Thymocyte depletion affects neurotrophin receptor expression in thymic stromal cells

Abstract

Thymocytes and thymic stromal cells cross-talk in a bidirectional manner within the thymus, thus contributing to the generation of mature T-cells. The thymic stromal cells in the rat express the high- (TrkA, TrkB) and low-affinity (p75NTR) receptors for neurotrophins. In this study we analysed the regulation of TrkA, TrkB and p75NTR expression in the rat thymus by thymocytes. We induced thymocyte apoptosis by administration of corticoids in rats, and then analysed the expression and distribution of these receptors 1, 4 and 10 days later. Thymocyte death was assessed by the activation of caspase-3 in cells undergoing apoptosis. We observed massive thymocyte apoptosis 1 day after injection and, to a lesser extent, after 4 days, which was parallel with a reduction in the density of thymic epithelial cells normally expressing TrkA and p75NTR. Furthermore, TrkA expression was found in cortical thymic epithelial cells, which normally lack this receptor. The expression of TrkB was restricted to a subset of macrophage-dendritic cells, and remained unchanged with treatment. The normal pattern of neurotrophin receptor expression was almost completely restored by day 10. The results demonstrate that the expression of neurotrophin receptors by thymic epithelial cells, but not by macrophage-dendritic cells, is regulated by thymocytes.

Fig. 1

Structure of the thymus in control rats (a), and 1 (b), 4 (c) and 10 days (d) after corticoid administration. Administration of corticoids resulted in increased death of cortical thymocytes at 1 and 4 days, which showed typical pyknotic nuclei. By 10 days after corticoid treatment the structure of the thymus was almost identical to that of the control animals. c, cortex; m, medulla; scale bar = 100 µm.

Fig. 2

Immunohistochemical detection of activated caspase-3 in the thymus of control rats (a) and after treatment of corticoids (b–d). In control animals only few cells were immunoreactive, whereas most cortical thymocytes were immunoreactive 1 day after corticoid administration (b), decreased by 4 days (c) and reached near normal levels at 10 days (d). Most of the capase-3-positive cells were localized in the cortex. In addition to thymocytes, large cells identified as macrophages were found displaying positive immunostaining 4 days after treatment (c′). c, cortex; m, medulla; scale bar = 100 µm.

Fig. 3

Immunohistochemical detection of TrkA in the rat thymus. Subcapsular-perivascular and medullary epithelial cells displayed TrkA immunoreactivity (a). After 1 and 4 days of corticoid treatment there was a strong reduction in immunostaining in cells that normally express TrkA (b,c), and in expression of TrkA in cells that normally do not express this protein. By 10 days after treatment the pattern of expression was consistent with normality (d). c, cortex; m, medulla; scale bar = 100 µm.

Fig. 4

The pattern of immunostaining for cytokeratins in the rat thymus paralleled that of TrkA, and followed an identical behaviour after corticoid treatment. c, cortex; m, medulla; scale bar = 100 µm.

Fig. 5

p75^NTR, the low-affinity neurotrophin receptor, is expressed in a subpopulation of medullary thymic epithelial cells (a). Corticoid administration resulted in a decrease of immunoreactive cells 1 day (b) and 4 days (c) after treatment, reaching normal values by day 10 (d). c, cortex; m, medulla; scale bar = 100 µm.

Fig. 6

TrkB immunoreactivity was detected in a subpopulation of macrophages localized at the cortico-medullary border (a), and this pattern of distribution remained unchanged after corticoid administration (b–d). c: cortex; m: medulla; scale bar = 100 µm.

Fig. 7

ED1-positive cells, identified as macrophages, were distributed in both the cortex and the medulla of the rat thymus (a). Corticoid treatment produced an increase in the density of ED1-positive cells 1 day (b) and 4 days (c) after treatment, and showed a normal density at 10 days. c, cortex; m, medulla; scale bar = 100 µm.