Thymic emigrants isolated by a new method possess unique phenotypic and functional properties

T cells that emigrate from the thymus have primarily been studied in vivo u sing fluorescent dye injection of the thymus. This study examined the prope rties of thymocytes that emigrate from cultured thymic lobes in organ cultu re. Under these conditions, thymic emigrants displayed the expected;phenoty pe, that of mature thymocytes expressing high levels of T-cell receptor (TC R-alpha beta) and either CD4 or CD8, and were observed to emigrate within 2 4 hours of positive selection. Emigration was inhibited by cytochalasin D, pertussis toxin, or Clostridium difficile toxin B, implicating an active mo tility process. Most of the surface markers on alpha beta -thymic emigrants (Thy1, CD44, CD69, CD25, leukocyte functional antigen-1, intercellular adh esion molecule-1, alpha (4)-integrin, alpha (5)- integrin, CD45, and CD28) were expressed at a surface density similar to that on mature intrathymic c ells and peripheral splenic T cells. Heterogeneous expression of L-selectin and heat stable antigen (HSA) suggested that subsets emerge from the thymu s with a commitment to different migration patterns. The only marker on emi grants not found on either intrathymic cells or mature spleen T cells was C TLA-4, which could dampen the response of emigrants to peripheral antigens. Antigen responsiveness measured in vitro against allogeneic dendritic cell s showed a proliferative response comparable to that of splenic T cells. In vivo, however, thymic emigrants failed to induce an acute graft-versus-hos t reaction in allogeneic severe combined immunodeficiency recipients. This suggests that a mechanism operating in vivo, perhaps tolerance or migration pattern, attenuates the response of emigrants against antigens that did no t induce their deletion in the thymus. (Blood, 2001;97:1360-1369) (C) 2001 by The American Society of Hematology.