International Immunology, Vol. 14, No. 3, 331-338,
March 2002
© 2002 Japanese Society for Immunology
Functional demonstration of intrathymic binding sites and microvascular gates for prothymocytes in irradiated mice
Department of Pathology, School of Medicine, University of Connecticut Health Center, 263 Farmington Avenue, Farmington, CT 06030-3105, USA
Correspondence to: I. Goldschneider; E-mail: igoldsch{at}neuron.uchc.edu
Quantitative intrathymic (i.t.) and i.v. adoptive transfer assays for prothymocytes show strict log dose saturation kinetics, consistent with a finite number of i.t. binding sites (microenvironmental niches). This inference is supported here by demonstration of competitive antagonism obeying one-on-one receptor occupancy kinetics during the establishment of thymic chimerism in irradiated adult mice. The results of primary and secondary transfer experiments suggested that hematogenous precursors (i) enter specific i.t. niches between 4 and 24 h after injection, (ii) compete reversibly with subsequently introduced precursors, (iii) establish unsurmountable competition within 5–7 days, (iv) mature through the initial stages of thymocytopoiesis preceding proliferative expansion, and (v) vacate the niches between 7 and 14 days after entry. The results also suggested that, as in non-irradiated mice, prothymocyte importation in irradiated mice is a gated phenomenon. Gate closure was indicated by the inability of i.v.-, but not i.t.-, injected bone marrow (BM) cells to induce thymic chimerism when administered 7–14 days after a primary injection and gate opening by the ability of i.v.-injected BM cells to induce thymic chimerism in competition with circulating host prothymocytes. Gate closing was log dose-responsive and could be induced in individual thymic lobes by unilateral i.t. injection, whereas gate opening, which occurs bilaterally, was not initiated until most of the niches for prothymocytes had been vacated. We therefore posit the existence of a series of associated microvascular gates and microenvironmental niches that act in concert to regulate prothymocyte importation and early thymocyte differentiation.
Keywords: bone marrow, lymphoid migration, lymphoid organization, thymus
Transmitting editor: A. Singer
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