Quantification of IgG anti-D bound to D-positive red cells infused into D-negative subjects after intramuscular injection of monoclonal anti-D

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.

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.

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.

Monoclonal anti-D antibodies to prevent alloimmunization: lessons from clinical trials

In the past 20 years, numerous monoclonal anti-D antibodies have been developed in order to replace the human plasma derived anti-D immunoglobulins, using different in vitro functional assays as screening methods. Some of these monoclonal antibodies have been evaluated in exploratory in vivo clinical trials, notably for their ability to mediate the clearance of D-positive red cells. A review of these reported trials is presented and the results are analyzed in the light of the newly published hypothesis conferring an important role to some Fc-FcgammaR interactions and to the glycosylation-dependent potency of the monoclonal anti-D antibodies.

Clearance of red cells by monoclonal IgG3 anti-D in vivo is affected by the VF polymorphism of Fcgamma RIIIa (CD16)

Human red cells (RBC) coated with IgG anti-D are cleared from the circulation to the spleen by macrophages which express IgG receptors (Fcgamma R). Polymorphisms of Fcgamma RIIa and Fcgamma RIIIa affect IgG binding in vitro, and may alter the efficiency of clearance of immune complexes in vivo. In a RBC clearance study, 22 Rh D-negative subjects were given 100-400 micro g human monoclonal or polyclonal IgG anti-D i.m. followed 48 h later by 51Cr-labelled D+ RBC. The half lives of the infused D+ RBC were determined, together with the coating levels of anti-D on the D+ RBC. Fcgamma RIIA and FcgammaRIIIA genotyping was performed. Large ranges of phagocytosis and extracellular lysis of RBC in vitro, and of half lives of RBC in vivo, were observed. Clearance of RBC coated with monoclonal IgG3 anti-D (BRAD-3) was more rapid in five subjects homozygous for Fcgamma RIIIa-F/F158 than in three subjects expressing the Fcgamma RIIIa-V158 allele (P = 0.024). This effect was not observed, however, for those individuals given polyclonal anti-D. There was also no significant difference in the efficiency of RBC destruction in vitro or of RBC clearance in vivo between the subjects analysed for individual genotypes or alleles or combinations of alleles. In conclusion, the presence of the Fcgamma RIIIa-V158 allele compromised the efficiency of removal of RBC coated with IgG3 anti-D.

Monoclonal anti-D development programme

Administration of anti-D immunoglobulin to D- women after delivery of a D+ infant has dramatically reduced the number of immunised women and cases of haemolytic disease of the fetus and newborn. The use of monoclonal anti-D might alleviate some of the pressures on maintaining adequate supplies of plasma sourced anti-D. Two human monoclonal antibodies, BRAD-3 (IgG1) and BRAD-5 (IgG3), with proven activity in in vitro functional (immunological) assays with cells bearing IgG Fc receptors (Fc gammaR) were selected for clinical studies. They were prepared by purification of IgG secreted by culture of the Epstein-Barr virus-transformed B cell lines in hollow fibre bioreactors. The mean half-lives of BRAD-3 and BRAD-5 in D- subjects were 10.2 and 22.2 days, respectively. The clearance of D+ red cells injected into D- subjects was accelerated by prior injection of the monoclonal antibodies, both individually and blended (3:1, BRAD-5/BRAD-3). The clearance rate was related to the amount of anti-D on the red cells. Clearance of the D+ red cells coated with BRAD-3 was more rapid in subjects homozygous for Fc gammaRIIIa-F/F158 than in those expressing the Fc gammaRIIIa-V158 allele. The subjects were protected from Rh D immunisation. A large multi-centre study evaluated the BRAD-3/5 blend for its ability to prevent Rh D immunisation in 95 D- subjects given 400 microg i.m. 24 h after injection of 5 ml D+ red cells. Challenge injections of D+ red cells alone were given 24 and 36 weeks later, and blood samples were taken every 4 weeks from the subjects throughout the study for detection of anti-D responses. There was one definite and one possible failure of protection; in one subject the plasma anti-D level rose from week 12 onwards, and in another individual rapid seroconversion was observed at week 28. Considering the relatively large dose of red cells and the number of subjects studied, it was concluded that the failure rate was much lower than in routine Rh D prophylaxis. The responder rate was 13% by week 36 and 24% by week 48. There was no relationship between HLA haplotype and Rh D immunisation. The low percentage of responders and the modest levels of endogenous anti-D produced suggested that administration of monoclonal anti-D had induced long-term specific suppression of anti-D responses in these subjects. The most likely mechanism of action was considered to be inhibition of B cells resulting from co-cross-linking antigen receptors with inhibitory Fc gammaR when the B cells contacted red cells that had bound passive anti-D.

On the mechanism of tolerance to the Rh D antigen mediated by passive anti-D (Rh D prophylaxis)

Anti-D prophylaxis is the most successful clinical application of antibody-mediated immune suppression. Passive IgG anti-D is given to Rh D-negative women to prevent immunisation to foetal Rh D-positive red blood cells (RBC) and subsequent haemolytic disease of the newborn. Despite its widespread use and efficacy, the mechanism of action of this therapy is unproven. The known facts about the antigen, antibody response, dose of anti-D, RBC clearance and effects of the passive anti-D on subsequent primary and secondary immune responses are discussed in relation to recent information on ways by which immune responses may be suppressed. Most Rh D antigen sites on RBC are not bound by passive anti-D, and thus epitope masking (which may occur in experimental murine models using xenogeneic RBC) is not the reason why anti-D responses are prevented by administration of prophylactic anti-D. It is hypothesised that although clearance and destruction of the antigenic RBC may be a contributing factor in preventing immunisation, down-regulation of antigen-specific B cells through co-ligation of B cell receptors and inhibitory IgG Fc receptors must also occur.

In vivo studies of monoclonal anti-D and the mechanism of immune suppression

Administration of anti-D immunoglobulin to D- women after delivery of a D+ infant has dramatically reduced the number of immunised women and cases of haemolytic disease of the fetus and newborn. The use of monoclonal anti-D might alleviate some of the pressures on maintaining adequate supplies of plasma sourced anti-D. Two human monoclonal antibodies, BRAD-3 (IgG1) and BRAD-5 (IgG3), with proven activity in in vitro functional (immunological) assays with cells bearing IgG Fc receptors (Fc gamma R) were selected for clinical studies. They were prepared by purification of IgG secreted by culture of the Epstein-Barr virus-transformed B cell lines in hollow fibre bioreactors. The clearance of D+ red cells injected into D- subjects was accelerated by prior injection of the monoclonal antibodies, both individually and blended (3:1, BRAD-5: BRAD-3). The subjects were protected from Rh D immunisation. A large multicentre study evaluated the BRAD-3/5 blend for its ability to prevent Rh D immunisation in 95 D- subjects given 400 micrograms i.m. 24 hours after injection of 5 ml D+ red cells. Challenge injections of D+ red cells alone were given 24 and 36 weeks later, and blood samples were taken every 4 weeks from the subjects throughout the study for detection of anti-D responses. There was one definite and one possible failure of protection; in one subject the plasma anti-D level rose from week 12 onwards, and in another individual rapid seroconversion was observed at week 28. Considering the relatively large dose of red cells and the number of subjects studied, it was concluded that the failure rate was much lower than in routine Rh D prophylaxis. The responder rate was 13% by week 36 and 24% by week 48. The low percentage of responders and the modest levels of endogenous anti-D produced suggested that administration of monoclonal anti-D had induced long-term specific suppression of anti-D responses in these subjects. The most likely mechanism of action was considered to be inhibition of B cells resulting from co-crosslinking antigen receptors with inhibitory Fc gamma R when the B cells contacted red cells that had bound passive anti-D.

CHO expression of a novel human recombinant IgG1 anti-RhD antibody isolated by phage display

Replacement of the hyperimmune anti-Rhesus (Rh) D immunoglobulin, currently used to prevent haemolytic disease of the newborn, by fully recombinant human anti-RhD antibodies would solve the current logistic problems associated with supply and demand. The combination of phage display repertoire cloning with precise selection procedures enables isolation of specific genes that can then be inserted into mammalian expression systems allowing production of large quantities of recombinant human proteins. With the aim of selecting high-affinity anti-RhD antibodies, two human Fab libraries were constructed from a hyperimmune donor. Use of a new phage panning procedure involving bromelin-treated red blood cells enabled the isolation of two high-affinity Fab-expressing phage clones. LD-6-3 and LD-6-33, specific for RhD. These showed a novel reaction pattern by recognizing the D variants D(III), D(IVa), D(IVb), D(Va), D(VI) types I and II. D(VII), Rh33 and DFR. Full-length immunoglobulin molecules were constructed by cloning the variable regions into expression vectors containing genomic DNA encoding the immunoglobulin constant regions. We describe the first, stable, suspension growth-adapted Chinese hamster ovary (CHO) cell line producing a high affinity recombinant human IgG1 anti-RhD antibody adapted to pilot-scale production. Evaluation of the Fc region of this recombinant antibody by either chemiluminescence or antibody-dependent cell cytotoxicity (ADCC) assays demonstrated macrophage activation and lysis of red blood cells by human lymphocytes. A consistent source of recombinant human anti-RhD immunoglobulin produced by CHO cells is expected to meet the stringent safety and regulatory requirements for prophylactic application.

Comparison of flow cytometric assays with isotopic assays of (51)chromium-labeled cells for estimation of red cell clearance or survival in vivo

BACKGROUND

A comparison was made between flow cytometric and conventional radioisotopic assays in the determination of the clearance or survival of small volumes of (51)chromium-labeled D+ red cells after injection into volunteers.

STUDY DESIGN AND METHODS

Four clearance studies were performed using 4 mL of autologous D+ cells coated with anti-D at two concentrations (5 or 10 microg anti-D/mL red cells) transfused to two subjects at separate times. Five survival studies were carried out using 5 mL of frozen-thawed D+ cells transfused to five D- subjects with no detectable anti-D. Sequential blood samples were taken for gamma counting and flow cytometry. Several methods were used to stain the transfused red cells, and the data were analyzed by using three flow cytometers.

RESULTS

The determination of red cell clearance or survival by radioactivity measurements gave results consistent with published data. However, none of the flow cytometric assays exhibited the necessary sensitivity or accuracy in quantitation of the rare events to provide reliable data for the calculation of the initial clearance rate, the red cell half-life, or the mean cell lifespan, although rough estimates of red cell clearance were obtained in some subjects. This inability to accurately enumerate rare fluorescence-labeled cells was due mainly to the presence of "background" events, which were a considerable problem in some samples, when the coating level of anti-D was less than 3000 molecules of IgG per cell.

CONCLUSION

Flow cytometry may enable the crude estimation of the percentage of small volumes (<5 mL) of transfused D+ red cells, but in this study it was found that this method was not sufficiently accurate to determine the initial clearance rate, red cell half-life, or mean cell lifespan. If the proportion of transfused cells in the recipient is about 0.2 percent or less, the use of radioisotopes for labeling cells for quantitative in vivo red cell clearance or survival data should remain the method of choice.

Monoclonal anti-D for prophylaxis of RhD haemolytic disease of the newborn

The incidence of Rh D haemolytic disease of the fetus and newborn has been dramatically reduced by the prophylactic administration of anti-D immunoglobulin to Rh D-negative women. This preventive treatment depends on adequate supplies of anti-D derived from plasma of immunised donors, and replacement with monoclonal anti-D would be advantageous. Two monoclonal antibodies, BRAD-3 (IgG3) and BRAD-5 (IgG1) have been produced from EBV-transformed B-lymphoblastoid cell lines in Bristol and extensively characterised. Both have shown good (but differing) functional activities, determined by study of interactions of anti-D coated red cells with effector cells bearing IgG Fc receptors. Phase I clinical trials using Rh D negative male volunteers were undertaken in Bristol. The plasma half lives of BRAD-3 and BRAD-5 were characteristic for their IgG subclass, and both anti-D mediated accelerated circulatory clearance of D-positive red cells infused two days after i.m. injection of the antibodies. BRAD-3 and BRAD-5 were then shown to protect the volunteers from mounting a primary anti-D response to these D-positive red cells, and thus they may be suitable for Rh D prophylaxis of Rh D-negative women.

Quantitation of Rh D antigen sites on weak D and D variant red cells by flow cytometry

Information on the number of D sites on weak D (Du) and D variant cells is limited and incomplete. The aim of this study was to use a simple non-isotopic technique utilising a combination of flow cytometry and ELISA to quantitate the number of D sites on an extensive range of these cells. Five human monoclonal IgG anti-D (BRAD-7 (JAC10), BRAD-5, 2B6, BRAD-3, H27) and one affinity-purified polyclonal IgG anti-D were each used at a saturating concentration of 20 micrograms/ml. In general, BRAD-3, BRAD-5 and 2B6 gave the highest number of D sites per cell (SPC), H27 and the polyclonal anti-D were slightly lower, while BRAD-7 gave the lowest SPC with all D-positive cells tested except DFR. Interestingly, BRAD-7 gave the highest SPC with DFR cells. Rh D antigen density for R2R2 cells was approximately double that seen with either R2r or Rzero (presumed R0r) cells. R1R1 cells gave only moderately higher SPC than R1r cells.Higher SPC were obtained with the R1 haplotype if the CW antigen was present. Weak D, Va and VI cells of the R1 haplotype had higher SPC than those of the Rzero or R2 haplotypes. The majority of D variant cells were found to have lower SPC than normal cells, and for polyclonal anti-D, which was the only anti-D to react with all D variants, SPC decreased in the order IIIc > IIIa > HMii > IVa > Va > DFR > DBT > IVb > VII > II > HMi > VI. The number of molecules of IgG anti-D bound to D variant cells varied by up to 10 times with reactive monoclonal antibodies. The highest SPC on weak D and D variant cells were obtained with BRAD-7 (DFR, 9,500), BRAD-3 (Va, 12,500), BRAD-5 (II, 5,500; VII, 5,300; weak D, 1,300), H27 (III, 24,000; VI, 2,900; HMi, 3,000; HMii, 16,800) and polyclonal anti-D (IVa, 9,300; IVb, 4,000; DBT, 4,300).

Coordinator's report: an assessment of the functional activity of human Rh monoclonal antibodies after their evaluation by nine laboratories

The functional activity of 55 anti-D and 26 MAbs of other Rh specificities was determined as part of the Third International Workshop and Symposium on Monoclonal Antibodies Against Human Red Blood Cell and Related Antigens. Most MAbs were IgG1 (45 anti-D, 16 Rh). There was a large range of anti-D and IgG concentrations of the anti-D MAbs (0.4-1680 IU/ml, 1.5-400 micrograms/ml). Correlation between quantitative data from different laboratories was good. Six laboratories tested the anti-Ds in monocyte ADCC assays. Methods varied greatly, especially the effector cells used, the methods of red cell sensitisation, the effector to target cell ratio and the assay incubation times. There was some correlation of results between most laboratories, but results were better when similar assays were performed. The extent of monocyte-mediated haemolysis was related to the number of molecules of IgG anti-D bound to the red cells. Monocyte phagocytosis assays resulted in a greater variability in results between the four evaluating laboratories, though IgG3 MAbs were found more active than IgG1, and mostly promoted red cell adherence rather than phagocytosis. Two monocyte chemiluminescence assays found comparatively low activity of the IgG3 MAbs; the most active IgG1 anti-Ds were MAbs 93 and 95. Correlation between different monocyte-mediated assays was generally poor unless the assays were performed by the same laboratory. Results of a macrophage ADCC assay showed that the IgG3 anti-D's promoted greater haemolysis than most IgG1 MAbs. Only two IgG1 anti-D MAbs (70 and 104) mediated high haemolysis in this assay. Lymphocyte ADCC assays were utilised in four laboratories. The IgG3 antibodies exhibited low activity but one IgG1 anti-D (104) consistently mediated high haemolysis. There was no relationship between quantitation and haemolysis in this assay. A few MAbs exhibited low or no activity in most assays, mainly due to low quantitation. Most of the MAbs of non-D specificity mediated low functional activity which in most cases was not due to low quantitation. However two MAbs, 24 (anti-CcEe) and 25 (anti-CD), consistently showed good activity. This study highlighted the great variability in assay methodology between the nine participating laboratories, which resulted in some apparent differences in functional activity of some of the MAbs. The use of standardised methods as well as fewer MAbs tested and quantitated at different concentrations may help to determine the relevant factors contributing to IgG function.

Characterization and functional activity of human Rh monoclonal antibodies

For quantification of the number of molecules of IgG anti-D bound to RBC, a combination of flow cytometry plus Sol-ELISA was found more accurate than Sol-ELISA alone. However, some Rh MAbs gave very low values of cell-bound IgG using flow cytometry. Monocyte-mediated haemolysis correlated well with the number of molecules of red cell-bound IgG1 anti-D. The monocyte phagocytosis assay was found more sensitive but less quantitative than the monocyte ADCC assay. Most monoclonal antibodies mediated little haemolysis by lymphocytes in ADCC assays, and haemolysis was not related to IgG concentration, titres or cell-bound IgG.