THE ROLE OF THYMOCYTES IN REGULATING THYMIC EPITHELIAL-CELL GROWTH AND FUNCTION

The basic tenet underlying the present work and supported by recent st udies is that there is a dialogue between developing thymocytes and th ymic stromal cells. One direction in this dialogue, i.e. thymic stroma l cell role in shaping thymocyte maturation, has been extensively stud ied. 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 a nalysis of this interaction may add substantial insight into the proce ss, as demonstrated by the present work. We made use of a culture syst em of either murine thymic epithelial cells (TEC line) cultured alone or cocultured with thymocytes. Unstimulated thymocytes or their supern atant caused 40-80% inhibition of TEC cell proliferation, as measured by H-3-thymidine incorporation. Cell cycle analysis by flow cytometry indicated that this inhibition can be attributed to reduction in G(2)/ M phase cell number pari passu with an increase in G(o)/G(1) cell numb er. This inhibitory effect was found to be partially mediated by TGF-b eta 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 t hymocyte pretreatment with formaldehyde or emetine. Furthermore, antib odies against thymocyte adhesion molecules (CD2, LFA-1) blocked the th ymocyte-induced IL-6 secretion. IL-6 was found to be an autocrine grow th factor of TEC in culture, since a combination of anti IL-6 and anti IL-6 receptor antibodies caused 70% inhibition of TEC proliferation a nd addition of exogenous recombinant IL-6 doubled the rate of prolifer ation. These results suggest that thymocytes regulate thymic epithelia l cell growth by a complex set of inhibitory and enhancing signals med iated through either soluble factors or direct contact. The ultimate e ffect is dependent on the balance between different signals and may be different in different microenvirommental settings in vivo. In cocult ure in vitro the dominant effect was growth inhibition of the epitheli al cells by thymocytes.