Characterization of LILRB3 and LILRA6 allelic variants in the Japanese population

Leukocyte immunoglobulin (Ig)-like receptors (LILRs) are encoded by members of a human multigene family, comprising 11 protein-coding genes and two pseudogenes. Among the LILRs, LILRB3 and LILRA6 show the highest homology with each other, along with high allelic and copy number variations. Therefore, it has been difficult to discriminate between them, both genetically and functionally, precluding disease association studies of LILRB3 and LILRA6. In this study, we carefully performed variant screening of LILRB3 and LILRA6 by cDNA cloning from Japanese individuals and identified four allelic lineages showing significantly high non-synonymous-to-synonymous ratios in pairwise comparisons. Furthermore, the extracellular domains of the LILRB3*JP6 and LILRA6*JP1 alleles were identical at the DNA level, suggesting that gene conversion-like events diversified LILRB3 and LILRA6. To determine the four allelic lineages from genomic DNA, we established a lineage typing method that accurately estimated the four allelic lineages in addition to specific common alleles from genomic DNA. Analysis of LILRA6 copy number variation revealed one, two, and three copies of LILRA6 in the Japanese-in-Tokyo (JPT) population. Flow cytometric analysis showed that an anti-LILRB3 antibody did not recognize the second most common lineage in the Japanese population, indicating significant amino acid differences across the allelic lineages. Taken together, our findings indicate that our lineage typing is useful for classifying the lineage-specific functions of LILRB3 and LILRA6, serving as the basis for disease association studies.