Immune response of silver pomfret (Pampus argenteus) to Photobacterium damselae subsp. Damselae: Virulence factors might induce immune escape by damaging phagosome

Photobacterium damselae subsp. damselae (PDD) is a dangerous bacterial disease that affects silver pomfret (Pampus argenteus) and its main virulence factors are pore-forming toxins. In this study, a highly pathogenic strain was obtained from naturally infected silver pomfret and was introduced into healthy fish for experimentation. Liver, gill, kidney and spleen samples were collected at 0, 24, 48, 72 and 96 h. Histopathological evaluations of these tissues, via HE staining, showed multi-organ bleeding, inflammatory infiltration and histopathocyte vacuolation as the main pathological changes. The levels of AKP, ACP, LZM, SOD, GSH and CAT in serum also displayed an increase following infection. Transcriptome sequencing of liver, kidney and spleen was performed using samples 96 h post-infection. The analysis of the resulting 770,878,120 clean reads collectively assembled into 89,310 unigenes. The WGCNA analysis demonstrated enrichment of phagosome and cytokine-cytokine receptor interaction in the liver, drug metabolism-cytochrome P450 in the kidney, and MAPK signaling pathway and other signaling pathways in the spleen. DEGs related to immune and immune-related pathways, such as Phagosome, Autophagy-animal and Glycerolipid metabolism, were significantly enriched. Selected key genes from these KEGG pathways were examined using RT-qPCR. The expression level of autophagy related genes (pi3k, bcl2) increased after infection, while the gene associated with MAPK and apoptosis (Caspase 9, mapk4) significantly increased after 72 h infection. The expression of phagosome related genes (atp6v1f, ctsl) increased initially, but later decreased. The genes related to innate immunity (ccl4, il15) were up-regulated, while those related to acquired immunity (mhc2a, mhc2b) showed a downward trend. Lysosome-related genes (gilta, giltb, cftr, c-type lectin, m6pra and m6prb) were up-regulated in liver and spleen and down-regulated in kidney and gill. The data obtained illustrated that PDD activated the immune defenses of the fish, increasing metabolic expenditure, but that the pore-forming toxins released by PDD could destroy the phagosome membrane and help the bacteria escape, which could disturb antigen presentation and inhibit the activation of the acquired immunity. This study reveals the potential immune escape mechanism of PDD in the host for the first time and has the potential to aid in the development of anti-PDD drugs.