A stressful predicament for blood bankers!

Monoclonal immunotherapeutic drugs are becoming an effective way to attack certain cancers and other conditions. Some of these antibodies recognise red blood cells (RBCs) as well as their intended cancer cell targets and can create considerable havoc in transfusion services when trying to provide compatible blood for transfusion to those patients who are receiving these medications. Anti-CD38 (daratumumab; DARA) is one example that has been used for a few years now and is FDA approved for use in multiple myeloma patients; unfortunately, it causes panagglutination in serologic tests. The good news is that these serologic reactions are relatively weak (1+ to 2+), and various ways have been reported around the anti-CD38 panagglutination, enabling elucidation of potentially clinically significant underlying alloantibodies. These various approaches include the use of dithiothreitol (DTT) and other means to work around this issue and allow more assurances that no potentially clinically significant alloantibodies underly the panagglutination. Unfortunately, another monoclonal antibody, termed Hu5F9-G4 (anti-CD47), is an antibody that targets CD47 on cells. CD47 is found in high density on many types of cancer cells and is especially highly expressed on RBCs. CD47 acts, presumably, as a signal to monocytemacrophages to not phagocytose the cells, and anti-CD47 can block the “don't eat me” signal and allow cells to be phagocytosed. When anti-CD47 is administered to certain cancer patients, it is given at very high dosage, which creates huge problems for transfusion services during their serologic evaluations related to type and screen and provision of compatible blood. In the paper by Reyland et al, in this issue of Transfusion Medicine, the authors present two case studies of patients who are receiving anti-CD47. These cases confirm a previous report of the type of problems that arise in a transfusion service or reference laboratory when faced with patients being administered this drug. There are two striking features of the serology found in these patients who present to the transfusion service. One of the more notable results of the serology is the enormous plasma titres of antiCD47 detectable. Titres > 16 000 have been previously reported, and in the two cases reported by Reyland et al, the titres in both instances are >65 000, making it impossible to elucidate underlying potentially clinically significant alloantibodies as the anti-CD47 titres are so high. In addition, not surprisingly with plasma titres so high is the fact that the antibody levels cannot be significantly reduced with autoor alloadsorption approaches to allow for alloantibody detection using polyclonal antihuman globulin (AHG) sera. Second is that the direct antiglobulin test (DAT) in these patients receiving anti-CD47 can be negative, yet high titres of anti-CD47 can be found in eluates. This is a result of the high antigen site density of CD47 on the RBCs coupled with the high titre of circulating antibody resulting in the “blockade” of the antigen sites and a false-negative DAT, also known as prozone effect. Unlike with anti-CD38, DTT treatment or other RBC antigenmodifying treatments have no effect on the CD47 antigen. Because the anti-CD47 is manufactured to be an IgG4 immunoglobulin, one can use Gamma-clone AHG (Immucor Inc., Norcross, Georgia) that lacks anti-IgG4 to interrogate underlying potentially clinically significant alloantibodies; however, reactivity at the indirect antiglobulin test (IAT) phase of testing can still be positive due to carryover. An approach that uses a combination of alloadsorption and Gamma-clone AHG could work. That is, using a multiple alloadsorption strategy to decrease the titre of >16 000 to 65 000 down to perhaps 512, as indicated in Reyland et al, may allow for antibody investigation using the Gamma-clone AHG. IAT results that would then be negative with Gamma-clone AHG for the anti-CD47 would still allow for the Gamma-clone to react with other IgG subclass alloantibodies that could be underlying the anti-CD47. With more immunotherapeutic antibodies that can react with RBCs coming into the clinic, a consistent strategy must be implemented by transfusion services and reference laboratories. One needs to make sure that a diagnosis and drug history is obtained on admission of patients requiring type and screen or crossmatch. Importantly, for those patients who will be administered anti-CD38, anti-CD47 or other RBC cross-reactive immunotherapeutic, it is highly recommended to perform ABO and Rh typing and to obtain an extended phenotype and/or genotype prior to patients going on these monoclonal antibody therapies. Therapeutic administration of anti-CD47 is going to be a challenge for blood bankers as its use increases, as well as other RBC cross-reactive monoclonal antibody clinical trials that may become therapies. This problem will not go away! However, as investigative immunohaematologists obtain more experience with anti-CD47, there may be additional approaches elucidated to get around the incredible serologic reactivity of these and other antibodies in the future.