Molecular characterization and expression analysis of four isotypes of immunoglobulin light chain genes in orange-spotted grouper, Epinephelus coioides

► 26 full-length OsgIgL cDNAs were obtained from Osg kidney cDNA library. ► 5 variable and 4 constant region isotypes were identified, and were subgrouped into κ and σ types. ► Head–kidney and spleen are the major OsgIgL expression areas, and CL-I is the major type. ► All OsgIgLs had each transcriptional pattern responded to PAMPs and grouper iridovirus. ► All OsgIgLs displayed significant induction upon inactive virus vaccination. To date, many immunoglobulin (Ig) genes have been identified in diverse teleost species, but the contributions of different types of light chain (IgL) to the immune response remain unclear. Screening of a stimulated kidney cDNA library from orange-spotted grouper (Osg, Epinephelus coioides) resulted in the identification of 26 full Ig light chain (OsgIgL) coding sequences. These 26 OsgIgLs encoded peptides from 235 to 248 amino acid residues and could be grouped into five variable (VL) and four constant (CL) isotypes. The CL regions contained three conserved cysteine residues that may participate in intra- or inter-chain disulfide bond formation. The four CL isotypes could be sub-grouped into two serological types: κ (CL-I, CL-II and CL-III) and σ (CL-IV), by phylogenetic analysis. The OsgIgL genes were found to be expressed in various tissues, with greatest levels of expression observed in the head–kidney and spleen. The major expression type was CL-I, which comprised 92% and 91% of total OsgIgL gene expression in the head–kidney and spleen, respectively. Transcription of all four CL isotypes was differentially affected in response to various immunostimulators, including lipopolysaccharide (LPS), poly I:C and grouper iridovirus (GIV). Induction of OsgIgL genes in response to immunostimulators was particularly dramatic in the spleen, suggesting this organ holds particular importance for the regulation of OsgIgL expression. Furthermore, vaccination of grouper with formalin-inactivated GIV also induced differential patterns of expression in all four OsgIgL isotypes. In summary, the significant and diverse patterns of transcriptional induction observed for OsgIgL isotypes in the spleen and head–kidney imply that each isotype may have unique roles in the immune response.