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Early Ontogeny of Thymocytes in Pigs: Sequential Colonization of the Thymus by T Cell Progenitors¹

Marek inkora^2,3,*, Jirí inkora^3,*, Zuzana Reháková^* and John E. Butler

^* Department of Immunology and Gnotobiology, Institute of Microbiology, Czech Academy of Sciences, Nov Hrádek, Czech Republic; and Department of Microbiology, University of Iowa, Iowa City, IA 52242

Successive colonization of the thymus by waves of thymocyte progenitors has been described in chicken-quail chimeras and suggested from studies in mice. In swine, we show that the first CD3-bearing thymocytes appear on day 40 of gestation (DG40). These early thymocytes were CD3^high and belonged to the T cell lineage. Mature CD3^high ß thymocytes were observed 15 days later (DG55), and their occurrence was preceded by the appearance of CD3^low thymocytes (DG45). Thereafter, we observed transient changes in thymocyte subset composition (DG56-DG74), which can be explained by a gap in pro-T cell delivery to the thymus. This delivery gap corresponds with the expression of the pan-leukocyte CD45 and pan-myelomonocytic SWC3a markers in fetal liver and bone marrow and is probably caused by shifting of primary lymphopoiesis between these organs. Therefore, we conclude that the embryonic thymus is colonized by at least two successive waves of hemopoietic progenitors during embryogenesis and that the influx of thymocyte progenitors is discontinuous. Surface immunophenotyping and cell cycle analysis of thymocyte subsets allowed us to compare thymocyte differentiation in pigs with that described for rodents and humans and to propose a model for T cell lymphopoiesis in swine. We also observed that the porcine IL-2R (CD25), a typical differentiation marker of pre-T cells in mice and humans, was not expressed on thymocyte precursors in pigs and could only be found on mature thymocytes. Finally, we observed a subset of TCR⁺ thymocytes that were cycling late during their development in the thymus.

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