Characterization and the potential immune role of class A scavenger receptor member 4 (SCARA4) in bacterial infection in turbot (Scophthalmus maximus L.).

The class A scavenger receptors play important roles in innate immunity and are distributed on plasma membrane of macrophages and other cell types. Notably, the class A scavenger receptor 4 (SCARA4) contains a typical C-type (calcium-dependent) lectin domain, which belongs to the collectin family of pattern recognition receptors and is involved in the immune response against infection. Here, one turbot SCARA4 gene was identified with a 2,292 bp open reading frame (ORF) encoding 763 amino acid residues. Multiple sequence analysis and phylogenetic analysis confirmed that Sm SCARA4 gene was more close to that of P. olivaceus. Gene structure and syntenic analysis showed conserved exon/intron organization pattern and syntenic pattern across selected vertebrate species. Tissue distribution analysis showed Sm SCARA4 was expressed in all the tested healthy tissues with the relative high expression levels in skin, gill and spleen. Following both E. tarda and V. anguillarum challenge in vivo , Sm SCARA4 was significantly repressed in gill and intestine. Remarkably, Sm SCARA4 showed the strongest binding ability to LPS and strongest upregulation in turbot head kidney macrophages in response to LPS. Knockdown and overexpression of Sm SCARA4 revealed its interactions with the two pro-inflammatory cytokines, TNF-α and IL-1β. Finally, repression of Sm SCARA4 via combined treatment of LPS and overexpression of Sm SCARA4 construct in turbot head kidney macrophages further indicated an inhibitory role of Sm SCARA4 in LPS-stimulated inflammation. Taken together, turbot Sm SCARA4 plays an important role in turbot immunity, especially in the mucosa-related systems; Sm SCARA4 possesses strong binding specificity to LPS, and exerts protective roles in response to LPS infection by reducing the release of pro-inflammatory cytokines. The mechanisms of inhibitory role of Sm SCARA4 in LPS-elicited inflammation await further investigation. • Gene structure and syntenic analysis of SCARA4 showed conserved patterns across selected vertebrate species. • Sm SCARA4 was expressed in all the tested healthy tissues with the relative high expression levels in skin, gill and spleen. • Following both E. tarda and V. anguillarum challenge, Sm SCARA4 was significantly repressed in gill and intestine. • Sm SCARA4 showed the strongest binding ability to LPS and strongest upregulation in turbot head kidney macrophages in response to LPS. • Knockdown and overexpression of Sm SCARA4 revealed its interactions with TNF-α and IL-1β. [ABSTRACT FROM AUTHOR]