In this study, we have isolated cells from the mesenchymal region of amnion and identified two subpopulations discordant for expression of the HLA-DR, CD45, CD14, and CD86 cellular markers. We have therefore referred to the unfractionated cell population derived from this region as amniotic mesenchymal tissue cells (AMTC). We have studied the suppressive and stimulatory characteristics of the unfractionated, HLA-DR-positive and HLA-DR-negative AMTC populations and demonstrate that all three fail to induce an allogeneic T cell response. However, unfractionated AMTC, which could inhibit T cell allogeneic proliferation responses, induced proliferation of T cells stimulated via the T cell receptor (TcR), in a cell-cell contact setting. We have shown that this stimulatory capacity can be attributed to the HLA-DR-positive AMTC subpopulation. Indeed, even though the HLA-DR-positive AMTC fraction surprisingly failed to induce proliferation of resting allogeneic T cells, they could cause strong proliferation of anti-CD3-primed allogeneic T cells. This stimulatory effect was not observed using the HLA-DR-negative AMTC fraction.

The revelation that human amniotic mesenchyme possesses cell populations with both suppressive or stimulatory properties sheds additional light on the immunomodulatory functions of this tissue and may contribute to the clarification of some ongoing controversies associated with mesenchymal stromal cells of other sources, such as the presence of HLA-DR-positive cells and the suppressive versus stimulatory properties of these cells.