BACKGROUND Alloimmunization against the high‐frequency Vel blood group antigen may result in transfusion reactions or hemolytic disease of fetus and newborn. Patients with anti‐Vel alloantibodies require Vel‐negative blood but Vel‐negative individuals are rare (1:4000). Identification of Vel‐negative donors ensures availability of Vel‐negative blood; however, accurate Vel blood group typing is difficult due to variable Vel antigen expression and limited availability of anti‐Vel typing sera. We report the production of a recombinant anti‐Vel that also identifies weak Vel expression. STUDY DESIGN AND METHODS A recombinant anti‐Vel monoclonal antibody was produced by cloning the variable regions from an anti‐Vel–specific B cell isolated from an alloimmunized patient into a vector harboring the constant regions of immunoglobulin (Ig)G1‐kappa or IgM‐kappa. Antibody Vel specificity was tested by reactivity to SMIM1‐transfected HEK293T cells and by testing various red blood cells (RBCs) of donors with normal, weak, or no Vel expression. High‐throughput donor screening applicability was tested using an automated blood group analyzer. RESULTS A Vel‐specific IgM class antibody was produced. The antibody was able to distinguish between Vel‐negative and very weak Vel antigen–expressing RBCs by direct agglutination and in high‐throughput settings using a fully automated blood group analyzer and performed better than currently used human anti‐Vel sera. High‐throughput screening of 13,288 blood donations identified three new Vel‐negative donors. CONCLUSION We generated a directly agglutinating recombinant anti‐Vel IgM, M3F5S‐IgM, functional in manual, automated agglutination assays and flow cytometry settings. This IgM anti‐Vel will improve diagnostics by facilitating the identification of Vel‐negative blood donors.

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Background Vel expression on erythrocytes is variable due to polymorphisms, complicating Vel typing. Weak Vel expression can be caused by mutations within SMIM1 in a heterozygous setting, suggesting a dominant negative effect of SMIM1 mutants on wild type (wt)SMIM1 expression. Here we report how SMIM1 expression is regulated during erythropoiesis, to understand its variable expression on erythrocytes. Study Design and Methods Peripheral blood reticulocytes at different stages, cultured erythroid precursors and HEK293T cells were used to investigate expression and putative competition between wtSMIM1 and mutated SMIM1 VEL*01W.01, (c.152T>A (p.Met51Lys)), VEL*01W.02 (c.152T>G (p.Met51Arg)), and VEL*01W.03 (c.161T>C (p.Leu54Pro)). Results Depending on the mutations in SMIM1 an effect on total and membrane expression of SMIM1 was observed in transfected HEK293T cells, but co‐expression of wtSMIM1 and mutatedSMIM1 did not have an effect on wtSMIM1 membrane expression. During differentiation of donors expressing VEL*01W.01, VEL*01W.03, Vel‐positive, Vel‐negative (homozygote SMIM1*64_80del), and Vel‐heterozygote SMIM1*64_80del primary human erythroblasts no overt defect was found in Vel expression dynamics or total SMIM1 expression levels when compared with wtSMIM1 erythroblasts. However, during enucleation, total Vel expression was significantly lower on reticulocytes of Vel‐weak donors expressing heterozygote mutated SMIM1 compared to Vel‐positive or Vel‐heterozygote SMIM1*64_80del donors, while Vel expression on extruded nuclei was maintained. In addition, reticulocyte maturation in vivo showed further loss of Vel expression in these individuals and nearly absent on erythrocytes. Conclusion These results suggest that SMIM1 mutations exert a dominant negative effect on wtSMIM1 probably by affecting SMIM1 multimerization and thereby Vel epitope presentation at the latest stages of erythroid differentiation.