Effect of CD44 signal axis in the gain of mesenchymal stem cell surface antigens from synovial fibroblasts in vitro

Tissue-residing mesenchymal stromal/stem cells (MSCs) have multipotent characteristics that are important for adult tissue homeostasis and tissue regeneration after injury. We previously reported that fibroblastic cells isolated from the synovial membrane in the knee joint give rise to cells with MSC characteristics in a two-dimensional culture. To explore the molecular mechanisms underlying these hyperplastic properties, we performed time-course surface antigen expression analyses during in vitro culture. Cells freshly isolated from the synovial membrane rarely contained cells that met the criteria (CD45(−)CD73(+)CD90(+)CD105(+)). However, the number of cells expressing MSC antigens increased on day 7. Flow cytometric analysis indicated that cells positive for either CD73 or CD90 were specifically derived from cells positive for CD44. CD44 expression was upregulated during culture, and CD105(+) cells were specifically derived from the CD44 highly expressing cells. In addition, depletion of hyaluronic acid (HA), a major ligand of CD44, decreased the number of CD105(+) cells, whereas supplementation with HA increased their number. These data suggest that intracellular signals activated by CD44 play an important role in the formation and/or maintenance of MSCs.