Distribution of IgW and IgM Transcripts in the Lymphoid Organs of the African Lungfish (Protopterus dolloi)

Understanding the evolution of the immune system is critical for understanding the immune system itself.  Lungfish are the closest relative to all 4-legged vertebrates (tetrapods) but little is known about their immune system.  The aim of this study is to characterize the B cell subsets expressing immunoglobulin (Ig) genes in different tissues from the spotted African lungfish (Protopterus dolloi).  Igs are soluble or membrane-bound, glycoprotein antibodies.  High-throughput sequencing identified 3 major Ig isotypes in lungfish spleen: IgW short, IgW long and IgM.  To quantify expression, fluorescence in situ hybridization studies were conducted on pre-and post-pyloric spleen and gut of P. dolloi cryosections and quantified by counting the number of mRNA (positive spots) per cell by using NIS Elements software (Nikon).  Expression was divided into the following categories: cells with 1-6 mRNA per cell (low), 7-14 mRNA per cell (medium), and 15-79 mRNA per cell (high).  Low transcript abundance was found in 69.77%-90.77% of the gut cells, pre- and post-pyloric spleen.  Medium expression was found in 8.12%-23.26% of cells and high expression in 1.11%-6.98% of cells.  Thus, most cells expressed IgW and IgM at low levels.  High expression was identified in putative plasma cells, which themselves are a small percentage of high expressing cells (1-3%).  Future studies will evaluate how these populations change during the course of an immune response.