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The availability of genetically modified mice has facilitated the study of mammalian T cells. No model has yet been developed to study these cells in chicken, an important livestock species with a high availability of γδ T cells. To investigate the role of γδ and αβ T cell populations in birds, we generated chickens lacking these T cell populations. This was achieved by genomic deletion of the constant region of the T cell receptor γ or αβ chain, leading to a complete loss of either γδ or αβ T cells. Our results show that a deletion of αβ T cells but not γδ T cells resulted in a severe phenotype in knockout chickens. The αβ T cell knockout chickens exhibited granulomas associated with inflammation of the spleen and the proventriculus. Immunophenotyping of αβ T cell knockout chickens revealed a significant increase in monocytes and the absence of CD4+T cells and FoxP3+regulatory T cells compared to wild type chickens. In addition, we observed a significant decrease in immunoglobulins, B lymphocytes, and changes in the bursa morphology. Our data reveal the consequences of T cell knockouts in chickens and provide new insights into their function in vertebrates.Significance statementThe lack of genetically modified chickens has severely limited research in avian immunology compared to other animal models. Here, we report the generation of two T cell knockout chicken lines that will contribute significantly to the understanding of T cell biology as a very important research model as well as an important livestock species. The generated animals reveal the function of different T cell populations in chickens and will help to better understand the role of these cells during the interaction with various pathogens in birds.