Heterogeneity of avian γδ T cells

γδ T cells are distinct with respect to tissue localisation, phenotype and biological functions and similarities between species are not very apparent. To elucidate local and functional heterogeneity of non-stimulated avian γδ T cells, the CD8-characterised γδ T cell subsets [CD8α+high (CD8αα+ and CD8αβ+); CD8α+dim; CD8−] of blood, spleen and caecum were flow cytometrically quantified and analysed for proliferation state as well as sorted for determination of immune-relevant gene expression by quantitative real-time RT-PCR. The number of avian CD8-characterised γδ T cell subsets differed in dependence on tissue and age of bird. Compared to blood and spleen, caecum showed the highest percentage of γδ T cells as well as of the CD8α+high γδ T cell subset in 7-week-old birds. Generally, the CD8αβ+ cells significantly outnumbered the CD8αα+ lymphocytes within the CD8α+high γδ T cell population of all organs. Additionally, the splenic CD8αβ+ subpopulation revealed the highest proliferation activity. By RT-PCR, mRNA expression of immune-relevant genes was proved in non-stimulated γδ T cell subsets, but on different levels. Generally, both CD8α+high cell subsets (CD8αα+ and CD8αβ+) of blood and spleen showed elevated expression levels for Fas ligand (FasL), XCL1 (lymphotactin) and interferon-gamma (IFNγ) compared to the CD8α− γδ T cell subset. In contrast, all caecal γδ T cell subsets showed similar high levels of these transcripts. Notably, the CD8αα+ cells of all locations showed unique expression of TLR4 and interleukin (IL)-2.The results demonstrated that avian γδ T cells are not only heterogeneous concerning their CD8 antigen characteristics and tissue localisation, but also with regard to functional features such as proliferation and mRNA expression.