Anti‐HLA‐B7/HLA‐B44 strong cross immunoreactivity observed in flow cytometry HLA‐B27 immunotyping

Abstract Cross reactivities are known for human leukocyte antigen inside HLA‐B7 related Cross‐Reactive Group (B7CREG). Some CE‐IVD flow‐cytometry kits use double monoclonal antibodies (mAb) to distinguish HLA‐B27 and HLA‐B7 but practice reveals more complexes results. This study explores the performances of this test. Analysis of 466 consecutive cases using HLA‐B27 IOTest™ kit on a Navios™ cytometer from Beckman‐Coulter, partially compared to their genotypes. Expected haplotypes HLA‐B27‐/HLA‐B7‐ (undoubtedly HLA‐B27 negative) and HLA‐B27+/HLA‐B7‐ (undoubtedly HLA‐B27+) were clearly identified according to the manufacturer's instructions. On the opposite, patients strongly labeled with anti‐HLA‐B7 showed three different phenotypes regarding anti‐HLA‐B27 labeling: (1) most of the cases were poorly labeled in accordance with cross reactivity inside B7CREG (HLA‐B27‐/HLA‐B7+ haplotype); (2) rare cases had strong B7 and B27 labeling corresponding to HLA‐B27+/HLA‐B7+ haplotype; (3) even less cases had strong labeling by anti‐HLA‐B7 but non for anti‐HLA‐B27, all expressing HLA‐B44 and no B7CREG molecules. Surprisingly, more cases were not labeled with anti‐HLA‐B7 antibody but partially labeled with anti‐HLA‐B27 suggesting another cross reactivity out of B7CREG. mAb HLA typing suggests new, cross reactivities of anti‐HLA‐B27 antibody to more molecules out of B7CREG and of anti‐HLA‐B7 antibody but not anti‐HLA‐B27 to HLA‐B44 molecule also out of B7CREG. HLA‐B27 could surely be excluded in most samples labeled with HLA‐B27, below a “grey zone” on intermediate intensity. More comparison is needed in future studies.