CD4+ T cells are at the nexus of the innate and adaptive arms of the immune system. However, little is known about the evolutionary history of CD4+ T cells, and it is unclear whether their differentiation into specialized subsets is conserved in early vertebrates. In this study, we have created transgenic zebrafish with vibrantly labeled CD4+ cells allowing us to scrutinize the development and specialization of teleost CD4+ leukocytes in vivo. We provide further evidence that CD4+ macrophages have an ancient origin and had already emerged in bony fish. We demonstrate the utility of this zebrafish resource for interrogating the complex behavior of immune cells at cellular resolution by the imaging of intimate contacts between teleost CD4+ T cells and mononuclear phagocytes. Most importantly, we reveal the conserved subspecialization of teleost CD4+ T cells in vivo. We demonstrate that the ancient and specialized tissues of the gills contain a resident population of il-4/13b–expressing Th2-like cells, which do not coexpress il-4/13a. Additionally, we identify a contrasting population of regulatory T cell–like cells resident in the zebrafish gut mucosa, in marked similarity to that found in the intestine of mammals. Finally, we show that, as in mammals, zebrafish CD4+ T cells will infiltrate melanoma tumors and obtain a phenotype consistent with a type 2 immune microenvironment. We anticipate that this unique resource will prove invaluable for future investigation of T cell function in biomedical research, the development of vaccination and health management in aquaculture, and for further research into the evolution of adaptive immunity.
The study was supported by the European Research Council (ERC-2011-StG-282059 PROMINENT to AH and 677501 – ZF_Blood to AC), the Biotechnology and Biological Sciences Research Council (BB/L007401/1 to AH and CS), the Dowager Countess Eleanor Peel Trust (TH-PRCL.FID2228 to AH and CD), a Medical Research Council and Department for International Development Career Development Award Fellowship (MR/J009156/1 to SJ), a Medical Research Foundation grant (R/140419 to SJ), Cancer Research UK (C45041/A14953 to AC and EIA) and a core support grant from the Wellcome Trust and MRC to the Wellcome Trust – Medical Research Council Cambridge Stem Cell Institute.