Piscirickettsia salmonis Imbalances the Innate Immune Response to Succeed in a Productive Infection in a Salmonid Cell Line Model

Abstract

Piscirickettsia salmonis is a facultative intracellular bacterium that causes the disease called “salmon rickettsial syndrome”. Attempts to control this disease have been unsuccessful, because existing vaccines have not achieved the expected effectiveness and the antibiotics used fail to completely eradicate the pathogen. This is in part the product of lack of scientific information that still lacks on the mechanisms used by this bacterium to overcome infected–cell responses and survive to induce a productive infection in macrophages. For that, this work was focused in determining if P. salmonis is able to modify the expression and the imbalance of IL-12 and IL-10 using an in vitro model. Additionally, we also evaluated the role the antimicrobial peptide hepcidin had in the control of this pathogen in infected cells. Therefore, the expression of IL-10 and IL-12 was evaluated at earlier stages of infection in the RTS11 cell line derived from Oncorhynchus mykiss macrophages. Simultaneously, the hepcidin expression and location was analyzed in the macrophages infected with the pathogen. Our results suggest that IL-10 is clearly induced at early stages of infection with values peaking at 36 hours post infection. Furthermore, infective P. salmonis downregulates the expression of antimicrobial peptide hepcidin and vesicles containing this peptide were unable to merge with the infective bacteria. Our results suggest that P. salmonis is able to manipulate the behavior of host cytokines and likely might constitute a virulence mechanism that promotes intracellular bacterial replication in leukocytes cells lines of trout and salmon. This mechanism involves the generation of an optimum environment for the microorganism and the downregulation of antimicrobial effectors like hepcidin.

Biofilm Produced In Vitro by Piscirickettsia salmonis Generates Differential Cytotoxicity Levels and Expression Patterns of Immune Genes in the Atlantic Salmon Cell Line SHK-1

“…This result is in agreement with what was described by Levipán et al [ 29 ], who reported different cytotoxicity degrees between P. salmonis LF-89 and EM-90 isolates in their sessile and planktonic state. Most of the studies aiming to characterize P. salmonis virulence, pathogenicity, antimicrobial resistance, and induction of the immune responses are routinely conducted using planktonic bacteria or colonies suspended from plaque growth, inoculating SHK-1, ASK [ 51 ], and RTS11 [ 52 ] fish cell lines. SHK-1 cells are derived from leukocytes that have some of the properties of a macrophages [ 53 ], which are activated by several pathogen associated molecular patterns (PAMPs) such as lipopolysaccharides (LPS) present in bacterial surface and which are recognized by TOLL-like receptors (TLRs) present in these cells.…”

“…Although it has been previously shown in vitro and in vivo that the virulence of planktonic P. salmonis is partly related to its ability to modulate the immune cell response and the host immune system [ 52 , 65 ], until now, no studies have compared the transcriptional response of immune genes against a planktonic and sessile P. salmonis infection. The NF-κB signaling pathway is an attractive target for exploitation by microbial pathogens in order to modulate host cell events, as activation of NF-κB is such a rapid response.…”

Comparison of gene expression in post‐smolt Atlantic salmon challenged by LF‐89‐like and EM‐90‐like Piscirickettsia salmonis isolates reveals differences in the immune response associated with pathogenicity

“…Alvarez et al. () demonstrated that in vitro infection of rainbow trout macrophages with P. salmonis induced an imbalance in the expression of IL‐10 and IL‐12. These results are consistent with the results of the present in vivo study, although the PS‐EM‐90 isolate promoted greater overexpression of ifng and a more significant imbalance between IL‐10 overexpression and IL‐12 underexpression than the PS‐LF‐89 isolate.…”

“…However, bacteria have developed mechanisms that interfere with antigen recognition and presentation in such a way as to modulate the evolution of persistent infections (Garcia-Del Portillo & Finlay, 1995). Alvarez, Gomez, Mercado, Ramirez, & Marshall (2016) demonstrated that in vitro infection of rainbow trout macrophages with P. salmonis induces IL-10 overexpression and IL-12 underexpression, a strategy that inactivates the antibacterial response and promotes intracellular survival and replication.…”

Single-step genome-wide association study for resistance toPiscirickettsia salmonisin rainbow trout (Oncorhynchus mykiss)

“…Within the region associated with TD on Omy3 we identified a gene coding for the receptor DC-SIGN related with the immune response and expressed on macrophage and dendritic-cell surfaces (Ahmed et al 2015). It has been previously described that Mycobacterium tuberculosis , interferes with the Toll-like receptor signaling by DC-SIGN, inhibiting interleukin-12 production (Gorvel et al 2014), a proinflammatory cytokine, which plays a key role in the performance of phagocytes in teleost fish (Alvarez et al 2016).…”