Ontogeny of the Hapuka (Polyprion oxygeneios) Immune System
In this study the ontogeny of the hapuka (Polyprion oxygeneios) immune system was studied during larval development. In teleost fish, the head kidney, thymus, and spleen are generally regarded as important immune organs. The head kidney was observed at 4 days post hatch (dph), the spleen at 16 dph and lastly the thymus at 20 dph and all 3 lymphoid organs were relatively well developed by 45 dph. The immune genes CSF1R, C3, MHCIIα, TCRα, TCRβ, RAG1, IgM and IgZ were examined by RT-PCR to investigate the leucocyte development. Macrophages appear to be present from hatch with both CSF1R and MHCIIα expression from 1 dph, while IgM is expressed at 9 dph. T-cells appear later in hapuka with TCRβ expression first detected at 32 dph whereas TCRα was not expressed until after 63 dph. Immunostaining using a monoclonal antibody against fish IgM detected IgM in the head kidney at 12 dph, the spleen at 32 dph, the intestinal tract at 45 dph and lastly the thymus at 50 dph. Comparison of the leucocyte populations in juveniles and adults indicated that innate cell populations are late to develop, while the adaptive cells mature earlier in hapuka than expected. Finally, the maternal transfer of immunity was examined and while lysozyme and IgM appear to be transferred, complement does not. Overall this study provides insight into the developmental sequence of immune organs and cells and will be useful in understanding the timing of immune competence in juveniles and adult hapuka.