Dev Comp Immunol 2026 May 22;:105629. doi: 10.1016/j.dci.2026.105629.

SpEchinoidin, a C-type lectin and a member of an expanded CTL family, is up-regulated by phagocytes in sea urchins responding to immune challenge.

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Transcript sequence analysis of coelomocytes from adult purple sea urchins (Strongylocentrotus purpuratus) responding to immune challenge identifies upregulated gene expression. Results included two sequences that encode a C-type lectin (CTL) with a single carbohydrate recognition domain similar to HcEchinoidin from the sea urchin, Heliocidaris crassispina, and designated as SpEchinoidin. SpEchinoidin has a leader, an O-linked glycosylation site, and seven cysteines in conserved positions for CTLs. The mature SpEchinoidin is 18 kDa with conserved Ca2+-binding sites and a sugar-binding motif specific for galactose and/or galactose derivatives. Isolation of lectins by sugar affinity after immune challenge resulted in diverse and dynamic lectin repertoires in the coelomic fluid. These include multiple lactose-binding lectins with subunit sizes of 15-20 kDa and others of 26-40 kDa. One of the isolated lectins was consistent in size, binding specificity, and partial amino acid sequence with the deduced SpEchinoidin sequence. The S. purpuratus genome (ver 5.0) harbors 27 genes annotated as echinoidin of which 22 are linked to other echinoidin genes or CTL genes. In addition to the galactose-binding lectins, other encoded CTLs are predicted to bind mannose or N-acetyl-glucosamine. Families of CTL genes that include echinoidin-like genes are present in several sea urchin species and cluster into monophyletic clades based on their taxonomic groups. Our findings suggest that sea urchin innate immunity includes an array of CTLs and their expression is consistent with an inducible lectin repertoire with wide recognition capabilities for a variety of sugars, of which many are present on microbial pathogens.

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