On the role of keratinocyte growth factor for thymic epithelial cell development and the protection from graft versus host disease

The function of the thymus is to provide the physiological microenvironment for the development of T lymphocytes. In this function, thymic epithelial cells play a critical role for the successful establishment and maintenance of the immune system’s capacity to distinguish between self and non-self. The cellular and architectural organizations of the thymus provide functionally distinct regions that control the separate and discrete maturational steps in the ordered differentiation from precursor cells to phenotypically mature and functionally competent T cells. The appropriate choice of the T cell antigen receptor (TcR) repertoire is the consequence of interactions with MHC classes I and II molecules on thymic epithelial cells (and other stromal cell types) underscoring the importance of thymic epithelial cells for thymocyte differentiation and selection. However, thymic epithelial cell homeostasis is severely disrupted after conditioning with radio-chemotherapy and allogeneic recognition by donor T cells in the context of allogeneic hematopoietic stem cell transplantation (HSCT). HSCT is the therapy of choice for a number of malignant and non -malignant diseases. The success of allogeneic HSCT depends on the efficiency by which the host’s immune system is restored. The thymus is one of the target organs of donor derived T cells and is severely damaged after HSCT. For this reason, the T-cell lineage is not re-generated as rapidly and efficiently as most other hematological lineages, which results in a protracted immune deficiency in the post-transplant period. Recent studies aimed at a better understanding of the regenerative mechanisms demonstrated that the donor-derived peripheral T-cell pool is restored through two independent pathways: (i) expansion of adoptively transferred mature T-cells, and (ii) intrathymic de novo generation of T-cells from donor-derived precursor cells. While transfer of mature T-cells during allogeneic HSCT may provide a short-term restoration of immune functions, but is limited by the complication of Graft versus Host Diseases (GVHD), the longlasting and complete reconstitution of the peripheral T cell pool depends ultimately on new T cell generation. Keratinocyte growth factor (KGF; a.k.a. Fibroblast Growth Factor 7) was described as a key effector molecule for the protection/restoration of epithelial cells following radio- chemotherapeutic conditioning; although the precise mechanism of action remain unknown. The presented thesis details the biological effects of KGF on the thymic microenvironment and its functions during postnatal homeostasis of the thymus and during T cell mediated damage in the course of GVHD. To investigate the molecular and cellular basis for the effect of KGF on function and architecture of thymic epithelial cells, different experimental systems were investigated. The in vitro proliferation capacity of different thymic epithelial cell lines in response to KGF exposure was tested and fetal thymic organ cultures (FTOC) were employed to analyze the effect of KGF in the complex ex vivo conditions. Additionally, thymic architecture and function were also investigated following the in vivo exposure to KGF. Under these conditions KGF induced the proliferation of thymic epithelial cells. Gene expression analysis of thymic epithelial cells treated ex vivo or in vivo with KGF revealed the upregulation of specific transcripts for Wnt and Bmp family members and IL-7, i.e. molecules known to regulate thymopoiesis. In consequence, KGF also stimulated an increase in thymic T lymphopoiesis resulting in an increase in size, weight and cellularity. Most importantly, KGF did not alter the architectural organization and the composition of the thymic epithelial subpopulation, thus allowing for a regular expansion and selection of thymocytes. These characteristics identified KGF as an ideal candidate for the preservation/restoration of thymic epithelial cell injury in the presence of acute GVHD. Indeed, when given to recipients of allogeneic T cells in a non-irradiated murine P->F1 transplantation model, KGF preserved normal thymic size, cellularity, epithelial cell architecture and thymocyte maturation. This normal function and phenotypic appearance of the thymus correlated with a decreased infiltration by donor-derived T cells. In addition, the typical GVHD-induced impairment in cell cycle progression of pro- and pre-T cells was also prevented by KGF. Moreover, the treatment with KGF decreased the number and in situ activation of infiltrating allogeneic T cells and drives the disease from an acute into a chronic course.
Taken together, these findings have detailed the role of KGF for thymic epithelial cell biology under physiological growth conditions, tissue maintenance, epithelial cell repair and protection in the presence of thymic GVHD.