Long Non-Coding RNA LOC109197777 Enhances Non-Specific Cytotoxic Cell Activity and Resistance to Streptococcus agalactiae in Nile Tilapia (Oreochromis niloticus)

Non-specific cytotoxic cells (NCCs) are critical first-line defenses against bacterial pathogens in teleosts, yet the molecular mechanisms governing their activity remain poorly understood. This study characterized LOC109197777, a novel long non-coding RNA (lncRNA) from Nile tilapia (Oreochromis niloticus), as a positive regulator of NCC-mediated immunity against Streptococcus agalactiae. Bioinformatic and experimental analyses confirmed its non-coding potential and predominant cytoplasmic localization. In vitro, overexpression of LOC109197777 in tilapia head kidney cells upregulated NCC activation receptors while downregulating effector molecules and pro-inflammatory cytokines, indicating a "primed but restrained" activation state; conversely, siRNA-mediated knockdown reduced NCC cytotoxicity. In vivo, intraperitoneal delivery of LOC109197777 significantly enhanced survival following S. agalactiae challenge and reduced bacterial burdens in the head kidney and spleen. Furthermore, tissue-specific expression profiling revealed that LOC109197777 orchestrates spatially distinct regulatory programs: rapid activation in the head kidney, sustained responses in the spleen, early responses in the liver, and prolonged mucosal immunity in the intestine. Notably, the lncRNA differentially regulated perforin-1 and granzyme B in a tissue and time-dependent manner, balancing effective pathogen clearance with immune homeostasis. These findings establish LOC109197777 as the first characterized lncRNA regulator of NCC activity in teleosts, operating through a tissue-specific, temporally coordinated mechanism. This study provides a novel molecular target for genetic enhancement of disease resistance in tilapia aquaculture and offers a theoretical foundation for developing lncRNA-based immunomodulatory strategies.