Quantitation of Lan antigen in Lan+, Lan+(w) and Lan- phenotypes.

Abstract

The Lan antigen (LAN1) is a clinically significant, high-frequency red blood cell (RBC) antigen. Lan was originally described in 1961 and assigned to the LAN blood group system in 20121,2. Lan is currently the only antigen within the LAN blood group system and Lan+, Lan−, Lan+w and Lan+w/− phenotypes have been defined1–4. The Lan− phenotype is very rare worldwide with a frequency of less than 1% in all populations tested to date2,5,6. For example, a Japanese screening study identified 14 Lan− individuals among 713,384 blood donors, giving a frequency of 0.002%2. Lan− individuals are usually identified due to the detection of anti-Lan during investigations into haemolytic disease of the foetus and newborn7,8 or when serological testing is performed to find compatible blood units for a patient5,9. Transfusion support for Lan− individuals is highly challenging due to the scarcity of both compatible blood and suitable anti-Lan reagents for screening for compatible blood. The first monoclonal anti-Lan antibody (OSK43) was produced in 2012 by Helias et al. and this antibody has proven to be of huge benefit as a reliable reagent for screening for Lan− individuals2. The carrier of the Lan antigen is the ABCB6 protein encoded by the ABCB6 gene at chromosome 2q36, containing 19 exons2. Numerous genetic variants have been identified as encoding for the Lan− phenotype2,4,10–12. The initial sequencing study identified ten novel alleles, including frameshift, nonsense and splice-site mutations, within 12 unrelated Lan− individuals and found that each individual was heterozygous for two ABCB6 null alleles2. Subsequently, Saison et al. characterised yet another ABCB6 null allele, a single nucleotide variant missense mutation (c.574C>T)10. To date this single nucleotide variant is the most common mutation causing the Lan− phenotype4,10. It has been suggested that the frequency of ABCB6 null alleles differs between populations as sequencing of 27 Japanese Lan− individuals identified a further ten novel alleles and revealed that none of the donors carried the c.574C>T variant12. The Lan+w phenotype has been described in individuals heterozygous for an ABCB6 null allele and a wild-type allele, and these individuals typically express 50% of the normal level of Lan antigen4,10. The fourth Lan phenotype is referred to as Lan+w/− as cells serologically type as either Lan+w or Lan− depending on the anti-Lan utilised3. A comprehensive study involving serological and molecular characterisation of Lan phenotypes was recently performed by Reid et al. and for the first time alleles encoding Lan+w/− phenotypes were defined4. Lan+w/− individuals are heterozygous for an ABCB6 null allele and a variant allele encoding for weakened Lan expression. Antigens can be quantified by flow cytometry by converting the fluorescent intensity of staining into an antibody-binding capacity (ABC), relating to the number of monoclonal antibody molecules bound to a cell. This is performed utilising populations of calibrated microspheres coated with defined amounts of a capture antibody, to create a calibration curve used for quantitation of ABC. The usefulness of determining ABC values has been well established, particularly for lymphocyte antigens13. In previous studies, calibrated microsphere-based assays have been validated to aid in the diagnosis, prognosis, and treatment monitoring of diseases including chronic lymphocytic leukaemia14,15 and human immunodeficiency virus infection16. In the context of RBCs, glycophorin A and RhD antigen expression has been investigated utilising traditional flow cytometric approaches17,18 and, more recently, calibrated microspheres19,20. The variability of Lan antigen expression has never been quantitatively investigated. In this study, we investigated the expression of Lan antigen by developing a novel indirect staining protocol capable of quantitating the number of Lan sites per RBC in samples reported as Lan+, Lan+w, and Lan−.