TLR22-mediated activation of TNF-α-caspase-1/IL-1β inflammatory axis leads to apoptosis of Aeromonas hydrophila-infected macrophages

Toll-like receptors (TLRs) represent first line of host defence against microbes. Amongst different TLRs, TLR22 is exclusively expressed in non-mammalian vertebrates, including fish. The precise role of TLR22 in fish-immunity remains abstruse. Herein, we used headkidney macrophages (HKM) from Clarias gariepinus and deciphered its role in fish-immunity. Highest tlr22 expression was observed in the immunocompetent organ - headkidney; nonetheless expression in other tissues suggests its possible involvement in non-immune sites also. Aeromonas hydrophila infection up-regulates tlr22 expression in HKM. Our RNAi based study suggested TLR22 restricts intracellular survival of A. hydrophila. Inhibitor and RNAi studies further implicated TLR22 induces proinflammatory cytokines TNF-alpha and IL-1 beta. We observed heightened caspase-1 activity and our results suggest the role of TLR22 in activating TNF-alpha/caspase-1/IL-1 beta cascade leading to caspase-3 mediated apoptosis of A. hydrophila-infected HKM. We conclude, TLR22 plays critical role in immune-surveillance and triggers proinflammatory cytokines leading to caspase mediated HKM apoptosis and pathogen clearance.