Two novel LRR and Ig domain-containing proteins from oyster Crassostrea gigas function as pattern recognition receptors and induce expression of cytokines

Leucine-rich repeat (LRR) domain and immunoglobulin (Ig) domain are both competent immune recognition modules, and the immunological roles of LRR and Ig domain containing- proteins (LRRIGs) are speculated to be multifunctional and worth investigating. In the present study, two novel LRRIGs, CgLRRIG-1 and CgLRRIG-2, were identified and characterized from oyster Crassostrea gigas. Both of them contained an N-terminal LRR region, an Ig domain, a transmembrane region, and a C-terminal cytoplasmic tail. The mRNA transcripts of CgLRRIG-1 and CgLRRIG-2 were constitutively expressed in muscle, gill, hepatopancreas, mantle, gonad and hemocytes with the highest expression level in hepatopancreas. Their mRNA expression levels in hemocytes were significantly up-regulated after the stimulations with four PAMPs including peptidoglycan (PGN), lipopolysaccharide (LPS), glucan (GLU) and polyinosinic-polycytidylic acid (poly I:C) and one bacteria Vibrio anguillarum. The recombinant proteins, rCgLRRIG-1 and rCgLRRIG-2, could bind to PGN, LPS, GLU and poly I:C, and rCgLRRIG-2 exhibited higher binding affinity. Additionally, rCgLRRIG-1 and rCgLRRIG-2 could significantly induce the expression of CgTNF-1 and CgIL17-5 in cultured oyster hemocytes, and the activity of rCgLRRIG-2 was higher than that of rCgLRRIG-1. All these results indicated that CgLRRIG-1 and CgLRRIG-2 could function as immune effectors or pro-inflammatory factors as well as PRRs in oyster.