Two paralogs of CXCR4 in the Japanese sea bass (Lateolabrax japonica) are involved in the immune response of B lymphocytes.

CXC chemokine receptor 4 (CXCR4), a member of the G-protein-coupled receptor family, plays an important role in host immune responses. Within the teleost lineage, there are two paralogs of CXCR4; however, the role of CXCR4 in teleost B cells is poorly understood. In this study, we determined the cDNA sequences of the two CXCR4 paralogs from the Japanese sea bass (Lateolabrax japonica; LjCXCR4a and LjCXCR4b). Sequence and phylogenetic tree analyses revealed that LjCXCR4a and LjCXCR4b are most closely related to CXCR4a and CXCR4b, respectively, in the large yellow croaker (Larimichthys crocea). CXCR4 transcripts were mainly expressed in the gills, and their expression in different tissues was altered upon infection with Vibrio harveyi. LjCXCR4a and LjCXCR4b protein levels were upregulated in infected B cells. Knockdown of LjCXCR4a and LjCXCR4b in B cells by RNA interference, the phagocytic activity of B cells was not affected. Furthermore, knockdown of LjCXCR4a, not of LjCXCR4b, was observed to inhibit LjIgM expression in lipopolysaccharide-stimulated B cells. In addition, knockdown of LjCXCR4a, not of LjCXCR4b, was found to reduce reactive oxygen species levels in B cells. Our results indicate that LjCXCR4a and LjCXCR4b modulate the immune response of Japanese sea bass B cells against bacterial infection, albeit via different pathways.