The monocyte monolayer assay (MMA) is an in vitro assay that utilizes isolated primary monocytes obtained from mammalian peripheral whole blood to evaluate Fcγ receptor (FcγR)-mediated phagocytosis.
Although originally developed for predicting transfusion outcomes of serologically incompatible blood, the monocyte monolayer assay (MMA) is a highly versatile in vitro assay that can be modified to examine different aspects of antibody and Fcγ receptor (FcγR)-mediated phagocytosis in both research and clinical settings. The assay utilizes adherent monocytes from peripheral blood mononuclear cells isolated from mammalian whole blood. MMA has been described for use in both human and murine investigations. These monocytes express FcγRs (e.g., FcγRI, FcγRIIA, FcγRIIB, and FcγRIIIA) that are involved in immune responses. The MMA exploits the mechanism of FcγR-mediated interactions, phagocytosis in particular, where antibody-sensitized red blood cells (RBCs) adhere to and/or activate FcγRs and are subsequently phagocytosed by the monocytes. In vivo, primarily tissue macrophages found in the spleen and liver carry out FcγR-mediated phagocytosis of antibody-opsonized RBCs, causing extravascular hemolysis. By evaluating the level of phagocytosis using the MMA, different aspects of the in vivo FcγR-mediated process can be investigated. Some applications of the MMA include predicting the clinical relevance of allo- or autoantibodies in a transfusion setting, assessing candidate drugs that promote or inhibit phagocytosis, and combining the assay with fluorescent microscopy or traditional Western immunoblotting to investigate the downstream signaling effects of FcγR-engaging drugs or antibodies. Some limitations include the laboriousness of this technique, which takes a full day from start to finish, and the requirement of research ethics approval in order to work with mammalian blood. However, with diligence and adequate training, the MMA results can be obtained within a 24-h turnover time.
The monocyte monolayer assay (MMA) is an in vitro assay originally developed to better predict blood transfusion outcomes in patients with auto- or alloantibodies to red blood cells (RBCs)1-5. By assessing the effect of anti-RBC antibodies in mediating Fcγ receptor (FcγR)-mediated phagocytosis using this in vitro assay, it is possible to predict the clinical outcome in vivo. Indeed, the MMA has been used successfully to avoid immune destruction of antibody-bound RBCs, despite the transfusion of serologically incompatible blood5. The typical pre-transfusion procedure for compatibility testing, also termed crossmatching, involves serological methods that include typing the patient’s blood for ABO and Rh antigens and screening for the presence of anti-RBC antibodies in the patient6. Blood matched for ABO/Rh is selected, and if antibodies are present, an attempt to identify them is made so that blood for transfusion can be further selected to avoid these antigens. An ideal crossmatch result occurs when all donor blood is serologically compatible with the patient’s blood, which reduces the risk of post-transfusion hemolysis7. However, this system falls short for the small group of patients who have become alloimmunized upon repeated transfusion or pregnancy. These patients produce alloantibodies against specific RBC antigens. Some produce antibodies to antigens of very high frequency in the general population, and thus become progressively more difficult to crossmatch8,9. Adding to the complexity, not all alloantibodies are clinically significant; in other words, the binding of an alloantibody to RBCs detected by a serology test does not necessarily result in hemolysis when antigen-positive, incompatible blood is transfused. The MMA was originally developed to assess the potential clinical significance of serologically incompatible blood in a transfusion setting1-5.
Since extravascular hemolysis of antibody-bound RBCs is known to be mediated by the mononuclear phagocyte system, primary monocytes/macrophages are utilized in the development of diagnostic assays. The first assay to study the interaction of monocytes, RBCs, and antibodies was published in 1975, but the sub-optimal conditions used led only to rosette formation (the binding of RBCs to the periphery of the monocyte), and no phagocytosis was observed10. Significant modifications to the assay were made by several groups, leading to an assay for which the level of phagocytosis of alloantibody-bound RBCs could be correlated to the clinical outcome of hemolysis1-5. Recently, the optimal storage conditions of clinical samples and further optimization of assay conditions were examined to enhance the utility of a clinical MMA crossmatch using autologous patient samples11.
Three other diagnostic techniques have been employed in addition to the MMA in predicting transfusion outcomes: the 51Cr release test, the rosette test, and the chemiluminescence test (CLT). In the 51Cr release test, the patient is injected with 51Cr-labeled donor RBCs, and the half-life of the labeled RBCs is monitored and is predictive of post-transfusion survival or clearance12,13. As this method uses radioactive materials, it is rarely performed anymore. The rosette test involves mixing and incubating monocytes with RBCs and quantifying the level of rosette formation (with no phagocytosis)14. The clinical significance of antibodies in vivo involves active phagocytosis by macrophages found in the spleen and/or the liver; thus, this method does not provide a relevant readout of phagocytosis. The CLT uses luminol to monitor the oxidative burst during monocyte phagocytosis of RBC, since luminol fluoresces blue when oxidized in the phagosome15. This method is good, but contamination by neutrophils can confound the readout. Parallel comparisons have been made to evaluate the sensitivity, practicality, and reproducibility of the four available methods, and both the CLT and MMA were ranked superior16. However, the CLT has been mainly utilized in assessing hemolytic disease of the fetus and newborn (HDFN), and the assay’s optimal pH of 8.0 might compromise the level of phagocytosis11.
In addition to its diagnostic and clinical utility, the MMA has been modified for other research purposes. Indeed, the MMA can not only serve as a functional assay to address discrepancies between serology and biology, it has also been used to retrospectively investigate the cause of hemolysis after intravenous immunoglobulin (IVIG) therapy17. It has also been used to examine the structure-function of chemical inhibitors of FcγR-mediated phagocytosis18-20 and to study the downstream signaling of FcγR-mediated phagocytosis21. In our laboratory, in addition to using a human MMA, we are developing a murine MMA using primary mouse peripheral blood mononuclear cells (PBMCs) and autologous RBCs. The rationale is to screen antibodies that can induce FcγR-mediated phagocytosis as an intermediate to developing an in vivo autoimmune hemolytic anemia (AIHA) mouse model (unpublished data). The various modifications focus on different aspects of the IgG antibody and FcγR interaction that induce phagocytosis.
MMA er en arbeidskrevende teknikk som krever kompetanse i både vev kultur og mikroskopi. Det er flere viktige tiltak for å sikre suksess: 1) generasjon av monocytt monolayer; 2) opsonization av RBC, og 3) manuell kvantifisering. Den monocytt-monolaget ikke fester seg meget sterkt til kammeret lysbilde, så fysiologisk pH må opprettholdes gjennom hele analysen 11 og et tilstrekkelig antall av PBMC skal podes. Under kraftig pipettering, som kan forstyrre det adherte celler, bør unngås. En tilnærming er å alltid fjerne og legge løsninger fra det samme hjørnet av kammeret og for å sikre at bevegelsen er langsom og jevn. Likeledes, i løpet av det siste vasketrinn for å fjerne overskudd av RBC, bør bevegelsen være langsom og jevn. Dette sikrer minimal forstyrrelse av monolaget og samtidig å fjerne mesteparten av de un-phagocytosed RBCs. Utilstrekkelig vasking vil føre til en høy bakgrunn av forurensende røde blodlegemer, noe som gjør manuell quantification vanskelig. Dernest må de a R R 2 RBC være tilstrekkelig opsonisert for å oppnå et gjennomsnitt fagocytisk indeks på 80-120 for fagocytose kontroll. Denne ønskede fagocytisk rekkevidde basert på en balanse mellom den enkle telle (f.eks monocytter med mer enn 5 phagocytosed RBCs er vanskelig å nøyaktig kvantifisere) og opprettholde en tilstrekkelig mengde av fagocytose for statistisk analyse. Graden av opsonization kan bekreftes ved en IAT, og en lese mellom 3+ til 4+ er nødvendig. R 2 R 2 RBC-er skal kastes når det er overflødig lyse under vask, når supernatanten blir mørk rød, eller når en betydelig reduksjon i fagocytose er observert i eksperimenter på grunn av aldring av cellene i lagring. Til slutt kan manuell kvantifisering ved hjelp av mikroskop være vanskelig, spesielt når man sammenligner teller mellom lab personell og mellom eksperimenter. Ved å undersøke den samme felt på hver brønn, eller ganske enkelt ved å telle flere cellerKan oppnås en mer konsistent teller. Side-ved-side trening med en erfaren tekniker og bruk av et utpekt sett med trenings slides anbefales.
En viktig kritikk av MMA er subjektivitet av håndboken kvantifisering trinn. Men med tilstrekkelig trening, konsistens kan oppnås på tvers av forskjellige tellere. En annen begrensning er den iboende donor-til-donor forskjeller i monocytt fagocytiske egenskaper og i R-2 R-2-overflateantigenet ekspresjonsnivåer, som er en kilde til variasjon av data når håndtere humane prøver.
Andre alternative teknikker er tilgjengelige for å undersøke FcγR-mediert fagocytose. Flertallet av kommersielle sett utnytte fluoriserende effekt for å overvåke fagocytose (f.eks bioparticles, pH-sensitive fluorescens-protein, eller IgG-merkede fluorescerende latekskuler). Bruk av fluoriserende effekt ikke tilbyr mer objektiv kvantifisering, men man trenger også å lureSider tilgjengelighet, kostnad, og trening i forbindelse med bruk av en fluorescerende mikroskop eller et flowcytometer, så vel som den påfølgende avhengighet av kommersielt tilgjengelige sett.
Endelig kan denne analysen modifiseres avhengig av problemstillingen. For eksempel, ved testing av medikament hemming av fagocytose, monocytter kan enten være forhåndsbehandlet eller ko-inkubert med både legemidler og opsonisert RBC-er (det vil si en konkurranseanalyse). Den nedstrøms signalisering av antistoffer med forskjellige undertyper, kimære antistoffer eller rekombinante konstruksjoner kan også bli testet. Med de siste gjennombrudd i utviklingen av en universell antigen-null blod 24, kan den MMA utnyttes i første skjermer av disse antigen-RBCs null med forskjellige antistoffer for å vurdere hvorvidt det er faktisk en lavere effekt i utløser fagocytose.
The authors have nothing to disclose.
The authors thank the Canadian Blood Services for a Graduate Fellowship Program Award to T.N.T. This research received financial support from the Canadian Blood Services’ Centre for Innovation, funded by the federal government (Health Canada) and the provincial and territorial ministries of health. The views herein do not reflect the views of the federal, provincial, or territorial governments in Canada.
Acid citrate dextrose (ACD) vacutainers | BD | REF364606 | |
RPMI 1640 | Sigma | R8758 | |
HEPES | Bioshop | HEP003.100 | |
Fetal bovine serum | Multicell | 080150 | |
Gentamicin | Gibco | 15710-64 | |
Ficoll-Paque PLUS | GE | 17-1440-03 | https://www.gelifesciences.com/ |
Phosphate buffered saline | Sigma | D8537 | |
8-chamber slides | Lab-Tek-ll | 154534 | |
R2R2 (cDE/cDE) red blood cells | Canadian Blood Services | Commercially available (e.g. http://www.bio-rad.com/en-ca/product/reagent-red-blood-cells) | |
Polyclonal anti-D from human serum | Gamma Biologics | DIN 02247724 | Can be substituted with commercially available monoclonal anti-D or with Rh immune globulin |
100% methanol | Caledon | 6700-1-42 | |
Polyvinyl alcohol resin | Sigma | P8136 | Can be substituted with commercially available mount |
UltraPure glycerine | Invitrogen | 15514-011 | |
Cover slips | VWR | 48366 067 | |
Novaclone anti-IgG | Immucorgamma | 5461023 | Optional for IAT (http://www.fda.gov/downloads/biologicsbloodvaccines/…/ucm081743.pdf) |