Hydrocortisone Increases the Numbers of KL1⁺Cells, a Discrete Thymic Epithelial Cell Subset Characterized by High Molecular Weight Cytokeratin Expression

Abstract

The thymic epithelium, a major component of the thymic microenvironment, is a heterogeneous tissue bearing distinct monoclonal antibody-defined subsets. Among these, KL1+ cells represent a mouse medullary subpopulation characterized by high mol wt cytokeratin expression. Given the fact that thymic epithelial cells (TEC) express glucocorticoid receptors and that glucocorticoid hormones are known to modulate the expression of keratins, we decided to study the in vivo effects of hydrocortisone on KL1+ cells in normal and autoimmune mice. Within 24 h after a single injection of this steroid we observed a significant increase in the number of KL1+ cells. Interestingly, this effect was reversible and was no longer detected 7 days after treatment. Parallel studies analyzing the effects of hydrocortisone on the secretion of thymulin, a chemically defined thymic hormone revealed a transient decrease in serum levels of this hormone, but with different kinetics than the effects on KL1+ cells. Ontogenetic studies showed that the responsiveness of TEC to hydrocortisone, in terms of high mol wt cytokeratin expression, appeared late in fetal life and disappeared in aging animals. Importantly, aging, but also young adult, autoimmune mice were not responsive. In vitro experiments using a mouse TEC line confirmed the data observed in vivo demonstrating that the increase in KL1+ cells is a direct effect of hydrocortisone on TEC. The bulk of the data presently reported demonstrates that glucocorticoid hormone can act on TEC modulating the expression of both secretory and cytoskeletal protein families.

Accelerated Maturation of the Thymic Stroma in the Progeny of Adrenalectomized Pregnant Rats

“…Although a direct effect of the absence of GCs on the thymic epithelial cells cannot be discarded, since GCs stimulate the expression of certain cytokeratins in these cells [8,22], the parallel differentiation of thymic epithelium and thymocytes seems to suggest an indirect influence of GCs on epithelial cell development through thymocytes. In this regard, the re-expression on thymic cortical epithelial cells of specific cell markers characteristic of the embryonic periods after hydrocortisone treatment has been related to the arrival of primitive cell precursors to recover the thymic lymphoid cell population rather than to a direct effect of GCs on the epithelial cells [9].…”

Repeated in vivo Hydrocortisone Treatment Promotes a Dual Modulation of Cytokeratin Expression by Mouse Thymic Epithelial Cells

“…To analyze the effects on KL1+ cells following repeated HHC injections, groups of animals were treated on days 0, 1, 3, 6, 7 and 9, where day 0 corresponded to the untreated group, with 3-6 individuals being sacrificed 24 h after each injection, and thymuses immediately frozen in liquid nitrogen until cryostat sectioning. Since a transient effect upon KL1+ cells was seen after a single HHC dose [15], further animals were treated until day 9 and then sacrificed 1 or 2 weeks later.…”

“…Between each incubation, the slides were washed in phosphate-buffered saline (PBS). Controls included the use of the fluorescent conjugates alone or the replacement of KL1 by an unrelated mouse MAb, as detailed elsewhere [11,15].…”

“…In this respect, the model described herein may be regarded as promising in studies concerning the effect of repeated severe stress conditions upon the immune system in general. Since glucocorticoid hormones can modulate the expression of the extracellular matrix ligands and receptors [20,21] as well as the thymic hormones [15,22], all influencing intrathymic T cell differentiation, including selection, one can imagine that the repertoire of T cells formed by the normal thymus may be different from that produced by the altered thymic microenvironment induced by repeated glucocorticoid hormone injection, probably resulting in a distinct pattern of immune response.…”

“…For example, in the mouse thymus medulla a few TEC specifically express the CK pair 3/10 that can be immunocytochemically detected with an anti-CK MAb, named KL1 [10]. Interestingly, we noticed that such CK 3/10 expression can be upregulated in vivo and in vitro by a single hydrocortisone (hydrocortisone sodium hemisuccinate; HHC) treatment [15,16]. Nonetheless, such an effect was transient, with complete recovery being detected 7 days following a single HHC injection.…”