Human mesenchymal stem cells require monocyte-mediated activation to suppress alloreactive T cells.

Human bone marrow-derived mesenchymal cells (MSCs) are precursors of nonhematopoietic mesenchymal cells of the bone marrow microenvironment. MSCs were shown to inhibit alloreactive T lymphocytes, but the mechanism and mediators of this effect are not fully understood. Here we describe a novel interaction between blood monocytes and bone marrow-derived, culture-expanded MSCs, which results in inhibition of T-lymphocyte activation. We found that CD14+ monocytes from blood activate MSCs to secrete inhibitory molecules that lead to inhibition of alloreactive T cells. This cellular communication is not contact-dependent, but rather is mediated by soluble factors that include interleukin (IL)-1beta. MSC-mediated inhibition of alloreactive T lymphocytes is associated with downregulation of activation markers CD25, CD38, and CD69 detected both in CD4+ and CD8+ T lymphocytes. The cytokines secreted by MSCs that mediate T-cell inhibition include transforming growth factor-beta1, but not IL-10. The interaction between blood monocytes and the MSCs represents a unique immune regulatory paradigm that can potentially be exploited in clinic.

Expansion of LTC-ICs and maintenance of p21 and BCL-2 expression in cord blood CD34(+)/CD38(-) early progenitors cultured over human MSCs as a feeder layer.

Allogeneic transplantation with umbilical cord blood (UCB) is limited in adult recipients by a low CD34(+) cell dose. Clinical trials incorporating cytokine-based UCB in vitro expansion have not demonstrated significant shortening of hematologic recovery despite substantial increases in CD34(+) cell dose, suggesting loss of stem cell function. To sustain stem cell function during cytokine-based in vitro expansion, a feeder layer of human mesenchymal stem cells (MSCs) was incorporated in an attempt to mimic the stem cell niche in the marrow microenvironment. UCB expansion on MSCs resulted in a 7.7-fold increase in total LTC-IC output and a 3.8-fold increase of total early CD34(+) progenitors (CD38(-)/HLA-DR(-)). Importantly, early CD34(+)/CD38(-)/HLA-DR(-) progenitors from cultures expanded on MSCs demonstrated higher cytoplasmic expression of the cell-cycle inhibitor, p21(cip1/waf1), and the antiapoptotic protein, BCL-2, compared with UCB expanded in cytokines alone, suggesting improved maintenance of stem cell function in the presence of MSCs. Moreover, the presence of MSCs did not elicit UCB lymphocyte activation. Taken together, these results strongly suggest that the addition of MSCs as a feeder layer provides improved conditions for expansion of early UCB CD34(+)/CD38(-)/HLA-DR(-) hematopoietic progenitors and may serve to inhibit their differentiation and rates of apoptosis during short-term in vitro expansion.