Can Chilodonella uncinata induce cross-protection in koi carp (Cyprinus carpio) against Ichthyophthirius multifiliis? Evidence from immune response and challenge experiments

The intraperitoneal injection of koi carp with inactivated Ichthyophthirius multifiliis or Chilodonella uncinata combined with Freund's adjuvant was evaluated for its capacity to protect the fish from I. multifiliis infection. Our results showed that the level of Ig M in serum significantly increased after vaccination, reaching up to 4.09 mu g/ mL on day 14 in fish vaccinated with C. uncinata, while C3 content also generally increased. In addition, genes related to immunity, including Ig M, IL-1 beta, TNF-alpha, TLR 9, TLR 20 and TLR 21, exhibited differential expression, which suggests that the koi carp did indeed produce immune responses after vaccination. In challenge experiments, each fish specimen was challenged with 30,000 theronts on day 14 post vaccination. The relative survival rate of fish vaccinated with 10,000 I. multifiliis theronts was 93.3%, while the survival rate of fish vaccinated with 10,000C. uncinata or with phosphate buffer saline (the latter being the control) was 0% in both cases. However, as the dose of C. uncinata was increased, the survival rate of the fish also increased, with fish vaccinated with 60,000C. uncinata cells achieving a higher survival rate than those vaccinated with 30,000 cells. Above all, we demonstrated that Ig M does not play a major role in protection against ichthyophthiriasis and the protection conferred by inactivated C. uncinata vaccination is related to the dose.