Hematopoietic stem cells (HSCs) differentiate into mature lineage restricted blood cells under the influence of a complex network of hematopoietic cytokines, cytokine-mediated transcriptional regulators, and manifold intercellular signaling pathways. The classical model of hematopoiesis proposes that progenitor cells undergo a dichotomous branching into myelo-erythroid and lymphoid lineages. Nonetheless, erythroid and lymphoid restricted progenitors retain their myeloid potential, supporting the existence of an alternative 'myeloid-based' mechanism of hematopoiesis. In this case, abnormal pathology is capable of dysregulating hematopoiesis in favor of myelopoiesis. The accumulation of immature CD11b+Gr-1+ myeloid-derived suppressor cells (MDSCs) has been shown to correlate with the presence of several hematopoietic cytokines, transcription factors and signaling pathways, lending support to this hypothesis. Although the negative role of MDSCs in cancer development is firmly established, it is now understood that MDSCs can exert a paradoxical, positive effect on transplantation, autoimmunity, and sepsis. Our conflicted understanding of MDSC function and the complexity of hematopoietic cytokine signaling underscores the need to elucidate molecular pathways of MDSC expansion for the development of novel MDSC-based therapeutics.
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