First evidence of comparative responses of Toll-like receptor 22 (TLR22) to relatively resistant and susceptible Indian farmed carps to Argulus siamensis infection.

Toll-like receptor 22 (TLR22) is present in teleost but not in mammals. Among Indian farmed carps, Catla catla is relatively more resistant than Labeo rohita to Argulus siamensis lice infection. TLR22 is believed to be associated with innate immunity against ectoparasite infection. To investigate the TLR22 mediated immunity against argulosis, we have cloned and characterized TLR22 genes of L. rohita (rTLR22) and C. catla (cTLR22). The full-length cDNAs of rTLR22 and cTLR22 contained an open reading frame of 2838 and 2841 nucleotides, respectively; bearing the typical structural features. Phylogenetically rTLR22/cTLR22 was most closely related to Cyprinus carpio (common carp) counterpart, having highest sequence identity of 86.0%. The TIR domain remained highly conserved with 90% identity within freshwater fishes. The sequence information of cDNA and genomic DNA together revealed that the rTLR22/cTLR22 genes are encoded by uninterrupted exons. The co-habitation challenge study with A. siamensis infection confirmed that C. catla is comparatively more resistant than L. rohita. Further, comparative mRNA expression profile in immune relevant tissues also suggested about the participatory role of TLR22 during lice infection. However, TLR22 might not solely be involved in conferring relative resistance among carp species against argulosis.