Rag1-/- Mutant Zebrafish Demonstrate Specific Protection following Bacterial Re-Exposure

Background: Recombination activation gene 1 deficient (rag1(-/-)) mutant zebrafish have a reduced lymphocyte-like cell population that lacks functional B and T lymphocytes of the acquired immune system, but includes Natural Killer (NK)-like cells and Non-specific cytotoxic cells (NCC) of the innate immune system. The innate immune system is thought to lack the adaptive characteristics of an acquired immune system that provide enhanced protection to a second exposure of the same pathogen. It has been shown that NK cells have the ability to mediate adaptive immunity to chemical haptens and cytomegalovirus in murine models. In this study we evaluated the ability of rag1(-/-) mutant zebrafish to mount a protective response to the facultative intracellular fish bacterium Edwardsiella ictaluri. Methodology/Principal Findings: Following secondary challenge with a lethal dose of homologous bacteria 4 and 8 weeks after a primary vaccination, rag1(-/-) mutant zebrafish demonstrated protective immunity. Heterologous bacterial exposures did not provide protection. Adoptive leukocyte transfers from previously exposed mutants conferred protective immunity to naive mutants when exposed to homologous bacteria. Conclusions/Significance: Our findings show that a component of the innate immune system mounted a response that provided significantly increased survival when rag1(-/-) mutant zebrafish were re-exposed to the same bacteria. Further, adoptive cell transfers demonstrated that kidney interstitial leukocytes from previously exposed rag1-/-mutant zebrafish transferred this protective immunity. This is the first report of any rag1(-/-) mutant vertebrate mounting a protective secondary immune response to a bacterial pathogen, and demonstrates that a type of zebrafish innate immune cell can mediate adaptive immunity in the absence of T and B cells.