Abstract

Morbilliviruses have been responsible for epizootic mortalities of pinnipeds and cetaceans throughout the world. Ethical and technical constraints limit the range of experimental studies that can be performed on live cetaceans, establishing a need for in vivo models using experimental animals that resemble, as closely as possible, the cetacean immune system. The present study utilized cetacean-reconstituted severe combined immunodeficient (SCID) mice as a model to study the protection afforded by vaccination with morbillivirus upon experimental challenge with live morbillivirus. SCID mice were reconstituted using leucocytes collected from four dolphins of the US Navy (San Diego, CA), two of which had previously received a morbillivirus DNA vaccine, while the other two were sham-vaccinated. Four weeks after reconstitution, cetacean-reconstituted mice were challenged with live canine distemper virus (CDV). Mice were sacrificed two weeks after challenge, and tissues were collected for the detection of virus. Blood and sub-samples of spleen were collected to assess the success of reconstitution, and cells from the spleen were also used to detect the presence of a cell-mediated response to CDV. A real-time RT-PCR was developed to detect RNA of CDV in blood and brain samples. Reconstitution by dolphin lymphocytes was detected in spleens of 13 out of 40 mice for B cells and 1 out of 40 mice for T cells, and in blood of 5 out of 15 mice for B cells and 3 out of 25 mice for T cells. Reconstitution was not observed in control un-reconstituted mice. Dolphin T cell-mediated responses to CDV were detected in all four individuals. There was no significant difference in brain virus titers between the vaccinated dolphins, sham-vaccinated dolphins, and the control group. However, brain virus titers of mice were highest (although not significantly) in the non-reconstituted SCID mice exposed to CDV, compared to the combined vaccinated or combined sham-vaccinated animals. This was expected, since SCID mice have no acquired immune system and would not be able to clear CDV infection, and confirms that our model can detect a reduction in viral loads as mediated by immunocompetent dolphin lymphocytes within a SCID mouse. While laboratory results suggest that previous exposure to morbillivirus was unlikely, two possible scenarios could explain the results. First, the experimental vaccine was not successful in eliciting a memory response and reducing the viral load upon later challenge in the mouse model. Second, it is possible that the primary immune response upon challenge was sufficient to account for equal control of infection, irrespective of previous vaccination. To better determine if our SCID mouse model can successfully be used to determine the efficacy of a vaccine, as measured by a reduction in viral loads, future studies will include cetacean-reconstituted mice vaccinated for CDV and then challenged with CDV. Results from these studies will help provide a useful model in the development of cetacean vaccines with experimentally infecting or harming any cetacean.

Acknowledgments

We would like to thank the US Navy Marine Mammal Program for providing blood samples for this project.