Organ-specific repertoires of IgNAR gene in a cartilaginous fish

Cartilaginous fish possess one of the most ancient adaptive immune systems, and they uniquely produce the heavy chain-only antibody, immunoglobulin novel antigen receptor (IgNAR). In this study, we explored the mRNA transcription of genes related to antibody production and IgNAR diversity in various organs in banded houndsharks. IgNAR and antibody production-related genes exhibited similar relative transcription levels, with the highest expression detected in the spleen. Subsequently, we examined the diversity of IgNAR using next-generation sequencing. The most frequent clones were dominant (25 %-40 %) in the epigonal organ and liver but less common in the spleen. Large individual variation was noted in the kidney and pancreas. The length of complementarity-determining region 3 ranged 2-39 amino acids. The region tended to have a narrow length distribution of approximately 13 amino acids in the epigonal organ and liver, whereas wider length variation was noted in the kidney, pancreas, and spleen. Type II IgNAR variable regions (VNARs) were predominant (60 %-96 %) in all organs, whereas Type IV and "other" not conventionally defined VNARs were present at low frequencies and in different proportion between organs. Type I VNARs were present in multiple organs. The VNAR sequences were commonly shared among the epigonal organ, liver, and/or pancreas, but few were shared in the kidney or spleen.