Abstract
Increasing evidence from small animal models shows that myeloid-derived suppressor cells (MDSCs) can play a crucial role in inhibiting allograft rejection and promoting transplant tolerance. We identified CD3(-)CD20(-)HLA-DR(-)CD14(+)CD33(+)CD11b(+) cells in peripheral blood of healthy rhesus macaques. These putative monocytic MDSCs constituted 2.1% ± 1.7% of lin(-)HLA-DR(-) peripheral blood mononuclear cells. Administration of granulocyte-macrophage colony-stimulating factor (CSF) and granulocyte CSF increased their incidence to 5.3% ± 3.4%. The total number of MDSCs that could be flow sorted from a single whole rhesus leukapheresis product was 38 ± 13 × 10(6) (n = 10 monkeys). Freshly isolated or cryopreserved MDSCs from mobilized monkeys incorporated in cultures of anti-CD3- and anti-CD28-stimulated autologous T cells markedly suppressed CD4(+) and CD8(+) T cell proliferation and cytokine secretion (interferon γ, IL-17A). Moreover, these MDSCs enhanced CD4(+)CD25(hi)Foxp3(+) regulatory T cell (Treg) expansion while inhibiting proliferation of activated memory T cells and increasing Treg relative to effector and terminally differentiated memory T cells. Inhibition of arginase-1, but not inducible nitric oxide synthase activity, partially reversed the inhibitory effect of the MDSCs on CD8(+) T cell proliferation. Consequently, functional MDSCs can be isolated from nonhuman primates for prospective use as therapeutic cellular vaccines in transplantation.