Characterization of lymph node stromal cells during Treg-mediated tolerance

Solid organs transplantation is therapy of choice for several human diseases. The success of this treatment is determined by the recipientÕs ability to prevent rejection. Rejection is currently controlled by immunosuppressive drugs, which unfortunately have side effects for patient quality of life as well as long-term graft survival. It is therefore important to better understand the molecular mechanisms underlying tolerance and rejection.
Regulatory T (Treg) cells are known to play an important role in suppressing immune responses, and have been demonstrated to be important for establishing allograft tolerance. To study Treg-mediated suppression we have generated a skin transplantation model in which the transfer of polyclonal Treg cells suppresses anti-Bm12 specific CD4 T cells (ABM cells) in a ratio dependent manner. Firstly, we characterized the migration pattern of both ABM and Treg cells to the skin graft and to the dLNs. High numbers of Treg cells delay the proliferation and reduce the IFN-g production of ABM cells in the LNs. The release of IFN-g by activated T cells was shown to influence the LN microenvironment comprised of stromal cells, which provide a structural environment for the homeostasis and differentiation of lymphocytes. Furthermore, T zone reticular cells (TRCs) were shown to provide survival factors and negatively regulate activated T cells during strong inflammatory responses. We showed that the presence of Treg cells was modifying the TRC activity during the establishment of tolerance. TRCs isolated from LNs of mice tolerating the graft exhibited enhanced transcription of the chemokines CCL19 and CCL21, the adhesion molecules ICAM-1 and VCAM-1 and the cytokines TSLP and IL-7 compared to TRCs isolated from mice transferred with ABM cells only. In our analysis we focused on the survival factor IL-7 and its role in activation. We found that IL-7 signaling in activated Treg cells leads to further upregulation of the IL-2Ra-chain CD25 and enhances Treg suppressive capacity in vitro.
In summary, Treg cells suppress activation and IFN-g release by effector T cells and promote TRC release of IL-7. We hypothesize that TRC derived IL-7 may be required to maintain high numbers of Treg cells in the LNs and to prevent skin graft alloresponses. In addition, IL-7 might sensitize Treg cells and facilitate their recognition of IL-2, leading to Treg cell expansion and better suppression of effector T cell responses.