compared to wild-type controls. Moreover, recipient inflammation with poly (I:C) enhanced RBC alloimmunization to similar degrees in both TRIM21 KO and wild-type control recipients. Together, these data rule out the hypothesis that decreased TRIM21 expression enhances transfusion induced humoral alloimmunization, in the context of a reductionist murine model.

Transfusion of minor histocompatibility antigen-mismatched platelets induces rejection of bone marrow transplants in mice.

Bone marrow transplantation (BMT) represents a cure for nonmalignant hematological disorders. However, compared with the stringent conditioning regimens used when performing BMT to treat hematological malignancies, the reduced intensity conditioning regimen used in the context of nonmalignant hematological disorders leads to substantially higher rates of BMT rejection, presumably due to an intact immune system. The relevant patient population typically receives transfusion support, often including platelets, and the frequency of BMT rejection correlates with the frequency of transfusion. Here, we demonstrate that immunity to transfused platelets contributes to subsequent BMT rejection in mice, even when the BMT donor and recipient are MHC matched. We used MHC-matched bone marrow because, although immunity to transfused platelets is best characterized in relation to HLA-specific antibodies, such antibodies are unlikely to play a role in clinical BMT rejection that is HLA matched. However, bone marrow is not matched in the clinic for minor histocompatibility antigens, such as those carried by platelets, and we report that transfusion of minor histocompatibility antigen-mismatched platelets induced subsequent BMT rejection. These findings indicate previously unappreciated sequelae of immunity to platelets in the context of transplantation and suggest that strategies to account for minor histocompatibility mismatching may help to reduce the chance of BMT rejection in human patients.

Minor histocompatibility antigens on transfused leukoreduced units of red blood cells induce bone marrow transplant rejection in a mouse model.

When successful, human leukocyte antigen (HLA)-matched bone marrow transplantation with reduced-intensity conditioning is a cure for several nonmalignant hematologic disorders that require chronic transfusion, such as sickle cell disease and aplastic anemia. However, there are unusually high bone marrow transplant (BMT) rejection rates in these patients. Rejection correlates with the number of transfusions before bone marrow transplantation, and it has been hypothesized that preimmunization to antigens on transfused blood may prime BMT rejection. Using a novel mouse model of red blood cell (RBC) transfusion and major histocompatibility complex-matched bone marrow transplantation, we report that transfusion of RBC products induced BMT rejection across minor histocompatibility antigen (mHA) barriers. It has been proposed that contaminating leukocytes are responsible for transfusion-induced BMT rejection; however, filter leukoreduction did not prevent rejection in the current studies. Moreover, we generated a novel transgenic mouse with RBC-specific expression of a model mHA and demonstrated that transfusion of RBCs induced a CD8(+) T-cell response. Together, these data suggest that mHAs on RBCs themselves are capable of inducing BMT rejection. Cellular immunization to mHAs is neither monitored nor managed by current transfusion medicine practice; however, the current data suggest that mHAs on RBCs may represent an unappreciated and significant consequence of RBC transfusion.

Antigen loss from antibody-coated red blood cells.

Clinically significant signs and symptoms of hemolysis can result from the transfusion of crossmatch incompatible blood, the development of anti-red blood cell (RBC) alloantibodies posttransfusion, or the development of autoantibodies to RBCs (ie, autoimmune hemolytic anemia). However, a less understood and poorly appreciated phenomenon is when an antibody induces the loss of its target antigen without significant damage to the cells themselves. This has been referred to as "depressed antigen," "antigen suppression," "weakened antigenicity," and "antigen loss." This phenomenon has been observed for multiple blood group antigens on human RBCs, in addition to antigens on leukocytes, platelets, neurons, and neoplastic cells. This review discusses the published human case reports of antigen loss in each of the contexts described above and describes several experimental models, including whole animal models and in vitro culture systems. Our current understanding of the cellular and molecular mechanisms is discussed, and the role of antigen loss phenomena in both normal immune function and in disease states is reviewed.

The spleen plays a central role in primary humoral alloimmunization to transfused mHEL red blood cells.

BACKGROUND: Several differences exist between antigens on transfused red blood cells (RBCs) and other immunogens, including anatomical compartmentalization. Whereas antigens from microbial pathogens and solid organ transplants drain into local lymph nodes, circulating RBCs remain segregated in the peripheral circulation, where they are consumed by antigen-presenting cells (APCs) in the spleen and liver. Accordingly, it was hypothesized that the splenic APCs play a central role in primary alloimmunization to transfused RBCs. STUDY DESIGN AND METHODS: Recipient mice were splenectomized and transfused with transgenic RBCs expressing the membrane-bound hen egg lysozyme (mHEL) model RBC antigen. In some experiments, mHEL-specific CD4+ T cells were adoptively transferred into recipient mice to allow investigation of helper T-cell responses. Unmanipulated or sham-splenectomized mice served as controls. Recombinant murine cytomegalovirus expressing mHEL (mHEL-MCMV) was used as a control non-RBC immunogen. Humoral responses were measured by mHEL-specific enzyme-linked immunosorbent assay and flow cytometric­based RBC cross-match. RESULTS: Control animals synthesized detectable anti-HEL immunoglobulin (Ig)G after a single mHEL RBC transfusion. mHEL-specific CD4+ T cells underwent robust expansion, and adoptive transfer of CD4+ T cells resulted in a 1000-fold increase in anti-HEL IgG. In contrast, minimal anti-HEL IgG was detectable in splenectomized mice, mHEL-specific CD4+ T cells did not proliferate, and adoptive transfer did not increase anti-HEL IgG. However, anti-HEL IgG response after exposure to mHEL-MCMV was equivalent in control and splenectomized mice. DISCUSSION: Together, these findings illustrate the distinct properties of transfused RBCs as immunologic stimuli, with the spleen playing a critical role in primary RBC alloimmunization at the level of CD4+ T-cell activation.

An autoanti-Kp b immunoglobulin M that simulates antigen suppression.

BACKGROUND: Patients may present with an antibody against a blood group antigen, a negative direct antiglobulin test (DAT), and a null phenotype. Typically, this represents an alloantibody in a null individual. However, on occasion, the antibody disappears coincident with conversion to a positive red blood cell (RBC) phenotype. This has been called antigen loss, antigen suppression, or weakened antigenicity. Herein, a unique serologic profile that mimics this pattern, when in fact antigen loss did not occur, is described. STUDY DESIGN AND METHODS: RBCs and serum were analyzed using a gel microtyping system and flow cytometry. Genomic DNA was amplified by polymerase chain reaction and sequenced. RESULTS: Initially, an anti-Kp(b) was detected in MTS gel, RBCs typed K-k-Kp(b-), and the DAT was negative for immunoglobulin G (IgG). Later, the anti-Kp(b) disappeared and RBCs phenotyped as K-k+Kp(b+). Analysis of initial specimens by flow cytometry identified an immunoglobulin M (IgM) anti-Kp(b) with a positive IgM-specific DAT; eluates contained an anti-Kp(b) at immediate spin. Supporting the presence of the Kell glycoprotein, RBCs agglutinated with anti-Js(b). Sequencing showed homozygosity for Kp(b) with no mutations surrounding the Kp(b) polymorphism. CONCLUSION: In antigen loss, antibody masking is excluded by a negative DAT. However, because typical DAT reagent does not detect IgM, such reasoning was inaccurate in the current case. In addition, an anti-Kp(b) resulted in RBCs typing k-, even though no anti-k was detected. Overall, this case suggests that an IgM may mask adjacent epitopes and illustrates the potential to mistake a non-IgG autoantibody as antigen loss.

Cytokine storm in a mouse model of IgG-mediated hemolytic transfusion reactions.

Cytokines are hypothesized to play a central role in the pathophysiology of IgG-mediated hemolytic transfusion reactions (HTRs), and deeper understanding is required for improving therapy for these events. After establishing well-defined mouse models of HTRs, we tested whether cytokines were involved. Red blood cells (RBCs) from human glycophorin A transgenic (hGPA-Tg) or wild-type (WT) mice were transfused into non-Tg recipients passively immunized with monoclonal antibodies (Mabs). Only transfusions of incompatible RBCs induced IgG-mediated HTRs, exemplified by rapid clearance and hemoglobinuria. Very high plasma levels of monocyte chemoattractant protein-1 (MCP-1) and interleukin-6 (IL-6), and lower levels of tumor necrosis factor-alpha (TNF-alpha), were induced after incompatible transfusion. No significant changes in IL-10, IL-12, or interferon-gamma (IFN-gamma) levels were observed. The proinflammatory cytokines elaborated in this in vivo mouse model are also implicated in the systemic inflammatory response syndrome (SIRS) and confirm the hypothesis that cytokine storm occurs as a result of HTRs.

Nonhemolytic antigen loss from red blood cells requires cooperative binding of multiple antibodies recognizing different epitopes.

Transfusion of crossmatch-incompatible red blood cells (RBCs) can result in antibody-mediated hemolysis. However, in some patients, crossmatch-incompatible RBCs lose the incompatible antigen from their surface, and then circulate normally ("antigen loss"). Although antigen loss has been reported in the settings of autoimmune hemolytic anemia and transfusion of crossmatch-incompatible RBCs, mechanistic understanding of this phenomenon is limited. Using an in vivo murine model of antigen loss, we report that, unlike polyclonal antisera, monoclonal antibodies did not induce antigen loss. However, the combination of 2 monoclonal antibodies that recognized separate epitopes on the same antigen induced antigen loss. This was not due to an increased number of Fc domains bound to the cell surface, because antigen loss still occurred when combining intact monoclonal IgG and F(ab')2 fragments recognizing different epitopes. Together, these data lead to the hypothesis that antigen-antibody crosslinking is required for nonhemolytic antigen loss to occur.