Intraembryonic haemopoietic cells and early T cell development

Characterization of prethymic progenitors within the chicken embryo

The thymic primordium in both birds and mammals is first colonized by cells emerging from the intra-embryonic mesenchyme but the nature of these precursors is poorly understood. We demonstrate here an early embryonic day 7 prethymic population with T lymphoid potential. Our work is a phenotypic analysis of, to date, the earliest embryonic prethymic progenitors arising in the avian para-aortic area during ontogeny. The phenotype of these cells, expressing the cell surface molecules alpha2beta1 integrin, c-kit, thrombomucin/MEP21, HEMCAM and chL12, reflects functional properties required for cell adhesion, migration and growth factor responsiveness. Importantly, the presence of these antigens was found to correlate with the recolonization of the recipient thymus following intrathymic cell transfers. These intra-embryonic cells were also found to express the Ikaros transcription factor, the molecular function of which is considered to be prerequisite for embryonic lymphoid development.

Chicken CD4, CD8alphabeta, and CD8alphaalpha T cell co-receptor molecules

New knowledge has recently been obtained about the evolutionary conservation of CD4, CD8alphaalpha, and CD8alphabeta T cell receptor (TCR) co-receptor molecules between chicken and mammals. This conservation extends from biochemical structure and tissue distribution to function. Panels of monoclonal antibodies and polyclonal antisera against different epitopes of chicken CD8 and CD4 molecules have proven their value in several recent studies. Chicken CD8 allotypes and homozygous strains carrying these allotypes have been established and these strains provide excellent models for further studies. The extensive polymorphism of CD8alpha in chickens has not been observed in any other species, suggesting that CD8alpha and CD8beta have evolved under different selective pressure in the chicken. A large peripheral blood CD4+CD8+ T cell population in chicken resembles that observed in some human individuals but the inheritance of peripheral blood CD4CD8alphaalpha T cells in the chicken is a unique observation, which suggests the presence of a single gene responsible for CD8alpha, but not CD8beta, specific expression. Despite these unique findings in chicken, the data on CD4, CD8alphaalpha, and CD8alphabeta molecules show that they have evolved before the divergence of mammalian and avian branches from their reptilian ancestors.

Antigenic phenotype of early intra-embryonic lymphoid progenitors in the chicken

The stem cells for the definitive haematopoiesis are derived from intra-embryonic sources originally described in an avian model and later also in mammals. However, the molecular make-up of the early embryonic haematopoietic progenitors is not yet clearly defined. We have recently characterized the phenotype of prethymic intra-embryonic progenitors capable of thymic colonization. Here we studied the ontogeny of cell-surface antigens HEMCAM, alpha2beta1 integrin, thrombomucin, chL12 and c-kit and their co-expression on prethymic T-cell progenitors. The early intra-embryonic expression of avian B-cell antigen chB6 was also demonstrated on cells derived from the intra-embryonic areas. We suggest that in the chicken, embryonic B-cell progenitors segregate earlier than T-cell progenitors in the differentiation of multipotent haematopoietic stem cells to committed progenitors.

Expression of chL12 surface antigen is associated with cell survival in the avian bursa of Fabricius

During B-cell development in the avian bursa of Fabricius most of the developing B cells die by apoptosis and only a minority survive to emigrate into the periphery. Recently, it has been shown that when developing bursal cells become mature and ready to migrate they start to express chL12 antigen. The expression of this cell-surface molecule was found to be associated with the survival of the bursal cells both after in vitro culture and after in vivo cyclophosphamide (CY) treatment. The frequency of early apoptotic cells in freshly isolated bursal cells was found to be high. The high susceptibility of these cells to apoptosis is in line with the finding of low bcl-2 mRNA expression. We conclude that expression of avian chL12 antigen is associated with the survival of bursal cells.