Expression of HLA class I on the cell surface of MSCs during in vitro NK cell development. (A) Representative micrographs (original magnification ×100)depicting the morphology of MSCs during NK cell differentiation from day 0 to day 28. MSCs keep their typical extended shape and are found in close contact with developing NK cells. (B) HLA class I expression on the surface of MSCs at the start and after 2 and 4 weeks of culture using the pan–HLA class I antibody W6/32. Gating was performed on the basis of a control staining with secondary antibody only. (C) Expression of HLA-C on MSCs at day 14 using isotype control (left panel) and FITC-conjugated HLA-C–specific monoclonal antibody DT9 (right panel).