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

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.

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.

Trogocytosis drives red blood cell antigen loss in association with antibody-mediated immune suppression

ABSTRACT

Red blood cell (RBC) alloimmunization to paternal antigens during pregnancy can cause hemolytic disease of the fetus and newborn (HDFN). This severe and potentially fatal neonatal disorder can be prevented by the administration of polyclonal anti-D through a mechanism referred to as antibody-mediated immune suppression (AMIS). Although anti-D prophylaxis effectively prevents HDFN, a lack of mechanistic clarity has hampered its replacement with recombinant agents. The major theories behind AMIS induction in the hematologic literature have classically centered around RBC clearance; however, antigen modulation/loss has recently been proposed as a potential mechanism of AMIS. To explore the primary mechanisms of AMIS, we studied the ability of 11 different antibodies to induce AMIS, RBC clearance, antigen loss, and RBC membrane loss in the HOD (hen egg lysozyme-ovalbumin-human Duffy) murine model. Antibodies targeting different portions of the HOD molecule could induce AMIS independent of their ability to clear RBCs; however, all antibodies capable of inducing a strong AMIS effect also caused significant in vivo loss of the HOD antigen in conjunction with RBC membrane loss. In vitro studies of AMIS-inducing antibodies demonstrated simultaneous RBC antigen and membrane loss, which was mediated by macrophages. Confocal live-cell microscopy revealed that AMIS-inducing antibodies triggered RBC membrane transfer to macrophages, consistent with trogocytosis. Furthermore, anti-D itself can induce trogocytosis even at low concentrations, when phagocytosis is minimal or absent. In view of these findings, we propose trogocytosis as a mechanism of AMIS induction.

Antibody-mediated antigen loss switches augmented immunity to antibody-mediated immunosuppression

Antibodies against fetal red blood cell (RBC) antigens can cause hemolytic disease of the fetus and newborn (HDFN). Reductions in HDFN due to anti-RhD antibodies have been achieved through use of Rh immune globulin (RhIg), a polyclonal antibody preparation that causes antibody-mediated immunosuppression (AMIS), thereby preventing maternal immune responses against fetal RBCs. Despite the success of RhIg, it is only effective against 1 alloantigen. The lack of similar interventions that mitigate immune responses toward other RBC alloantigens reflects an incomplete understanding of AMIS mechanisms. AMIS has been previously attributed to rapid antibody-mediated RBC removal, resulting in B-cell ignorance of the RBC alloantigen. However, our data demonstrate that antibody-mediated RBC removal can enhance de novo alloimmunization. In contrast, inclusion of antibodies that possess the ability to rapidly remove the target antigen in the absence of detectable RBC clearance can convert an augmented antibody response to AMIS. These results suggest that the ability of antibodies to remove target antigens from the RBC surface can trigger AMIS in situations in which enhanced immunity may otherwise occur. In doing so, these results hold promise in identifying key antibody characteristics that can drive AMIS, thereby facilitating the design of AMIS approaches toward other RBC antigens to eliminate all forms of HDFN.

Factor VIII antibody immune complexes modulate the humoral response to factor VIII in an epitope-dependent manner

Introduction

Soluble antigens complexed with immunoglobulin G (IgG) antibodies can induce robust adaptive immune responses in vitro and in animal models of disease. Factor VIII immune complexes (FVIII-ICs) have been detected in individuals with hemophilia A and severe von Willebrand disease following FVIII infusions. Yet, it is unclear if and how FVIII-ICs affect antibody development over time.

Methods

In this study, we analyzed internalization of FVIII complexed with epitope-mapped FVIII-specific IgG monoclonal antibodies (MAbs) by murine bone marrow-derived dendritic cells (BMDCs) in vitro and antibody development in hemophilia A (FVIII-/-) mice injected with FVIII-IC over time.

Results

FVIII complexed with 2-116 (A1 domain MAb), 2-113 (A3 domain MAb), and I55 (C2 domain MAb) significantly increased FVIII uptake by BMDC but only FVIII/2-116 enhanced antibody titers in FVIII-/- mice compared to FVIII alone. FVIII/4A4 (A2 domain MAb) showed similar FVIII uptake by BMDC to that of isolated FVIII yet significantly increased antibody titers when injected in FVIII-/- mice. Enhanced antibody responses observed with FVIII/2-116 and FVIII/4A4 complexes in vivo were abrogated in the absence of the FVIII carrier protein von Willebrand factor.

Conclusion

These findings suggest that a subset of FVIII-IC modulates the humoral response to FVIII in an epitope-dependent manner, which may provide insight into the antibody response observed in some patients with hemophilia A.

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

Passively transferred IgG enhances humoral immunity to a red blood cell alloantigen in mice

Antibodies are typically thought of as the endpoint of humoral immunity that occur as the result of an adaptive immune response. However, affinity-matured antibodies can be present at the initiation of a new immune response, most commonly because of passive administration as a medical therapy. The current paradigm is that immunoglobulin M (IgM), IgA, and IgE enhance subsequent humoral immunity. In contrast, IgG has a "dual effect" in which it enhances responses to soluble antigens but suppresses responses to antigens on red blood cells (RBCs) (eg, immunoprophylaxis with anti-RhD). Here, we report a system in which passive antibody to an RBC antigen promotes a robust cellular immune response leading to endogenous CD4+ T-cell activation, germinal center formation, antibody secretion, and immunological memory. The mechanism requires ligation of Fcγ receptors on a specific subset of dendritic cells that results in CD4+ T-cell activation and expansion. Moreover, antibodies cross-enhance responses to a third-party antigen, but only if it is expressed on the same RBC as the antigen recognized by the antibody. Importantly, these observations were IgG subtype specific. Thus, these findings demonstrate that antibodies to RBC alloantigens can enhance humoral immunity in an IgG subtype-specific fashion and provide mechanistic elucidation of the enhancing effects.

IgG-mediated suppression of antibody responses: Hiding or snatching epitopes?

Antibodies forming a complex with antigen in vivo can dramatically change the antibody response to this antigen. In some situations, the response will be a 100-fold stronger than in animals immunized with antigen alone, and in other situations, the response will be completely suppressed. IgG is known to suppress the antibody response, for example to erythrocytes, and this is used clinically in Rhesus prophylaxis. The mechanism behind IgG-mediated immune suppression is still not understood. Here, we will review studies performed in experimental animal models and discuss the various hypotheses put forward to explain the profound suppressive effect of IgG. We conclude that an exclusive role for negative regulation of B cells through FcγRIIB, increased clearance of erythrocytes from the circulation or complement-mediated lysis is unlikely. Epitope masking, where IgG hides the epitope from B cells, or trogocytosis, where IgG removes the epitope from the erythrocyte, is compatible with many observations. These two mechanisms are not mutually exclusive. Moreover, it cannot be ruled out that clearance, in combination with other mechanisms, plays a role.

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.

IgG Suppresses Antibody Responses to Sheep Red Blood Cells in Double Knock-Out Mice Lacking Complement Factor C3 and Activating Fcγ-Receptors

Antigen-specific IgG antibodies, passively administered together with erythrocytes, prevent antibody responses against the erythrocytes. The mechanism behind the suppressive ability of IgG has been the subject of intensive studies, yet there is no consensus as to how it works. An important question is whether the Fc-region of IgG is required. Several laboratories have shown that IgG suppresses equally well in wildtype mice and mice lacking the inhibitory FcγIIB, activating FcγRs (FcγRI, III, and IV), or complement factor C3. These observations consistently suggest that IgG-mediated suppression does not rely on Fc-mediated antibody functions. However, it was recently shown that anti-KEL sera failed to suppress antibody responses to KEL-expressing transgenic mouse erythrocytes in double knock-out mice lacking both activating FcγRs and C3. Yet, in the same study, antibody-mediated suppression worked well in each single knock-out strain. This unexpected observation suggested Fc-dependence of IgG-mediated suppression and prompted us to investigate the issue in the classical experimental model using sheep red blood cells (SRBC) as antigen. SRBC alone or IgG anti-SRBC together with SRBC was administered to wildtype and double knock-out mice lacking C3 and activating FcγRs. IgG efficiently suppressed the IgM and IgG anti-SRBC responses in both mouse strains, thus supporting previous observations that suppression in this model is Fc-independent.

Imprinting, immunodominance, and other impediments to generating broad influenza immunity

Natural influenza virus infections and seasonal vaccinations often do not confer broadly neutralizing immunity across diverse influenza strains. In addition, the virus is capable of rapid antigenic drift in order to evade pre-existing immunity. The surface glycoproteins, hemagglutinin, and neuraminidase can easily mutate their immunodominant epitopes without impacting fitness. Skewing human antibody repertoires to target more conserved epitopes is thus an expanding area of research: Many groups are attempting to produce universal influenza vaccines that can protect across a wide variety of strains. Achieving this goal will require a detailed understanding of how infection history impacts humoral responses. It will also require the ability to manipulate or enhance B cell selection in order to expand clones that can recognize subdominant but protective epitopes. In this review, we will discuss what immune imprinting means to immunologists and describe efforts to overcome or silence imprinting in order to improve vaccination efficiency.

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.

Specific IgM and Regulation of Antibody Responses

Specific IgM, administered together with the antigen it recognizes, enhances primary antibody responses, formation of germinal centers, and priming for secondary antibody responses. The response to all epitopes on the antigen to which IgM binds is usually enhanced. IgM preferentially enhances responses to large antigens such as erythrocytes, malaria parasites, and keyhole limpet hemocyanine. In order for an effect to be seen, antigens must be administered in suboptimal concentrations and in close temporal relationship to the IgM. Enhancement is dependent on the ability of IgM to activate complement, but the lytic pathway is not required. Enhancement does not take place in mice lacking complement receptors 1 and 2 (CR1/2) suggesting that the role of IgM is to generate C3 split products, i.e., the ligands for CR1/2. In mice, these receptors are expressed on follicular dendritic cells (FDCs) and B cells. Optimal IgM-mediated enhancement requires that both cell types express CR1/2, but intermediate enhancement is seen when only FDCs express the receptors and low enhancement when only B cells express them. These observations imply that IgM-mediated enhancement works through several, non-mutually exclusive, pathways. Marginal zone B cells can transport IgM-antigen-complement complexes, bound to CR1/2, from the marginal zone and deposit them onto FDCs. In addition, co-crosslinking of the BCR and the CR2/CD19/CD81 co-receptor complex may enhance signaling to specific B cells, a mechanism likely to be involved in induction of early extrafollicular antibody responses.

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.

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.

ABO incompatibility and RhIG immunoprophylaxis protect against non-D alloimmunization by pregnancy

BACKGROUND

Hemolytic disease of the fetus and newborn (HDFN) is caused by maternal antibodies against fetal red blood cell antigens, most often anti-D, -K, or -c. ABO incompatibility between mother and child and anti-D immunoprophylaxis (RhIG) are known to reduce the risk of D immunization and subsequent HDFN. However, no immunoprophylaxis has been developed to prevent non-D immunizations.

STUDY DESIGN AND METHODS

We evaluated whether ABO incompatibility has a preventive effect on formation of non-D alloantibodies, by performing a case-control study including pregnant women with newly detected non-D antibodies, identified within a nationwide data set, immunized during their first pregnancy and/or delivery. Subsequently, we assessed a possible protective effect of RhIG in a subgroup with non-Rh antibodies only. The proportions of previous ABO incompatibility and of RhIG administrations of these women were compared to the known rate of 19.4% ABO incompatibility and 9.9% RhIG administrations (D- women carrying a D+ child) in the general population of pregnant women.

RESULTS

A total of 11.9% of the 232 included immunized women had a possible ABO incompatibility in their first pregnancy (vs. expected 19.4%; 95% confidence interval [CI], 7.3-18.8; p = 0.036). Furthermore, 1.0% women with non-Rh antibodies were D-, delivered a D+ child, and had therefore received RhIG, whereas 9.9% was expected (95% CI, 0.18-5.50; p = 0.003).

CONCLUSION

We found that ABO incompatibility and RhIG reduce the risks not only for D, but also for non-Rh immunizations, suggesting that antibody-mediated immune suppression in this condition is not antigen specific.

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.

Red blood cell alloimmunization: new findings at the bench and new recommendations for the bedside

PURPOSE OF REVIEW

To summarize recent discoveries from clinical studies and animal models that contribute to understanding the alloimmune response to non-ABO blood group antigens.

RECENT FINDINGS

Several studies have confirmed high rates of alloimmunization among patients requiring chronic red blood cell (RBC) transfusion. Moreover, 'triggers' for alloantibody development in the transfusion setting have been identified, with a number of investigations linking recipient inflammation to a higher likelihood of alloimmunization. Additional associations between human leukocyte antigen expression and CD4 T-cell markers in 'responder' or 'nonresponder' humans have been revealed. Recent animal studies have described novel mechanistic properties by which the alloimmune response is governed, including the critical role played by dendritic cells in transfusion-associated alloimmunization. New light has also been shed on the properties of alloantibodies developed as a result of pregnancy, as well as mechanisms through which such alloimmunization may be prevented.

SUMMARY

Many of the clinical/biological factors that contribute to the RBC alloimmune response have been further elucidated. This knowledge will be applied to identify individuals most likely to mount an immune response to RBC antigens, such that appropriate resources and strategies for preventing alloimmunization (or mitigating its harmful effects) can be implemented.

Mice Immunized with IgG Anti-Sheep Red Blood Cells (SRBC) Together With SRBC Have a Suppressed Anti-SRBC Antibody Response but Generate Germinal Centers and Anti-IgG Antibodies in Response to the Passively Administered IgG

Antigen-specific IgG antibodies, passively administered together with large particulate antigens such as erythrocytes, can completely suppress the antigen-specific antibody response. The mechanism behind has been elusive. Herein, we made the surprising observation that mice immunized with IgG anti-sheep red blood cells (SRBC) and SRBC, in spite of a severely suppressed anti-SRBC response, have a strong germinal center (GC) response. This occurred regardless of whether the passively administered IgG was of the same allotype as that of the recipient or not. Six days after immunization, the GC size and the number of GC B cells were higher in mice immunized with SRBC alone than in mice immunized with IgG and SRBC, but at the other time points these parameters were similar. GCs in the IgG-groups had a slight shift toward dark zone B cells 6 days after immunization and toward light zone B cells 10 days after immunization. The proportions of T follicular helper cells (T_FH) and T follicular regulatory cells (T_FR) were similar in the two groups. Interestingly, mice immunized with allogeneic IgG anti-SRBC together with SRBC mounted a vigorous antibody response against the passively administered suppressive IgG. Thus, although their anti-SRBC response was almost completely suppressed, an antibody response against allogeneic, and probably also syngeneic, IgG developed. This most likely explains the development of GCs in the absence of an anti-SRBC antibody response.

Expression of Fc<gamma>Rs on monocytes among Kawasaki disease patients with coronary artery lesions

OBJECTIVE

To study expression of Fc gamma receptors (Fc<gamma>Rs) on monocytes in Kawasaki disease (KD) patients with coronary artery lesions (CAL).

METHODS

160 newly diagnosed KD patients and 80 health children were enrolled in this study. All patients were scheduled to receive both aspirin and intravenous immunoglobulin (IVIG). Serial blood samples were obtained before and 3days after completing IVIG therapy. The first two-dimensional echocardiographic examination was performed for all KD patients within 10days, and was repeated at 3weeks. CAL was defined by coronary artery Z-scores≥2.5 by echocardiography. Expression of inhibitory and activating Fc<gamma>Rs on CD14⁺ monocytes (MCs) was assessed by flow cytometry. Cytokine expression in MC was evaluated by PCR.

RESULTS

Of the 160KD patients enrolled in this study, 36 had coronary artery lesions (KD-CAL⁺ group), while 124 did not (KD-CAL^- group). There was no significant difference in Fc<gamma>RI expression on MCs from KD patients and that of Ctrls. Although Fc<gamma>RIII and Fc<gamma>RIIa levels were significantly higher in KD patients compared with those in Ctrls, there were no significant differences between the KD-CAL⁺ and KD-CAL^- groups. Fc<gamma>RIIb expression in the KD patients was lower than that of Ctrls, meanwhile expression in the KD-CAL⁺ group was lower than that in the KD-CAL^- group. After IVIG therapy, Fc<gamma>RIIb expression increased in KD-CAL⁺, but did not reach the normal range. A negative correlation was observed between the levels of IL-6, TNF-α and Fc<gamma>RIIb expression.

CONCLUSION

Decreased Fc<gamma>RIIb expression on MCs may contribute to the development of coronary artery lesions in patients with KD.

Epitope-Specific Suppression of IgG Responses by Passively Administered Specific IgG: Evidence of Epitope Masking

Specific IgG, passively administered together with particulate antigen, can completely prevent induction of antibody responses to this antigen. The ability of IgG to suppress antibody responses to sheep red blood cells (SRBCs) is intact in mice lacking FcγRs, complement factor 1q, C3, or complement receptors 1 and 2, suggesting that Fc-dependent effector functions are not involved. Two of the most widely discussed explanations for the suppressive effect are increased clearance of IgG–antigen complexes and/or that IgG “hides” the antigen from recognition by specific B cells, so-called epitope masking. The majority of data on how IgG induces suppression was obtained through studies of the effects on IgM-secreting single spleen cells during the first week after immunization. Here, we show that IgG also suppresses antigen-specific extrafollicular antibody-secreting cells, germinal center B-cells, long-lived plasma cells, long-term IgG responses, and induction of memory antibody responses. IgG anti-SRBC reduced the amount of SRBC in the spleens of wild-type, but not of FcγR-deficient mice. However, no correlation between suppression and the amount of SRBC in the spleen was observed, suggesting that increased clearance does not explain IgG-mediated suppression. Instead, we found compelling evidence for epitope masking because IgG anti-NP administered with NP-SRBC suppressed the IgG anti-NP, but not the IgG anti-SRBC response. Vice versa, IgG anti-SRBC administered with NP-SRBC, suppressed only the IgG anti-SRBC response. In conclusion, passively transferred IgG suppressed all measured parameters of an antigen-specific antibody/B cell response and an important mechanism of action is likely to be epitope masking.

Antigen modulation as a potential mechanism of anti-KEL immunoprophylaxis in mice

Red blood cell (RBC) alloimmunization is a serious complication of transfusion or pregnancy. Despite the widespread use of Rh immune globulin to prevent pregnancy associated anti-D alloimmunization, its mechanism of action remains elusive. We have previously described a murine model in which immunoprophylaxis with polyclonal anti-KEL sera prevents alloimmunization in wild-type recipients transfused with transgenic murine RBCs expressing the human KEL glycoprotein. To investigate the mechanism of action, we have now evaluated the outcome of immunoprophylaxis treatment in mice lacking Fcγ receptors (FcγRs), complement (C3), both, or none. Whereas polyclonal anti-KEL sera completely prevented alloimmunization in wild-type and single-knockout (KO) mice lacking FcγRs or C3, double-KO mice lacking both FcγRs and C3 became alloimmunized despite immunoprophylaxis. Rapid clearance of essentially all transfused RBCs with detectable KEL glycoprotein antigen occurred within 24 hours in wild-type and single-KO recipients treated with immunoprophylaxis, with the transfused RBCs remaining in circulation having minimal KEL glycoprotein antigen detectable by flow cytometry or western blot. In contrast, transfused RBCs with the KEL glycoprotein antigen fully intact continued to circulate for days in double-KO mice despite treatment with immunoprophylaxis. Further, in vitro phagocytosis assays showed no consumption of opsonized murine RBCs by double-KO splenocytes. Taken in combination, our data suggest that modulation of the KEL antigen (and potentially RBC clearance) by redundant recipient pathways involving both FcγRs and C3 may be critical to the mechanism of action of polyclonal anti-KEL immunoprophylaxis. These findings could have implications for the development of immunoprophylaxis programs in humans.

Antibody-mediated immune suppression is improved when blends of anti-RBC monoclonal antibodies are used in mice

Although the prevention of hemolytic disease of the fetus and newborn is highly effective using polyclonal anti-D, a recombinant alternative is long overdue. Unfortunately, anti-D monoclonal antibodies have been, at best, disappointing. To determine the primary attribute defining an optimal antibody, we assessed suppression of murine red blood cell (RBC) immunization by single-monoclonal antibodies vs defined blends of subtype-matched antibodies. Allogeneic RBCs expressing the HOD antigen (hen egg lysozyme [HEL]-ovalbumin-human transmembrane Duffy(b)) were transfused into naïve mice alone or together with selected combinations of HEL-specific antibodies, and the resulting suppressive effect was assessed by evaluating the antibody response. Polyclonal HEL antibodies dramatically inhibited the antibody response to the HOD antigen, whereas single-monoclonal HEL antibodies were less effective despite the use of saturating doses. A blend of monoclonal HEL-specific antibodies reactive with different HEL epitopes significantly increased the suppressive effect, whereas a blend of monoclonal antibodies that block each other's binding to the HEL protein did not increase suppression. In conclusion, these data show that polyclonal antibodies are superior to monoclonal antibodies at suppressing the immune response to the HOD cells, a feature that can be completely recapitulated using monoclonal antibodies to different epitopes.

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.