Mechanism of anti-D-mediated immune suppression--a paradox awaiting resolution?

Influenza virus antibodies inhibit antigen-specific de novo B cell responses in mice

Antibody responses to influenza vaccines tend to be focused on epitopes encountered during prior influenza exposures, with little production of de novo responses to novel epitopes. To examine the contribution of circulating antibodies to this phenomenon, we passively transferred a hemagglutinin (HA)-specific monoclonal antibody (mAb) into mice before immunizing with whole inactivated virions. The HA mAb inhibited de novo HA-specific antibodies, plasmablasts, germinal center B cells, and memory B cells, while responses to a second antigen in the vaccine, neuraminidase (NA), were uninhibited. The HA mAb potently inhibited de novo antibody responses against epitopes near the HA mAb binding site. The HA mAb also promoted IgG1 class switching, an effect that, unlike the inhibition of HA responses, relied on signaling through Fc-gamma receptors. These studies suggest that circulating antibodies inhibit de novo B cell responses in an antigen-specific manner, which likely contributes to differences in antibody specificities elicited during primary and secondary influenza virus exposures.IMPORTANCEMost humans are exposed to influenza viruses in childhood and then subsequently exposed to antigenically drifted influenza variants later in life. It is unclear if antibodies elicited by earlier influenza virus exposures impact immunity against new influenza virus strains. Here, we used a mouse model to investigate how an anti-hemagglutinin (HA) monoclonal antibody (mAb) affects de novo B cell and antibody responses to the protein targeted by the monoclonal antibody (HA) and a second protein not targeted by the monoclonal antibody [neuraminidase (NA)]. Collectively, our studies suggest that circulating anti-influenza virus antibodies can potently modulate the magnitude and specificity of antibody responses elicited by secondary influenza virus exposures.

A Phase 1b PK/PD Study to Demonstrate Antigen Elimination with RLYB212, a Monoclonal Anti-HPA-1a Antibody for FNAIT Prevention

BACKGROUND

 Fetal and neonatal alloimmune thrombocytopenia (FNAIT) is a rare bleeding disorder of the fetus/newborn caused by development of maternal alloantibodies against fetal human platelet antigens (HPAs), predominantly HPA-1a. Currently there are no treatments available to prevent maternal alloimmunization to HPAs or FNAIT.

METHODS

 This proof-of-concept study (EudraCT Number: 2021-005380-49) was designed to assess the ability of subcutaneous (SC) RLYB212, a monoclonal anti-HPA-1a antibody, to eliminate HPA-1a-positive platelets in an antigen challenge model of a 30 mL fetal-maternal hemorrhage. Subjects were randomized to receive a single SC dose of RLYB212 or placebo on day 1 in a single-blinded manner, followed by transfusion of 10 × 109 HPA-1a-positive platelets on day 8.

RESULTS

 Four subjects received 0.09 mg SC RLYB212, five received 0.29 mg SC RLYB212, and two received placebo. RLYB212 achieved rapid elimination of HPA-1a-positive platelets in a concentration-dependent manner, with concentrations as low as 3.57 ng/mL meeting the prespecified proof-of-concept criterion of ≥90% reduction in platelet elimination half-life versus placebo. Following HPA-1a-positive platelet transfusion, a rapid decline was observed in the concentration of RLYB212 over a period of 2 to 24 hours, corresponding to the time needed for RLYB212 to bind to ∼10% of HPA-1a on cell surfaces. RLYB212 was well tolerated with no reports of drug-related adverse events.

CONCLUSION

 The data from this study are consistent with preclinical efficacy data and support the potential use of RLYB212 as a prophylactic treatment for FNAIT that prevents maternal HPA-1a alloimmunization during at-risk pregnancies.

Antibody feedback regulation

Antibodies are able to up- or downregulate antibody responses to the antigen they bind. Two major mechanisms can be distinguished. Suppression is most likely caused by epitope masking and can be induced by all isotypes tested (IgG1, IgG2a, IgG2b, IgG3, IgM, and IgE). Enhancement is often caused by the redistribution of antigen in a favorable way, either for presentation to B cells via follicular dendritic cells (IgM and IgG3) or to CD4+ T cells via dendritic cells (IgE, IgG1, IgG2a, and IgG2b). IgM and IgG3 complexes activate complement and are transported from the marginal zone to follicles by marginal zone B cells expressing complement receptors. IgE-antigen complexes are captured by CD23+ B cells in the blood and transported to follicles, delivered to CD8α+ conventional dendritic cells, and presented to CD4+ T cells. Enhancement of antibody responses by IgG1, IgG2a, and IgG2b in complex with proteins requires activating FcγRs. These immune complexes are captured by dendritic cells and presented to CD4+ T cells, subsequently helping cognate B cells. Endogenous feedback regulation influences the response to booster doses of vaccines and passive administration of anti-RhD antibodies is used to prevent alloimmunization of RhD-negative women carrying RhD-positive fetuses.

Influenza virus antibodies inhibit antigen-specific de novo B cell responses in mice

Antibody responses to influenza vaccines tend to be focused on epitopes encountered during prior influenza exposures, with little production of de novo responses to novel epitopes. To examine the contribution of circulating antibody to this phenomenon, we passively transferred a hemagglutinin (HA)-specific monoclonal antibody (mAb) into mice before immunizing with whole inactivated virions. The HA mAb inhibited de novo HA-specific antibodies, plasmablasts, germinal center B cells, and memory B cells, while responses to a second antigen in the vaccine, neuraminidase (NA), were uninhibited. The HA mAb potently inhibited de novo antibody responses against epitopes near the HA mAb binding site. The HA mAb also promoted IgG1 class switching, an effect that, unlike the inhibition of HA responses, relied on signaling through Fc-gamma receptors. These studies suggest that circulating antibodies inhibit de novo B cell responses in an antigen-specific manner, which likely contributes to differences in antibody specificities elicited during primary and secondary influenza virus exposures.

Complement C3 and marginal zone B cells promote IgG-mediated enhancement of RBC alloimmunization in mice

Administration of anti-RhD immunoglobulin (Ig) to decrease maternal alloimmunization (antibody-mediated immune suppression [AMIS]) was a landmark clinical development. However, IgG has potent immune-stimulatory effects in other settings (antibody-mediated immune enhancement [AMIE]). The dominant thinking has been that IgG causes AMIS for antigens on RBCs but AMIE for soluble antigens. However, we have recently reported that IgG against RBC antigens can cause either AMIS or AMIE as a function of an IgG subclass. Recent advances in mechanistic understanding have demonstrated that RBC alloimmunization requires the IFN-α/-β receptor (IFNAR) and is inhibited by the complement C3 protein. Here, we demonstrate the opposite for AMIE of an RBC alloantigen (IFNAR is not required and C3 enhances). RBC clearance, C3 deposition, and antigen modulation all preceded AMIE, and both CD4+ T cells and marginal zone B cells were required. We detected no significant increase in antigen-specific germinal center B cells, consistent with other studies of RBC alloimmunization that show extrafollicular-like responses. To the best of our knowledge, these findings provide the first evidence of an RBC alloimmunization pathway which is IFNAR independent and C3 dependent, thus further advancing our understanding of RBCs as an immunogen and AMIE as a phenomenon.

Antigen-specific IgG subclass composition in recipient mice can indicate the degree of red blood cell alloimmunization as well as discern between primary and secondary immunization

BACKGROUND

Despite the vast antigen disparity between donor and recipient red blood cells (RBCs), only 2%-6% of transfusion patients mount an alloantibody response. Recently, RBC antigen density has been proposed as one of the factors that can influence alloimmunization, however, there has been no characterization of the role of antigen density along with RBC dose in primary and secondary immunization.

STUDY DESIGN AND METHODS

To generate RBCs that express distinct antigen copy numbers, different quantities of hen egg lysozyme (HEL) were coupled to murine RBCs. The HEL-RBCs were subsequently transfused into recipient mice at different RBC doses and their HEL-specific IgM, IgG, and IgG subclass response was evaluated.

RESULTS

Productive immune responses could be generated through a high copy number antigen transfused at low RBC doses or a low copy number transfused at high RBC doses. Further, primary but submaximal humoral immunization predominantly induced the IgG2b and IgG3 subclasses. In contrast, a maximal primary immunization or a secondary immunization induced all four IgG subclasses.

DISCUSSION

Our results confirm the existence of an antigen threshold for productive immune responses but indicate that a high antigen copy number alone might not be enough to induce a response, but rather a combination of both antigen copy number and cell dosage may determine the outcome of immunization. Further, this study provides a proof of concept that the IgG subclass composition can be an indicator of the level of RBC alloimmunization as well as discern between primary and secondary immunization at least in this murine model.

Anti-RhD antibody therapy modulates human natural killer cell function

Anti-RhD antibodies are widely used in clinical practice to prevent immunization against RhD, principally in hemolytic disease of the fetus and newborn. Intriguingly, this disease is induced by production of the very same antibodies when an RhD negative woman is pregnant with an RhD positive fetus. Despite over five decades of use, the mechanism of this treatment is, surprisingly, still unclear. Here we show that anti-RhD antibodies induce human natural killer (NK) cell degranulation. Mechanistically, we demonstrate that NK cell degranulation is mediated by binding of the Fc segment of anti-RhD antibodies to CD16, the main Fcγ receptor expressed on NK cells. We found that this CD16 activation is dependent upon glycosylation of the anti-RhD antibodies. Furthermore, we show that anti-RhD antibodies induce NK cell degranulation in vivo in patients who receive this treatment prophylactically. Finally, we demonstrate that the anti-RhD drug KamRho enhances the killing of dendritic cells. We suggest that this killing leads to reduced activation of adaptive immunity and may therefore affect the production of anti-RhD antibodies

Hemolytic Disease of the Fetus and Newborn: Historical and Current State

Hemolytic disease of the fetus and newborn (HDFN) is an immune-mediated disorder affecting neonates globally, with a range of clinical presentations from severe and life threatening to mild or even asymptomatic. Historically, HDFN has been responsible for a large proportion of perinatal mortality, and, despite advances in diagnosis and management, this morbidity and mortality has not been eradicated. Blood banking techniques and blood transfusion have contributed to improved prophylaxis and management, drastically improving the outcome of newborns with HDFN over the last century.

Aggregates in blood filter chambers used from the plasma donations of anti-D donors: evaluation for monoclonal antibody discovery using phage display

BACKGROUND

RhD-immunoglobulin (RhIg) prevents anti-D alloimmunisation in D-negative pregnant women when the fetus is D-positive, reducing the incidence of haemolytic disease of the fetus and newborn. Manufacturing RhIg is reliant on the limited supply of plasma donations with anti-D antibodies. Monoclonal antibody (mAb) development platforms such as phage display, require blood samples to be collected from anti-D donors, which may be a complicated process. The blood filter chamber (BFC) discarded after an anti-D donor's donation might provide a source of Ig-encoding RNA. This study aims to evaluate whether used BFCs are a suitable source of Ig-encoding RNA for phage display.

MATERIAL AND METHODS

Haemonetics PCS2 BFCs were obtained from 10 anti-D donors for total RNA extraction, cDNA synthesis and amplification of VH and VL IgG sequences for assembly of single-chain variable fragments (scFvs). A scFv-phage display library was constructed and 3 rounds of biopanning were performed using D-positive and D-negative red blood cells (RBCs). Positive phage clones were isolated, Sanger sequenced and, where possible, reformatted into full-length human IgGs to define specificity. The BFC aggregates from 2 anti-D donors underwent a Wright-Giemsa stain and hematological cell count.

RESULTS

Of 10 BFCs, a sufficient yield of total RNA for library construction was obtained from BFCs containing cellular aggregates (n=5). Aggregate analysis showed lymphocytes were the cellular source of Ig-encoding RNA. From the 5 samples with aggregates, scFvs were assembled from amplified IgG variable regions. The library constructed from 1 of these samples resulted in the isolation of clones binding to D-positive RBCs with IGHV3 gene usage. Of the 4 reformatted IgG, 3 were anti-D and 1 had undefined specificity.

DISCUSSION

BFC aggregates are a new and convenient source of Ig-encoding RNA which can be used to construct Ig gene libraries for mAb isolation and discovery via antibody phage display.

A consideration of convalescent plasma and plasma derivatives in the care of Severely-ill patients with COVID-19

The pathogenesis and immunopathological damage of severe forms of COVID-19 resemble acute autoimmune disease sparked by SARS-CoV-2, including an early systemic overproduction of proinflammatory cytokines. Such immunopathological features provide a rationale for the use of passive immunotherapy with convalescent plasma as a source of neutralizing anti-viral antibodies and of anti-inflammatory plasma components. While convalescent plasma therapy is now being evaluated in prospective clinical trials, we further consider the therapeutic potential of human hyper immune globulins, and of heterologous, engineered and monoclonal neutralizing antibodies as anti-viral agents to treat COVID-19. Good medical practice procedures are still needed and is why we also discuss the potential use of polyclonal polyspecific immunoglobulins (IVIG), a therapeutic plasma derivative, with potent anti-inflammatory activity, in severe forms of Covid-19.

IgG Subclass Determines Suppression Versus Enhancement of Humoral Alloimmunity to Kell RBC Antigens in Mice

It has long been appreciated that immunoglobulins are not just the effector endpoint of humoral immunity, but rather have a complex role in regulating antibody responses themselves. Donor derived anti-RhD IgG has been used for over 50 years as an immunoprophylactic to prevent maternal alloimmunization to RhD. Although anti-RhD has dramatically decreased rates of hemolytic disease of the fetus and newborn (for the RhD alloantigen), anti-RhD also fails in some cases, and can even paradoxically enhance immune responses in some circumstances. Attempts to generate a monoclonal anti-RhD have largely failed, with some monoclonals suppressing less than donor derived anti-RhD and others enhancing immunity. These difficulties likely result, in part, because the mechanism of anti-RhD remains unclear. However, substantial evidence exists to reject the common explanations of simple clearance of RhD + RBCs or masking of antigen. Donor derived anti-RhD is a mixture of 4 different IgG subtypes. To the best of our knowledge an analysis of the role different IgG subtypes play in immunoregulation has not been carried out; and, only IgG1 and IgG3 have been tested as monoclonals. Multiple attempts to elicit alloimmune responses to human RhD epitopes in mice have failed. To circumvent this limitation, we utilize a tractable animal model of RBC alloimmunization using the human Kell glycoprotein as an antigen to test the effect of IgG subtype on immunoregulation by antibodies to RBC alloantigens. We report that the ability of an anti-RBC IgG to enhance, suppress (at the level of IgM responses), or have no effect is a function of the IgG subclass in this model system.

Anti-D monoclonal antibodies from 23 human and rodent cell lines display diverse IgG Fc-glycosylation profiles that determine their clinical efficacy

Anti-D immunoglobulin (Anti-D Ig) prophylaxis prevents haemolytic disease of the fetus and newborn. Monoclonal IgG anti-Ds (mAb-Ds) would enable unlimited supplies but have differed in efficacy in FcγRIIIa-mediated ADCC assays and clinical trials. Structural variations of the oligosaccharide chains of mAb-Ds are hypothesised to be responsible. Quantitative data on 12 Fc-glycosylation features of 23 mAb-Ds (12 clones, 5 produced from multiple cell lines) and one blood donor-derived anti-D Ig were obtained by HPLC and mass spectrometry using 3 methods. Glycosylation of mAb-Ds from human B-lymphoblastoid cell lines (B) was similar to anti-D Ig although fucosylation varied, affecting ADCC activity. In vivo, two B mAb-Ds with 77–81% fucosylation cleared red cells and prevented D-immunisation but less effectively than anti-D Ig. High fucosylation (>89%) of mouse-human heterohybridoma (HH) and Chinese hamster ovary (CHO) mAb-Ds blocked ADCC and clearance. Rat YB2/0 mAb-Ds with <50% fucosylation mediated more efficient ADCC and clearance than anti-D Ig. Galactosylation of B mAb-Ds was 57–83% but 15–58% for rodent mAb-Ds. HH mAb-Ds had non-human sugars. These data reveal high galactosylation like anti-D Ig (>60%) together with lower fucosylation (<60%) as safe features of mAb-Ds for mediating rapid red cell clearance at low doses, to enable effective, inexpensive prophylaxis.

Murine models for studying treatment, prevention and pathogenesis of FNAIT

Maternal alloimmunization to paternally inherited antigens on fetal/neonatal platelets can cause fetal/neonatal alloimmune thrombocytopenia (FNAIT) after antibody-mediated removal of platelets from the fetal circulation. The complications vary from mild bleeding symptoms to severe intracranial hemorrhage and subsequent neurological impairment or death. Studies on in vivo mechanisms are challenging to measure directly in pregnant women, rendering murine models as valuable and attractive alternatives, despite some critical differences between mice and men affecting the translational value. Here we present and discuss, the different murine models that substantially have increased our knowledge and understanding of FNAIT pathogenesis - as well as pre-clinical evaluation of therapeutic and preventive strategies.

The third international standard for anti-D immunoglobulin: international collaborative study to evaluate candidate preparations

BACKGROUND AND OBJECTIVES

The purpose of the study was to evaluate a lyophilized anti-D immunoglobulin preparation to serve as a replacement WHO International Standard for the calibration of potency assays of anti-D immunoglobulin products. Such products are used to prevent haemolytic disease of the foetus and newborn due to maternal alloanti-D.

MATERIALS AND METHODS

The candidate 3rd International Standard for anti-D immunoglobulin (16/332) was evaluated and calibrated against the 2nd International Standard for anti-D immunoglobulin (01/572), along with a coded duplicate, a second candidate preparation (16/278) and a comparability sample (16/272) in an international collaborative study. Twenty of 21 laboratories in 15 countries performed one or more of the three European Pharmacopoeia reference methods.

RESULTS

The overall geometric mean potency (from all methods) of the candidate 3rd International Standard, 16/332, was 296·6 IU/ampoule, with inter-laboratory variability, expressed as % GCV, of 4·7%. SE-HPLC of the immunoglobulin preparations demonstrated combined monomeric and dimeric IgG peak areas of >95% for all samples. Accelerated stability studies have shown both 16/332 and 16/278 to be very stable for long-term storage at -20°C.

CONCLUSIONS

Preparation 16/332 was established by the World Health Organisation Expert Committee on Biological Standardization as the 3rd International Standard for anti-D immunoglobulin with an assigned potency of 297 IU/ampoule.

Antibody-mediated immunosuppression can result from RBC antigen loss independent of Fcγ receptors in mice

BACKGROUND

Anti-RhD administration can prevent de novo anti-RhD formation following RhD+ red blood cell (RBC) exposure, termed antibody-mediated immunosuppression (AMIS). Recent studies suggest that AMIS may occur through target antigen alterations, known as antigen modulation. However, studies suggest that AMIS may occur independent of antigen modulation. In particular, AMIS to RBCs that transgenically express the fusion hen egg lysozyme-ovalbumin-Duffy (HOD) antigen have been shown to occur independent of activating Fcγ receptors (FcγRs) thought to be required for antigen modulation. Therefore, we sought to determine the mechanism behind AMIS following HOD RBC exposure.

STUDY DESIGN AND METHODS

Following transfer of HOD RBCs into wild-type or FcγR-chain knockout recipients in the presence or absence of monoclonal anti-hen egg lysozyme (HEL) antibody, individually or in combination, HOD antigen levels and anti-HOD antibody formation were examined.

RESULTS

Our results demonstrate that anti-HEL antibodies individually or in combination suppressed anti-HOD IgM, which correlated with the rate of detectable decrease in HEL on HOD RBCs. Furthermore, exposure to anti-HEL antibodies alone or in combination equally suppressed anti-HOD IgG formation. Unexpectedly, combination or individual anti-HEL antibodies induced AMIS and antigen modulation in an FcγR-independent manner. Pre-exposure of HOD RBCs to anti-HEL antibodies reduced antigen levels and suppressed anti-HOD antibody formation following HOD RBC exposure.

CONCLUSION

These results suggest that antibody-mediated antigen modulation may reflect a mechanism of AMIS that can occur independent of activating FcγRs and may provide a surrogate to identify antibodies capable of inducing AMIS against different RBC antigens.

Antibody-mediated immune suppression by antigen modulation is antigen-specific

Alloantibodies developing after exposure to red blood cell (RBC) alloantigens can complicate pregnancy and transfusion therapy. The only method currently available to actively inhibit RBC alloantibody formation is administration of antigen-specific antibodies, a phenomenon termed antibody-mediated immune suppression (AMIS). A well-known example of AMIS is RhD immune globulin prophylaxis to prevent anti-D formation in RhD- individuals. However, whether AMIS is specific or impacts alloimmunization to other antigens on the same RBC remains unclear. To evaluate the specificity of AMIS, we passively immunized antigen-negative recipients with anti-KEL or anti-hen egg lysozyme (HEL) antibodies, followed by transfusion of murine RBC expressing both the HEL-ovalbumin-Duffy (HOD) and human KEL antigens (HOD × KEL RBC). Significant immunoglobulin G deposition on transfused HOD × KEL RBC occurred in all passively immunized recipients. Complement deposition and antigen modulation of the KEL antigen occurred on transfused RBC only in anti-KEL-treated recipients, whereas HEL antigen levels decreased only in the presence of anti-HEL antibodies. Western blot analysis confirmed the specificity of antigen loss, which was not attributable to RBC endocytosis and appears distinct for the 2 antigens. Specifically, removal of KEL was attenuated by clodronate treatment, whereas loss of HEL was unaffected by clodronate in vivo but sensitive to protease treatment in vitro. Antigen-specific modulation correlated with antigen-specific AMIS, with anti-KEL treated recipients forming antibodies to the HOD antigen and anti-HEL-treated recipients developing antibodies to the KEL antigen. Together, these results demonstrate that passively administered antibodies can selectively inhibit the immune response to a specific antigen.

IgG-mediated immune suppression in mice is epitope specific except during high epitope density conditions

Specific IgG antibodies, passively administered together with erythrocytes, suppress antibody responses against the erythrocytes. Although used to prevent alloimmunization in Rhesus (Rh)D-negative women carrying RhD-positive fetuses, the mechanism behind is not understood. In mice, IgG suppresses efficiently in the absence of Fcγ-receptors and complement, suggesting an Fc-independent mechanism. In line with this, suppression is frequently restricted to the epitopes to which IgG binds. However, suppression of responses against epitopes not recognized by IgG has also been observed thus arguing against Fc-independence. Here, we explored the possibility that non-epitope specific suppression can be explained by steric hindrance when the suppressive IgG binds to an epitope present at high density. Mice were transfused with IgG anti-4-hydroxy-3-nitrophenylacetyl (NP) together with NP-conjugated sheep red blood cells (SRBC) with high, intermediate, or low NP-density. Antibody titers and the number of single antibody-forming cells were determined. As a rule, IgG suppressed NP- but not SRBC-specific responses (epitope specific suppression). However, there was one exception: suppression of both IgM anti-SRBC and IgM anti-NP responses occurred when high density SRBC-NP was administered (non-epitope specific suppression). These findings answer a longstanding question in antibody feedback regulation and are compatible with the hypothesis that epitope masking explains IgG-mediated immune suppression.

Erythrocyte Saturation with IgG Is Required for Inducing Antibody-Mediated Immune Suppression and Impacts Both Erythrocyte Clearance and Antigen-Modulation Mechanisms

Anti-D prevents hemolytic disease of the fetus and newborn, and this mechanism has been referred to as Ab-mediated immune suppression (AMIS). Anti-D, as well as other polyclonal AMIS-inducing Abs, most often induce both epitope masking and erythrocyte clearance mechanisms. We have previously observed that some Abs that successfully induce AMIS effects could be split into those that mediate epitope masking versus those that induce erythrocyte clearance, allowing the ability to analyze these mechanisms separately. In addition, AMIS-inducing activity has recently been shown to induce Ag modulation (Ag loss from the erythrocyte surface). To assess these mechanisms, we immunized mice with transgenic murine RBCs expressing a single Ag protein comprising a recombinant Ag composed of hen egg lysozyme, OVA sequences comprising aa 251-349, and the human Duffy transmembrane protein (HOD-Ag) with serial doses of polyclonal anti-OVA IgG as the AMIS-inducing Ab. The anti-OVA Ab induced AMIS in the absence of apparent epitope masking. AMIS occurred only when the erythrocytes appeared saturated with IgG. This Ab was capable of inducing HOD-RBC clearance, as well as loss of the OVA epitope at doses of Ab that caused AMIS effects. HOD-RBCs also lost reactivity with Abs specific for the hen egg lysozyme and Duffy portions of the Ag consistent with the initiation of Ag modulation and/or trogocytosis mechanisms. These data support the concept that an AMIS-inducing Ab that does not cause epitope masking can induce AMIS effects in a manner consistent with RBC clearance and/or Ag modulation.

Anti-Human Platelet Antigen-1a Immunoglobulin G Preparation Intended to Prevent Fetal and Neonatal Alloimmune Thrombocytopenia

Fetal and neonatal alloimmune thrombocytopenia (FNAIT) is a severe disease that is caused by maternal alloantibodies generated during pregnancy or at delivery as a result of incompatibility between maternal and fetal human platelet antigens (HPAs) inherited from the father. Antibody-mediated immune suppression using anti-HPA-1a immunoglobulins is thought to be able to prevent FNAIT caused by HPA-1a. A fractionation process to prepare anti-HPA-1a immunoglobulin (Ig) G (IgG) from human plasma was therefore developed. Anti-HPA-1a plasma was obtained from volunteer mothers who underwent alloimmunization against HPA-1a during a previous pregnancy. Plasma was cryoprecipitated and the supernatant treated with caprylic acid and solvent/detergent (S/D), purified by chromatography, nanofiltered, concentrated, and sterile-filtered. The anti-HPA-1a immunoglobulin fraction was characterized for purity and safety. PAK12 and quantitative monoclonal antibody immobilization of platelet antigen (MAIPA) assays were used to detect anti-HPA-1a IgG. Hepatitis C virus (HCV) removal during nanofiltration was assessed by spiking experiments, using cell culture-derived reporter HCV and luciferase analysis. The caprylic acid treatment precipitated non-Ig proteins yielding a 90% pure Ig supernatant. S-HyperCel chromatography of the S/D-treated supernatant followed by HyperCel STAR AX provided high IgG recovery (>80%) and purity (>99.5%), and efficient IgA and IgM removal. Concentrations of complement factors C3 and C4 were < 0.5 and < 0.4 mg/dL, respectively. The final IgG could be nanofiltered on Planova 20N under conditions removing more than 3 log HCV infectivity to baseline mock infection level, and concentrated to ca. 30 g/L. Proteolytic activity and thrombin generation were low in the final fraction. The Pak12 and MAIPA assays showed good recovery of anti-HPA-1a throughout the process. Clinical-grade HPA-1a IgG can be prepared using a process compliant with current quality requirements opening perspectives for the prevention of FNAIT.

IgG Suppresses Antibody Responses in Mice Lacking C1q, C3, Complement Receptors 1 and 2, or IgG Fc-Receptors

Antigen-specific IgG antibodies, passively administered to mice or humans together with large particulate antigens like erythrocytes, can completely suppress the antibody response against the antigen. This is used clinically in Rhesus prophylaxis, where administration of IgG anti-RhD prevents RhD-negative women from becoming immunized against RhD-positive fetal erythrocytes aquired transplacentally. The mechanisms by which IgG suppresses antibody responses are poorly understood. We have here addressed whether complement or Fc-receptors for IgG (FcγRs) are required for IgG-mediated suppression. IgG, specific for sheep red blood cells (SRBC), was administered to mice together with SRBC and the antibody responses analyzed. IgG was able to suppress early IgM- as well as longterm IgG-responses in wildtype mice equally well as in mice lacking FcγRIIB (FcγRIIB knockout mice) or FcγRI, III, and IV (FcRγ knockout mice). Moreover, IgG was able to suppress early IgM responses equally well in mice lacking C1q (C1qA knockout mice), C3 (C3 knockout mice), or complement receptors 1 and 2 (Cr2 knockout mice) as in wildtype mice. Owing to the previously described severely impaired IgG responses in the complement deficient mice, it was difficult to assess whether passively administered IgG further decreased their IgG response. In conclusion, Fc-receptor binding or complement-activation by IgG does not seem to be required for its ability to suppress antibody responses to xenogeneic erythrocytes.

Rh(O)D immune globulin products for prevention of alloimmunization during pregnancy

PURPOSE

The pharmacologic properties of Rhesus (Rh) immune globulin (RhIG) and clinical data on its effectiveness in preventing Rh-antigen alloimmunization in pregnant women are reviewed.

SUMMARY

RhIG is a human plasma derivative that targets red blood cells (RBCs) positive for Rh(O) antigen (also called D antigen). In the United States and other countries, the widespread use of RhIG has markedly reduced the occurrence of hemolytic disease of the fetus and newborn (HDFN), a devastating condition caused by D-antigen sensitization of a pregnant woman via exposure to fetal RBCs (usually during detachment of the placenta in labor) that results in a maternal immune response leading to severe hemolysis in the fetus. Routine administration of RhIG at 26-30 weeks' gestation and again within 72 hours of delivery has been shown to be highly effective in preventing maternal Rh alloimmunization, with very low rates of D-antigen sensitization (in the range of 0-2.2%) reported in multiple studies of at-risk women. The four RhIG products currently available in the United States have common clinical indications but differ in certain attributes. Pharmacists can play an important role in guiding other clinicians on the rationale for the use of RhIG, important differences between products, and appropriate timing of RhIG therapy.

CONCLUSION

Routine administration of RhIG to women at risk for Rh alloimmunization is clinically effective and has made HDFN a rare clinical event. The available RhIG products are not the same and should be carefully reviewed to ensure that they are administered safely.

Recent progress in understanding the pathogenesis of fetal and neonatal alloimmune thrombocytopenia

Fetal and neonatal alloimmune thrombocytopenia (FNAIT) occurs in c. 1 in 1000 births and is caused by maternal antibodies against human platelet alloantigens that bind incompatible fetal platelets and promote their clearance from the circulation. Affected infants can experience bleeding, bruising and, in severe cases, intracranial haemorrhage and even death. As maternal screening is not routinely performed, and first pregnancies can be affected, most cases are diagnosed at delivery of a first affected pregnancy. Unlike its erythrocyte counterpart, Haemolytic Disease of the Fetus and Newborn, there is no prophylactic treatment for FNAIT. This report will review recent advances made in understanding the pathogenesis of FNAIT: the platelet alloantigens involved, maternal exposure and sensitization to fetal platelet antigens, properties of platelet Immunoglobulin G antibodies, maternal-fetal antibody transport mechanisms and efforts to develop an effective FNAIT prophylaxis.

Anti-D administration in pregnancy for preventing Rhesus alloimmunisation

BACKGROUND

During pregnancy, a Rhesus negative (Rh-negative) woman may develop antibodies when her fetus is Rhesus positive (Rh-positive). These antibodies may harm Rh-positive babies.

OBJECTIVES

To assess the effects of antenatal anti-D immunoglobulin on the incidence of Rhesus D alloimmunisation when given to Rh-negative women without anti-D antibodies.

SEARCH METHODS

We searched the Cochrane Pregnancy and Childbirth Group's Trials Register (31 May 2015) and reference lists of retrieved studies.

SELECTION CRITERIA

Randomised trials in Rh-negative women without anti-D antibodies given anti-D after 28 weeks of pregnancy, compared with no treatment, placebo or a different regimen of anti-D.

DATA COLLECTION AND ANALYSIS

Two review authors independently assessed trials for inclusion and risk of bias, extracted data and checked them for accuracy.

MAIN RESULTS

We included two trials involving over 4500 women, comparing anti-D prophylaxis with no anti-D during pregnancy in this review. Overall, the trials were judged to be at moderate to high risk of bias. The quality of the evidence for pre-specified outcomes was also assessed using the GRADE (Grades of Recommendation, Assessment, Development and Evaluation) approach.In regards to primary review outcomes, there did not appear to be a clear difference in the risks of immunisation when women who received anti-D at 28 and 34 weeks' gestation were compared with women who were not given antenatal anti-D: risk ratio (RR) for incidence of Rhesus D alloimmunisation during pregnancy was 0.42 (95% confidence interval (CI) 0.15 to 1.17, two trials, 3902 women; GRADE: low quality evidence); at birth of a Rh-positive infant the RR was 0.42 (95% CI 0.15 to 1.17, two trials, 2297 women); and within 12 months after birth of a Rh-positive infant the average RR was 0.39 (95% CI 0.10 to 1.62, two trials, 2048 women; Tau²: 0.47; I²: 39%; GRADE: low quality evidence). Neither of the trials reported on incidence of Rhesus D alloimmunisation in subsequent pregnancies.Considering secondary outcomes, in one trial, women receiving anti-D during pregnancy were shown to be less likely to register a positive Kleihauer test (which detects fetal cells in maternal blood) in pregnancy (at 32 to 25 weeks) (RR 0.60, 95% CI 0.41 to 0.88; 1884 women; GRADE: low quality evidence) and at the birth of a Rh-positive infant (RR 0.60, 95% CI 0.46 to 0.79; 1189 women; GRADE: low quality evidence). No clear differences were seen for neonatal jaundice (RR 0.26, 95% CI 0.03 to 2.30; 1882 infants; GRADE: very low quality evidence). Neither of the trials reported on adverse effects associated with anti-D treatment.

AUTHORS' CONCLUSIONS

Existing studies do not provide conclusive evidence that the use of anti-D during pregnancy benefits either mother or baby in terms of incidence of Rhesus D alloimmunisation during the pregnancy or postpartum, or the incidence of neonatal morbidity (jaundice) (low to very low quality evidence). However women receiving anti-D may be less likely to register a positive Kleihauer test in pregnancy and at the birth of a Rh-positive infant (low quality evidence). Fewer women who receive anti-D during pregnancy may have Rhesus D antibodies in a subsequent pregnancy, with benefits for the baby, however this needs to be tested in studies of robust design.

IgG-Mediated Immune Suppression to Erythrocytes by Polyclonal Antibodies Can Occur in the Absence of Activating or Inhibitory Fcγ Receptors in a Full Mouse Model

Polyclonal anti-D has been used to prevent RhD-negative mothers from becoming immunized against RhD positive fetal erythrocytes, and this mechanism has been referred as Ab or IgG-mediated immune suppression (AMIS). Although anti-D has been highly successful, the inhibitory mechanisms remain poorly understood. Two major theories behind AMIS involve the binding of IgG to activating or inhibitory FcγR, which can induce either erythrocyte clearance or immune inhibition, respectively. In this work, we explored the absolute role of activating and inhibitory FcγR in the AMIS mechanism using the HOD mouse model of RBC immunization. HOD mice contain a RBC-specific recombinant protein composed of hen egg lysozyme (HEL), OVA and human transmembrane Duffy Ag, and erythrocytes from HOD mice can stimulate an immune response to HEL. To assess the contribution of activating and inhibitory FcγR to AMIS, C57BL/6 versus FcRγ-chain(-/-) or FcγRIIB(-/-) mice were used as recipients of HOD-RBC alone or together with anti-HEL Abs (i.e., AMIS) and the resulting immune response to HEL evaluated. We show that anti-HEL polyclonal Abs induce the same degree of AMIS effect in mice lacking these IgG binding receptors as compared with wild-type mice. In agreement with this, F(ab')2 fragments of the AMIS Ab also significantly reduced the Ab response to the HOD cells. In conclusion, successful inhibition of in vivo Ab responses to HOD-RBC by polyclonal IgG can occur independently of activating or inhibitory FcγR involvement. These results may have implications for the understanding of RhD prophylaxis.

Characterization of a human platelet antigen-1a-specific monoclonal antibody derived from a B cell from a woman alloimmunized in pregnancy

Human platelet Ag (HPA)-1a, located on integrin β3, is the main target for alloantibodies responsible for fetal and neonatal alloimmune thrombocytopenia (FNAIT) in the white population. There are ongoing efforts to develop an Ab prophylaxis and therapy to prevent or treat FNAIT. In this study, an mAb specific for HPA-1a, named 26.4, was derived from an immortalized B cell from an alloimmunized woman who had an infant affected by FNAIT. It is the only HPA-1a-specific human mAb with naturally paired H and L chains. Specific binding of mAb 26.4, both native and recombinant forms, to platelets and to purified integrins αIIbβ3 (from platelets) and αVβ3 (from trophoblasts) from HPA-1a(+) donors was demonstrated by flow cytometry and surface plasmon resonance technology, respectively. No binding to HPA-1a(-) platelets or integrins was detected. Moreover, the Ab binds with higher affinity to integrin αVβ3 compared with a second HPA-1a-specific human mAb, B2G1. Further in vitro experimentation demonstrated that mAb 26.4 can opsonize HPA-1a(+) platelets for enhanced phagocytosis by monocytes, inhibit binding of maternal polyclonal anti-HPA-1a Abs, and weakly inhibit aggregation of HPA-1a-heterozygous platelets, the latter with no predicted clinical relevance. Thus, mAb 26.4 is highly specific for HPA-1a and could potentially be explored for use as a prophylactic or therapeutic reagent for FNAIT intervention and as a phenotyping reagent to identify women at risk for immunization.

Risky Bodies in the Plasma Bioeconomy: A Feminist Analysis

In 2003 the UK National Blood Service introduced a policy of 'male donor preference' which involved women's plasma being discarded following blood collection. The policy was based on the view that data relating to the incidence of Transfusion-Related Acute Lung Injury (TRALI) was linked to transfusion with women's plasma. While appearing to treat female donors as equal to male donors, exclusion criteria operate after donation at the stage of processing blood, thus perpetuating myths of universality even though only certain 'extractions' from women are retained for use in transfusion. Many women in the UK receive a plasma-derived product called Anti-D immunoglobulin which is manufactured from pooled male plasma. This article examines ways in which gender has significance for understanding blood relations, and how the blood economy is gendered. In our study of relations between blood donors and recipients, we explore how gendered bodies are produced through the discursive and material practices within blood services. We examine both how donation policies and the manufacturing and use of blood products produces gendered blood relations.

Maternal anti-D prophylaxis during pregnancy and risk of hemolysis among preterm infants

OBJECTIVE

To evaluate whether Rh-positive preterm newborn infants born to Rh-negative mothers treated with prophylactic anti-D immunoglobulins exhibited signs of hemolytic reaction, including anemia and hyperbilirubinemia.

STUDY DESIGN

Retrospective data were collected for 94 Rh-positive preterm newborns born at gestational age (GA) 28 to 34 weeks to 76 Rh-negative mothers and for matched controls.

RESULT

We found 11.7% positive Coombs' tests among infants in the study group and slightly higher bilirubin levels at birth and on the following 3 days. No differences were recorded between the study and the control groups for hematocrit levels throughout hospitalization, maximal bilirubin level, phototherapy treatment or the need for blood transfusion.

CONCLUSION

Among preterm Rh-positive newborn infants born to Rh-negative mothers, there appears to be no evidence of significant hemolytic reaction derived from placental anti-D transfer. Further prospective studies are needed to confirm these findings in order to support anti-D administration close to preterm birth.

Prophylactic anti-D preparations display variable decreases in Fc-fucosylation of anti-D

BACKGROUND

RhIG is obtained from hyperimmunized healthy anti-D donors (HIDs) boosted with D+ red blood cells (RBCs). One hypothesis for its mechanism of action is fast clearance of opsonized D+ RBCs through Fcγ receptor (FcγR)III. Levels of immunoglobulin (Ig)G Fc-fucosylation influence interactions with FcγRIII, with less Fc-fucosylation strengthening the interaction.

STUDY DESIGN AND METHODS

Anti-D IgG1 Fc-glycosylation patterns in 93 plasma samples from 28 male and 28 female Dutch HIDs and RhIG were analyzed with mass spectrometry. The Fc-glycosylation profiles of HIDs were evaluated with regard to their immunization history.

RESULTS

HID sera demonstrated clearly lowered anti-D Fc-fucosylation compared to normal IgG fucosylation (93%); this was more pronounced for female than for male HIDs (47% vs. 65%, p = 0.001). RhIG preparations from seven manufacturers varied greatly in the level of Fc-fucosylation (56%-91%). The level of fucosylation slightly increased upon repeated immunization, although it remained fairly constant over time. The RhIG from the different manufacturers all demonstrated increased Fc-galactosylation (64%-82%) compared to total IgG (38%-51%).

CONCLUSION

RhIG preparations vary in Fc-fucosylation and all demonstrate increased galactosylation. Despite not knowing the exact working mechanism, immunoprophylaxis could perhaps be optimized by selection of donors whose anti-D have low amounts of Fc-fucose, to increase the clearance activity of anti-D preparations, as well as high amounts of galactosylation, for anti-inflammatory effects. Implementing a biologic assay in the standardization of RhIG preparations might be considered.

Mice expressing RHAG and RHD human blood group genes

Anti-RhD prophylaxis of haemolytic disease of the fetus and newborn (HDFN) is highly effective, but as the suppressive mechanism remains uncertain, a mouse model would be of interest. Here we have generated transgenic mice expressing human RhAG and RhD erythrocyte membrane proteins in the presence and, for human RhAG, in the absence, of mouse Rhag. Human RhAG associates with mouse Rh but not mouse Rhag on red blood cells. In Rhag knockout mice transgenic for human RHAG, the mouse Rh protein is “rescued” (re-expressed), and co-immunoprecipitates with human RhAG, indicating the presence of hetero-complexes which associate mouse and human proteins. RhD antigen was expressed from a human RHD gene on a BAC or from RHD cDNA under control of β-globin regulatory elements. RhD was never observed alone, strongly indicative that its expression absolutely depends on the presence of transgenic human RhAG. This first expression of RhD in mice is an important step in the creation of a mouse model of RhD allo-immunisation and HDFN, in conjunction with the Rh-Rhag knockout mice we have developed previously.

Does Rh immune globulin suppress HLA sensitization in pregnancy?

BACKGROUND

How Rh immune globulin (RhIG) prevents sensitization to D antigen is unclear. If RhIG Fc delivers a nonspecific immunosuppressive signal, then RhIG may inhibit sensitization to antigens other than D. HLA antibody prevalence was compared in previously pregnant D- versus D+ women to investigate whether RhIG suppresses HLA sensitization.

STUDY DESIGN AND METHODS

In the Leukocyte Antibody Prevalence Study (LAPS), 7920 volunteer blood donors were screened for anti-HLA and surveyed about prior pregnancies and transfusions. A secondary analysis of the LAPS database was performed.

RESULTS

D- women not more than 40 years old (presumed to have received antenatal with or without postpartum RhIG in all pregnancies) had a significantly lower HLA sensitization rate than D+ women (relative risk, 0.58; 95% confidence interval [CI], 0.40-0.83). When stratified by deliveries (one, two, three, or four or more), D- women not older than 40 were HLA sensitized less often than D+ women in every case. In contrast, a clear relationship between D type and HLA sensitization was not seen in older previously pregnant women whose childbearing years are presumed to have preceded the use of routine RhIG prophylaxis. In a multivariable logistic regression model, D- women not more than 40 years old remained significantly less likely to be HLA sensitized compared with D+ women after adjusting for parity, time from last pregnancy, lost pregnancies, and transfusions (odds ratio [OR], 0.55; 95% CI, 0.34-0.88).

CONCLUSION

Consistent with a nonspecific immunosuppressive effect of RhIG, younger previously pregnant D- women were less likely than previously pregnant D+ women to be HLA sensitized.

Anti-rhesus D prophylaxis in pregnant women is based on sialylated IgG antibodies

Red blood cells (RBCs) from a rhesus D (RhD)-positive fetus that reach the bloodstream of an RhD-negative pregnant woman during birth can induce a pathogenic antibody (Ab) response against the RhD-positive RBCs, leading to fetal hemolytic disease in subsequent pregnancies. To prevent a pathogenic immune reaction, the RhD-negative mother receives serum immunoglobulin G (IgG) containing polyclonal RhD-specific IgG Abs that is purified from healthy RhD-negative men immunized with RhD-positive RBCs. However, the protective mechanism of these polyclonal RhD-specific IgG Abs is unclear. It has become increasingly clear that the effector function of IgG Abs is regulated by the glycan pattern linked to the Fc region of IgG Abs. Non-fucosylated (afucosylated) IgG Abs have a higher affinity for activating Fc gamma receptors, and thus induce a stronger Ab-dependent cellular cytotoxicity (ADCC) reaction than do fucosylated IgG Abs. Agalactosylated and asialylated, autoantigen-specific serum IgG Abs correlate with pro-inflammatory immune responses and disease activity in patients with rheumatoid arthritis. In contrast, galactosylated and sialylated IgG Abs are immunosuppressive and inhibit in form of immune complexes (ICs) dendritic cell (DC) maturation and pro-inflammatory T and B cell immune responses in an antigen-specific manner. However, the galactosylation and sialylation levels of the protective polyclonal RhD-specific IgG Abs are unknown. Here, we purified RhD-specific IgG Abs from the approved commercial product Rhophylac® (CSL Behring) and found that these RhD-specific IgG Abs were even more galactosylated and sialylated than the total Rhophylac® IgG Abs. This result suggests that these galactosylated and sialylated polyclonal RhD-specific IgG Abs are immunosuppressive and induce tolerance against RhD, which would be in strong contrast to a low fucosylated, low galactosylated and low sialylated monoclonal RhD-specific IgG Ab developed to prevent fetal hemolytic disease that has recently passed a clinical phase II study.

Role of IgG antibodies in association with placental function and immunologic diseases in human pregnancy

During human pregnancy, the maternal immune system develops and changes, providing protection for the growing placenta and fetus. These protective changes provide mechanisms allowing two genetically different individuals to interact with each other without allograft rejection. In addition to normal pregnancy, some pregnancies may develop under immunologic diseases, during which specific monitoring and medical treatments are essential. The aim of this current review is to provide information regarding the development of human placental function during pregnancy, the immunology of human pregnancy and the role of the placenta in providing the fetal tissue with antibodies (IgG and its subclasses 1-4), which are required for the passive immunization of the newborn. In addition, the available methods for the determination of placental function will be explored. Furthermore, immunologic diseases observed during pregnancy and the possible therapies for these diseases will be assessed.

Alloantibodies to a paternally derived RBC KEL antigen lead to hemolytic disease of the fetus/newborn in a murine model

Exposure to nonself red blood cell (RBC) antigens, either from transfusion or pregnancy, may result in alloimmunization and incompatible RBC clearance. First described as a pregnancy complication 80 years ago, hemolytic disease of the fetus and newborn (HDFN) is caused by alloimmunization to paternally derived RBC antigens. Despite the morbidity/mortality of HDFN, women at risk for RBC alloimmunization have few therapeutic options. Given that alloantibodies to antigens in the KEL family are among the most clinically significant, we developed a murine model with RBC-specific expression of the human KEL antigen to evaluate the impact of maternal/fetal KEL incompatibility. After exposure to fetal KEL RBCs during successive pregnancies with KEL-positive males, 21 of 21 wild-type female mice developed anti-KEL alloantibodies; intrauterine fetal anemia and/or demise occurred in a subset of KEL-positive pups born to wild type, but not agammaglobulinemic mothers. Similar to previous observations in humans, pregnancy-associated alloantibodies were detrimental in a transfusion setting, and transfusion-associated alloantibodies were detrimental in a pregnancy setting. This is the first pregnancy-associated HDFN model described to date, which will serve as a platform to develop targeted therapies to prevent and/or mitigate the dangers of RBC alloantibodies to fetuses and newborns.

Isolated anaemia as a manifestation of Rh isoimmunisation

Rh isoimmunisation leads to haemolytic anaemia and hyperbilirubinaemia in the first h of life. Isolated early onset neonatal anaemia has rarely been reported. The authors describe the case of a term infant, born to an 'A' negative, second gravida mother. On the second day of life, pallor was noticed. His haemoglobin (Hb) was 6.8 g/dl, he had reticulocytosis and a positive direct antiglobulin test. However, he did not have a high total serum bilirubin (TSB) (87.2 μmol/l). He was transfused with red blood cells and kept under phototherapy for 3 days. Three weeks later, he received another transfusion for severe anaemia (Hb 6 5 g/dl). During this period, he was never jaundiced and the maximum level of TSB was 122 μmol/l. On follow-up, his Hb stabilised and he had no further problems. This report highlights the possibility of early onset anaemia without jaundice as the sole manifestation of Rh isoimmunisation.

Unexpected suppression of anti-Fya and prevention of hemolytic disease of the fetus and newborn after administration of Rh immune globulin

BACKGROUND

Rh immune globulin (RhIG) has been used successfully for many years for the antenatal suppression of anti-D in D- mothers carrying D+ babies to prevent hemolytic disease of the fetus and newborn. Although the mechanism of RhIG-induced immunosuppression remains unknown, a recent report (TRANSFUSION 2006;46:1316-22) has shown that women receiving RhIG produce elevated levels of transforming growth factor (TGF)β-1, a powerful immunosuppressant cytokine. It was suggested that induction of TGFβ-1 and immunosuppression may be independent of cognate antigen recognition by RhIG. Herein, we present a description of a mother and baby that supports this hypothesis.

STUDY DESIGN AND METHODS

Red blood cells and serum were analyzed using saline-tube indirect antiglobulin test methods. RhIG (RhoGAM) was administered after each amniocentesis performed at 28, 31, and 36 weeks' gestation.

RESULTS

A group A, D-(cde), K+, Fy(a-b+), MNs, Jk(a+b+) mother with no detectable anti-D had an anti-Fy(a) titer of 4096 before RhIG but only 256 after RhIG. Mother gave birth to a group O, D-(cde), Fy(a+b+) healthy baby boy having a weak-positive direct antiglobulin test with anti-Fy(a) eluted from his cells and the titer in the cord serum was 4.

CONCLUSION

This case demonstrates the potential immunosuppressive properties of RhIG for down regulation of a possible clinically significant alloantibody, not anti-D, where no D+ antigen is in the circulation of the mother. The case illustrates the potential utility for using RhIG to modulate antibody levels in situations other than for classical suppression of anti-D production. Although the mechanism in this case is unknown, TGFβ-1-mediated or antibody-mediated immunosuppression to soluble nonparticulate antigens are possible mechanisms.

Mechanisms of anti-D action in the prevention of hemolytic disease of the fetus and newborn: what can we learn from rodent models?

PURPOSE OF REVIEW

Hemolytic disease of the fetus and newborn can be effectively prevented by administration of anti-D to the mother. In this setting, the IgG purified from the plasma of D-alloimmunized donors prevents the maternal immune response to D-positive red blood cells (RBC). Several monoclonal anti-D antibodies have recently been developed for potential use in the setting of hemolytic disease of the fetus and newborn; the functional assays used to assess the potential success of these antibodies have often assumed antigen clearance as the predominant mechanism of anti-D. Unfortunately, the in-vivo success of these monoclonal antibodies has thus far been limited. A similar inhibitory effect of IgG has been observed in animal models with a vast array of different antigens, referred to as antibody-mediated immune suppression (AMIS). Here, studies of AMIS are reviewed and the relevance of these findings for anti-D-mediated immunoprophylaxis is discussed.

RECENT FINDINGS

In animal models of AMIS, IgG-mediated antigen clearance was not sufficient for prevention of the antibody response to RBC. Furthermore, anti-RBC IgG inhibited B-cell priming to foreign RBC, but failed to prevent a T-cell response and immunological memory.

SUMMARY

The applicability of AMIS models for determining the true mechanism of anti-D, though uncertain, may nevertheless provide knowledge as to potential mechanisms of action of anti-RBC antibodies.

Specificity and affinity of human Fcγ receptors and their polymorphic variants for human IgG subclasses

Distinct genes encode 6 human receptors for IgG (hFcγRs), 3 of which have 2 or 3 polymorphic variants. The specificity and affinity of individual hFcγRs for the 4 human IgG subclasses is unknown. This information is critical for antibody-based immunotherapy which has been increasingly used in the clinics. We investigated the binding of polyclonal and monoclonal IgG1, IgG2, IgG3, and IgG4 to FcγRI; FcγRIIA, IIB, and IIC; FcγRIIIA and IIIB; and all known polymorphic variants. Wild-type and low-fucosylated IgG1 anti-CD20 and anti-RhD mAbs were also examined. We found that (1) IgG1 and IgG3 bind to all hFcγRs; (2) IgG2 bind not only to FcγRIIAH131, but also, with a lower affinity, to FcγRIIAR131 and FcγRIIIAV158; (3) IgG4 bind to FcγRI, FcγRIIA, IIB and IIC and FcγRIIIAV158; and (4) the inhibitory receptor FcγRIIB has a lower affinity for IgG1, IgG2, and IgG3 than all other hFcγRs. We also identified parameters that determine the specificity and affinity of hFcγRs for IgG subclasses. These results document how hFcγR specificity and affinity may account for the biological activities of antibodies. They therefore highlight the role of specific hFcγRs in the therapeutic and pathogenic effects of antibodies in disease.

Mechanisms of anti-D action in the prevention of hemolytic disease of the fetus and newborn

Anti-D is routinely and effectively used to prevent hemolytic disease of the fetus and newborn (HDFN) caused by the antibody response to the D antigen on fetal RBCs. Anti-D is a polyclonal IgG product purified from the plasma of D-alloimmunized individuals. The mechanism of anti-D has not been fully elucidated. Antigenic epitopes are not fully masked by anti-D and are available for immune system recognition. However, a correlation has frequently been observed between anti-D-mediated RBC clearance and prevention of the antibody response, suggesting that anti-D may be able to destroy RBCs without triggering the adaptive immune response. Anti-D-opsonized RBCs may also elicit inhibitory FcgammaRIIB signaling in B cells and prevent B cell activation. The ability of antigen-specific IgG to inhibit antibody responses has also been observed in a variety of animal models immunized with a vast array of different antigens, such as sheep RBCs (SRBC). This effect has been referred to as antibody-mediated immune suppression (AMIS). In animal models, IgG inhibits the antibody response, but the T-cell response and memory may still be intact. IgG does not mask all epitopes, and IgG-mediated RBC clearance or FcgammaRIIB-mediated B-cell inhibition do not appear to mediate the AMIS effect. Instead, IgG appears to selectively disrupt B cell priming, although the exact mechanism remains obscure. While the applicability of animal models of AMIS to understanding the true mechanism of anti-D remains uncertain, the models have nevertheless provided us with insights into the possible IgG effects on the immune response.

Lessons learnt from many years of experience using anti-D in humans for prevention of RhD immunization and haemolytic disease of the fetus and newborn

For 40 years prophylactic anti-D has been given to D-negative women after parturition to prevent haemolytic disease of the fetus and newborn. Monoclonal or recombinant anti-D may provide alternatives to the current plasma-derived polyclonal IgG anti-D, although none of them have yet proved as effective in phase 1 clinical trials. The variation in efficacy of the antibodies may have been influenced by heterogeneity in glycosylation of anti-D produced from different cell lines. Some aspects of the conduct of the human studies, most notably the use of low doses of anti-D and target D positive red cells in vivo, may aid the design of the clinical development of other immunomodulatory drugs in order to minimize adverse effects.

Transfusion of IgG-opsonized foreign red blood cells mediates reduction of antigen-specific B cell priming in a murine model

Hemolytic disease of the fetus and newborn can be effectively prevented by administration of anti-D to the mother. The administered IgG results in the attenuation of RBC-specific Ab production, a process termed Ab-mediated immune suppression (AMIS). Because in animal models of AMIS no major effect on T cell priming occurs, we hypothesized that the effect of the IgG on the immune system under AMIS conditions may involve a deficiency in B cell priming. We therefore challenged mice with either untreated RBCs or IgG-opsonized RBCs (AMIS) and assessed B cell priming. B cells from mice transfused with untreated RBCs, but not from mice treated under AMIS conditions, were primed as assessed by their ability to function as Ag-specific APCs to appropriate T cells. To our knowledge, this is the first report demonstrating that AMIS inhibits the appearance of Ag-primed RBC-specific B cells.

Immunoglobulin G-mediated regulation of the murine immune response to transfused red blood cells occurs in the absence of active immune suppression: implications for the mechanism of action of anti-D in the prevention of haemolytic disease of the fetus and newborn?

Anti-D has been widely and effectively used in Rhesus blood group D negative mothers for the prevention of haemolytic disease of the fetus and newborn; its mechanism of action however, often referred to as antibody-mediated immune suppression (AMIS), remains largely unresolved. We investigated, in a murine model, whether active immune suppression or clonal deletion mediated by anti-red blood cell (RBC) immunoglobulin G (IgG) could explain the phenomenon of AMIS. Transfusion of IgG-opsonized foreign RBCs (i.e. AMIS) strongly attenuated antibody responses compared to transfusion of untreated foreign RBCs. When the AMIS-mice were subsequently transfused with untreated RBCs, no immune suppression was observed at 5 and 35 days after AMIS induction; in fact, the mice responded to retransfusion with untreated RBCs in a manner that was characteristic of a secondary immune response. When IgG-opsonized RBCs were transfused concurrently with untreated RBCs, a dose-dependent reduction of the antibody response was observed. This work suggests that the attenuation of the antibody responsiveness by anti-RBC IgG is not associated with active immune suppression or clonal deletion at either the T-cell or B-cell level; rather, the effect appears more characteristic of B-cell unresponsiveness to IgG-opsonized RBCs. These results may have implications for the understanding of the mechanism of action of anti-D in haemolytic disease of the fetus and newborn.

Immune regulation by B cells and antibodies a view towards the clinic

B lymphocytes contribute to immunity in multiple ways, including production of antibodies, presentation of antigen to T cells, organogenesis of secondary lymphoid organs, and secretion of cytokines. Recent clinical trials have shown that depleting B cells can be highly beneficial for patients with autoimmune diseases, implicating B cells and antibodies as key drivers of pathology. However, it should be kept in mind that B cell responses and antibodies also have important regulatory roles in limiting autoimmune pathology. Here, we analyze clinical examples illustrating the potential of antibodies as treatment for immune-mediated disorders and discuss the underlying mechanisms. Furthermore, we examine the regulatory functions of activated B cells, their involvement in the termination of some experimental autoimmune diseases, and their use in cell-based therapy for such pathologies. These suppressive functions of B cells and antibodies do not only open new ways for harnessing autoimmune illnesses, but they also should be taken into account when designing new strategies for vaccination against microbes and tumors.