Human monocyte-derived suppressor cells control graft-versus-host disease by inducing regulatory forkhead box protein 3-positive CD8⁺ T lymphocytes

Background Adoptive transfer of immunosuppressive cells has emerged as a promising strategy for the treatment of immune-mediated disorders. However, only a limited number of such cells can be isolated from in vivo specimens. Therefore efficient ex vivo differentiation and expansion procedures are critically needed to produce a clinically relevant amount of these suppressive cells. Objective We sought to develop a novel, clinically relevant, and feasible approach to generate ex vivo a subpopulation of human suppressor cells of monocytic origin, referred to as human monocyte-derived suppressive cells (HuMoSCs), which can be used as an efficient therapeutic tool to treat inflammatory disorders. Methods HuMoSCs were generated from human monocytes cultured for 7 days with GM-CSF and IL-6. The immune-regulatory properties of HuMoSCs were investigated in vitro and in vivo. The therapeutic efficacy of HuMoSCs was evaluated by using a graft-versus-host disease (GvHD) model of humanized mice (NOD/SCID/IL-2Rγ_c ^-/- [NSG] mice). Results CD33⁺ HuMoSCs are highly potent at inhibiting the proliferation and activation of autologous and allogeneic effector T lymphocytes in vitro and in vivo. The suppressive activity of these cells depends on signal transducer and activator of transcription 3 activation. Of therapeutic relevance, HuMoSCs induce long-lasting memory forkhead box protein 3-positive CD8⁺ regulatory T lymphocytes and significantly reduce GvHD induced with human PBMCs in NSG mice. Conclusion Ex vivo-generated HuMoSCs inhibit effector T lymphocytes, promote the expansion of immunosuppressive forkhead box protein 3-positive CD8⁺ regulatory T cells, and can be used as an efficient therapeutic tool to prevent GvHD.