Cancer is associated with a profound perturbation in myelopoiesis that results in the accumulation of myeloid-derived suppressor cells (MDSCs) to promote disease progression. Recent studies in mice suggest that tumor-derived factors could regulate the differentiation of hematopoietic stem and progenitor cells (HSPCs) in the bone marrow and subsequently contribute to dysregulation of hematopoiesis. However, the nature and role of HPSCs in patients with cancer remain unknown. Here we show, in detailed studies of the peripheral blood from 133 untreated patients with seven different types of tumors, that the composition of circulating HSPCs was significantly altered in patients with solid tumors. The frequencies of circulating granulocyte-monocyte progenitors (GMPs) were increased four to seven fold in all types of tumors examined, and the circulating hematopoietic precursors exhibited myeloid bias with a skew toward granulocytic differentiation in patients with solid tumors. These myeloid precursors are selectively enriched in tumor tissues, and the high levels of circulating GMPs were positively correlated with disease progression. By using cord blood-derived CD34 + cells, we developed an in vitro short-term culture model to effectively induce the rapid generation of MDSCs. We found that, among the factors produced by various tumors, GM-CSF, granulocyte colony-stimulating factor, and IL-6 could not only promote the myeloidbiased differentiation, but also induce the differentiation of myeloid precursors into functional MDSCs. These findings suggest that the altered circulating HSPCs may serve as an important link between dysregulated bone marrow hematopoiesis and accumulated MDSCs in patients with cancer.

Cancer progression is associated with alterations of intra- and extramedullary hematopoiesis to support a systemic tumor-promoting myeloid response. However, the functional specialty, mechanism, and clinical relevance of extramedullary hematopoiesis (EMH) remain unclear. Here, we showed that the heightened splenic myelopoiesis in tumor-bearing hosts was not only characterized by the accumulation of myeloid precursors, but also associated with profound functional alterations of splenic early hematopoietic stem/progenitor cells (HSPCs). With the distinct capability to produce and respond to granulocyte-macrophage CSF (GM-CSF), these splenic HSPCs were "primed" and committed to generating immunosuppressive myeloid cells. Mechanistically, the CCL2/CCR2 axis-dependent recruitment and the subsequent local education by the splenic stroma were critical for eliciting this splenic HSPC response. Selective abrogation of this splenic EMH was sufficient to synergistically enhance the therapeutic efficacy of immune checkpoint blockade. Clinically, patients with different types of solid tumors exhibited increased splenic HSPC levels associated with poor survival. These findings reveal a unique and important role of splenic hematopoiesis in tumor-associated myelopoiesis.

BACKGROUND. Despite an increasing appreciation of the roles that myeloid cells play in tumor progression and therapy, challenges remain in interpreting the tumor-associated myeloid response balance and its translational value. We aimed to construct a simple and reliable myeloid signature for hepatocellular carcinoma (HCC). METHODS. Using in situ immunohistochemistry, we assessed the distribution of major myeloid subtypes in both peri-and intratumoral regions of HCC. A 2-feature-based, myeloid-specific prognostic signature, named the myeloid response score (MRS), was constructed using an L1-penalized Cox regression model based on data from a training subset (n = 244), a test subset (n = 244), and an independent internal (n = 341) and 2 external (n = 94; n = 254) cohorts. RESULTS. The MRS and the MRS-based nomograms displayed remarkable discriminatory power, accuracy, and clinical usefulness for predicting recurrence and patient survival, superior to current staging algorithms. Moreover, an increase in MRS was associated with a shift in the myeloid response balance from antitumor to protumor activities, accompanied by enhanced CD8 + T cell exhaustion patterns. Additionally, we provide evidence that the MRS was associated with the efficacy of sorafenib treatment for recurrent HCC. CONCLUSION. We identified and validated a simple myeloid signature for HCC that showed remarkable prognostic potential and may serve as a basis for the stratification of HCC immune subtypes.

Myeloid cells are key components of the tumor microenvironment and critical regulators of disease progression. These innate immune cells are usually short-lived and require constant replenishment. Emerging evidence indicates that tumors alter the host hematopoietic system and induce the biased differentiation of myeloid cells to tip the balance of the systemic immune activities toward tumor-promoting functions. Altered myelopoiesis is not restricted to the bone marrow and also occurs in extramedullary organs. In this review, we outline the recent advances in the field of cancer-associated myelopoiesis, with a focus on the spleen, the major site of extramedullary hematopoiesis in the cancer setting. We discuss the functional specialization, distinct mechanisms, and clinical relevance of cancer-associated myeloid cell generation from early progenitors in the spleen and its potential as a novel therapeutic target.

We recently identified CXCR4 as a novel vascular marker for vessel sprouting in hepatocellular carcinoma (HCC) tissues. Thus, CXCR4 endothelial cells (ECs) could serve as a potential predictor for patients who may benefit from sorafenib treatment; however, the mechanism that regulates vascular CXCR4 expression in HCC remains largely unknown. Here, we revealed a large number of monocytes/macrophages (Mo/Mϕ) to be selectively enriched in the perivascular areas of CXCR4 vessels in HCC samples. The depletion of Mo/Mϕ with gadolinium chloride (GdCl) or zoledronic acid (ZA) treatment significantly reduced vascular CXCR4 expression in HCC tumors. This phenomenon was also confirmed in CCR2-KO mice, which exhibited reduced infiltration of inflammatory Mo/Mϕ in tumor tissues. Mechanistic studies revealed that inflammatory cytokines derived from tumor conditioned Mo/Mϕ, especially TNF-α, could up-regulate CXCR4 expression on ECs. TNF-α-induced activation of the Raf-ERK pathway, but not Notch signaling, was responsible for the expression of CXCR4. Moreover, the combination treatment of sorafenib with ZA was associated with improved anti-tumor efficacy by significantly reducing vascular CXCR4 expression. These findings revealed that Mo/Mϕ could regulate CXCR4 expression in the tumor vasculature. Thus, the inhibition of Mo/Mϕ inflammation might enhance the treatment efficacy of sorafenib in HCC.