Upregulated expression of fibronectin receptors underlines the adhesive capability of thymocytes to thymic epithelial cells during the early stages of differentiation: Lessons from sublethally irradiated mice

A 250-cGy whole-body gamma-radiation dose was used to induce thymus regression in mice, and to study the expression and function of extracellular matrix (ECM) receptors in distinct thymocyte subsets emerging during repopulation of the organ. The onset of regeneration was detected from day 2 to 3 postirradiation (P-Ir), when a remarkable increase in the absolute counts of CD3(-)CD25(hi)CD44(+) and CD3(-)CD25(in/hi)CD44(-) cells occurred. Enhanced expression of L-selectin, alpha 4, and alpha 5 integrin chains (L-sel(hi) alpha 4(hi) alpha 5(hi)) was also exhibited by these cells. This pattern of expression was maintained until the CD4(+)CD8(+) (DP) young stage was achieved. Afterward, there was a general downregulation of these ECM receptors in DP as well as in CD4(+) or CD8(+) single positive (SP) thymocytes (L-sel(in) alpha 4(in) alpha 5(in)). In some recently generated SP cells, alpha 4 expression was downregulated before the alpha 5 chain, and L-selectin was upregulated in half of more mature cells. The expression of the alpha 6 integrin chain was downregulated only in maturing CD4(+) cells. Importantly, the increased expression of L-selectin and alpha 4 and alpha 5 chains in thymocytes was strongly correlated with their adhesiveness to thymic epithelial cells (TEC) in vitro. Blocking experiments with monoclonal antibody or peptides showed the following: (1) that the LDV rather than the REDV cell attachment motif in the IIIC segment of fibronectin is targeted by the alpha 4 integrin during thymocyte/TEC adhesion; (2) that the RGD motif of the 120-kD fragment of fibronectin, a target for alpha 5 integrin, has a secondary role in this adhesion; and (3) that the YIGSR cell attachment motif of the beta 1 chain of laminin/merosin recognized by a nonintegrin receptor is not used for thymocyte adherence. In conclusion, our results show that an upregulated set of receptors endows CD25(+) precursors and cells up to the young DP stage with a high capability of interacting with thymic ECM components. (C) 1999 by The American Society of Hematology.