Delineation of chicken thymocytes by CD3 - TCR complex, CD4 and CD8 antigen expression reveals phylogenically conserved and novel thymocyte subsets

To further define the relationship between thymocyte subsets and their developmental sequence, multi-parameter flow cytometry was used to determine the distribution of the CD3-TCR complex and the accessory molecules CD4 and CD8 on chicken thymocytes. As in mammals, adult thymocytes could be subdivided into CD3^-, CD3^lo, and CD3^hi staining populations. CD4 and CD8 distribution on such populations revealed the presence of CD3^-CD4⁺CD8^- and CD3^-CD4^-CD8⁺ thymocytes, putative precursors to CD4⁺CD8⁺ cells, detectable in the adult and at high frequency during ontogeny. Of particular interest was the existence of CD3^lo expression on CD4⁺CD8^- and CD4^-CD8⁺, and in some instances, on CD4^-CD8^- thymocytes. Such phenotypes are not easily detectable in the mammalian thymus but were readily observed in both adult and embryonic chicken thymus from 16 days of embryogenesis. Further analysis of the TCR lineage of these CD3^lo cells revealed that they were essentially all of the αβ TCR type. Mature CD3^hl thymocytes were found within the CD4⁺CD8⁺ and CD4⁺CD8⁺ and CD4^-CD8⁺ subsets. Both αβ and γδ TCR lineage thymocytes were detected within all CD4^- and CD8-defined subsets, thus identifying novel thymocyte subsets in the chicken thymus, namely αβ TCR⁺CD4^-CD8^- and γδ TCR⁺ CD4⁺CD8^- cells. Hence, this analysis of chicken thymocytes, while confirming the phylogenically conserved nature of the thymus, has revealed novel T cell subsets, providing further insight into the complexity of mainstream thymocyte maturation pathways.