Molecular Cloning and Expression Responses to Streptococcus agalactiae and Aeromonas veronii of TLR19, TLR20, and TLR21 in Schizothorax prenanti

Toll-like receptors (TLRs) are essential pattern recognition receptors of the innate immune system and play critical roles in pathogen invasion in teleosts. In this study, we identified and characterized full-length open reading frames of three TLRs belonging to the TLR11 subfamily from <i>Schizothorax prenanti</i>, termed <i>spTLR19</i> (2868 bp), <i>spTLR20</i> (2835 bp), and <i>spTLR21</i> (2946 bp), encoding 955, 944, and 981 amino acids, respectively. All three proteins exhibited the conserved domain architecture typical of TLRs, comprising a leucine-rich repeat (LRR) domain, a transmembrane region, and a Toll/IL-1 receptor (TIR) domain. Phylogenetic and homology analyses revealed that <i>sp</i>TLR19 and <i>sp</i>TLR20 clustered most closely with their homologues from <i>Cyprinus carpio</i>, while <i>sp</i>TLR21 showed the highest similarity to <i>Onychostoma macrolepis</i> TLR21. Expression profiling showed that these TLRs were ubiquitously expressed across examined tissues, with relatively higher expression in immune-related tissues such as spleen and gills. Furthermore, challenge with <i>Streptococcus agalactiae</i> and <i>Aeromonas veronii</i> significantly up-regulated the expression of <i>spTLR19</i>, <i>spTLR20</i>, and <i>spTLR21</i> in spleen, liver, and gills, suggesting their involvement in antibacterial immune responses. These findings enhance the functional understanding of the teleost TLR11 subfamily and provide a foundation for elucidating disease resistance and immune regulation in <i>S. prenanti</i>.