ER stress sensor PERK drives tumor-promoting myelopoiesis by reprogramming hematopoietic progenitor cells in the spleen

The spleen, as the prominent site for extramedullary myelopoiesis in cancer, generates significant amounts of myeloid cells to promote tumor progression. Cancer-associated splenic myelopoiesis is mediated by a population of functionally distinct hematopoietic stem/progenitor cells (HSPCs) that are committed to immunosuppressive myeloid cell generation, but the molecular mechanism regulating this response remains unclear. Here, we found that HSPCs in the spleen of tumor-bearing mice exhibited accelerated protein synthesis and engaged in a robust endoplasmic reticulum (ER) stress response. Single-cell RNA sequencing showed that the ER stress response was concomitant with the myeloid-biased lineage commitment and dysfunction of HSPCs in the spleen. PKR-like ER resident kinase (PERK), a molecular sensor of ER stress, directed HSPC differentiation into myeloid-derived suppressor cells (MDSCs) via PERK–eIF2α–ATF4–C/EBPβ signaling. Inhibition of this signaling prevented the emergence of granulocyte/macrophage colony-stimulating factor-expressing HSPCs in the spleen and disrupted the positive feedback-driven splenic myelopoiesis. Consequently, a blockade of PERK abated the suppressive function of tumor-infiltrating MDSCs and increased cytotoxic T cell infiltration, reshaping the tumor microenvironment to effectively suppress disease progression and potentiate both immuno- and targeted therapies. In accordance, activation of PERK–ATF4–C/EBPβ signaling was indispensable for the differentiation of human umbilical cord blood-derived HSPCs into functionally competent MDSCs. Together, these results indicate a crucial role for the cellular ER stress sensor PERK in modulating splenic HSPC activities in cancer, suggesting novel therapeutic opportunities for targeting the tumor-promoting myeloid response at its source. Raw data are available on Genome Sequence Archive, Bioproject accession number: PRJCA003519 LSK cells from the BM and spleen of Hepa mice were isolated and purified by FACS. The cells were then processed for scRNA-seq using the Chromium Single Cell 3’ Library & Gel Bead Kit v3 (10x Genomics) following the manufacturer’s protocol. Samples were sequenced at an average of 27,000 reads per cell. The resulting scRNA-seq reads were aligned to the GRCm38 (mm10) reference genome and quantified using cellranger count (Cell Ranger pipeline version 3.1.0, 10x Genomics).