Identification, expression pattern and potential role of variable lymphocyte receptor Aj-VLRA from Apostichopus japonicus in response to bacterial challenge

The variable lymphocyte receptors (VLRs) are found in jawless vertebrates (agnathans), and specifically recognize bacteria and viruses via their leucine-rich repeats (LRRs). VLRs are believed to be adaptive immune response molecules. Echinoderms do not have adaptive immune systems; however, in the present study, a VLR cDNA named Aj-VLRA was cloned and characterized from sea cucumber, Apostichopus japonicus. The complete cDNA of Aj-VLRA was 3072 bp, including a 1995 bp open reading frame encoding 664 amino acids comprising LRR domains, a predicted transmembrane helix and an N-terminal signal peptide. As determined by quantitative real-time reverse transcription polymerase chain reaction, Aj-VLRA transcripts are ubiquitously expressed in the body wall, longitudinal muscles, intestine and respiratory tree of A. japonicus. The expression level of Aj-VLRA was upregulated after challenge with four common marine bacteria. In situ hybridization showed that the expression of Aj-VLRA was widely distributed in the four tissues, particularly in the cytoplasm of epidermal cells. Recombinantly expressed Aj-VLRA (including the LRR domains) could bind to bacteria including Micrococcus lysodeikticus (Gram+) and Vibrio anguillarum (Gram−). Collectively, the results suggested that Aj-VLRA is related to an innate immune response of A. japonicus.