Distinguishing between monozygotic twins' blood samples through immune repertoire sequencing.

Monozygotic (MZ) twins with highly similar genomic DNA sequences can not be distinguished by conventional forensic DNA testing. The immune repertoire (IR) reflects an individual's immune history, which is unique between individuals, has been applied to individualized treatment in precision medicine. However, the application of IR in forensic genetics has not been reported to date. In this study, the diversity in the complementary determining region 3 (CDR3) of both the T-cell receptor ß chain (TCRß) and B-cell receptor heavy chain (also known as immunoglobulin heavy chain, IGH) in four pairs of MZ twins were analyzed. The results showed that the amino acid sequences length distribution frequency of TCRß CDR3 had 4-10 differences, and the nucleic acid sequences length distribution frequency of TCRß CDR3 had 2-7 differences between MZ twins. The shared difference of four pairs of MZ twins focused on the length distribution frequency of 34 bp nucleotide sequences in TCRß. By analyzing the usage frequency of V and J genes in TCRß and IGH CDR3 DNA sequence rearrangements, we also found that there were biases between each pair of MZ twins, and the usage frequency of TRBJ2-3 showed common differences between each pair of MZ twins. Furthermore, each pair of MZ twins had its own unique V-J genes combination mode in TCRß and IGH CDR3 DNA sequences. This study, for the first time, suggested that IR can be used as a potential biological marker to distinguish MZ twins.