Harmful effects of transfusion of older stored red blood cells: iron and inflammation

Retrospective studies suggest that the transfusion of older, stored red blood cells (RBCs) may be associated with increases in mortality, serious infections, multiorgan failure, thrombosis, and hospital length of stay. Our research is based on the overarching hypothesis that the adverse effects associated with transfusion of older, stored RBCs result from the acute delivery of hemoglobin iron to the monocyte-macrophage system. To test this "iron hypothesis," we are recruiting healthy human volunteers to donate double, leukoreduced, RBC units. We then transfuse them with one autologous fresh unit (i.e., after 3-7 days of storage) and one older, stored unit (i.e., at 40-42 days of storage). The primary study outcome will compare laboratory iron measures and proinflammatory cytokines after transfusion of fresh or older, stored RBCs. Similar studies using allogeneic RBC transfusions will be performed in chronically transfused patients with either sickle cell disease or β-thalassemia. Although prospective, randomized studies will ultimately determine the existence of adverse effects from transfusing older, stored RBCs, our goal is to determine the mechanism(s) for this potential effect.

Transfusion of fresh murine red blood cells reverses adverse effects of older stored red blood cells

BACKGROUND

Although a subset of recent studies have suggested that red blood cell (RBC) storage length is associated with adverse patient outcomes, others have shown no such relationship. Adults may be transfused with RBC units of different storage lengths, and existing studies do not take into consideration that fresh RBCs may alter responses to concurrently transfused stored RBCs. To test this possibility, we utilized a murine model and investigated transfusion outcomes of fresh, stored, or fresh-plus-stored RBCs.

STUDY DESIGN AND METHODS

Fresh, 14-day-stored or fresh plus 14-day-stored leukoreduced RBCs from HOD-transgenic donors (with RBC-specific expression of hen egg lysozyme, ovalbumin, and human Duffy(b)) were transfused into naïve C57BL/6 recipients. Serum cytokines and anti-HOD alloimmunization were evaluated after transfusion.

RESULTS

In six of six experiments (n = 90 mice total), a proinflammatory serum cytokine storm of interleukin-6, keratinocyte-derived chemokine/CXCL1, and monocyte chemoattractant protein-1 was observed in transfusion recipients of stored but not fresh RBCs, along with high degrees of anti-HOD alloimmunization. However, concurrent transfusion of fresh HOD RBCs along with stored HOD RBCs significantly decreased these adverse outcomes (p < 0.05).

CONCLUSIONS

These results are consistent with fresh murine HOD RBCs losing protective properties during storage, and introduce a previously unrecognized variable in RBC storage studies. If translatable to humans, uniform "old blood" groups may be needed in future clinical studies to more accurately investigate the biologic effects of older RBC units.

Discriminative accuracy of novel and traditional biomarkers in children with suspected appendicitis adjusted for duration of abdominal pain

OBJECTIVES

The objective was to assess the accuracy of novel and traditional biomarkers in patients with suspected appendicitis as a function of duration of symptoms.

METHODS

This was a prospective cohort study, conducted in a tertiary care emergency department (ED). The authors enrolled children 3 to 18 years old with acute abdominal pain of less than 96 hours and measured serum levels of interleukin-6 (IL-6), interleukin-8 (IL-8), C-reactive protein (CRP), white blood cell (WBC) count, and absolute neutrophil count (ANC). Final diagnosis was determined by histopathology or telephone follow-up. Trends in biomarker levels were examined based on duration of abdominal pain. The accuracy of biomarkers was assessed with receiver operating characteristic (ROC) curves. Optimal cut-points and test performance characteristics were calculated for each biomarker.

RESULTS

  Of 280 patients enrolled, the median age was 11.3 years (interquartile range [IQR] = 8.6 to 14.8), 57% were male, and 33% had appendicitis. Median IL-6, median CRP, mean WBC count, and mean ANC differed significantly (p < 0.001) between patients with nonperforated appendicitis and those without appendicitis; median IL-8 levels did not differ between groups. In nonperforated appendicitis, median IL-6, WBC, and ANC levels were maximal at less than 24 hours of pain, while CRP peaked between 24 and 48 hours. In perforated appendicitis, median IL-8 levels were highest by 24 hours, WBC count and IL-6 by 24 to 48 hours, and CRP after 48 hours of pain. The WBC count appeared to be the most useful marker to predict appendicitis in those with fewer than 24 or more than 48 hours of pain, while CRP was the most useful in those with 24 to 48 hours of pain.

CONCLUSIONS

In this population, the serum levels and accuracy of novel and traditional biomarkers varies in relation to duration of abdominal pain. IL-6 shows promise as a novel biomarker to identify children with appendicitis.

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.

A novel mouse model of red blood cell storage and posttransfusion in vivo survival

BACKGROUND

Storage of red blood cells (RBCs) is necessary for an adequate blood supply. However, reports have identified potential negative sequelae of transfusing stored RBCs. An animal model would be useful to investigate the pathophysiology of transfusing stored RBCs. However, it has been reported that storage of rat RBCs in CPDA-1 resulted in an unexpected sudden decline in posttransfusion survival. A mouse model of RBC storage and transfusion was developed to assess survival kinetics of mouse RBCs.

STUDY DESIGN AND METHODS

RBCs expressing green fluorescent protein were collected in CPDA-1, filter leukoreduced, adjusted to a 75% hematocrit, and stored at 4°C. At weekly intervals, stored RBCs were transfused into C57BL/6 recipients. RBC survival was measured by flow cytometry and chromium-51 labeling. Phosphatidylserine externalization and CD47 expression was also evaluated.

RESULTS

Mean 24-hour survivals of transfused RBCs were 99, 91, 64, 54, 30, and 18% after 0, 7, 14, 21, 28, and 35 days of storage, respectively. Stored RBCs showed an initial rapid clearance with subsequent extended survival. Increased surface phosphatidylserine and decreased CD47 expression were also observed.

CONCLUSIONS

Mouse RBCs showed a progressive decline in survival, as a function of storage time, unlike the precipitous loss of viability reported for rat RBCs. Moreover, changes in the measured surface markers were analogous to trends reported for human RBCs. Together, these findings provide an initial characterization of a novel mouse model of RBC storage with the potential to serve as an experimental platform for studying the pathophysiologic consequences of transfusing stored RBCs.

CXCL1 and its receptor, CXCR2, mediate murine sickle cell vaso-occlusion during hemolytic transfusion reactions

Hemolytic transfusion reactions (HTRs) can produce serious and potentially life-threatening complications in sickle cell disease (SCD) patients; however, the mechanisms underlying these complications remain undetermined. We established a model of alloimmune, IgG-mediated HTRs in a well-characterized humanized murine model of SCD. HTRs induced acute vaso-occlusive crisis (VOC), resulting in shortened survival of SCD mice. Acute VOC was associated with elevated circulating inflammatory chemokine levels, including striking elevation of the levels of the neutrophil chemoattractant CXCL1. Recombinant CXCL1 administration was sufficient to induce acute VOC in SCD mice, characterized by leukocyte recruitment in venules, capture of circulating red blood cells, reduction of venular flow, and shortened survival. In contrast, blockade of the CXCL1 receptor, CXCR2, prevented HTR-elicited acute VOC and prolonged survival in SCD mice. These results indicate that CXCL1 is a key inflammatory mediator of acute VOC in SCD mice. Targeted inhibition of CXCL1 and/or CXCR2 may therefore represent a new therapeutic approach for acute VOC in SCD patients.

Current problems and future directions of transfusion-induced alloimmunization: summary of an NHLBI working group

In April 2010, a working group sponsored by the National Heart, Lung, and Blood Institute was assembled to identify research strategies to improve our understanding of alloimmunization caused by the transfusion of allogeneic blood components and to evaluate potential approaches to both reduce its occurrence and manage its effects. Significant sequelae of alloimmunization were discussed and identified, including difficulties in maintaining chronic transfusion of red blood cells and platelets, hemolytic disease of the newborn, neonatal alloimmune thrombocytopenia, and rejection of transplanted cells and tissues. The discussions resulted in a consensus that identified key areas of future research and developmental areas, including genetic and epigenetic recipient factors that regulate alloimmunization, biochemical specifics of transfused products that affect alloimmunization, and novel technologies for high-throughput genotyping to facilitate extensive and efficient antigen matching between donor and recipient. Additional areas of importance included analysis of unappreciated medical sequelae of alloimmunization, such as cellular immunity and its effect upon transplant and autoimmunity. In addition, support for research infrastructure was discussed, with an emphasis on encouraging collaboration and synergy of animal models biology and human clinical research. Finally, training future investigators was identified as an area of importance. In aggregate, this communication provides a synopsis of the opinions of the working group on the above issues and presents both a list of suggested priorities and the rationale for the topics of focus. The areas of research identified in this report represent potential fertile ground for the medical advancement of preventing and managing alloimmunization in its different forms and mitigating the clinical problems it presents to multiple patient populations.

Educating medical students in laboratory medicine: a proposed curriculum

As the 100th anniversary of the Flexner report nears, medical student education is being reviewed at many levels. One area of concern, expressed in recent reports from some national health care organizations, is the adequacy of training in the discipline of laboratory medicine (also termed clinical pathology). The Academy of Clinical Laboratory Physicians and Scientists appointed an ad hoc committee to review this topic and to develop a suggested curriculum, which was subsequently forwarded to the entire membership for review. The proposed medical student laboratory medicine curriculum defines goals and objectives for training, provides guidelines for instructional methods, and gives examples of how outcomes can be assessed. This curriculum is presented as a potentially helpful outline for use by medical school faculty and curriculum committees.

Use of mouse models to study the mechanisms and consequences of RBC clearance

Mice provide tractable animal models for studying the pathophysiology of various human disorders. This review discusses the use of mouse models for understanding red-blood-cell (RBC) clearance. These models provide important insights into the pathophysiology of various clinically relevant entities, such as autoimmune haemolytic anaemia, haemolytic transfusion reactions, other complications of RBC transfusions and immunomodulation by Rh immune globulin therapy. Mouse models of both antibody- and non-antibody-mediated RBC clearance are reviewed. Approaches for exploring unanswered questions in transfusion medicine using these models are also discussed.

High-sensitivity C-reactive protein predicts mortality but not stroke: the Northern Manhattan Study

OBJECTIVE

To determine whether high-sensitivity C-reactive protein (hsCRP) and serum amyloid A (SAA) predict stroke, vascular events, and mortality in a prospective cohort study.

BACKGROUND

Markers of inflammation have been associated with risk of myocardial infarction (MI). Their association with stroke is controversial.

METHODS

The Northern Manhattan Study includes a stroke-free community-based cohort study in participants aged > or =40 years (median follow-up 7.9 years). hsCRP and SAA were measured using nephelometry. Cox proportional hazards models were used to calculate hazard ratios (HR) and 95% confidence intervals (CI) for the association of markers with risk of ischemic stroke and other outcomes after adjusting for demographics and risk factors.

RESULTS

hsCRP measurements were available in 2,240 participants (mean age 68.9 +/- 10.1 years; 64.2% women; 18.8% white, 23.5% black, and 55.1% Hispanic). The median hsCRP was 2.5 mg/L. Compared with those with hsCRP <1 mg/L, those with hsCRP >3 mg/L were at increased risk of ischemic stroke in a model adjusted for demographics (HR = 1.60, 95% CI 1.06-2.41), but the effect was attenuated after adjusting for other risk factors (adjusted HR = 1.20, 95% CI 0.78-1.86). hsCRP >3 mg/L was associated with risk of MI (adjusted HR = 1.70, 95% CI 1.04-2.77) and death (adjusted HR = 1.55, 95% CI 1.23-1.96). SAA was not associated with stroke risk.

CONCLUSION

In this multiethnic cohort, high-sensitivity C-reactive protein (hsCRP) was not associated with ischemic stroke, but was modestly associated with myocardial infarction and mortality. The value of hsCRP and serum amyloid A may depend on population characteristics such as age and other risk factors.

Storage of murine red blood cells enhances alloantibody responses to an erythroid-specific model antigen

BACKGROUND

Red blood cell (RBC) alloimmunization can be a serious complication of blood transfusion, but factors influencing the development of alloantibodies are only partially understood. Within FDA-approved time limits, RBCs are generally transfused without regard to length of storage. However, recent studies have raised concerns that RBCs stored for more than 14 days have altered biologic properties that may affect medical outcomes. To test the hypothesis that storage time alters RBC immunogenicity, we utilized a murine model of RBC storage and alloimmunization.

STUDY DESIGN AND METHODS

Blood from transgenic HOD donor mice, which express a model antigen (hen egg lysozyme [HEL]) specifically on RBCs, was filter leukoreduced and stored for 14 days under conditions similar to those used for human RBCs. Fresh or 14-day-stored RBCs were transfused into wild-type recipients. The stability of the HOD antigen and posttransfusion RBC survival were analyzed by flow cytometry. RBC alloimmunization was monitored by measuring circulating anti-HEL immunoglobulin levels.

RESULTS

Transfusion of 14-day-stored, leukoreduced HOD RBCs resulted in 10- to 100-fold higher levels of anti-HEL alloantibodies as detected by enzyme-linked immunosorbent assay than transfusion of freshly collected, leukoreduced RBCs. RBC expression of the HOD antigen was stable during storage.

CONCLUSIONS

These findings demonstrate that HOD murine RBCs become more immunogenic with storage and generate the rationale for clinical trials to test if the same phenomenon is observed in humans. Length of storage of RBCs may represent a previously unappreciated variable in whether or not a transfusion recipient becomes alloimmunized.

2008 ACLPS panel discussion on resident education in clinical pathology

A curriculum in clinical pathology (CP) was developed under the auspices of the Academy of Clinical Laboratory Physicians and Scientists (ACLPS) in 2006. At the 2008 ACLPS meeting in Philadelphia, PA, a panel was convened to address the current challenges in resident education and how to overcome them. Current challenges include the heterogeneity of the discipline (which requires analytical, medical, and managerial knowledge), the diverse repertoire of clinical laboratory testing, and the need to better integrate the resident into the work flow of the laboratory, especially with respect to clinical consultation. Recommendations of the panel include the incorporation of active learning, clinical consultation, and competency assessment into CP resident training. A summary of the panel discussion is presented herein.

Transfusion therapy in postpartum hemorrhage

Postpartum hemorrhage (PPH) is an obstetric emergency that can occur following vaginal or cesarean delivery. Rapid diagnosis of PPH using laboratory and clinical parameters is an important first step in its management. Traditional blood components, including packed red blood cells, platelets, plasma, and cryoprecipitate, should be used in patients with significant bleeding. Recent studies underline the utility of transfusing these components in defined ratios to prevent dilutional coagulopathy. Disseminated intravascular coagulation (DIC) should be considered in severely bleeding obstetric patients and should be treated aggressively using blood components. Newer hemostatic agents, such as activated factor VII, will play significant roles in patients with bleeding that is refractory to standard therapy. Implementation of an obstetric bleeding protocol that integrates new knowledge in coagulation should aid physicians in improving outcomes for the mother and her fetus.

Hypothesis: hemolytic transfusion reactions represent an alternative type of anaphylaxis

Classical anaphylaxis is the most severe, and potentially fatal, type of allergic reaction, manifested by hypotension, bronchoconstriction, and vascular permeability. Similarly, a hemolytic transfusion reaction (HTR) is the most feared consequence of blood transfusion. Evidence for the existence of an alternative, IgG-mediated pathway of anaphylaxis may be relevant for explaining the pathophysiology of IgG-mediated-HTRs. The purpose of this review is to summarize the evidence for this alternative pathway of anaphylaxis and to present the hypothesis that an IgG-mediated HTR is one example of this type of anaphylaxis.

Transfusion-induced autoantibodies and differential immunogenicity of blood group antigens: a novel hypothesis

Blood bank serology has identified hundreds of red blood cell (RBC) antigens contained within numerous blood group systems. Although most blood group antigens are defined by amino acid polymorphisms in the extracellular domain of membrane proteins, it is also possible that additional nonexofacial polymorphisms (NEPs) may exist within cytoplasmic or transmembrane domains. To assess this possibility, we analyzed several blood group molecules by searching the SNPper database for nonsynonymous single-nucleotide polymorphisms. We report the identification of a number of NEPs in the Kell, Kidd, and Duffy molecules. Because the identified NEPs are not exposed on the surface of intact RBCs and are, thus, not accessible to recipient antibodies, they would neither be detected by blood bank serology in vitro, nor would they be recognized targets in hemolytic transfusion reactions in vivo. The presentation of peptides containing NEPs by recipient MHC Class II molecules, however, would nevertheless produce helper T-cell epitopes. In addition to identifying NEPs in human blood group molecules, we explore a novel hypothesis that the presence of NEPs contributes to the immunogenicity of blood group antigens. We further hypothesize that NEPs provide a mechanism by which transfusion can lead to anti-RBC autoantibodies, which are known to occur in humans after transfusion. The scientific basis, existing evidence, approaches to testing, and predicted biology of this hypothesis are presented.

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.

Mouse models of IgG- and IgM-mediated hemolysis

Well-characterized mouse models of allo-immune antibody-mediated hemolysis would provide a valuable approach for gaining greater insight into the pathophysiology of hemolytic transfusion reactions. To this end, mouse red blood cells (mRBCs) from human glycophorin A transgenic (hGPA-Tg) donor mice were transfused into non-Tg recipients that had been passively immunized with IgG or IgM hGPA-specific monoclonal antibodies (mAbs). In this novel murine "blood group system," mRBCs from hGPA-Tg mice are "antigen positive" and mRBCs from non-Tg mice are "antigen negative." Passive immunization of non-Tg mice with the IgG1 10F7 and IgG3 NaM10-2H12 anti-hGPA mAbs each induced rapid clearance of incompatible transfused hGPA-Tg-mRBCs in a dose-response manner. Using various knockout mice as transfusion recipients, both the complement system and activating Fcgamma receptors were found to be important in the clearance of incompatible mRBCs by each of these IgG mAbs. In addition, the IgM E4 anti-hGPA mAb induced complement-dependent intravascular hemolysis of transfused incompatible hGPA-Tg-mRBCs accompanied by gross hemoglobinuria. These initial studies validate the relevance of these new mouse models for addressing important questions in the field of transfusion medicine.

Saliva and the clinical pathology laboratory

There have been increasing numbers of applications using oral fluids, saliva in particular, as the target substrate for performing clinical diagnostic tests. These have focused primarily on point-of-care (POC) testing. These POC testing approaches range from, for example, currently available, highly specialized screening tests for the presence of antibodies recognizing HIV to the potential development of "lab-on-a-chip" platforms. Broad claims have been made that the latter will revolutionize clinical laboratory testing. From the perspective of large centralized clinical laboratories, multiple issues must be considered before implementing individual tests using saliva as the target fluid in a POC format or using saliva as a universal test fluid for measuring multiple analytes in a centralized laboratory format. The current scope of laboratory testing is large and comprehensive, involving both POC and centralized testing. Current academic laboratory programs have the ability to qualitatively identify and/or quantitatively measure several thousand analytes in various target matrices including blood, plasma, serum, urine, joint fluid, pleural fluid, peritoneal fluid, cerebrospinal fluid, and tissue. These tests fall into multiple clinical pathology disciplines, including clinical chemistry, hematology, coagulation, transfusion medicine, microbiology, cytogenetics, molecular diagnosis, and immunology. In addition, before implementing a given test, multiple issues need to be evaluated to ensure the validity of the reported result; these include considerations involving the three major phases of testing: preanalytical (e.g., patient identification and specimen collection, stability, and transport), analytical (e.g., sensitivity, specificity, accuracy, and precision), and postanalytical (e.g., reporting results, quality improvement, and turn-around-time).

Glycosylation and cold platelet storage

Platelets, unlike red blood cells and plasma, are stored at room temperature because platelets transfused after refrigeration at 4 degrees C are rapidly cleared from the circulation. Storage at room temperature promotes bacterial proliferation, however, and transfusion-transmitted bacteremia has become an increasing problem. Traditionally, the cold storage lesion has been attributed to a change in platelet shape from disc to sphere, but Hoffmeister et al. revisited this issue and have shown that the shape change induced by cold storage does not result in poor platelet survival. Instead, they showed that poor survival results from a virtually irreversible clustering of alpha subunits of glycoprotein Ib (GPIbalpha)) on the platelet surface. In a series of elegant papers, these researchers change the way we view platelet clearance. That is, they show that exposed, terminal, beta-linked N-acetylglucosamine (beta-GlcNAc) residues on clustered GPIbalpha are recognized by the lectin domain of type 3 complement receptors on liver macrophages, leading to rapid clearance by phagocytosis. They also demonstrate that phagocytosis of chilled platelets can be inhibited--and in vivo survival prolonged--by enzymatically galactosylating the terminal beta-GlcNAc residues on GPIbalpha. Disguising the exposed beta-GlcNAc residues on the N-glycans of the clustered GPIbalpha molecules by galactosylation is a promising approach to storing platelets at 4 degrees C without affecting platelet function. Cold storage would limit bacterial proliferation and extend the duration of platelet storage, reducing the incidence of transfusion-transmitted bacteremia and improving the availability of this scarce resource.

Crystallographic analysis of the NNA7 Fab and proposal for the mode of human blood-group recognition

The NNA7 Fab antibody fragment recognizes the human N-type blood-group antigen comprised of the N-terminal glycopeptide of glycophorin A (GPA). A mutant form of this Fab fragment, NNA7-G91S, exhibits markedly reduced antigen binding. To provide insight into how these Fab fragments recognize this glycopeptide antigen, the crystal structures of NNA7 and NNA7-G91S were solved and refined to 1.83 and 1.97 A resolution, respectively. Both molecules are composed of the same heavy (H) chain Fd fragment, but each contains a slightly different light (L) chain owing to the G91S substitution. Specifically, the G91S mutation pushes the backbone of the neighboring H chain away from complementarity-determining region 3 (CDR3) of the L-chain variable region, allowing an additional glycerol cryoprotectant molecule to enter the antigen-combining site near the putative location of O-linked glycosylation. Each Fab fragment also possesses a well defined 2-(N-morpholino)ethanesulfonic acid (MES) molecule trapped in its antigen-combining site, as well as a crystallographic symmetry-related molecule comprising an amino-acid sequence that is virtually identical to the N-terminus of GPA. The MES molecule interacts with the H-chain CDR in a manner reminiscent of antibody-carbohydrate complexes. These results suggest a model for recognition of the glycopeptide antigen that accounts for the deleterious effect of the G91S substitution.

N-glycosylation at one rabies virus glycoprotein sequon influences N-glycan processing at a distant sequon on the same molecule

Rabies glycoprotein (RGP(WT)) contains N-glycosylation sequons at Asn(37), Asn(247), and Asn(319), although Asn(37) is not efficiently glycosylated. To examine N-glycan processing at Asn(247) and Asn(319), full-length glycosylation mutants, RGP(-2-) and RGP(--3), were expressed, and Endo H sensitivity was compared. When the Asn(247) sequon is present alone in RGP(-2-), 90% of its N-glycans are high-mannose type, whereas only 35% of the N-glycans at Asn(319) in RGP(--3) are high-mannose. When both sequons are present in RGP(-23), 87% of the N-glycans are of complex type. The differing patterns of Endo H sensitivity at sequons present individually or together suggests that glycosylation of one sequon affects glycosylation at another, distant sequon. To explore this further, we constructed soluble forms of RGP: RGP(WT)T441His and RGP(--3)T441His. Tryptic glycopeptides from these purified secreted proteins were isolated by HPLC and characterized by a 3D oligosaccharide mapping technique. RGP(WT)T441His had fucosylated, bi- and triantennary complex type glycans at Asn(247) and Asn(319). However, Asn(247) had half as many neutral glycans, more monosialylated glycans, and fewer disialylated glycans when compared with Asn(319). Moreover, when comparing the N-glycans at Asn(319) on RGP(--3)T441His and RGP(WT)T441His, the former had 30% more neutral, 28% more monosialylated, and 33% fewer disialylated glycans. This suggests that the N-glycan at Asn(247) allows additional N-glycan processing to occur at Asn(319), yielding more heavily sialylated bi- and triantennary forms. The mechanism(s) by which glycosylation at one sequon influences N-glycan processing at a distant sequon on the same glycoprotein remains to be determined.