The Role of Thymocytes in Regulating Thymic Epithelial Cell Growth and Function

Abstract

The basic tenet underlying the present work and supported by recent studies is that there is a dialogue between developing thymocytes and thymic stromal cells. One direction in this dialogue, i.e. thymic stromal cell role in shaping thymocyte maturation, has been extensively studied. The other direction, thymocyte effect on stromal cell development and function, started to emerge only recently on the basis of in vivo observations in SCID and knockout mice. An in vitro approach to the analysis of this interaction may add substantial insight into the process, as demonstrated by the present work. We made use of a culture system of either murine thymic epithelial cells (TEC line) cultured alone or cocultured with thymocytes. Unstimulated thymocytes or their supernatant caused 40-80% inhibition of TEC cell proliferation, as measured by 3H-thymidine incorporation. Cell cycle analysis by flow cytometry indicated that this inhibition can be attributed to reduction in G2/M phase cell number pari passu with an increase in Go/G1 cell number. This inhibitory effect was found to be partially mediated by TGF-beta produced by thymocytes. On the other hand, thymocytes augmented IL-6 production by TEC cells in coculture, an effect which could not be reproduced by thymocyte culture supernatant and was not inhibited by thymocyte pretreatment with formaldehyde or emetine. Furthermore, antibodies against thymocyte adhesion molecules (CD2, LFA-1) blocked the thymocyte-induced IL-6 secretion. IL-6 was found to be an autocrine growth factor of TEC in culture, since a combination of anti IL-6 and anti IL-6 receptor antibodies caused 70% inhibition of TEC proliferation and addition of exogenous recombinant IL-6 doubled the rate of proliferation. These results suggest that thymocytes regulate thymic epithelial cell growth by a complex set of inhibitory and enhancing signals mediated through either soluble factors or direct contact. The ultimate effect is dependent on the balance between different signals and may be different in different microenvironmental settings in vivo. In coculture in vitro the dominant effect was growth inhibition of the epithelial cells by thymocytes.

A Unique Thymic Fibroblast Population Revealed by the Monoclonal Antibody MTS-15

“…Indeed, previous studies have indicated IL-6 is an autocrine proliferative factor for TECs (25), supported here by the overlapping expression profiles of IL-6 and IL-6R on fibroblast and TEC subsets. A role for IL-6 in thymic atrophy and regeneration has previously been implied by studies of mice rendered deficient in STAT-3 (a transducer of, among other factors, IL-6 signaling) in TEC, which showed increased susceptibility to thymic atrophy (46).…”

“…TEC exhibited significant increases in growth factors supporting early thymocyte development (SCF and IL-7) and intrathymic migration (CXCL12, CCL25, and CCL21) (all p Ͻ 0.01; Student's t test). In MTS-15 ϩ fibroblasts the highest increases were observed in the growth factors IL-6, FGF-7 (keratinocyte growth factor), and FGF-10, which support TEC proliferation (7,25) (Fig. 6B).…”

Effect of Transmitters and Co-Transmitters of the Sympathetic Nervous System on Interleukin-6 Synthesis in Thymic Epithelial Cells

“…IL-6 induces the expression of the p55 IL-2 receptor [22] and serves as a second signal for the production of IL-2 by CD4+ thymocytes activated by a submitogenic concentration of mitogen or by T-cell receptor cross-linking [23]. In addition to its effects on thymocytes, IL-6 has been shown to act as an autocrine (or thymocyte-derived) growth factor for TEC themselves [24,25]. A thymic hyperplasia found in patients with myasthenia gravis may be caused by a de-regulated IL-6 production in TEC resulting in an increased TEC proliferation [26].…”

Expression of cyclin D1 in epithelial tissues of transgenic mice results in epidermal hyperproliferation and severe thymic hyperplasia.

“…Cyclin Dl overexpression might enable thymic epithelial cells to escape negative growth regulatory signals and continue to proliferate, providing an ever growing environment for T-cell development. In this regard, a recent study showed that thymocytes could inhibit thymic epithelial cell proliferation in culture, in part through production of transforming growth factor 1B (TGF-3) (37). Also, cyclin Dl overexpression in an esophageal epithelial cell line resulted in reduced TGF-,B growth inhibition (38).…”