Mononuclear phagocyte assays, autoanalyzer quantitation and IgG subclasses of maternal anti-RhD in the prediction of the severity of haemolytic disease in the fetus before 32 weeks gestation

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.

Étude quantitative des sous-classes d’IgG anti-D par Élisa au cours de la maladie hémolytique néonatale

The quantification of IgG anti-D subclasses is one of the most important parameters considered in the assessment of the severity of hemolytic disease of the newborn. Traditionally IgG subclassing is performed using qualitative haemagglutination methods, difficult to interpret. A quantitative enzyme-linked immunosorbent assay (Elisa) was implemented for measuring IgG anti-D subclasses in 20 sera collected from 14 RhD-immunized pregnant women. All 4 IgG subclasses were detected in the 20 sera tested. The mean proportion of IgG1 was 52.8%. The mean proportion of IgG3 was 30.7%. The mean proportions of IgG2 and IgG4 were 14.5 and 1.9% respectively. A good correlation between the sum of IgG subclasses and the severity of HDN was found. Severe HDN occurred when both IgG1 and IgG3 were present. IgG1 anti-D was the predominant subclass in 4 of the 8 severe cases.

Laboratory assays for predicting the severity of haemolytic disease of the fetus and newborn

Haemolytic disease of the fetus and newborn (HDFN) is characterised by the presence of IgG antibodies in the maternal circulation which cause haemolysis in the fetus by crossing the placenta and sensitising red cells for destruction by macrophages in the fetal spleen. Serological, quantitative and cellular assays have all been developed to predict the severity of HDFN. These assays measure and/or characterise alloantibodies in the maternal circulation. Quantitative assays which accurately measure antibody levels correlate with disease severity better than serological assays which are inherently less precise. Nevertheless, high antibody levels are found in some cases of mild HFDN and relatively low antibody levels are found in some severe cases. This suggests that disease severity is influenced by factors in addition to antibody concentration. These factors remain to be fully elucidated but may include: the subclass and glycosylation of maternal antibodies; the structure, site density, maturational development and tissue distribution of blood group antigens; the efficiency of IgG transport to the fetus; the functional maturity of the fetal spleen; polymorphisms which affect Fc receptor function; and the presence of HLA-related inhibitory antibodies. Cellular assays which are sensitive to factors affecting antibody function have, therefore, been developed in an attempt to improve the prediction of disease severity. Although these assays are cumbersome, there are now sufficient data to suggest that some cellular assays provide clinically useful information to complement serological and quantitative assays.

A comparison of in vitro tests for predicting the severity of haemolytic disease of the fetus and newborn

Haemolytic disease of the newborn (HDN) is characterized by the presence of IgG antibodies in the maternal circulation which cause haemolysis in the fetus by crossing the placenta and sensitizing red cells for destruction by macrophages in the fetal spleen. Numerous serological, quantitative and cellular assays have been developed to predict the severity of HDN. These assays all measure and/or characterize alloantibodies in the maternal circulation. Quantitative assays which accurately measure antibody levels correlate with disease severity better than serological assays which are inherently less precise. Nevertheless, high antibody levels are found in some cases of mild HDN and relatively low antibody levels are found in some severe cases. This suggests that disease severity is influenced by factors in addition to antibody concentration. These factors remain to be fully elucidated but may include the subclass and glycosylation of maternal antibodies, the structure, site density, maturational development and tissue distribution of blood group antigens, the efficiency of IgG transport to the fetus, the functional maturity of the fetal spleen, polymorphisms which affect Fc receptor function, and the presence of HLA-related inhibitory antibodies. Cellular assays which are sensitive to factors affecting antibody function have therefore been developed in an attempt to improve the prediction of disease severity. Although these assays are cumbersome, there are now sufficient data to suggest that some cellular assays, when used as part of a structured approach to diagnostic testing, may provide clinically-useful information to complement serological and quantitative assays.

Placental transport of maternal immunoglobulin G in pregnancies at risk of Rh (D) hemolytic disease of the newborn

PROBLEM

The following questions were addressed: Is the placental transport of immunoglobulin (Ig)G, IgG1, and IgG3 diminished in pregnancies at risk of hemolytic disease of the newborn? Is the placental transport of IgG, IgG1, and IgG3 correlated with the hemoglobin concentration in the fetus and AutoAnalyzer (AA) quantitations of maternal anti-D?

METHOD OF STUDY

IgG concentrations were determined retrospectively in 41 paired fetal/maternal (f/m) samples in 31 Rh (D) alloimmunized pregnancies. IgG1 and IgG3 concentrations were determined in those 23 cases in which the results of fetal hemoglobin concentration and quantitations of maternal anti-D were available. The results were compared with values found in normal pregnancy and correlated with maternal anti-D AA quantitations and fetal hemoglobin concentrations.

RESULTS

Fetal IgG, IgG1, and IgG3 concentrations, and the corresponding fetomaternal ratios in Rh (D) alloimmunized pregnancies, increased with gestational age according to the following formulas (obtained by simple regression): Fetal IgG = -8.846 + 0.491.gestational age (GA), (R2 = 0.544); fetal IgG = 10.021 + 0.46.GA (R2 = 0.463); fetal IgG3 = -0.865 + 0.039.GA, (R2 = 0.327); f/m IgG = -1.006 + 0.054.GA, (R2 = 0.557); f/m IgG1 = -1.876 + 0.085.GA, (R2 = 0.654); f/m IgG3 = -0.199 + 0.026.GA, (R2 = 0.146).

CONCLUSIONS

The placental transport of IgG, IgG1, and IgG3 in women with Rh (D) immunizations is not diminished compared with normal pregnancy. However, AA quantitations of anti-D are inversely correlated with f/m IgG ratio, f/m IgG1 ratio, and fetal IgG and IgG1 concentrations (P = 0.002, P = 0.004, P = 0.02, and P = 0.02 respectively). The placental transport of IgG3 is significantly higher in pregnancies at risk of hemolytic disease of the newborn compared with IgG3 concentrations in normal pregnancy.

Application of the gel test in IgG subclassing--a comparison of two agglutination assays

BACKGROUND AND OBJECTIVES

IgG subclasses of anti-D seem to play some role in hemolytic disease of the newborn, but there has been disagreement as to its exact nature. The aim of our study was to evaluate a new technique for IgG subclassing of anti-D and to compare it to an established test.

MATERIALS AND METHODS

In 31 cases of RhD immunization, we have compared two simple agglutinating assays for subclassing anti-D: an established V-well microtiter tray assay and a new gel test assay. Polyclonal rabbit anti-IgG agglutinating antibodies were tested against sensitized D-positive red blood cells by both assays.

RESULTS

The anti-D concentration in tested sera varied between 0.1 and 81 micrograms/ml (0.5-405 IU/ml). Both assays were simple to perform and the results correlated well.

CONCLUSIONS

The gel test had two advantages: quick performance and easy interpretation.

Human Rh monoclonal antibodies: assessment of functional activity by chemiluminescence and RhD antibody quantitation

In vitro cellular assays have been described which are capable of evaluating the interactions between sensitised red cells and monocyte or K cells. The chemiluminescence assay (CL) has several advantages over other cellular assays used to assess functional activity. The CL assay unlike the ADCC assays does not require the use of radioisotopes and therefore can be easily integrated into the work of a Reference Serology laboratory. The CL assay is an objective test and not labour intensive which is the main criticism of the monocyte monolayer assay. Seventy-four monoclonal anti-Ds and 29 other Rh specificities have been evaluated by a CL assay. The use of the chemiluminescent response produced by erythrophagocytosis of sensitised red cells has been shown to correlate well with the in vivo response to red cells sensitized with polyclonal IgG antibodies. This study aimed at investigating whether the CL assay could identify and differentiate monoclonal antibodies that are capable of eliciting a response from human monocytes. Poor correlation was obtained between the CL assay results and anti-D quantitation (r = 0.236). The chemiluminescence assay discriminated between anti-D's with high quantitation levels but low predicted functional activity and anti-Ds of low quantitation levels which produced elevated CL responses. Only 3 of the 29 non-Rh D specificities tested produced a response in the CL assay emphasising the importance of specificity in the functional activity of monoclonal antibodies. The demonstration of significant differences in the functional capabilities of monoclonal antibodies has important implications for reviewing the possible use of monoclonal anti-D preparations for Rh immune prophylaxis and highlights the requirement for factors other than the antibody concentration to be examined.

Prediction of the severity of haemolytic disease of the newborn. Quantitative IgG anti-D subclass determinations explain the correlation with functional assay results

Sera containing anti-D, taken from 44 RhD-negative women with RhD-positive infants, were tested in antibody-dependent cellular cytotoxicity (ADCC) and monocyte monolayer assays (MMA) which used similar target and effector cell populations. In addition, the anti-D concentration was measured in the Auto Analyzer and the number of IgG1 and IgG3 anti-D molecules bound to the target red cells was measured by flow cytometry. The results of the functional assays and Auto Analyzer quantitation were examined for correlation with IgG subclass quantitation and all results were compared for their ability to predict the severity of haemolytic disease of the newborn (HDN). ADCC correctly predicted HDN in 39/44 (88.6%) cases, Auto Analyzer quantitation in 35/44 (79.5%) and the MMA in 32/44 (72.7%). For all three assays, the number of correct predictions was highest when the maternal serum contained both IgG1 and IgG3 anti-D. ADCC activity and HDN were correlated with the number of cell-bound IgG1 molecules (r > or = 0.58), but MMA activity was most closely correlated with the number of cell-bound IgG3 molecules (r = 0.68). Hence the superior predictive value of ADCC is due to its ability to reflect the IgG1 component of maternal anti-D, which has a better correlation than IgG3 anti-D with the severity of HDN.

The predictive value of maternal serum testing for detection of fetal anemia in red blood cell alloimmunization

OBJECTIVE

Current management protocols for pregnancies complicated by red blood cell alloimmunization use the maternal antibody titer to predict the need for invasive testing for detection of fetal anemia. We investigated the use of three maternal serum tests to assess their usefulness in predicting fetal disease: indirect Coombs' titer, Marsh score, and monocyte monolayer assay.

STUDY DESIGN

Forty-seven serum samples from pregnant women with red blood cell antibodies associated with fetal anemia were analyzed at cordocentesis. Fetal blood was analyzed for hematocrit (corrected for gestational age) and antigen status. Fetal anemia was defined as a hematocrit value of < 2 SD from the mean value for gestational age. Fetuses were classified into three groups: Antigen positive with anemia (n = 19), antigen positive without anemia (n = 17), antigen negative (n = 11). Statistical methods included Kruskal-Wallis test, Newman-Keuls test, Spearman's rank correlation, and receiver-operator characteristic curves; p < 0.05 was considered significant.

RESULTS

The median monocyte monolayer assay (phagocytosis, adherence, and association) did not differ among the three groups. Both maternal titers and Marsh scores were significantly higher in fetuses with anemia compared with the other two groups of fetuses (256 vs 64 vs 64, p < 0.001, and 86 vs 69 vs 64, p = 0.02, respectively). Both titer and Marsh score exhibited significant correlations with corrected fetal hematocrit (r = -0.70, p < 0.001; r = -0.63, p < 0.001, respectively). Comparison of the overall receiver-operator characteristic curves for titer and Marsh score revealed no statistical difference; however, a Marsh score of 57 was noted to have a superior specificity than a titer of 16 (p = 0.02).

CONCLUSION

The maternal Marsh score can be performed in conjunction with standard indirect Coombs' titers to enhance the predictability of fetal anemia.

Transplacental IgG subclass concentrations in pregnancies at risk of haemolytic disease of the newborn

The relationship of haemolytic disease of the newborn (HDN) to the transplacental passage of the four IgG subclasses was assessed at various gestational ages by comparing the maternal and fetal IgG subclass concentrations in 34 pregnancies at risk of HDN with those in 30 pregnancies not at risk. Higher maternal and fetal IgG1 levels were attained in pregnancies at risk of HDN than in pregnancies not at risk. In contrast, a slight decrease in maternal IgG2 and IgG4 levels occurred in pregnancies at risk of HDN, as compared with a slight rise in maternal IgG2 and IgG4 levels in pregnancies not at risk of HDN. Changes in fetal IgG2 and 4 concentrations in either type of pregnancy were very similar, showing only slight increases between the 19th and 34th week of gestation. A slight decrease in maternal IgG3 occurred in both types of pregnancy. In contrast, higher and fairly steady levels of fetal IgG3 were observed in fetuses not at risk of HDN throughout gestation, when compared with those in 'at risk' pregnancies. However, the statistical reliability of these results is not clear since only small numbers of samples were tested and because wide variations in IgG concentrations were observed. The IgG subclass concentrations in 50 paired maternal and cord blood samples were also measured and revealed that IgG1 levels were substantially higher in cord rather than maternal blood; cord and maternal IgG2, 3 and 4 levels, on the other hand, were fairly similar.

Clinical application of functional assays for assessing the red cell antibody activity

Functional assays (the monocyte monolayer assay, the chemiluminescence test, and the antibody dependent cellular cytotoxicity assay) aim to reflect an interaction between sensitized red calls and Fc-gamma receptor bearing mononuclear cells, which plays the main role in immune destruction of erythrocytes in vivo. These assays have been used for predicting the clinical significance of antibodies in hemolytic disease of the newborn, in patients with alloantibodies requiring transfusion and patients suspected of autoimmune hemolytic anemia. This review presents a brief summary of the literature on this subject. In many cases (especially in hemolytic disease of the newborn, RhD) the results of bioassays can assess the functional activity of antibodies in vivo, but further investigation is needed to establish their real value for clinical practice.

Anti-D concentrations in fetal and maternal serum and amniotic fluid in rhesus allo-immunised pregnancies

OBJECTIVE

To investigate the relation between anti-D concentrations in maternal serum, fetal serum and amniotic fluid, and the development of fetal anaemia.

DESIGN

Observational cross sectional and longitudinal study.

SETTING

Regional referral centre.

SUBJECTS

Sixty-one women undergoing fetal blood sampling at 19 to 36 weeks' gestation for fetal blood and haematocrit estimation for the management of Rh (D) allo-immunisation. Thirty-eight pregnancies (7 with an Rh (D) negative fetus) were tested only once but the rest had two to five fetal blood samplings.

INTERVENTIONS

Ultrasound guided fetal blood sampling and amniocentesis, and automated analysis of anti-D antibody quantitation.

RESULTS

There were strong correlations between maternal serum, fetal serum and amniotic fluid anti-D concentrations. Analyses of both longitudinal and cross sectional data demonstrated a decrease of the maternal/fetal serum anti-D ratio with gestation. In pregnancies with Rh (D) negative fetuses the maternal/fetal anti-D ratio was significantly lower (P < 0.0001) than in those with Rh (D) positive fetuses. The degree of fetal anaemia (delta haematocrit) was correlated with maternal serum and amniotic fluid anti-D concentrations (r = -0.55, n = 54, P < 0.0001; r = -0.57, n = 44, P < 0.0001, respectively) but there was a weaker correlation with fetal serum anti-D (r = 0.37, n = 54, P < 0.01).

CONCLUSION

Anti-D concentrations in maternal serum, fetal serum and amniotic fluid are correlated with fetal anaemia. The decrease in maternal/fetal anti-D ratio with gestation suggests an increase in placental permeability for anti-D with advancing pregnancy.

The role of Rh antibodies in haemolytic disease of the newborn

Recent insights into the structure-function relationship of IgG, the nature of Fc receptors and their interactions with antibodies, and the cellular mechanisms involved in the immune destruction of IgG-sensitized cells have all contributed to a fuller understanding of the role of Rh antibodies in HDN. As this understanding has increased, so different diagnostic and therapeutic approaches have been developed and evaluated in order either to predict or ameliorate disease severity. The role of Rh antibodies in HDN can be considered in three contexts: maternal anti-D, monoclonal anti-D and prophylactic anti-D. Anti-D formed after maternal alloimmunization may be transported across the placenta, resulting in destruction of sensitized red cells by mononuclear phagocytes in the fetus and infant. The use of monoclonal anti-D has given an insight into the cellular and molecular mechanisms involved in red cell destruction, and has facilitated the development and evaluation of assays which use maternal anti-D to predict the severity of HDN. Polyclonal anti-D, given prophylactically, can prevent maternal alloimmunization to D-positive fetal red cells. Future developments are likely in several areas. Prophylactic polyclonal anti-D may be replaced, wholly or partially, with monoclonal anti-D. The development and introduction of cellular assays as non-invasive tests for predicting disease severity is likely to continue as preliminary results are encouraging. Finally, new strategies for ameliorating disease severity may be assessed including the role of ivIgG and Fc gamma R-blocking antibodies.