Effects of Live Attenuated Vaccine and Wild Type Strains of Edwardsiella ictaluri on Phagocytosis, Bacterial Killing, and Survival of Catfish B Cells

Trained Immunity Provides Long-Term Protection against Bacterial Infections in Channel Catfish

Beta glucan exposure induced trained immunity in channel catfish that conferred long-term protection against Edwardsiella ictaluri and Edwardsiella piscicida infections one month post exposure. Flow cytometric analyses demonstrated that isolated macrophages and neutrophils phagocytosed higher amounts of E. ictaluri and E. piscicida. Beta glucan induced changes in the distribution of histone modifications in the monomethylation and trimethylation of H3K4 and modifications in the acetylation and trimethylation of H3K27. KEGG pathway analyses revealed that these modifications affected expressions of genes controlling phagocytosis, phagosome functions and enhanced immune cell signaling. These analyses correlate the histone modifications with gene functions and to the observed enhanced phagocytosis and to the increased survival following bacterial challenge in channel catfish. These data suggest the chromatin reconfiguration that directs trained immunity as demonstrated in mammals also occurs in channel catfish. Understanding the mechanisms underlying trained immunity can help us design prophylactic and non-antibiotic based therapies and develop broad-based vaccines to limit bacterial disease outbreaks in catfish production.

Molecular and functional characterization of teleost-specific Interleukin-17N in yellow catfish (Pelteobagrus fulvidraco)

In mammals, six interleukin-17 (IL-17) genes, as potent pro-inflammatory cytokines, all accelerate the inflammatory responses. In teleosts, seven IL-17 genes have been found in various species, but little is known about the function of teleost-specific IL-17N. In this study, teleost IL-17N and IL-17A/F2 genes all had six conserved cysteine residues forming three intrachain disulfide bridges, the length of three exons of teleost IL-17N gene was similar to that of teleost IL-17A/F2 gene, and the neighbor-joining (NJ) phylogenetic tree showed that teleost IL-17N was clustered with vertebrate IL-17A/F, implying that teleost IL-17N gene may be a paralog of teleost IL-17A/F gene. Pelteobagrus fulvidraco (Pf) IL-17N gene was highly expressed in the blood, brain and kidney of healthy yellow catfish. Pf_IL-17N transcript and protein were notably up-regulated in the spleen, head kidney, gill and kidney detected after Edwardsiella ictaluri infection. Lipopolysaccharides (LPS), polyinosinic-polycytidylic acid (Poly I:C) and peptidoglycan (PGN) also remarkably induced the expression of Pf_IL-17N in the isolated peripheral blood leucocytes (PBLs) of yellow catfish. These results reveal that Pf_IL-17N may play important roles in preventing the invasion of pathogens. Furthermore, the recombinant (r) Pf_IL-17N protein could significantly induce the mRNA expressions of inflammatory cytokines, chemokines and antimicrobial peptide genes in yellow catfish in vivo and in vitro, and it also notably promoted the phagocytosis of myeloid cells in the PBLs and the chemotaxis of the PBLs and gill leucocytes (GLs) in yellow catfish. Besides, though the rPf_IL-17N protein could induce and aggravate inflammation infiltration in the kidney of yellow catfish, it did not effectively and notably increase the survival rate of yellow catfish after E. ictaluri infection. Furthermore, the rPf_IL-17N protein could induce the mRNA expressions of nuclear factor kappa-B (NF-κB) and mitogen-activated protein kinase (MAPK) signal pathways related genes, and the inhibitor of NF-κB and MAPK signal pathways could restrain the rPf_IL-17N protein-induced inflammatory response. This study provides crucial evidence that the Pf_IL-17N may mediate inflammatory response to eliminate invasive pathogens.

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.

Live attenuated Edwardsiella ictaluri vaccines enhance the protective innate immune responses of channel catfish B cells

Edwardsiella ictaluri causes enteric septicemia of catfish. Our group developed two E. ictaluri live attenuated vaccines (LAVs). However, their effects on the innate functions of catfish B cells are still unexplored. We evaluated phagocytosis and killing of wild-type (WT) E. ictaluri opsonized with sera from vaccinated fish and the survival of B cells exposed to E. ictaluri strains. We assessed phagocytosis of the opsonized WT at 30 °C and 4 °C. B cells killed the internalized E. ictaluri opsonized with sera from vaccinated fish with LAVs more efficiently than other groups at 30 °C. However, catfish B cells were unable to destroy E. ictaluri at 4 °C. Furthermore, E. ictaluri opsonized with serum from fish exposed to WT induce apoptosis and decreased live B cells numbers. Results indicate that opsonization of E. ictaluri with sera from vaccinated fish enhanced phagocytosis and killing activity in B cells and inhibited apoptotic changes in the infected B cells.