Correlation of in vivo alloantibody significance or insignificance with an in vitro monocyte-macrophage phagocytosis assay

Red blood cells, still vital after all these years: Commentary on Canadian Blood Services' International Symposium 2017

Canadian Blood Services (CBS), Canada's national blood transfusion service, has for many years sponsored an annual conference, for the education and awareness of interested participants, showcasing the latest evidence-based understanding of both basic science and clinical issues in transfusion medicine and science. The 15th iteration of this symposium took place September 9, 2017 and focused on some of the vital aspects of red blood cells (RBC), in line with the" 3Rs" concept, namely the provision of the Right red blood cell (RBC) product to the Right patient at the Right time. Presentations touched upon: the evolution of blood banking in North America; the monocyte monolayer assay as a predictor of post-transfusion hemolysis; hemoglobin-based oxygen carriers; RBC alloimmunization; serological approaches to complex RBC antibody problems; randomized clinical trials related to the age of stored RBC; RBC genotyping; pathophysiology, prevention and treatment of hemolytic disease of the fetus and newborn (HDFN); and testing and timing in perinatal serology. This commentary provides summaries of all speakers' presentations annotated with relevant references. Special thanks are due to all contributors for their praiseworthy approaches in sharing their experiences and knowledge on this interesting scientific/clinical and management theme.

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.

Inhibition of phagocytic recognition of anti-D opsonized Rh D+ RBC by polymer-mediated immunocamouflage

The Rh D antigen posed both a significant clinical risk and inventory supply issue in transfusion medicine. The successful development of the immunocamouflaged RBC has the potential to address both the risk of acute anti‐D transfusion reactions and to improve D− blood inventory in geographic locations where D− blood is rare (e.g., China). The immunocamouflage of RBC was mediated by the covalent grafting of methoxy(polyethylene glycol) to the cell membrane thereby obscuring the D protein from the immune system. To determine the potential efficacy of mPEG‐D+ RBC in D− recipients, anti‐D alloantibodies from previously alloimmunized individuals were utilized. The effects of polymer chain size (2–30 kDa) and grafting concentration (0–4 mM) on antibody binding and erythrophagocytosis were determined using the clinically validated monocyte monolayer assay (MMA) and flow cytometry. The immunocamouflage of D was polymer size and grafting concentration dependent as determined using human anti‐D alloantibodies (both pooled [RhoGAM] and single donors). Importantly, the 20 kDa polymer provided excellent immunocamouflage of D and reached a clinically significant level of protection, as measured by the MMA, at grafting concentrations of ≥1.5 mM. These findings further support the potential use of immunocamouflaged RBC to reduce the risk of acute transfusion reactions following administration of D+ blood to D− recipients in situations where D− units are unavailable or supply is geographically constrained. Am. J. Hematol. 90:1165–1170, 2015. © 2015 Wiley Periodicals, Inc.

Successful transfusion of antigen positive blood to alloimmunised patients using a monocyte monolayer assay

BACKGROUND

Providing adequate transfusion support for alloimmunised patients for whom antigen negative blood is not readily available is hampered by the risk of a haemolytic reaction. The monocyte monolayer assay (MMA) has shown good correlation between the antibody clinical significance and the fate of antigen positive blood.

MATERIALS AND METHODS

From 2006 to 2013, the clinical significance of red cell alloantibodies produced by 61 patients was evaluated using a MMA; and antigen positive blood offering the best survival advantage was selected for transfusion following a secondary MMA crossmatch. Post-transfusion, patients were evaluated for clinical signs of haemolysis.

RESULTS

Overall, 19 of 61 (31·1%) of our antibodies were potentially clinically significant, with a monocyte index (MI) > 5%. There was no correlation between the clinical significance as showed by the MMA, and the specificity of the antibody or the strength of reactivity at antihuman globulin (AHG) phase. Using the MMA as a secondary crossmatch method, 31 alloimmunised patients (including: eight anti-hr(B), four anti-Yt(a), one each anti-Rg1, -Co(a), Er(a), Le(b), -LW, -Sl1) received 103 antigen positive blood units with no clinical sign of a post-transfusion reaction. For three patients (one each anti-Jo(a), -AnWj, unidentified 'HTLA'), initial MMA was performed as part of an investigation of a suspected haemolytic reaction. In each case, the MMA accurately identified the unit responsible for the reaction.

CONCLUSION

Used as a crossmatch surrogate, the MMA provided valuable information in the decision of transfusing antigen positive blood to alloimmunised patients, avoiding delay because of the search of rare antigen negative units.

Rosetting, phagocytosis and immune red cell damage

Observations on rosette formation of red blood cells sensitized with a known number of IgG1 or IgG3 polyclonal or monoclonal anti-Rh(D) antibodies with monocytes, lymphocytes and granulocytes have provided further evidence for the essential role of monocytes/macrophages in red cell destruction. The rosette test with monocytes and the monocyte-monolayer assay (MMA) have confirmed, on a quantitative basis, the greater binding ability of IgG3 than of IgG1 antibodies and that the rate of interaction with red cells increases in proportion to the level of their sensitization. These observations have suggested that the MMA may reflect haemolysis in vivo. It has appeared that the MMA is suitable for assessing haemolysis in many cases of haemolytic disease of the newborn, in autoimmune haemolytic anaemia and in patients with alloantibodies to be transfused. Since in individual cases a clear correlation between the MMA results and haemolysis in vivo was not observed, the factors which may influence such discrepancies are discussed.

Transfusion of incompatible RBCs to a patient with alloanti-Kp(b)

BACKGROUND

The finding of an antibody that reacts against a high-incidence blood group antigen always constitutes a complex transfusion problem because of the difficulty in finding compatible units. When the transfusion of incompatible RBCs is imperative, it would be of great interest to have access to techniques facilitating the prediction of the transfusion outcome.

STUDY DESIGN AND METHODS

The case of a patient with alloanti-Kp(b) who required RBC transfusions is reported. The functional activity of this antibody was assessed by both the chemiluminescence test (CLT) and the survival of 51Cr-labeled RBCS:

RESULTS

The CLT showed an opsonic index of 0.8 with Kp(b)-positive RBCs (normal values up to 1.6) in pretransfusion studies. During an elective surgical procedure, the patient required the transfusion of one incompatible unit of RBCs, which did not produce hemolysis. Two weeks after this incompatible transfusion, the opsonic index had risen to 11. Results of the 51Cr in vivo study, also performed at that time, indicated 24.3 percent survival of Kp(b)-positive RBCs at 60 minutes and 2.0 percent at 24 hours.

CONCLUSION

Results of the CLT correlated with the in vivo transfusion outcome and later with the 51Cr survival study.

Monocyte monolayer assay (MMA) reactivity of alloantibodies reacting by the manual polybrene technique but not by an antiglobulin test

The manual polybrene technique (MP) has been shown to be useful in pretransfusion testing as a rapid method capable of detecting a wide range of red cell antibodies. The clinical significance of alloantibodies reacting by the MP but not by an antiglobulin test (MP+/AGT-) has not been determined. The monocyte monolayer assay (MMA) is an established in vitro technique for assessing the likely clinical significance of red cell antibodies. This was used to examine 21 MP+/AGT- alloantibodies and 41 alloantibodies reacting by a LISS antiglobulin method (LISS-AGT). Two of the 21 MP+/AGT- antibodies (9.5%) produced positive results by the MMA compared with 26/41 (63.4%) of LISS-AGT-reactive antibodies. The incidence of positive reactivity of the latter antibodies in the MMA was similar between those that were reactive and nonreactive by the MP. These results suggest that antibodies reacting by MP and not by an antiglobulin test are less likely to be clinically significant.

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.

Autoanti-Ge associated with severe autoimmune hemolytic anemia

Severe autoimmune hemolytic anemia due to anti-Ge is described. The patient's red cells had a positive direct antiglobulin test, and they typed as Ge+ using saline reactive reagents. Anti-Ge was eluted from her RBCs, and her serum had an IgG and IgM anti-Ge2,3.

Correlation of monocyte-monolayer assay results, number of erythrocyte-bound IgG molecules, and IgG subclass composition in the study of red cell alloantibodies other than D

Comparisons have been made between the serological and immunological characteristics of 42 blood group alloantibodies (other than D) covering twelve systems using a monocyte-monolayer assay (MMA), a radiometric antiglobulin test for antibody binding and IgG subclass determinations. The results of the MMA correlated well with the level of IgG molecules bound on incompatible cells, and the highest levels in both cases were associated with the presence of the IgG3 subclass. However, limited clinical data shows that, while in general the MMA clearly identifies the clinically significant antibodies, the correlation with the degree of clinical outcome is less well defined, and in some instances other factors may be operating to ameliorate the in vivo effect of the antibody.

Serological and immunological characteristics of maternal anti-Rh(D) antibodies in predicting the severity of haemolytic disease of the newborn

A number of factors were analyzed for their predictive value in indicating the severity of haemolytic disease of the newborn (HDN) in 72 infants. The factors investigated were: maternal antibody titre in the indirect antiglobulin test, the number of antibody molecules on sensitized standard red cells evaluated by a radiometric antiglobulin test, the IgG subclass specificity and the reactivity in monocyte-monolayer assay (MMA) and in the rosette assays with lymphocytes and granulocytes from healthy individuals. The results of the MMA correlate much better with the severity of HDN than the antibody titre. In clinically unaffected infants the reactivity in the MMA never exceeded 20%, while in the severe/very severe group it was always greater than 20% (in 95% of very severe cases even above 50%). The number of IgG-bound molecules was also shown to closely correlate with the clinical severity and there was a much greater proportion of severe/very severe cases exhibiting combined IgG1 and IgG3 specificity. Of all the evaluations performed the rosette assays with lymphocytes and granulocytes were found to be less useful in predicting the severity of HDN.

Phagocytosis of erythrocytes sensitized with known amounts of IgG1 and IgG3 anti-Rh antibodies

Phagocytosis was investigated using human peripheral monocytes and erythrocytes sensitized with known amounts of subclass-specific IgG anti-Rh antibodies. The erythrocyte-bound IgG was quantitated by a radiometric antiglobulin test. This evaluation revealed the following: (1) there is a relationship between phagocytosis and the number of erythrocyte-bound IgG molecules; (2) phagocytosis is IgG subclass-dependent, since a similar degree of phagocytosis is observed with fewer IgG3 than IgG1 molecules and also the minimum number of IgG3 molecules for phagocytosis is 150-640, whilst for IgG1 the minimum is 1,230-4,020; (3) the minimum levels of sensitization for phagocytosis should be detectable by the serological antiglobulin test; (4) the phagocytosis assay is no more sensitive than the monocyte rosette assay for the detection of anti-Rh alloantibodies, and (5) phagocytosis of adherent erythrocytes observed by video-enhanced microscopy indicated that erythrocytes may adhere to monocytes for a considerable time before phagocytosis, but that phagocytosis itself was rapid.