Thymocyte expansion and maturation: crosstalk of CD44v6 on thymocytes and panCD44 on stroma cells

Mesenchymal stromal cells support the viability and differentiation of thymocytes through direct contact in autologous co-cultures

The development of thymocytes and generation of mature T cells is a complex process that requires spatio-temporal interactions of thymocytes with the other cells of the thymus microenvironment. Recently, mesenchymal stromal cells were isolated from the neonatal human thymus and differentiated into chondrogenic, osteogenic, and adipogenic lineages, just like their bone marrow counterparts. However, their function in thymocyte homeostasis is unknown. In our autologous co-cultures of rat mesenchymal stromal cells and thymocytes, the stromal cells preserve the viability of cultured thymocytes and stimulate the development of CD4-CD8- double-negative and the maturation of mainly CD4+ single-positive thymocytes. Thymocytes also influence the stemness of bone marrow mesenchymal stromal cells, as their expression of CD44, a marker associated with cellular proliferation and migration, is reduced in co-cultures. Mesenchymal stromal cells' influence on thymocyte development requires direct physical contact between the two cells and is not mediated by a soluble factor. When the two types of cells were physically separated, the stimulative effects of mesenchymal stromal cells on thymocytes did not occur. Electron microscopy confirmed the close contact between the membranes of thymocytes and mesenchymal stromal cells. Our experiments suggest that membrane exchanges could occur between mesenchymal stromal cells and thymocytes, such as the transfer of CD44 from mesenchymal stromal cells to the thymocytes, but its functional significance for thymocytes development remains to be established. These results suggest that mesenchymal stromal cells could normally be a part of the in vivo thymic microenvironment and form a niche that could sustain and guide the development of thymocytes.

Immunohistochemical investigation of extracellular matrix components in the lymphoid organs of healthy pigs and pigs with systemic disease caused by circovirus type 2

The extracellular matrix (ECM) provides a scaffold for cell growth, impacts on cellular behaviour and plays an important role in pathological conditions. Several components of the ECM of lymphoid tissues have been shown to be crucial in the maturation, differentiation and migration of lymphocytes and other immune cells and, therefore, in the development of immune responses. Little is known of the composition and function of the ECM in porcine lymphoid tissues. The present study characterizes immunohistochemically the expression of several ECM-related molecules (i.e. hyaluronan [HA] and its receptor CD44, tenascin-C [TN-C] and versican) in primary and secondary lymphoid organs of healthy pigs and animals affected by porcine circovirus type 2-systemic disease (PCV2-SD). These ECM molecules displayed a highly defined expression pattern in healthy animals, suggesting that they may have a role in the compartmentalization of immune cells within lymphoid tissues. HA was abundant in the medulla of the thymus and follicles of secondary organs; CD44 and TN-C were present in the thymic medulla and parafollicular areas of secondary lymphoid organs; however, there was minimal expression of versican in healthy tissues. In PCV2-SD-affected animals, HA and CD44 showed a similar but more diffuse distribution. TN-C was increased in the T-cell-dependent areas and in tonsillar crypts, and versican was more abundantly expressed, with expression restricted to vascular structures and trabeculae and also surrounding tonsillar crypts. The altered expression in PCV2-SD-affected pigs was most probably related to a higher content of connective tissue secondary to tissue destruction and remodelling attempts as part of the disease process.