S100A9 and myelodysplastic syndrome: In addition, MDSCs can exert a direct suppressive effect on the growth of erythroid and myeloid progenitor cells in vitro (88), and very recent data from Cheng et al. showed that the immune checkpoint receptor programmed cell death protein-1 (PD-1) and its ligand programmed cell death-ligand 1 (PD-L1) are upregulated in response to S100A9 signaling and contribute to MDSC-induced HSPC death in MDS (90).