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.

Adipocytes differentiated in vitro from rat mesenchymal stem cells lack essential free fatty acids compared to adult adipocytes.

Adult bone marrow mesenchymal stem cells (BMSCs) can be differentiated in vitro to become adipocyte-like cells with lipid vacuoles, similar to adipocytes derived from adult adipose tissue. Little is known regarding the composition of free fatty acids (FFAs) of the in vitro-differentiated adipocytes, or whether it resembles that of native adult adipocytes. We used gas chromatography-mass spectrometry to identify FFA species in BMSC-derived adipocytes and compared them with FFAs found in adipocytes derived from adult adipose tissue. We found that adult adipocytes contained significant percentages of saturated and monounsaturated FFAs, including palmitic acid (C16:0), stearic acid (C18:0), and oleic acid (C18:1); some polyunsaturated FFAs, such as linoleic acid (C18:2), a small percentage of arachidonic acid (C20:4), and very little linolenic acid (C18:3). In comparison, 80%-90% confluent BMSCs contained comparable percentages of palmitic and oleic acids, significantly more arachidonic and stearic acids, very little linoleic acid, and no linolenic acid. After differentiation, compared with adult adipocytes, BMSC-derived adipocytes contained a comparable percentage of palmitic acid, more stearic and arachidonic acids, less oleic acid, almost no linoleic acid, and no detectable linolenic acid. This composition was quite similar to that of undifferentiated BMSCs. The differentiation medium contained only palmitic and stearic acids, with traces of oleic acid; it did not contain the essential polyunsaturated fatty acids. Thus, the composition of FFAs in BMSC-derived adipocytes was altered compared with adult adipocytes. BMSC-derived adipocytes had an altered composition of saturated and monounsaturated FFAs and lacked essential FFAs that may directly affect signaling related to their lipolysis/lipogenesis functions.