TLR5 mediated Reactivation of Quiescent Ranavirus FV3 in Xenopus peritoneal macrophages

Ranaviruses such as Frog virus 3 (FV3) are large dsDNA viruses causing emerging infectious diseases leading to extensive morbidity and mortality of amphibians and other ectothermic vertebrates worldwide. Among the hosts of FV3, some are highly susceptible, whereas others are resistant and asymptomatic carriers that can take part in disseminating the infectious virus. To date, the mechanisms involved in the processes of FV3 viral persistence associate with subclinical infection transition to lethal outbreaks remain unknown. Investigation in has revealed that in the asymptomatic FV3 carrier animals, inflammation induced by heat-killed (HK) stimulation can provoke the relapse of active infection. Since Toll-like receptors (TLRs) are critical for recognizing microbial molecular patterns, we investigated their possible involvement in inflammation-induced FV3 reactivation. Among the 10 different TLRs screened for changes in expression levels following FV3 infection and HK stimulation, only TLR5 and TLR22 that both recognize bacterial products showed differential expression, and only the TLR5 ligand, flagellin, was able to induce FV3 reactivation similar to HK Furthermore, only the TLR5 ligand flagellin induced FV3 reactivation in peritoneal macrophages both and These data indicate that the TLR5-signalling pathway can trigger FV3 reactivation, and suggests a role of secondary bacterial infections or microbiome alterations (stress, pollution) in initiating sudden deadly disease outbreaks in amphibian populations with detectable persistent asymptomatic ranavirus. This study in the amphibian provides new evidence of the critical role of macrophages in the persistence of ranaviruses in a quiescent state as well as in the reactivation of these pathogens into a virulent infection. Among the multiple microbial sensors expressed by macrophages, our data underscore the preponderant involvement of TLR5 stimulation in triggering reactivation of quiescent FV3 in resident peritoneal macrophages, unveiling a mechanistic connection between reactivation of persisting ranavirus infection and bacterial co-infection. This suggests a role for secondary bacterial infections or microbiome alterations (stress, pollution) in initiating sudden deadly disease outbreaks in amphibian populations with detectable persistent asymptomatic ranavirus.