Purple sea urchins (Strongylocentrotus purpuratus) express a highly variable set of immune genes called Sp185/333 by two subtypes of coelomocytes: the polygonal and small phagocytes. We report that the Sp185/333 genes and their encoded proteins are also expressed in all of the major organs in the adult sea urchin, including the axial organ, pharynx, esophagus, intestine and gonads. After immune challenge, there is an increase in the level of Sp185/333 mRNA in cells associated with the intestine and axial organ. The Sp185/333 proteins increase in the axial organ, pharynx, esophagus and intestine after challenge. However, the proportion of Sp185/333-positive cells only increases in the axial organ, while there is no change in that proportion in the other organs after challenge. The size range of the major Sp185/333 proteins expressed by organs is broader (5 kDa to > 250 kDa) compared with those in coelomocytes (∼40 kDa to < 250 kDa). Images of the different organs do not clarify whether coelomocytes or parenchymal cells express the Sp185/333 proteins. The increase in levels of Sp185/333 transcripts, protein expression and Sp185/333-positive cells in the axial organ in response to challenge suggests that this organ may have an important role in immunity for this species.
Keywords: Axial organ; coelomocytes; echinoderm; immune diversity; innate.