The immune system can be subdivided into two main arms: the innate and the adaptive. Recently, it was identified a new family of cells belonging to the innate system but that phenotypically and functionally resemble the specific subtypes of adaptive T lymphocytes. In mice and in humans, these innate lymphoid cells (ILCs) play critical roles in host defense, in shaping the microbiota and in tissue repair at mucosal tissues via secreting cytokines such as IL-22. The innate system is more ancient than the adaptive, however, the existence and diversity of ILCs in organisms other than mammals remains unknown. Also unknown is how ILCs interact with other cells within tissues. This project will address the existence and role of the different subsets of ILCs and the conservation of IL-22 function in a distant relative, the zebrafish, a genetically tractable vertebrate with remarkable optical accessibility. To identify ILCs, cell populations likely enriched in these cells will be purified from mucosal sites such as the intestine and the gills by flow cytometry, guided by morphology and expression of reporter transgenes for immune cell types. Their transcriptome profile will be assessed in bulk and at the single-cell level adapting the microfluidic-based Biomark system to the zebrafish. Also, to know if cytokines produced by T cells can also be produced by non-adaptive lymphocytes – i.e., ILCs, as described in mouse, immune responses of zebrafish mutants lacking adaptive lymphocytes or all types of lymphocytes will be compared to that of wild-type fish. To study IL-22 function, mutants for this cytokine and its receptor will be generated. Together with a transgenic il22 reporter line allowing the identification of IL-22-producing cells, they will be challenged with pathogens or inflammatory stimuli, and studied by various approaches including in vivo imaging. This work will allow a detailed in vivo study of the ontogeny, role and behavior of ILCs and IL-22- producing cells.