Immune-oxidative and apoptotic response to titanium dioxide nanoparticle (TiO₂-NP) exposure in an aquatic lower vertebrate, rohu (<i>Labeo rohita</i>)

Titanium dioxide nanoparticles (TiO₂-NPs) are one of the most commercially manufactured and widely applied NPs. However, often TiO₂-NPs leak into the environment and make aquatic animals exposure inevitable. Consequently, a deeper comprehension of TiO₂-NPs toxicity is utmost important. The 96-hour lethal concentration of TiO₂-NP in rohu (<i>Labeo rohita</i>) was 77.49 mg/L. An <i>in-vivo</i> toxicity assessment of TiO₂-NP was conducted at sub lethal concentration of 1 mg/L (2%), 2.5 mg/L (5%), and 5 mg/L (10%) at 24 hours post exposure (hpe), 4 days post exposure (dpe), and 14 dpe in an aquatic lower vertebrate, rohu. Quantitative bioaccumulation analysis showed highest TiO₂-NPs bioaccumulation in intestine followed by liver, gill, kidney, spleen, and negligible in muscle. TiO₂-NP at 5 mg/L concentration induced the immunotoxic response by destabilization of serum lysozyme and antiprotease activity which was further potentiated by increased production of myeloperoxidase, respiratory burst activity leading to higher production of reactive oxygen species that contribute to oxidative stress, inflammation and cellular damage. Molecular study demonstrated that TiO₂-NP is recognized and processed by signaling PRR, TLR22 leading to initiation of the downstream immune-signaling cascade and pro-inflammatory cytokines production. The TiO₂-NP induced the oxidative stress gene (SOD, CAT, and GPx) expression significantly at 1, 2.5 and 5 mg/L. Nevertheless, apoptotic biomarker (caspase3, BAX and p53) were induced significantly on 14th dpe at 5 mg/L dose exposure. Our study infer that TiO₂-NP induced immunotoxic response at higher concentration of 5 mg/L, nevertheless it acts as immunostimulator at lower concentration of 1 mg/L in <i>L. rohita</i>.