| Enteric septicemia of catfish (ESC), caused by the gram negative bacterium Edwardsiella ictaluri, is the most economically important bacterial disease of farm raised channel catfish. In the research presented here, innate immune responses of channel catfish to E. ictaluri were investigated by evaluating expression of genes encoding proteins involved in neutrophil (end binding protein 1 (EB1) and beta1-integrin), macrophage (natural resistance associated macrophage protein (Nramp) and TNF-alpha (tumor necrosis factor-alpha), acute phase (serum amyloid P (SAP) and transferrin) and the cellular stress (heat shock protein 70 (Hsp70) response.; Channel catfish were intraperitoneally challenged with E. ictaluri and sub-sampled at different time points over 96 hours post infection. Expression profiling was performed in head kidney, spleen, liver, gut, and peripheral blood of channel catfish. All the genes evaluated, except SAP and transferrin, were constitutively expressed in all tissues tested from healthy channel catfish, suggesting functions in normal physiological processes. SAP and transferrin were expressed only in the liver. The selected genes were found to be differentially regulated early in response to E. ictaluri and displayed tissue specificity. Expression data were correlated to tissue levels of neutrophils and macrophages to evaluate factors contributing to the observed gene expression profiles. Histochemical analysis showed significant increase in neutrophil numbers in both spleen and liver early during infection, indicating cellular infiltration and a rapid inflammatory response. Comparison of gene expression with neutrophil and macrophage numbers in the spleen of individual fish demonstrated strong correlations of EB1, beta1-integrin, and Hsp70 with neutrophil numbers and a weak correlation of Nramp with macrophage numbers. In the liver, a weak correlation between EB1 and neutrophil numbers was observed. These results indicate that cell infiltration is at least partially responsible for up-regulation of the neutrophil responsive genes and an associated cellular stress response, in comparison, the macrophage gene expression response appears to be more based on transcriptional regulation. Overall, this study provides useful information on the critical time points and the response of channel catfish to ESC early during infection. |
