Identification of monocyte-like precursors of granulocytes as a mechanism for accumulation of PMN-MDSC in cancer [microarray]

Polymorphonuclear myeloid-derived suppressor cells (PMN-MDSC) inhibit immune responses in cancer, limit the effects of therapies and promote tumor cell metastasis. Here, we have identified a new precursor that differentiates into granulocytes in vitro and in vivo yet belong to the monocytic lineage. We have termed these cells monocyte-like precursors of granulocytes (MLPG). Under steady state conditions MLPG were absent in the spleen and barely detectable in the bone marrow (BM). In contrast, these cells significantly expanded in tumor-bearing mice. Selective depletion of monocytic cells had no effect on the number of granulocytes present in naïve mice but decreased the population of PMN-MDSC in tumor-bearing mice by 50%. The expansion of MLPG was found to be controlled by the down-regulation of the protein Rb1 and not the IRF8 transcription factor that is known to regulate the expansion of PMN-MDSC from classical granulocytes precursors. In cancer patients, putative MLPG were found within the population of CD15 CD14+HLA-DR-/lo M-MDSC. CXCR1+CD15 CD14+HLA-DR-/lo cells lacked immune suppressive activity but had potential to differentiate to neutrophils in contrast to monocytes and CXCR1- suppressive M-MDSC. These findings describe a mechanism of abnormal myelopoiesis in cancer and suggest potential new approaches for selective targeting of MDSC. Microarray gene expression of monocyte-like precursors of granulocytes (MLPG) and granulocytic progenitors (GP)