Ryley S Crow Leon Grayfer L Courtney Smith ^*

• Department of Biological Sciences, George Washington University, Washington, D.C., DC, United States

The California purple sea urchin, Strongylocentrotus purpuratus, relies exclusively on an innate immune system to survive in its pathogen rich marine environment. Central to this defense is the SpTransformer (SpTrf) gene family that is unique to the euechinoid group of echinoderms and is strikingly upregulated in response to immune challenge. The encoded SpTrf proteins are structurally similar but show a very wide range of sequence diversity within and among individual sea urchins. A recombinant (r)SpTrf protein interacts specifically with a variety of non-self targets whereas several other rSpTrf proteins show distinct functions for binding and augmenting phagocytosis when cross-linked to inert beads. However, whether the rSpTrf proteins bind to sea urchin phagocytes, and the cellular consequences of binding are largely unexplored. Results from cytology, flow cytometry, binding competition, and gene expression show that soluble rSpTrf proteins bind specifically and exclusively to both live and fixed polygonal and small phagocytes. The different rSpTrf proteins compete for shared binding site(s) on the phagocytes. rSpTrf proteins bound to phagocytes results in modulated gene expression of the SpTrf gene family as well as other immune-related genes. These findings underscore the multifaceted and dynamic functions of SpTrf proteins in sea urchins. The varied functions of these proteins enable a robust immune response while providing a unique modulatory mechanism by which immune response levels are controlled and adjusted relative to the threat.