NK cell development in a human stem cell niche: KIR expression occurs independently of the presence of HLA class I ligands

The development of mature natural killer (NK) cells expressing killer cell immunoglobulin-like receptors (KIRs) depends on cell contact-dependent signals from nonhematopoietic cells. So far, detailed studies of this process have been hampered by the lack of an appropriate in vitro model. Here, human bone marrow-derived mesenchymal stem cells (MSCs), generated under good manufacturing practice (GMP) conditions, are established as a supportive niche for in vitro NK cell differentiation. In the presence of MSCs, cord blood and bone marrow-derived hematopoietic stem and progenitor cells (HSPCs) effectively and reproducibly differentiated into mature KIR-expressing NK cells. Notably, the novel in vitro differentiation assay enabled us to analyze the impact of HLA class I ligands on KIR repertoire development. To this end, a panel of MSC lines divergent for expression of the major KIR ligands C1, C2, and Bw4 was used for NK cell differentiation. The resulting NK cell repertoires were independent of the presence of specific KIR ligands on MSCs and were, in fact, invariably dominated by expression of the C1-specific inhibitory KIR2DL3. Similarly, short hairpin RNA-mediated knockdown of HLA class I ligands on MSCs did not delay or change the course of KIR expression. Our data suggest that the initial acquisition of KIRs during NK cell development is biased toward recognition of C1 ligands, irrespective of the presence of self-ligands. Altogether, the MSC/HSPC model constitutes a novel platform to study NK cell development in a human stem cell niche. Moreover, the system constitutes a promising GMP-compliant platform to develop clinical-grade NK cell products from cord blood HSPCs.