Advancing Threespine Stickleback Fish as an Outbred Immunogenetics Model by Pinpointing the Onset of Adaptive Immunity

Abstract

Understanding the onset of the adaptive immune system is important for understanding host-microbe interactions and the development of disease phenotypes. While the onset of adaptive immunity has been previously studied in model organisms such as mice and zebrafish, these inbred laboratory models are challenged by a lack of genetic diversity and may not be appropriate for all immunological studies. We advance threespine stickleback fish (Gasterosteus aculeatus) as an outbred immunogenetics model in order to study the onset of the adaptive immune system in the context of genetic variation. Threespine stickleback fish exist in various coastal habitats throughout the Northern hemisphere and exhibit natural genetic diversity within families and between populations. Although this teleost model has been effective in previous immunological studies, there are foundational questions still left unanswered including, when does the onset of the adaptive immune system occur in threespine stickleback? To pinpoint the onset of adaptive immunity, we looked at two populations of threespine stickleback over a developmental time series and analyzed the expression of a gene involved in the development of T-lymphocytes. T-lymphocytes are a primary adaptive immune cell type able to recognize and elicit a response against pathogens. Early development of these cells involves the TCR/CD3 protein complex composed of six subunits that are necessary for proper T-cell receptor (TCR) expression and cell activation in mature T-lymphocytes. Genes encoding the TCR/CD3 complex have been previously used to study the ontogeny of immune cells and have provided important insights into the development of the adaptive immune system. In this study, we chose to focus on cd3d, a gene encoding one subunit of the TCR/CD3 complex. Similar work determining the timing of onset of adaptive immunity in other fish species has produced a wide range of results, from 11 hours post fertilization to 26 days post hatching (dph). We found that by 10 dph, cd3d was expressed in all individuals, with population level variation indicating some individuals may exhibit expression earlier in development.