T cell migration during ontogeny and T cell repertoire generation

CD4+CD25+ Regulatory T cell ontogeny and preferential migration to the cecal tonsils in chickens

Thymic CD4(+)CD25(+) cells have regulatory-T-cell-like properties in chickens. This study examined the ontogeny of CD4(+)CD25(+) cells in the thymus and in peripheral compartments in chickens. CD4(+)CD25(+) cells started to appear in the thymus at day 15 of incubation (E15), although at low percentages. Expressed as a percentage of CD4(+) cells, CD4(+)CD25(+) cells increased (P<0.01) from 1.7% at E20 to 7.3% at 0 d post-hatch (D0). CD4(+)CD25(+) cells did not appear in the spleen or cecal tonsils of embryos. Expressed as a percentage of CD4(+) cells, CD4(+)CD25(+) cells increased (P<0.01) from 0% at D0 to 27% at D1 in cecal tonsils and from 0% at D0 to 11% at D1 in the spleen. Expressed as a percentage of all mononuclear cells, cecal tonsils at D1 had approximately 3.5-fold higher percentage of CD4(+)CD25(+) cells than the spleen at D1. CD4(+)CD25(+) cells from cecal tonsils of chicks at D1 were suppressive. CD4(+)CD25(+) cells from D0 thymus, when injected back into MHC-compatible chicks, migrated to cecal tonsils and lungs and were detected until 10 d post-injection. CD4(+)CD25(+) cells from cecal tonsils had a higher (P = 0.01) relative amount of CCR9 mRNA than CD4(+)CD25(+) cells from the thymus. It could be concluded that in chickens CD4(+)CD25(+) cells migrate from the thymus immediately post-hatch and preferentially colonize the gut associated lymphoid tissues. CD4(+)CD25(+) cells' preferential migration to cecal tonsils is likely directed through the CCR9 pathway in chickens.

Effects of hypophyseal or thymic allograft on thymus development in partially decerebrate chicken embryos: expression of PCNA and CD3 markers

Changes in chicken embryo thymus after partial decerebration (including the hypophysis) and after hypophyseal or thymic allograft were investigated. Chicken embryos were partially decerebrated at 36–40 h of incubation and on day 12 received a hypophysis or a thymus allograft from 18-day-old donor embryos. The thymuses of normal, sham-operated and partially decerebrate embryos were collected on day 12 and 18. The thymuses of the grafted embryos were collected on day 18. The samples were examined with histological method and tested for the anti-PCNA and anti-CD3 immune-reactions. After partial decerebration, the thymic cortical and medullary compartments diminished markedly in size. Anti-PCNA and anti-CD3 revealed a reduced immunereaction, verified also by statistical analysis. In hypophyseal or grafted embryos, the thymic morphological compartments improved, the anti-PCNA and anti-CD3 immune-reactions recovered much better after the thymic graft, probably due to the thymic growth factors and also by an emigration of thymocytes from the same grafted thymus.

Variant isoforms of CD44 are required in early thymocyte development

The earliest T cells homing to the thymus (CD3-CD4loCD8-) express CD117 (c-kit), CD43 (leukosialin), and the integrins CD11a (alphaL), CD11b (alphaM), CD29 (beta1), CD49f (alpha6), and CD44. Using reagents specific for CD44 variant isoforms (CD44v), we demonstrated that CD44v were expressed on virtually all early thymocytes,whereas cells carrying only the standard molecule (CD44s, not containing any variant domains), which is ubiquitously found on mature lymphocytes later, are very sparse. The expression of CD44v was closely correlated with CD43 and CD117 and was restricted to the CD3-CD4loCD8- stage. CD44v were detected on lymphocyte progenitor populations in the fetal blood, liver, thymus and spleen, as well as in the adult bone marrow. Functional studies demonstrated that only cells expressing CD44v from fetal liver and adult bone marrow could efficiently populate fetal thymic stroma and develop into mature T cells. In fetal thymic organ cultures anti-CD44v antibodies specifically blocked thymocyte development. We also present evidence that CD44v were required for the initial interaction of hematopoietic progenitor cells with the thymic stroma. Our data imply that CD44v are not only a useful marker for hematopoietic progenitors, but also play a functional role in the initiation of thymocyte development.

Ontogeny of the immune system: gamma/delta and alpha/beta T cells migrate from thymus to the periphery in alternating waves

The embryonic thymus is colonized by the influx of hemopoietic progenitors in waves. To characterize the T cell progeny of the initial colonization waves, we used intravenous adoptive transfer of bone marrow progenitors into congenic embryos. The experiments were performed in birds because intravenous cell infusions can be performed more efficiently in avian than in mammalian embryos. Progenitor cells, which entered the vascularized thymus via interlobular venules in the capsular region and capillaries located at the corticomedullary junction, homed to the outer cortex to begin thymocyte differentiation. The kinetics of differentiation and emigration of the T cell progeny were analyzed for the first three waves of progenitors. Each progenitor wave gave rise to gamma/delta T cells 3 d earlier than alpha/beta T cells. Although the flow of T cell migration from the thymus was uninterrupted, distinct colonization and differentiation kinetics defined three successive waves of gamma/delta and alpha/beta T cells that depart sequentially the thymus en route to the periphery. Each wave of precursors rearranged all three TCR Vgamma gene families, but displayed a variable repertoire. The data indicate a complex pattern of repertoire diversification by the progeny of founder thymocyte progenitors.